WO2016142859A1

WO2016142859A1 - Method and system for illuminating seeding particles in flow visualisation

Info

Publication number: WO2016142859A1
Application number: PCT/IB2016/051306
Authority: WO
Inventors: Michael Atkins; Tongbeum KIM
Original assignee: University Of The Witwatersrand, Johannesburg
Priority date: 2015-03-08
Filing date: 2016-03-08
Publication date: 2016-09-15

Abstract

This invention concerns a flow visualisation system and in particular a Particle Image Velocimetry (P!V) system for performing flow visualisation around a test body. The system includes a laser beam generator and laser optics for transforming a laser beam into a substantially planar laser disc. The system further has means for recording the iliumination of seeding particles, carried by a fluid flowing around the test body, by the light disc. Means for analysing the recordings is used to determine the flow around the test body. In accordance with the invention the laser optics is located at least partially inside the test body such that the laser disc is emitted from within the test body. This invention also concerns a method of performing flow visualisation.

Description

METHOD AND SYSTEM FOR ILLUMINATING SEEDING PARTICLES IN

FLOW VISUALISATION

BACKGROUND TO THE INVENTION

The present invention relates to flow visualisation. In particular, but not exclusively, this invention relates to a system and method for determining fluid flow behaviour using Particle image Velocimetry (PIV). The invention also relates to system and method for performing flow measurement.

Velocity field measurement techniques can be broadly classified as either intrusive or non-intrusive [ ,2]. Intrusive techniques are typically point measurement techniques where the flow velocity is determined directly through the use of probes that are placed directly in the flow, such as a Pitot tube or hotwire sensor. Non-intrusive velocity measurement methods are generally optical techniques where the flow velocity within a local region is obtained by measuring the displacements of seeding or tracer particles that are typically introduced into the flow field [2]. These seeding particles should be small and neutrally buoyant to follow the flow field closely so that the trajectory of the particles is similar to the local flow field [3], By determining the change of position of the pattern of these particles within a finite time interval it is possible to determine the local velocity field. Particle Image Veiocimetry (PIV) is an example of such a measurement technique that can provide quantitative velocity field information. P!V is an optical measurement technique whereby numerous velocity vectors can be determined simultaneously for a specific region of the flow field, which is also referred to as flow field mapping [4].

A PIV system generally consists of five sub-systems, namely: 1) a fiow test rig, such as a wind tunnel; 2) a seeding generator; 3) a pulsed laser system with light sheet optics; 4) a Charge Coupled Device (CCD) digital camera system; and 5) PIV data acquisition and processing software. In a conventional PIV experiment involving a wind tunnel as shown in Figure 1 , seeding particles 120 are supplied using a seeding generator and are introduced into the flow field of the test rig 100. The test rig 100 has transparent walls 102 to allow the illumination of the seeding particles using the pulsed laser 104 and to enable the recording of the illuminated seeding particles in the PIV measurement area by a CCD camera 106. A primary objective in P!V is to create high quality PIV recordings of the pattern of seeding particles that are moving with the flow inside the test rig 100. These recordings will facilitate further analysis using the data acquisition and processing software to determine the velocity field in the PIV measurement area 108 (Figure 1). An outcome of optical flow measurement methods such as the PIV method is also the visualisation of certain flow structures that may or may not be distinguishable from the raw flow images. Accordingly, it shouid be understood that the broad concept of fiow visualisation also includes flow field mapping techniques.

The PIV recordings are created by synchronising high energy laser light sheet 110 pulses with the exposure of the CCD camera 106. In two dimensional (2D) PIV, the beam emitted from the laser is transformed into a thin laser light sheet using laser light sheet optics 112. The thin laser light sheet illuminates the seeding particles that fail within the plane of the light sheet. These seeding particles scatter the laser light which is then captured by a single or multiple CCD camera(s). The optical axis of the camera is typically perpendicular, to the light sheet.

Scattered incident laser light is transmitted through the camera lens and received by a photosensitive sensor within the CCD camera. The photosensitive sensor is typically a square array consisting of a number of small square photosensitive elements referred to as pixels [1]. The CCD camera outputs digital images related to the mean intensity of the incident laser light that falls onto each pixel during the exposure period. The digital images, which are also referred to as PIV recordings, have a horizontal and vertical resolution that is equivalent to the number of pixels that make up the photo-sensor.

In order to implement PIV methods that employ spatial cross correlation [3,5], single exposure double frame images are needed. This involves capturing two images of the flow field that are separated by the time interval (At) which is equal to the time between the light sheet pulses. Consequently, this requires that the laser is capable of firing two pulses of laser light that is in the visible light spectrum and in close succession to one another. This is typically achieved with PIV specific frequency doubled, dual cavity lasers [1,2]. This type of laser has two isolated laser cavities that emit green laser light, which for example has a wavelength of 532nm, and can be fired independently. The paths of the two laser beams are combined inside the laser unit and directed through a single aperture where they exit the laser unit. Similarly, PIV has special requirements of the CCD camera that allows a pair of images to be acquired in close succession to each other. CCD camera technology has been developed for PIV applications to enable dual image acquisition [1 ,4,6],

Using the data acquisition and processing software, the PIV recordings are interrogated to yield the measured velocity field. Accordingly, the accuracy and reliability of the evaluated velocity field is determined by the quality of the recordings.

Significant advances have been made in PIV over the last coupie of decades, which have enabled this measurement technique to become commercially viable [4]. However, PIV has an intrinsic limitation that stems from the manner in which the laser light is introduced into the flow test rig. Referring now to Figure 2, which shows a conventional test rig, the laser light sheet is first formed externally so that the light sheet optics is outside the flow field created by the test rig. The laser light sheet is then introduced into the flow area through the transparent walls of the test rig. if a solid test body 114 such as an aerofoil or a cylinder is placed in the path of the light sheet, a shadow 116 is formed as shown in Figure 2.

A significant drawback is that the seeding particles that are in the shadow region created by the test body cannot be illuminated and, as a result, they are invisible on the PiV recordings. Without illumination of the seeding particles no reliable velocity field information can be extracted from the PIV recordings. As a result of this limitation, conventional PIV methods and systems to obtain PIV velocity field measurements are limited to the regions of the flow field where there are no shadows [7-9].

It is an object of this invention to alleviate at least some of the problems experienced with conventional PIV methods and systems. It particular, it is an object of this invention to provide a method and system for the illumination of the seeding particles in flow visualisation in which the shadow region is eliminated.

It is a further object of this invention to provide a method and system that wit! be useful alternatives to existing PiV methods and systems. SUMMARY OF THE INVENTION in accordance with a first aspect of the invention there is provided a flow visualisation system for performing flow visualisation around a test body, the system including:

a light beam generator;

light optics for transforming a light beam generated by the light beam generator into a substantially planar light sheet; and

means for recording the illumination of seeding particles by the light sheet;

wherein the light optics are, in use, located at least partially inside the test body such that the light sheet is emitted from within the test body.

In the preferred embodiment at least a portion of the test body is transparent so as to allow the light sheet to be emitted through the transparent portion of the test body. Preferably, the test body is completely transparent.

The planar light sheet created by the light optics is preferably substantially isotropic.

The light beam originating from the light beam generator, the light optics and the means for recording the illumination of seeding particles may be substantially aligned, i.e. located in linear line.

The light optics may include a mirror-shroud assembly which includes an optical shroud and a mirror located inside the shroud. The mirror is preferably a cone mirror which has a cone angle of between 80 and 100 degrees, preferably 90 degrees, so as to produce a 360 degree light disc in a plane perpendicular to the light beam.

The mirror-shroud assembly may be movable along an axis which is substantially in line with the light beam so as to vary the light sheet thickness. The cone mirror is preferably movable inside the optical shroud so as to vary the brightness of the light sheet by moving the cone mirror relative to the optical shroud.

Preferably, the optical shroud has sharp edges. The edges may have a wedge angle of about 30 degrees.

The system may further include an opaque shield to prevent light scattering of the test body. Preferably, the system has a second optical shroud for supporting the shield. The second optical shroud may have sharp edges which have the same wedge angle as the optical shroud housing the cone mirror.

The system may further have an adaptor for connecting the fight beam generator to the test body.

In one embodiment of the system the light beam generator is a dual cavity, pulsed laser generator for generating two laser beams from the cavities.

Preferably, the system is a Particle Image Veiocimetry (PIV) system.

In accordance with a second aspect of the invention there is provided a method of performing flow visualisation including the following steps:

providing a test body;

locating laser optics at least partially inside the test body;

generating a light beam and directing the beam towards the light optics;

transforming the light beam into a substantially planar, isotropic light sheet by means of the light optics from within the test body;

introducing seeding particles in a fluid flowing around the test body; recording the illumination of the seeding particles by the light sheet; and

analysing the recordings.

Preferably, the method includes emitting the light sheet through a transparent portion of the test body. The method may include aligning the light beam generator, the light optics and the means for recording the illumination of seeding particles, i.e. locate them along a linear line.

The step of transforming the light beam into a substantially planar, isotropic light sheet may include using a mirror-shroud assembly which includes an optical shroud and a mirror located inside the shroud. The mirror is preferably a cone mirror which has a cone angle of between 80 and 100 degrees, preferably 90 degrees, so as to produce a 360 degree light disc in a plane perpendicular to the light beam.

The method may include controlling the light sheet thickness by moving the mirror-shroud assembly along an axis which is substantially in line with the light beam.

The method may further include controlling the brightness of the light sheet by moving the cone mirror relative to the optical shroud.

The method may also include minimising diffraction of the light sheet by providing a wedge angle on the optical shroud. The wedge angle may be about 30 degrees.

The method may include the step of shielding the test body by means of an opaque shield to prevent light scattering of the test body. Preferably, the method includes supporting the shield by means of a second optical shroud. The second optica! shroud may have sharp edges which have the same wedge angle as the optical shroud housing the cone mirror.

The method may include connecting the light beam generator or an auxiliary light delivery device to the test body using an adaptor.

The step of generating a light beam may include generating two pulsed laser beams from a dual cavity laser generator. Preferably, the method is a Partic!e Image Velocimetry (PIV) flow visualisation method.

According to another aspect of the invention there is provided a fiow visualisation system to measure the flux and paths of particles within a fluid system wherein the required thin layer laser beam sheet is emitted from within the fluid system such that an isotropic planar beam sheet is produced with 360 degree coverage.

Typically, the laser beam sheet is emitted from a transparent materia! inserted into the fluid system.

Preferably, the transparent material is a hollow rod, and the planar laser sheet is formed by reflecting the incoming laser beam along the axis of the rod using a 45 degree conical mirror inserted in the hollow rod, such that the laser sheet is formed in a plane perpendicular to the axis of the rod.

Preferably, the flow visualisation system is a particle image velocimetry system.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows a schematic representation of a conventional two- dimensional PIV setup;

Figure 2 shows a schematic representation of a conventional two- dimensional PIV setup in which a shadow created by the test body is visible; Figure 3 shows a schematic representation of a flow visualisation system in accordance with a first embodiment of the invention in which a plan view is shown in Figure 3(a) and a side view in Figure 3(b);

Figure 4 shows a cross-sectiona! view of laser disc optics of the system of Figure 3;

Figure 5 shows a schematic representation of a flow visualisation system in accordance with a second embodiment of the invention for multiple test bodies in which a side view is shown in Figure 5(a) and a front view in Figure 5(b);

Figure 6 shows a schematic representation of an experimental setup in which a closed type wind tunnel is shown in plan view in Figure 6(a) and in which a side view of a test section is shown in Figure 6(b);

Figure 7 shows a schematic representation of an experimental setup in which a test section for conventional external illumination is shown in Figure 7(a) and in which a test section for IIP illumination according to the invention is shown in Figure 7(b);

Figure 8 shows a cross-sectional view of laser disc optics of the conventional experimental setup of Figure 7(a);

Figure 9 shows the time averaged velocity vector field of the flow around the cylinders of the setup of Figure 6 in which the results for the conventional external illumination are shown in Figure 9(a), while the results for internal isotropic-planar (MP) illumination of the invention are shown in Figure 9(b); and Figure 10 shows the time averaged streamlines of the flow around the cylinders of the setup of Figure 6 in which the results for the conventional external illumination are shown in Figure 9(a), while the results for JiP illumination of the invention are shown in Figure 9(b).

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Referring to the drawings, in which like numerals indicate like features, a non-!imiting example of a flow visualisation system in accordance with a first embodiment of the invention is generaliy indicated by reference numeral 10.

It is envisaged that the system 10 could find particular application in the field of Particle image Velocimetry (PIV) to determine the fluid flow characteristics around a test body 12 as described above. However, the invention is not limited to the field of Particle Image Velocimetry (PIV) and could be used in other applications without departing from the spirit and scope of the invention. In this specification the term "flow visualisation" should be interpreted to mean the act of making fluid flow patterns visible. The system and method in accordance with the invention could also be used to perform flow measurement, which should, in turn, be understood to mean the quantification of fluid flow. It should further be understood that flow measurements typicaiiy follows flow visualisation seeing that the results obtained in flow visualisation, i.e. visible fluid flow, are used to perform flow measurement. It should therefore be understood that in this specification the term flow visualisation also covers flow measurement.

Referring now to Figure 3 it can be seen that the system 10 includes a light beam generator 14. In the preferred embodiment the light beam generator is in the form of a dual cavity, pulsed laser beam generator. The paths of the two laser beams generated in the two cavities of the laser beam generator 14 are combined inside the laser beam generator and exit through a single aperture of the laser beam generator, in use, the laser beam generator emits a circular, coliimated laser beam, (n a test rig, such as the one described below with under the heading Experimental Setup and Results, the laser beam generator is positioned in line with the test body 12. However, the laser beam generator 14 does not necessarily have to be in line with the test body 12. A light delivery device, such as an articulated arm for example, could be used to deliver the light beam into the test body 12. The only requirement is that the light beam be coaxial with the longitudinal central axis of the test body 12. In Figure 3 the test body is shown as a cylinder, the longitudinal, centre axis of which lies in the x-z plane. The laser beam generator 14 and test body 12 are arranged so that the laser beam is emitted along the centre axis of the test body.

Although the test body 12 is shown to be a cylinder in the accompanying drawings it should be understood that it could take any form. The invention is therefore not limited to the use of a test body of any particular shape. Accordingly, the invention could be used to perform flow visualisation around any article.

Figure 4 shows a schematic illustration of laser optics for transforming a laser beam generated by the laser beam generator into a substantially planar, isotropic laser sheet. The laser optics is located at least partially, preferably completely, inside the test body 12, thereby being located in the path of the laser beam. From Figure 4 it can be seen that the test body 12 is a thin walled, cylinder and houses the laser optics. The laser optics includes a mirror-shroud assembly 16. The assembly has an optical shroud 8 and a mirror 20 carried by the shroud. In the illustrated embodiment the mirror 20 is seated inside the shroud 18. The mirror 20 is movable relative to the shroud 18, thereby allowing the position of the mirror to be adjusted.

The mirror 20 is illustrated as a cone mirror which in this particular embodiment has a 90 degree cone angle. Although a 90 degree cone angle is preferred in this particular embodiment, it is envisaged that a cone angle of between 80 and 100 degrees could be used. As a result of the 90 degree cone angle the laser beam is transformed into a laser sheet (i.e. a type of light sheet) in a plane which is substantially perpendicular to the path of the laser, in Figure 4 the laser beam and iaser sheet are indicated by the reference numerals 22 and 24 respectively, in this embodiment the sheet 24 can also be referred to as a laser disc in view of the fact that it extends through 360 degrees about the centre axis or path of the Iaser beam 22. ft should be clear that the 360 degree coverage of the iaser disc is due to the 90 degree cone mirror.

For a given Iaser beam diameter and energy level, adjusting the depth of the cone mirror 20 inside of the shroud 18 varies the energy intensity of the laser light that is reflected from the cone mirror. If it is assumed that the energy intensity distribution of the laser beam has a Gaussian profile [1, 11], the maximum energy intensity is located at the centre of the Iaser beam 22. The energy then decays exponentially with increasing distance away from the centre of the laser beam 22. The brightness of the Iaser disc 24 can be increased if the light that passes through an aperture of the shroud 18 is reflected from a section of the incident beam that has the highest energy intensity, i.e. the centre of the Iaser beam 22.

Returning to the adjustable mirror 20, the brightness of the laser disc 24 is controlled by adjusting the position at which the mirror is seated inside the shroud 18. In other words, the brightness of the laser disc 24 is controlled by moving the mirror 20 relative to the optical shroud. Movement of the mirror 20 relative to the shroud 18 moves the mirror along the centre axis, which is also along the direction of the Iaser path.

Still referring to Figure 4, the optical shroud 18 is also movable along the centre axis of the test body, which is substantially in line with the Iaser beam as mentioned above. It must be understood that by moving the shroud 18 the entire mirror-shroud assembly 16 is moved. It can therefore be said that the mirror-shroud assembly 16 is movable. As shown in Figure 4, the terminating ends of the optical shroud 18 have sharp edges 26. As can be seen in the drawings the terminating end of the optical shroud 18 is the annular end which defines the shroud aperture. In the illustrated embodiment the edges have a wedge angle of about 30 degrees.

The system 10 also has a second optical shroud 28. The second shroud 28 is similar to the shroud 18 in that it has ends terminating in sharp edges 30 which define its aperture. The sharp edges 30 have a wedge angie substantially equal to that of the shroud 18. In other words, the wedge angle of the second shroud 28 is also about 30 degrees. As best seen in Figure 4 the second optical shroud 28 is also annular. In this illustrated embodiment the centre axis of the shroud 28 is coaxial with the laser beam 22. in use, the sharp edges 26 and 30 of both of the optical shrouds 18 and 28 are used to cut the laser disc 24. The optical shrouds 18 and 28, and more particularly their sharp edges 26 and 30, act to create a more desirable light disc for PIV measurements. In particular, the sharp edges 26, 30 define the boundaries of the light disc more sharply. By moving the shrouds 18, 28 relative to each other along the central axis, the thickness of the light disc 24 can be varied seeing that the light disc 24 is only allowed to pass through the gap between the two sets of sharp edges 26 and 30 of the shrouds 18, 28. In use, the wedge angle minimises diffraction of the laser disc 24. Without the inclusion of the shrouds 18, 28 and their sharp edges 26, 30 a high degree of diffraction will occur as a result of the spreading of the light disc 24. Diffraction could results in a rapid increase in the thickness of the light disc 24 as the distance from the central light source increases. This, in turn, could cause the boundaries of the light disc 24 to become blurred, which is undesirable for PIV measurements. These drawbacks are addressed by the inclusion of the shrouds 18 and 28. Although not strictly necessary for the operation of the system 10, the inclusion of the second shroud 28 gives superior control of the thickness of the laser disc 24 and improves the quality of the laser disc 24.

It is envisaged that in an embodiment of the system in accordance with the invention not shown in the accompanying drawings, the axial position of the movable mirror-shroud assembly 16 in relation to the test body 12 along the centre axis can be controlled by means of a control arrangement such as an adjusting screw and thread arrangement, for example. It is envisaged that the mechanical device could be connected to the test body 12 and the optical shroud 18. This control arrangement provides for fine adjustment of the light sheet thickness by moving the mirror-shroud assembly 16 in an axial direction along the axis of the test body 12. The axia! position of the cone mirror 20 in relation to the optical shroud 18 can also be controlled independently from the mirror-shroud assembly 16 using a similar adjustment arrangement which may again be in the form of a thread and screw arrangement, for example. This second adjustment arrangement for adjusting the cone mirror 20 is located between the cone mirror 20 and the optical shroud 18, This second adjustment arrangement, in turn, provides for fine adjustment of the brightness of the laser disc 24.

Stili referring to Figure 4 the system 10 includes an opaque shield 32 to prevent light scattering and illumination of the test body 12. In this embodiment, the shield 32 is in the form of a tube or cylinder mounted inside the test body 12. As shown in Figure 4 the shield 32 is mounted inside the test body 12 by means of the second optical shroud 28. This second shroud 28 has the same internal and externa! diameters as the optica! shroud 18.

While the one end of the shield 32 is located by the second shroud 28 an adapter 34 is used to locate the other end of the shield. The adapter 34 is also shaped to engage the test body 12, thereby locating the test body in the test rig and, accordingly, relative to the laser beam 24. in is envisaged that the shield 32 could be telescopic, thereby allowing the position of the second shroud 28 to be adjustable. Extension and contraction of the shield 32 would adjust the position of the second shroud 28 along centre axis.

It is also envisaged that additional optical elements, such as beam reducer or expander lenses, or an arrangement of cylindrical lenses for example (not shown in the accompanying drawings), could be mounted inside the test body 12. The optical axes of the additional optical elements will typically be aligned with the laser beam 22. in use, the additional optical elements may be used to manipulate the laser beam 22. For example, the arrangement of cylindrical lenses may be added to condition the laser beam 22 further before striking the cone mirror 20. This further conditioning could be carried out in order to create a laser beam that has a uniform, circular profile. The additional optical elements may further condition or manipulate the incoming laser beam 22 before it impinges on the cone mirror, which could be used to change certain properties of the light disc 24 such as the disc thickness.

In the illustrated embodiment of Figure 4, the adapter 34 is also used to connect the laser beam generator 14 to the test body 12. The adapter 34 can also be used to align the laser beam generator 14 with the test body 12 in a substantially linear line. In this particular arrangement the laser beam 22 generated by the laser beam generator 14 is emitted along the longitudinal centre axis of the test body 12.

The concealed laser beam 22 has laser safety benefits seeing that the laser beam 22 is introduced directly into the test body 12. This is in contrast to conventional PIV setups where the operator is exposed to significant health and safety risks associated with the external laser light sheet reflecting from the transparent walls of the test rig.

Returning to the illustrated embodiment, it must be understood that at least a portion of the test body 12 should be transparent to allow the laser disc 24 to be emitted radially outward as shown in Figure 4. However, in the illustrated embodiment of Figure 4 the entire side wall of the test body 12 is transparent. It should also be mentioned that although the test body 12 should preferably be transparent so as to allow the maximum amount of light to pass through it, the test body 12 could also be translucent. The quality of the laser disc 24 will however be superior if a transparent test body 12 is used in comparison to a translucent test body. Returning now to Figure 3, it can be seen that the system 10 further includes means for recording the illumination of seeding particles by the laser disc 24, In the preferred embodiment of the invention the means for recording the illumination of the seeding particles is in the form of a CCD camera 36. As shown in Figure 3 the CCD camera 36 is aligned with the centre axis of the test body 12. In this arrangement the CCD camera is also substantially in line with the laser beam 22 and substantially perpendicular to the plane of the laser disc 24. When using a single camera 36 it is preferably to align it with the test body 12. However, in alternative embodiments the camera 36 does not necessarily have to be aligned with the test body 12. Factors such as the interested measurement area, the shape of the test body 12 and the positioning of the test body could dictate that the camera 36 should be angled relative to the test body, in the event that multiple cameras are used, typically when conducting 3-dimensional flow visualisation and/or measurement, the cameras will generally be set up so that their optical axes are not perpendicular to the plane of the laser disc 24.

In order to capture the illumination of the seeding particles, which is indicated by the reference numeral 38 in Figure 3, the test rig has side wa!ls 40.1 and 40.2, of which at least one side wall 40.1 is transparent. A measurement area is created where the viewing angie of the CCD camera 36 intersects the plane of the laser disc 24. In Figure 3 the viewing angle of the CCD camera 36 is indicated by the numeral 42. The viewing window or field of view, which is also referred to as the PiV measurement area, is shown in Figure 3{b) and is indicated by the reference numeral 44. This viewing window 44 fails within the boundaries of the test rig as defined by the side walls 40.1 , 40.2 and walls 46.1 and 46.2 which are, in use, top and bottom wa!ls respectively. From Figure 3(b) it can also be seen that the laser disc 24 spans the entire distance between the wails 46.1 and 46.2, thereby covering the entire viewing window 44.

A second embodiment of a flow visualisation system in accordance with the invention will now be described with reference to Figure 5. The system in accordance with the second embodiment is indicated by the reference numeral 50. In the drawings like numerals indicate like features.

This second embodiment of the system 50 is intended for use in performing flow visualisation around multiple test bodies. In Figure 5 the illuminated test bodies are indicated by the reference signs 50.1 to 50.4. The test bodies are shown as tubes or cylinders, which are arranged as part of a bank 52. In the bank 52, the cylinders are arranged so that their longitudinal centre axes are substantially parallel to one another, in the cylinder bank of Figure 5 the illuminated test bodies 50.1 to 50.4 form part of a larger cylinder bank 52. in the particular bank illustrated in Figure 5 the cylinder bank has 63 cylinders, of which 4 are used as test bodies. It should be understood that only the test bodies are illuminated during testing. This arrangement of cylinders in the bank 52 is similar to that of a tube bundle used in heat exchangers, for example.

In order to perform flow visualisation through the cylinder bank 52 the measurement area or viewing window is increased compared to that of the first embodiment of the system 10. The viewing window focuses on a region covering multiple cylinders, including the test bodies 50.1 to 50.4. To achieve the enlarged viewing window the same internal illumination configuration as described above with reference to the first embodiment of the system 10 is used and placed inside each of the test bodies 50.1 to 50.4. in other words, lasers optics is located inside each of the test bodies 50.1 to 50.4. This arrangement wherein each test body carries laser optics provides simultaneous illumination of the enlarged viewing window. Accordingly, the laser optics inside each of the test bodies 50.1 to 50.4 creates a planar, isotropic laser disc as described above with reference to the first embodiment of the system 10. Essentially these laser discs overlap to create a planar, isotropic laser sheet 54 (best seen in Fig. 5(a)) that covers the enlarged viewing area of this second embodiment of the system 50. ln this second illustrated embodiment of the system 50, multiple cylinder illumination is achieved by splitting a single laser light beam 56. The laser beam 56 is split into a number of laser beams that correspond to the number of test bodies. Accordingly, in this second illustrated embodiment of the system 50, the laser beam 56 is split into four laser beams 58.1 to 58.4, i.e. one for each test body 50.1 to 50.4. The configuration of optical elements required to split the laser beam 56 into the four laser beams 58.1 to 58.4 is shown in Figure 5(b). In use, the laser beam 56 is again produced by a dual cavity, pulsed laser beam generator 60. Although the laser beam generator used in the system 50 is pulsed, a continuous laser beam generator could also be used.

As shown in Figure 5(b) the laser beam generator 60 is arranged such that the path of the laser beam 56 emitted therefrom is substantially parallel to the centre axes of the test bodies 50.1 to 50.4. The laser beam 56 is split into two laser beams 62.1 and 62.2 by a laser beam splitting optical element 64. The paths of the laser beams 62.1 and 62.2 are substantially perpendicular to that of the laser beam 56 and extend in substantially opposite directions. Laser beam turning mirrors 66.1 and 66.2 are used to turn the laser beams 62.1 and 62.2 by 90 degrees so that their paths are again substantially parallel to the centre axes of the test bodies 50.1 to 50.4. Two additional laser beam splitting optical elements 68.1 and 68.2 are used to split each of the laser beams 62.1 and 62.2 again into separate laser beams. The laser beam 62.1 is split into two separate laser beams 58.1 and 58.2, while the laser beam 62.2 is split into two separate laser beams 58.3 and 58.4. The laser beams 58.1 and 58.2 are again turned by 90 degrees by means of laser beam turning mirrors 70.1 and 70.2 so that their paths are substantially in line with the centre axes of two of the test bodies 50.1 and 50.2. The laser beams 58.3 and 58.4 are similarly turned by 90 degrees by means of laser beam turning mirrors 72.1 and 72.2 so that their paths are substantially in line with the centre axes of two of the test bodies 50.3 and 50.4. The laser beam 56 is split sequentially over a number of stages. In this second embodiment of the system 50, the laser beam 56 is split into four separate laser beams 58.1 to 58.4 in two stages. It should however be understood that in another embodiment not illustrated in the drawings, the laser beam 56 emitted from the laser generator 60 cou!d be split into any number of separate laser beams. The number of separate laser beams will typically correspond to the number of test bodies, such that each test body is, in use, illuminated by a separate laser beam.

The beam splitting components of the system 50 are housed in a casing 76. Similarly to the first embodiment of system 10, adaptors 78 are used to connect the test bodies 50.1 to 50.4 to the casing 76. The position of the beam splitting optical elements, turning mirrors and adaptors are adjusted precisely relative to the outer casing so as to direct the multiple laser beams 68.1 to 68.4 along the centre axes of the test bodies towards the laser light disc forming optics 74 for an arbitrary transverse centre-to-centre spacing (T) between the test bodies. As mentioned above, it is envisaged that additional stages may be included to increase the number of individual laser beams further and, therefore, increase the illumination area.

!t is further envisaged that the laser beam may be delivered to the laser splitting components, which are also collectively referred to as the laser beam distribution assembly, from the laser generator 60 either directly where the laser generator 60 is connected to the outer casing 76 or via an articulated laser arm (or any other laser light delivery apparatus).

Although the method of performing flow visualisation in accordance with the invention should be clear from the above description it will nevertheless now be described. The method of performing flow visualisation, and in particular Particle Image Velocimetry (PIV) flow visualisation, in accordance with the invention is also described in greater detail below with reference to the experimental testing of the system 10 as set out under the heading Experimental Setup and Results below. The method commences by providing the test body 12 and locating it inside the test rig. The laser optics is located at least partially inside the test body 12 so that the test body is illuminated from within. Next, the laser beam 22 is emitted from the laser beam generator 14 and directed towards the laser optics located inside the test body 12. To allow the laser beam 22 to strike the laser optics, and in particular the cone mirror 20, the laser beam generator 14 and the light optics are aligned, i.e. located along a linear line. The laser optics transforms the laser beam 22 into a substantially planar, isotropic laser disc 24. The transformation of the laser beam 22 into the laser disc 24 is achieved through the mirror-shroud assembly 16 and in particular the 90 degree cone mirror. As a result of the 90 degree cone angle of the mirror 20 a 360 degree laser disc 24 is created in a plane perpendicular to the laser beam 22. This laser disc 24 in turn illuminates seeding particles introduced into the fluid, which is air in this particular example, which is flowing around the test body 12 in the test rig. The illumination of the seeding particles by the laser disc 24 is recorded by recording means, which may be in the form of a CCD camera as mentioned above. The recordings are then typically analysed by computer software to determine the flow characteristics around the test body 2.

In order to achieve recordings that are of desirable quality and accuracy, the properties of the laser disc 24 may be controlled or adjusted. For example, the thickness of the laser disc 24 can be controlled by moving the mirror-shroud assembly 16 along an axis which is substantially in line with the laser beam 22. In other words, the thickness of the laser disc 24 is controlled by moving the mirror-shroud assembly 16 linearly towards and away from the laser generator 14. The brightness of the laser disc 24 may in turn be controlled by adjusting the position that the mirror cone 20 is seated inside the optical shroud 18. In the preferred fluid visualisation method, the diffraction of the laser disc 24 is also minimised by providing a wedge angle of about 30 degrees on the optical shroud 18. To improve the recordings further the test body 12 is also shielded by means of the opaque shield 32 to prevent light induced florescence thereof. From the above description it must be understood that the test body 12 is illuminated from within. In other words the laser light beam 22 is directed into the test body and the laser light disc 24 is projected through the test body. To allow the laser disc 24 to extend beyond the side wail of the test body, at least a portion of the test body, preferably the complete test body, is transparent, in other words, for the laser light creating the laser disc 24 to pass through the side wall of the test body 12, at least of portion of the test body is transparent. A major advantage of the systems 10, 50 and the method in accordance with the invention is that the problem associated with shadows in conventional flow visualisation is eliminated. By creating a laser disc from inside the test body no shadow region exists. As mentioned above, the 360 degree laser disc 24 illuminates the entire measurement area. Seeding particles can therefore be illuminated anywhere within the measurement area, thereby resulting in more accurate flow analysis around the test body.

EXPERIMENTAL SETUP AND RESULTS

To demonstrate the effectiveness of the flow visualisation system 10, 50 and method in accordance with the invention, a P1V investigation of the flow field around five side-by-side circular cylinders immersed in uniform flow was conducted. The experimental setup and results are discussed below with reference to Figures 6 to 10.

The Test Rig and Instrumentation

Figure 6 shows a schematic of the experimental setup. The testing was conducted in a closed-type wind-tunnel 200 with test section 202 dimensions of 0.312mx0.32mx 1.0m (width χ height χ length). The walls 204 of the test section 202 are made from transparent acrylic sheet, such as Perspex, that has a thickness of about 10.0mm. A glass observation window (not shown) for the CCD camera was inserted into the side of the test section 202. The window has a thickness of about 4.0mm. A single layer of honeycomb 206 is placed ahead of the test section 202 to improve the uniformity of the free-stream flow. The flow velocity of the fluid, in this instance air, in the test section 202 is controlled by a frequency inverter 208 that adjusts the speed of an axial fan 210 that drives the flow through the wind tunnel. The flow velocity was set to maintain a subcritical Reynolds number based on the single cylinder diameter of eD= 6000. Prior to the actual testing the free stream turbulence intensity was measured to be Tu= 1.9% (Dantec MiniCTA 54T42).

A bank of cylinders 212, which includes five side-by-side circular cylinders 212.1 to 212.5, was mounted in the test section 202. Each cylinder was made from an acrylic sheet with a diameter of 15.0mm and a span of 0.312m. The resulting area blockage within the test section 202 was 23.5%. The spacing ratio, T/D = 1.7 was selected where T is the transverse centre- to-centre spacing and D is the cylinder diameter, which is 15.0mm.

To measure the free stream flow conditions, the velocity profile was traversed at a distance Lu = 15.0D upstream of the cylinders (Figure 6(b)). The axial velocity was measured using a Pitot probe mounted on a linear traverse system (along the y-axis) that was positioned at the mid-span of the cylinders 212. The pressure data from the Pitot probe was read by a differential pressure transducer (DSA 3217, Scanivalve Inc).

In order to compare the conventional, external illumination configuration with the internal, isotropic-planar illumination configuration in accordance with the invention, the two configurations were respectively setup experimentally as shown in Figure 7(a) and (b). In comparing Figure 7(b) with Figure 3(a), it is apparent that the setups are substantially identical apart from the orientation of the test body as can be seen from the x-y-z axes indicated in the drawings. In the two configurations of Figure 7(a) and (b) the laser generator 214 and the CCD camera 216 were positioned differently with respect to the wind tunnel 200. The two experimental setups for the conventional external illumination and the internal isotropic-pianar illumination in accordance with the invention are described below.

Conventional External Illumination Setup

The conventional externally illuminated PIV setup is shown in Figure 7(a). In this setup the plane of the laser sheet is parallel to the x-z plane, because the base of the laser is mounted horizontally on a table secured to the floor of the test facility. Consequentially, the five side-by-side circular cylinders 212 were mounted so that the axes of the cylinders are perpendicular to the x-z plane. It has been previously noted for 2D PiV experiments that the optical axis of the CCD camera is also perpendicular to the x-z plane or parallel to the cylinder axis. The CCD camera 216 is positioned and focused towards the cylinders so the PIV measurement area is substantially similar to the square region 218 that is schematically illustrated in Figure 6(b).

To create the laser sheet as shown in Figure 7(a) and indicated by the numeral 220, light optics 222 is connected to the laser beam generator 214 to transform the emitted laser generator beam into the laser sheet. A schematic of the light optics 222 is shown in Figure 8, which illustrates the arrangement of multiple !enses necessary to form the thin laser sheet 220. A laser beam 224 emitted by the laser generator is inter alia transmitted through a Plano-concave lens 226, a cylindrical Plano-convex lens 228 and cylindrical Plano-concave lenses 230. The laser light sheet thickness (Ay) is manually adjustable and was set to about 1.0mm.

The objective of the present experiment was to quantify the velocity field of the upstream, gap and downstream regions of the cylinders 212 simultaneously. However, this is not achievable using the conventional external light sheet 220 due to the large shadow regions 232 that form. The shadow regions 232 are illustrated in Figure 7(a). The upstream region was therefore illuminated to observe how the upstream flow field behaves as it approaches the cylinders 212. The laser beam 224 was offset by an angle α of about 37 degrees, which is measured from the transverse z-axis of the five side-by-side cylinders 212. This is the maximum angle that could be obtained within the confines of the test facility. Naturally, the angular offset of the laser beam 224 means that the majority of the gap region and the downstream regions were obstructed by the shadows 232 created by the cylinders 212. This is a significant drawback of conventional flow visualisation as discussed above.

In an attempt to address the problem experienced with the lack of illumination of these show regions 232, transparent cylinders were also tested using the external illumination method. However, this had limited benefits for illuminating the gap and downstream regions because a significant proportion of the energy from the laser sheet 220 was scattered and dissipated while passing through the transparent cylinders. Consequently, insufficient energy remained within the laser sheet 220 to illuminate the downstream seeding particles. internal isotropic-planar illumination setup in accordance with the invention

The internal illumination technique in accordance with the invention is also hereinafter referred to as Internal Isotropic-Planar (IIP) illumination. As mentioned above, the primary object of HP illumination is to eliminate the shadow regions so that the upstream, gap and downstream velocity fields of the five side-by-side cylinders 212 can be measured simultaneously using PIV. From the description of the illustrated embodiments . of the invention it is clear that the shadows are eliminated by introducing the laser light beam internally through the test body. Figure 7(b) schematically illustrates the experimental setup for the IIP illumination in accordance with the invention. Again, the setup of Figure 7(b) is substantially identical to that of the system 10 shown in Figure 3 apart from the orientation of the test bodies. Accordingly the description of the system 10 applies equally to the test setup of Figure 7(b), and vice versa. The same reference numerals are therefore used when describing the system 10 and the IIP illumination setup. in the experimentai IIP setup illustrated in Figures 7(b), the Iaser beam 22 is introduced into the central or middle cylinder 212.3 and radiated radially outwards through the cylinder side waii by means of the iaser optics. The arrangement of the laser optics is identical to that of the system 10 shown in Figures 3 and 4 and will therefore not be described in detail again. The laser light beam 22 essentially consists of two beams generated by the dual cavity Iaser which have a circular shape with the same beam diameters and energy. In this experimental setup the middle cylinder 212.3 was transparent and manufactured from optical glass.

The optical axes of the iaser generator 14 and the CCD camera 36 are substantially perpendicular to the plane of the Iaser light disc 24, where the Iaser light disc is parallel to the x-y plane. The Iaser generator is again connected by an adaptor 34 to the end of the middle cylinder 212.3. The p!anar Iaser light disc 34 is created and radiates outwards in all directions resulting in 360 degree Iaser light coverage around the middle cylinder 212.3. With the 360 degree coverage, the upstream, gap and downstream flow regions are now illuminated simultaneously.

In the MP illumination setup the mirror-shroud assembly 16 located inside of the outer clear glass circular cylinder 212.3 is complementally shaped and dimensioned to the cylinder 212.3. In this particular arrangement the cylinder 2 2.3 has an internal diameter of 1 .5 mm.

The optical shroud 18 and cone mirror 20 were adjusted so that the Iaser light disc 24 had a thickness about 1.0mm. The fluence of the pulsed Iaser and the beam profile were characterised and controiled to ensure that the energy density limit or damage threshold of the cone mirror 20 was not exceeded. The cone mirror 20 used in the HP illumination setup had a threshold of about 0.2 J/cm². It should be understood that the laser energy has to be controiled seeing that the application of excessive Iaser energy in the attempt to increase the illumination of the seeding particles would result in permanent damage to the cone mirror reflective surface. Particle image Velocimetry (PiV) velocity measurement

For direct comparison of the conventional external illumination and the IIP illumination in accordance with the invention, the PIV data acquisition and processing procedures followed were similar for both cases.

The flow field was seeded with an atomised mineral oil with a mean particle diameter of 1 pm. The particles carried by the flow were then illuminated by a frequency doubled, dual cavity Nd:YAG laser (New Wave Research), with a light wavelength of 532nm, i.e. green light. It was further assumed that the measured flow field is predominately two dimensional so that the majority of particles entering into the thin light sheet remain within the light sheet during the measurement period. The measurement period was defined by two sequential pulses of the laser light sheet that are separated by a finite time interval of about 33με.

The recorded flow field is then represented by the random pattern of particle images that are mapped onto the image plane of the CCD sensor. The images are recorded on the sensor frame that has a 2048x2048 pixel resolution and a pixel pitch of 7.4pm. The field of view that was obtained within the tight sheet is shown in Figure 6(b) where the PIV measurement area 218 is about lOQmm* 100mm.

Acquisition of the velocity field depends on first estimating the displacement field. This was achieved by tnterrogating the PIV images using the Dantec {DynamicStudio V1.45) imaging software. The PiV images were divided into sub-regions referred to as interrogation areas which were 32^32 pixels in size and which overlapped by 50%.

The evaluation of the vector field between two successive frames yields an instantaneous vector map. The time averaged velocity field for a specific location in the flow field is then evaluated over an ensemble of eighty instantaneous vector maps sampled at a frequency of 5Hz.

The Reynolds number based on the single cylinder diameter D and the upstream mean flow velocity U°° is defined as:

where p« and μ« are the density and dynamic viscosity of air, respectively. The Reynolds number was fixed at 6000 (sub-critical flow regime).

The experimental uncertainty of the Reynolds number and axial flow velocity were estimated using a method reported by Ho!man [12] based on 20:1 odds and were within 1.1 %. Uncertainty of the cylinder dimensions and the transverse centre-to-centre cylinder spacing were within about 0,015mm. Pitot probe traverse positions along the y-axis were measured using a digital calliper with a resolution of 0.01 mm.

The uncertainty of the instantaneous PIV velocity measurements are primarily related to the estimation of the average particle displacement within an interrogation area. The uncertainty regarding the timing of the light sheet pulses, camera synchronization and particle lag are not considered as significant sources of error. The determination of measurement uncertainty relating to particle displacement have been quantified analytically and by the generation of synthetic images with known parameter values [3,11 ,13]. In order to provide a reasonable estimate of measurement error, the results given in Westerweei [1 1] are considered to be applicable to the PiV algorithm used in this investigation. The displacement measurement error based on the mean particle image diameter of 2-3 pixels is 0.05 pixels. Therefore, the full scale relative measurement error is 0.6%.

Discussion of Results A comparison of the velocity field measured using conventional external illumination and HP illumination in accordance with the invention are shown in Figure 9. Figure 9(a) shows the velocity field measured using conventional external illumination. It is apparent that the gap and downstream regions provide no velocity field information, which is a direct consequence of having these regions 232 cast in shadow. Because the laser was offset by 37 degrees with respect to the transverse plane of the cylinders 212 and the spacing ratio between the cylinders is relatively large (e.g., T/D =1.7), partial illumination of the downstream region occurs. The partial illumination of the downstream area results in the appearance of strips or sections of laser light. Incomplete illumination of the downstream region creates spurious vectors or outliers due to the inadequate number of seeding particle pairs appearing within the interrogation areas during the PiV interrogation process. However, due to the angular offset the upstream velocity field ahead of the cylinder has been well resolved.

Figure 9(b) in turn shows the velocity field measured using HP illumination in accordance with the invention, where the entire flow field around the illuminated cylinder 212.3 is revealed. It is also evident that the gap flow, i.e. the flow passing between the cylinders above and below the middle cylinder 21 .3, is deflected towards the adjacent cylinders 212.2 and 212.4 respectively. Consequently, the near wake structure behind the middle cylinder 212.3 is wide, whereas the near wake behind the adjacent cylinders 212.2 and 212.4 is narrow.

The calculation of derived quantities based on the velocity vector field such as streamlines, reveal with a greater degree of clarity how the entire flow field around the cylinders 212 develops. Referring now to Figure 10(a), which shows the stream lines for the conventional external illumination, only the upstream streamlines can be calculated due to only the upstream flow region being illuminated. No physical insight of the flow behaviour in the gap and downstream or behind the cylinders 212 can be obtained. Figure 10(b) in turn shows the streamlines measured using IIP illumination in accordance with the invention. As a result of simultaneously capturing the upstream, gap and downstream regions, it is now possible to ascertain how the approaching flow field changes when it encounters the cylinders 212. The upstream streamlines disperse ahead of the cylinders 212. The dispersion indicates that the flow velocity ahead of the cylinder 212.3 decreases relative to the free stream, it is evident that there is also a high degree of flow acceleration in the gap region between the cylinders 212 as the streamlines now bunch together. In the near wake, as it was also shown in Figure 9(b), the gap flow is generally deflected away from the middle cylinder 212.3 and towards its adjacent neighbours 212.2 and 212.4, thereby forming a narrow near wake. However, a wide near wake develops behind the middle cylinder 212.3 due to the deflected gap flow.

Conclusion

The HP illumination technique in accordance with the invention iiiuminates regions of the flow field that were previously cast into shadow using conventional flow visualisation techniques. The IIP illumination configuration creates a 360 degree laser disc 24 which is generated from an optical arrangement that is placed inside of a transparent test body 12. The test body 12 is therefore utilised as a means of introducing the laser light beam 24 internally into the test section. This is a significant advantage over conventional flow visualisation in which the laser sheet is introduced externally. The comparison of results clearly indicates that MP illumination is effective at illuminating regions of the flow field that were previously in shadow using conventional methods in which external light sheet illumination was used. Using MP illumination according to the invention the upstream, gap and downstream regions can be illuminated simultaneously. Consequently, this allowed the velocity field in these regions to be determined using a PIV measurement technique. EFERENCES M. Raffel, C. E. Willert, and J. Kompenhans, Particle Image Veiocimetry: A Practical Guide (Springer, Heidelberg , 1998).

Bryanston-Cross PJ, Towers CE, Judge TR, Towers DP, Harasgama, SP, Hopwood ST., "The application of particle image veiocimetry (PIV) in a short-duration transonic annular turbine cascade". ASME J Turbomachinery ,1 14: 504 (1992).

J. Westerweel, "Fundamentals of Digital particle image veiocimetry" Meas. Sci. Technoi. 8, pp1379-1392 (1997)

R. J. Adrian, " Twenty years of particle image veiocimetry," Exp. Fluids 39, 159-169, (2005)

C. E. Willert, and M. Gharib, " Digital particle image veiocimetry," Exp. Fluids 10, pp 181-193 (1991).

M. P. Wernet, "Application of DPIV to study both steady state and transient turbomachinary flows," Optics & Laser Technology 32, pp 497-525 (2000).

D. Sumner, S. S. T. Wong, S. J. Price, and . P. Paidoussis, "Fluid behavior of side-by side circular cylinders in steady cross-flow," J. Fluids Struct. 13, pp. 309-338 (1999).

. S. J. Xu, Y. Zhou, and R. M. C. So, "Reynolds number effect on the flow structure behind two side-by-side cylinders," Physics. Fluids 15, 1214 (2003).

. Z. J. Wang, Y.Zhou, " Vortex interactions in a two side-by-side cylinder near-wake," Int. J. Heat and Fluid Flow 26, pp. 362-377 (2005).

0. Dantec Dynamics A/S, 80X70 Light-sheet series Installation & User's guide, Publication no. 9040U6052 ( 1 ,July, 2002).

1. J. Westerweel, "Theoretical analysis of the measurement precision in particle image veiocimetry," Exp. Fluids [SuppL] S3-S-12 (2000).2. J. P. Holman & W. J. Gajda, Jr., Experimental Methods For Engineers (McGraw-Hill , New York, 1978, pp 7-46).

3. J. Westerweel, "On velocity gradients in PIV interrogation," Exp.

Fluids 44, 831 (2008).

Claims

CLA1MS

1. A flow visualisation system for performing flow visualisation around a test body, the system including:

a light beam generator;

means for recording the illumination of seeding particles by the light sheet;

2. A flow visualisation system according to claim 1 , wherein at least a portion of the test body is transparent so as to allow the light sheet to be emitted through the test body.

3. A flow visualisation system according to either claim 1 or 2, wherein the planar light sheet created by the light optics is substantially isotropic.

4. A flow visualisation system according to any one of claims 1 to 3, wherein the light optics includes a mirror-shroud assembly which includes an optical shroud and a mirror located inside the shroud.

5. A flow visualisation system according to claim 4, wherein the mirror is a cone mirror which has a cone angle of between 80 and 100 degrees so as to produce a 360 degree light disc in a plane substantially perpendicular to the light beam.

6. A flow visualisation system according to either claim 4 or 5, wherein the mirror-shroud assembly is movable along an axis which is substantially in line with the light beam.

7. A flow visualisation system according to any one of claims 4 to 6, wherein the cone mirror is movable inside the optical shroud so as to vary the brightness of the light sheet by moving the cone mirror relative to the optical shroud.

8. A flow visualisation system according to any one of claims 4 to 7, wherein the optical shroud has sharp edges.

9. A flow visualisation system according to claim 8, wherein the sharp edges have a wedge angle of about 30 degrees.

10. A flow visualisation system according to any one of claims 4 to 9, including an opaque shield to prevent light scattering and illumination of the test body.

11. A flow visualisation system according to claim 10, including a second optical shroud for supporting the shield.

12. A flow visualisation system according to claim 11, wherein the second optical shroud has sharp edges which have the same wedge angle as the optical shroud housing the cone mirror.

13. A flow visualisation system according to any one of claims 1 to 12, including an adaptor for connecting the light beam generator to the test body:

14. A flow visualisation system according to any one of claims 1 to 13, wherein the light beam generator is a dual cavity, pulsed laser generator for generating two laser beams from the cavities.

15. A flow visualisation system according to any one of claims 1 to 14, wherein the system is a Particle Image Velocimetry (PIV) system.

16. A method of performing flow visualisation including the following steps:

providing a test body;

locating laser optics at least partially inside the test body;

generating a light beam and directing the beam towards the light optics;

transforming the light beam into a substantially planar light sheet by means of the light optics from within the test body;

introducing seeding particles in a fluid flowing around the test body;

recording the illumination of the seeding particles by the light sheet; and

analysing the recordings.

17. A method according to claim 16, including emitting the light sheet through a transparent portion of the test body. 8. A method according to either claim 16 or 17, wherein the planar light sheet is substantially isotropic.

19. A method according to any one of claims 16 to 18, wherein the step of transforming the light beam into a substantially planar light sheet includes using a mirror-shroud assembly which includes an optical shroud and a mirror located inside the shroud.

20. A method according to claim 19, wherein the mirror is a cone mirror which has a 90 degree cone angle so as to produce a 360 degree light disc in a plane perpendicular to the light beam.

21. A method according to either claim 19 or 20, including controlling the light sheet thickness by moving the mirror-shroud assembly along an axis which is substantially in line with the light beam.

22. A method according to any one of claims 19 to 21 , including controliing the brightness of the light sheet by moving the cone mirror relative to the optical shroud.

23. A method according to any one of claims 19 to 22, including minimising diffraction of the light sheet by providing a wedge angle on the optical shroud.

24. A method according to claim 23, wherein the wedge angle is about 30 degrees.

25. A method according to any one of claims 19 to 24, including shielding the test body by means of an opaque shield to prevent light scattering of the test body.

26. A method according to claim 25, including supporting the shield by means of a second optical shroud.

27. A method according to claim 26, wherein the second optical shroud has sharp edges which have the same wedge angle as the optical shroud housing the cone mirror.

28. A method according to any one of claims 16 to 27, including connecting the light generator to the test body using an adaptor.

29. A method according to any one of claims 16 to 1 , wherein the step of generating a light beam includes generating two pulsed laser beams from a dual cavity laser generator.

30. A method according to any one of claims 16 to 29, wherein the method is a Particle Image Velocimetry (PfV) flow visualisation method.