Planar particle imaging and doppler velocimetry system and method

[0002] This invention relates to planar velocity measurements. Specifically this invention relates to a planar velocity measurement system and method which requires only a single line of sight to measure all three velocity components of a fluid field flow across an illuminated plane.

[0003] Systems for planar velocity measurers of fluid field flows are well known. Examples of planar velocity measurement techniques include particle imaging velocimetry (PIV) and Doppler global velocimetry (DGV). In both of these techniques a laser is used to illuminate a fluid flow seeded with particles. With PIV the particle positions at multiple instances in time are recorded with CCD cameras. With DGV the Doppler shift in scattered laser light by particles in the flow is measured.

[0004] In general PIV is capable of providing 3-component flow field measurements using a two camera, stereo viewing configuration. An example of a PIV configuration 10 is shown in FIG. 1. Here two cameras 11, 12 view light 18 scattered from a seeded fluid 20 which pass through a pulsed laser illuminated sheet 14. The cameras are positioned in a common horizontal plane and are spatially separated. The accuracy of the out-of-plane velocity component is inversely proportional to the sine of the coupling angle 16 between the two cameras. The minimum error occurs at a coupling angle of about ±forty-five degrees. As a result the technique requires either two separate optical access ports to the fluid flow, or at least one large rectangular optical access port to the fluid flow.

[0005] DGV is also capable of providing 3-component flow field measurements. A typical DGV system installation is shown in FIG. 2. The technique 40 requires three receiver systems 42, 44, 46. located at oblique angles from the illuminated measurement plane 48. As shows in FIG. 3, each receiver system 42 includes two CCD cameras 62, 64, which share a common oblique view of the illumination sheet 48 through a beam splitting cube or plate beam splitter 60. The first camera 62 corresponds to a reference camera and views the illuminated flow directly. The second camera 64 corresponds to a signal camera and views the illuminated flow through an Iodine vapor cell 66.

[0006] The intensity of the Doppler shifted light passing through the Iodine cell is proportional to the frequency shift of the illuminated particles in the fluid flow 50. Because the frequency. shift is a function of the velocity of the particles in the fluid flow, the Iodine cell 66 acts as an intensity-to-velocity transducer. A velocity component of the fluid flow is derived from the ratio of the signal to reference images from the two CCD cameras 62, 64.

[0007] With DGV, the coupling angle between the different receiver systems 42, 44, 46 determines the accuracy of the different velocity components measured. If optical access points to a fluid point are not at oblique angles, poor discrimination in the resolved velocity components will result. Therefore, as with PIV, to achieve accurate 3-component velocity measurements, multiple access ports or very wide access ports are required.

[0008] Unfortunately with many flow field velocity measurement platforms such as aerospace propulsion testing platforms, optical access is extremely limited. Limited optical access is typically due to concerns regarding mismatched boundary conditions on walls or due to concerns regarding high thermal or mechanical stresses. In many of these platforms only a single optical access port is available. Consequently there exists a need for a new technique for obtaining planar velocity measurements of fluids which can measure all three components of velocity through a limited access optical viewing port.

[0009] It is an object of the exemplary form of the present invention to provide a system and method for obtaining planar velocity measurements of fluid flows.

[0010] It is a further object of the exemplary form of the present invention to provide a system and method for obtaining planar 3-component velocity measurements of fluid flows.

[0011] It is a further object of the exemplary form of the present invention to provide a system and method for obtaining planar 3-component velocity measurements of fluid flows through a limited access optical viewing port.

[0012] Further objects of the present invention will be made apparent in the following Best Modes For Carrying Out Invention and the appended claims.

[0013] The foregoing objects are accomplished in an exemplary embodiment of the invention by a velocimetry system and method that includes a single, component DGV receiver system configured to simultaneously acquire PIV image data. In one exemplary embodiment of the present invention, a seeded fluid flow field is illuminated with at least two laser pulses to form an illuminated light sheet. At least one of the laser pulses is generated from an injection seeded laser to satisfy the requirements of a DGV system. The second laser pulse may come from either an injection seeded laser or a standard Nd:YAG laser, for example.

[0014] The exemplary receiver system is orientated at an angle which is generally perpendicular to the illuminated light sheet plane. The receiver system includes two PIV “frame-straddling” cameras: a reference camera and a signal camera. The scattered light from particles in the illuminated light sheet are split into two beams by a beam splitter. One beam is imaged by the reference camera. The second beam is imaged by the signal camera after being passed through a molecular filter such as an Iodine vapor cell. The exemplary system enables the acquisition of PIV image frame pairs using both the reference and signal cameras. Each camera in the receiver obtains a pair of image frames responsive to at least two laser pulses. The image frames are processed using PIV processing techniques to acquire the complete 2-component in-plane velocity field of the fluid flow.

[0015] In the exemplary embodiment, the image frames from the signal and reference cameras are also processed using DGV processing techniques. The orientation of the receiver perpendicular to the illuminated light sheet enables the DGV system to measure the component of the flow velocity that lies forty-five degrees out-of-the plane of the illuminated light sheet. As a result the DGV velocity measurement can be combined with the in-plane PIV measurements to obtain the full 3-component velocity fields across the illuminated plane, through a single optical access port.

[0016] FIG. 1 is a schematic view representative of a prior art stereo viewing PIV optical configuration.

[0017] FIG. 2 is a schematic view representative of a prior art DGV optical configuration.

[0018] FIG. 3 is a schematic view representative of a prior art DGV receiving system illustrating oblique viewing of an illuminated light sheet.

[0019] FIG. 4 is a schematic view representative of an exemplary planar particle imaging and Doppler velocimetry system of the present invention.

[0020] FIGS. 5 and 6 are graphs showing vectors involved with the calculation of the out-of-plane velocity components.

[0021] FIG. 7 is a schematic view representative of an exemplary frequency monitoring device of the present invention.

[0022] Referring now to the drawings and particularly to FIG. 4, there is shown therein a schematic view representative of a planar particle imaging and Doppler velocimetry system 100 of one exemplary embodiment of the present invention. The system includes a pulsed laser light source 102 that is operative to output pulses of light 110. In the exemplary embodiment the light source 102 includes a lens or lenses 104 which is operative to form an illuminated light sheet 108 for each pulse of laser light source 102. The laser light source 102 is positioned such that the illuminated light sheet is projected across a fluid flow 106 being measured by the system.

[0023] In one exemplary embodiment of the present invention, the light source 102 includes two lasers 112, 114 which are operatively configured to output pulses of light 110 which are coaxially aligned along a common line. As a result, the corresponding light sheets 108 formed by each pulse of each laser 112, 114 are coincident in position across the fluid flow 106. In the exemplary embodiment, at least one of the lasers 112, 114 is comprised of a laser that is suitable for DGV measurements such as an injection seeded laser with a line width on the order of 100 MHz. The second one of the lasers may be comprised of a laser that is suitable for PIV measurements such as a standard Nd:YAG laser. However, in alternative exemplary embodiments both lasers 112, 114 may also be comprised of an injection seeded laser.

[0024] In the exemplary embodiment, the fluid flow 106 is seeded with particles which are operative to scatter light in the illuminated light sheet 108. Examples of particles for seeding the fluid flow 106, may include atomized liquid particles such as vegetable oil and silicone oil; solid particles such as alumina. Exemplary embodiments of the present invention may further include seed particles such as titanium dioxide generated by mixing titanium tetrachloride vapor and moist air in the flow. Particles may also be generated by combustion. Monodisperse (uniform distribution of size) particles such as polystyrene latex suspended in a volatile liquid may also be used. It is to be understood that the present invention encompasses measuring velocity flow fields seeded with any particles that are operative to scatter light for performing both PIV and DGV measurements.

[0025] The exemplary embodiment of the present invention includes one receiver system 120 which is orientated to view light scattered generally perpendicular to the plane of the illuminated light sheet 108. In alternative exemplary embodiments oblique viewing may also be performed. The calibration of the system using a dot card target can be used to perform a warping of the distorted optical plane back to a rectilinear coordinate system.

[0026] The exemplary receiver system 120 includes a reference camera 122, a signal camera 124 and a beam splitter 126. The beam splitter 126 is operative to split the scattered light 128 viewed by the receiver 120 into two beams 130, 132. The first one of the beams 130 is directed to the reference camera 122. The second one of the beams 132 is directed to the signal camera 124. In the exemplary embodiment the cameras 122 and 124 are operative to capture a pair of single exposure image frames at high effective framing rates suitable for performing the PIV technique on air flows greater than 1 meter/second. In one exemplary embodiment, the cameras 122 and 124 are frame-straddling cameras. With frame straddling cameras the timing of the laser pulse is adjusted so that the first laser pulse occurs just at the end of the integration period for the first frame and the second laser pulse occurs just at the beginning of the integration period for the second frame. Although though the cameras may only operate at 30 or 15 frames/second, the frame stradling technique produces a high effective framing rate between the frame straddling image pairs.

[0027] In the exemplary embodiment of the present invention, the receiver 120 includes a molecular filter 134 such as an Iodine vapor cell between the beam splitter and the signal camera 124. The molecular filter is operative to absorb Doppler shifted light in the beam 132 in proportion to the frequency shift of the scattered light. Because the frequency shift is a function of the flow velocity, the resulting filtered beam 136 yields an image of the flow in which the intensity is proportional to the local flow velocity. Hence the molecular filter 134 is operative as an intensity-to-velocity transducer. In this described exemplary embodiment the light pulses are emitted at the second harmonic of the Nd:YAG Laser (λ=532 nm) in order to be within the I₂absorption bands of the Iodine vapor cell.

[0028] In the exemplary embodiment of the present invention, the absorption profile slope and optical density of the molecular filter 134 are adjusted to encompasses a range of anticipated Doppler frequency shifts by changing the temperature and pressure of iodine vapor in the cell. The laser injection seeder voltage on the Nd:YAG laser is adjusted so that the center frequency of the laser light is in general at the midpoint of the transition edge in between the maximum absorption and maximum transmission of the filter. With this arrangement, the molecular filter transmission is directly related to the Doppler shift of the flow velocity component. In exemplary embodiments of the present invention, the molecular filter may include Doppler-broadened filters and Lorentz or pressure broadened filters.

[0029] The exemplary configuration of the present invention is operative to acquire 3-component velocity measurements of the fluid flow 106 from a single optical access point 138 of the system 100. As a result the present invention may be used to measure high speed fluid flows in wind tunnels, turbines, nozzles, or any other fluid test platform which may have only one optical access point. Measurements are performed by pulsing the light source at least twice to produce two light pulses 109, wherein at least one of the pulses originates from an injected seeded laser 112. Images of the light scattered from these pulses are captured by both the reference and signal cameras 122, 124 and are processed by at least one computer processing system 116 in operative connection with the cameras. In embodiments where the lasers are pulsed twice, two image frames are captured from each of the two cameras 122, 124. A first set of image frames R1 and R2 are captured by the reference camera 122. A second set of image frames S1 and S2 are captured by the signal camera 124.

[0030] The exemplary processing system 116 is operatively programmed to perform both PIV and DGV image processing on the image frames collected by the signal and reference cameras 122, 124 during the laser pulses. The exemplary processing system 116 is operative to perform standard PIV data processing techniques to obtain the complete 2-component in-plane velocity field. The PIV data processing includes the processing of the R1 and R2 image frames captured by the reference camera 122. In the exemplary embodiment, the PIV technique provides velocity estimates that are theoretically accurate to about 1% of fullscale.

[0031] The PIV processing by the processing system 116 includes either dividing the images up into small interrogation regions and determining the average displacement of the particles or by determining the individual particle displacement captured by the cameras between successive pulses of the illuminated light sheet. Knowledge of the time interval between light sheet pulses permits the exemplary processing system 116 to compute the in-plane flow velocity components using the formula: $\begin{array}{cc}V=\frac{\Delta x}{M·\Delta t}& \left(1\right)\end{array}$

[0032] where Δx is the measured displacement, M is the optical system magnification, and Δt is the time between exposures.

[0033] The exemplary processing system 116 is further operative to perform standard DGV processing techniques from at least one of the sets of image frames (R1,S1 or R2,S2) captured by the reference and signal cameras 122, 124 from the pulse of the at least one injection seeded laser 112. The processing system 116 is operative to perform DGV processing to measure the component of the flow velocity that lies forty-five degrees out-of-the plane of the illuminated light sheet 106. If both sets of image frames (R1,S1 or R2,S2) are generated from injection seeded lasers, the exemplary system is operative to average the resulting velocity calculations to improve the accuracy of the velocity measurement. When performing the DGV processing, the exemplary processing system 116 is operative to normalize the images (S1, S2) captured with the signal camera with the images (R1, R2) captured by the reference camera 122. Normalization is performed to account for the effects of nonuniform particle size, nonuniform particle number density distributions and variations in laser intensity across the light sheet. Based on the measured intensity change of the normalized image, the exemplary system is operative to calculate the Doppler shift across the illuminated light sheet. Once the optical shift has been obtained using DGV processing techniques, the exemplary processing system 116 is operative to obtain the full 3-component velocity field 222 across the illuminated light sheet 108, by combining the DGV velocity measurement 224 with the in-plane PIV measurements 226.

[0034] FIG. 5 illustrates the illumination direction ({right arrow over (k)}_i) 140 mind observation direction ({right arrow over (k)}_o) 142 involved in one exemplary embodiment of the present invention The out-of-plane velocity component ({right arrow over (K)}) 144 is obtained from the DGV technique which measures the Doppler shift of the scattered light along a component direction which lies forty-five degrees to the plane of the light sheet, in a plane defined by the light sheet propagation direction {right arrow over (k)}_iand the camera observation direction {right arrow over (k)}_o. Thee resulting out-of-plane velocity component {right arrow over (K)} corresponds to {right arrow over (k)}_i−{right arrow over (k)}_oand makes an angle θ with the incident light sheet propagation direction {right arrow over (k)}_i.

[0035] In the exemplary embodiment, the processing system 116 obtains the velocity measurements in a 3-dimensional velocity space consisting of the components U, V, and W, which point in the {right arrow over (i)}, {right arrow over (j)}, and {right arrow over (k)} unit vector directions. The PIV technique yields the in-plane velocity components (U and V), which point in the {right arrow over (i)} and {right arrow over (j)} directions. The Doppler frequency shift measured by the DGV technique is given by the equation: $\begin{array}{cc}\Delta f=\frac{\overrightarrow{A}·\overrightarrow{K}}{{\lambda }_o}& \left(2\right)\end{array}$

[0036] where {right arrow over (A)} is a 3-dimension velocity vector (U_i+V_j+W_k), {right arrow over (K)} is the DGV measured velocity component direction, Δf is the measured Doppler shift (in Hz), and λ_ois the laser light frequency (in units of length). The velocity measurement obtained from the DGV technique is:

[0037] Z=Δfλ_o  (3)

[0038] where Z points in the {right arrow over (K)} direction. The desired results calculated by the exemplary processing system 116 are estimates of the U, V, and W components of velocity. The Z component measured by the DGV system is not in the {right arrow over (i)}, {right arrow over (j)}, {right arrow over (k)} coordinate system. As a result the exemplary processing system 116 is operative to rotate the Z component into the correct coordinate system in order to obtain the W component of velocity in terms of Z and −U. Note that due to the optical system configuration in the exemplary embodiment, the DGV measured component of velocity lies in the {right arrow over (i)}−{right arrow over (k)} plane.

[0039] FIG. 6 shows these relationships in terms of vector notation. Here the coordinate vector 150 aligned with the DGV measured Z velocity component is {right arrow over (i)}_θ. The U and W components are aligned in the {right arrow over (i)} and {right arrow over (k)} unit vector directions 152, 154, respectfully. The unit vector pointing in the direction of the DGV measured velocity component Z, written in terms of {right arrow over (i)} and {right arrow over (k)} is:

[0040] {right arrow over (i)}_θ=−{right arrow over (i)}(cos θ)+{right arrow over (k)}(sin θ)  (4)

[0041] With the vector length:

[0042] Z{right arrow over (i)}_θ=−U{right arrow over (i)}(cos θ)+W{right arrow over (k)}(sin θ)  (5)

[0043] Rearranging and solving for W produces: $\begin{array}{cc}W\overrightarrow{k}=\frac{Z{\overrightarrow{i}}_{\theta }}{\sin \theta }+U\overrightarrow{i}\left(\frac{\cos \theta }{\sin \theta }\right)& \left(6\right)\end{array}$

[0044] which is the expression for obtaining the out-of-plane velocity estimate W in terms of the measured DGV velocity estimate Z (which lies in the {right arrow over (K)} direction and which is the same as the {right arrow over (i)}_θ direction) and the PIV velocity estimate for U. In the exemplary embodiment the V component of velocity measured with the PIV technique is not involved in the estimate of W since the camera observation plane lies in the {right arrow over (i)}−{right arrow over (k)} plane. In alternative exemplary embodiments, if the observation direction is moved out of the {right arrow over (i)}−{right arrow over (k)} plane, then the W component will also be a function of the V component of velocity.

[0045] In general the PIV technique requires imaging of individual particles, while the DGV technique requires multiple particles per pixel on the CCD of the cameras. DGV is theoretically capable of providing velocity measurements at every pixel in the CCD array provided there is sufficient particle seeding in the flow and that laser speckle is eliminated. Laser speckle results from the interference of light of nonuniform phase caused when coherent light is scattered by particles of a nonuniform distribution. The exemplary processing system is operative to remove the adverse effects of laser speckle by performing N×N pixel spatial averaging during DGV processing, where N=1, 2, 3, . . . as needed to remove speckle noise.

[0046] To reduce the need for high seed particle concentrations typically required in DGV techniques, the exemplary processing system is operative to spatially average the DGV processed images over subregions corresponding to the size of the PIV subregions, thereby yielding the same spatial resolution as the PIV measurements. In one exemplary embodiment of the present invention 1024×1024 pixel images are captured and are processed using 32×32 pixel subregions. In other exemplary embodiments, other image resolutions and subregion sizes may be used depending on the flow velocity being measured, seed particle concentration, and the desired precision of the velocity measurements.

[0047] Alternative exemplary embodiments of the present invention may further use a single camera to capture the reference and signal image frames for each pulse of the lasers. In such an alternative exemplary embodiment, the reference beam would be directed to a first portion of the CCD of the single camera and the signal beam would be directed to a second portion of the CCD of the single camera.

[0048] In addition alternative exemplary embodiments of the present invention may use the captured single image frames (S1, S2) to further increase the accuracy of the PIV calculations. In this described exemplary embodiment, the Doppler shift information may be used to assist in the correlation of particles captured in the images for each of the pulses of the lasers.

[0049] Exemplary embodiments of the present invention may further include a frequency monitoring device. Laser light frequency generally varies from pulse to pulse of the laser. In addition, there is often a significant variation in frequency across the laser beam which may produce variations in laser frequency across the light sheet. These variations may result is velocity estimate errors in the estimated out-of-plane velocity component W. In one exemplary embodiment of the present invention a laser diode may be used to provide a single point estimate of the laser frequency. A further exemplary embodiment of the present invention may include a frequency monitory device that is operative to measure the spatial variation and pulse to pulse variations in laser light frequency. The exemplary processing system is operative to use the measured light frequency information in its calculations to provide a more accurate estimate for the out-of-plane velocity component W.

[0050] FIG. 7 shows an exemplary embodiment of a frequency monitoring system 160. Here a portion of the laser light 162 is directed with a beam splitter 164 to a monitoring receiver 166. The monitoring receiver 166 includes a pair of CCD cameras 168, 170. If the laser light source 180 includes two injection seeded lasers, the pair of cameras 168, 170 may be frame stradling cameras.

[0051] A first one of the receivers 168, is operative to capture reference image frames of each pulse of light. The second one of the receivers 170 is operative to capture signal image frames of each pulse of light after being filtered by a molecular filter 172. The exemplary processing system 116 is operative to process the reference and signal image frames to estimate the spatial variation in laser frequency for each pulse of the laser 180. The exemplary processing system is further operative to calculate the out-of-plane velocity components responsive to the measured spatial variation in laser frequency.

[0052] Thus the planar particle imaging and Doppler velocimetry system and method of the present invention achieves one or more of the above stated objectives, eliminates difficulties encountered in the use of prior devices and systems, solves problems and attains the desirable results described herein.

[0053] In the foregoing description certain terms have been used for brevity, clarity and understanding, however no unnecessary limitations are to be implied therefrom because such terms are used for descriptive purposes and are intended to be broadly construed. Moreover, the descriptions and illustrations herein are by way of examples and the invention is not limited to the exact details shown and described.

[0054] In the following-claims any feature described as a means for performing a function shall be construed as encompassing any means known to those skilled in the art to be capable of performing the recited function, and shall not be limited to the features and structures shown herein or mere equivalents thereof.

[0055] Having described the features, discoveries and principles of the invention, the manner in which it is constructed and operated, and the advantages and useful results attained; the new and useful structures, devices, elements, arrangements, parts, combinations, systems, equipment, operations, methods and relationships are set forth in the appended claims.