PIV and Volumetric 3D Measurements of Separated Turbulent Boundary Layer on a NACA4412 Hydrofoil

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1 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition January 2012, Nashville, Tennessee AIAA PIV and Volumetric 3D Measurements of Separated Turbulent Boundary Layer on a NACA4412 Hydrofoil Redha Wahidi 1, Jonathan A. Smith 2 and Amy Lang 3 The University of Alabama, Tuscaloosa, AL, Time-resolved PIV and three-dimensional three-component measurements of the flowfield were carried out on a two-dimensional NACA4412 hydrofoil with turbulent separation. The location of turbulent separation is investigated by the time-averaged mean velocities and the backflow coefficient. Regions of backflow and backflow coefficients of 50% were investigated against the Cp results obtained from XFLR5 software. The location of a backflow coefficient of 50% agreed with the beginning of the constant-pressure region on the Cp curve near the trailing edge. Backflow coefficient results obtained from the two different measurement techniques were compared. These results only agreed in one case. However, the volumetric three-component measurement results better agreed with the Cp results. X Y Z Re c Cp C t U V W BFC Nomenclature = coordinate in the streamwise direction = coordinate in the spanwise direction = coordinate in the wall-normal direction = Reynolds number based on the freestream velocity and chord = coefficient of pressure = chord = time step = mean velocity in the streamwise direction = mean velocity in the spanwise direction = mean velocity in the wall-normal direction = backflow coefficient I. Introduction HE understanding of turbulent separation is of great importance due to its occurrence in many engineering T applications, such as jet engine diffusers, turbo machinery blades, automobiles and wings. The separated boundary layer causes an increase in drag and in some cases a severe degradation of the performance of wings due to loss of lift and increase of drag. Additionally, since turbulent separation normally occurs near the trailing edge of the hydrofoil, its presence may reduce the effectiveness of the control devices which affects the maneuverability of the airplane. Although turbulent separation has been studied for decades, there is still need for better understanding of the details of turbulent separation to assist in achieving more accurate predictions and better designs of passive and active separation controls. There is a shortage of experimental data of turbulent separation on hydrofoils due to the difficulties associated with carrying out experimental investigations. Furthermore, a large percentage of the experimental work that has been conducted in the past involved point-wise measurement techniques, such as hot wire or laser Doppler velocimetry (LDV), which did not fully explore the two- and three-dimensionality of the turbulent separation phenomenon. The experimental results of Ref. 1 and the DNS study of Ref. 2, for example, showed that the instantaneous turbulent separation and reattachment lines are three dimensional. Therefore, planar or volumetric measurement techniques will assist in achieving a better understanding of the separated flow structure in space and time. Consequently, measurements of the flowfield in this study were carried out using time-resolved 1 Postdoctoral Associate, Department of Aerospace Engineering and Mechanics, 216 Hardaway Hall, Member. 2 Graduate Research Assistant, Department of Aerospace Engineering and Mechanics, Student Member. 3 Associate Professor, Department of Aerospace Engineering and Mechanics, 213 Hardaway Hall, Member. 1 Copyright 2012 by the, Inc. All rights reserved.

2 particle image velocimetry (TRPIV) and volumetric three-component velocimetry (V3V) measurement techniques. The purpose of this study is to determine the location of turbulent separation and explore some of the details in the recirculation region. The results of this investigation will be used to design a passive flow separation control based on the microgeometries found on fast-swimming sharks. Also, due to the recent development of the volumetric three-component measurement technique, it is needed and valuable to explore its capabilities in flow cases where turbulent separation occurs. This assessment will be accomplished by investigating how the time-averaged mean velocities obtained from the V3V measurements and backflow coefficient indicate regions of separated flows. Moreover, the V3V results will be compared with PIV results. II. Experimental Apparatus and Procedure The experimental investigation was carried out in a custom-built Eidetics 1520-EXT water tunnel facility located at the Department of Aerospace Engineering and Mechanics at The University of Alabama (UA). The length of the test section of the tunnel is 108 inches (2.743 m) where the width and height of the test section are 15 inches (0.381 m) and 30 inches (0.762 m), respectively. The total volume of the standard Model 1520 tunnel is approximately 1000 gallons; the capacity of the University of Alabama's model is slightly higher due to the extended test section (108 inches versus 60 inches). Maximum freestream velocity delivered by the standard tunnel is 1.0 ft/s (0.305 m/s), but with the installation of a high-performance impeller and the reduction of the tunnel water level, the UA tunnel is capable of reaching freestream velocities of over 1.64 ft/s (0.5 m/s). The turbulence level in the tunnel was reported to be 0.4% at a freestream velocity of 2 in/sec (0.051 m/s). A two-dimensional NACA4412 hydrofoil model with a chord length of 12 inches ( m) and a span of 24 inches (0.610 m) was used in this experimental investigation. The model was cut from a 2-inch thick sheet of clear plexiglass by inputting the hydrofoil coordinates into a precision CNC machine. To hold the model securely in place in the water tunnel, it was attached to a mounting bracket composed of aluminum and stainless steel plates. The hydrofoil model was placed vertically in the tunnel and connected to the mounting bracket by a steel rod at the top and a bushing at the bottom, allowing it to rotate freely until locked in place by the angle of attack dial (see Fig. 1). The mounting system positioned the chord line of the hydrofoil at the midpoint of the tunnel width in order to maximize the distance between the hydrofoil surface and the tunnel walls. The hydrofoil extended 4 inches ( m) above the free surface and had a gap of 1 inch ( m) between the spanwise end of the hydrofoil and the bottom wall of the water tunnel. The hydrofoil model was painted using standard matte black spray paint to minimize reflections from the PIV laser sheet (see Fig. 2). It was then sanded to eliminate the grain present in the paint and ensure the smoothness of the surface. The flow over the upper surface was tripped by a 2mm-diameter copper tube placed at the 5% chord location across the entire span of the hydrofoil. Figure 3 shows the coordinates of the NACA4412 hydrofoil. Figure 1. Angle of attack selection dial on top of hydrofoil pivot plate. 2

3 Figure 2. Painted NACA 412 hydrofoil and mounting system, view from bottom. Figure 3. Coordinates of NACA4412 Airfoil. Volumetric Three-Component Velocimetry (V3V) was used for the flowfield measurements. This system is licensed and manufactured by TSI Incorporated and was developed based on the defocused digital Particle Image Velocimetry (DDPIV) concept. 3,4 Flowfield measurements with the V3V system provide the three components of the fluid velocity vectors in a volume of standard dimensions of 120mm by 120mm by 100mm. The V3V system consists of a camera probe, shown in Fig. 4, with three 4-million-pixel cameras outfitted with 50 mm lenses fixed at f#16. The lenses record the X, Y and Z locations of the seeding particles in the measurement volume at two instants of time separated by a time interval ( t) and the velocity vectors are extracted by the Insight V3V TM software. The V3V system also includes a pulsed Nd:YAG laser rated at 425mJ per pulse, synchronization unit, and controlling and analyzing software. The measurement volume was illuminated by the Nd:YAG laser and the flow was seeded using 50 m Polyamide particles. The camera probe was aligned with the test section of the tunnel where the lenses were perpendicular to the flow direction. An in situ calibration was performed by traversing the calibration target through the measurement volume. This calibration procedure is standard with the V3V system and 3

4 yields a set of polynomial-fit equations that are used during the data processing to obtain the particles X, Y and Z locations and the corresponding velocity vectors. Four steps comprise the data processing of the V3V system. In the first step, the particles are identified in each of the six (6) images obtained from the three cameras at two instants of time based on an intensity threshold value. The triplet identification is performed in the second step of the data processing routine. In this step, the calibration polynomial fit equations are used to find a triplet match of each of the particles in one of 2D images from the other two apertures. This step results in a three-dimensional particle cloud for each set of the two instants of time. In the next step, a three-dimensional relaxation tracking method 5 is used to track the particles and produce a randomly spaced vector field. Finally in the fourth step, the data are presented on a regular grid by performing a Gaussianweighted interpolation. A picture of the V3V system and the water tunnel facility is shown in Fig. 5 and a diagram of the experimental setup with the V3V system is shown in Fig. 6. Figure 4. V3V 3-Aperture Camera. Flow Direction Figure 5. Experimental setup showing the V3V camera, water tunnel test section and hydrofoil. The second system used for the flowfield measurement is a time-resolved digital particle image velocimetry (TR- DPIV). The system consisted of a pulsed solid-state laser, a high speed digital camera, image acquisition software, and PIV processing software. The particles used to seed the flow for the PIV measurements were silver-coated hollow glass spheres with an average diameter of 14 μm manufactured by Potters Industries, Inc. The laser used was a Quantronix Falcon 30 series Nd:YLF system. This laser generates a beam with a wavelength of 532 nm and a maximum power output of 20 watts. All of the experiments in this research were run with the laser pulsed at 400 Hz. 4

5 The beam was spread into a triangular sheet by a concave lens to create an illuminated plane for image acquisition. A diagram of the experimental setup with the PIV system is shown in Fig. 7. Figure 6. Top-down diagram of V3V experimental setup in water tunnel test section. The camera used to record the raw PIV images was a Basler A504k 8-bit high-speed digital camera with a Nikon AF Micro Nikkor 105mm lens. This camera was capable of acquiring images with a resolution of 1280x1024 pixels at a frame rate of up to 500 frames per second. Higher frame rates could be achieved by reducing the camera's resolution. For all runs in these experiments, the camera frame rate was matched with the laser pulse rate, and the full resolution of the camera was used. The camera was linked to a PC by a National Instruments PCIe-1429 frame grabber, which worked in conjunction with a LabView program to capture and store the images. Each run of image acquisition saved 1200 images, and four runs were taken for each combination of experimental conditions for a total of 4800 images per case. After capture, the images were converted into sequential, overlapping pairs by another LabView program. PIV processing and preliminary analysis was performed using Insight 3G software. This program takes the aforementioned image pairs as input and computes the resulting vector field for each pair in the 1200-image run being processed, given the change in time ( t) between the first and second image in the pair. The length scale calibration of the images must also be input into the program. This was determined by placing a ruler in the illuminated measurement plane of the flow and capturing a test image, then measuring the distance in pixels between the centimeter tick marks on the ruler in the image. The measurement locations were laid out such that the flowfield over the entire chord length of the upper surface of the hydrofoil could be analyzed. For the PIV experiments, the illuminated plane of the laser sheet was set at middepth of the water tunnel test section, and the vertical position of the camera below the water tunnel was adjusted accordingly to produce the correct width of each image location. A total of 5 measurement regions were used to cover the length of the model; the upstream edge of each window location was aligned with the point corresponding to 0, 25, 50, 66.7, and 83.3 percent chord, respectively. A field of view of approximately 7 cm was used at each measurement window location. The first and second measurement locations were aligned with the X- and Z- coordinates of the centerline of the water tunnel, whereas the last three locations were aligned with the local coordinates of the hydrofoil surface, due to the lower curvature at these locations. A diagram of the orientation of these measurement regions is shown in Fig. 8. For the V3V measurements, only three measurement volumes were required to cover the entire chord length of the hydrofoil. 5

6 Figure 7. Top-down diagram of the PIV experimental setup in water tunnel test section. Camera is located under the test section directly below the measurement plane. Figure 8. Diagram of measurement window locations and orientations. The angles of attack tested were 8, 12, and 16 degrees. The combination of 5 measurement windows and 4 angles of attack resulted in 20 unique cases (20 cases for the PIV measurements and 9 cases for the V3V measurements). In order to minimize the movement and adjustment of the laser and optics, all runs for a given angle of attack and image location were completed consecutively. The angle of attack of the hydrofoil was then changed and the orientation of the camera was adjusted as needed; this procedure was repeated for all angles of attack at a given image location. Once these runs were completed, the entire hydrofoil model and mounting system was moved forward so that the illuminated plane of the laser sheet would coincide with the next measurement location, and the process was repeated. After the runs for all cases had been taken and the images converted into consecutive pairs, each run was loaded into the Insight 3G TM software individually for processing. A separate processing mask was applied at each measurement location to block out the regions of the image below the hydrofoil surface. The processor was set to use a recursive Nyquist grid with a final interrogation window size of 32x16 pixels. For the V3V measurements, the laser cone was adjusted for each measurement location whereas the camera was stationary. After acquiring the images, then angle of attack was changed. Since the camera was stationary, changing the angle of attack only required slight adjustments of the laser cone due to the curvature of the hydrofoil. After all 6

7 runs at different angles of attack were completed, the entire hydrofoil model and mounting system were moved forward similarly to the PIV measurement procedure and the process was repeated. At each run in the V3V measurements, 500 images were taken at 5 frames per second and the images were processed using the Insight V3V TM software. All experiments were carried out at a freestream velocity of m/s, resulting in a Reynolds number (Re c ) of 110,000 and at angles of attack of 8, 12 and 16 degrees. III. Results A. Volumetric Three-Component Measurement Results The surface pressure (Cp) results obtained from XFLR5 (free version of the software XFOIL) were used to estimate the location of separation. The XFLR5 software uses the e N method to predict the location of transition. This software can also be used with a forced transition option to cause a transition at a location defined by the user. The results for free and forced transition were obtained to compare the cases with no trip and with a boundary layer trip at X/C = 0.05 as in the case of this experimental study. Figure 9 shows a typical distribution of Cp for a hydrofoil with laminar and turbulent separations. Figure 9. Typical Cp curve with laminar and turbulent separations. If the flow is not tripped at the Reynolds number of this study (Re c = 100,000), the boundary layer will be laminar in the region of favorable pressure gradient. As the boundary layer encounters the adverse pressure gradient downstream of the location of minimum pressure on the surface of the hydrofoil, it separates and then transition occurs in the separated shear layer. The separated shear layer then reattaches as a turbulent boundary layer. The region enclosed between the locations of separation and reattachment is referred to as the laminar separation bubble. Typically, 6,7,8,9 the forward portion of the laminar separation bubble appears in the Cp plot as a region of constant pressure. The beginning of the constant-pressure region, pressure plateau, is the location of separation whereas the end of the pressure plateau is the location of transition. Reattachment is located where the slope of the actual (viscous) Cp curve is nearly equal to the slope of the fully turbulent (or inviscid) Cp curve. The locations of separation, transition and reattachment are shown in Fig. 9. It is also possible (depending on the hydrofoil geometry, pressure distribution and Reynolds number) for the flow to separate again near the trailing edge if the angle of attack is increased to moderate values that do not cause the laminar separation bubble to burst and fail to reattach. This possible case occurs because the laminar separated shear layer reattaches as a turbulent boundary layer. As this turbulent boundary layer encounters an adverse pressure gradient that is larger than it can negotiate, the boundary layer separates. This turbulent separation will appear as a second region of constant pressure near the trailing edge of the hydrofoil as indicated in Fig. 9. The distribution of Cp for the NACA4412 hydrofoil at Re c = 100,000 at = 8 degrees is shown in Fig. 10. Three Cp distributions are shown in the figure representing the inviscid solution and the cases with free transition using the 7

8 e N transition prediction method, and with forced transition at X/C = According to the preceding description of the laminar and turbulent separations, the locations of laminar separation and reattachment are determined from the Cp plot at X/C = and 0.508, respectively. The location of transition is obtained directly from the XFLR5 solution at X/C = Turbulent separation does not appear to occur at this angle of attack as there is no constantpressure region near the trailing edge. When the boundary layer is forced to transition at X/C = 0.05, the Cp curves does not show any sign of a laminar separation bubble. However, a region of constant pressure appears near the trailing edge starting at X/C = Figure 10. Distribution of Cp for the case at = 8 degrees. The V3V data presented in this section have the following coordinate system: X, Y and Z representing the streamwise, spanwise and wall-normal directions, respectively. The velocity components corresponding to this coordinate system are U, V and W, respectively. Figure 11 shows slices in the spanwise direction (X-Z plane) flooded with (U/Uinf) for the case at = 8 degrees. Also, the figure includes a slice in the streamwise (Y-Z plane) at the location of the trip wire colored in black. The results from each of the three measurement volumes are combined in this figure and the reversed flow is colored with grey for clarity. It can be seen in the figure that the boundary layer was not sufficiently resolved in the region 0<X/C<0.5. However, downstream of X/C = 0.5, it appears that the boundary layer is adequately resolved. Several reasons are considered for this observation. Since the boundary layer thickness is very small in the forward portion of the hydrofoil, it is possible that the V3V system in its current configuration did not provide the required spatial resolution for measurement in this region. Another possible reason could be the measurement procedure where the illumination of the region close to the surface may have not been sufficient. Resolving this issue, though, requires further investigation. A region of reversed flow appears between X/C = 0.12 and This region is indicated by an iso-surface where U/Uinf < 0. If the trip wire is effective in forcing the boundary layer to transition at X/C = 0.05, or shortly after, it will not be expected for the flow to separate near the leading edge. Additionally, even if no trip wire is used, or if it is ineffective, laminar separation is expected to occur at X/C = based on the Cp results with free transition. Therefore, one possible reason for this small reversed flow region is that it was induced by the relatively large trip wire (2mm in diameter). The other possible reason is that the estimated location of laminar separation from the Cp curve (if the trip wire was not effective) is not accurate. A further investigation of the Cp values in the region between X/C = 0.12 and 0.17 shows that these values only differ by 2.5% which could be assumed as a region of constant pressure. Furthermore, the Cp values in the region between X/C = 0.12 and (transition was predicted to occur at this location) shows that they vary by 4% only. Hence, it could be possible that the boundary layer trip was ineffective and laminar separation occurred at X/C = 0.12 and the previous estimate of laminar separation at X/C = was not accurate. 8

9 Figure 11. Slices in the spanwise direction (X-Z plane) flooded with (U/Uinf) for the case at = 8 degrees. Figure 12. Slices in the spanwise direction (X-Z plane) flooded with (U/Uinf) for the case at = 12 degrees. 9

10 Figure 12 represents the case at = 12 degrees and is plotted similarly to Fig. 11. A reversed-flow region can be seen between X/C = 0.06 and The Cp curve for the case at = 12 degrees is shown in Fig. 13. According to the criteria used in this study to determine the locations of laminar separation and reattachment from the Cp curve, it was found that they are at X/C = 0.06 and 0.12, respectively. The transition location obtained from XFLR5 is at X/C = Therefore, there is an agreement between the locations of laminar separation and reattachment obtained from the Cp curve and V3V results. However, these results and those of the case at = 8 degrees may suggest that the trip wire was ineffective in causing an early transition. Nevertheless, this suggestion does not violate the purpose of this investigation where it concentrates on the turbulent separation which, presumably, should occur near the trailing edge regardless of when the boundary layer transitioned. This conclusion is supported by observing the Cp curves of the forced and free transition cases shown in Fig. 13 as the two curves appear identical downstream of reattachment at X/C = Turbulent separation is estimated from the Cp curves to be near the location where the constant-pressure region near the trailing edge begins. The Cp curves in Fig. 13 show that this region starts at X/C = 0.88 where the variations in the Cp values become less than 4%. Scattered reversed-flow regions near the trailing edge appear in Fig. 12. The region of scattered reversed flow begins at X/C = 0.87 and extends to X/C = Although this region is not continuous, it strongly suggests a separated flow. Figure 13. Distribution of Cp for the case at = 12 degrees. However, the beginning of the constant-pressure region is not necessarily the location of turbulent separation. It was explained in Ref. 10 that turbulent separation must not be characterized by flow reversal only. Several occurrences were defined in the separated flow based on the fraction of the time the flow is reversed 10. These occurrences are: incipient detachment (ID) where the flow is reversed 1% of time; intermittent transitory detachment (ITD) occurs when the flow is reversed 20% of the time; transitory detachment (TD) takes place as 50% of time the flow is reversed and detachment (D), at which the time-averaged wall shear stress is zero ( w = 0). He also explained that there is experimental evidence that (TD) and (D) occur at the same location. Figure 14 focuses on the reversed flow region near the trailing edge. The presented iso-surfaces are defined where U/Uinf < 0 and flooded with the back-flow coefficient (BFC) defined as the percent of time the flow is reversed. Although only scattered spots of reversed flow appear in this case, these regions of reversed flow have on average a BFC of 50% or more. A possible reason for the not observing larger reversed flow region could be that the backflow mean velocity is much slower than mean velocity in the downstream direction. Therefore, when the instantaneous velocities are averaged over a period of time, the resultant will be a positive value for the mean velocity. This can be illustrated in Fig. 15 where it shows an iso-surface defined at BFC = 50% and flooded with U/Uinf. The figure shows that the region where BFC = 50% is much larger than the regions of reversed flow appeared in Fig. 14. Also, the mean velocities at this BFC are between 1 to 10% of the freestream velocity. It can be concluded, then, that for the time average mean velocity to be positive, the backflow velocities must have been less than 0.1*Uinf 50% of the 10

11 time. Additionally, it is observed in the figure that the beginning of the region where BFC = 50% is at X/C = 0.87 which is in agreement with the beginning of the constant-pressure region at X/C = 0.88 observed in Fig. 13. However, the beginning of this region is not constant in the spanwise direction. Figure 14. Iso-surfaces defined where U/Uinf < 0 and flooded with BFC for the case at = 12 degrees. Figure 15. Iso-surfaces defined where BFC = 50% and flooded with U/Uinf for the case at = 12 degrees. 11

12 Figure 16 shows a slice in the spanwise direction (X-Z plane) at Y/C = 0 flooded with the BFC values. The location of the incipient detachment (BFC = 1%) can be located at X/C = 0.82 whereas the location of the intermittent transitory detachment (BFC = 20%) is at X/C = It is worth mentioning here that at angles of attack of 12 and 16 degrees, there was a spanwise flow induced by the free surface in the water tunnel. To quantify this spanwise flow, an iso-surface defined where U/Uinf = 1.2 and flooded with V/Uinf is presented in Fig. 17. It is seen in this figure that the spanwise flow is moving towards the bottom of the hydrofoil (hydrofoil is mounted vertically) at velocities as large as 5% of the freestream velocity. In the mid chord region, this flow reaches values of 10% of Uinf. However, it can be noticed that this spanwise flow weakens downstream of X/C = 0.8 where turbulent separation is expected to occur. Additionally, the strength of this spanwise flow extends between Y/C = 0.2 and Therefore, separation results can still be interpreted as twodimensional separation with a reasonable confidence. Figure 16. A slice in the spanwise direction flooded with BFC for the case at = 12 degrees. 12

13 Figure 17. Iso-surfaces defined where U/Uinf = 1.2 and flooded with V/Uinf for the case at = 12 degrees. The case at = 16 degrees is shown Fig. 18 in a similar fashion as the cases at = 8 and 12 degrees in Figs. 11 and 12. This figure illustrates several important observations. The results presented in this figure show that the boundary layer is better resolved even in regions near the leading edge of the hydrofoil. This may be due to the thicker boundary layer at the higher angle of attack. Another observation is the appearance of a reversed-flow region between X/C = 0.06 and As was argued above that in the cases at = 8 and 12 degrees this reversed-flow region represent the location of the laminar separation bubble which suggested that the trip wire was ineffective. The appearance of this reversed-flow region at the same location does not necessarily contradict the above suggestion and does not indicate that, in all cases, this reversed flow region is induced by the trip wire. If it were induced by the trip wire, it would have appeared at the same location at = 8 degrees. However, in that case, this separated flow appeared much downstream than the location of its appearance in the case at = 16 degrees. Also, even if the trip wire induced a small separation, it may, still, have not caused the boundary layer to transition. Therefore, it is concluded that the trip wire may not have been effective in transitioning the flow for the cases at = 8 and 12 degrees and did not induce any separation. However, at = 16 degrees, the trip wire induced separation. The surface pressure results obtained from XFLR5 are presented in Fig. 19. The Cp curves of the free and forced transition cases are identical in this case because the free transition location is at X/C = which is upstream of the forced transition location at X/C = The constant-pressure region starts slightly downstream of X/C = Similarly to Fig. 15, an iso-surface defined where BFC = 50% and flooded with U/Uinf for the case at = 16 degrees is shown in Fig. 20. As was discussed above, the location of detachment (D) corresponds to the location where BFC = 50%. In this case, the location of (D) is slightly upstream of X/C = It can also be seen in the figure that when BFC = 50%, i.e. 50% of time the flow is moving in the upstream direction, the time-averaged mean velocities are positive. The effect of the spanwise flow on the separated flow is well illustrated in this case as the region closer to the free surface does not show separation. However, the region closer to the bottom of the tunnel, i.e < Y/C < 0, does not show any variation due to the spanwise flow. This observation further support the conclusion that it can be reasonably assumed that for this part of the hydrofoil the separated flow is two dimensional. 13

14 Figure 18. Slices in the spanwise direction (X-Z plane) flooded with (U/Uinf) for case at = 16 degrees. Figure 19. Distribution of Cp for the case at = 16 degrees. 14

15 Figure 20. Iso-surfaces defined where BFC = 50% and flooded with U/Uinf for the case at = 16 degrees. The iso-surfaces colored in grey displayed in Fig. 18 are defined where U/Uinf < 0 and represent the reversedflow region. These iso-surfaces are shown in Fig. 21 and flooded with the BFC values. The BFC values for the reversed-flow region are more than 70%. This result and those presented in Fig. 20 illustrate that for the timeaveraged mean velocities to appear moving upstream, the BFC values have to be more much more than 50%. The region of reversed-flow starts at X/C = 0.55 which is slightly downstream of the detachment location at X/C = To shed light on the maximum backflow velocities, iso-surfaces defined where U/Uinf = and flooded with BFC are presented in Fig. 22. The BFC values are more than 85% for these iso-surfaces. This indicates that the mean backflow velocities reach 15% of the freestream velocity in the core of the recirculation region. Figure 23 summarizes the relationship between the time-averaged mean velocities and the BFC values. In this figure, a slice in the spanwise direction (X-Z plane) is shown and flooded with the BFC values. Also shown in the figure, lines defined and colored by the time-averaged velocities. It is clear in the figure that the time-averaged back flow corresponds with the core of the recirculation region with values of BFC reaching 85%. 2 Iso-surfaces defined at the maximum Reynolds shear stresses (- u w U inf ) are flooded with BFC values (Fig. 24). For the most part, the values of BFC are less than 20%. This suggests that the maximum Reynolds shear stresses occur away from the wall and core of the recirculation region. To further illustrates this, Fig. 25 shows that the Reynolds shear stresses increase away from the flow and beyond the reversed-flow region and reaches maximum values where U/Uinf >

16 Figure 21. Iso-surfaces defined where U/Uinf < 0 and flooded with BFC for the case at = 16 degrees. Figure 22. Iso-surfaces defined where U/Uinf = and flooded with BFC for the case at = 16 degrees. 16

17 Figure 23. A Slice in the spanwise direction (X-Z plane) flooded with BFC and lines defined and colored by (U/Uinf) for the case at = 16 degrees. Figure 24. Iso-surfaces defined at maximum (- u w 2 U inf ) and flooded with BFC values. 17

18 Figure 25. A Slice in the spanwise direction (X-Z plane) flooded with (U/Uinf) and lines colored by (- u w ) for the case at = 16 degrees. 2 U inf B. PIV results The PIV results are briefly discussed in the section as a comparison with the V3V results. The coordinate system in presenting the PIV results has X as the streamwise direction whereas Y is the wall-normal direction. Figure 26 shows lines defined by the BFC values for the case at = 8 degrees. BFC values of 10% appear near the trailing edge starting at X/C = 0.94 indicating some backflow. This back flow was not observed in the V3V results. As the angle of attack is increased to 12 degrees, BFC values of 10% start to appear at X/C = 0.82 and their values reach 50% at X/C = 0.93 as shown in Figs. 27 and 28. The V3V results showed that the BFC values for this case reached 50% at X/C = 0.97 and 20% at X/C = Therefore, the locations of (D) and (ITD) detected by the V3V measurements are slightly downstream of those obtained from the PIV measurements. Figures show the BFC values for the case at = 16 degrees for measurement locations 3, 4 and 5, respectively. The locations where BFC values for this case reach 10, 20 and 50% are X/C = 0.52, 0.62 and 0.73, respectively. These values occur downstream of those detected by the V3V measurement. This trend is opposite to the trend found for the case at 12 degrees. It is unclear at this point why the difference in the locations of these BFC values is large and requires further investigation of the PIV results. However, one plausible reason would be the time period over which the results are averaged. The V3V results were averaged over a period of 100 seconds whereas the PIV results where averaged over 3 seconds for each set of the 1200 images. Therefore, further analyses of the frequency at which the separated flow cycles between moving upstream and downstream is required. 18

19 Figure 26. Lines defined by BFC values for case at = 8 degrees. Figure 27. Lines defined by BFC values for case at = 12 degrees at measurement location 4. 19

20 Figure 28. Lines defined by BFC values for case at = 12 degrees at measurement location 5. Figure 29. Lines defined by BFC values for case at = 16 degrees at measurement location 3. 20

21 Figure 30. Lines defined by BFC values for case at = 16 degrees at measurement location 4. Figure 31. Lines defined by BFC values for case at = 16 degrees at measurement location 5. IV. Conclusion Volumetric three-component measurement technique was used to investigate turbulent separation on a hydrofoil at different angles of attack. The results obtained from this measurement technique are encouraging and provided reasonable assessment of the separation process. The V3V results were also compared to those obtained from PIV. Satisfactory agreement was only achieved at the case at 12 degrees and the results varied significantly at the case at the higher angle of attack. However, the V3V agreed better with the surface pressure distribution obtained from XFLR5 software. It is concluded that the PIV requires further investigation to resolve the disagreement between the two measurement techniques especially since the time-averaged results of the PIV and V3V were averaged over significantly different periods. This observation requires more analyses of the frequency of the separated flow. Future work will also include analysis of the Reynolds normal and shear stresses and vortex structure about the X, Y and Z axes. 21

22 Acknowledgments This material is based upon work supported by the National Science Foundation under Grants No and The support of The University of Alabama is also acknowledged through the Research Stimulation Post- Doctoral Fellow Program and the Graduate Council Fellowship. References 1 Angele, K. P., and Muhammad-Klingmann, B., PIV Measurements in a Weakly Separating and Reattaching Turbulent Boundary Layer, European Journal of Mechanics B/Fluids, Vol. 25, 2006, pp Na, Y., and Moin, P., Direct Numerical Simulation of a Separated Turbulent Boundary Layer, Journal of Fluid Mechanics, Vol. 374, 1998, pp Willert, C.E., and Gharib, M., Three-Dimensional Particle Imaging with a Single Camera, Experiments in Fluids, Vol. 12, 1992, pp Pereira F., Gharib M., Dabiri D., Modarress D., Defocusing Digital Particle Image Velocimetry: a 3- component 3-Dimensional DPIV Measurement Technique. Application to Bubbly Flows, Experiments in Fluids Suppl. S78 S84, Pereira F., Stuer H., Graff, E. C., and Gharib, M., Two-frame 3D particle tracking, Measurement Science and Technology, Vol. 17, 2006, pp Wahidi, R., and Bridges, D. H., Experimental Investigation of the Boundary Layer and Pressure Measurements on Airfoils with Laminar Separation Bubbles, Proceedings of the 39 th AIAA Fluid Dynamics Conference, AIAA Paper , San Antonio, TX, June O Meara, M. M., and Mueller, T. J., Laminar Separation Bubble Characteristics on an Airfoil at Low Reynolds Numbers, AIAA Journal, Vol. 25, No. 8, 1987, pp Gaster, M., The Structure and Behaviour of laminar Separation Bubbles, ARC Rept. R&M 3595, Arena, A. V., and Mueller, T. J., Laminar Separation, Transition, and Turbulent Reattachment near the Leading Edge of Airfoils, AIAA Journal, Vol. 18, No. 7, 1980, pp Simpson, R. L., Aspects of Turbulent Boundary-Layer Separation, Prog. Aerospace Sci., Vol. 32, 1996, pp