stereo piv measurements of suction and pulsed...

18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics・LISBON | PORTUGAL ・JULY 4 – 7, 2016

Stereo PIV measurements of Suction and Pulsed Blowing Interaction with a

Laminar Boundary Layer

L. Marom1, N. Shay1,*, A.Seifert2 1: Graduate Student, School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Israel

2: Prof., School of Mechanical Engineering, Faculty of Engineering, Tel Aviv University, Israel, Associate Fellow AIAA. * Correspondent author: [email protected]

Keywords: Stereo PIV, Active Flow Control, Boundary Layers

ABSTRACT

The focus of the current paper is a fundamental study of active flow control in the form of steady suction and oscillatory blowing actuators (SaOB) and quantifies its effect on a laminar boundary layer with nearly zero pressure gradient. Recent experiments had shown the effective and efficient action of the SaOB actuator as a drag reduction device on the flow in a separating boundary layer and also as a lift enhancement device. However, in order to utilize it to its fullest extent, a deeper fundamental understanding of the boundary layer interaction with suction and oscillatory blowing and the combination of these two effects in close proximity is required. In the current study, new results have been acquired using stereo PIV measurements in the cross flow plane in order to validate previous HW results and to investigate 3D vortical structures which their existence can only be hypothesized based on single HW measurements. Different methods to identify vortices were applied to the new acquired data and the phase-locked velocity field was reconstructed. The results show the development of 3D vortical structures from the steady suction and from the oscillatory blowing. As will be detailed in the paper, the pulsed jet is generating a complex 3D structure with stream-wise vortices which spread in the span-wise direction to at least twice the width of the SaOB exit nozzle.

Introduction Drag reduction and expansion of the effective operation envelope through active flow control (AFC) has been a goal in aviation, energy and flow machinery industries for decades. Recent experimental efforts have shown the possible performance benefits of several AFC applications. In those studies it has shown its effectiveness in preventing boundary layer separation around axisymmetric body [1] and bluff bodies including circular cylinders [2] and large-heavy ground transportation vehicles [3]. It is also proven for lift enhancement [4] and fluidic Gurney flap [5]. From these experiments, fluidic devices have emerged as promising candidates for future full scale applications. However, the fundamental flow control mechanism and interactions with the boundary layer are still relatively unexplored. Prior knowledge of the optimal actuator placement and configurations targeting energy efficient flow modification would be beneficial and are essential in order to incorporate AFC in future realistic designs. Developing predictive


capability for new flow control devices requires detailed understanding of operating principles and flow physics of the actuator and its effect on the baseline flow. A novel type of flow control system that incorporates steady suction with un-steady sideways oscillating jets is the Suction and Oscillatory Blowing (SaOB) actuator. Earlier Experimental studies were conducted using Hot-Wire (HW) measurements of steady suction alone [6], of pulsed blowing alone [7] and of the combination of the two effects [8]. The objective is to investigate the fundamental flow physics of SaOB flow control through highly resolved measurements with the ultimate goal of improving the effectiveness, efficiency, and our predictive capability of fluidic AFC as it interacts with separating laminar or turbulent boundary layers. This study is focused on SaOB actuators interaction with nearly zero pressure gradient (ZPG) laminar boundary layer. Measurements are conducted in laminar boundary layer interaction with steady suction and pulsed blowing. The insight gained from this fundamental study provides additional step towards development of understanding and tools required for an active flow control flight application. Experimental setup Experiments were conducted on a small, open-loop wind tunnel with rectangular cross-section at Tel Aviv University's Meadow Aerodynamics laboratory. The boundary layer integral parameters were measured by a single HW on a 3D traversed system confirmed that the flow has slightly accelerating laminar boundary layer behavior. All PIV data was acquired using LaVision’s system (Davis 7.2) with double pulsed Nd:YAG Laser (120mJ max per pulse, 7 Hz max repetition rate) generating light sheet of wave length 532 nm and thickness of about 2mm. The image acquisition system contains two, 14 bit B/W CCD cameras with 4Mpixel resolution and a NIKON lens with a focal length of 60mm each. The flow was seeded using two theatrical fog machines with mean particle size of 5 µm: one for the main flow in the tunnel and the other was connected to the SaOB actuator air feeding system in order to highlight the oscillatory jets emerging from the SaOB exit nozzles. Fig. 1 illustrates the wind tunnel and the SPIV system as installed. The coordinate system is set such that X is the streamwise direction, Y is the vertical-wall normal direction and Z is the spanwise direction. The origin is located at the test sction entrance entrance, on the floor and at the symmetry line (spanwise half width). The SaOB actuator is placed inside a plenum chamber which is connected to the floor of the tunnel through a pair of nozzles, angled at about 45 degree to the X downstream direction, as seen in Fig. 2. Seven (7) suction holes, 4 mm diameter each , spaced 15mm in the Z direction, are used in these


experiments and are located at x=476.5mm. The two-oscillatory blowing jets that issue from the SaOB actuator are located at x=548 mm (see Fig. 2). The entire SPIV system is mounted on a traversing mechanism that allows its motion in the X direction without need for re-calibration. The SPIV calibration was made using a custom made 3D printed calibration plate.

Fig. 1 Top view of the wind tunnel and SPIV setup.

Fig. 2 (Left) Side view of the SaOB actuator box mounted under the test section floor, (Right) view of the test panel

with suction holes taped over, a hot-wire and the pulsed blowing slots. The suction holes are at X=476.5 mm and

pulsed blowing slots at X=548 mm. SPIV system is absent.

For the mean flow measurements, 500 image pairs were acquired at each x location. Later on, and for the purpose of time dependent calculations, more detailed data acquisition tests were made using 1000 image pairs (about 50 pairs for each phase, grouped into 20 deg “bins”) taken

Flow

Camera2Camera1

Laser

LightSheet

X

Z

Y

LightSheetformingoptics


for each x location. During these measurements, the cameras trigger signal was measured as well as the SaOB’s actuator’s oscillation signal (from its feedback tube, [7]) to find the phase at which each image pair was acquired and to sort the images into phase-locked dataset. The image processing uses a stereo PIV cross-correlation method with a standard cyclic FFT algorithm [9]. The interrogation window is of square shape with decreasing size, starting from 128 pixels down to 12 pixels with 50% overlap and 4 passes. The eventual resolution of the SPIV is close to 0.3mm (18 pixels/mm) in the Z & Y directions and close to 2 mm in the streamwise direction, due to the light sheet thickness. Experimental Results Validation In order to validate the calibration of the SPIV system certain baseline uncontrolled boundary layer measurements were taken and verified against H.W. measurements. Those measurements were acquired at 6 different x locations between x=350 mm to 600 mm and with a free-stream velocity of 6.5 m/s (that was held constant throughout). Fig. 3 shows an example of the results, the best agreement can be seen at X=600mm (left) and the "worst" at X=350mm (right) while all the rest of the results fall somewhere in between. Overall a good agreement between the HW and the U component (out of plane velocity) SPIV measurements was achieved. Another validation of the PIV measurements was made using the 3D continuity equation. The divergence of the entire scanned fraction of the test section volume was calculated in order to obtain the flow rate at each cross-section. When comparing the flow rate between the different X-sections a maximum error of 2.3% was obtained compared to the flow rate at the tunnel’s entrance. This calculation translates into an error in the continuity equation as a mass “lost” between different cross sections. Note that the interrogation region does not reach the top or side-walls such that boundary layer spreading is not accounted for.


Fig. 3 Velocity profiles comparison between H.W and PIV measurements

at x=600 mm (left) and x=350 mm (right). The distance to the wall was found using a linear fit of the data points

within u/U∞<0.3, for both HW and PIV.

SPIV results Measurements were acquired at different x locations and the 3 components of velocity and vorticity were calculated. Fig. 4 shows the Y vorticity component, perpendicular to the surface. The bottom part shows a cross section at X location of 540mm, which is about 65mm downstream from the suction hole’s location. It can be seen that the effects on the vertical vorticity is reaching a height of about 5 mm. As hypothesized in previous studies [6], a single suction hole is generating a pair of counter rotating stream-wise vortices, causing an increase in the near-wall velocity directly down-stream from the suction hole. This effect spreads downstream, to at least three times the hole’s diameter in the span-wise direction. This rise in the near-wall stream-wise velocity is causing a strong vertical vorticity component along with the expected stream-wise vortices. Downstream of the blowing jets the vertical vorticity component starts to vanish except for at the symmetry line (span-wise), this can be expected as the jets are emerging with much higher momentum than the flow and are spreading wide in the span-wise direction creating a different flow structure than the suction effect alone. This combined effect will be discussed next.

0

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4

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10

0 1 2 3 4 5 6 7 8

Y[mm]

Umean [m/s]

VelocityprofileatX=600mm

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0 1 2 3 4 5 6 7 8Umean [m/s]

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Fig 4. Surfaces of vertical vorticity component, Top view (upper) at vertical height of Y=2.5 mm, blowing nozzles are

indicated as dashed black rectangles, and cross-section view (lower chart) at stream-wise location of X=540 mm.

Fig. 5 shows a cross-section surface of the mean velocity (V) wall-normal component and the velocity vectors W,V. It shows data at the location of x=590 mm which is about 40 mm downstream of the pulsed blowing jets. Only half of the tunnel cross-section width is shown to emphasis on the effects of one sideways oscillating jet with time varying magnitude. As shown in our previous recent studies [6-8], 3 main structures can be identified in the figure, two clockwise rotating vortices located at -55 < z < -35 mm and at -35 < z < -20 mm, and a stronger vortex which starts at z=-15 mm and collides with its mirror-image generated from the other jet (not shown). The two left CW vortices seem to conflict with each other and further downstream the left one loses some of its angular momentum. We assume that this phenomenon is a result of angular momentum transferring from the main vortex (right one in Fig 5) to the middle vortex, causing it to overcome the left CW vortex.


Fig. 5 Mean vertical velocity component cross-section surface at X=590mm and W,V velocity vectors on top. The

vectors are scaled by 8 and only every 4th vector is shown.

The strong effect of the blowing can also be seen in Fig. 6, mean surfaces of the vertical velocity component are shown. In the upper part of the chart a top view is presented, as discussed before the jets do not emerge uniformly in the span-wise direction, they have a profile which includes two velocity peaks, a strong one near the symmetry line of the tunnel and actuator (0 < |z| < 15 mm) as can be seen in the upper part of the chart indicated by the high vertical velocity, and a weaker one near the outer edge of the jet's nozzle (25 < |z| < 35 mm). The blue shapes which correspond to negative vertical velocity are a clear indication of the strong stream-wise vortices generated from the emerging jets, which play a key role in mixing the low momentum boundary layer flow with higher momentum layers, helping to prevent flow separation, when it would be present


Fig 6. Mean surfaces of the vertical velocity component. Top view (upper) at vertical height of Y=3 mm, blowing

nozzles are indicated as dashed black rectangular, and cross section view (lower) at stream-wise location of X=590

mm

Conclusions Results from an ongoing experimental study were presented, the measurements were acquired using SPIV system and the complete 3D flow field was analyzed. The new acquired data was first used to validate previous measurements, and then a more in-depth analysis was done in order to quantify the spatial effects of the SaOB interaction with laminar boundary layer. As seen in previous studies, the suction effect spread downstream to at least 3 times the hole's diameter in the span-wise direction. The high vertical vorticity component indicates the strong effect of the suction. The oscillatory blowing jets add momentum to the near-wall flow as well as generating streamwise vortices which help with mixing the low momentum layer with higher momentum layers. Those effects eventually causing a significant reduction in the boundary layer thickness and assisting in preventing separation. The oscillatory blowing effects spread to at least 1.5 times the nozzles' width and can be measured quite strongly for at least 100 mm downstream of the nozzles.


Acknowledgments The authors would like to acknowledge technical support by V. Palei, M. Vassermann, Shlomo Pasteur and T. Bachar. Support and helpful discussions with all members of the TAU Meadow Aerolab are acknowledged. References

[1] Wilson, J., et al., Suction and Pulsed-Blowing Flow Control Applied to an Axisymmetric Body. AIAA Journal, 2013: p. 1-15.

[2] Shtendel, T. and A. Seifert, Three-dimensional aspects of cylinder drag reduction by suction and oscillatory blowing. International Journal of Heat and Fluid Flow, 2013.

[3] Seifert, A., et al., Large trucks drag reduction using active flow control, in The Aerodynamics of Heavy Vehicles II: Trucks, Buses, and Trains. 2009, Springer. p. 115-133.

[4] Dmitri Sarkorov, D., Seifert, A., Detinis, I., Bauminger, S. and Moshe Steinbuch, M., Active Flow Control and Part Span Slat Interactions. AIAA J., 2015.

[5] Dolgopyat, D.a.S., A., Conventional Airfoil Active Flow Control “Virtual” Maneuvering System, in 55th ISR Aero Conf. 2015. [6] Schatzman, D., et al., Suction and Oscillatory Blowing Interaction with Boundary Layers. AIAA

Journal, January 2015. [7] Marom, L., V. Palei, and A. Seifert, Suction and Pulsed Blowing Interaction with a Laminar

Boundary Layer. 55th ISR Aero Conf, Feb. 2015. [8] Seifert, A. and L. Marom, Interaction of Suction and Pulsed Blowing with a Laminar Boundary

Layer. Bulletin of the American Physical Society, 2015. 60. [9] Prasad, A.K., Stereoscopic particle image velocimetry. Experiments in Fluids 29 (2000) 103-116.

stereo piv measurements of suction and pulsed...

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