Three-dimensional span effects of highaspect ratio synthetic jet forcing for separation control on a low-reynolds number airfoil

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1 TSpace Research Repository tspace.library.utoronto.ca Three-dimensional span effects of highaspect ratio synthetic jet forcing for separation control on a low-reynolds number airfoil Mark Feero, Philippe Lavoie, Pierre E. Sullivan Version Post-Print/Accepted Manuscript Citation (published version) Feero, M.A., Lavoie, P. & Sullivan, P.E. "Three-dimensional span effects of high-aspect ratio synthetic jet forcing for separation control on a low Reynolds number airfoil". Journal of Visualization (2016), 1-7. doi: /s Publisher s Statement The final publication is available at Springer via How to cite TSpace items Always cite the published version, so the author(s) will receive recognition through services that track citation counts, e.g. Scopus. If you need to cite the page number of the TSpace version (original manuscript or accepted manuscript) because you cannot access the published version, then cite the TSpace version in addition to the published version using the permanent URI (handle) found on the record page.

2 Feero MA, Lavoie P, Sullivan PE. Three-dimensional span effects of high-aspect ratio synthetic jet forcing for separation control on a low Reynolds number airfoil. Journal of Visualization. 2016;1 7. DOI: /s Journal of Visualization manuscript No. (will be inserted by the editor) Three-dimensional span effects of high-aspect ratio synthetic jet forcing for separation control on a low-reynolds number airfoil Mark Feero Philippe Lavoie Pierre E. Sullivan Received: date / Accepted: date Abstract The three-dimensional structure of the reattached flow caused by synthetic jet actuation on an airfoil was investigated using surface flow visualization. Without active control, the flow was stalled with laminar boundary layer separation occurring near the leading edge. Tuft and oil visualization showed the shape and spanwise extent of the attached flow due to a finite span synthetic jet where the effect of excitation frequency and blowing ratio was the focus. For all excitation frequencies tested, a similar contraction of the spanwise extent of the attached flow towards the trailing edge was observed due to edge effects of the finite span jet. Increasing the blowing ratio was found to decrease the amount by which the attached flow contracted. Keywords Flow control Synthetic jet Flow separation 1 Introduction Boundary layer separation on airfoils at low Reynolds number occurs even at relatively low angles of attack due to the interaction of a laminar boundary layer on the suction surface with an adverse pressure gradient. At low Reynolds number (i.e., less than [9]), conventional airfoil profiles experience stall at lower angles of attack and therefore a significant increase in drag and decrease in lift. Active flow control using synthetic jet actuators is a common technique for mitigating laminar boundary layer separation and improving aerodynamic performance (e.g., [3], [5], [1]). A synthetic jet is a zero-net-mass-flux fluidic device that alternately expels and ingests the working fluid through a slot or orifice, thereby adding momentum but not mass to the flow [14]. Synthetic jets are advantageous M. Feero Institute for Aerospace Studies, University of Toronto, Toronto, Ontario M3H 5T6, Canada Tel.: m.feero@mail.utoronto.ca P. Lavoie Institute for Aerospace Studies, University of Toronto, Toronto, Ontario M3H 5T6, Canada P. Sullivan Mechanical and Industrial Engineering, University of Toronto, Toronto, Ontario M5S 3G8, Canada

3 2 Mark Feero et al. in that they require no external fluid source/sink and typically can be used to provide excitation over a range of reduced frequencies F + = f e c/u (where f e is the excitation frequency, c is the airfoil chord length and U is the freestream velocity). It is often also possible to vary the excitation amplitude, characterized by the blowing ratio C B = U j /U (where U j is the mean expulsion velocity), independent from the frequency. Effective excitation frequencies for flow reattachment can range from low-frequencies on the order of the convective time scale, F + = O(1), to high-frequencies that can be considered time-invariant relative to the flow [8]. Typical applications of fluidic flow control on airfoils involve single [6, 7] or multiple slots [13, 15] oriented in the spanwise direction, or a spanwise row of orifices [11], that may or may not cover the entire span of the model. To quantify the effect of control on the flow, measurements are often restricted to the plane of symmetry with the assumption that the flow in the controlled region is quasi two-dimensional (or may be considered as such over a certain spanwise extent). The purpose of this study was to investigate the three-dimensionality of the reattached flow using a single finite span synthetic jet slot covering approximately 1/3 of the model span. The three-dimensionality of the resulting controlled flow is often not considered. The effects of both F + and C B were considered and surface streamlines were observed using several flow visualization techniques. This type of visualization has not been presented to date. 2 Experimental Setup 2.1 Test facility Experiments were conducted in a low-speed, low-turbulence recirculating wind tunnel located in the Mechanical and Industrial Engineering Department at the University of Toronto. The flow enters a 5 m long octagonal test section that is 0.91 m wide and 1.22 m tall after passing through seven screens and a 9:1 contraction. The freestream velocity in the test section can be varied from 2.8 m/s to 18 m/s and the turbulence intensity is 0.08%. The freestream velocity is monitored by a pitot-static tube located at the test section entrance. The experiments in the current study were performed at Re c = , which corresponds to U 6.4 m/s. A NACA 0025 airfoil with a chord length c = 300 mm was studied in these experiments. The aluminum airfoil model is 885 mm wide and spans the width of the test section (Figure 1). The model is divided into three spanwise sections: two solid outer sections and a centre section that is hollow and houses sensors, such as pressure taps, and the synthetic jet actuator. A row of 64 static pressure taps evenly distributed between the upper and lower surfaces is installed at midspan. The pressure taps have a diameter of mm at the surface and are connected by flexible tubing to a Scanivalve multiplexer located outside the test section. The surface pressure is measured by an MKS Baratron 223B differential pressure transducer with a range of 100 Pa and an accuracy of ±0.3 Pa. Circular end plates with a diameter of 2.25c are mounted to the ends of the model to isolate the model from the tunnel wall boundary layer and ensure spanwise uniformity of the baseline flow. The end plate geometry was designed according to Boutilier and Yarusevych [2] and includes a 30 chamfer of the outer edge to avoid flow separation. Using this end plate configuration, measurements confirmed that the flow can be considered two-dimensional over at least 60% of the central model span (1.8c). The synthetic jet actuator consists of a high-aspect-ratio mm 294 mm slot and a cavity driven by 16 circular piezoelectric diaphragms with 35 mm diameter. The motion of the diaphragms in the cavity is perpendicular to the motion of the air in the slot and opposing cavity walls each contain eight diaphragms moving in-phase. At an excitation frequency of 1000 Hz, the mean expulsion velocity of the jet can reach 16 m/s. Measurement of the synthetic jet velocity as a function of applied voltage

4 Title Suppressed Due to Excessive Length 3 U 300 mm Pressure taps Synthetic jet slot End plates 885 mm Fig. 1: Top view of the airfoil model. and frequency was performed using hot-wire anemometry prior to installing the synthetic jet in the model. Detailed hot-wire measurements were also used to confirm that the synthetic jet velocity is approximately two-dimensional, as it is uniform in both magnitude and phase within ±15% over 80% of the slot major axis. When installed in the airfoil, the synthetic jet slot is located at x/c = 11.7% (where the streamwise coordinate x originates at the leading edge and is parallel with the chord line). 2.2 Visualization techniques Surface flow visualization techniques were used to investigate the three-dimensional effects of the actuation on the reattached flow. Two surface visualization techniques were employed: tuft and oil film. Tuft visualization requires attaching a number of short segments of string/thread to the surface. Tufts approximately follow the instantaneous motion of the flow and highlight regions of unsteadiness. For the present study, yarn tufts of approximately 0.75 mm in diameter and 25 mm in length were fixed to the surface in four spanwise rows at x/c = 30%, 47%, 63% and 85%. Tufts were not placed further upstream to avoid interfering with the flow. The tape used to hold the tufts was approximately 0.04 mm thick. The spanwise spacing between tufts was approximately 25 mm, except at x/c = 85% where they were spaced by 37.5 mm to avoid tufts tangling near the trailing edge. The oil film technique is used to visualize time-averaged surface streamlines. The oil mixture used for the present study was a light mineral oil combined with lampblack pigment in a ratio of approximately 10:1 by volume. Prior to applying the oil with a foam brush, a portion of the model surface was covered with a white adhesive plastic film that was 0.05 mm thick. The purpose of the film

5 4 Mark Feero et al. was two-fold: to protect the pressure taps from clogging and better contrast with the black pigment. Photographs were taken after 15 minutes of run time. Through repeated trials it was determined that this was a sufficient amount of time for the oil streaks to form. 3 Results and discussion 3.1 Surface pressure distributions Prior to the flow visualization, static surface pressure measurements were used to determine the state of the flow over the airfoil (i.e., fully attached, laminar separation bubble, leading edge separation, etc.). Stalled flow results in a large region of approximately constant static pressure extending to the trailing edge, while a laminar separation bubble is indicated by a pressure plateau followed by recovery [10]. As shown by the pressure coefficient C p = p/ρu 2 distribution in Figure 2, at α = 12 and Re c = the flow is stalled and the suction surface boundary layer separates at approximately x/c = 13%. The baseline C p distributions in Figure 2 with and without the tufts show negligible differences, indicating that the tufts are not affecting the flow in a substantial way. Also, at C B = 1 and F + = 47 the pressure distribution shows the flow is completely reattached at midspan. This result was typical of other F + values at the same blowing ratio Cp x/c Baseline Baseline (tufts) C B = 1, F + = 47 Fig. 2: Midspan pressure coefficient distribution at α = 12 and Re c = under baseline conditions and with control. 3.2 Tuft visualization The flow near the surface of the airfoil under baseline and control conditions was visualized using tufts for C B = 1 and reduced frequencies F + =, 1, 2, 12 and 47. Excitation at F + =, 1 and 2 affects

6 Title Suppressed Due to Excessive Length 5 the frequency associated with the large scale vortex shedding in the wake, f w + = f w c/u 1, along with its sub- and super-harmonics. Excitation at F + = 12 is within the band of frequencies associated with the separated shear layer, centred at f + sl = f sl c/u 15, and F + = 47 can be considered high-frequency control that is time-invariant relative to the flow. Previous work has shown that high-frequency excitation and excitation near f + sl leads to steadily reattached flow, while excitation near f w + causes the flow to lock in at the control frequency [4]. The frequencies f w and f sl associated with the wake and shear layer instabilities, respectively, were determined from hot-wire measurements in the corresponding regions of the flow. To visualize unsteady flow regions, photographs were taken with an exposure time of 2 seconds. This resulted in tufts in a region of unsteady flow will appearing blurry and an approximate range of motion can be seen. The tuft flow visualization results are shown in Figure 3. In the baseline case, motion can be seen in almost all the tufts and few are aligned with the mean flow direction, typical of a region where the flow is separated. Most motion is observed in the row of tufts closest to the trailing edge. When control is applied, the following general observations can be made for each F + value: the flow is not reattached over the entire span of the synthetic jet and the spanwise extent of the attached flow decreases towards the trailing edge, there is significant flow in the spanwise direction at the edges of the attached region, and the size of the attached region appears similar. The lines in Figure 3 indicate the approximate bounds of the attached region. The amount by which the span of the attached flow region contracts in the streamwise direction can be quantified as the ratio of the spanwise length of the attached flow in the first row of tufts to the fourth row. The approximate spanwise extent of the attached flow at x/c = 30% is 0.9c and reduces to c at x/c = 85%, giving a ratio of 44%. At F + = and 1, significant unsteadiness can be observed in the tufts in the attached region other than very near z/c = 0, which is indicative of the expected unsteadily attached flow. For F + 2, the flow appears to be steady in the attached region. While three-dimensionality of the reattached flow due to the finite span of the synthetic jet and edge effects is not unexpected, the extent by which the span of the reattached flow decreases in the streamwise direction is substantial. 3.3 Oil visualization Because oil film visualization is not well suited for unsteady flows, the oil experiments were limited to F + = 12 and 47 where steadily reattached flow occurs [4]. The effect of F + on the shape of the attached region for constant C B = 1 is shown in Figure 4. The solid black region indicates separated flow because the oil has not moved due to the low shear stress at the wall. Near the edges of the area where the oil was applied, there are some streaks from accumulated oil flowing due to gravity. The results show that the spanwise extent of the attached flow near mid chord and the trailing edge are similar for the two F + values, while just downstream of the slot it appears that the flow is attached over almost the entire span of the slot for F + = 12. For F + = 47, the flow is attached over approximately z/c just downstream of the slot. The effect of C B on the three-dimensionality of the reattached flow was also investigated to determine whether increasing C B would decrease the severity of the edge effects that cause the contraction of the attached flow region. At F + = 47, three blowing ratios were considered: C B = 1, 2 and 2.5 (the largest blowing ratio possible at the given U ). As C B is increased (Figure 5), the spanwise extent of the attached region increases and the edge effects become less severe (i.e., there is less of a contraction of the attached flow in the streamwise direction). As estimated from the oil images, the spanwise extent of the attached flow at x/c = 0.9 increases from 3c, 9c, to 6c for C B = 1, 2 and 2.5, respectively. The spanwise extent of the attached flow near the trailing edge at C B = 1, 3c, compares very well to the value of c estimated from the tuft visualization. It is also interesting

7 6 Mark Feero et al. U z/c =0 (a) Baseline (b) F + = (c) F + = 1 (d) F + = 2 (e) F + = 12 (f) F + = 47 Fig. 3: Tuft flow visualization for a range of reduced frequencies with constant blowing ratio CB = 1. to note the finger-like structures in the attached region. While it is possible that this is a product of the oil visualization, these may indicate qualitatively secondary spanwise structures. Sahni et al.

8 Title Suppressed Due to Excessive Length - - z/c=0 (a) F + = 12 7 x/c= z/c=0 x/c= (b) F + = 47 Fig. 4: Oil visualization at F + = 12 and 47 with constant blowing ratio CB = 1. [12] performed experiments with a finite span synthetic jet on a NACA 4421 airfoil and observed the presence of secondary structures for blowing ratios from CB = to 1.2. Furthermore, they found that the spanwise wavelength of these structures increased with increasing blowing ratio and the channeling of the tracer material in Figure 4 seems to show similar behaviour. In their case, the baseline flow appears to be attached (likely with a laminar separation bubble), and may differ from the case of a baseline stalled flow. Smith and Glezer [14] showed the existence of spanwise structures forming in the near-field region of a rectangular synthetic jet with aspect ratio 150 in quiescent conditions. The spanwise structures, observed by smoke visualization, were found the have a spacing of approximately 2.5 slot widths. If similar behaviour occurred in the presence of a pressure gradient, this may be the cause of the apparent spanwise structures observed in this study. 4 Conclusions Surface flow visualization techniques were used to investigate the interation of a finite span and nominally two-dimensional synthetic jet with the stalled flow on a NACA 0025 airfoil at Rec = and α = 12. The results showed that for a range of excitation frequencies from F + = to 47 (with both steady and unsteady reattachment), the reattached flow demonstrated three-dimensionality found through a contraction of its spanwise extent from just downstream of the actuator towards the trailing edge. Tuft and oil visualization suggested that the size and shape of this attached region did not vary with F + for CB = 1. Oil visualization revealed that as CB was increased from 1 to 2.5, the contraction of the attached flow decreased and the flow became attached over a larger spanwise extent. The oil visualization also revealed the possible secondary spanwise structures within the attached flow. When considering the global effect of control on the flow, the results demonstrate that increasing the blowing ratio increases the size of the reattached flow region and would therefore increase the aerodynamic performance. This has implications for measurement techniques that quantify lift/drag improvements at midspan versus those that measure the global forces. Acknowledgements The authors graciously acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada.

8 Mark Feero et al. - - z/c=0 - - z/c=0 x/c=0.1 x/c=0.1 0.3 0.3 0.7 0.7 0.9 0.9 (a) CB = 1 (b) CB = 2 - - z/c=0 x/c=0.1 0.3 0.7 0.9 (c) CB = 2.5 Fig.

: Aerodynamic Flow Control over an Unconventional Airfoil Using Synthetic Jet Actuators. AIAA Journal 39(3), 361 370 (2001) 2. Boutilier, M.S., Yarusevych, S.

: Influence of znmf jet flow control on the spatio-temporal flow structure over a naca-0015 airfoil. Experiments in fluids 54(3), 1 14 (2013) 4. Feero, M.A., Goodfellow, S.D., Lavoie, P., Sullivan, P.

9 8 Mark Feero et al. - - z/c=0 - - z/c=0 x/c=0.1 x/c= (a) CB = 1 (b) CB = z/c=0 x/c= (c) CB = 2.5 Fig. 5: Oil visualization at F + = 47 with increasing blowing ratio. References 1. Amitay, M., Smith, D.R., Kibens, V., Parekh, D.E., Glezer, A.: Aerodynamic Flow Control over an Unconventional Airfoil Using Synthetic Jet Actuators. AIAA Journal 39(3), (2001) 2. Boutilier, M.S., Yarusevych, S.: Effects of end plates and blockage on low-reynolds-number flows over airfoils. AIAA journal 50(7), (2012) 3. Buchmann, N., Atkinson, C., Soria, J.: Influence of znmf jet flow control on the spatio-temporal flow structure over a naca-0015 airfoil. Experiments in fluids 54(3), 1 14 (2013) 4. Feero, M.A., Goodfellow, S.D., Lavoie, P., Sullivan, P.E.: Flow reattachment using synthetic jet actuation on a low-reynolds-number airfoil. AIAA Journal 53(7), (2015) 5. Glezer, A., Amitay, M., Honohan, A.M.: Aspects of low-and high-frequency actuation for aerodynamic flow control. AIAA Journal 43(7), (2005) 6. Goodfellow, S.D., Yarusevych, S., Sullivan, P.E.: Momentum coefficient as a parameter for aerodynamic flow control with synthetic jets. AIAA Journal 51, (2013) 7. Greenblatt, D.: Dual location separation control on a semispan wing. AIAA journal 45(8), (2007) 8. Greenblatt, D., Wygnanski, I.J.: The control of flow separation by periodic excitation. Progress in Aerospace Sciences 36(7), (2000) 9. Lissaman, P.B.S.: Low-Reynolds-Number Airfoils. Annual Review of Fluid Mechanics 15, (1983) 10. O Meara, M., Mueller, T.: Laminar separation bubble characteristics on an airfoil at low reynolds numbers. AIAA journal 25(8), (1987)

10 Title Suppressed Due to Excessive Length Packard, N.O., Thake Jr, M.P., Bonilla, C.H., Gompertz, K., Bons, J.P.: Active control of flow separation on a laminar airfoil. AIAA journal 51(5), (2013) 12. Sahni, O., Wood, J., Jansen, K.E., Amitay, M.: Three-dimensional interactions between a finite-span synthetic jet and a crossflow. Journal of Fluid Mechanics 671, (2011) 13. Sefcovic, J.A., Smith, D.R.: Proportional aerodynamic control of a swept divergent trailing edge wing using synthetic jets. In: 48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, p. 92 (2010) 14. Smith, B.L., Glezer, A.: The formation and evolution of synthetic jets. Physics of fluids 10, 2281 (1998) 15. Troshin, V., Seifert, A.: Performance recovery of a thick turbulent airfoil using a distributed closed-loop flow control system. Experiments in fluids 54(1), 1 19 (2013)

INFLUENCE OF ACOUSTIC EXCITATION ON AIRFOIL PERFORMANCE AT LOW REYNOLDS NUMBERS

ICAS 2002 CONGRESS INFLUENCE OF ACOUSTIC EXCITATION ON AIRFOIL PERFORMANCE AT LOW REYNOLDS NUMBERS S. Yarusevych*, J.G. Kawall** and P. Sullivan* *Department of Mechanical and Industrial Engineering, University