Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R20: Flow Control: Separation |
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Chair: Kunihiko Taira, Florida State University Room: 2008 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R20.00001: Flow control at low Reynolds numbers using periodic airfoil morphing Gareth Jones, Matthew Santer, George Papadakis, Yann Bouremel, Marco Debiasi The performance of airfoils operating at low Reynolds numbers is known to suffer from flow separation even at low angles of attack as a result of their boundary layers remaining laminar. The lack of mixing --- a characteristic of turbulent boundary layers --- leaves laminar boundary layers with insufficient energy to overcome the adverse pressure gradient that occurs in the pressure recovery region. This study looks at periodic surface morphing as an active flow control technique for airfoils in such a flight regime. It was discovered that at sufficiently high frequencies an oscillating surface is capable of not only reducing the size of the separated region --- and consequently significantly reducing drag whilst simultaneously increasing lift --- but it is also capable of delaying stall and as a result increasing $C_{L_{max}}$. Furthermore, by bonding Macro Fiber Composite actuators (MFCs) to the underside of an airfoil skin and driving them with a sinusoidal frequency, it is shown that this control technique can be practically implemented in a lightweight, energy efficient way. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R20.00002: Effect of Synthetic Jet Actuator Spacing on the Performance Enhancement of a Vertical Tail Model Marianne Monastero, Nicholas Rathay, Edward Whalen, Michael Amitay The use of synthetic-jet-based active flow control to augment the side force produced by vertical tail models through rudder separation control was experimentally investigated in the Rensselaer Polytechnic Institute Subsonic Wind Tunnel. Increasing the side force generated by the vertical tail may lead to a reduction in tail size and, therefore, less drag and fuel consumption. Stereo particle image velocimetry and aerodynamic load data were acquired with a focus on the effect of non-dimensional spacing between jets on the resulting flowfield and forces for a 1/19$^{\mathrm{th}}$ scale model based on a Boeing 767 commercial airplane. For some rudder deflections, differing results with active flow control were found when force data for the 1/19$^{\mathrm{th}}$ scale model were compared to force data obtained on a larger, 1/9$^{\mathrm{th}}$ scale model. Actuator spacing was varied and individual jet momentum coefficient was held constant for these experiments. These results show the need for more fundamental testing to understand why jets are beneficial or detrimental to the augmented side force and how those effects scale-up. A new model was designed to enable a fundamental study of the effect on the flowfield of various jet and model parameters such as sweep angle, jets spacing, rudder chord extent, and rudder deflection. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R20.00003: Effect of superhydrophobic surfaces on the flow over a hydrofoil at low Reynolds number Hyunseok Kim, Nayoung Kim, Hyungmin Park In the present study, we experimentally investigate the effect of superhydrophobic surface on the flow over a hydrofoil at low $Re_{c} < 10^{4}$, where $c$ is the chord length of a hydrofoil. As a hydrofoil, we consider the cross-sections typically used for airfoils like NACA0012, NACA0024, and NACA4412, which stand for thin, thick and cambered hydrofoils, respectively. Spray-coating of hydrophobic nanoparticles are applied onto the hydrofoil surface and subsequent velocity fields are measured in a water tunnel using two-dimensional particle image velocimetry at different angles of attack, $\alpha = 0^{\circ}-20^{\circ}$. At small $\alpha$'s (for example, less than $10^{\circ}$), it is found that the surface slip tends to affect the flow separation slightly and also modify the size of recirculation region in the wake. Since a massive separation occurs at the leading edge at larger $\alpha$'s, however, the effect of superhydrophobic surface becomes diminished. In the talk, the dependence of the hydrodynamic role of surface slip on the hydrofoil shape and $Re_{c}$ will be presented. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R20.00004: Control of the near wake behind a circular cylinder using superhydrophobic surfaces Nayoung Kim, Hyunseok Kim, Hyungmin Park In the present study, the effect of superhydrophobic (SHPo) surface on turbulent wake behind a circular cylinder is studied. Using 2D particle image velocimetry, velocity fields are measured in a water tunnel at $Re_{D}=0.7 - 2.5 \times 10^{4}$. For SHPo surfaces, spray-coating of hydrophobic nanoparticles and roughened Teflon (with a sandpaper) are applied. The griding direction of a Teflon surface is varied as streamwise and spanwise ones, respectively, to see the effect of slip direction as well. It is found that the surface slip increases the turbulence in the flows above the circular cylinder and along the separating shear layers, which result in the delay of flow separation and early vortex roll-up in the wake. As a result, the recirculation bubble in the wake is reduced by up to 50\%, and the wake survey estimates the drag reduction of about 10\%. On the other hand, the spanwise slip is found to be more effective than streamwise one in flow control, supporting the suggested mechanism. Finally, the SHPo surfaces are applied locally by varying its installation angle and SHPo surface applied around the separation point was most effective, indicating that the surface slip directly controls the flow separation. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R20.00005: Flow Control in a Transonic Diffuser Jeremy Gartner, Michael Amitay In some airplanes such as fighter jets and UAV, short inlet ducts replace the more conventional ducts due to their shorter length. However, these ducts are associated with low length-to-diameter ratio and low aspect ratio and, thus, experience massive separation and the presence of secondary flow structures. These flow phenomena are undesirable as they lead to pressure losses and distortion at the Aerodynamic Interface Plane (AIP), where the engine face is located. It causes the engine to perform with a lower efficiency as it would with a straight duct diffuser. Different flow control techniques were studied on the short inlet duct, with the goal to reattach the flow and minimize the distortions at the AIP. Due to the complex interaction between the separation and the secondary flow structures, the necessity to understand the flow mechanisms, and how to control them at a more fundamental level, a new transonic diffuser with an upper ramp and a straight floor was designed and built. The objective of this project is to explore the effectiveness of different flow control techniques in a high subsonic (up to Mach 0.8) diffuser, so that the quasi two-dimensional separation and the formation of secondary flow structure can be isolated using a canonical flow field. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R20.00006: Control of Pitching Airfoil Aerodynamics by Vorticity Flux Modification using Active Bleed John Kearney, Ari Glezer Distributed active bleed driven by pressure differences across a pitching airfoil is used to regulate the vorticity flux over the airfoil's surface and thereby to control aerodynamic loads in wind tunnel experiments. The range of pitch angles is varied beyond the static stall margin of the 2-D VR-7 airfoil at reduced pitching rates up to $k =$ 0.42. Bleed is regulated dynamically using piezoelectric louvers between the model's pressure side near the trailing edge and the suction surface near the leading edge. The time-dependent evolution of vorticity concentrations over the airfoil and in the wake during the pitch cycle is investigated using high-speed PIV and the aerodynamic forces and moments are measured using integrated load cells. The timing of the dynamic stall vorticity flux into the near wake and its effect on the flow field are analyzed in the presence and absence of bleed using proper orthogonal decomposition (POD). It is shown that bleed actuation alters the production, accumulation, and advection of vorticity concentrations near the surface with significant effects on the evolution, and, in particular, the timing of dynamic stall vortices. These changes are manifested by alteration of the lift hysteresis and improvement of pitch stability during the cycle, while maintaining cycle-averaged lift to within 5{\%} of the base flow level with significant implications for improvement of the stability of flexible wings and rotor blades. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R20.00007: Active Flow Control of the Near Wake of an Axisymmetric Body in Prescribed Motion Thomas Lambert, Bojan Vukasinovic, Ari Glezer Controlled interactions between fluidic actuators and the cross flow over the aft end of a wire-mounted axisymmetric moving wind tunnel bluff body model are exploited for modification of its near wake and thereby its global unsteady aerodynamic loads. The model is supported by eight servo-controlled wires, each including a miniature inline force transducer for measurements of the time-resolved tension. The body moves along a prescribed trajectory controllable in six degrees of freedom using closed loop feedback from an external camera system. Actuation is effected by an integrated azimuthally-segmented array of four aft-facing synthetic jet modules around the perimeter of the tail end. In the present investigations, the aerodynamic loads are controlled during time-periodic axial and rotational motions with varying reduced frequencies of up to 0.259. It is shown that this flow control approach modifies the near wake and induces aerodynamic loads that are comparable to the baseline model dynamic loads. Control of the model's unsteady aerodynamic characteristics may be adopted for in flight stabilization. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R20.00008: The effects of momentum and vorticity injection for suppressing separation on a NACA 0012 airfoil Phillip Munday, Kunihiko Taira Flow control actuators are used to modify the behavior of fluid flows by adding forcing input that can consist of mass, momentum, vorticity, and energy. The present computational study focuses on the effects of steady momentum and wall-normal vorticity injection (swirling jets) on separated flow over a NACA 0012 airfoil at Re = 23,000 and angles of attack of 6$^{\circ}$ and 9$^{\circ}$. Large eddy simulations are performed for three-dimensional, spanwise periodic flow, with control input prescribed through velocity boundary conditions near the natural separation point. We observe that the addition of wall-normal momentum mitigates flow separation for a moderate angle of attack of 6$^{\circ}$ with reduction in drag. For massively separated flows at an increased angle of attack of 9$^{\circ}$, the superposition of wall-normal vorticity to wall-normal momentum injection shows significant enhancement in reattaching the flow. It is found that notable lift increase and drag reduction can be achieved in such case. To further understand the roles that vorticity addition plays in separation control, the vorticity flux on the surface of the airfoil is examined in detail. The current findings are compared to the results from the ongoing effort in linear global stability analysis. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R20.00009: Use of thermoacoustic excitation for control of turbulent flow over a wall-mounted hump Chi-An Yeh, Phillip Munday, Kunihiko Taira We numerically examine the effectiveness of high-frequency acoustic excitation for drag reduction control of turbulent flow over a wall-mounted hump at a free stream Reynolds number of 500,000 and Mach number of 0.25. Actuation frequencies around Helmholtz number of 3 are considered based on the characteristics of recently developed graphene/carbon nanotube-based surface compliant loud speakers. The present study utilizes LES (CharLES) with an oscillatory heat flux boundary condition to produce high-intensity acoustic waves, which interact with the turbulent flow structures by introducing small-scale perturbations to the shear layer in the wake of the hump. With thermoacoustic control, the recirculation zone downstream of the hump becomes elongated with thinner shear layer profile compared to the uncontrolled case. This change in the flow shifts the low-pressure region of the wake further downstream and results in reduction in drag by 10\% for two-dimensional and 15\% for three-dimensional flows. The influence of actuation frequency and amplitude is also examined. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R20.00010: Control of Flow Structure on Low Swept Delta Wing with Steady Leading Edge Blowing Ilhan Ozturk, Mohammadreza Zharfa, Mehmet Metin Yavuz Interest in unmanned combat air vehicles (UCAVs) and micro air vehicles (MAVs) has stimulated investigation of the flow structure, as well as its control, on delta wings having low and moderate values of sweep angle. In the present study, the flow structure is characterized on a delta wing of low sweep 35-degree angle, which is subjected to steady leading edge blowing. The techniques of laser illuminated smoke visualization, laser Doppler anemometry (LDA), and surface pressure measurements are employed to investigate the steady and unsteady nature of the flow structure on delta wing, in relation to the dimensionless magnitude of the blowing coefficient. Using statistics and spectral analysis, unsteadiness of the flow structure is studied in detail. Different injection locations are utilized to apply different blowing patterns in order to identify the most efficient control, which provides the upmost change in the flow structure with the minimum energy input. The study aims to find the optimum flow control strategy to delay or to prevent the stall and possibly to reduce the buffeting on the wing surface. Since the blowing set-up is computer controlled, the unsteady blowing patterns compared to the present steady blowing patterns will be studied next. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R20.00011: Characterization of counter-rotating vortices past trapezoidal tabs: simulations and visualization via 3D digitized reconstruction Jeongmoon Park, Axy Pagan-Vazquez, Jorge Alvarado, Leonardo P. Chamorro, Scott Lux, Charles Marsh Characterization of the turbulence generated by passive vortex generators has been a matter of intense research due to their relevance in applications ranging from aerodynamic efficiency to turbulence mixing. The advection diffusion patterns of the induced vortical structures are heavily controlled by the topology of the vortex generators. In this study, self-sustaining counter-rotating vortex pairs (CVP) generated from a series of trapezoidal tabs have been characterized numerically and experimentally to understand the role of the tab geometries on the flow turbulence. The trapezoidal tabs were fabricated using a 3D printer and defined in terms of inclination and taper angles. Reynolds-Averaged Navier--Stokes (RANS) and Large Eddy Simulation (LES) were performed to quantify turbulence statistics and vorticity in the wake of the tabs. Flow fields were experimentally visualized via smoke technique and qualitatively compared with the numerical simulations. 3D vortices were digitally reconstructed by interpolating several 2D images taken at various spanwise planes. The role of the tabs geometry on the stability and features of the vortical structures is discussed for a Reynolds number of 2100 based on the channel depth. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R20.00012: Observability of the turbulent wake behind an axisymmetric bluff body Rowan Brackston, Andrew Wynn, Jonathan Morrison Closed loop control of bluff body wakes has been a topic of active research for some time, with one key objective being the reduction of the aerodynamic drag of road vehicles. The implementation of closed loop control requires continuous measurements of system properties from which the `state' of the system can be deduced. In the context of bluff body flows surface pressure is one of the easiest variables to measure, however it remains an open question as to how much of the structure of the flow can be deduced from surface pressure information. In this investigation the wake behind an axisymmetric bluff body (Re $ = 2\times 10^{5}$) was observed using time resolved PIV and simultaneous pressure measurements on the base of the body. Analysis and decomposition of the velocity field allows an improved understanding of the three dimensional features in the wake that can be used for modeling and control purposes. Furthermore, by examining the simultaneous velocity and pressure data it is possible to establish which of these features can be deduced based upon surface pressure data alone. This understanding will enable the future design of closed loop flow control systems that use surface pressure measurements as the controller input signal. [Preview Abstract] |
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