Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P26: Flow Control: Separation |
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Chair: Avraham Seifert, Tel Aviv University Room: 608 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P26.00001: The Unsteady Suction Actuator Nimrod Shay, Bar Mizrahi1, Ofek Drori, Ariel Yaniv, Avraham Seifert Steady suction is known to be significantly more effective than steady blowing for boundary layer separation control. Pulsed blowing is known to be significantly more effective than steady blowing, owing to the instability and favorable timescales imposed by unsteady 3D vorticity components created by a suitable device. Recently it was shown that unsteady suction can be more effective even when compared to steady suction. This paper describes the development and characterization of a new fluidic device, capable of creating unsteady suction with no moving parts. The device uses at least one fluidic oscillator and at least one ejector to create unsteady suction. Detailed numerical and experimental rapid prototyping techniques were used and the results of the device characterization in still fluid are described. It is shown that the new device can create the required output signals for external flow separation control using relatively low input mass flux and low power. Further to its development, it was already tested interacting with a turbulent boundary layer and compared directly to other flow control devices in keeping an airfoil boundary layer attached. [1] Seifert, A. and Pack, L.G. ``Active flow separation control on wall-mounted hump at high Reynolds numbers,'' AIAA J., vol. 40, pp. 1363--1372, 2012. [2] Morgulis, N. and Seifert, A., ``Fluidic flow control for improved performance of small Darrieus type wind turbines,'' Wind Energy, Volume 19,~Issue 9, September 2016, Pages~1585-1602. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P26.00002: On the Interaction of Unsteady Suction and Turbulent Flat Plate Boundary Layer Bar Mizrahi, Avraham Seifert This paper describes the interaction of unsteady suction created by the new oscillatory suction (OSUB) actuator with a turbulent, flat plate boundary layer. Unsteady suction is imposed on the boundary layer from four discrete holes in the plate in the transitional region of the boundary layer. The boundary layer is first characterized without excitation then two magnitudes and two frequencies for each magnitude are imposed. The resulting flow is very effectively modified by the imposed suction. Hot-wire scans of planes parallel to the wall at two different heights and full boundary layer velocity profiles along selected rays from the origin of the disturbances reveal a fascinating 3D complex yet quite coherent patterns. Phase-locked analysis decomposes the controlled flow field and reveals the unsteady flow created by the unsteady suction. The disturbances are compared to those created by discrete steady suction through the same array of four spanwise holes and reveal the parameter range in which unsteady suction has significant benefits as compared to steady suction. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P26.00003: A Comparison Between Oscillatory and Steady Suction for Airfoil Separation Control Avraham Seifert, Bar Mizrahi1 This paper presents a direct comparison between two classes of flow control actuators. The first actuator uses steady suction while the other actuator uses unsteady suction. Both use also pulsed blowing (PB) downstream of the suction locations. Both actuators were installed on a custom-designed and 3D printed airfoil, specifically designed to have two distinct separation locations. At the upstream location, four suction holes each are placed symmetrically to the chord and are connected either to steady or unsteady suction devices. Further downstream, where the boundary layer tends to separate again, a pair of PB slots are placed, connected each pair to each device, again symmetrically to the chord line. The airfoil is mounted on a load cell and aerodynamic forces are measured at low speeds. The actuators were previously calibrated on a benchtop setup. Tests are conducted first in the absence of actuation, at low Reynolds numbers (up to 200K), then the two actuation concepts are directly compared. The range of parameters in which unsteady suction has benefits over steady suction for separation control are identified. Actuation energy expenditure is shown to be significantly lower when using unsteady suction. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P26.00004: Preventing boundary layer separation by non-uniform suction control James Ramsay, Mathieu Sellier, Wei Hua Ho Suction of the boundary layer has been studied as a method of flow control for over a hundred years. However, its use has failed to migrate to mainstream engineering applications. There are two main reasons for this: one, the relative complexity and broad parameter space for the control makes it difficult and costly to design; two, the energy required to generate the suction can often outweigh the savings from reduced drag or improved performance. To address these issues, an investigation of suction control on laminar flow around the circular cylinder and within an axisymmetric diverging pipe has been performed. Numerical simulations were performed with optimisation to determine the most effective and efficient applications of suction to achieve a variety of objectives. The models without suction were validated against experimental data from the literature. With the aim of making the design of suction control more accessible in real applications, the focus of the results was on the relationships between the uncontrolled flow and the optimised control parameters. Particular attention was paid to the objective of controlling the separation of the boundary layer and its potential use as an objective, or as an important feature, in determining the best control. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P26.00005: Control of Shock-Induced Separation and Vorticity Concentrations in a Serpentine Diffuser Travis Burrows, Bojan Vukasinovic, Ari Glezer Advanced propulsion inlet systems utilize complex serpentine diffusers, whose geometry engenders large-scale streamwise vortices and boundary-layer separation coupled to shock formation at high flow rates that limit engine efficiency operating range due to severe losses and distortion. The present experimental investigation demonstrates active control of the diffuser transonic shock by utilization of surface-mounted fluidic oscillating jets, thereby indirectly control streamwise vorticity concentrations by exploiting the coupling to the shock. It is demonstrated that flow control modifies the shock topology and footprint, confining it towards the diffuser's sidewalls. Consequently, the streamwise vortices are displaced, ultimately mitigating their advection of low-momentum fluid into core diffuser flow. This active flow control leads to a 35{\%} reduction in average diffuser circumferential distortion and a concomitant increase in pressure recovery, indicating the capability of this flow control approach to extend the engine operation range beyond its nominal operating condition. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P26.00006: Flow Physics and Scaling for Discrete Jet Forcing on a Wall-Mounted Hump Christopher Otto, Benjamin Champion, Jesse Little, Rene Woszidlo An experimental study is conducted to explore flow physics and scaling parameters (e.g., aspect ratio, exit area, spacing) for various types of fluidic oscillators in support of the development of active flow control technology. Various actuation modules are designed, built and tested on an existing model of the NASA hump geometry. Experiments are carried out at a Reynolds numbers of 1.0 x 10$^{\mathrm{6}}$ (Ma $=$ 0.09). Time-averaged pressure measurements are conducted along both the chord and span of the model. Stereoscopic PIV is performed downstream of the actuation location to investigate the underlying control mechanisms in detail. Flow control using various spatially distributed fluidic oscillators was applied for spacings of $\Delta $z/c $=$ 4.55{\%} {\&} 9.09{\%}. Performance curves were compared based on a momentum, mass flow, and energy coefficient, and revealed regions of different efficiency. A higher slope of the performance curve was found for lower momentum inputs (separation control), whereas a smaller slope was found for higher inputs (circulation control). Counter-rotating vortex pairs in the time-averaged field are the main driver to enhance mixing and thereby reduce separation especially at low momentum input. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P26.00007: Fluidic Control of Round Inlet Flow in Cross Wind D.A. Nichols, B. Vukasinovic, A. Glezer, M. Defore, B. Rafferty The suction flow within a round inlet in the presence of cross wind is investigated experimentally with specific emphasis on characterization and control of separation over the surface of windward lip using arrays of surface static pressure ports and radial total pressure rakes, surface oil-flow visualization, and particle image velocimetry. Of specific interest are the roles of independently-interchangeable variations between the inlet flow (M < 0.8) and crosswind speed (up 35 knots) on the the onset and evolution of topology of separation. It is shown that for a given inlet flow the presence of sufficiently high cross wind leads to the formation of a three-dimensional separation domain that has an azimuthal, horseshoe-like boundary with its tip near the windward edge. As the cross wind speed increases, separation spreads by the formation of secondary interacting azimuthal separation cells whose topology resembles the main separation domain. Fluidic control of the separation leads to significant reduction in losses that are manifested by reduction in cross stream total pressure deficit and a concomitant increase in the inlet’s mass flow rate. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P26.00008: Flow Topology and Control of a Closed Separation Domain Over a Curved Surface C.J. Peterson, N.K. Koukpaizan, B. Vukasinovic, M.J. Smith, A. Glezer The unsteady interactions between fluidic oscillating jets and the vorticity concentration within a separation bubble formed by a subsonic cross flow over a curved surface is investigated experimentally. The nominally 2-D curved surface is designed to promote separation representative of that seen in rotorcraft applications. A spanwise array of fluidic oscillating jets located upstream of the separation provides dissipative, high-frequency actuation, which in turn controls the characteristic scale of the separation domain. The effect of the actuation and its progression from the upstream to the downstream edges of the separated bubble are assessed using stereoscopic particle image velocimetry. Emphasis is placed on local entrainment and flow features near the actuators and their ensuing downstream progression throughout the separated region. Additionally, the relationship between the local flow features at separation and characteristics of the ensuing separated flow will be assessed with respect to the global flow control effectiveness. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P26.00009: Controlled Aerodynamic Forces on Slender Axisymmetric Bodies at High Incidence E. Lee, Y. Huang, B. Vukasinovic, A. Glezer The flow and aerodynamic loads over slender axisymmetric bodies at high incidence are dominated by interactions of a hierarchy of vortical structures resulting from streamwise- successive, wake instabilities of the cylinder’s forebody, main, and aft segments. These vortical interactions are driven by the onset of a counter-rotating vortex pair that form over the forebody and are extremely receptive to small perturbations and can rapidly evolve asymmetrically resulting in significant side force and yawing moment. The present wind tunnel investigations utilize an axisymmetric model (L/D = 10, incidence up to 65 o , Re D = 810 4 ) to exploit the receptivity of the forebody flow to small perturbations for controlling the evolution of the forebody vortices and thereby the aerodynamic loads (side forces and roll and yaw moments). Upwind actuation is effected using synthetic jet actuators at the juncture of several forebodies of different aspect ratios. It is shown that the flow is extremely receptive to actuation at high incidence angles yielding side force changes of up to C S = 5 (corresponding C L about 6.5). The effected side forces are used to control the model’s lateral stability. [Preview Abstract] |
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