Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session Z24: Separated Flows: General |
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Chair: Filipe Pereira, Los Alamos National Laboratory; Mark Tachie Room: 232 |
Tuesday, November 22, 2022 12:50PM - 1:03PM |
Z24.00001: Study of turbulent separations induced by a forward-facing step using particle tracking velocimetry Xingjun Fang, Sedem Kumahor, Martin Agelin-Chaab, Douglas Neal, Mark F Tachie Turbulent separations induced by a forward-facing step (FFS) submerged in a thick turbulent boundary layer (TBL) were experimentally studied using a time-resolved particle tracking velocimetry. The Reynolds number based on the free-stream velocity and step height was 13200. The oncoming TBL was developed over a cube-roughened wall, and its thickness was 6.5 times the step height. Consequently, the FFS was exposed to excessively strong mean shear, turbulence intensity and large-scale motion (LSM) in the oncoming TBL. The complete set of Reynolds stresses and triple velocity correlations are, for the first time, reported for turbulent flows over FFS. The Reynolds stresses in the streamwise-vertical plane exhibit dual local peaks: one upstream of FFS and the other over FFS, whereas the spanwise Reynolds normal stress peak immediately upstream of the leading edge. The local peaks of Reynolds stresses upstream of FFS signify the interaction of oncoming LSM with FFS. On the frontal surface of FFS, a pair of counter-rotating vortices are dominant. Particles approaching the step are deflected towards either positive or negative spanwise directions before passing over the step in helical trajectories. This mechanism induces strong ejection events immediately upstream of the leading edge. |
Tuesday, November 22, 2022 1:03PM - 1:16PM |
Z24.00002: Coherent Vortical Structures in the Separated Flow over a 3-D Hump Thomas C Corke, Patrick Gray, Igal Gluzman, Flint O Thomas, Kevin Mejia The separated flow over a three-dimensional hump model in an approaching fully turbulent boundary layer is investigated as part of a program to generate a data set used in validating CFD simulations of 3-D separated flows. The flow regimes include a range of Mach numbers from 0.05 to 0.2 that correspond to Reynolds numbers based on the hump height of 8.3e4 to 3.3e5. Single-camera and dual-camera-stereo PIV sampling several y-z planes downstream of the hump apex are used to quantify the characteristics of the flow separation and reattachment. Over the range of Mach numbers, the flow past the hump apex transitioned from being fully attached to fully separated. When fully separated, the PIV measurements revealed the presence of lifted large-scale, streamwise-oriented symmetric vortical structures whose presence was initially identified with surface flow visualization. These vortical structures are believed to be a universal feature of all 3-D separated flows. Their spatial evolution is characterized for the range of freestream Mach numbers. Of special interest is the role that these coherent vortical motions might have on flow reattachment. |
Tuesday, November 22, 2022 1:16PM - 1:29PM |
Z24.00003: Sensor-based Temporal Super-Resolution for Turbulent Separated Flows Kevin H Manohar, Owen Williams, Robert J Martinuzzi, Christopher R Morton The high Reynolds number turbulent separated flow over a Gaussian speed-bump benchmark geometry has presented challenges for predicting flow separation. Moreover, the lack of time-resolved (TR) experimental data on the Bump has limited progress on understanding the link between the unsteady dynamics and energy transfer mechanisms, which would advance turbulence models. The above challenges motivate the present work: to provide TR estimates of the velocity field from undersampled particle image velocimetry (PIV) data. We propose a data-driven estimation technique that uses oversampled surface-mounted pressure sensors and long short-term memory (LSTM) neural networks to predict transient dynamics that are inherently aliased from the undersampled PIV time-series. The method leverages the TR pressure dynamics to estimate a low-dimensional representation of the velocity field. Spectral analysis of the flowfields after up-sampling the 15 Hz PIV data to 3000 Hz reveals (i) a low-frequency breathing mode that is associated with the contraction and expansion of the separation bubble, and (ii) a medium-frequency mode describing the flapping of the shear layer. The modal dynamics are then used to explore physical mechanisms that govern the turbulence transport within the flow. |
Tuesday, November 22, 2022 1:29PM - 1:42PM |
Z24.00004: Measurement and Analysis of a Ship Superstructure Wake Under Static and Dynamic Conditions Cameron J Eggart, Fernando Zigunov, Rhylan A Huss, Farrukh S Alvi This experimental study focuses on the flow field above the landing deck and in the nearfield wake of a simplified canonical model of a surface ship while in static and dynamic conditions. The landing pad region is of interest because launch and recovery helicopters use the region for take-off and landing. Due to the bluff nature of a typical naval ship superstructure, the flow field around the ship produces a wake that poses a challenge for landing air vehicles (helicopters and VTOL aircraft) under certain conditions. A characterization and analysis of the flow field through particle image velocimetry, along with other diagnostics, will be presented for static and dynamic cases. For the latter, the motion of a ship is simulated by a 6-axis Stewart platform. A potential interaction between the ship motion and the wake dynamics at certain wave motion frequencies is expected, which may provide predictive capabilities of wake behavior based on surface measurements. |
Tuesday, November 22, 2022 1:42PM - 1:55PM |
Z24.00005: Control of Turbulent Boundary Layer Separation by a 3D Printed Shark Skin Model with Passive Bristling Andrew Bonacci, Amy W Lang, Kaila Wong, Leonardo M Santos Turbulent boundary layer separation can be problematic in many engineering applications. However, nature may have a solution in the form of flexible shark scales found on the shortfin mako which have been proven to passively bristle under reversing flow conditions and control flow separation in past experiments. An investigation of how these shark scales interact with reversing flow in the near-wall regions of the boundary layer is of interest to better understand the fluid-shark scale interactions. Enlarging the geometry and constructing 3D printed models of shark skin is the best route forward to developing a bio-inspired surface for real applications. Using a rotating cylinder above a flat plate in a water tunnel setup, an adverse pressure gradient was induced creating a separated region over a tripped turbulent boundary layer with approximate Reynolds numbers up to 8 x 105. 3D printed shark scales were mounted into a plate using wire to replicate low-resistance passive bristling angles of 50 degrees. Rigid scale models were also constructed to observe how the motion of the scales during separation is important to passive control. The model scales were constructed with crown lengths of 3 mm, fifteen times greater than those observed on a real shark. This low-speed flow study makes the boundary layer dynamics and shark scale motions more measurable while allowing for actuation heights of the scales to be within the bottom 10% of the boundary layer. Baseline studies document flow separation and reversing flow development in the presence of an adverse pressure gradient over a smooth plate. The same experiments are then repeated with the flexible and rigid shark skin models to document control of flow separation and observe how reversing flow induces scale bristling. |
Tuesday, November 22, 2022 1:55PM - 2:08PM |
Z24.00006: Bio-inspired hybrid flow control and gust mitigation using reinforcement learning Nirmal Nair, Andres Goza Passive flow control using a covert-feather inspired passively deployable flap attached on an airfoil provides significant lift improvements at post-stall angles of attack involving massive flow separation and vortex shedding. In this talk, we will describe a hybrid flow control strategy to achieve even greater aerodynamic benefit, where the stiffness of the hinge is actively varied in time to yield more favorable fluid-structure interaction (FSI) than for a constant stiffness. The control of such a strongly-coupled FSI system is challenging due to the nonlinear coupling between the passively oscillating flap and the unsteady flow. Therefore, we design a closed-loop feedback controller for temporally varying the stiffness using reinforcement learning (RL). In RL, the optimal control strategy is learned by trial-and-error where the actuator performs a variety of control actions, analyzes the resulting performance of the system and updates the controller strategy to maximize performance. The controller is constructed to (a.) maximize aerodynamic lift under steady free-stream conditions and (b.) minimize fluctuations in lift due to vortex gusts. The appropriate cost functions and learning algorithms for achieving the lift maximization and gust mitigation goals will be presented. |
Tuesday, November 22, 2022 2:08PM - 2:21PM |
Z24.00007: Transverse gust mitigation via closed-loop control Girguis Sedky, Antonios Gementzopoulos, Francis Lagor, Anya R Jones Unsteady flow conditions present significant challenges to stable flight, and gust rejection remains a concern for stability and control in many modern flight environments. Examples of gust-dominated environments include stormy conditions, aircraft takeoffs and landings in strong crosswinds or ship air wakes, and micro air vehicle flight in urban settings. In this work, we present lift mitigation experiments in large-amplitude transverse wing-gust encounters using simple proportional closed-loop control based on pitch actuation. The chosen control law was found to mitigate the lift transient peaks by 75 % for gusts inducing up to 35° effective angles of attack without a priori knowledge of the gust onset time or strength. The physics behind the success of simple control in a highly unsteady nonlinear environment is discussed. In addition, we present time-resolved lift, pitching moment, and flowfield measurements to bring about a deeper understanding of the unsteady flow physics for wings maneuvering in strong transverse gusts. Highlights of these findings include the effect of the wing’s maneuver on the gust’s structure, the evolution of the leading- and trailing-edge vortices, and the integral contribution of added mass in gust mitigation. |
Tuesday, November 22, 2022 2:21PM - 2:34PM |
Z24.00008: Optimal blade pitch control for a vertical-axis wind turbine blade undergoing dynamic stall Sébastien Le Fouest, Karen Mulleners Vertical-axis wind turbines feature many advantages to complement traditional wind turbines in power production including omni-directionality, low noise production, and scalability. The inherent aerodynamic complexity of vertical-axis wind turbines has challenged their development for large-scale power production. The blades of these turbines undergo periodic variations in effective angle of attack and incident flow velocity, leading to the occurrence of dynamic stall. This phenomenon causes large cycle-to-cycle load fluctuations, jeopardising the turbine’s structural integrity. We investigate the potential of individual blade pitching to control the occurrence of dynamic stall on a single-bladed wind turbine model. We optimise the blade’s pitching profile using a genetic algorithm with two objectives: 1) maximising the net power production and 2) minimising load fluctuations related to flow separation. We performed over 1500 experiments visiting a large envelope of pitching kinematics. We obtain time-resolved aerodynamic force measurements using a customised load cell built into the blade’s shaft and compute the wind turbine performance for each individual. The strongest individuals achieve a 300% increase in net power production compared to a non-actuated wind turbine blade. |
Tuesday, November 22, 2022 2:34PM - 2:47PM |
Z24.00009: Control of shock-induced flow separation using elliptical air-jet vortex generators Deepak Prem Ramaswamy, Anne-Marie Schreyer Flow separation, induced by strong shock waves interacting with a boundary layer, is harmful to many aerospace applications. An established technique to control the flow is by using air-jet vortex generators (AJVGs). Here, streamwise vortices are generated by injecting steady air-jets into the crossflow, which redistribute the boundary-layer momentum, thereby improving their resistance to separation. The effectiveness of AJVGs depends on a number of parameters. An encouraging parameter is the jet-orifice shape, which affects the topology and dynamics of the jet-induced vortices and consequently the control effectiveness. In this study, we investigate the potential of AJVGs with elliptical jet-orifices in controlling a 24o ramp-induced interaction at Me= 2.52 and Reθ = 8225. |
Tuesday, November 22, 2022 2:47PM - 3:00PM |
Z24.00010: Low Frequency Dynamics of a Pressure-Gradient Induced Turbulent Separation Bubble Ross Richardson, YANG ZHANG, Wen Wu, Louis Cattafesta A pressure-gradient induced (PGI) turbulent separation bubble (TSB) is experimentally investigated on an elliptic leading edge flat-plate model at a Reynolds number based on the momentum thickness of the incoming turbulent boundary layer of approximately ReΘ0 = 640. Separation of the incoming turbulent boundary layer is caused by a suction-only boundary condition from a small cutout on the top wall of the test section imposed by an axial fan. Surface oil flow visualization is used to capture the evolution and dimensions of the PGI-TSB, highlighting the relevant features of the surface flow field. Unsteady pressure measurements in the experiment show significant energy at Strouhal numbers (StLsep= fLsep/U0) of approximately 0.47 and 0.007 which is close to what is documented in the literature to be associated to the high frequency shear-layer shedding and low frequency ‘breathing’ mode of a PGI-TSB, respectively. Synchronous low-speed PIV (non-time resolved) combined with time-resolved unsteady surface pressure measurements near the mid-span of the flat plate model are acquired to utilize the spectral analysis modal method (DOI: 10.1007/s00348-020-03057-8) to determine the dynamically coherent modes associated to the two characteristic frequencies of the PGI-TSB. |
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