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 Q24: Separated Flows: Modeling, Theory, and Simulation |
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Chair: Karen Mulleners, EPFL Room: 232 |
Monday, November 21, 2022 1:25PM - 1:38PM |
Q24.00001: Experimental and LES analysis of the flow around a 5:1 rectangular cylinder Alessandro Mariotti, Gianmarco Lunghi, Mario Morello, Maria Vittoria Salvetti The high Reynolds number flow around a 5:1 rectangular cylinder is experimentally and numerically analyzed. The flow is characterized by shear layer separation from the upstream edges. Vortical structures of different sizes form from the roll-up of these shear layers, move downstream and interact with the classical vortex shedding in the wake. The corresponding mean flow is characterized by a recirculation region along the lateral surface of the cylinder, ending close to the trailing edge. The mean flow features on the cylinder side have been shown to be highly sensitive to the shear layer dynamics, which is influenced by set up parameters both in numerical simulations and in experiments. The results of 5 experimental set-ups and 35 Large Eddy Simulations are analyzed herein to highlight the impact of the shear layer dynamics on the lateral mean recirculation characteristics and, in turn, on the near wake flow features and on some bulk quantities of practical interest. Experiments and numerical simulations are carried out at zero angle of attack and Reynolds number Re=DU/ν=40000, being D the crossflow dimension, U the freestream velocity and ν the kinematic viscosity of air. Almost-sharp upstream corners (r/D=0.0005) and different values of the corner roundings are considered. LES simulations are carried out with sharp edges and different values of the rounding of the upstream edges matching the experimental ones and for different numerical and modeling settings. The shear-layer dynamics and the dimensions of the mean recirculation zone vary considerably in these experiments and simulations, allowing us to single out meaningful trends. The growth of the velocity fluctuations along the shear layers detaching from the upstream corners is highly correlated with the location of the onset of Kelvin-Helmholtz instability and, in turn, with the length of the mean recirculation on the cylinder side. This, in turn, influences the near wake features. |
Monday, November 21, 2022 1:38PM - 1:51PM |
Q24.00002: Rating effective pitch rates for dynamic stall prediction Sahar Rezapour, Karen Mulleners The delay in the onset of dynamic stall varies with the unsteadiness of the pitching kinematics. The unsteadiness of linear ramp-up pitching motions is characterised by the non-dimensional pitch rate. With increasing pitch rate, dynamic stall onset occurs at higher angles of attack and the temporal stall delay decreases. For sinusoidal motions, an effective unsteadiness parameter is introduced as the rate of change of angle of attack when the static stall angle is exceeded. For large amplitude pitching motions around a mean angle of attack close to the static stall angle, the effective unsteadiness predicts the dynamic stall delay well as non-linear variations in the angle of attack remain small. Here, we explore the effect of highly non-linear pitching kinematics on the onset and delay of dynamic stall using time-resolved force and flow-field measurements. We prescribe non-linear pitching profiles with a constant mean pitch rate but different values of the pitch rate and the pitch acceleration around the static stall angle. The non-linearity of the kinematics significantly influences the dynamic stall development and delay. New candidate effective unsteadiness parameters are proposed to characterise the effect of pitch angle accelerations on the dynamic stall onset and delay. |
Monday, November 21, 2022 1:51PM - 2:04PM |
Q24.00003: Deformations of a Highly-Flexible Circular Shell in Uniform Crossflow Daniela Caraeni, Yahya Modarres-Sadeghi We study the response of a flexible circular shell placed in uniform crossflow using computational fluid dynamics at Re < 500, calculated based on the incoming flow velocity and the initial diameter of the cylinder. The surface compliance effects of the shell are investigated as a fully coupled fluid-structure interactions problem. The circular shell is deformed due to its interactions with the incoming flow and undergoes some oscillations. The wake is observed to deviate from that of a rigid cylinder. The influence of the structure’s support on its response and its wake is studied as well. |
Monday, November 21, 2022 2:04PM - 2:17PM Author not Attending |
Q24.00004: Investigation of Separated and Reattached Flow on a Blunt Flat Plate Arun K Saha The separated–reattached flow plays an important role in fluid transport and mixing; hence it is of practical relevance to the engineering community. Turbine blades, aircraft wings, structures, bridge piers, and heat sinks linked to electronic components are some of the potential places of application. In the present work, Direct Numerical Simulation (DNS) study has been carried out for flow past a long blunt plate placed along the flow direction. The finite difference representations are second-order accurate in time and space. The mesh is uniform in the cross-stream direction, while in both streamwise and wall-normal directions, a non-uniform mesh is used. The overall algorithm is time-explicit. Simulations are reported a Reynolds number (based on free-stream velocity and plate thickness) range of 400-1200. Results show that the flow experiences a series of temporal and spatial transitions with increasing Reynolds numbers. The flow field reaches a well-defined steady state at a low Reynolds number. However, the flow undergoes a Hopf bifurcation at around Re=415. It is also found that beyond a threshold value of the Reynolds number, the two-dimensional shear layers become three-dimensional, giving rise to hairpin structures. At Reynolds number close to the transitional value (Re=425), the flow reveals switching between two-dimensional and three-dimensional disturbances. |
Monday, November 21, 2022 2:17PM - 2:30PM |
Q24.00005: Large-Eddy Simulation of Vortex-Induced Transition in a Separation Bubble Sonalika Srivastava, Subrata Sarkar The transitional behaviour of laminar separation bubbles (LSBs) can be modulated significantly by periodically passing convective vortices. Such low Reynolds number aerodynamic features are commonly encountered in cascade flows in turbomachinery, flows over aerofoils, flow past surface mounted obstacles, dynamic stall, blade vortex interaction and impulsive motion of bluff bodies. In the present study, an LSB has been created on a flat plate by imposing a suction-blowing velocity profile in the free-stream resembling the pressure distribution on the suction side of a NACA 643-618 aerofoil. The Reynolds number is kept constant at Re=U∞L∞/ν=3200. Realistic free-stream turbulence, with a turbulence intensity Tu∞=3.3%, has been imposed in the inlet to mimic grid turbulence. A high-fidelity LES has been performed using a dynamic subgrid-scale model to study the receptivity of the LSB to periodically passing vortices, instabilities induced by these vortices, transition mechanism and the associated flow structures as compared to an LSB without passing vortices. The passing frequency of the vortices (fv) is varied as a multiple of the natural frequency of vortex shedding (fn) from the LSB (without the presence of passing vortices) in the range (fv/ fn =1-10). The passing vortices with the same sign of vorticity as the separated shear layer lead to an earlier breakdown of the shear layer leading to a decrease in mean bubble length with decreasing frequency ratio (fv/ fn). The vortex breaks the bubble into multiple instantaneous separated regions, and the flow field relaxes for a significantly long time after a single vortex is passed. The time-averaged statistics show a disappearance of the mean bubble for fv/ fn ≤ 3. The phase-averaged statistics, calculated at the vortex passing frequency, depict a phase-dependent bubble structure dependent on the frequency ratio. |
Monday, November 21, 2022 2:30PM - 2:43PM |
Q24.00006: Law of incipient separation for turbulent flows over airfoils as inferred by RANS Shao-Chi Huang, Abhiram B Aithal, Antonino Ferrante Lu, Aithal & Ferrante (AIAA J., 2021) discovered a law that predicts the incipience of flow separation over curved ramps by knowing only a few geometrical parameters of the ramp and the Reynolds number of the flow. In that spirit, we have discovered a similar law for airfoils by performing Reynolds averaged Navier-Stokes (RANS) simulations of incompressible turbulent flows over thirty-two NACA airfoils. First, we have carried out verification and validation of RANS against the experimental measurements by A. J. Wadcock (NASA-CR 177450, 1987) which show the accuracy of the RANS prediction at small angles of attack when flow separation begins to occur on the upper side of the airfoil. Then, we have investigated the effects of the angle of attack, airfoil thickness, and camber on the incipience of flow separation for Reynolds number based on airfoil chord Rec∈[1.64×106, 6×106]. From the analysis of the RANS results, we have determined a law for predicting the incipience of turbulent flow separation over airfoils that relies only on airfoil's newly defined characteristic slope, thickness, camber and Rec (Huang, Aithal & Ferrante, Phys. Fluids, 2022). |
Monday, November 21, 2022 2:43PM - 2:56PM |
Q24.00007: On the utility of nonlinear data-driven models for separation control A. Leonid Heide, Bjoern F Klose, Gustaaf B Jacobs, Maziar S Hemati Flow separation can adversely impact aircraft performance by reducing lift and increasing drag. Active flow control has been proposed as a means of mitigating this performance degradation by temporarily reattaching the boundary layer. However, the placement of flow control actuators can have a significant influence on achievable performance. Prior work on optimal actuator selection for separation control has utilized linear data-driven models of the fluid dynamics, which disregards nonlinear flow interactions that could potentially be harnessed for improved control. In this work we investigate the utility of using nonlinear models to facilitate the optimal actuator selection task. We leverage a physically constraine data-driven approach called the Sparse Identification of Nonlinear Dynamics (SINDy). The SINDy model is constrained to preserve the energy-conserving property of the quadratic nonlinearity in the incompressible Navier-Stokes equations. The SINDy framework is also modified to provide output equations that predict flow-dependant parameters, such as the separation angle or lift. The model is obtained from direct numerical simulation data of two-dimensional and three-dimensional separated flows. Opportunities for optimal control analysis are also discussed. |
Monday, November 21, 2022 2:56PM - 3:09PM Author not Attending |
Q24.00008: Reduced-order modeling of separated flow on an airfoil via the Principle of Minimum Pressure Gradient. Cody Gonzalez, Haithem E Taha Analytical determination of circulation over airfoils has classically required ad hoc closure to potential-flow solutions via the Kutta-Zhukovsky condition. This empirically-derived closure, being kinematic in nature, is not strictly applicable to transient flows with appreciable boundary-layer thickness, or flow separation. Recently, "A Variational Theory of Lift," proposed minimality of curvature in the potential flow field, i.e. the outer solution, and subsequently "What Does Nature Minimize In Every Incompressible Flow?" supplements this foundation beyond potential-flows by a generalized Principle of Minimum Pressure Gradient, (PMPG). We continue the study into determining circulation in the "outer solution," by applying PMPG to a potential-flow outer solution coupled with an integral boundary-layer (IBL) method. The flow solutions determined using PMPG are compared to classical IBL solutions, and experimental data. |
Monday, November 21, 2022 3:09PM - 3:22PM |
Q24.00009: Predicting Lift in Unsteady Separated Flows using Classical Aerodynamics Antonios Gementzopoulos, Girguis Sedky, Anya R Jones Large force transients produced during transverse wing-gust encounters can lead to permanent structural damage or catastrophic loss of control. Predicting these force transients is an important task that can aid in the development of effective and robust flight controllers. Despite its simplicity, Kussner's unsteady aerodynamics model has shown tremendous agreement with data for a wide range of gust ratios and gust profiles. This is a surprising result as large-amplitude gust encounters are known to possess substantial amounts of flow separation and the model was developed for wings under attached flow conditions. In this work we elucidate the mechanisms by which this potential flow model predicts the lift transients of separated flows. It is found that the bound circulation in Kussner's model approximates the circulation of the leading-edge vortex (LEV) - bound vortex pair in large-amplitude transverse gust experiments. Furthermore, it is found that for short-duration gusts, the LEV remains attached to the wing, thus permitting the model to capture the effective impulse of the flow. These findings make it possible to extend application of Kussner's model to large-amplitude, short-duration gusts, as well as offer perspective on its limitations and possible extensions. |
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