Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session M25: Aerodynamics: Fixed, Flapping and Rotating Wing III |
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Chair: Vibhav Durgesh, University of Idaho Room: North 225 AB |
Monday, November 22, 2021 1:10PM - 1:23PM |
M25.00001: The Aerodynamics of the Transition from Thrust Production to Dynamic Stall on a Pitching Airfoil David Lee, Colin Stutz, John T Hrynuk Many swimmers maneuver using a thrust producing propulsor, which is typically modelled as a pitching airfoil and has been heavily studied. Vortices are generated and produce the classical reverse von Karman vortex street and thus thrust at select Strouhal numbers. Other pitching foils, such as helicopter blades, experience similar pitching oscillations at similar reduced frequencies but instead experience a dynamic stall event. Typically in dynamic stall the vortices are shed from the leading edge. The clear difference between the two problems is the differing mean angle of attack. This presentation will experimentally examine how mean angle changes cause a transition from thrust production to dynamic stall behavior. A NACA 0012 airfoil was oscillated about ¼ chord at reduced frequencies of k =0.25 and a Reynolds number of 12,000. High speed particle image velocimetry (PIV) measurements were taken to analyze the formation location and trajectory of vortex structures formed on the wing. The transition between thrust generating flow structures and dynamic stall will also be discussed. |
Monday, November 22, 2021 1:23PM - 1:36PM |
M25.00002: The Importance of Strain-Dominant Regions in Vortical Flows: Application to Dynamic Stall Karthik Menon, Rajat Mittal The pressure loads induced by vortices on immersed surfaces are central to numerous problems in fluid dynamics. While past studies in unsteady aerodynamics have focused mostly on the role of vortices in force production, we show using a force partitioning method (FPM; Menon & Mittal, JFM 918, R3, 2021) that strain-dominated regions associated with vortices can in fact have a significant effect on aerodynamic loads in some situations. FPM allows us to quantify the loads induced on immersed surfaces by individual vortices as well as their associated regions of strain. By analyzing the forces on a pitching airfoil undergoing dynamic stall, we show that our current understanding of vortex-dominated phenomena could be incomplete without considering the substantial, and sometimes dominant, effect of strain-dominated regions that are associated with vortices. |
Monday, November 22, 2021 1:36PM - 1:49PM |
M25.00003: Experimental Investigations of Dynamic Stall on a Pitching Airfoil at Moderate Reynolds Numbers Katie Wu, Marcus Hultmark The unsteady aerodynamics of stall play a key role in determining the performance of engineered and biological flight systems, wind turbines, etc. An airfoil undergoing a rapid pitching motion in which its angle of attack increases above its static stall angle experiences a phenomenon known as dynamic stall, in which aerodynamic forces and moments exhibit large deviations from their steady values. In the present work, the flow around a pitching NACA0021 airfoil is investigated using Particle Image Velocimetry with a focus on the mechanisms of stall, the time scales of dynamic stall events, and associated flow features. Experimental conditions cover reduced frequencies up to 0.1 for chord-based Reynolds numbers up to 2.5 × 105. This work complements similar studies performed in the High Reynolds number Test Facility. |
Monday, November 22, 2021 1:49PM - 2:02PM |
M25.00004: Modal analysis of dynamic stall on a cross-flow turbine blade Mukul Dave, Jennifer A Franck A cross-flow turbine blade, that rotates on an axis perpendicular to the flow, undergoes dynamic stall due to the cyclical variation of relative flow speed and angle of attack it experiences. In this work we investigate the control of dynamic stall through modal analysis of highly resolved flow fields from large-eddy simulation with the goal of enhancing power generation. An optimized intracycle variation of angular velocity, by modifying the dynamic stall cycle, has been shown to enhance the power conversion efficiency of a two-blade straight-bladed turbine in confined flow by 54%. A generalized intracycle control strategy requires identifying the flow mechanisms causing this change. A proper orthogonal decomposition (POD) is able to identify the dominant flow modes at the cross-flow turbine blade such as the fully attached, fully separated, and vortex formation modes and their temporal activation functions. Comparing these for a constant and intracycle variation of angular velocity, while correlating the mode activation with the temporal variation of relative flow and power, highlights the modes which are prominently modified. Such a reduced-order approach for modeling the performance can lead to an efficient design process with the need for less experiments and computations. |
Monday, November 22, 2021 2:02PM - 2:15PM |
M25.00005: On the time scales governing dynamic stall at high Reynolds numbers Claudia E Brunner, Janik Kiefer, Martin O. L. Hansen, Marcus Hultmark Dynamic stall refers to the time-resolved stall process of an airfoil that is rapidly pitched above its static stall angle. It involves the role-up of the suction-side boundary layer into a dynamic stall vortex, which causes a momentary lift overshoot before convecting downstream. Here, the non-dimensional time scales governing various stages of the dynamic stall process are elucidated. To this end, we experimentally investigate a NACA 0021 airfoil undergoing ramp motions from below to above the stall angle. The Reynolds number for all test cases is 3.0 x 10^6 and the reduced frequency is varied over 0.01 ≤ k ≤ 0.40. This parameter space is achieved using a high-pressure wind tunnel. The transient pressure field provides insight into the time-resolved stall behavior. It reveals that at sufficiently high reduced frequencies, there is a distinct point in time at which the stall process becomes independent of the airfoil kinematics and is instead purely convection-governed. |
Monday, November 22, 2021 2:15PM - 2:28PM |
M25.00006: Evolution of the Leading-Edge Vortex on a Revolving Wing and its Effect on Aerodynamic Loading Rajat Mittal, Jung-Hee Seo, Vrishank Raghav The leading-edge vortex (LEV) plays a dominant role in the aerodynamic loading of wings in many configurations, but particularly so for wings undergoing dynamic motion such as for pitching, revolving or flapping wings. While a growing LEV can generate a high lift force, it can also be a precursor to dynamic stall. For revolving wings, additional forces such as centripetal and Coriolis forces affect the stability and evolution of the LEV, and these mechanisms are far from being well understood. In the present study, the evolution of the LEV on a two bladed rotor, and its effect on the aerodynamic forces are investigated by performing high-fidelity, direct numerical simulations. The Reynolds number based on the chord and tip-velocity is 5000, and the simulations are designed to match experiments that employ 3D PIV measurements. The mechanisms for the LEV instability and shedding, and their effects on the aerodynamic forces are analyzed by applying a Force Partitioning Method (Menon & Mittal, JFM, Vol. 918, 2021), which enables precise quantification of the effect of the LEV and other vortices on the aerodynamic loading of the revolving wing. |
Monday, November 22, 2021 2:28PM - 2:41PM |
M25.00007: Leading-Edge Separation Initiation of Large Amplitude Oscillating Airfoils using a Discrete Vortex Model Kiana Kamrani Fard, Vickie Ngo, James A Liburdy Leading edge separation of an oscillating airfoil operating in the energy harvesting regime is predicted in this study. This prediction can be implemented in discrete vortex models as a criteria to shed point vortices from the leading edge. This criteria uses the transient local wall stress distribution determined from computational fluid dynamics (CFD) simulations, which indicates the time and location of zero wall shear stress on the foil surface. The occurrence of separation is found to collapse during the cycle when scaled using the leading edge shear layer velocity which is determined from the airfoil kinematics. The advantage of the proposed separation criteria is that it can be fully determined from the motion kinematics and then applied to a wide range of low order models for design purposes. Results are obtained for a thin flat airfoil undergoing sinusoidal heaving and pitching motions for a range of reduced frequencies k=fc/U∞ = 0.06 – 0.16 where f is the heaving frequency of the foil, c is the chord length and U∞ is the freestream velocity. The heaving and pitching amplitudes are h0 = 0.5c and q0 = 70°, respectively, and the airfoil pitches about the mid-chord. This study uses a panel method with the proposed leading-edge vortex (LEV) shedding criteria which is applicable to a wide range of foil geometries an empirical trailing-edge separation correction is also applied to the transient force results. Lastly, the effects of a wide range of Reynolds numbers on the leading-edge separation is shown for the given range of reduced frequencies. Lastly, the low order model results of transient lift force are calculated and compared with the CFD simulations. |
Monday, November 22, 2021 2:41PM - 2:54PM |
M25.00008: Experimental investigation of aerodynamics of pitching wings in tandem Syed Hassan Raza Shah, Anwar Ahmed The aerodynamic interaction of two wings in tandem undergoing pitching motion was experimentally investigated at a Reynolds number of 100,000. Rectangular wings of the NACA 0012 airfoil section with a 4-inch chord and fixed aspect ratio of 5 were utilized. Longitudinal spacing between the wings varied from 0.5 to 2.5 chord lengths. The wings were pitched at constant pitch rates in the quasi-static range from -30 to 90 degrees and back. The direct force and moment measurements were recorded for three cases; (a) front wing fixed, rear wing pitching, (b) front wing pitching, rear wing fixed and (c) both the wings pitching at different rates and phases. The wings proximity had a significant influence on the aerodynamic characteristics. The aerodynamic coefficients were changed considerably for wings in tandem even for a wing at a fixed angle of attack when the other wing was pitching and resulted in better aerodynamic efficiency in some instances. The effects of interaction remained similar but started to diminish with increasing spacing, however, changed considerably by the vertical offset (transverse spacing). Results revealed that at low pitch rates i.e., in the quasi-static range, the aerodynamic coefficients depended only on the geometric angles of attack of two wings at any instant. |
Monday, November 22, 2021 2:54PM - 3:07PM |
M25.00009: Effect of Reynolds number and reduced frequency on instantaneous pressure field and loads on a harmonically pitching airfoil Jibu T Jose, Yuhui Lu, Joseph Katz Experimental investigation of the effect of Reynolds number and reduced frequency on an airfoil undergoing dynamic stall were performed on a harmonically pitching NACA 0015 airfoil. The experiments were performed in a refractive index matched water tunnel, with the airfoil oscillating between 5o and 25o, the Reynolds number ranging from 13.6-91x103, and the reduced frequency, between 0.047-1.57. Time-resolved stereo PIV data covering both sides of the airfoil simultaneously was acquired at mid span at frame rates up to 1250Hz. Using in-house, GPU based, parallel-line, omni-directional code, the pressure field around the airfoil was computed by direct integration of the material acceleration calculated from the time- resolved velocity field. The lift and pitching moment were determined from integration of the surface pressure distribution. Data analysis examined the effects of Reynolds number and reduced frequencies on the phase lag between the airfoil orientation and the suction side flow structure, and the resulting variations in lift, pitching moments, and trailing edge velocity during upstroke and downstroke. The mechanisms involved were elucidated and compared to prior publications, such as the roles of the leading-edge and dynamic stall vortices. |
Monday, November 22, 2021 3:07PM - 3:20PM |
M25.00010: Numerical simulation of the critical reduced velocity (U*) in a 2-D pitching airfoil (NACA 0012) using fluid-structure interaction. Ricardo Castillo Villalpando One of the most important assemblies in a wind turbine is the pitch control system that is responsible of feathering the wind turbine blades in strong winds. In this study we present the numerical analysis of the interaction between a laminar flow and an oscillating symmetric airfoil. This oscillations are generated using an undamped linear axial spring. The shear stresses and the pressure distribution of the flow on the surface of the airfoil are responsible for the spring behavior. This causes a deformation in the spring that modifies the airfoil motion hence the fluid flow is disturbed. This interaction is a function of different parameters as the angular position θ, the spring stiffness k, the reduced velocity U*=U∞/fsC where U∞ is the flow velocity, fs is the natural frequency of the spring, and C is the chord of the airfoil, as well as the Reynolds number. The equations are solved in OpenFOAM using PIMPLE and sixDoFRigidBodyMotion. The first case is analyzed without a flow field, where a damping in the amplitude of the oscillation as a function of time is observed. As the Re increases, the damping also increases. As the Re number increases further, at the beginning the flow prevents the airfoil motion however as time passes the oscillation suddenly begins to increase. |
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