Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A16: Aerodynamics: Unsteady Effects I |
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Chair: Jeff Eldredge, University of California, Los Angeles Room: Georgia World Congress Center B303 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A16.00001: Viscous Extension of Theodorsen's Lift Frequency Response Haithem E Taha, Amir Rezaei Many studies invoked an alternative auxiliary condition to the Kutta condition for the analysis of unsteady flows around wings with sharp edges. Some of these reports, because of the observed discrepancies at zero lift conditions, suggested a fundamental revision to the classical theory of unsteady aerodynamics. That is, since the vorticity generation and lift development are essentially viscous processes, a purely inviscid theory of unsteady aerodynamics might be fundamentally flawed. In this work, an unsteady boundary layer approach (triple deck theory) is adopted to develop a viscous correction to the unsteady lift frequency response; i.e., a Reynolds-number-dependent extension of Theodorsen function. The developed model is compact and efficient so that it may be used in flight dynamics, control, and aeroelastic analyses. It is found that the viscous correction induces a significant phase shift to the circulatory lift component, particularly at low Reynolds numbers and high-frequencies, that matches previous experimental results and current computational simulations. This enhancement in the predicted phase of the unsteady loads is envisaged to boost the current predictability of aeroelastic flutter boundaries. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A16.00002: Deep Learning for Gust Detection in Unsteady Aerodynamics Wei Hou, Jeff Eldredge Recent successes of deep learning in various fields aroused the interest to integrate such algorithms into aerodynamics. Here, we present several deep learning structures to identify the unknown characteristics of incident gusts and rapid changes of angle of attack in the measured surface pressures on a flat plate. The flow field is simulated with a two-dimensional inviscid vortex model, in which the separated flow is modeled by the leading edge suction parameter (LESP) criterion. When the instantaneous value of LESP exceeds a varying critical value, the model releases vorticity proportional to the excess to simulate the effect of the gusts. We present several deep learning algorithms that use the surface pressure history to determine the characteristics of disturbances. The first algorithm aims to detect the LESP history at a constant angle of attack. The second algorithm aims to detect the angle of attack history during a pitch-up maneuver with constant critical LESP. The third and fourth algorithms concern mixtures of these two scenarios. The four algorithms achieve satisfactory results in their own tasks, showing the possibility of employing deep learning on a broader scale for gust detection in aerodynamics. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A16.00003: FTLE structure of the unsteady flow around accelerated non-slender swept wings Han Tu, Matthew Marzanek, David E Rival, Melissa A Green Previous experiments have shown that for a 45° sweep delta wing with an NACA 0012 cross section, the flow around the wing is fully separated by 25° in steady translation. To study the influence of an axial wing acceleration on the flow structure, time-resolved planar PIV and surface pressure measurement were conducted at angles of attack of 20° and 30°, with axial acceleration 1≤g*≤6, where g*=s_{gust}/c is the distance over which the acceleration takes place, normalized by the chord. Distance traveled is denoted by s*=s/c. Finite-time Lyapunov exponent (FTLE) analysis, combined with instantaneous pressure and velocity profiles, showed that axial acceleration contributes to flow reattachment under certain conditions. Future work will also study vertical accelerations, and those results combined with the axial acceleration results may provide a foundation to develop methods for predicting forces and moments on wings in highly unsteady environments. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A16.00004: Characterization and understanding of unsteady airfoil behavior using modal analysis Jonathan W Naughton, Tanner D Harms, Pourya Nikoueeyan Characterizing unsteady airfoil behavior including dynamic stall in a methodical way has been a challenge due to lack of an objective means of classifying these complex flows. Using modal analysis techniques with data from experiments and computations promises to make the process more objective. Modal techniques have recently been applied to experimental data taken for a range of airfoils under different conditions to investigate their potential. Modal analysis has been applied to both surface pressure data as well as flow-field data in an attempt to develop characterization approaches. Analysis of phase-averaged data has led to classification strategies for different dynamic stall types based on the coupling of the shape of spatial modes with the time dependence of the coefficients multiplying the modes. Similarly, modal analysis of the unsteady data has led to a means of identifying and understanding cycle-to-cycle variations in these flows. In addition to developing better classification schemes, these modal analysis methods promise to aid in better understanding of these complex flows. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A16.00005: Unsteady Lift on a Fixed-AoA Airfoil in Unsteady Freestream James W. Gregory, Wenbo Zhu, George Altamirano, Jack Plank, Jeffrey P. Bons Unsteady lift production of an airfoil in a time-varying freestream velocity flow is studied in this work. Various theories such as those of Isaacs (1945), Greenberg (1947), van der Wal & Leishman (1994), and Strangfeld et al. (2016) have been formulated to analytically describe the time-dependent airloads. These theories generally attribute the unsteady response to the effects of unsteady shedding and convection of vorticity in the airfoil wake. However, these theories have been only partially validated on a thick airfoil at low Reynolds number, and the limits of applicability of the theory are presently unknown. This work provides a comprehensive set of experimental data on a range of airfoils at high Reynolds number (> 10^{6}), under varying freestream velocity amplitudes, reduced frequencies, and airfoil angles of attack. Surprisingly, the scope of the validity of the theoretical approaches is limited: significant nonlinear aerodynamic effects are found to dominate the lift and moment response in many cases, even at high Reynolds number. This work documents some of those limits and discusses the physical mechanisms underlying the departure of theory from experimental results. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A16.00006: Airfoil surface pressure measurements in unsteady flow conditions Janik Kiefer, Mark Miller, Claudia E Brunner, Martin Otto Laver Hansen, Marcus Hultmark The combination of high Reynolds numbers and unsteady flow conditions depict a challenge in experimental wind tunnel studies. Unsteady airfoil aerodynamics are commonly described by the reduced frequency k = ωc/2/U, where a range of 0 ≤ k ≤ 0.25 characterizes steady to highly unsteady flow conditions. The Reynolds number scales proportionally with the flow velocity U, whereas the reduced frequency scales inversely proportional. In regular wind tunnels, this leads to unrealistically high pitching frequencies in experimental attempts to achieve high Reynolds numbers simultaneously with high reduced frequencies. The present study takes advantage of a high-pressure flow facility, in which the density of compressed air promotes high Reynolds numbers, while low velocities below 10 m/s allow for unsteady flow conditions. A symmetric NACA0021 airfoil is equipped with surface pressure sensors to investigate distributed pressures and integrated forces at a constant Reynolds number of 3 million. The present study focuses on the effect of altered mean angle of attack α and varied pitching amplitudes Δα at a constant reduced frequency k. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A16.00007: Dynamic effects on airfoil performance under unsteady inflow conditions at high Reynolds numbers Claudia Brunner, Janik Kiefer, Mark Miller, Marcus Hultmark Dynamic stall is a flow phenomenon where an instantaneous change in flow conditions leads to a momentary increase in the lift force on an airfoil due to the formation of a low-pressure vortex at the leading edge. It occurs on the airfoils of helicopters, wind turbines, and on aircraft wings during maneuvers or fluctuating wind conditions. Most existing experimental data on airfoil performance at high Reynolds numbers was taken under steady conditions, due to the practical challenges of conducting experiments at high Reynolds numbers and the appropriate time scales. As such, data from previous experiments gives limited insight into dynamic stall, or an airfoil’s performance in unsteady conditions at Reynolds numbers relevant for many applications. Here, we use a pressurized facility to achieve very high Reynolds numbers at low velocities, which enables unsteady investigations at realistic time scales. Lift forces on a two-dimensional symmetrical airfoil, exposed to unsteady inflow conditions, are investigated in attached and separated conditions at high Reynolds numbers. The unique experimental set-up enables tests in real-world flow conditions which can inform both our understanding of this phenomenon and predictive models. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A16.00008: Critical evolution of leading edge suction during dynamic stall Julien Deparday, Karen Mulleners Dynamic stall dominates the aerodynamic performance, robustness, and wake dynamics of aircrafts in gusts, or on rotating blades such as helicopter or wind turbine blades. To assess dynamic stall onset and its associated unsteady effects, we analyse the evolution of the leading edge suction on a sinusoidally pitching airfoil based on time-resolved surface pressure measurements and particle image velocimetry. During the dynamic stall development stage, we link the shear layer evolution with the evolution of the leading edge suction. The dynamic stall development prior to stall onset consists of two stages: a primary instability stage and a vortex formation stage. The transition between stages is marked by a maximum in the leading edge suction. During the primary instability stage, both the leading edge suction and the shear layer height with respect to the airfoil's surface increase linearly with convective time. During the vortex formation stage, the leading edge suction drops and decreases linearly in time while the shear layer rolls up and creates a dynamic stall vortex. The maximum leading edge suction increases with normalized effective unsteadiness of the pitching motion. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A16.00009: The Effect of Aerodynamic Loads on Nonlinear Vibration Behavior of a Ferromagnetic Beam-Plate under Magnetic Fields Nariman Ashrafi Khorasani In order to study the mechanical behavior of a ferromagnetic rectangular thin beam-plate under the magnetic field and aerodynamic load, the electrodynamic equation are derived based on the Maxwell equations and the electromagnetic constitutive relations. The basic set of equations for nonlinear electromagnetic elasticity vibration expressed by the displacement of a thin beam-plate in a transverse magnetic field is obtained. In addition, we study the nonlinear principal resonance and the solution stability of a thin plate with two opposite sides simply supported and subjected to an aerodynamic pressure in a constant transverse magnetic field. The coupled nonlinear differential equations of the system, are discarded by Galerkin method and solved numerically to characterize the phenomena of magnetoelastic buckling and instability of the plates. Then the effect of different parameters on the vibration characteristics of the soft ferromagnetic beam-plate and the effects of magnetic tractions and thermal fields which created by plate vibration is investigated |
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