Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R30: Aerodynamics: Theory and Vehicles |
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Chair: Jeff Eldredge, University of California at Los Angeles Room: 2016 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R30.00001: Unsteady Nonlinear Aerodynamic Response Modeling: A Data-Driven Perspective Maziar Hemati, Scott Dawson, Clarence Rowley Current real-time-capable aerodynamic modeling strategies are greatly challenged in the face of aggressive flight maneuvers, such as rapid pitching motions that lead to pronounced leading-edge vortex shedding and aerodynamic stall. The inability to accurately and robustly predict, in a low-dimensional manner, the nonlinear aerodynamic force/moment response of an aircraft to sharp maneuvers puts practical approaches for aerodynamic optimization and control out of reach. Here, we propose a parameter-varying model to approximate the response of an airfoil to arbitrarily prescribed rapid pitching kinematics. An output-minimization procedure is invoked to identify the nonlinear model from input-output data gathered from direct numerical fluid dynamics simulations. The resulting nonlinear models have noteworthy predictive capabilities for arbitrary pitching maneuvers that span a broad range of operating regimes, thus making the models especially useful for aerodynamic optimization and real-time control and simulation. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R30.00002: A reduced model for unsteady laminar flow past a solid body using matched asymptotics Ponnulakshmi Vadivelnadar Kartheeswaran, Xinjun Guo, Shreyas Mandre We present a reduced order method for unsteady, laminar flow past a smooth but otherwise arbitrarily shaped body at high Reynolds number. Inspired by matched asymptotic expansion of Navier-Stokes equation, the flow domain is divided into two regimes: (i) an outer inviscid region where the flow field is represented using potential flow and point vortices, and (ii) a boundary layer around the body where the flow field obeys Prandtl's boundary layer equations. Since both representations of the flow field are governed by identical process (viscous effects becoming negligible sufficiently away from the solid body), it is possible to match the flow field at the interface between the two domains. Matching the flow field at the interface dictates the strength and location of vorticity shed from the boundary layer to the outer region. An approximately 100-fold increase in computational speed may be achieved using this method. In this talk, we present results for the flow surrounding a 2D oscillating elliptic hydrofoil, a configuration employed for energy extraction from tides. Simulations are performed for various pitching and heaving parameters in an effort to optimize the stroke for maximum energy extraction. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R30.00003: Aerodynamics of yacht sails: viscous flow features and surface pressure distributions Ignazio Maria Viola The present paper presents the first Detached Eddy Simulation (DES) on a yacht sails. Wind tunnel experiments on a 1:15th model-scale sailing yacht with an asymmetric spinnaker (fore sail) and a mainsails (aft sail) were modelled using several time and grid resolutions. Also the Reynolds-average Navier-Stokes (RANS) equations were solved for comparison with DES. The computed forces and surface pressure distributions were compared with those measured with both flexible and rigid sails in the wind tunnel and good agreement was found. For the first time it was possible to recognise the coherent and steady nature of the leading edge vortex that develops on the leeward side of the asymmetric spinnaker and which significantly contributes to the overall drive force. The leading edge vortex increases in diameter from the foot to the head of the sail, where it becomes the tip vortex and convects downstream in the direction of the far field velocity. The tip vortex from the head of the mainsail rolls around the one of the spinnaker. The spanwise twist of the spinnaker leads to a mid-span helicoidal vortex, which has never been reported by previous authors, with an horizontal axis and rotating in the same direction of the tip vortex. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R30.00004: Aerodynamic drag on intermodal railcars Philip Kinghorn, Daniel Maynes The aerodynamic drag associated with transport of commodities by rail is becoming increasingly important as the cost of diesel fuel increases. This study aims to increase the efficiency of intermodal cargo trains by reducing the aerodynamic drag on the load carrying cars. For intermodal railcars a significant amount of aerodynamic drag is a result of the large distance between loads that often occurs and the resulting pressure drag resulting from the separated flow. In the present study aerodynamic drag data have been obtained through wind tunnel testing on 1/29 scale models to understand the savings that may be realized by judicious modification to the size of the intermodal containers. The experiments were performed in the BYU low speed wind tunnel and the test track utilizes two leading locomotives followed by a set of five articulated well cars with double stacked containers. The drag on a representative mid-train car is measured using an isolated load cell balance and the wind tunnel speed is varied from 20 to 100 mph. We characterize the effect that the gap distance between the containers and the container size has on the aerodynamic drag of this representative rail car and investigate methods to reduce the gap distance. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R30.00005: Geometric theory of garden hose instability Vakhtang Putkaradze, Francois Gay-Balmaz We derive a fully three-dimensional, geometrically exact theory for flexible tubes conveying fluid. The theory also incorporates the change of the cross-section available to the fluid motion during the dynamics. Our approach is based on the symmetry-reduced, exact geometric description for elastic rods, coupled with the fluid transport and subject to the volume conservation constraint for the fluid. We analyze the fully nonlinear behavior of the model when the axis of the tube remains straight. We then proceed to the linear stability analysis and show that our theory introduces important corrections to previously derived results, both in the consistency at all wavelength and in the effects arising from the dynamical change of the cross-section. Finally, we derive and analyze several analytical, fully nonlinear solutions of traveling wave type in two dimensions. We show that in all cases, the change of cross-section plays an important role in the dynamics. [Preview Abstract] |
(Author Not Attending)
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R30.00006: Flow and Stall Characteristics of Transonic Compressor with a Single Circumferential Casing Groove Yasunori Sakuma In the present study, CFD analysis on transonic rotor, NASA Rotor 37 with and without circumferential single grooved casing treatment was carried out, with the purpose of clarifying the effect which casing groove has on the near casing flow field of compressors and to obtain productive guidelines for designing robust and effective casing treatment. Based on the evaluation of blockage flow distribution and its development process in near stall condition, the onset of stall in NASA Rotor 37 was shown to be caused by the blockage flow local to the tip region of the passage. The implementation of simple, single circumferential groove enabled to assess the exact effect it has on the near tip flow field and its stability enhancement capability. The principal effect of the casing groove was the reduction of flow momentum of tip leakage flow. This effect contributed to the stability enhancement of NASA Rotor 37 by suppressing the accumulation of blockage near the blade tip region. In the series of analysis, the axial location and the depth of the groove were parametrically varied. The effect of the casing groove tended to show distinctive change according to the groove location, and deeper groove showed a higher potential in suppressing the onset of stall. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R30.00007: Aerodynamics on a Finite Wing from the Perspective of Force Elements Chun-Fei Kung, Chien-Cheng Chang, Chin-Chou Chu This paper is aimed to examine various contributions to the forces on an impulsively started finite plate from the perspective of a force-element representation. The wing plate has an aspect ratio (AR) between 1 and 3, and is placed at low and high angles of attack, while the Reynolds number \textit{Re} is varied between 100 or 300. The force theory enables us to quantify the contributions to the forces exerted on the plate in terms of all the fluid elements with nonzero vorticity, such as in the tip vortices (TiVs), leading- and trailing-edge vortices (LEV and TEV) as well on the plate surface. The present vorticity force analysis (VFA) was made parallel to the pressure force analysis (PFA) by examining the sectional force contributions along the wing span, but can further extend to include the outer regions (of TiVs). The interplay between the LEV and the TiVs by assessing the relative importance of the transverse as well as the longitudinal vorticity components at various time stages leads to insightful physical explanations of the force mechanisms. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R30.00008: A reduced-order vortex model of three-dimensional unsteady non-linear aerodynamics Jeff D. Eldredge Rapid, large-amplitude maneuvers of low aspect ratio wings are inherent to biologically-inspired flight. These give rise to unsteady phenomena associated with the interactions among the coherent structures shed from wing edges. The objective of this work is to distill these phenomena into a low-order physics-based dynamical model. The model is based on interconnected vortex loops, composed of linear segments between a small number of vertices. Thus, the dynamics of the fluid are reduced to tracking the evolution of the vertices, whose motions are determined from the velocity field induced by the loops and wing motion. The feature that distinguishes this method from previous treatments is that the vortex loops, analogous to point vortices in our two-dimensional model, have time-varying strength. That is, the flux of vorticity from the wing is concentrated in the constituent segments. Chains of interconnected loops can be shed from any edge of the wing. The evolution equation for the loop vertices is based on the impulse matching principle developed in previous work. We demonstrate the model in various maneuvers, including impulse starts of low aspect ratio wings, oscillatory pitching, etc., and compare with experimental results and high-fidelity simulations where applicable. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R30.00009: Towards a predictive vortex model for 2D non-linear aerodynamics Darwin Darakananda, Jeff D. Eldredge In previous work (Hemati et al 2014), we presented a framework in which a low-order point vortex model can be optimized to capture the non-linear aerodynamics of a wing undergoing arbitrary rigid body motion. Rather than determine the time-varying vortex strengths with the Kutta condition, these strengths were chosen to minimize the difference between the force predicted by the model and pre-existing empirical data. Here, we present ongoing extensions of this model. With the help of tools from dynamical systems theory, we develop a means to incrementally optimize the model against new data. This opens the possibility for using the model in a dynamic estimator context. Self-sustained vortex shedding from wings is achieved using a criterion based on the leading edge suction parameter. We demonstrate the model on a variety of canonical problems, including pitch-up, oscillatory heaving and pitching, and impulsive translation of a plate at various angles of attack. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R30.00010: ABSTRACT WITHDRAWN |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R30.00011: Aerodynamics of Drag Reduction Devices for Semi-Trucks Jason Ortega, Kambiz Salari An increasing number of semi-trucks throughout the United States are being retrofitted with aerodynamic drag reduction devices to improve the vehicle fuel economy. Such devices typically include both trailer skirts and boattails to mitigate trailer underbody drag and base drag, respectively. Since full-scale measurements of the device performance are especially prone to experimental noise due to the effects of the driver, route, payload, or atmospheric conditions, more precise data must be obtained within a wind tunnel. In this experimental study, the wind-averaged drag coefficient is measured for a detailed 1/8th scale semi-truck model. The Reynolds number based upon the vehicle width is 1.7e6. A number of trailer skirt and boattail device configurations are considered, as well as the effects of the boattail deflection angle. The results of this study demonstrate that a combination of a trailer skirt and boattail reduces the aerodynamic drag of a semi-truck by as much as 25{\%}. If such a combination were applied to each of the semi-trucks throughout the United States, several billion dollars in fuel savings could be achieved each year. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-657810. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R30.00012: Light Vehicle-Trailer Systems' Aerodynamics Testing and Simulation Henry Boyer, Lorenz Sigurdson, Carlos Lange A wide range of trailers with very poor aerodynamics are hauled long distances across a vast North American highway system. Our goal was to use preliminary smoke-wire flow visualizations to learn: the characteristic flow patterns over models representing modern Vehicle-Trailer Systems (VTS); what improvements need to be made in the experimental set-up; and if there is an opportunity for reduction in aerodynamic drag. Visualization tests were done in an open circuit wind tunnel, with a cross-sectional area of 0.3 m$^2$. Detailed models of light duty trucks and trailers were used at a Reynolds number of 13,700. Images of the streaklines indicated two characteristic features. One was the presence of a stagnation point on the leading face of the trailer followed by a separation bubble on its top. The other feature was an unexpected separation bubble on the hood of the towing vehicle. We determined that it did not have a significant effect on the downstream flow pattern. By adding a small wedge deflector on the cab of the vehicle it was concluded that there is an opportunity for significant improvement of the VTS aerodynamics. Computational simulation of the flow is underway. [Preview Abstract] |
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