Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session NK: Aerodynamic Computations |
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Chair: Jason Ortega, Lawrence Livermore National Laboratory Room: Salt Palace Convention Center 250 E |
Tuesday, November 20, 2007 11:35AM - 11:48AM |
NK.00001: Delayed Detached Eddy Simulation of Flow Over an Airfoil with Synthetic Jet Control Omar Lopez, Godse Uday, Robert Moser Delayed Detached-Eddy Simulation (DDES) is a hybrid RANS-LES model similar to DES but with modifications to reduce the influences of ambiguous grid densities in the numerical results. This model was implemented in CDP, a parallel unstructured grid incompressible flow solver, developed at the Center for Integrated Turbulence Simulations (CITS) at Stanford University. CDP has the advantage of being nearly energy conserving. Several simulations at static angles of attack at a Re based on the chord of 5.7e5 showed good agreement with experiments and other computational studies. Simulations of pitching and plunging cases have also yield good results. This simulation capability is being used to model closed-loop flow control of the airfoil. The action of tangential-blowing synthetic jet actuators mounted near the trailing edge of the airfoil are modeled as local momentum sources, based on detailed measurements of this configuration conducted at Georgia Tech. Resulting simulations show the effects of the actuators on the vortical structure of the flow, as well as on the aerodynamic properties. By integrating actuator and sensor models with a model of the flow controller, we will be able to simulate experiments currently being conducted by A.Glezer and his group at Georgia Tech. on the control of an airfoil in a wind tunnel. [Preview Abstract] |
Tuesday, November 20, 2007 11:48AM - 12:01PM |
NK.00002: ABSTRACT WITHDRAWN |
Tuesday, November 20, 2007 12:01PM - 12:14PM |
NK.00003: Drag reduction of a heavy vehicle Jason Ortega, Kambiz Salari During the 1970's and 1980's, a number of first-generation drag reduction devices were designed to reduce the aerodynamic losses of heavy vehicles (Cooper, 2003). The result of this effort led to the development of a number of devices that improved the aerodynamics of a heavy vehicle tractor. Additionally, a number of second-generation devices were developed for heavy vehicle trailers. Unfortunately, these trailer devices did not enter into the market on a wide-scale basis and, as a result, the modern heavy vehicle trailer largely remains a ``box on wheels'' with minimal aerodynamic consideration taken into its design. The primary obstacle to implementing trailer devices was not their effectiveness in reducing drag, but rather operational, maintenance, and ultimately, economic concerns. However, with rising fuel costs and potentially unstable fuel supplies, there is a renewed objective to further reduce heavy vehicle fuel usage. To accomplish this purpose, the present study investigates the drag reduction capability of a trailer device, which neither reduces the trailer cargo capacity, nor limits access to the trailer doors. RANS simulations are performed on a full-scale tractor-trailer that is traveling at highway conditions with and without the trailer device. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. [Preview Abstract] |
Tuesday, November 20, 2007 12:14PM - 12:27PM |
NK.00004: Direct Numerical Simulation of Low Reynolds Number Separated Flow around an Eppler 387 Mehmet Sahin, Kamran Mohseni Low Reynolds number aerodynamic flows are important for various applications including micro-arial vehicles, sailplanes, leading edge control devices, high-altitude vehicles and wind turbines. These flows are generally characterized by the presence of laminar separation bubbles. These bubbles are generally unsteady and have a significant effect on the overall resulting aerodynamic forces. In this study, the time-dependent unsteady calculations of low Reynolds number flows are carried out over an Eppler 387 airfoil in both two- and three-dimensions. Various instantaneous and time-averaged aerodynamic parameters including pressure, lift, and drag coefficients are calculated in each case and compared with the available experimental data. An observed anomaly in the pressure coefficient around the location of the separation bubble in two-dimensional simulations is attributed to the lack of spanwise flow in two-dimensional simulations that results in vorticity extraction from the forming vortices in this region in three-dimensional flows. Preliminary three-dimensional calculations verified this argument. [Preview Abstract] |
Tuesday, November 20, 2007 12:27PM - 12:40PM |
NK.00005: Corner separation and the onset of stall in an axial compressor Aicha Thiam, Robert Whittlesey, Candace Wark, David Williams Axial compressor performance is limited by the onset of stall between the diffusing passageways of the rotors and stators. The flow physics responsible for the stall depends on the blade geometry of the machine, and in this experiment stall develops from a blade-hub corner separation. The 1.5 stage axial compressor consists of inlet guide vanes, a rotor and stator section. Separate motors drive the downstream fan and rotor, which makes it possible to change the compressor pressure ratio and flow coefficient by changing either the wheel speed or the bulk flow rate through the machine. Detailed maps of the flow behind the stators and in front of the rotors were obtained using a Kulite stagnation pressure probe. Mean pressure measurements show the growth of the corner flow separation and divergence of the ``through flow'' toward the outer casing. Spectra show a sensitivity of the separated region to small amplitude external disturbances, in this case originating from the downstream fan. The onset of rotating stall appears as the first subharmonic of the rotor frequency, 0.5 f$_{r}$, then shifts to a slightly lower frequency 0.45 f$_{r}$ as the flow coefficient is decreased. [Preview Abstract] |
Tuesday, November 20, 2007 12:40PM - 12:53PM |
NK.00006: Stability of the Leading Edge Vortex on Insect Wings B. Bush, K. Duraisamy, J. Baeder A defining characteristic of the flowfield associated with insect flight is a stable leading edge vortex that persists over a majority of the flapping stroke. While it is known that spanwise flow, coupled with the effect of wing rotation and interaction with the tip vortex can result in stability, the specific mechanisms by which this stability is achieved have not been clearly identified. Towards a clarification of this issue, two idealized cases are computationally simulated. First, computations of the flow over a rectangular plate in linear translation are compared with experimental data to provide both code validation and a basis for comparison with the rotational cases. Secondly, a model wing, similar in planform to a fruit fly (Drosophila), is simulated both in steady translation and in an impulsively started steady rotation at a constant angle of attack. The stability of the resulting vortex system is compared against the translational cases at various Reynolds numbers and angles of attack to better understand the role that rotation and planform shape play in the leading edge vortex development and stability. [Preview Abstract] |
Tuesday, November 20, 2007 12:53PM - 1:06PM |
NK.00007: Direct Numerical Simulations of the Flow around a Golf Ball: Methodologies and Approach Nikolaos Beratlis, Clinton Smith, Elias Balaras, Kyle Squires, Masaya Tsunoda An approach to Direct Numerical Simulations (DNS) for the flow around a golf ball is being developed. The spatio-temporal resolution that is required to capture all essential scales of the flow at realistic Reynolds numbers is severe, and highly efficient numerical tools that can scale up to hundreds of processors are required. In the present study an embedded boundary approach is adopted, where the Navier-Stokes equations are solved on a structured, staggered grid in cylindrical coordinates, and boundary conditions on the solid surface, which is not aligned with the grid, are imposed using interpolation. The code is parallelized using a domain decomposition strategy and message passing interface (MPI). In this first part, an assessment of the computational approach is presented, including an examination of grid dependence and a study of the parallel performance for grids up to 1.2 billion nodes on 500 processors. [Preview Abstract] |
Tuesday, November 20, 2007 1:06PM - 1:19PM |
NK.00008: Direct Numerical Simulations of the Flow around a Golf Ball: Flow Structure and Forces Clinton Smith, Nikolaos Beratlis, Kyle Squires, Elias Balaras, Masaya Tsunoda The drag on a golf ball can be reduced by as much as $50\%$ compared to a smooth sphere. There have been very few studies, primarily experimental, that provide quantitative information on the details of the underlying mechanisms. To illuminate the underlying mechanisms, Direct Numerical Simulation (DNS) is applied to the flow around a golf ball using an immersed boundary method. Computations are performed using up to 500 processors on a range of mesh resolutions from 61 million points to 1.2 billion points. Results are presented from simulations performed at Reynolds numbers of $Re=UD/\nu =0.25 \times 10^5$ and $1.0 \times 10^5$. Flow visualizations reveal the location of turbulent transition, as well as the delay of complete separation due to shear layer instability and the local separation within individual dimples. Prediction of the drag coefficient appears in reasonable agreement with measurements. Time-averaged statistics of the velocity and pressure are being acquired and will be presented at the meeting. [Preview Abstract] |
Tuesday, November 20, 2007 1:19PM - 1:32PM |
NK.00009: Analysis of Non-symmetrical Flapping Airfoils Wee Beng Tay, Kah Bin Lim Simulations have been done to assess the performance of different types of non-symmetrical airfoils on lift, thrust and propulsive efficiency under different flapping configurations at a Reynolds number of 10,000. The variables studied include the Stroudal number, reduced frequency, pitch angle and phase angle difference. In order to analyze the variables more efficiently, the Design of Experiments using the response surface methodology is applied. The simulation results show that besides the flapping configuration, airfoil shape also has a profound effect on the efficiency, thrust and lift production. The 4 factors have different levels of significance on the responses, indicating the shape of the airfoil plays a part as well. Thrust production depends more heavily on these parameters, rather than the shape of the airfoil. On the other hand, lift production is primarily dominated by its airfoil shape. Efficiency falls somewhere in between. Two-factor interactions among the variables also exist in efficiency and thrust production. Vorticity plots are analyzed to explain some of the results. Overall, the s1020 airfoil is able to provide relatively good efficiency and at the same time generate high thrust and lift force. These results can be used to help in the design of a better ornithopter's wing. [Preview Abstract] |
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