Gregory Zilliac

Numerical/experimental study of a wingtip vortex in the near field

The near-field behavior of a wingtip vortex flow has been studied computationally and experimentally in an interactive fashion. The computational approach involved using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations with experimentally determined boundary conditions and a modified Baldwin-Barth turbulence model. Inaccuracies caused by the finite difference technique, grid resolution, and turbulence modeling have been explored. The complete geometry case was computed using 1.5 million grid points and compared with mean velocity measurements on the suction side of the wing and in the near wake. Good agreement between the computed and measured flowfields has been obtained. The velocity distribution in the vortex core compares to within 3% of the experiment.

Mean and Turbulence Measurements in the Near Field of a Wingtip Vortex

The rollup of a wingtip vortex, at a Reynoldsnumber based on chord of 4 .6 £ 10 6 , was studied with an emphasis onsuctionsideandverynear-wakemeasurements(upto x/c = 0.678 downstreamofthetrailingedge).Theresearch was conducted in a 32 £ 48 in. (0.81 £ 1.22 m), low-speed wind tunnel. The rectangular half-wing model had a semispan of 36 in. (0.91 m), a chord of 48 in. (1.22 m), and a rounded tip. Seven-hole pressure probe measurements of the velocity ® eld surrounding the wingtip showed that a large axial velocity up to 1.77 U 1 developed in the vortex core. This high a level of core axial velocity has not been measured previously. Triple-wire probes were used to measure all components of the Reynolds stress tensor. It was determined from correlation measurements that

On numerical errors and turbulence modeling in tip vortex flow prediction

The accuracy of tip vortex flow prediction in the near-field region is investigated numerically by attempting to quantify the shortcomings of the turbulence models and the flow solver. In particular, some turbulence models can produce a ‘numerical diffusion’ that artificially smears the vortex core. Low-order finite differencing techniques of the convective and pressure terms of the Navier–Stokes equations and inadequate grid density and distribution can also produce the same adverse effect. The flow over a wing and the near-wake with the wind tunnel walls included was simulated using 2.5 million grid points. Two subset problems, one using a steady, three-dimensional analytical vortex, and the other, a vortex obtained from experiment and propagated downstream, were also devised in order to make the study of vortex preservation more tractable. The method of artificial compressibility is used to solve the steady, three-dimensional, incompressible Navier–Stokes equations. Two one-equation turbulence models (Baldwin–Barth and Spalart–Allmaras turbulence models), have been used with the production term modified to account for the stabilizing effect of the nearly solid body rotation in the vortex core. Finally, a comparison between the computed results and experiment is presented. Published in 1999 by John Wiley & Sons, Ltd.

Measurement of Continuous Pressure and Shear Distributions Using Coating and Imaging Techniques

The pressure-sensitive paint (PSP) method and the shear-sensitive liquid crystal coating (SSLCC) method were sequentially applied to measure the areal pressure and shear stress vector distributions on a planar test surface beneath an inclined, axisymmetric, turbulent impinging jet. The combined results provide the first-ever consolidated measurements of the continuous normal and tangential force distributions beneath a fundamental flowfield. Results indicate that the PSP method can be extended to the measurement of small pressure differences (< ∼0.1 psig) encountered in low-speed atmospheric flows. Further, these results provide the first demonstration of the capability of the SSLCC method to measure continuous shear stress vector distributions on planar surfaces beneath flowfields where shear vectors of all possible orientations are present

Progress in Reducing Aerodynamic Drag for Higher Efficiency of Heavy Duty Trucks (Class 7-8)

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. In addition, greater use of newly developed computational tools holds promise for reducing the number of prototype tests, for cutting manufacturing costs, and for reducing overall time to market. Experimental verification and validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California. Companion computer simulations are being performed by Sandia National Laboratories, Lawrence Livermore National Laboratory, and California Institute of Technology using state-of-the-art techniques, with the intention of implementing more complex methods in the future.

Measurements in the near-field of a turbulent wingtip vortex

The roll-up of a wingtip vortex, at Reynolds number based on chord of 4.6 million, was examined with emphasis on suction side and near wake measurements. Surface oil flow and laser smoke flow studies are performed, demonstrating the highly 3D nature of the flow around the wingtip, the formation of the feeding sheet, and the viscous core region of the vortex. Flow visualization and correlation measurements show that meandering of the vortex is small and does not appreciably contribute to the turbulence measurements.

Turbulent structure of a wingtip vortex in the near field

The turbulent rollup of a vortex generated by a rectangular wing has been investigated. Extensive mean and turbulence measurements of the flowfield on a wingtip and in the near field have been completed. Velocity fluctuation measurements show that the near-field core is not laminar. A large axial velocity excess was found to exist in the core of the vortex. A momentum balance in the near-field of the wingtip showed that the magnitude of the core Reynolds-stress gradient terms are the same order as the largest terms in the governing equations. Navier-Stokes computations of the identical configuration, including wind tunnel walls and using measured inflow and outflow boundary conditions, reproduced many of the features of the experiment. Inherent limitations of the Baldwin-Barth turbulence model combined with limited grid resolution caused the computed vortex core to be more diffuse than desired. The momentum balance also demonstrated that the level of numerically generated false diffusion in the vortex core is relatively high.

Aerodynamic Drag of Heavy Vehicles (Class 7-8): Simulation and Benchmarking

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. Experimental validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California (USC). Companion computer simulations are being performed by Sandia National Laboratories (SNL), Lawrence Livermore National Laboratory (LLNL), and California Institute of Technology (Caltech) using state-of-the-art techniques.

Accuracy assessment of a wingtip vortex flowfield in the near-field region

Flow prediction of the near-field behavior of a wingtip vortex flowfield has been studied and compaied extensively with experiment. The computational approach involved using the method of artificial compressibility to solve the three-dimensional, incompressible, Navier-Stokes equations. Two oneequation models (Baldwin-Earth turbulence model and Spalart-Allmaras turbulence model} have been used with the production term modified to account for the stabilizing effect of the nearly solid-body rotation in the vortex core. A grid refinement study using 0.6 million, 1.1 million, 1.5 million and 2.5 million grid points is also presented and discussed. An accuracy assessment study revealed that turbulence modeling errors and errors due to numerics are both important factors which influence the resolution of the tip vortex flow. This study attempts to quantify to what extent each of these two error sources affect the accuracy of tip vortex flow prediction in the near-field.

Oil-Film Interferometry Shear Stress Measurements in Large Wind Tunnels - Technique and Applications (Invited)

The oil-film interferometry skin-friction technique is described and applied to a wide variety of flows. The paper reviews recent advances in oil-film interferometry and highlights the utility of skin-friction measurements in aerodynamic flows. Over the last few years, the technique has been widely used in large-scale wind-tunnel tests, it has provided data useful for identifying problematic aspects of designs, and has been used for validation of turbulence models.