On the sensitization of turbulence models to rotation and curvature

▪ Abstract An optical technique is described that is often used nowadays to measure surface pressures on wind tunnel models and flight vehicles. The technique uses luminescent coatings, which are painted on the model surface, excited by light of appropriate wavelength, and imaged with digital cameras. The intensity of the emitted light is inversely proportional to the surface pressure. Hence, the surface pressures can be measured efficiently and affordably with a high spatial resolution. The theory and chemistry of how such coatings work and the parameters that affect them are presented. The required hardware and software are described, with emphasis on the different measurement systems and procedures. The various error sources are discussed, and correction schemes that can be used to minimize them are presented. Sample results, covering a wide range of conditions and applications, are presented and discussed.

Surface Pressure Measurements Using Luminescent Coatings

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.

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

▪ Abstract This review surveys the formation, motion, and persistence of trailing vortices as relevant to the safety and productivity of air travel. It highlights findings or shifts made since Widnall's (1975) review in this series. This review also examines the predictability of the vortices (particularly in terms of lifespan), the durability of multiple vortex pairs, the controversy between expectations of vortex decay and of vortex collapse, the many types of turbulence that may influence the flow, the rich interplay between the rotational and the axial velocity fields in the vortex, the various atmospheric and ground-related factors that dominate the late behavior, a few instances of wakes rising back, and the still unexplained bursting of the vortices. The article also briefly covers prospects for detection and control.

Airplane trailing vortices

http://www.pa.op.dlr.de/wirbelschleppe/publ/PAS.pdf

Commercial aircraft wake vortices

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

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

The symmetric and asymmetric leeward-side flow fields on an inclined ogive-cylinder have been investigated using a number of experimental techniques. Naturally occurring and perturbed flow fields were studied at a moderate Reynolds number and at many incidence angles. By close examination of the steady side force behavior at different roll orientations of the tip, it has been established that micro-variations in the tip geometry of the model have a large influence on the downstream development of the flow field. Under certain conditions, a bistable flow field was observed.

Asymmetric Vortices on a Slender Body of Revolution

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.

On numerical errors and turbulence modeling in tip vortex flow prediction

The 1/8-scale Ground Transportation System (GTS) model was studied experimentally in the NASA Ames 7- by 10-Ft Wind Tunnel. Designed for validation of computational fluid dynamics (CFD), the GTS model has a simplified geometry with a cab-over-engine design and no tractor-trailer gap. As a further simplification, all measurements of the GTS model were made without wheels. Aerodynamic boattail plates were also tested on the rear of the trailer to provide a simple geometry modification for computation. The experimental measurements include body-axis drag, surface pressures, surface hot-film anemometry, oil-film interferometry, and 3-D particle image velocimetry (PIV). The wind-averaged drag coefficient with and without boattail plates was 0.225 and 0.277, respectively. PIV measurements behind the model reveal a significant reduction in the wake size due to the flow turning provided by the boattail plates. Hot-film measurements on the side of the cab indicate laminar separation with turbulent reattachment within 0.08 trailer width for zero and +/- 10 degrees yaw. Oil film interferometry provided quantitative measurements of skin friction and qualitative oil flow images. A complete set of the experimental data and the surface definition of the model are included on a CD-ROM for further analysis and comparison.

/pdf/an-experimental-study-of-the-ground-transportation-system-8xkzo2axhc.pdf

Gregory G. Zilliac

Papers

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

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

Asymmetric Vortices on a Slender Body of Revolution

On numerical errors and turbulence modeling in tip vortex flow prediction

An Experimental Study of the Ground Transportation System (Gts) Model in the NASA Ames 7 by 10-FT Wind Tunnel