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A study of shock-buffet onset and instability mechanism via Reynolds-averaged Navier-Stokes simulations on several airfoils is presented. The numerical setup and the Spalart-Allmaras turbulence closure are validated based on wind-tunnel data from NACA 0012 and RA16SCI airfoils. The paper presents simulations of the flow past three airfoils: the subsonic NACA 0012, the supercritical RA 16SC1, and the thin, transonic/supersonic NACA 64A204, at pre- and postbuffet conditions, and within a cycle of developed shock buffet. The shock-buffet cycle is found to be similar in nature for all airfoils, originating in unstable interaction of the shock and the separation bubble. Simulation results support the notion that buffet onset is not related to the bursting of the separation bubble behind the shock. Shock-buffet categorizing is posited as a transonic prestall instability phenomenon that depends on the shock strength and location. Shock-buffet onset conditions occur when the shock position is behind and sufficiently close to the upper-surface maximum curvature location. Additionally, it is suggested that offset conditions are when the shock is at an upstream location and the flow aft of it is fully separated. [ABSTRACT FROM AUTHOR]

A computational fluid-dynamics investigation, including passive vortex generators (VGs) that generate streamwise counter-rotating vortex structures, usually requires a grid with fully resolved VG geometries and vortex structures with a corresponding large number of grid points to obtain an accurate solution. An efficient way to avoid such a setup and time-consuming process in turbulent shear-layer flows is to introduce statistics-based vortex-generator modeling. The second-order statistics of the initial vortices are computed by using a vortex model in combination with the lifting-line theory. The statistics are added as additional turbulence stress terms to the equations within a differential Reynolds stress-turbulence model. In this investigation, results from statistical VG model computations for zero- and adverse-pressure-gradient flat-plate boundary-layer flows, as well as for the flow in a plane asymmetric diffuser, are evaluated against results from fully resolved VG computations and experiments. It could be shown that the initial near-field forcing is too weak for the proposed VG model. An improved VG model description removes some drawbacks by adding additional statistical forcing terms. Results become more comparable, resulting in improved predictions when compared to experiments and fully resolved computations. [ABSTRACT FROM AUTHOR]

Finite-volume discretization schemes for viscous fluxes on general grids are compared using node-centered and cell-centered approaches. The grids range from regular grids to highly irregular grids, including random perturbations of the grid nodes. Accuracy and complexity are studied for four nominally second-order accurate schemes: a node-centered scheme and three cell-centered schemes (a node-averaging scheme and two schemes using least-squares face-gradient reconstruction). The two least-squares schemes use either a nearest-neighbor or an adaptive-compact stencil at a face. The node-centered and least-squares schemes have similarly low levels of complexity. The node-averaging scheme has the highest complexity and can fail to converge to the exact solution when clipping of the node-averaged values is used. On highly anisotropic grids, typical of those encountered in grid adaptation, the least-squares schemes, the node-averaging scheme without clipping, and the node-centered scheme demonstrate similar second-order accuracies per degree of freedom. On anisotropic grids over a curved body, typical of turbulent flow simulations, the node-centered scheme is second-order accurate. The node-averaging scheme may degenerate on mixed-element grids. The least-squares schemes have to be amended to maintain second-order accuracy by either introducing a local approximate mapping or modifying the stencil to reflect the direction of strong coupling. Overall, the accuracies of the node-centered and the best cell-centered schemes are comparable at an equivalent number of degrees of freedom on isotropic and curved anisotropic grids. On stretched, randomly perturbed grids in a rectangular geometry, both gradient and discretization errors for all schemes are orders of magnitude higher than corresponding errors on regular grids. [ABSTRACT FROM AUTHOR]

The near-field mean flow and turbulence characteristics of a turbulent jet of air issuing from a sharp-edged equilateral triangular orifice into still air surroundings have been examined experimentally using hot-wire anemometry and a pitot-static tube. For comparison, some measurements were made in a round free air jet, also issuing from sharp-edged orifice. The Reynolds number, based on the equivalent diameter, was 1.84 ⊗ 105 in each jet. The three components of the mean velocity vector, the Reynolds normal and primary shear stresses, the distribution of the autocorrelation coefficients and the one-dimensional energy spectra of the fluctuating velocity signals, and the mean static pressure were measured. The mean streamwise vorticity, the half-velocity widths, mass entrainment into the jet, and the local shear in the mean streamwise velocity were obtained from the measured data. It was found that mixing in the equilateral triangular jet is faster than in the round jet. The mean streamwise vorticity field was found to be dominated by counter-rotating pairs of vortices, which facilitated mixing and entrainment in the equilateral triangular jet. The distribution of the autocorrelation coefficients and the one-dimensional energy spectra results indicated the presence of coherent structures in the near field of the equilateral triangular jet. [ABSTRACT FROM AUTHOR]

The commercial computational fluid dynamics (CFD) code CFX-5 of ANSYS, Inc., has been used to compute the engine installation drag for the German Aerospace Center (DLR) F6 aircraft configuration as part of the Second AIAA Drag Prediction Workshop. The computations were performed with the standard hexahedral meshes provided by ICEM-CFD to the workshop. The full drag polar for the DLR-F6 configuration has been computed. For all cases, good agreement between the experiment and the predictions were obtained for lift, drag, and pitching moment coefficients. All simulations where based on the shear stress transport turbulence model, and additional computations have indicated that turbulence modeling issues are largely responsible for the overprediction of the lift curve slope that was observed by many of the workshop participants. [ABSTRACT FROM AUTHOR]

Flows with extensive crossflow separation appear in many flying vehicles. Traditionally these types of flows are difficult to simulate accurately, especially if the separation is extensive (flow at high incidence or strong swept shock waves impinging on boundary layers). The present author has shown in previous publications that if the three- dimensional structure of these flows is considered for modifying a conventional turbulence model the efficiency of the model is enhanced considerably. This concept has been demonstrated for the case of the supersonic fin/plate interaction, and it is based on the observation that in turbulent flows characterized by extensive crossflow separation a layer of low turbulence exists close to the surface (underneath the core of the separation vortex). This flow feature has been incorporated into the Baldwin-Lomax algebraic turbulence model by the introduction of a new relation for the calculation of the eddy viscosity within the separation domain. In the present paper, the modified turbulence model is extended for application to more complicated shock-wave/turbulent boundary-layer interactions (crossing shocks) and to high-angle-cf-attack flows. [ABSTRACT FROM AUTHOR]

Analyzes the development of turbulence inside short bubble formed on an airfoil. Turbulent energy balances; Growth of turbulent energy; Production terms for short bubbles.

Presents a study which determined the effects of small particles on fluid turbulence in a round water jet using a two-channel discriminator laser Doppler velocimeter. Experimental methods; Results; Conclusions.

This article is the second of two companion papers which document the concept and the application of a coupled computational fluid dynamics system which was designed to incorporate the prediction of laminar-turbulent transition into a hybrid Reynolds-averaged Navier-Stokes solver. Whereas the first part deals with the description of the transition prediction methodology and the sensitivities of the coupled system, the second part documents its practical application. The complete coupled system consists of the Reynolds- averaged Navier-Stokes code, a laminar boundary-layer code, and a fully automated local, linear stability code. The system predicts and applies transition locations due to Tollmien-Schlichting and crossflow instabilities using the eN method based on the two-N-factor approach. The coupled system was designed to be applied to three-dimensional aircraft configurations which are of industrial relevance. The application of the coupled system to a wing-body configuration with a three-element wing consisting of slat, main wing, and flap is described and documented in this paper. The prediction of the laminar-turbulent transition lines was done in a fully automatic manner. It is shown that complex aircraft configurations can be handled without a priori knowledge of the transition characteristics of the specific flow problem. [ABSTRACT FROM AUTHOR]

A new flow-physics-based modeling of surface-roughness effects is developed for the Reynolds-averaged Navier-Stokes equations numerical calculations of high-Reynolds-number turbulent flows over rough walls. In the proposed approach, we model the fluid dynamics of the volume-averaged flow in the near-wall rough layer by using the Brinkman equation. The porosity can be calculated based on the volumetric characteristics of the roughness, and the permeability is modeled. The Reynolds-averaged Navier-Stokes equations are solved numerically above the near-wall rough layer, and a low-Reynolds-number k-" model is employed in all regions. In this paper, we present the computational results, including the skin-friction coefficient, the log-law mean velocity, the roughness function, the turbulent kinetic energy, and the Reynolds shear stress. The results show that the new rough-wall-layer modeling approach well predicts the skin-friction coefficient, the log-law mean velocity, the roughness function, and the Reynolds shear stress. [ABSTRACT FROM AUTHOR]

The article focuses on an artificial dissipation model that preserves vorticity for use in predictions of vortical wakes. It states that when vorticities are shed from a bluff body, they often form several streamwise vortical flows which can last for long distances downstream. It mentions that computational fluid dynamics has difficulty in accurately predicting these vortical flows as the computed vortices usually dissipate prematurely. It comments on the use of large eddy simulations (LES) in tracking vortical flows over long distances, and mentions that LES has an advantage because the smaller eddy viscosity predicted in the wake compared to conventional turbulence model predictions.

The current study explored the mean and turbulent flowfield features of a confined transverse slot jet. The slot jet spans 95%of the full channel spanwise dimension, a geometrical feature found to result in a highly three-dimensional mean flowfield. The transverse slot jet produces a recirculation bubble that has similarities to that found for the flow downstream of a rearward-facing step. The flowfield results were compared with a 2:1 expansion ratio step flow and the observations were discussed in the context of how the transverse slot jet may provide advantages compared with a sudden expansion for subsonic combustors. High turbulence levels are achieved and large turbulent length scales are produced for strong transverse slot jets. The momentum ratio of the jet to that of the channel is found to be a governing parameter, and the dimensions of the recirculation zone scale with this parameter. A series of models constructed in ANSYS/CFX-10 were done to complement the experimental work and showed that the three-dimensionality of the mean flow disappears when the slot jet extends fully across the channel. [ABSTRACT FROM AUTHOR]

Results of computations of aerodynamic flows with attached and separated boundary layers with adverse pressure gradients are presented. The objective is to compare the predictive accuracy of 11 eddy-viscosity turbulence models frequently employed for computing aeronautical flows. The turbulence models considered are the algebraic model of Baldwin and Lomax; the one-equation model of Spalart and Allmaras along with two additional variants of it; six different k, ω models; and the nonlinear eddy-viscosity model of Wallin and Johansson. Starting with a boundary-layer flow with a nominally zero pressure gradient, flows with a successively increased adverse pressure gradient are investigated. Computational results are compared with each other and to experimental data. It is found that the agreement between computational and experimental results varies between the turbulence models also for the simpler flow cases. Predictive accuracy obtained with a given model for a given flow case varies between the different flow variables evaluated. The "best" turbulence model changes from flow case to flow case. Models using transport equations for the eddy viscosity (or a related quantity) typically show better response to a pressure gradient than the algebraic Baldwin-Lomax model. However, no general conclusion on which model to use for adverse-pressure-gradient flows is drawn. [ABSTRACT FROM AUTHOR]

When turbulence models are used in the simulation of a flow for which experimental results do not exist, there is, as yet, no reliable procedure for choosing a model or for quantifying the uncertainty of the results. In fact, even if experimental data are available, there appears to be no general procedure to quantify uncertainty in simulations. The present study explores the potential of employing the Dempster-Shafer theory of evidence to address these issues and airfoil proposes an approach to quantify uncertainty in turbulent flow simulations. The approach is tested using a subsonic flow around the RAE 2822 airfoil. [ABSTRACT FROM AUTHOR]