James R. Forsythe

Detached-Eddy Simulation With Compressibility Corrections Applied to a Supersonic Axisymmetric Base Flow

Detached-eddy simulation is applied to an axisymmetric base flow at supersonic conditions. Detached-eddy simulation is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes and large-eddy simulation approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras turbulence model or Menter’s shear stress transport model; in the largeeddy simulation mode, it is based on the Smagorinski subgrid scale model. The intended application of detached-eddy simulation is the treatment of massively separated, highReynolds number flows over complex configurations (entire aircraft, automobiles, etc.). Because of the intented future application of the methods to complex configurations, Cobalt, an unstructured grid Navier-Stokes solver, is used. The current work incorporates compressible shear layer corrections in both the Spalart-Allmaras and shear stress transport-based detached-eddy simulation models. The effect of these corrections on both detached-eddy simulation and Reynolds-averaged Navier-Stokes models is examined, and comparisons are made to the experiments of Herrin and Dutton. Solutions are obtained on several grids—both structured and unstructured—to test the sensitivity of the models and code to grid refinement and grid type. The results show that predictions of base flows using detached-eddy simulation compare very well with available experimental data, including turbulence quantities in the wake of the axisymmetric body. @DOI: 10.1115/1.1517572#

Detached-Eddy Simulation of the F-15E at High Alpha

Detached-eddy simulation (DES) is used to predict the massively separated flow around an F-15E at 65-deg angle of attack. The calculations are performed at flight conditions corresponding to a chord-based Reynolds number of 13.6 × 10 6 and Mach number of 0.3. Flowfield solutions are obtained using unstructured grids with the commercial solver Cobalt: the average mesh spacing from solid surfaces to the first cell center from the wall under one viscous unit. The influence of the mesh size is assessed using a series of three grids ranging from 2.85 × 10 6 cells to 10 × 10 6 cells. In addition, the influence of the time step is investigated using three simulations with varied time steps. DES predictions are assessed via comparison to Boeings stability and control database as well as to solutions of the Reynolds-averaged Navies-Stokes (RANG) equations. These are steady, with the Spalart-Allmaras model. Both RANS and DES predictions of integrated forces exhibit a relatively weak dependence on the grid density for the range examined. In the DES the wake region is characterized by complex and chaotic three-dimensional structures with direct resolution of a reasonable range of length and timescales

Computational Challenges in High Angle of Attack Flow Prediction

Abstract Aircraft aerodynamics have been predicted using computational fluid dynamics for a number of years. While viscous flow computations for cruise conditions have become commonplace, the non-linear effects that take place at high angles of attack are much more difficult to predict. A variety of difficulties arise when performing these computations, including challenges in properly modeling turbulence and transition for vortical and massively separated flows, the need to use appropriate numerical algorithms if flow asymmetry is possible, and the difficulties in creating grids that allow for accurate simulation of the flowfield. These issues are addressed and recommendations are made for further improvements in high angle of attack flow prediction. Current predictive capabilities for high angle of attack flows are reviewed, and solutions based on hybrid turbulence models are presented.

Analysis of Delta-Wing Vortical Substructures Using Detached-Eddy Simulation

An understanding of the vortical structures that comprise the vortical flowfield around slender bodies is essential for the development of highly maneuverable and high-angle-of-attack flight. This is primarily because of the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current computational fluid dynamics methods in the high-Reynolds-number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal here to demonstrate the ability of detached-eddy simulation (DES), a hybrid Reynolds-averaged Navier-Stokes/large-eddy-simulation method, to accurately model the vortical flowfield over a slender delta wing at Reynolds numbers above one million. DES has successfully predicted the location of the vortex breakdown phenomenon, and the goal of the current effort is to analyze and assess the influence of vortical substructures in the separating shear layers that roll up to form the leading-edge vortices. Very detailed experiments performed at ONERA using three-dimensional laser-Doppler-velocimetry measurement will be used to compare simulations utilizing DES turbulence models. The computational results provide novel insight into the formation and impact of the vortical substructures in the separating shear layers on the entire vertical flowfield.

Detached-Eddy Simulations and Reynolds-Averaged Navier-Stokes Simulations of Delta Wing Vortical Flowfields

An understanding of vortical structures and vortex breakdown is essential for the development of highly maneuverable vehicles and high angle of attack flight. This is primarily due to the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current CFD methods in the high Reynolds number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal of this paper to demonstrate the ability of detached-eddy Simulation (DES), a hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy Simulation (LES) method, to accurately predict vortex breakdown at Reynolds numbers above 1 ×10 6 . Detailed experiments performed at Onera are used to compare simulations utilizing both RANS and DES turbulence models

Unsteady Computations of Abrupt Wing Stall Using Detached-Eddy Simulation

Unsteady computational fluid dynamics calculations are presented of the abrupt wing stall phenomenon on the preproduction F/A-18E using detached-eddy simulation. Detached-eddy simulation combines the efficiency of a Reynolds-averaged turbulence model near the wall with the fidelity of large-eddy simulation in separated regions. Because it uses large-eddy simulation in the separated regions, it is capable of predicting the unsteady motions associated with separated flows. Detached-Eddy Simulation has been applied to predict the unsteady shock motion present on the F/A-18E at transonic speeds over several angles of attack. Solution-based grid adaption is used on unstructured grids to improve the resolution in the separated region

Numerical investigation of flow past a prolate spheroid

The flowfield around a 6:1 prolate spheroid at angle of attack is predicted using solutions of the Reynolds-averaged Navier-Stokes (RANS) equations and detached-eddy simulation (DES). The calculations were performed at a Reynolds number of 4.2×10 6 , the flow is tripped at x/L=0.2, and the angle of attack a is varied from 10 to 20 deg. RANS calculations are performed using the Spalart-Allmaras one-equation model. The influence of corrections to the Spalart-Allmaras model accounting for streamline curvature and a nonlinear constitutive relation are also considered. DES predictions are evaluated against experimental measurements, RANS results, as well as calculations performed without an explicit turbulence model. In general, flowfield predictions of the mean properties from the RANS and DES are similar

DES GRID RESOLUTION ISSUES FOR VORTICAL FLOWS ON A DELTA WING AND AN F-18C

An assessment of unstructured grids for use in Detached-Eddy Simulations (DES) of vortical flowfields over two configurations, a 70 degree delta wing and an F-18C are presented. The role of the grid in detached eddy simulations of vortical flowfields, including complex features such as vortex breakdown, is assessed on a delta wing with comparison to wind tunnel data. Adaptive mesh refinement is applied to the delta wing grid to improve the focus region aft of the vortex breakdown where massively separated flow exists and unsteady pressures are generated that could impact the loads on vertical tails of more complex configurations. The adaptively refined mesh is compared to the baseline mesh to determine the advantage of the adaptive mesh refinement approach for vortex breakdown. The focus region grid resolution is then applied to an F-18C in the region of the vortex generat ed from the leading edge extension (LEX). The resulting unsteady tail loads are compared to flight test data from the NASA F-18 HARV database. This paper represents one of the first times adaptive mesh refinement will be applied to a detached eddy simulation of a flight vehicle configuration. INTRODUCTION Many of todays military vehicles exhibit vortex dominated flowfields. At a recent NATO Air Vehicle Technology conference, D. A. Lovell presented a review of “Military Vortices,” where he discussed the declining research budget in this area and the importance of understanding the phenomena. He classified vortex flows into three categories, “those designed into a vehicle to improve performance, those which cannot be avoided and whose adverse affects must be minimized, and those that were not expected to occur.” He gives examples of many of these vortex dominated flowfields: tip vortices on wings having low sweep, leading edge extension vortices from the F-18 and F-16 aircraft, foreplanes on the Rafale, and flow over the MK-82 bomb, to name just a few. He also discusses the fact that governments are relying ever increasingly on the aerospace industry to perform research. Since the aerospace industry concentrates on cruise conditions for optimization of commercial aircraft, these vortical flowfields common in military aircraft are losing their place in research budgets. This is occurring at a time when the three largest US fighter development programs (F/A18E/F, F-22, and F-35) incorporate twin tail configurations and high angle-of-attack maneuvering. The F-18 High Angle of Attack Research Vehicle (HARV; see Fig. 1) has proven to be an excellent source of data for researchers working on high angle of attack flowfields. Extensive flight testing of the HARV has been conducted that provides a rich source of flow visualization, surface pressures, and aeroelastic information. The F-18 utilizes wing leading edge extensions (LEX) to generate * Associate Professor of Aeronautics, AIAA Associate Fellow. # USAF Academy Undergraduate Student, AIAA Student Member. % Distinguished Visiting Professor, AIAA Associate Fellow. & Associate Professor of Aeronautics, AIAA Senior Member. This paper is declared a work of the US government and is not subject to copyright protection in the United States. vortices which enhance the wing lift, and the twin vertical tails are canted to intercept the strong vortex field and increase maneuverability. At large incidence, the LEX vortices breakdown upstream of the vertical tails, resulting in a loss of yaw control power and severe aeroelastic effects. This tail buffet phenomenon was reduced by using extensive flight tests to design a LEX fence. The ultimate goal of computationally modeling the flowfield shown in Fig. 1 would be to accurately simulate the aeroelastic impact of the LEX vortices on the twin vertical tails. The current level of simulation technology, however, has not allowed for accurate prediction of vortex breakdown, and the unsteady flow downstream of breakdown, at flight Reynolds numbers. Because of this, researchers have used simpler geometries, such as slender forebodies and delta wings, to improve their simulation capabilities. Figure 1 : NASA F-18 High Angle of Attack Research Vehicle (HARV). The delta wing vortex breakdown phenomena has been studied extensively since Henri Werlé first photographed it in 1954, during water tunnel tests of a slender delta wing model at Onera. This work was quickly confirmed by Peckham and Atkinson, Elle and Lambourne and Bryer

Detached-Eddy Simulation of the Separated Flow around a Forebody Cross-Section

Detached-Eddy Simulation (DES) is used to predict the massively separated flow around a forebody cross section. The configuration is the flow at 10° angle of attack over a rounded-corner square. The spanwise coordinate of the flow is statistically homogeneous with periodic end conditions employed in the calculations. Simulations are performed at sub- and super-critical Reynolds numbers for which experimental measurements show a reversal of the lateral (side) force acting on the body. DES predictions are evaluated using experimental measurements and unsteady Reynolds-averaged Navier-Stokes (URANS) results for a modest range of grid refinement, in calculations with a doubling of the spanwise period, and using simulations performed without an explicit turbulence model.

Prediction of Separated Flow Characteristics over a Hump

Predictions of the flow over a wall-mounted hump are obtained using solutions of the Reynolds-averaged Navier‐ Stokes (RANS) equations and detached-eddy simulation (DES). The upstream solution is characterized by a two-dimensional turbulent boundary layer with a thickness approximately half of the maximum hump thickness measured at a location about two chord lengths upstream of the leading edge. The Reynolds number based on the hump chord length is 9.75 × 10 5 . A slot at approximately 65% chord C is used for flow control via a spatially uniform (with respect to the spanwise coordinate) steady suction and with alternating suction/blowing. Solutions of the two- and three-dimensional RANS equations are obtained using the Spalart‐Allmaras (S-A) and shear-stresstransport (SST) turbulence models. DES is applied to a three-dimensional geometry corresponding to an extruded section of the hump. DES predictions of the baseline case exhibit a three-dimensional chaotic structure in the wake, with a mean reverse-flow region that is 20% shorter than predicted by the two-dimensional RANS computations and a mean reattachment length that is in good agreement with measurements. DES predictions of the pressure coefficient in the separated-flow region for the baseline case also exhibit good agreement with measurements and are more accurate than either the S-A or SST RANS results. The simulations also show that blockage effects in the experiments used to assess the predictions are important: three-dimensional RANS predictions more accurately predict the pressure distribution upstream and over the front portion of the hump. Predictions of the steady suction case show a reduction in the length of the reverse-flow region, though are less accurate compared to the baseline configuration. Unsteady two-dimensional RANS predictions of the sinusoidal suction/blowing case are used to investigate impedance affects associated with increases in the driving velocity. The simulations show that a factor of four increase in the cavity driving velocity increases the average velocity through the slot by only a factor of 2.7.