John A. Benek

Microramps Upstream of an Oblique-Shock/Boundary-Layer Interaction

To examine the potential of micro vortex generators for shock/boundary-layer interaction control, a detailed experimental and computational study in a supersonic boundary layer at M = 3.0 was undertaken. The experiments employed a flat-plate boundary layer with an impinging oblique shock with downstream total-pressure measurements. The moderate Reynolds number of 3800 allowed the computations to use monotone-integrated large eddy simulations. The monotone-integrated large eddy simulations predictions indicated that the shock changes the structure of the turbulent eddies and the primary vortices generated from the microramp. Furthermore, they generally reproduced the experimentally obtained mean velocity profiles, unlike similarly resolved Reynolds-averaged Navier-Stokes computations. The experiments and monotone-integrated large eddy simulations results indicate that the microramps, for which the height is h ≈ 0.5δ, can significantly reduce boundary-layer thickness and improve downstream boundary-layer health as measured by the incompressible shape function H. Regions directly behind the ramp centerline tended to have increased boundary-layer thickness, indicating the significant three-dimensionality of the flowfield. Compared with baseline sizes, smaller microramps yielded improved total-pressure recovery. Moving the smaller ramps closer to the shock interaction also reduced the displacement thickness and the separated area. This effect is attributed to decreased wave drag and the closer proximity of the vortex pairs to the wall.

Assessment of Computational Fluid Dynamics and Experimental Data for Shock Boundary-Layer Interactions

A workshop on the computational fluid dynamics (CFD) prediction of shock boundary-layer interactions (SBLIs) was held at the 48th AIAA Aerospace Sciences Meeting. As part of the workshop, numerous CFD analysts submitted solutions to four experimentally measured SBLIs. This paper describes the assessment of the CFD predictions. The assessment includes an uncertainty analysis of the experimental data, the definition of an error metric, and the application of that metric to the CFD solutions. The CFD solutions provided very similar levels of error and, in general, it was difficult to discern clear trends in the data. For the Reynolds-averaged Navier-Stokes (RANS) methods, the choice of turbulence model appeared to be the largest factor in solution accuracy. Scale-resolving methods, such as large-eddy simulation (LES), hybrid RANS/LES, and direct numerical simulation, produced error levels similar to RANS methods but provided superior predictions of normal stresses. Copyright

Validation Issues for Engine-Airframe Integration

The engine-airframe integration methodology is reviewed and some process deficiencies are identified. The use of computational fluid dynamics as an alternative simulation source is discussed. An approach toward combining computations with the existing integration methodology is suggested. Validation issues associated with the computational procedures are examined. Computational fluid dynamics as a source of test facility corrections is found to be well developed, but validation efforts remain ad hoc. The use of computational fluid dynamics to evaluate inlet drag increments and to generate high-resolution total pressure maps is discussed. Related validation issues include the simulation of unsteady flow with steady-state approximations.

The effect of wind tunnel size on incident shock boundary layer in-teraction experiments

We examine the effects of varying the tunnel width to height ratio on the shock boundary layer interac-tion of an incident oblique shock with a turbulent boundary layer. The computational domain is a simpli-fied representation of typical wind tunnel experiments; the top wall of the tunnel is not modeled; only the flow conditions imposed by the shock are modeled on the top of the computational domain. A hy-pothesis of the expected effect of width to height ratio is presented and tested computationally. All flows are found to be three dimensional for the single shock strength range of width to height ratios considered. The effect of tunnel width is a function of the boundary layer thickness which decreases the effective width.

Discontinuous Galerkin Scheme Applied to Chimera Overset Viscous Meshes on Curved Geometries

The Chimera overset method is a powerful technique for modeling uid ow associated with complex engineering problems. The use of structured meshes has enabled engineers to develop a number of high-order schemes, such as the WENO and compact di erencing schemes. This paper demonstrates a Discontinuous Galerkin (DG) scheme with a Chimera overset method applied to viscous meshes on curved geometries. The small stencil of the DG scheme makes it particularly suitable for Chimera meshes. The small stencil simpli es the hole cutting and partitioning of grids that contain holes. In addition, because the DG scheme represents the solution as cell local polynomials, it does not require an interpolation scheme with a large stencil to establish the inter-grid communication in overlapping regions. Furthermore, the DG scheme is capable of using curved cells to represent geometric features. The curved cells resolve issues associated with linear non-co-located Chimera viscous meshes used for nite volume and nite di erence schemes. The DG-Chimera method is demonstrated on a set of viscous Chimera meshes, which would produce erroneous results for a nite volume or nite di erence scheme without corrections to the interpolation.

Extending the Discontinuous Galerkin Scheme to the Chimera Overset Method

The Chimera overset method is a powerful technique for modeling uid ow associated with complex engineering problems. The use of structured meshes has enabled engineers to develop a number of high-order schemes, such as the WENO and compact di erencing schemes. This paper demonstrates a methodology for using the Discontinuous Galerkin (DG) scheme with Chimera overset meshes. The small stencil of the DG scheme makes it particularly suitable for Chimera meshes as it simpli es the inter-grid communication scheme as well as hole cutting procedures for parallel computing. In addition, because the DG scheme represents the solution as cell local polynomials, the DG-Chimera scheme does not require a donor interpolation method with a large stencil. The DG-Chimera method also does not require the use of fringe points to maintain the interior stencil across inter-grid boundaries. Thus, inter-grid communication can be established so long as the receiving boundary is enclosed by or abuts to the donor mesh. This makes the inter-grid communication procedure applicable to both Chimera and zonal meshes. Details of DGChimera scheme are presented, and the method is demonstrated on a set of inviscid ow problems.

The effect of wind tunnel size and shock strength on incident shock boundary layer interaction experiments

We computationally examine the effects of varying the tunnel width to height ratio, the incident shock strength, Mach number, and boundary thickness on the shock boundary layer interaction of an incident oblique shock with a turbulent boundary layer. The computational domain is a simplified representation of typical wind tunnel experiments; the top wall of the tunnel is not modeled; only the flow conditions imposed by the shock are modeled on the top of the computational domain. We compare the sizes of the flow separation zone on the tunnel centerline resulting from the variation of the tunnel sizes and shock strengths with a conjecture of the expected effect of width to height ratio. The flows are found to be three dimensional for the range of shock strengths and width to height ratios considered. The effect of tunnel width on the separation is a function of increasing boundary layer thickness which decreases the effective width. We show that the effect of shock strength and Mach number on separation length can be collapsed into a single curve by proper scaling.

A Discontinuous Galerkin Chimera Scheme with Implicit Artificial Boundaries

The Chimera overset methods use of structured meshes has enabled engineers to utilize a number of high-order schemes, such as the WENO and compact di erencing schemes, for modeling uid ow associated with complex engineering problems. However, the large stencil of these high-order schemes leads to complications with establishing arti cial intergrid communication boundaries. This paper demonstrates a Discontinuous Galerkin (DG) scheme with a Chimera overset method that utilizes implicit arti cial boundaries to accelerate the convergence rate to a steady solution and reduces execution time for implicit time accurate calculations. The small stencil of the DG scheme makes it particularly suitable for Chimera meshes. The small stencil simpli es the hole cutting and partitioning of grids that contain holes. In addition, because the DG scheme represents the solution as cell local polynomials, it does not require an interpolation scheme with a large stencil to establish the inter-grid communication in overlapping regions. The convergence rate of the Chimera schemes is dramatically increased by including the linearization of the inter-grid communication in comparison with traditional Quasi-Newton Chimera schemes where only the right hand side vector is updated through the inter-grid communication.

Hole Cutting of Curved Discontinuous Galerkin Chimera Overset Meshes using a Direct Cut Method

The Chimera overset method has enabled the use of high-order nite di erence and nite volume approaches such as WENO and compact di erencing schemes, which require structured meshes for modeling uid ow associated with complex geometries. The exibility of the Chimera scheme is due, in large measure, to its ability to exclude regions of grids that lie outside the computational domain. The exclusion process is commonly known as hole cutting. However, the large stencils required by high-order nite di erence and nite volume schemes complicate the process of generating holes. Unlike the high-order nite di erence and nite volume schemes, the Discontinuous Galerkin (DG) method always retains a stencil that involves only the immediate neighbors of a given cell regardless of the order of approximation. The small stencil implies hole cutting can be performed without regard to maintaining a minimum stencil, which greatly simpli es the hole cutting process. However, DG schemes also require the use of curved cells to correctly represent curved geometries. This paper presents a form of the Direct Cut hole cutting method that is generalized for curved cells.

On the Effect of Test Section Aspect Ratio for Shock Wave - Boundary Layer Interactions

© 2017 by the American Institute of Aeronautics and Astronautics, Inc. Many test sections used to study shock wave boundary layer interaction (SBLI) phenomena employ rectangular geometries which exhibit various levels of three-dimensionality in the generated shock structure. Previous studies have suggested the three-dimensional nature to be heavily influenced by the compression fans generated by the corner separation. It is hypothesized test section geometry has a large influence on the flowfield due to the role it plays on how the compression fans will affect the SBLI. This study uses a Reynolds-averaged Navier-Stokes flow solver to investigate the effect of test section aspect ratio (width/height) for a normal SBLI at Mach 1.6. Geometries include an aspect ratio of 4.3 (10.635” x 2.462”), and two 2 aspect ratios (4.924” x 2.462” and 10.635” x 5.3175”). Analysis is focused on how the test section height influences the compression fans generated by corner flow blockage. Two flow states governed by a viscous height ratio (δ* /H) are used to describe the corner shock system. Additionally, a Buckingham Pi analysis was performed using simulation data of oblique SBLIs to determine physics-based functional relationships relating geometric and fluid dynamic parameters important to SBLIs. This paper provides insight on geometric influences on SBLIs and serves as a reference for SBLI test designs.