Unstructured Grid Approaches for Accurate Aeroheating Simulations

39th AIAA Fluid Dynamics Conference

Jentink

et al. 2009

Self Cite

The Hypersonic International Flight Research and Experimentation (HIFiRE) 5 flight experiment by Air Force Research Laboratories and Australian Defense Science and Technology Organization is designed to provide in-flight boundary-layer transition data for a canonical 3D configuration at hypersonic Mach numbers. This paper outlines the progress, to date, on boundary layer stability analysis for the HIFiRE-5 flight configuration, as well as for selected test conditions from the wind tunnel experiments supporting the flight test. At flow conditions corresponding to the end of the test window, rather large values of linear amplification factor are predicted for both second mode (N>40) and crossflow (N>20) instabilities, strongly supporting the feasibility of first in-flight measurements of natural transition on a fully three-dimensional hypersonic configuration. Additional results highlight the rich mixture of instability mechanisms relevant to a large segment of the flight trajectory, as well as the effects of angle of attack and yaw angle on the predicted transition fronts for ground facility experiments at Mach 6. Backgroundue to its influence on surface heat transfer, skin-friction drag, and flow-separation characteristics, prediction of boundary layer transition constitutes an important aspect of hypersonic vehicle design. Laminar to turbulent transition over realistic vehicle surfaces is often caused by surface roughness of sufficiently large magnitude. However, when the surface is relatively smooth, the transition process is initiated by linear instabilities of the laminar boundary layer. Typically, the second (or Mack) mode instability dominates transition in 2D and axisymmetric boundary layers with hypersonic edge Mach numbers, although centrifugal (i.e., Gortler) instabilities may also come into play when the surface has concave curvature along the streamwise direction. Three-dimensional boundary layers involve the additional mechanisms of stationary and traveling modes of crossflow instability and, depending on the geometric configuration, may include the attachment line instability as well [1].Regardless of the speed regime, linear stability correlations have been quite successful in predicting the onset of transition when a single instability mechanism dominates the transition process [2]. Mixed mode transition has been more challenging to predict because of possible nonlinear interactions between the relevant modes of instability. To account for the effect of such interactions, an ad hoc composite metric based on the linear amplification factors for the different instability mechanisms has sometimes been used for transition correlation (see, for instance, [3]). Recent work on crossflow instability in low-speed boundary layers [4] has exposed additional shortcomings in applying purely linear predictive models to crossflow dominated transition in 3D boundary layers.The simplest canonical configuration that includes the necessary elements for studying both mixed mode transition and crossflow evolu...

Section: Flow Conditions and Analysis Codesmentioning

confidence: 99%

Transition Analysis for the HIFiRE-5 Vehicle

39th AIAA Fluid Dynamics Conference

Jentink

et al. 2009

Self Cite

“…When applied to multi-dimensional flows, such one-dimensional approximations give rise to undesired numerical problems, especially for triangular or tetrahedral unstructured meshes [5]. For hypersonic applications, the carbuncle phenomenon takes place near the blunt nose region [6] and accurate computation of viscous flows with tetrahedral meshes remains a challenge [7]. Remedies for these numerical issues have been proposed and investigated but in general, lack robustness.…”

Section: Introductionmentioning

confidence: 99%

Numerical Computations of Hypersonic Boundary-Layer over Surface Irregularities

48th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

2010

Surface irregularities such as protuberances inside a hypersonic boundary layer may lead to premature transition on the vehicle surface. Early transition in turn causes large localized surface heating that could damage the thermal protection system. Experimental measurements as well as numerical computations aimed at building a knowledge base for transition Reynolds numbers with respect to different protuberance sizes and locations have been actively pursued in recent years. This paper computationally investigates the unsteady wake development behind large isolated cylindrical roughness elements and the scaled wind-tunnel model of the trip used in a recent flight measurement during the reentry of space shuttle Discovery. An unstructured mesh, compressible flow solver based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for the flow past a roughness element under several wind-tunnel conditions. For a cylindrical roughness element with a height to the boundary-layer thickness ratio from 0.8 to 2.5, the wake flow is characterized by a mushroom-shaped centerline streak and horse-shoe vortices. While time-accurate solutions converged to a steady-state for a ratio of 0.8, strong flow unsteadiness is present for a ratio of 1.3 and 2.5. Instability waves marked by distinct disturbance frequencies were found in the latter two cases. Both the centerline streak and the horse-shoe vortices become unstable downstream. The oscillatory vortices eventually reach an early breakdown stage for the largest roughness element. Spectral analyses in conjunction with the computed root mean square variations suggest that the source of the unsteadiness and instability waves in the wake region may be traced back to possible absolute instability in the front-side separation region.

“…Remedies exist but in general, lack robustness. For hypersonic applications, carbuncle phenomenon takes place near the blunt nose region [2] and accurate computation of viscous flows with tetrahedral meshes remains a challenge [3].…”

Section: Introductionmentioning

confidence: 99%

Hypersonic viscous flow over large roughness elements

Theor. Comput. Fluid Dyn.

2010

Viscous flow over discrete or distributed surface roughness has great implications for hypersonic flight due to aerothermodynamic considerations related to laminar-turbulent transition. Current prediction capability is greatly hampered by the limited knowledge base for such flows. To help fill that gap, numerical computations are used to investigate the intricate flow physics involved.An unstructured mesh, compressible Navier-Stokes code based on the space-time conservation element, solution element (CESE) method is used to perform time-accurate Navier-Stokes calculations for two roughness shapes investigated in wind tunnel experiments at NASA Langley Research Center. It was found through 2D parametric study that at subcritical Reynolds numbers of the boundary layers, absolute instability resulting in vortex shedding downstream, is likely to weaken at supersonic free-stream conditions. On the other hand, convective instability may be the dominant mechanism for supersonic boundary layers. Three-dimensional calculations for a rectangular or cylindrical roughness element at post-shock Mach numbers of 4.1 and 6.5 also confirm that no self-sustained vortex generation is present.