Boundary-layer Flows Past an Hemispherical Roughness Element: DNS, Global Stability and Sensitivity Analysis

We investigate the full three-dimensional instability mechanism arising in the wake of an hemispherical roughness element immersed in a laminar Blasius boundary layer. The inherent three-dimensional flow pattern beyond the critical Reynolds number is characterized by coherent vortical structures called hairpin vortices. Direct numerical simulation is used to analyze the formation and the shedding of hairpin packets inside the shear layer. The first bifurcation characteristics are investigated by global stability tools. We show the spatial structure of the linear direct and adjoint global eigenmodes of the linearized Navier-Stokes operator and use structural sensitivity analysis to locate the region where the instability mechanism acts. Results show that the “wavemaker” driving the self-sustained instability is located in the region immediately past the roughness element, in the shear layer separating the outer flow from the wake region

Effects of Cavities and Protuberances on Transition over Hypersonic Vehicles

Surface protuberances and cavities on a hypersonic vehicle are known to cause several aerodynamic or aerothermodynamic issues. Most important of all, premature transition due to these surface irregularities can lead to a significant rise in surface heating. To help understand laminar-turbulent transition induced by protuberances or cavities on a Crew Exploration Vehicle (CEV) surface, high-fidelity numerical simulations are carried out for both types of trips on a CEV wind tunnel model. Due to the large bluntness, these surface irregularities reside in an accelerating subsonic boundary layer. For the Mach 6 wind tunnel conditions with a roughness Reynolds number Re(sub kk) of 800, it was found that a protuberance with a height to boundary layer thickness ratio of 0.73 leads to strong wake instability and spontaneous vortex shedding, while a cavity with identical geometry only causes a rather weak flow unsteadiness. The same cavity with a larger Reynolds number also leads to similar spontaneous vortex shedding and wake instability. The wake development and the formation of hairpin vortices for both protuberance and cavity were found to be qualitatively similar to that observed for an isolated hemisphere submerged in a subsonic, low speed flat-plate boundary layer. However, the shed vortices and their accompanying instability waves were found to be slightly stabilized downstream by the accelerating boundary layer along the CEV surface. Despite this stabilizing influence, it was found that the wake instability spreads substantially in both wall-normal and azimuthal directions as the flow is evolving towards a transitional state. Similarities and differences between the wake instability behind a protuberance and a cavity are investigated. Computations for the Mach 6 boundary layer over a slender cylindrical roughness element with a height to the boundary layer thickness of about 1.1 also shows spontaneous vortex shedding and strong wake instability. Comparisons of detailed flow structures associated with protuberances at subsonic and supersonic edge Mach numbers indicate distinctively different instability mechanisms

Numerical Simulation of Flow over a Surface-mounted Cube with the Vorticity Confinement Method

Over the last several years, Vorticity Confinement has been shown to be a very efficient method to simulate the vortex-dominated flows over complex configurations. To calculate these flows, no high-order numerical scheme and body conforming grids are required for this method and only a fixed, uniform Cartesian grid is employed. First, an overall description of the original Vorticity Confinement method (VC1) is presented, followed by an introduction of the newly developed Vorticity Confinement method (VC2). The advantage of VC2 over VC1 is the ability to conserve the Momentum. Two different numerical schemes are shown for VC1 and VC2. The one for VC2 is much simpler than that of VC1. Results of VC1 and VC2 for convecting vortices and scalars in 1-D and 2-D will be presented. Numerical results are presented for the three dimensional flow over a surface-mounted cube. Comparisons have been made with experimental and Large Eddy Simulation (LES) data. It is observed that with a coarse uniform Cartesian grid, Vorticity Confinement is able to get results in better agreement with the experimental results than the LES simulation results on a fine grid. This method is shown to be more effective than trying to model and discretize more complex system of equations in the traditional methods, when solving complex high Reynolds number flow problems

NO PLIF Study of Hypersonic Transition Over a Discrete Hemispherical Roughness Element

Nitric oxide (NO) planar laser-induced fluorescence (PLIF) has been use to investigate the hypersonic flow over a flat plate with and without a 2-mm (0.08-in) radius hemispherical trip. In the absence of the trip, for all angles of attack and two different Reynolds numbers, the flow was observed to be laminar and mostly steady. Boundary layer thicknesses based on the observed PLIF intensity were measured and compared with a CFD computation, showing agreement. The PLIF boundary layer thickness remained constant while the NO flowrate was varied by a factor of 3, indicating non-perturbative seeding of NO. With the hemispherical trip in place, the flow was observed to be laminar but unsteady at the shallowest angle of attack and lowest Reynolds number and appeared vigorously turbulent at the steepest angle of attack and highest Reynolds number. Laminar corkscrew-shaped vortices oriented in the streamwise direction were frequently observed to transition the flow to more turbulent structures

Hypersonic Viscous Flow Over Large Roughness Elements

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

Progress Towards Petascale Applications in Biology: Status in 2006

Petascale computing is currently a common topic of discussion in the high performance computing community. Biological applications, particularly protein folding, are often given as examples of the need for petascale computing. There are at present biological applications that scale to execution rates of approximately 55 teraflops on a special-purpose supercomputer and 2.2 teraflops on a general-purpose supercomputer. In comparison, Qbox, a molecular dynamics code used to model metals, has an achieved performance of 207.3 teraflops. It may be useful to increase the extent to which operation rates and total calculations are reported in discussion of biological applications, and use total operations (integer and floating point combined) rather than (or in addition to) floating point operations as the unit of measure. Increased reporting of such metrics will enable better tracking of progress as the research community strives for the insights that will be enabled by petascale computing.This research was supported in part by the Indiana Genomics Initiative and the Indiana Metabolomics and Cytomics Initiative. The Indiana Genomics Initiative of Indiana University and the Indiana Metabolomics and Cytomics Initiative of Indiana University are supported in part by Lilly Endowment, Inc. The authors also wish to thank IBM, Inc. for support via Shared University Research Grants and partnerships via IU’s relationship as an IBM Life Sciences Institute of Innovation. Indiana University also thanks the TeraGrid partners; IU’s participation in the TeraGrid is funded by National Science Foundation grant numbers 0338618, 0504075, and 0451237. The early development of this paper was supported by a Fulbright Senior Scholars award from the Council for International Exchange of Scholars (CIES) and the United States Department of State to Dr. Craig A. Stewart; Matthias Mueller and the Technische Universität Dresden were hosts. Many reviewers contributed to the improvement of the ideas expressed in this paper and are gratefully appreciated; Thom Dunning, Robert Germain, Chris Mueller, Jim Phillips, Richard Repasky, Ralph Roskies, and Allan Snavely are thanked particularly for their insights

Procedures for the identification of coherent structures in oscillatory and pulsating flows over a wavy bottom

Oscillatory flows over a wavy bottom play an important role in sediment dynamics, coastal circulation and bottom - water column biogeochemical interactions. The most important process that controls the energy and mass flux in such flows is the generation, advection and dissipation of coherent structures. This thesis gives a working definition of a coherent structure in an instantaneous and averaged flow and compares several identification methods using velocity and pressure fields obtained from computer simulations using a Large Eddy Simulation (LES) numerical scheme. The effectiveness of each identification method is assessed for several flows with different Reynolds number and relevant physical properties of each flow, obtained from the study of such coherent structures, are described

Diseño de un prototipo de sistema de realidad virtual inmersivo simplificado

This paper introduces the development of a low cost prototype of a virtual immersion system with two screens. The main implementation details are presented, so that the advantages and drawbacks from many physic assembly architectures. Three prototypes were constructed, one table system and the other two at real size. Thus, different tests were implemented with animations, which were created using a 3D modeling and animation software.Este artículo presenta el diseño y construcción de un prototipo de bajo costo de un sistema inmersión virtual de dos pantallas. Se describen los detalles más importantes de la implementación y se muestran las ventajas y problemas de diferentes arquitecturas posibles para el montaje físico. Se construyeron tres prototipos, uno de mesa y dos de tamaño real, sobre los cuales se realizaron diferentes pruebas con animaciones creadas en un software de modelado y animación 3D