Browsing by Subject "Hypersonic"

Now showing 1 - 6 of 6
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    Computational study of hypersonic boundary layer stability on cones
    (2012-12) Gronvall, Joel Edwin
    Due to the complex nature of boundary layer laminar-turbulent transition in hypersonic flows and the resultant effect on the design of re-entry vehicles, there remains considerable interest in developing a deeper understanding of the underlying physics. To that end, the use of experimental observations and computational analysis in a complementary manner will provide the greatest insights. It is the intent of this work to provide such an analysis for two ongoing experimental investigations. . The first focuses on the hypersonic boundary layer transition experiments for a slender cone that are being conducted at JAXA’s free-piston shock tunnel HIEST facility. Of particular interest are the measurements of disturbance frequencies associated with transition at high enthalpies. The computational analysis provided for these cases included two-dimensional CFD mean flow solutions for use in boundary layer stability analyses. The disturbances in the boundary layer were calculated using the linear parabolized stability equations. Estimates for transition locations, comparisons of measured disturbance frequencies and computed frequencies, and a determination of the type of disturbances present were made. It was found that for the cases where the disturbances were measured at locations where the flow was still laminar but nearly transitional, that the highly amplified disturbances showed reasonable agreement with the computations. Additionally, an investigation of the effects of finite-rate chemistry and vibrational excitation on flows over cones was conducted for a set of theoretical operational conditions at the HIEST facility. . The second study focuses on transition in three-dimensional hypersonic boundary layers, and for this the cone at angle of attack experiments being conducted at the Boeing/AFOSR Mach-6 quiet tunnel at Purdue University were examined. Specifically, the effect of surface roughness on the development of the stationary crossflow instability are investigated in this work. One standard mean flow solution and two direct numerical simulations of a slender cone at an angle of attack were computed. The direct numerical simulations included a digitally-filtered, randomly distributed surface roughness and were performed using a high-order, low-dissipation numerical scheme on appropriately resolved grids. Comparisons with experimental observations showed excellent qualitative agreement. Comparisons with similar previous computational work were also made and showed agreement in the wavenumber range of the most unstable crossflow modes.
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    Consistent modeling of hypersonic nonequilibrium flows using direct simulation Monte Carlo
    (2013-08) Zhang, Chonglin
    Hypersonic flows involve strong thermal and chemical nonequilibrium due to steep gradients in gas properties in the shock layer, wake, and next to vehicle surfaces. Accurate simulation of hypersonic nonequilibrium flows requires consideration of the molecular nature of the gas including internal energy excitation (translational, rotational, and vibrational energy modes) as well as chemical reaction processes such as dissociation. Both continuum and particle simulation methods are available to simulate such complex flow phenomena. Specifically, the direct simulation Monte Carlo (DSMC) method is widely used to model such complex nonequilibrium phenomena within a particle-based numerical method. This thesis describes in detail how the different types of DSMC thermochemical models should be implemented in a rigorous and consistent manner. In the process, new algorithms are developed including a new framework for phenomenological models able to incorporate results from computational chemistry. Using this framework, a new DSMC model for rotational energy exchange is constructed. General algorithms are developed for the various types of methods that inherently satisfy microscopic reversibility, detailed balance, and equipartition of energy in equilibrium. Furthermore, a new framework for developing rovibrational state-to-state DSMC collision models is proposed, and a vibrational state-to-state model is developed along the course. The overall result of this thesis is a rigorous and consistent approach to bridge molecular physics and computational chemistry through stochastic molecular simulation to continuum models for gases in strong thermochemical nonequilibrium.
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    Geometric effects on the amplification of first mode instability waves
    (2013-05) Kirk, Lindsay Christine
    The effects of geometric changes on the amplification of first mode instability waves in an external supersonic boundary layer were investigated using numerical techniques. Boundary layer stability was analyzed at Mach 6 conditions similar to freestream conditions obtained in quiet ground test facilities so that results obtained in this study may be applied to future test article design to measure first mode instability waves. First, geometric parameters such as nose radius, cone half angle, vehicle length, and surface curvature for an axisymmetric cone geometry were examined separately to determine the individual effects on the first mode amplification. The DAKOTA optimization software package was then used to optimize the geometry to maximize the amplification of waves at first mode frequencies and to minimize the amplification of the waves at second mode frequencies, as computed by the 2D STABL hypersonic boundary layer stability analysis tool. This was accomplished by allowing all geometric parameters in the sensitivity study to vary to produce a shape optimized to maximize the amplification of first mode instability waves while minimizing the amplification of second mode instability waves. During this process, boundary layer edge properties were recorded to investigate any correlations. Results of the sensitivity analysis indicate that an axisymmetric cone with a sharp nose or an axisymmetric cone with a high degree of concave curvature under the Mach 6 freestream conditions used here will cause the largest amplification of first mode instability waves.
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    Hypersonic Boundary Layer Stability Analysis Using Momentum Potential Theory
    (2020-09) Houston, Mary
    Linear Stability Theory (LST) and the Parabolized Stability Equations (PSE) have provided valuable tools for analysis and prediction of laminar to turbulent transition for plates, sharp cones, and geometries for which parallel-flow or a slowly-varying boundary layer can be assumed. However, these techniques struggle to capture the complex flow-physics present near the tip of blunt-cones. Input-output analysis has been used in conjunction with direct numerical simulation to capture the effects of nose bluntness on downstream stability. Using the results of the input-output analysis we apply momentum potential theory (MPT) to preform fluid-thermodynamic (FT) decomposition, separating disturbances into their vortical, thermal and acoustic components. A reference case of Mach 6 flow over a flat-plate is computed and output responses are compared to the results for Mach 6 flow over a blunt-cone of $7^{o}$ half angle. Perturbation eigenfunctions and structures are examined in the areas of second-mode amplification. For both the flat-plate and blunt-cone the vortical components are the largest, followed by the thermal then acoustic components. Fluid-thermodynamic structures in the second-mode amplification region of blunt-cone show wall-normal stretching above the critical layer. Fluid-thermodynamic decomposition of full-domain input and output results for the blunt-cone geometry are considered. It is found that input sensitivity is highest at the top of the entropy layer and along the boundary layer edge for the fore-half of the cone. Output response in the streamwise direction is highest in the regions between the generalized inflection point (GIP) and the boundary layer edge and dissipates near the surface, whereas wall-normal response extends to the surface and shows a local minimum between the GIP and boundary layer edge. To compliment existing studies on hypersonic boundary layer response to surface roughness/ vibration we look at input sensitivity and output response at the surface. It is found that there is greater sensitivity to wall-normal forcing than streamwise forcing at the surface and among the three FT components in this direction the vortical had the highest relative output amplitude. Finally, total fluctuating enthalpy (TFE) is considered for both the flat-plate and blunt-cone, in both cases the thermal terms provides the strongest source of TFE.
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    Modeling and simulation of high-speed wake flows.
    (2009-08) Barnhardt, Michael Daniel
    High-speed, unsteady flows represent a unique challenge in computational hypersonics research. They are found in nearly all applications of interest, including the wakes of reentry vehicles, RCS jet interactions, and scramjet combustors. In each of these examples, accurate modeling of the flow dynamics plays a critical role in design performance. Nevertheless, literature surveys reveal that very little modern research effort has been made toward understanding these problems. The objective of this work is to synthesize current computational methods for high-speed flows with ideas commonly used to model low-speed, turbulent flows in order to create a framework by which we may reliably predict unsteady, hypersonic flows. We wish to validate the new methodology for the case of a turbulent wake flow at reentry conditions. Currently, heat shield designs incur significant mass penalties due to the large margins applied to vehicle afterbodies in lieu of a thorough understanding of the wake aerothermodynamics. Comprehensive validation studies are required to accurately quantify these modeling uncertainties. To this end, we select three candidate experiments against which we evaluate the accuracy of our methodology. The first set of experiments concern the Mars Science Laboratory (MSL) parachute system and serve to demonstrate that our implementation produces results consistent with prior studies at supersonic conditions. Second, we use the Reentry-F flight test to expand the application envelope to realistic flight conditions. Finally, in the last set of experiments, we examine a spherical capsule wind tunnel configuration in order to perform a more detailed analysis of a realistic flight geometry. In each case, we find that current 1st order in time, 2nd order in space upwind numerical methods are sufficiently accurate to predict statistical measurements: mean, RMS, standard deviation, and so forth. Further potential gains in numerical accuracy are demonstrated using a new class of flux evaluation schemes in combination with 2nd order dual-time stepping. For cases with transitional or turbulent Reynolds numbers, we show that the detached eddy simulation (DES) method holds clear advantage over heritage RANS methods. From this, we conclude that the current methodology is sufficient to predict heating of external, reentry-type applications within experimental uncertainty.
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    Simulations of injection, mixing, and combustion in supersonic flow using a hybrid RANS/LES approach.
    (2011-09) Peterson, David Michael
    There is a great need for accurate and reliable numerical simulation of injection, mixing, and combustion in supersonic combustion ramjet engines. This study seeks to improve the accuracy and reliability which these flow can be simulated with by investigating the use of recent improvements in turbulence modeling and numerical methods. The present numerical simulations use implicit time integration and low-dissipation flux evaluation schemes in an unstructured grid framework. A hybrid Reynolds-Averaged Navier-Stokes and large-eddy simulation approach is used to model turbulence. The large-scale turbulent structure of the flow is resolved, while the near-wall structure is fully modeled. The effects of numerics, grid resolution, and boundary conditions are investigated. The simulation approach is thoroughly validated against available experimental data at a variety of flow conditions. The simulations focus on the injection of fuel through circular injector ports that are oriented either normal to the supersonic crossflow, or at a low angle with respect to the crossflow. The instantaneous flow structure resolved by the simulations is qualitatively compared to experimental flowfield visualization. Quantitative comparisons are made to mean wall pressure, mean velocity, turbulence quantities, and mean mixing data. The simulations are found to do very well at predicting the mean flowfield as well as fluctuations in velocity and injectant concentration. The simulation approach is then used to simulate the flow within a model supersonic combustor. The focus is on the non-reacting case. The simulation results are found to agree well with experimental measurements of temperature and species concentrations. The flow is examined to improve understanding of the mixing within the model combustor. Preliminary results for a simulation including hydrogen combustion are also presented.