Michael Holden

Effect of Vibrational Nonequilibrium on Hypersonic Double-Cone Experiments

Recent numerical simulations of hypersonic double-cone experiments overpredict the heat-transfer rate to the model by about 20%. We present a systematic analysis of the experimental facility and the physical modeling to explainthisdiscrepancy.Nozzlee owe eldsimulationsareusedtoinvestigatetheeffectofvibrationalnonequilibrium in the test section. These simulations show that the vibrational modes of the nitrogen gas freeze near the nozzle throatconditions, resulting inanelevated vibrationaltemperatureinthetestsection. Thislowersthekineticenergy e ux, reducing the heat transfer to the model. The effect of slip boundary conditions is also studied, and it is shown that weak accommodation of vibrational energy at the surface further reduces the heat-transfer rate to the model. The combination of these two effects brings the predicted heat-transfer rate into agreement with the experiments. In addition, weak e ow nonuniformity in the test section is shown to slightly modify the predicted separation zone, further improving the agreement.

Experimental Shock-Wave Interference Heating on a Cylinder at Mach 6 and 8

The study, which was conducted at Mach numbers of 6.3, 6.5, and 8.0, has provided 1) detailed pressure and heat-transfer-rate distributions for a two-dimensional shock-wave interference on a cylinder and 2) insight into the effects of temperature-dependent specific heats on the phenomena. The peak levels and their gradients increased with Mach number. Variation in specific heats and, hence, in the ratio of specific heats with temperature is manifested in slightly lower loads and amplification factors than for corresponding perfect-gas conditions

Boundary-Layer Stability Analysis of the Hypersonic International Flight Research Transition Experiments

Boundary-layer stability analysis is performed by computational fluid dynamics simulation of experiments conducted in theCalspan–University at BuffaloResearchCenter Large EnergyNational ShockTunnel in support of the first flight of the Hypersonic International Flight Research Experimentation program. From the laminar flow solutions, disturbances are calculated using the linear parabolized stability equations method and instability is quantified by integrating the resulting disturbance growth rates. Comparisons aremade between the experimentally measured transition locations and the results of the parabolized stability equations analysis. The results show that for the cases tested, the e transition correlation works better than the commonly usedRe =Me engineering criterion for predicting the onset of boundary-layer transition from laminar to turbulent flow.

CFD Validation for Hypersonic Flight: Hypersonic Double-Cone Flow Simulations

Abstract : At the 2001 AIAA Aerospace Sciences Meeting there was a blind comparison between computational simulations and experimental data for hypersonic double-cone and hollow cylinder-flare flows. This code validation exercise showed that in general there was good agreement between the continuum CFD simulations and experiments. Also, in general, there was good agreement between direct simulation Monte Carlo (DSMC) calculations and the experiments in regions of attached flow. However, in almost all of the computations, the heat transfer rate on the forebody of the cone was over-predicted by about 20%. The purpose of this paper is to report on our analysis of this difference. We perform CFD simulations of the hypersonic nozzle flow to assess the importance of vibrational nonequilibrium on the test conditions. We then recompute the flows using a new set of vibrational nonequilibrium conditions and consider the effects of a slip boundary condition at the model surface. Additionally, we analyze new heat transfer rate data on sharp and blunt 25-degree cones over a wider range of test conditions. This analysis appears to explain the discrepancy between the previous calculations and the experiments.

Transition Onset and Turbulent Heating Measurements for the Mars Science Laboratory Entry Vehicle

An investigation of transitional/turbulent heating on the Mars Science Laboratory entry vehicle has been conducted. Laminar, transitional, and turbulent heating data were obtained in a perfect-gas, Mach 6 air wind tunnel and in a high-enthalpy shock tunnel in CO2. Flow field solutions were computed using a Navier-Stokes solver at the test conditions and comparisons were made between measured and predicted heating levels. Close agreement was obtained for all laminar perfect-gas cases. For the high-enthalpy CO2 cases, close agreement with the data was achieved when a fully-catalytic wall boundary condition was employed, whereas the predictions exceeded the data by more than 25% if a noncatalytic boundary condition was used. Turbulent heating predictions fell below the perfectgas air data by 25% but exceeded the CO2 data by 60%. Transition onset locations were determined through comparisons with laminar heating predictions, and boundary-layer parameters from the flow field solutions were used to develop correlations for the transition onset location and the turbulent heating augmentation on the leeside of the vehicle.

EXPERIMENTAL STUDIES IN THE LENS SHOCK TUNNEL AND EXPANSION TUNNEL TO EXAMINE REAL-GAS EFFECTS IN HYPERVELOCITY FLOWS

Measurements and analyses are presented from a combined experimental and numerical program to examine the separate and combined effects of viscous/inviscid interaction and real gas chemistry on ground test facility performance and the aerothermal characteristics of vehicles in hypervelocity flows. The results of earlier studies to examine real gas effects are reviewed. The major features of the LENS reflected shock tunnels and the LENS X expansion tunnel are presented together with measurements and numerical simulations to calibrate and validate their performance for velocities up to 15,000 ft/s. The results of the most recent experimental studies conducted in the LENS I and 48-inch shock tunnel together with “state-of-the-art” Navier-Stokes and DSMC predictions are presented demonstrating that, in the absence of real gas effects, complex regions of laminar shock wave/boundary layer interaction in hypervelocity flow can be accurately described by experienced computationalists. Surface and flowfield measurements on the double cone configuration in studies in LENS I and LENS X with nitrogen and air at velocities of 14 kft/sec indicate that real gas effects can significantly decrease the separation length and the heating in the reattachment/shock/shock interaction regions. Comparisons with NavierStokes predictions suggest that the current models for air chemistry used in these codes are not sufficiently accurate to allow good predictions of the size and properties of the interaction region for airflow velocities of 14 kft/s.

Experimental study of shock wave interference heating on a cylindrical leading edge at Mach 6 and 8

This paper presents the details of an experimental study of shock wave interference heating on a cylindrical leading edge representative of the cowl of a rectangular hypersonic engine inlet. The study was conducted at Mach numbers of 6.3, 6.5 and 8.0. This study has provided the first (1) detailed pressure and heat transfer rate distributions for a two-dimensional shock wave interference on a cylinder and (2) insight into the effects of temperature dependent specific heats on the phenomena. The peak pressure and heat transfer rates were 10 times the undisturbed flow stagnation point levels. The peak levels and their gradients increased with Mach number. Variation in specific heats and hence the ratio of specific heats with temperature manifest in slightly lower loads and amplification factors than for corresponding perfect gas conditions.

Experimental Studies in the LENS Supersonic and Hypersonic Tunnels for Hypervelocity Vehicle Performance and Code Validation

A review is presented of experimental studies conducted in the LENS I and II shock tunnels and the LENS X expansion tunnel to evaluate models of turbulence and flow chemistry employed in the numerical codes and to examine aerothermal performance of hypersonic vehicles at fully duplicated flight conditions. Experimental studies have been conducted in the HiFire-1 program to evaluate the performance of numerical techniques to predict boundary layer transition and the characteristics of regions of shockwave/boundary layer interactions over compression surfaces. These studies were conducted in support of the design of the HiFire flight vehicle. Measurements of the flow characteristics over the flap and wind sections of a shuttle model have been performed to evaluate the real gas and viscous interaction phenomena associated with the shuttle flap anomaly. Additional studies to examine real gas effects in hypervelocity flows were conducted in the LENS I and X tunnels with blunt capsule and double cone configurations. Here we demonstrate that at velocities above 3 Km/sec the models of real gas chemistry and surface and flow field interaction employed in most numerical codes do not agree with measurement. A summary is presented of measurements made in a series of vehicle performance studies conducted with the X-51, HyFly, and HyCAUSE configurations with emphasis on boundary layer transition, turbulent interacting heating phenomena and the characteristics of unsteady shock wave/ boundary layer interactions associated with mode switching. As in all our studies, comparisons have been made between the measurements and predictions using advanced numerical codes.

Experimental studies of separated flows at hypersonic speeds. II - Two-dimensional wedge separated flow studies.

This paper describes an experimental study of the flow patterns and distribution of heat transfer developed in regions of adverse pressure gradient and separated flow over two-dimensional models in a Mach 10 airflow at a freestream Reynolds number of 1.35 X 10/in. Schlieren photographs and heat-transfer distributions were obtained in regions of adverse pressure gradient and moderately separated flow, induced by both forward-facing wedges and externally generated shock, to detect the difference between a flow in which the boundary layer had been merely thickened by an adverse pressure gradient and one in which there was a region of reverse flow. A separation criterion, based on a distinctive change in heat-transfer profile at the beginning of the interaction, is suggested to distinguish between unseparated and separated flows. A study was made to determine the effect of reattachment angle, boundary-layer thickness at separation, and downstream expansion on the heat transfer generated in the reattachment region on the wedge of a flat-plate-wedge model. The reattachment heat-transfer rates were found to be strongly dependent on reattachment angle; whereas for the configurations tested, there was little influence of the boundary-layer thickness at separation or downstream expansion. The measurements were correlated in terms of the viscous interaction parameters MC# and \R and also compared with calculations based on simple models for the separated flowfield.

Numerical and Experimental Characterization of High Enthalpy Flow in an Expansion Tunnel Facility

Several tools have been developed to perform rapid simulations of the relevant physics of an expansion tunnel facility in preparation for the operation of the LENS-XX tunnel. These tools have been assembled to provide multiple mechanisms to analyze the performance of the facility. Two algorithms have been employed in this effort to provide nearly instantaneous computations of the freestream state of the test gas – a characteristics based code and a quasi one-dimensional, unsteady code. These two codes have been modified to include thermal and chemical excitation that has been shown to be essential for this type of simulation. In addition, full unsteady Navier-Stokes simulations have been performed to study viscous effects in the facility and two-dimensional behavior to contrast the simplified algorithms. These codes are currently undergoing validation with available data from the facility to anchor the numerical codes and to assess and understand the experimental data.