Timothy Wadhams

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 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.

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.

Characterization of the New LENS Expansion Tunnel Facility

LENS XX, a new large-scale expansion tunnel facility, has been designed and constructed at CUBRC. This stand-alone facility was designed after successful testing in the prototype LENS X facility. This facility is the largest known expansion tunnel in the world with an inner diameter of 24 inches and an end to end length of more than 240 ft. With the addition of LENS XX, CUBRC now has the ability to test duplicated supersonic or hypersonic flight conditions at any practical flight condition from low supersonic launch trajectories to planetary reentry. Expansion tunnels have showed much promise in recent years with their ability to generate a wide range of hypervelocit y conditions with reduced chemistry effects at high-enthalpy conditions in comparison with shock tunnels; however, short test times and large-amplitude test gas disturbances are still practical limitations of this type of facility. The large scale of this new facility offers a large core-flow with increased test time far exceeding other expansion tubes/tunnels. The large diameter tube will generate lower frequency test gas disturbances and reduced viscous effects, resulting in a higher quality coreflow. Additionally, an electrically heated high-pressure hydrogen driver allows highenthalpy testing with stagnation enthalpies up to 90 MJ/kg in a standard configuration and up to 120 MJ/kg or more in a four-chamber configuration. LENS XX will also have test times over 4 ms, test gas Mach numbers over 30, Reynolds numbers over 10 7 per meter, test gas velocities greater 13 km/s and shock speeds up to 15 km/s are expected. Shock speeds up to 12.4 km/s have been successfully demonstrated. The present work is meant to validate and begin to characterize the large range of possible test conditions through shock speed and freestream pressure measurements. Shock speed measurements are constant from 5-25 tube diameters over a large range of conditions i ndicating that viscous effects are minimal. Additionally, facility operation has proven to be very repeatable with primary shock speeds varying less than ±1.5%.

A Computational Analysis of Ground Test Studies of the HIFiRE-1 Transition Experiment

*† ‡ Comparisons to measurements made in the CUBRC LENS-I facility on a full-scale HIFiRE-1 vehicle at duplicated flight conditions have been made with the computational fluid dynamics code DPLR and the parabolized stability code STABL. These comparisons include laminar heating, transition onset, turbulent heating, and turbulent shock-induced separation covering all major aspects of the ground test experiment. These results and comparisons serve as a design package for the future flight article. It has been found that several issues remain with regards to state-of-the-art RANS modeling, both on the attached forebody flow and in the interaction region. On the attached forebody, heating predictions compared to ground test measurements have shown that the turbulence models can overpredict the measurements by up to 30% and initial investigations suggest that this discrepancy may be linked to total to wall temperature ratio. In the interaction region, the comparison with experiment has shown the importance of proper stress-limiting of the Reynolds stress tensor to obtain good agreement.

Experimental Studies in LENS I and X to Evaluate Real Gas Effects on Hypervelocity Vehicle Performance

An experimental program combined with numerical analysis has been conducted to examine real gas effects on the performance of hypervelocity re-entry vehicles. These studies which were conducted in the LENS I Shock Tunnel and LENS X Expansion Tunnel examine the flows over simple nose shapes and the Apollo, Mars Lander and Space Shuttle configurations. Measurements were made in air, nitrogen and CO2 at enthalpy levels from 2 MJ/kg to 12 MJ/kg to evaluate the nonequilibrium characteristics of the flow in the test facilities. The laser diode measurements which were made to determine the velocity and NO species concentration in the facility for the full range of enthalpies were in excellent agreement with predictions of freestream velocity but indicated NO mole fractions half of the predicted values. Measurements with the Apollo configuration in the LENS I Shock Tunnel and LENS X Expansion Tunnel were in good agreement with each other and with predictions of shock layer geometry. However, in the higher enthalpy flows, the heating levels were poorly predicted. Measurements in the two facilities with the Mars Lander configuration and CO2 as a test gas demonstrated that in high enthalpy flows there are significant differences in the standoff distances which are believed to result from a nonequilibrium nature of the CO2 flow in the shock tunnel. The measurements in the LENS X tunnel were in excellent agreement with DPLR predictions of the shock shape and standoff distance. Studies were also conducted to examine real gas effects on the control surface characteristics of a shuttle configuration. These studies which were conducted in air and nitrogen in the LENS I Shock Tunnel showed that real gas effects reduced the scale of the separated interaction region over the flap increasing its effectiveness. However, real gas effects reduce the pressure over the curved surface of the wing resulting in a reduced pitching moment and net reduction in vehicle stability.

Investigation of Blunt Bodies with CO2 Test Gas including Catalytic Effects

The capability to test blunt bodies using a carbon dioxide test gas with applications appropriate for Martian atmospheric reentry vehicles has been demonstrated for the LENS I facility at Calspan-UB Research Center (CUBRC). The facility was extensively calibrated at conditions of nominally 5 MJ/kg and 10 MJ/kg increased total enthalpy over a range of Reynolds numbers. This capability was used to test a 70 O sphere-cone geometry for the Mars Science Laboratory (MSL) mission, primarily to assess laminar, transitional, and turbulent heating levels as well as leeward transition criteria. The 61 cm (24 inch) diameter model was studied at 0 O , 11 O , and 16 O angles of attack. Numerical predictions of the laminar heating levels on the model were found to be inadequate without the inclusion of wall catalytic heating induced by 100% CO2 recombination at the surface of the stainless steel model.