Matthew MacLean

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.

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.

Improved Predictions of PICA Recession in Arc Jet Shear Tests

A new capability is incorporated into the DPLR CFD code which includes surface oxidation reactions, pyrolysis gas production, and time resolved surface shape change (due to ablation). These capabilities make it possible to predict the recession of carbon ablators as well as the effects of the ablator on the boundary layer. Simulations using DPLR (with ablation) can more accurately reproduce the relatively high recession rates seen in arc jet shear tests of Phenolic Impregnated Carbon Ablator (PICA); these same shear tests are under-predicted by the heat transfer based approach (FIAT). The simulations indicate that the high recession rate (relative to FIAT) seen in shear tests can be attributed to the test fixture (copper upstream of PICA) that disrupts the similarity between heat and oxygen transfer in the boundary layer – the copper fixture produces an enthalpy depleted boundary layer, while leaving the boundary layer oxygen rich. Alternative test fixture designs are proposed. This study also shows that the heat transfer analogy is very good for flows where similarity is preserved, such as stagnation flows in the arc jet and in flight. Shape change effects were simulated and improved the agreement with arc jet shear test data.

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.

Understanding High Recession Rates of Carbon Ablators Seen in Shear Tests in an Arc Jet

High rates of recession in arc jet shear test s of Phenolic Impregnated Carbon Ablator (PICA) inspired a series of tests and analysis on FiberForm (a carbon preform used in the fabrication of PICA). Arc jet tests were p erformed on FiberForm in both air and pure nitrogen for stagnation and shear configurations . The nitrogen tests showed little or no recession, while the air tests of FiberForm show ed recession rates similar to that of PICA (when adjusted for the difference in density). While mechanical erosion can not be ruled out, this is the first step in doing so. Anal ysis using a carbon oxidation boundary condition within DPLR was used to predict the recession rate of FiberForm. The analysis indicates that much of the anomalous recession behavior seen in shear tests may simply be an artifact of the non-flight like test conf iguration (copper upstream of the test article) a result of dis- similar enthalpy and oxygen concentration profiles on the copper. Shape change effects were also investigated and shown to be relatively small.

Numerical Assessment of Data in Catalytic and Transitional Flows for Martian Entry

The conditions for a typical run from the MSL phase two study of transition that was performed in the LENS facility have been analyzed to understand the sensitivity to the freestream conditions of the facility. A simplified analysis technique has been used based on energy accounting to freeze specified portions of the chemical or vibrational energy during the expansion process in the nozzle. The effect of freezing this energy results in increased shock standoff distance that better matches the measured shock shape. Based on several cases, it was found that freezing approximately 42% of the total enthalpy of the flow in the vibration mode results in the best agreement with the measured shock shape. This modified condition also results in significantly better agreement with the measured surface heat transfer at the stagnation point and with the measured pressure at the shoulders of the model. Based on this adjusted freestream condition, the surface heat transfer data shows behavior generally consistent with fully-catalytic recombination on the cold wall. This behavior is consistent with previous results obtained in shock tunnel facilities in carbon dioxide, air, and nitrogen. Although the mechanism causing this frozen energy in the flow has not been identified, the sensitivity of the transition onset point of the flowfield to this phenomenon has been estimated to be less than 10% based on a simple transition criterion.

Numerical evaluation of flow conditions in the LENS reflected shock-tunnel facilities

The calculation process of the flowfield of the nozzles of the Calspan-UB Research Center reflected shock tunnel facilities has been described. These calculations utilize a full NavierStokes solver based on the robust data-parallel line relaxation method coupled with chemical and vibrational non-equilibrium effects as well as the inclusion of a non-ideal equation of state necessitated by the high pressures in the reservoir of the shock tunnel. The calculation of the reservoir conditions and the modeling of the turbulent boundary layer in the throat region have been found to be the two most significant issues associated with the computation. With the use of the Spalart-Allmaras one-equation model, the error calculated by comparing the solution to measured Pitot pressure profiles in the test section are nominally +/5% or less for all the cases considered. In addition to good agreement with the available experimental data, the calculations have proven to be important in the understanding of the flow interactions during the nozzle expansion and have provided a means of evaluating new nozzle throat inserts to generate new capability ranges for the facilities.

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.