Tomoaki Ishihara

Numerical Study on Anomalous Heating over Blunt-body in Free-Piston Shock Tunnel HIEST

Anomalous heating phenomena observed in free-piston shock tunnel HIEST is numeri- cally investigated in terms of radiation from upstream high-temperature gas. For assessing recently reported heating levels that depends on model size, werst estimate heat uxes over two different size test models through axi-symmetric Navier-Stokes simulations with thermochemical nonequilibrium chemistry. For the small test model, the convective heat ux is signicantly larger than the radiative heat ux, and the anomalous heating does not appear remarkably even with an assumption of the radiative heating from the upstream gas. On the other hand, the anomalous heating can be generated for the large test model with relatively large radiative heat ux compared to the convective one. If the anomalous heating is attributed to the radiation from the upstream gas, difference in contributions of the radiative heating between two models can explain the scaling effect. We also calculate oweld in the acceleration sections of HIEST and directly evaluate the radiative heat ux from the upstream gas. The accelerated ow is stagnated at the diaphragms and releases the considerable radiation. In the low enthalpy condition, the computed radiative ux reaches a level of the experiments, while that for the high enthalpy case becomes higher than the measured one.

A fitting formula for radiative cooling based on non-local thermodynamic equilibrium population from weakly-ionized air plasma

A fitting formula for radiative cooling with collisional-radiative population for air plasma flowfield has been developed. Population number densities are calculated from rate equations in order to evaluate the effects of nonequilibrium atomic and molecular processes. Many elementary processes are integrated to be applied to optically-thin plasmas in the number density range of 1012/cm3 ≤ N ≤ 1019/cm3 and the temperature range of 300 K ≤ T ≤ 40,000 K. Our results of the total radiative emissivity calculated from the collisional-radiative population are fitted in terms of temperature and total number density. To validate the analytic fitting formula, numerical simulation of a laser-induced blast wave propagation with the nonequilibrium radiative cooling is conducted and successfully reproduces the shock and plasma wave front time history observed by experiments. In addition, from the comparison between numerical simulations with the radiation cooling effect based on the fitting formula and those with a gray gas radiation model that assumes local thermodynamic equilibrium, we find that the displacement of the plasma front is slightly different due to the deviation of population probabilities. By using the fitting formula, we can easily and more accurately evaluate the radiative cooling effect without solving detailed collisional-radiative rate equations.

Global stability analysis on cone models under the conditions of the HIEST experiments

Global stability analysis by time-stepping approach with a thermochemical nonequilibrium code was conducted for hypersonic flows around the HIFiRE-1 model under conditions of the HIEST experiments. The flowfields were unstable for all the conditions as expected, and characteristic structures were extracted as an eigen-mode corresponding to the largest real eigenvalue. The structures obtained on the edge of the boundary layer are periodic and have the features of the second mode. The frequency of pressure fluctuation on the wall also indicates prominence of the second mode, while it is different in a few 100 kHz from the experimental data. Another periodic structure like the second mode is also found in the middle of the shock layer, and turbulent transition points observed in the experiments are located close to the points influenced by this structure via acoustic waves. Moreover, this periodic structure may result from the interference between the acoustic waves emitted from the second mode and the shock surface.

LES Computation of Turbulent Heat Flux on Reentry Capsule Afterbody with Forced Transition

A high order CFD code was developed for LES computation of turbulent heat flux in hypersonic flow. Aeroheating measurement tests with forced transition on an Apollo capsule model and an HTV-R which was a manned space capsule under development by JAXA were performed by the free-piston shock tunnel HIEST in JAXA Kakuda Space Center. Measured data set indicate that heat flux on the forebody of the Apollo capsule model was 1.5-2 times larger than one in laminar flow. On the other hand, the heat flux on the afterbody of HTV-R became significantly larger. Furthermore, in the separation region which exists afterbody of capsules, the Baldwin-Lomax model which can reproduce turbulent heat flux in the attached flow, tends to underestimate. In order to reproduce the turbulent heat flux on the after body by numerical simulation, high order CFD code towards LES in hypersonic flow is needed. Since the robustness near the shock and high resolution in the boundary layer is needed for LES in hypersonic flow, improved WENO method is employed. We calculated the hypersonic flow (M∞ = 17) around cylinder and investigated the robustness for strong shock by the method. Smooth pressure distribution was obtained agree well with the calculated heat flux by NASA LAURA code. Our developed code applied to the hypersonic flowfield around HTV-R with a trip and the heat flux was examined.

Numerical Analysis on Aerothermodynamic Characteristics of Blunt-nosed Cone in Free-piston Shock Tunnel HIEST

The measured pressure distributions of the blunt-nosed cone test model in Free-piston shock tunnel HIEST placed in JAXA Kakuda space center were compared with calculated results. At high total enthalpy conditions, the calculated pressure distribution was higher than the experimental one. As the cause for this discrepancy, first we focused on the chemical nonequilibrium model in the supersonic expanding region. Flowfield calculation with changes of chemical reaction rates of Park’s model was conducted and the effect on pressure distribution was examined. Accelerating the dissociation reaction rate of O2 had most effect on the pressure coefficients. A good agreements with experimental values was obtained. Second, the effect of chemical composition in freestream was also investigated. The more undisociated O2 in freestream resulted in the more reduction of pressure. This implies that it is necessary to identify chemical compositions in freestream and evaluation of aerodynamic characteristics.

Experimental Study on Heat Flux Augmentation in High-enthalpy Shock Tunnels

A wind-tunnel test campaign has helped identify the cause of the stagnation heat-flux augmentation observed under high-enthalpy and high-pressure conditions in several major shock tunnels. The test was carried out in the free-piston high-enthalpy shock tunnel HIEST under stagnation enthalpies from H0 = 8 to 20 MJ/kg and with stagnation pressure of P0 = 50 MPa. A flat plate with miniature coaxial thermocouples was used to measure surface-heat flux. To measure radiation and convection heating independently, optical filter windows passing different wavelengths were placed in the front of the thermocouples. There was significant radiation heating from wavelengths, ranging from visible to near infrared, which supplied approximately 40% of total heating. It was concluded that this radiation was a major cause of the heat-flux augmentation. Comparison of heat-flux records with and without optical filters respectively showed that the heat-flux increase was significantly delayed by the filter windows. Sequential Schlieren images showed that the delay was consistent with the time of establishment of the shock layer, indicating that the radiation was mainly emitted from the shock layer.

Computational Code for Air Plasma Flow Field with Atomic and Molecular Processes

T 40,000 K. We then compute collisional-radiative populations and total radiative emissivities. Results of the total radiative emissivity calculated from the collisional-radiative population aretted in terms of temperature and total number density. For the validation of curve-�tted radiative emissivity, we compute a laser-induced blast wave propagation with radiative cooling, and compare with experimentally observed shock wave and plasma front displacement. We could fairly reproduce the oweld for the blast wave induced by a pulse laser heating. From the comparison of the reduced emissivity model with black-body radiation, displacement of ionization front was slightly different due to the deviation of population probabilities.