Yousuke Ogino

An Attempt to Improve Prediction Capability of Transonic Buffet Using URANS

A numerical approach to improve prediction capability of transonic buffet onset based on URANS solutions is presented. In the computation of unsteady flowfield over NACA0012 wing section, numerical perturbation is artificially introduced to avoid convergence to a pseudo steady solution where buffet phenomenon is expected to take place. It is shown that the buffet onset can be easily determined from variation of RMS of unsteady flowfield. The computed boundary of buffet onset favorably agrees with the experimental data.

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.

Nonequilibrium Plasma Flow Computation with Atomic and Molecular State Transitions

A computational code for collisional-radiative rate equations coupled with momentum and energy conservation equations has been developed to study the effects of nonequilibrium atomic and molecular processes on the population densities in an air plasma flowfield. This model consists of fifteen air species: e−, N, N, N, O, O, O, O−, N2, N + 2 , NO, NO, O2, O + 2 , and O − 2 with their major electronic excited states. Many elementary processes are considered in the number density range of 10/cm ≤ N ≤ 10/cm and the temperature range of 300 K ≤ T ≤ 60,000 K. The state population, the chemical composition and temperature profiles in a high enthalpy flow condition assuming the Fire II reentry problem are computed. Presented computational results indicated that the amount of radiative intensity emitted from nitrogen atoms has a possibility to be reduced more than 10 % due to the atomic and molecular state transitions.

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.

Computational Study of Flow Generation by Charged Particles in DBD Plasma Actuators

We have developed a two-dimensional numerical code based on PIC-MCC method with representative reactions for N2 to perform discharge dynamics in DBD plasma actuator and estimate generated force resulting from ion-neutral collision. Depending on the applied voltage to the exposed electrode, two types of discharge process are found in the plasma actuator. The dielectric surface potential changes as time advances by following the fact that the generated plasma severely affected on the electric field. Moreover, the resultant thrust from the ion-neutral collision is several times smaller than the electric force. These flow properties should be included appropriately in a macroscopic model for CFD.

Numerical Study on Air Plasma Floweld Coupled with Transition Rate Equations

A collisional-radiative rate equation coupled with energy equations has been developed to study the effects of nonequilibrium atomic and molecular processes on the population densities in an air plasma flowfield. This model consists of fifteen air species: e−, N, N, N, O, O, O, O−, N2, N + 2 , NO, NO , O2, O + 2 , and O − 2 with their major electronic excited states. Many elementary processes are considered in the number density range of 10/cm ≤ N ≤ 10/cm and the temperature range of 300 K ≤ T ≤ 40,000 K. We then compute the time evolution of chemical composition and temperature in a high enthalpy flow condition assuming the Fire II reentry problem. From comparison with an existing collisional-radiative code, we have confirmed that the molecular dissociation rates and thermal relaxation time should be determined by a suitable physical model to describe the plasma state achieved in the ionization phase by shock heating.