Rotational and vibrational nonequilibrium effects in rarefied, hypersonic flow

Results are reported for an investigation into the methods by which energy transfer is calculated in the Direct Simulation Monte Carlo method. Description is made of a recently developed energy exchange model that deals with the translational and rotational modes. A new model for simulating the transfer of energy between the translational and vibrational modes is also explained. This model allows the vibrational relaxation time to follow the temperature dependence predicted by the Landau-Teller theory at moderate temperatures. For temperatures in excess of about 8000K the vibrational model is extended to include an empirical result for the relaxation time. The effect of introducing these temperature dependent collision numbers into the DSMC technique is assessed by making calculations representative of the stagnation streamline of a hypersonic space vehicle. Both thermal and chemical nonequilibrium effects are included while the flow conditions have been chosen such that ionization and radiation may be neglected. The introduction of these new models is found to significantly affect the degree of thermal nonequilibrium observed in the flowfield. Larger, and more widely ranging, differences in the results obtained with the different energy exchange probabilities are found when a significant amount of internal energy is included in the calculation of chemical nonequilibrium

Predicting Breakdown of the Continuum Equations Under Rarefied Flow Conditions

The breakdown of the continuum equations of gas dynamics under conditions of rarefied flow is considered. The conditions of breakdown are of interest in assessing the physical accuracy of continuum solutions, in understanding the mathematical relations between different sets of continuum transport equations, and play a significant role in the development of hybrid methods that use both continuum and kinetic simulation techniques. A number of previous breakdown parameters are reviewed. A new method, based on kinetic theory analysis, is introduced for detecting conditions where the continuum approach is valid. Continuum breakdown is discussed in detail for gas expansions and shock waves by considering the basic physical phenomena involved in each case. Success of the breakdown parameters for these flows is assessed. It is shown that continuum breakdown in gas expansions and shock waves proceeds through different physical mechanisms that are best predicted by different types of breakdown parameters. The new method for detecting continuum onset is applied to shock waves and shown to offer promising results. © 2003 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87928/2/899_1.pd

Hybrid Particle-Continuum Methods for Nonequilibrium Gas and Plasma Flows

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98680/1/APC000531.pd

Modelling of satellite control thruster plumes

Deleterious effects such as surface heating and turning moments can arise as a consequence of the impingement of thruster plumes with spacecraft surfaces. Such thrusters are normally fired for attitude control. The prediction of such effects must be undertaken at the design stage of the vehicle.In this study, the modelling of such plumes was undertaken. The following prediction techniques were implemented into computer programmes: (a) the Simons model, (b) the Method of Characteristics (MOC), and (c) the Direct Simulation Monte Carlo method (DSMC). The first two methods are derived from continuum equations whilst the third adopts a discrete particle approach.Several DSMC schemes exist for treating the collisional behaviour of the gas, and it was unclear which would be best suited for the intended application. A thorough assessment of the implementation and performance of several such schemes was therefore completed.Having determined the most suitable DSMC scheme, the three modelling techniques were then applied to the isentropic core expansion of a small hydrazine thruster plume. It was found that significant errors occur in the determination of impingement quantities through application of the continuum methods in the flow regime lying between the continuum and free molecular limits.The DSMC technique was also used to calculate the nozzle lip and backflow expansion regions of the same hydrazine thruster. A significant degree of backflow was found with flow angles of up to 140º. The sensitivity of the calculations to the conditions initially assumed were assessed and found to be important

Computation of atmospheric entry flow about a Leonid meteoroid

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76701/1/AIAA-2000-583-980.pd

Computation of Hypersonic Flows Using the Direct Simulation Monte Carlo Method

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140680/1/1.A32767.pd

Modeling of Plasma Formation in Rarefied Hypersonic Entry Flows

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76173/1/AIAA-2007-206-924.pd

Particle kinetic simulation of high altitude hypervelocity flight

A new model for simulating the transfer of energy between the translational and rotational modes was derived for a homogeneous gas of diatomic molecules. The model was developed specifically for use in discrete particle simulation methods where molecular motion and inter-molecular collisions are treated at the molecular level. In such methods it is normal to assume a constant rotational collision number for the entire flowfield. The new model differs in that a temperature dependence is introduced which has been predicted by theory and observed in experiment. The new model is applied to the relaxation of rotational temperature, and is found to produce significant differences in comparison with the model normally employed at both high and low temperatures. Calculations have also been performed for a Mach 7 normal shock wave. Large differences in the solutions are again observed, with the new model offering an improved correspondence to the available experimental data

Particle kinetic simulation of high altitude hypervelocity flight

In this grant period, the focus has been on the effects of thermo-chemical nonequilibrium in low-density gases, and on interactions between such gases and solid surfaces. Such conditions apply to hypersonic flows of re-entry vehicles, and to the expansion plumes of small rockets. Due to the nonequilibrium nature of these flows, a particle approach has been adopted. The method continues to undergo refinement and application to typical flows of interest. A number of studies have been performed for flows in thermo-chemical nonequilibrium. The effects of vibrational nonequilibrium on the rate of dissociation were studied for diatomic nitrogen. It was found that a new model reproduced the nonequilibrium behavior observed experimentally

On the magnetic mirror effect in Hall thrusters

The magnetic mirror effect is studied in the channel of a Hall thruster. It is shown that gradients in magnetic field affect the presheath structure and electric potential distribution. The length of the radial presheath region decreases in the presence of a magnetic field gradient. The two-dimensional potential shape can be affected by proper choice of the magnetic mirror ratio. In particular, it is possible to obtain a concave shape of the potential profile in the channel even in the case of a primarily radial magnetic field. This, in turn, can be used to efficiently control the ion dynamics in the acceleration region.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87826/2/121501_1.pd