John K. Harvey

Some observations of the vortex breakdown phenomenon

This paper describes an experiment in which a cylindrical vortex, formed in a long tube, was used to study the ‘vortex breakdown’ that has been previously reported in investigations of the flow over slender delta wings. By varying the amount of swirl that was imparted to the fluid before it entered the tube, it was found that the breakdown was the intermediate stage between the two basic types of rotating flows, that is, those that do and those that do not exhibit axial velocity reversal. In addition, it was shown that an unusual flow pattern was established after the breakdown and that certain features of this pointed to it being a ‘critical’ phenomenon. The tests were concluded by measuring the swirl angle distribution a short distance ahead of the breakdown and comparing these results with the prediction of Squires theory (1960).

Nonequilibrium thermal radiation from air shock layers modeled with direct simulation Monte Carlo

At reentry velocities it is generally agreed that the radiation associated with transitions between excited electronic states of atoms and molecules is responsible for the bulk of the thermal radiation emitted from the shock wave area. This article deals with the evaluation of thermal radiation emitted from hypersonic shock waves in real air using the direct simulation Monte Carlo method. The calculation of electronic excitation is made without assuming equilibrium for the distribution of the energy states, and measured or theoretically evaluated cross sections are used to determine the electronic excitation of atoms and molecules in the flow and the subsequent thermal radiation. The results with this new scheme are compared with available experimental data and existing numerical methods. The test cases are based on an AVCO Everett shock-tube experiment and on the axisymmetric flowfield of a blunted Mars-net reentry vehicle. The method is in good agreement with both experimental data and results given by other methods. Discrepancies are evaluated and discussed. 34 refs.

Experimental and computational studies of the flow over a sting mounted planetary probe configuration

This paper summarizes the results of a series of experimental studies in the LENS shock tunnel and computations with DSMC and Navier Stokes codes which have been made to examine the aerothermal and flowfield characteristics of the flow over a sting-supported planetary probe configuration in hypervelocity air and nitrogen flows. The experimental program was conducted in the LENS hypervelocity shock tunnel at total enthalpies of 5and 10 MJkg for a range of reservoir pressure conditions from 70 to 500 bars. Heat transfer and pressure measurements were made on the front and rear face of the probe and along the supporting sting. High-speed and single shot schlieren photography were also employed to examine the flow over the model and the time to establish the flow in the base recirculation region. Predictions of the flowfield characteristics and the distributions of heat transfer and pressure were made with DSMC codes for rarefied flow conditions and with the Navier-Stokes solvers for the higher pressure conditions where the flows were assumed to be laminar. Analysis of the time history records from the heat transfer and pressure instrumentation on the face of the probe and in the base region indicated that the base flow was fully established in under 4 milliseconds from flow initiation or between 35 and 50 flow lengths based on base height. The measurements made in three different tunnel entries with two models of identical geometries but with different instrumentation packages, one prepared by NASA Langley and the second prepared by CUBRC, demonstrated good agreement between heat transfer measurements made with two different types of thin film and coaxial gage instrumentation. The measurements of heat transfer and pressure to the front face of the probe were in good agreement with theoretical predictions from both the DSMC and Navier Stokes codes. For the measurements made in low density flows, computations with the DSMC code were found to compare well with the pressure and heat transfer measurements on the sting, although the computed heat transfer rates in the recirculation region did not exhibit the same characteristics as the measurements. For the 10MJkg and 500 bar reservoir match point condition, the measurements and heat transfer along the sting from the first group of studies were in agreement with the Navier Stokes solutions for laminar conditions. A similar set of measurements made in later tests where the model was moved to a slightly different position in the test section indicated that the boundary layer in the reattachment compression region was close to transition or transitional where small changes in the test environment can result in larger than laminar heating rates. The maximum heating coefficients on the sting observed in the present studies was a small fraction of similar measurements obtained at nominally the same conditions in the HEG shock tunnel, where it is possible for transition to occur in the base flow, and in the low enthalpy studies conducted in the NASA Langley high Reynolds number Mach 10 tunnel where the base flow was shown to be turbulent. While the hybrid Navier- StokedDMSC calculations by Gochberg et al. (Reference 1) suggested that employing the Navier- Stokes calculations for the entire flowfield could be seriously in error in the base region for the 10 MJkg, 500 bar test case, similar calculations performed by Cornell, presented here, do not.

An evaluation of some collision models used for Monte Carlo calculations of diatomic rarefied hypersonic flows

Three intermolecular collision models have been used in Monte Carlo direct-simulation computations. Their merits have been assessed by comparing the predictions given for two contrasting flows with experimental results. In one flow viscous effects were predominant; in the other the rapid compression ahead of a blunt body was the feature concentrated upon. In both examples the flows were rarefied and hypersonic and the gas was diatomic and rotationally excited.

The modeling of chemical reactions and thermochemical nonequilibrium in particle simulation computations

The treatment of chemical reactions and nonequilibrium energy exchange in Direct Simulation Monte Carlo calculations is examined. Details of a Maximum Entropy chemical reaction model are presented that is based on the classical scheme devised by Levine and Bernstein. Data are given for all of the significant reactions that occur in hypersonic reentry flight into the atmospheres of the Earth, Mars, and Venus. The method is an extension of that described and used previously by the authors (Gallis and Harvey [J. Fluid Mech. 312, 149 (1996); AIAA J. 34(7), 1378 (1996)]) and now includes carbon dioxide/nitrogen and ionic reactions. The model allows an appropriate dependence of each reaction on its controlling energy mode and avoids inappropriate use of equilibrium distributions to determine the reaction probabilities and post-collision energy reallocation. Sample flow solutions are given and comparisons are made with results obtained using continuum solvers.

A numerical simulation of the rarefied hypersonic flat-plate problem

The direct-simulation Monte-Carlo method for the full Boltzmann equation is applied to the problem of rarefied hypersonic flow of rotationally excited N 2 past the leading edge of a two-dimensional flat plate aligned with the free stream. An approximate collision model representing rotational–translational energy exchanges is developed for use in the calculations. The effects of this and other inelastic collision models and of the single-parameter Maxwell gas–surface interaction law on the flow in the kinetic/transition regime are discussed.

Experimental and Numerical Studies on Hypersonic Vehicle Performance in the LENS Shock and Expansion Tunnels

In this paper, we review the results of a series of experimental and numerical studies which are being conducted in the LENS facilities at CUBRC to provide measurements with which to evaluate and improve the modeling of turbulence and chemical processes with the emphasis on four major areas associated with the prediction of the aerothermal characteristics of blunt and slender vehicles traveling in hypervelocity flows. In the first area, we are examining real-gas effects in nitrogen, air and CO2 with the objective of validating codes to predict both the structure of the shock layer over the body, and also the heat transfer characteristics of flows where real gas effects are associated with significant enhanced heating resulting from catalytic wall effects. The second area of experimental research is focused on providing measurements of the unsteady characteristics of boundary layer transition on blunt and slender vehicles to provide data with which to evaluate the physical models of the transition process employed in Reynolds averaged and unsteady versions of the Navier-Stokes prediction techniques. In this work, we obtained measurements in transitional flows over a blunt capsule configuration, on slender re-entry vehicles and on scramjet inlets. A third area of study has been focused on the important and difficult problem of measuring and predicting the characteristics of regions of shock wave/turbulent boundary layer interaction in supersonic and hypersonic flows. Here we are specifically interested in predicting the size and characteristics of shock-induced turbulent separated regions similar to those developed in the bipod and control surface areas over the shuttle, and similar shock/turbulent boundary layer interaction regions over maneuverable hypersonic vehicles. Of equal importance are the turbulent interactions which occur inside a scramjet engine which strongly influence fuel injection, flameholding, and the release of energy during the combustion process in the engine. Finally, we are engaged in extensive experimental work to develop and employ techniques in our tunnels, with which to evaluate unsteady shock interaction phenomena, which are associated with shroud separation, the starting and mode switching associated with ram- and scramjet engines, and the multi-body separation which can occur during the dispensing of stores and the separation of a two-stage vehicle. This is an extremely challenging and dangerous activity and our success in this area has demonstrated that a short-duration facility can be employed to safely study the separation of free flying two stage vehicles.

Modelling of chemical reactions in hypersonic rarefied flow with the direct simulation Monte Carlo method

In this paper the phenomenon of chemical reactivity in hypersonic rarefied Bows is examined. A new model is developed to describe the reactions and post-collision energy exchange processes that take place under conditions of molecular non-equilibrium. The new scheme, which is applied within the framework of the direct simulation Monte Carlo (DSMC) method, draws its inspiration from the principles of maximum entropy which were developed by Levine & Bernstein. Sample hypersonic flow fields, typical of spacecraft re-entry conditions in which reactions play an important role, are presented and compared with results from experiments and other DSMC calculations. The latter use traditional methods for the modelling of chemical reactions and energy exchange. The differences are discussed and evaluated. Envisaged expeditions to planets within the solar system and the requirement to improve the payload-to-weight ratio of spacecraft have revived the Apollo-era interest in the study of rarefied hypersonic flow. These vehicles travel at near orbital velocity through the upper levels of the Earth’s atmosphere and the high temperatures met under such flight conditions (typically above 10000 K) provoke dissociation, chemical reactions and even ionization of the fluid through which they are travelling. The variety of real gas effects that take place substantially modify the energy flux to the vehicle. Endothermic reactions (principally oxygen and nitrogen dissociation) dominate the shock layer ahead of a re-entry vehicle and absorb energy, reducing the rise in temperature due to the comparison. Generally this leads to an alleviation of the heat transfer to the vehicle, especially if chemical recombination can be prevented from occurring on its surface through the use of non- or only slight catalytic coatings. The ceramic tiles used to cover the surface of Shuttle Orbiter were chosen for this reason. Efficient modern spacecraft design depends on having a precise understanding of the fluid dynamics, including the non-equilibrium chemical activity. Such understanding enables reliable estimates to be made of the aerodynamic heating, forces and moments that will be experienced during the re-entry. From these it should be possible to make an accurate estimation of the minimum amount of thermal protection required to maintain the vehicle’s integrity. Reliable numerical predictions of continuum high-enthalpy flow fields can be obtained using the Navier-Stokes equations. Species mass conservation equations, based on equilibrium chemical reaction rates, are coupled to these equations to provide a complete description for reacting flows. During re-entry, peak heating rates are usually experienced at very high altitude (above about 55 km) where the use of the

Flat-ended circular cylinder in hypersonic rarefied flow

The paper describes a study of the rarefied hypersonic flow about a flat-ended circular cylinder at various angles of attack. Examples of direct simulation Monte Carlo (DSMC) calculations for nonreacting nitrogen flows are presented, covering a range of Knudsen numbers from 0.034 to 1.84. These include the effects of rotational and vibrational energy exchange. Contour plots of density and temperature show the structure of the disturbed flowfield. For zero angle-of-attack flows, the density and rotational temperature predictions are compared with measurements made in a hypersonic wind tunnel at a Mach number of about 25 for a range of Knudsen numbers. Close agreement between the two is seen, although a lag in rotational temperature suggests that the coupling to this mode is underestimated. The DSMC calculations have been extended to cover angles of attack up to 40 deg. Flowfield and surface flux patterns are presented for these cases. For the denser flows, the maxima in heat transfer occur away from the plane of symmetry.

Review of Code Validation Studies in High-Speed Low-Density Flows

Introduction Few would dispute that when Bird in 1970 first formulated the Direct Simulation Monte Carlo (DSMC) method, it marked a milestone in the evolution of fluid mechanics. It brought within grasp solutions to engineering problems involving rarefied flow that at the time were completely impossible to obtain using any other technique. Although probably not then appreciated, the method had enormous power and, as was later to be demonstrated, it had the potential to solve very complex flows involving chemical reactions and high degrees of molecular non-equilibrium.