Garry L. Brown

Stable and Unstable Accretion Flows with Angular Momentum near a Point Mass

The properties of axisymmetric accretion flows of cold adiabatic gas with zero total energy in the vicinity of a Newtonian point mass are characterized by a single dimensionless parameter, the thickness of incoming flow. In the limit of thin accretion flows with vanishing thickness, we show that the governing equations become self-similar, involving no free parameters. We study numerically thin accretion flows with finite thickness as well as those with vanishing thickness. Mass elements of the incoming flow enter the computational regime as thin rings. In the case with finite thickness, after a transient period of initial adjustment, an almost steady-state accretion shock with a small oscillation amplitude forms, confirming the previous work by Molteni, Lanzafame, \& Chakrabarti (1994). The gas in the region of vorticity between the funnel wall and the accretion shock follows closed streamlines, forming a torus. This torus, in turn, behaves as an effective barrier to the incoming flow and supports the accretion shock which reflects the incoming gas away from the equatorial plane. The postshock flow, which is further accelerated by the pressure gradient behind the shock, goes through a second shock which then reflects the flow away from the symmetry axis to form a conical outgoing wind. As the thickness of the inflowing layer decreases (or if the ratio of the half thickness to the distance to the funnel wall along the equatorial plan is smaller than

Creation of Steering Moments in Supersonic Flow by Off-Axis Plasma Heat Addition

\sim0.1

Turbulent shear layers and wakes

), the flow becomes unstable. In the case with vanishing thickness, the accretion shock formed to stop the incoming flow behind the funnel wall oscillates quasi-periodically with an amplitude comparable to the thickness. The structure between the funnel wall and the accretion shock is destroyed as the shock moves inwards toward the central mass and re-generated

Experiments on stability and transition at Mach 3

In this paper, the change in aerodynamic forces as a result of local heat addition, upstream of a cone, to supersonic flow (Mach 3.0) is studied numerically. In principle, such an effect on the forces and moments can be used for vehicle steering as well as drag reduction. Local energy addition to the flow is achieved by the use of microwave radiation as a heating source and an electron beam to control the air conductivity and consequently the location of the energy deposition. Since steering creation requires heating in a localized pre-ionized region, the strength of the microwave field has to be much lower than the critical value of the electric field at breakdown. Results show the potential effects of heat addition on the aerodynamic forces. The corresponding power and the optimized location require d to achieve these effects are discussed. 1. Introduction

Steering Moments Creation in Supersonic Flow by Off-Axis Plasma Heat Addition

This paper considers the lectures on turbulent shear layers and wakes presented and discussed at the Marseille meeting in 1961 and provides our perspective on progress in understanding the mechanics of these flows since that time. The initial discussion is based on the understanding in 1961 gained from prior work. Particular emphasis is then placed on the subsequent experimental revelation of the large-scale vortical structure (coherent structure) found to be essential to understanding the mechanics of the turbulent shear layer. Critical insight into the mechanics that determines the growth rate (the shear stress), for example, is provided by the Biot–Savart relationship. Conclusions are drawn from the experiments and some unresolved questions posed. This is followed by a discussion of plane wakes. Four regions of the plane wake are identified and experimental results on the large-scale structure are discussed. Again emphasis is placed on the vorticity and the vorticity fluxes that contribute directly to the derivative of the principal Reynolds stress. Results from numerical calculations offer new insights into the mechanics, especially through the vorticity and vorticity fluxes that could not previously be measured. For this case too, conclusions are drawn and outstanding questions posed.

Fluid Mechanics of a Mach 7-12 Electron Beam Driven Missile Scale Hypersonic Wind Tunnel: Modeling and Predictions

The results of an experimental study of stability, receptivity and transition of the flat-plate laminar boundary layer at Mach 3 are discussed. With a relatively low free-stream disturbance level (∼0.1%), spectra, growth rates and amplitude distributions of naturally occurring boundary layer waves were measured using hot wires. Physical (mass-flux) amplitudes in the boundary layer and free stream are reported and provide stability and receptivity results against which predictions can be directly compared. Comparisons are made between measurements of growth rates of unstable high-frequency waves and theoretical predictions based on a non-parallel, mode-averaging stability theory and receptivity assumptions; good agreement is found. In contrast, it was found that linear stability theory does not account for the measured growth of low-frequency disturbances. A detailed investigation of the disturbance fields in the free stream and on the nozzle walls provides the basis for a discussion of the source and the development of the measured boundary layer waves. Attention is drawn to the close matching in streamwise wavelengths for instability waves and the free-stream acoustic disturbances. It was also found that a calibration of the hot wire in the free stream yields a double-peak boundary layer disturbance amplitude distribution, as has been found by previous investigators, which is not consistent with the predictions of linear stability theory. This double peak was found to be an experimental anomaly which resulted from assumptions that are frequently made in the free-stream calibration procedure. A single-peak amplitude distribution across the boundary layer was established only when the hot-wire voltage was calibrated against the mean boundary layer profile. Finally, the late stages of transition, at a higher Reynolds number with a higher free-stream disturbance level, were explored. Calibrated amplitude levels are provided at locations where nonlinearities are first detected and where the mean boundary layer profile is first observed to depart from the laminar similarity solution. A qualitative discussion of the character of ensuing nonlinearities is also included.

Predictions for the Heat Transfer and Boundary Layer Growth in the Radiatively Driven Hypersonic Wind Tunnel and Comparisons with Experiment at Ultra High Reynolds Number (Invited)

The change in aerodynamic forces as a result of local plasma heat addition to supersonic flow (Mach 3.0), upstream of a cone, is studied numerically by solving the three-dimensional compressible Euler equations. In principle, such an effect on the forces and moments can be used for vehicle steering as well as drag reduction. Local energy addition to the flow is achieved by the use of microwave radiation as a heating-source and an electron beam to control the air conductivity and consequently the location of the energy deposition. This approach requires heating only in a localized preionized region, and so the strength of the microwave field has to be much lower than the critical value of the electric field at breakdown. Results show the potential effects of heat addition on the aerodynamic forces. The corresponding power and the optimized location required to achieve these effects are discussed.

RDHWT/MARIAH II Energy Addition Modeling and Experiments Review

Models of increasing complexity have been developed for the design and simulation of the axisymmetric inviscid fluid mechanics and energy addition of an electron-beam driven hypersonic wind tunnel for missile-scale testing and development. The principal target has been a Mach-12 capability at altitudes of approximately 25 km and above with a test section of 1.3 m. Also, results for lower Mach numbers at lower altitudes down to Mach-7 at 2 km have been obtained. The fully coupled e-beam and Euler flow simulation shows that with a magnetically guided, 3 MeV e-beam and a nozzle geometry determined from the solution to an optimization problem, shock waves can be eliminated notwithstanding the very high radiative power (200 MW) that is deposited into the core flow (away from the boundary layer). While there remain many issues to be resolved, we have not yet found an intrinsic problem with either the concept or its application to such long run time, missile-scale, facilities.

VORTICITY TRANSPORT IN MODELING THREE-DIMENSIONAL UNSTEADY SHEAR FLOWS

American Institute of Aeronautics and Astronautics For permission to copy or to republish, contact the copyright owner named on the first page. For AIAA-held copy-right, write to AIAA Permissions Department, 1801 Alexander Bell Drive, Suite 500, Reston, VA, 20191-4344. 22 AIAA Aerodynamic Measurement Technology and Ground Testing Conference 24-26 June 2002 / St. Louis, MO AIAA-2002-3128 Predictions for the Heat Transfer and Boundary Layer Growth in the Radiatively Driven Hypersonic Wind Tunnel and Comparisons with Experiment at Ultra High Reynolds Number (Invited)

Probe for measurements of density/conductivity in flows of conducting fluids

The viability of the Radiatively-Driven Hypersonic Wind Tunnel (RDHWT)/ MARIAH II project rests on our ability to controllably and predictably add energy to a supersonic air flow downstream of the throat with a high power electron beam. Experiments conducted at Sandia National Laboratories during the Spring of 2003 deposited over 800 KW of power into the flow in a successful demonstration of that concept. A detailed analysis of the streamwise pressure distribution and other flow variables led to several quantitative disagreements with the predictive models that are currently being used. Much of this disagreement appears to have arisen because the electron beam was clipped by apertures inside the accelerator, leading to large fluctuations in beam current during the run. Recent models suggest that, in addition to electron beam loss at these apertures, scattering may have introduced helicity into the trajectory of the electrons in the electron beam, leading to reduced penetration depth.