The growth rate and turbulent structure of the compressible, plane shear layer are investigated experimentally in a novel facility. In this facility, it is possible to flow similar or dissimilar gases of different densities and to select different Mach numbers for each stream. Ten combinations of gases and Mach numbers are studied in which the free-stream Mach numbers range from 0.2 to 4. Schlieren photography of 20-ns exposure time reveals very low spreading rates and large-scale structures. The growth of the turbulent region is defined by means of Pitot-pressure profiles measured at several streamwise locations. A compressibility-effect parameter is defined that correlates and unifies the experimental results. It is the Mach number in a coordinate system convecting with the velocity of the dominant waves and structures of the shear layer, called here the convective Mach number. It happens to have nearly the same value for each stream. In the current experiments, it ranges from 0 to 1.9. The correlations of the growth rate with convective Mach number fall approximately onto one curve when the growth rate is normalized by its incompressible value at the same velocity and density ratios. The normalized growth rate, which is unity for incompressible flow, decreases rapidly with increasing convective Mach number, reaching an asymptotic vaue of about 0.2 for supersonic convective Mach numbers.

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The compressible turbulent shear layer: an experimental study

Advances in scientific computing have made modelling and simulation an important part of the decision-making process in engineering, science, and public policy. This book provides a comprehensive and systematic development of the basic concepts, principles, and procedures for verification and validation of models and simulations. The emphasis is placed on models that are described by partial differential and integral equations and the simulations that result from their numerical solution. The methods described can be applied to a wide range of technical fields, from the physical sciences, engineering and technology and industry, through to environmental regulations and safety, product and plant safety, financial investing, and governmental regulations. This book will be genuinely welcomed by researchers, practitioners, and decision makers in a broad range of fields, who seek to improve the credibility and reliability of simulation results. It will also be appropriate either for university courses or for independent study.

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Verification and Validation in Scientific Computing

Applications of detonations to propulsion are reviewed. First, the advantages of the detonation cycle over the constant pressure combustion cycle, typical of conventional propulsion engines, are discussed. Then the early studies of standing normal detonations, intermittent (or pulsed) detonations, rotating detonations, and oblique shock-induced detonations are reviewed. This is followed by a brief discussion of detonation thrusters, lasersupported detonations and oblique detonation wave engines. Finally, a more detailed review of research during the past decade on ram accelerators and pulsed detonation engines is presented. The impact of the early work on these recent developments and some of the outstanding issues are also discussed.

Review of Propulsion Applications of Detonation Waves

Noncircular jets have been the topic of extensive research in the last fifteen years. These jets were identified as an efficient technique of passive flow control that allows significant improvements of performance in various practical systems at a relatively low cost because noncircular jets rely solely on changes in the geometry of the nozzle. The applications of noncircular jets discussed in this review include improved large- and small-scale mixing in low- and high-speed flows, and enhanced combustor performance, by improving combustion efficiency, reducing combustion instabilities and undesired emissions. Additional applications include noise suppression, heat transfer, and thrust vector control (TVC). The flow patterns associated with noncircular jets involve mechanisms of vortex evolution and interaction, flow instabilities, and fine-scale turbulence augmentation. Stability theory identified the effects of initial momentum thickness distribution, aspect ratio, and radius of curvature on the initial flow evolution. Experiments revealed complex vortex evolution and interaction related to selfinduction and interaction between azimuthal and axial vortices, which lead to axis switching in the mean flow field. Numerical simulations described the details and clarified mechanisms of vorticity dynamics and effects of heat release and reaction on noncircular jet behavior.

Flow control with noncircular jets

It is argued that because of the lack of intrinsic length and time scales in the core part of the jet flow, the radiated noise spectrum of a high-speed jet should exhibit similarity. A careful analysis of all the axisymmetric supersonic jet noise spectra in the data-bank of the Jet Noise Laboratory of the NASA Langley Research Center has been carried out. Two similarity spectra, one for the noise from the large turbulence structures/instability waves of the jet flow, the other for the noise from the fine-scale turbulence, are identified. The two similarity spectra appear to be universal spectra for axisymmetric jets. They fit all the measured data including those from subsonic jets. Experimental evidence are presented showing that regardless of whether a jet is supersonic or subsonic the noise characteristics and generation mechanisms are the same. There is large turbulence structures/instability waves noise from subsonic jets. This noise component can be seen prominently inside the cone of silence of the fine-scale turbulence noise near the jet axis. For imperfectly expanded supersonic jets, a shock cell structure is formed inside the jet plume. Measured spectra are provided to demonstrate that the presence of a shock cell structure has little effect on the radiated turbulent mixing noise. The shape of the noise spectrum as well as the noise intensity remain practically the same as those of a fully expanded jet. However, for jets undergoing strong screeching, there is broadband noise amplification for both turbulent mixing noise components. It is discovered through a pilot study of the noise spectrum of rectangular and elliptic supersonic jets that the turbulent mixing noise of these jets is also made up of the same two noise components found in axisymmetric jets. The spectrum of each individual noise component also fits the corresponding similarity spectrum of axisymmetric jets.

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On the Two Components of Turbulent Mixing Noise from Supersonic Jets

Compressibility Effects in Turbulent Shear Layers

A new method for accelerating projectiles from velocities of ~0.7 km/s up to -12 km/s using chemical energy is presented in this paper. The concept, called the "ram accelerator," is based on gasdynamic principles similar to those of an airbreathing ramjet but operates in a different manner. The projectile, which resembles the center body of a ramjet, travels through a tube filled with a premixed gaseous fuel and oxidizer mixture. The tube becomes the outer cowling of the ramjet, and the energy release process travels with the projectile. By tailoring the propellant mixture along the tube, a nearly constant acceleration can be achieved. In principle, the ram accelerator can be scaled for projectile masses ranging from grams to hundreds of kilograms and is capable of ballistic efficiencies as high as 30%. A straightforwar d, quasisteady, one-dimensional approach is used to model the acceleration process. The experimental facility developed to investigate the concept is described, and the results of recent experiments are presented. The velocity range of 690-1500 m/s has been explored in a 4.88-m long, 38-mm bore accelerator tube. Using methane, oxygen, and various diluents, accelerations of up to 16,000 g have been achieved with 75 gm projectiles and gas fill pressures of 20 atm. Proof of concept has been demonstrated, and agreement between theory and experiment has been found to be very good.

Ram accelerator - A new chemical method for accelerating projectilesto ultrahigh velocities

Operational characteristics of the thermally choked ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, are investigated theoretically and experimentally. The projectile resembles the centerbody of a conventional ramjet and travels through a stationary tube filled with premixed gaseous fuel and oxidizer at high pressure. The combustion process travels with the projectile, its thermal choking producing a pressure field which results in thrust on the projectile. The results of experiments with 45-75 gm projectiles in a 12.2 m long, 38 mm bore accelerator, using methane-based propellant mixtures, are presented in the velocity range of 1150-2350 m/s. Acceleration of projectiles with staged propellants and transitions between different mixtures are investigated and the velocity limits in several propellant mixtures are explored. Agreement between theory and experiment is found to be very good.

Operational characteristics of the thermally choked ram accelerator

Ramjet-in-tube techniques have been proposed to accelerate masses up to thousands of kilograms to velocities of 0.7-12.0 km/s by chemical means. CFD calculations for three oblique detonation ramjet-in-tube drive modes are presented; the operational velocities for the present results range from 3.5 to 10.0 km/s. The first drive mode achieves ignition on the reflection of the nose cone bow shock. The second drive mode relies on a sudden, steep, but small increase in projectile radius to initiate a detonation wave, following a deliberately gentle, gradual compression process. The third drive mode is similar to the second mode except that the projectile is thermally protected by flying it through a core of pure hydrogen gas surrounded by a detonable mixture. At optimum operating conditions, the thrust pressure ratios (defined as the effective thrust pressure divided by the maximum cycle pressure) for the three modes range from 0.12 to 0.30, and the efficiencies (defined as the thrust times the velocity divided by the rate of chemical energy release) range from 0.09 to 0.26. Tables of thrust pressure ratios and efficiency data and representative plots of the pressure fields around the projectiles are presented.

D. W. Bogdanoff

Papers

Compressibility Effects in Turbulent Shear Layers

Ram accelerator - A new chemical method for accelerating projectilesto ultrahigh velocities

Operational characteristics of the thermally choked ram accelerator

Computational investigation of oblique detonation ramjet-in-tube concepts

Investigation of gasdynamic phenomena associated with the ram accelerator concept