Effects of Specific Types of Surface Roughness on Boundary-Layer Transition
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PRESSURE DISTRIBUTION - AIRFOILS↗
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Results for “boundary effects”
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Reynolds number effects on the turbulence field in compressible boundary layers
Detailed experiments were conducted in a zero pressure gradient, supersonic turbulent boundary layer, including measurements of the three components of velocity fluctuations and the turbulent shear stress, for Reynolds numbers ranging from 11.7 million to 105 million at a freestream Mach number of 2.3. The mean flow measurements established the fully developed and equilibrium nature of the boundary layer. Measurements of the turbulence field show that the vertical and transverse fluctuations are essentially equal throughout the boundary layer at all Reynolds numbers, a feature that is different from observations in incompressible flows. The data show that the boundary layer exhibits similarity in the turbulence profiles for the entire Reynolds number range and agrees with previous compressible and incompressible data using Morkovin's scaling to account for compressibility effects.
Acharya, M.↗
Local structural ordering determines the mechanical damage tolerance of amorphous grain boundary complexions
Amorphous grain boundary complexions act as toughening features within a microstructure because they can absorb dislocations more efficiently than traditional grain boundaries. This toughening effect should be a strong function of the local internal structure of the complexion, which has recently been shown to be determined by grain boundary crystallography. To test this hypothesis, molecular dynamics are used here to simulate dislocation absorption and damage nucleation for complexions with different distributions of structural short-range order. Here, the complexion with a more disordered structure away from the dislocation absorption site is actually found to better resist crack nucleation, as damage tolerance requires delocalized deformation and the operation of shear-transformation zones through the complexion thickness. The more damage tolerant complexion accommodates plastic strain efficiently within the entire complexion, providing the key mechanistic insight that local patterning and asymmetry of structural short-range order controls the toughening effect of amorphous complexions.
36 MATERIALS SCIENCE↗
Laminar-flow research and experiments.
Critical parameters and flow characteristics of swept wings with full-chord laminar flow, noting boundary layer disturbance effects
FLOW CHARACTERISTICS↗
Aeroacoustics
An analysis was conducted to determine the properties of sound generated by aerodynamic forces or motions originating in a flow, such as the unsteady aerodynamic forces on propellers or by turbulent flows around an aircraft. The acoustics of moving media are reviewed and mathematical models are developed. Lighthill's acoustic analogy and the application to turbulent flows are analyzed. The effects of solid boundaries are calculated. Theories based on the solution of linearized vorticity and acoustic field equations are explained. The effects of nonuniform mean flow on the generation of sound are reported.
Goldstein, M., E.↗
Grain boundary oxidation and oxidation accelerated fatigue crack nucleation and propagation
Fatigue life at elevated temperatures is often shortened by oxidation. Grain boundary oxidation penetrates deeper than the surface oxidation. Therefore, grain boundary oxide penetration could be the primary cause of accelerated fatigue crack nucleation and propagation, and the shortened fatigue life at elevated temperatures. Grain boundary oxidation kinetics was studied and its statistical scatter was analyzed by the Weibull's distribution function. The effects of grain boundary oxidation on shortened fatigue life was analyzed and discussed. A model of intermittent microruptures of the grain boundary oxide was proposed for the fatigue crack growth in the low frequency region. The proposed model is consistent with the observations that fatigue crack growth rate in the low frequency region with hold time at K sub max is inversely proportional to cyclic frequency and that crack growth is intergranular.
Liu, H. W.↗
Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator
A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. Previous analyses of this concept were based on inviscid assumptions. The present results indicate that viscous effects are of primary importance; in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure, and turbulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. In the unstable case, a detonation wave is formed, which propagates upstream and unstarts the ram accelerator. In the stable case, a solution that converges to steady-state is obtained, in which the combustion wave remains stationary with respect to the ram accelerator projectile. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. In addition to these studies, two numerical techniques were tested. These two techniques are vector extrapolation to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices that arise in chemically reacting flows.
Yungster, Shaye↗
Navier-Stokes simulation of the supersonic combustion flowfield in a ram accelerator
A computational study of the ram accelerator, a ramjet-in-tube device for accelerating projectiles to ultrahigh velocities, is presented. The analysis is performed using a fully implicit TVD scheme that efficiently solves the Reynolds-averaged Navier-Stokes equations and the species continuity equations associated with a finite rate combustion model. The present results indicate that viscous effects are of primary importance in all the cases studied, shock-induced combustion always started in the boundary layer. The effects of Mach number, mixture composition, pressure and tubulence are investigated for various configurations. Two types of combustion processes, one stable and the other unstable, were observed depending on the inflow conditions. The possibility of stabilizing the detonation wave by means of a backward facing step is also investigated. Two numerical techniques were tested: vector extrapolation, to accelerate convergence, and a diagonal formulation that eliminates the expense of inverting large block matrices which arise in chemically reacting flows.
Yungster, Shaye↗
Lateral straining of turbulent boundary layers. I - Streamline divergence
The effect of prolonged streamline divergence on developing turbulent boundary layers is investigated using an experimental approximation of the source flow over a flat plate to achieve a simple divergence. Results are presented of hot-wire measurements for the planes of symmetry of two layers which had the same (low) Reynolds number and were developed in the presence of the same amount of simple divergence with a maximum divergence parameter of about 0.075 but with different (by a factor of 2) pressure-gradient parameters. It was found that there were two overlapping stages of development. In the initial stage, which covered a distance of about 20 initial boundary-layer thicknesses from the start of divergence, the coupled effects of both the pressure gradient and divergence were present. In the second region, which lasts nearly to the end of the diverging section, the pressure-gradient effects were negligible.
Saddoughi, Seyed G.↗
The effect of the transport property models on the shuttle boundary layer
Theoretical solutions for the nonsimilar, laminar boundary-layer were computed for four points along the shuttle entry trajectory. Since the boundary layer is that region of the flow field where the effects of viscosity and of thermal conductivity are most important, numerical solutions for the boundary layer were generated using different models for the transport properties. These solutions indicate that the displacement thickness and the heat-transfer rates are very sensitive to changes in the models for thermal conductivity and for specific heat. Thus, the solutions are sensitive to the assumed transport-property model.
Bertin, J. J.↗
Elasto visco-plastic flow with special attention to boundary conditions
A simple but nontrivial steady-state creeping elasto visco-plastic (Maxwell fluid) radial flow problem is analyzed, with special attention given to the effects of the boundary conditions. Solutions are obtained through integration of a governing equation on stress using the Runge-Kutta method for initial value problems and finite differences for boundary value problems. A more general approach through the finite element method, an approach that solves for the velocity field rather than the stress field and that is applicable to a wide range of problems, is presented and tested using the radial flow example. It is found that steady-state flows of elasto visco-plastic materials are strongly influenced by the state of stress of material as it enters the region of interest. The importance of this boundary or initial condition in analyses involving materials coming into control volumes from unusual stress environments is emphasized.
Shimazaki, Y.↗
The impact of defects on the photovoltaic potential of RTR silicon ribbon
Silicon ribbon grown at a high rate by the ribbon-to-ribbon (RTR) method contains a variety of defects and can exhibit an unusual dendritic morphology. This paper is a study of the effect of the defect structure and dendritic morphology on the photovoltaic potential of RTR ribbon. Defects found in RTR ribbon include grain boundaries, twin planes, stacking faults and dislocations. The electrical activity of these defects is investigated by measuring the diffusion length in the ribbon; by fabricating small area diodes on the ribbon and evaluating their photoresponse; and by using a scanning electron microscope in the electron beam induced current mode. It is concluded that although grain boundaries serve very effectively as recombination centers, they do not occur frequently enough in RTR ribbons to significantly affect the generation current. Dislocation densities up to 1,000,000/sq cm can be tolerated without strongly reducing the photoresponse. Similarly, linear boundaries such as twin planes or stacking faults do not affect the cell performance up to densities of 1000/sq cm.
Baghdadi, A.↗
Effects of leading-edge bluntness and ramp deflection angle on laminar boundary-layer separation in hypersonic flow
Leading edge bluntness and ramp deflection angle effects on laminar boundary layer separation in hypersonic flow
TWO-DIMENSIONAL FLOW↗
Graph-theoretical KMC Framework for Calculating Effective Diffusivity in TPBAR Components: Effective Diffusivity of Tritium in α-Zr Grain Boundaries
We report the development of a computer simulation tool based on the Graph-theoretical kinetic Monte Carlo (GT-KMC) approach that can be used to simulate diffusion and derive effective diffusivity of species in complex structures of TPBAR components. The GT-KMC framework was implemented in AKSOME, an on-lattice self-learning kinetic Monte Carlo tool developed at PNNL and was previously used to study solute diffusion in metal alloys. This newly developed tool will be helpful in simulating atom diffusion and extracting their effective diffusivities in grain boundaries and interfaces in various TPBAR components. Once benchmarked, the tool will be used to simulate the diffusion of tritium (hydrogen) along grain boundaries in α-Zr using the activation energy barrier data previously obtained from Density Functional Theory calculations.
36 MATERIALS SCIENCE↗
Theorem on extensive spectral degeneracy for systems with rigid higher symmetries in general dimensions
We establish, in the spirit of the Lieb-Schultz-Mattis theorem, lower bounds on the spectral degeneracy of quantum systems with higher (gaugelike) symmetries with rather generic physical boundary conditions in an arbitrary number of spatial dimensions. Contrary to applying twists or equivalent adiabatic operations, we exploit the effects of modified boundary conditions. We find that when a general choice of boundary geometry is immaterial in determining spectral degeneracies while approaching the thermodynamic limit, systems that exhibit rigid noncommuting gaugelike symmetries, such as the orbital compass model, must have an exponential (in the size of the boundary) degeneracy of each of their spectral levels.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Design requirements for the NASA Langley supersonic low-disturbance wind tunnel
The high intensity, high frequency acoustic disturbances that cause large adverse boundary layer transition effects on test models in conventional supersonic wind tunnels consist of finite length wavelets radiating from eddies in the turbulent boundary layers of the wind tunnel walls. NASA Langley has undertaken 'quiet' supersonic tunnel research that demonstrates the ability to maintain laminarity at high unit Reynolds numbers on limited upstream regions of the nozzle wall boundary layers in small, Mach 3.0, 3.5, and 5.0 pilot tunnels. The high level acoustic disturbances are then eliminated, and the transition Reynolds numbers measured on cones approach those for atmospheric flight. Attention is presently given to the design requirements that can be extrapolated from these results for a large wind tunnel facility; high quality air filtering, noise attenuation, nozzle coordinate accuracy, and surface finish, are quantified with pilot tunnel data.
Beckwith, I. E.↗
The effect of small streamwise velocity distortion on the boundary layer flow over a thin flat plate with application to boundary layer stability theory
Researchers show how an initially linear spanwise disturbance in the free stream velocity field is amplified by leading edge bluntness effects and ultimately leads to a small amplitude but linear spanwise motion far downstream from the edge. This spanwise motion is imposed on the boundary layer flow and ultimately causes an order-one change in its profile shape. The modified profiles are highly unstable and can support Tollmein-Schlichting wave growth well upstream of the theoretical lower branch of the neutral stability curve for a Blasius boundary layer.
Goldstein, M. E.↗
Effect of transducer size, shape, and surface sensitivity on the measurement of boundary- layer pressures.
Various boundary layer parameters and transducer shape effect on Measurement error of random pressure field by finite size transducer
PRESSURE FIELD↗