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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Validation of three-dimensional incompressible spatial direct numerical simulation code: A comparison with linear stability and parabolic stability equation theories for boundary-layer transition on a flat plate

Spatially evolving instabilities in a boundary layer on a flat plate are computed by direct numerical simulation (DNS) of the incompressible Navier-Stokes equations. In a truncated physical domain, a nonstaggered mesh is used for the grid. A Chebyshev-collocation method is used normal to the wall; finite difference and compact difference methods are used in the streamwise direction; and a Fourier series is used in the spanwise direction. For time stepping, implicit Crank-Nicolson and explicit Runge-Kutta schemes are used to the time-splitting method. The influence-matrix technique is used to solve the pressure equation. At the outflow boundary, the buffer-domain technique is used to prevent convective wave reflection or upstream propagation of information from the boundary. Results of the DNS are compared with those from both linear stability theory (LST) and parabolized stability equation (PSE) theory. Computed disturbance amplitudes and phases are in very good agreement with those of LST (for small inflow disturbance amplitudes). A measure of the sensitivity of the inflow condition is demonstrated with both LST and PSE theory used to approximate inflows. Although the DNS numerics are very different than those of PSE theory, the results are in good agreement. A small discrepancy in the results that does occur is likely a result of the variation in PSE boundary condition treatment in the far field. Finally, a small-amplitude wave triad is forced at the inflow, and simulation results are compared with those of LST. Again, very good agreement is found between DNS and LST results for the 3-D simulations, the implication being that the disturbance amplitudes are sufficiently small that nonlinear interactions are negligible.

Joslin, Ronald D.↗
NASA NTRS1992

On the instability of boundary layers on heated flat plates

The stability of a boundary layer on a heated flat plate is investigated in the linear regime. The flow is shown to be unstable to longitudinal vortex structures which in general develop in a nonparallel manner in the streamwise direction. Solutions of the nonparallel equations are obtained numerically at O(1) values of the appropriate stability parameter, i.e., the Grashof number. The particular cases investigated relate to the situations when the instability is induced by localized or distributed wall roughness or nonuniform wall heating. The case when the vortices are induced by freestream disturbances is also considered. The fastest growing mode is found to be governed by a quasi-parallel theory at high wavenumbers. The wavenumber and growth rate of the fastest growing mode are found in closed form. At low wavenumbers the vortex instability is shown to be closely related to Tollmein-Schlichting waves. The effect of wall heating or cooling on the latter type of instability is discussed.

Hall, Philip↗
NASA NTRS1993

Numerical simulation of laminar-turbulent transition in a spatially-developing flat plate wake

Laminar-turbulent transition of an incompressible flat-plate wake is investigated by direct numerical integration of the Navier-Stokes equations. For the numerical integration, a combination of finite-difference and spectral methods along with an ADI/Crank-Nicolson/Adams-Bashforth time integration scheme is employed. Subject to 2D forcing, the wake exhibited a rapidly-growing fundamental disturbance that quickly saturated. This saturation was due partly to the stabilizing effect of the mean flow distortion. Downstream of the saturation point, disturbance energy was concentrated in the fundamental disturbance, the second harmonic, and the mean flow distortion component. At large amplitude levels, a Karman vortex street formed. Variations in the 2D forcing level did not alter the qualitative behavior of the disturbances. Simulations of 3D breakdown indicates that the presence of large-amplitude, 2D disturbances tends to initially suppress small-amplitude 3D disturbance growth. Following this initial suppression, a resumption of 3D growth is observed that may have been due to a secondary instability mechanism. For high levels of 3D disturbance energy, lambda-shaped vortical structures formed between adjacent Karman vortices.

Dratler, D. I.↗
NASA NTRS2016

Changes in Flat Plate Wake Characteristics Obtained With Decreasing Plate Thickness

The near and very near wake of a flat plate with a circular trailing edge is investigated with data from direct numerical simulations. Computations were performed for four different Reynolds numbers based on plate thickness (D) and at constant plate length. The value of θ/D varies by a factor of approximately 20 in the computations (θ being the boundary layer momentum thickness at the trailing edge). The separating boundary layers are turbulent in all the cases. One objective of the study is to understand the changes in wake characteristics as the plate thickness is reduced (increasing θ/D). Vortex shedding is vigorous in the low θ/D cases with a substantial decrease in shedding intensity in the largest θ/D case (for all practical purposes shedding becomes almost intermittent). Other characteristics that are significantly altered with increasing θ/D are the roll-up of the detached shear layers and the magnitude of fluctuations in shedding period. These effects are explored in depth. The effects of changing θ/D on the distributions of the time-averaged, near-wake velocity statistics are discussed.

Wakes↗
NASA NTRS1972

Characteristics of the unsteady shock-induced laminar boundary layer on a flat plate.

The unsteady shock-induced laminar boundary layer on a flat plate is theoretically studied for shock speeds ranging from 1.12 to 9 km/sec. Boundary layer flows for real air, assumed to be in thermochemical equilibrium, are analyzed by extending Lam's work on shock-induced laminar boundary layers to equilibrium dissociated and ionized flows. A complete description of the unsteady nature of the boundary layer is presented in terms of heat transfer and several boundary layer thickness quantities as functions of a single time-position variable. Boundary layer development is considered for two points of view. Time-dependent boundary layer development and approach to steady state for any fixed position on the plate is described, as is the configuration of the boundary layer with position on the plate at any point in time.

Cook, W. J.↗
NASA NTRS1977

High-performance flat-plate solar collector

Concentric glass-tube-envelope device surrounds flat-plate absorber having spectrally selective coating. Transparent envelope has antireflection coating. Heat-transfer medium is gas that circulates along hairpin path.

Reynolds, R. K.↗
NASA NTRS1987

The economic payoff for a state-of-the-art high-efficiency flat-plate crystalline silicon solar cell technology

In 1986 during the flat-plate solar array project, silicon solar cells 4.0 sq cm in area were fabricated at the Jet Propulsion Laboratory (JPL) with a conversion efficiency of 20.1 percent (AM1.5-global). Sixteen cells were processed with efficiencies measuring 19.5 percent (AM1.5 global) or better. These cells were produced using refined versions of conventional processing methods, aside from certain advanced techniques that bring about a significant reduction in a major mechanism (surface recombination) that limits cell efficiency. Wacker Siltronic p-type float-zone 0.18-ohm-cm wafers were used. Conversion efficiencies in this range have previously been reported by other researchers, but generally on much smaller (0.5 vs. 4.0 cm) devices which have undergone sophisticated and costly processing steps. An economic analysis is presented of the potential payoffs for this approach, using the Solar Array Manufacturing Industry Costing Standards (SAMICS) methodology. The process sequence used and the assumptions made for capturing the economies of scale are presented.

Bickler, Donald B.↗
NASA NTRS1989

Part A: Nonprincipal-plane scattering from flat plates: Second-order and corner diffractions

Two models of a flat plate for nonprincipal-plane scattering are explored. The first is a revised version of the Physical Optics/Physical Theory of Diffraction (PO/PTD) model with second-order PTD equivalent currents included to account for second-order interactions among the plate edges. The second model uses a heurisitcally derived corner diffraction coefficient to account for the corner scattering mechanism. The patterns obtained using the newer models were compared to the data of previously reported models, the Moment Method (MM), and experimental results. Near normal incidence, all the models agreed; however, near grazing incidence a need for higher-order and corner diffraction mechanisms was noted. In many instances the second-order and corner-scattered fields which were formulated improved the results.

Balanis, Constantine A.↗
NASA NTRS2019

The Intermediate Wake of a Thin Flat Plate with a Circular Trailing Edge

The intermediate wakes of thin flat plates with circular trailing edges (TEs) are investigated here with direct numerical simulations (DNSs). The separating boundary layers are turbulent in all cases. The near wake in two thin-plate cases (IN & NS), with a focus on the vortex shedding process, was explored in a recent article. Intermittent shedding was observed in Case IN. Case NS, with half the TE diameter of Case IN, was an essentially non-shedding case. A third case (ST) with a sharp trailing edge was also investigated and found to exhibit an intermittent wake instability. The objectives of the present study are twofold. The first is to determine if the wake instability found in Case ST exists in Cases IN and NS as well. The second is to provide the distributions of the turbulent normal intensities and shear stress in the wake and to understand these distributions via the budget terms in the corresponding transport equations. The results show that both Cases IN & NS exhibit a wake instability in the intermediate wake region, that is similar to that found earlier in Case ST. We note that in Case IN, the presence of an intermediate-wake instability results in the co-existence of two different types of instability within a single wake. The distributions of the turbulent normal intensities and shear stress, and the budget terms for the streamwise intensity are included and discussed here. All the budget terms contribute appreciably to the overall budget in the transport equation for streamwise normal intensity.

Rai, Man Mohan↗