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At least 1,675 records · Page 93
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

A numerical scheme to solve unstable boundary value problems

The considered scheme makes it possible to determine an unstable steady state solution in cases in which, because of lack of symmetry, such a solution cannot be obtained analytically, and other time integration or relaxation schemes, because of instability, fail to converge. The iterative solution of a single complex equation is discussed and a nonlinear system of equations is considered. Described applications of the scheme are related to a steady state solution with shear instability, an unstable nonlinear Ekman boundary layer, and the steady state solution of a baroclinic atmosphere with asymmetric forcing. The scheme makes use of forward and backward time integrations of the original spatial differential operators and of an approximation of the adjoint operators. Only two computations of the time derivative per iteration are required.

Kalnay-Rivas, E.↗
NASA NTRS1978

Prediction and monitoring of clear air turbulence: An evaluation of the applicability of the rawinsonde system

The applicability of rawinsonde data for either detection or prediction of clear air turbulence is discussed. The mechanism currently believed responsible for the development of CAT is reviewed. Since previous studies determined that the most significant factor for the existence of turbulent layers is the magnitude of the vertical shear, certain theoretically derived shear criteria was applied to statistical and diagnostic comparisons of rawinsonde and Jimsphere/Jimsonde conjunctive vertical profiles. It is determined that the Rawinsonde system cannot reliably be used as a CAT detector in a single-station sense.

Keller, J. L.↗
NASA NTRS1982

Reducing the non-axisymmetry of a planetary dynamo and an application to Saturn

A simple model for the tendency toward axisymmetrization observed in planets is developed. The model is presented in general but linear form, assuming that the differentially rotating fluid is thin, which means that Lorentz forces or Ohmic dissipation are neglected. Two cases are considered: uniform shear throughout the shell and shear concentrated within a very thin boundary layer. In each case, explicit expressions are obtained for the spatial attenuation of the non-spin-axisymmetric field components. The substantial nonlinear effects which prevent these results from being directly applicable to planets are discussed, with particular emphasis on the Taylor constraint. The model is applied to Saturn and found to give a satisfactory semiquantitative explanation for the near-axisymmetry of the field. The parameter choices required to reproduce the observed tilt are entirely reasonable and potentially testable. The model explains why Jupiter and Saturn are so different.

Stevenson, D. J.↗
NASA NTRS1982

Measurement of the structure of the streamwise vorticity field in a turbulent boundary-layer

A survey of measurements of vorticity in bounded turbulent shear flows is presented. It is shown that the measured rms streamwise vorticity is systematically too large by as much as 45% when measured with a four-sensor Kovasznay-type array. Instantaneous values can be much more in error. Therefore this probe should not be used to detect instantaneous vortical structures. New attempts to measure vorticity are described.

Wallace, J. M.↗
NASA NTRS1984

Numerical simulation of turbulent flows

Computational models of turbulence in incompressible Newtonian fluids governed by the Navier-Stokes equations are reviewed. The governing equations are presented, and both direct and large-eddy-simulations are examined. Resolution requirements and numerical techniques of spatial representation, definition of initial and boundary conditions, and time advancement are considered. Results of simulations of homogeneous turbulence in uniform shear, the evolution of a turbulent mixing layer, and turbulent channel flow are presented graphically and discussed.

Rogallo, R. S.↗
NASA NTRS1984

Some structural features of unsteady separating turbulent shear flows

Some physical features of unsteady separating turbulent boundary layers are presented for practical Reynolds numbers and reduced frequencies for helicopter and turbomachinery flows. Upstream of detachment in moderate amplitude flows, the flow is quasi steady, i.e., the phase averaged flow is described by the steady-free stream flow structure. Results show that oscillation waveform and amplitude strongly influence the detached flow behavior.

Simpson, R. L.↗
NASA NTRS1989

Three-dimensional structure of a plane mixing layer

An experimental study is reported which shows the spanwise structure of a plane mixing layer originating from laminar boundary layers. Quantitative measurements are obtained for the first time which show the origin and evolution of streamwise vortices within the mixing layer. After the first spanwise vortex rollup occurs, streamwise structures are triggered in clusters within the braid region of the mixing layer. Further downstream, they realign to form pairs of counterrotating vortices. The maximum vorticity diffuses rapidly with increasing downstream distance while the vortices grow, scaling with the mixing layer vorticity thickness. The presence of the streamwise vortices leads to significant spanwise distortions in the mean and turbulence properties of the mixing layer. A consistent peak in the secondary shear stress exists for each streamwise vortex.

Bell, James H.↗
NASA NTRS1989

Use of piezoelectric foil for flow diagnostics

A laboratory investigation was conducted to characterize two piezoelectric-film sensor configurations, a rigidly mounted sensor and a sensor mounted over an air cavity. The sensors are evaluated for sensitivity and frequency response, and methods to optimize data are presented. The cavity-mounted sensor exhibited a superior frequency response and was more sensitive to normal pressure fluctuations and less sensitive to vibrations through the structure. Both configurations were sensitive to large-scale structural vibrations. Flight-test data are shown for cavity-mounted sensors, illustrating practical aspects to consider when designing sensors for application in such harsh environments. The relation of the data to skin friction and maximum shear stress, transition detection, and turbulent viscous layers is derived through analysis of the flight data.

Carraway, Debra L.↗
NASA NTRS1990

The University of Minnesota oscillating flow experiments

Hot-wire anemometry techniques used to document laminar-to-turbulent transition in oscillating flow are discussed, and two transition mechanisms proposed by Seume (1988) are reviewed. Reliable near-wall data, and the convergence of ensemble-averaged velocities and rms velocity fluctuations will aid in evaluating wall shear stresses and calculating various boundary-layer parameters. Special consideration was given to a detailed measurement program which documents transition at different axial positions for the space power research engine (SPRE) operating point. Future work on the effect of entry geometry and measurements of heat transfer are outlined. In order to better simulate flow conditions in actual Stirling engine heat exchangers, the effects of various inflow geometries are investigated. Once the hydrodynamic conditions in the test section are documented, a measurement program to assess the effect of flow oscillation on heat transfer will commence.

Friedman, G. T.↗
NASA NTRS1992

The gabbro-eclogite phase transition and the elevation of mountain belts on Venus

Among the four mountain belts surrounding Lakshmi Planum, Maxwell Montes is the highest and stands up to 11 km above the mean planetary radius and 7 km above Lakshmi Planum. The bulk composition and radioactive heat production of the crust on Venus, where measured, are similar to those of terrestrial tholeiitic basalt. Because the thickness of the low-density crust may be limited by the gabbro-garnet granulite-eclogite phase transitions, the 7-11 km maximum elevation of Maxwell Montes is difficult to understand except in the unlikely situation that the crust contains a large volume of magma. A possible explanation is that the base of the crust is not in phase equilibrium. It has been suggested that under completely dry conditions, the gabbro-eclogite phase transition takes place by solid-state diffusion and may require a geologically significant time to run to completion. Solid-state diffusion is a strongly temperature-dependent process. In this paper we solve the thermal evolution of the mountain belt to attempt to constrain the depth of the gabbro-eclogite transition and thus to assess this hypothesis quantitatively. The one-dimensional heat equation is solved numerically by a finite difference approximation. The deformation of the horizontally shortening crustal and mantle portions of the thermal boundary layer is assumed to occur by pure shear, and therefore the vertical velocity is given by the product of the horizontal strain rate and depth.

Namiki, Noriyuki↗
NASA NTRS1993

AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, 34th and AIAA/ASME Adaptive Structures Forum, La Jolla, CA, Apr. 19-22, 1993, Technical Papers. Pts. 1-6

Topics addressed include the prediction of helicopter component loads using neural networks, spacecraft on-orbit coupled loads analysis, hypersonic flutter of a curved shallow panel with aerodynamic heating, thermal-acoustic fatigue of ceramic matrix composite materials, transition elements based on transfinite interpolation, damage progression in stiffened composite panels, a direct treatment of min-max dynamic response optimization problems, and sources of helicopter rotor hub inplane shears. Also discussed are dynamics of a layered elastic system, confidence bounds on structural reliability, mixed triangular space-time finite elements, advanced transparency development for USAF aircraft, a low-velocity impact on a graphite/PEEK, an automated mode-tracking strategy, transonic flutter suppression by a passive flap, a nonlinear response of composite panels to random excitation, an optimal placement of elastic supports on a simply supported plate, a probabilistic assessment of composite structures, a model for mode I failure of laminated composites, a residual flexibility approach to multibody dynamics,and multilayer piezoelectric actuators.

Source record↗
NASA NTRS1998

Interaction of Vascular Smooth Muscle Cells Under Low Shear Stress

The blood vessel wall consists of three cellular layers, an outer adventitial, a middle medial and an inner intimal layer. When the blood vessel forms in the embryo it begins as a tube composed of a single cell type called endothelial cells. Over time, other cells are recruited from the surrounding tissue to form additional layers on the outer surface of the endothelial tube. The cells that are recruited are called mesenchymal cells. Mesenchymal cells are responsible for the production of connective tissue that holds the blood vessel together and for developing into vascular smooth muscle cells that are responsible for regulating the diameter of the vessel (1) and therefore, blood flow. In a fully developed blood vessel, the endothelial cells make- up the majority of cells in the intimal layer while the mesenchymal cells make-up the majority of cells in the medial and adventitial layers. Within the medial layer of a mature vessel, cells are organized into multiple circular layers of alternating bands of connective tissue and cells. The cell layer is composed of a mixture of mesenchymal cells that have not developed into smooth muscle cells and fully developed smooth muscle cells (2). The assembly and organization of complex tissues is directed in part by a signaling system composed of proteins on the cell surface called adhesion molecules. Adhesion molecules enable cells to recognize each other as well as the composition of the connective tissue in which they reside (3). It was hypothesized that the different cell types that compose the vascular wall possess different adhesion molecules that enable them to recognize each other and through this recognition system, form the complex layered organization of the vascular wall. In other words, the layered organization is an intrinsic property of the cells. If this hypothesis is correct then the different cells that make up the vessel wall, when mixed together, should organize themselves into a layered structure resembling an intact blood vessel. Experiments described below were designed to test this hypothesis.

Seidel, Charles L.↗
NASA NTRS2004

Progress on Acoustic Measurements of the Bulk Viscosity of Near-Critical Xenon (BVX)

We plan to determine the bulk viscosity of xenon 10 times closer [in reduced temperature tau = (T-Tc)/Tc] to its liquid-vapor critical point than ever before. (Tc is the critical temperature.) To do so, we must measure the dispersion and attenuation of sound at frequencies 1/100 of those used previously. In general, sound attenuation has contributions from the bulk viscosity acting throughout the volume of the xenon as well as contributions from the thermal conductivity and the shear viscosity acting within thin thermoacoustic boundary layers at the interface between the xenon and the solid walls of the resonator. Thus, we can determine the bulk viscosity only when the boundary layer attenuation is small and well understood. We present a comparison of calculations and measurements of sound attenuation in the acoustic boundary layer of xenon near its liquid-vapor critical point.

Gillis, Keith A.↗
NASA NTRS2010

A Simple Technique for Creating Regional Composites of Sea Surface Temperature from MODIS for Use in Operational Mesoscale NWP

This paper describes a simple technique for creating regional, high-resolution, daytime and nighttime composites of sea surface temperature (SST) for use in operational numerical weather prediction (NWP). The composites are based on observations from NASA s Moderate Resolution Imaging Spectroradiometer (MODIS) aboard Aqua and Terra. The data used typically are available nearly in real time, are applicable anywhere on the globe, and are capable of roughly representing the diurnal cycle in SST. The composites resolution is much higher than that of many other standard SST products used for operational NWP, including the low- and high-resolution Real-Time Global (RTG) analyses. The difference in resolution is key because several studies have shown that highly resolved SSTs are important for driving the air sea interactions that shape patterns of static stability, vertical and horizontal wind shear, and divergence in the planetary boundary layer. The MODIS-based composites are compared to in situ observations from buoys and other platforms operated by the National Data Buoy Center (NDBC) off the coasts of New England, the mid-Atlantic, and Florida. Mean differences, mean absolute differences, and root-mean-square differences between the composites and the NDBC observations are all within tenths of a degree of those calculated between RTG analyses and the NDBC observations. This is true whether or not one accounts for the mean offset between the skin temperatures of the MODIS dataset and the bulk temperatures of the NDBC observations and RTG analyses. Near the coast, the MODIS-based composites tend to agree more with NDBC observations than do the RTG analyses. The opposite is true away from the coast. All of these differences in point-wise comparisons among the SST datasets are small compared to the 61.08C accuracy of the NDBC SST sensors. Because skin-temperature variations from land to water so strongly affect the development and life cycle of the sea breeze, this phenomenon was chosen for demonstrating the use of the MODIS-based composite in an NWP model. A simulated sea breeze in the vicinity of New York City and Long Island shows a small, net, but far from universal improvement when MODIS-based composites are used in place of RTG analyses. The timing of the sea breeze s arrival is more accurate at some stations, and the near-surface temperature, wind, and humidity within the breeze are more realistic.

Knievel, Jason C.↗
NASA NTRS2012

Experimental Studies of Low-Pressure Turbine Flows and Flow Control

This report summarizes research performed in support of the NASA Glenn Research Center (GRC) Low-Pressure Turbine (LPT) Flow Physics Program. The work was performed experimentally at the U.S. Naval Academy faculties. The geometry corresponded to "Pak B" LPT airfoil. The test section simulated LPT flow in a passage. Three experimental studies were performed: (a) Boundary layer measurements for ten baseline cases under high and low freestream turbulence conditions at five Reynolds numbers of 25,000, 50,000, 100,000, 200,000, and 300,000, based on passage exit velocity and suction surface wetted length; (b) Passive flow control studies with three thicknesses of two-dimensional bars, and two heights of three-dimensional circular cylinders with different spanwise separations, at same flow conditions as the 10 baseline cases; (c) Active flow control with oscillating synthetic (zero net mass flow) vortex generator jets, for one case with low freestream turbulence and a low Reynolds number of 25,000. The Passive flow control was successful at controlling the separation problem at low Reynolds numbers, with varying degrees of success from case to case and varying levels of impact at higher Reynolds numbers. The active flow control successfully eliminated the large separation problem for the low Reynolds number case. Very detailed data was acquired using hot-wire anemometry, including single and two velocity components, integral boundary layer quantities, turbulence statistics and spectra, turbulent shear stresses and their spectra, and intermittency, documenting transition, separation and reattachment. Models were constructed to correlate the results. The report includes a summary of the work performed and reprints of the publications describing the various studies.

Volino, Ralph J.↗
NASA NTRS2019

On the Use of Optimization Techniques for Turbulence Model Calibration

This article discusses the use of numerical optimization procedures to aid in the calibration of turbulence model coefficients. Such methods would increase the rigor and repeatability of the calibration procedure by requiring clearly defined and objective optimization metrics, and could be used to identify unique combinations of coefficient values for specific flow problems. The approach is applied to the re-calibration of an explicit algebraic Reynolds stress model for the incompressible planar mixing layer using the Nelder-Mead simplex algorithm and a micro-genetic algorithm with minimally imposed constraints. Three composite fitness functions, each based upon the error in the mixing layer growth rate and the normal and shear components of the Reynolds stresses, are investigated. The results demonstrate a significant improvement in the target objectives through the adjustment of three pressure-strain coefficients. Adjustments of additional coefficients provide little further benefit. Issues regarding the effectiveness of the fitness functions and the efficiency of the optimization algorithms are also discussed.

Yoder, Dennis A.↗
NASA NTRS1993

Analysis of passive damping in thick composite structures

Computational mechanics for the prediction of damping and other dynamic characteristics in composite structures of general thicknesses and laminations are presented. Discrete layer damping mechanics that account for the representation of interlaminar shear effects in the material are summarized. Finite element based structural mechanics for the analysis of damping are described, and a specialty finite element is developed. Applications illustrate the quality of the discrete layer damping mechanics in predicting the damped dynamic characteristics of composite structures with thicker sections and/or laminate configurations that induce interlaminar shear. The results also illustrate and quantify the significance of interlaminar shear damping in such composite structures.

Saravanos, D. A.↗
DOE OSTI2022

Numerical investigation of the influence of shear and thermal stratification on the wind turbine tip‐vortex stability

Summary The interaction between wind turbine wakes and atmospheric turbulence is characterised by complex dynamics. In this study, two major components of the atmospheric boundary layer dynamics have been isolated, namely, the mean velocity profile shear and the thermal stratification, to examine their impact on the near‐wake development by undertaking a series of highly resolved large‐eddy simulations. Subsequently, instantaneous flow fields are extracted from the simulations and used to conduct Fourier analysis and proper orthogonal decomposition (POD) and compute the mean kinetic energy fluxes by different POD modes to better understand the tip‐vortex instability mechanisms. Our findings indicate that shear can significantly affect the breakup of the wind turbine tip‐vortices and the shape and stable length of the wake, whereas thermal stratification seems to only have limited contribution to the spatial development of the near‐wake field. Finally, our analysis shows that the applied perturbation frequency determines the tip‐vortex breakup location as it controls the onset of the mutual inductance instability.

17 WIND ENERGY↗