Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “SHEAR”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1993

Experimental investigations of the time and flow-direction responses of shear-stress-sensitive liquid crystal coatings

Time and flow-direction responses of shearstress-sensitive liquid crystal coatings were explored experimentally. For the time-response experiments, coatings were exposed to transient, compressible flows created during the startup and off-design operation of an injector-driven supersonic wind tunnel. Flow transients were visualized with a focusing Schlieren system and recorded with a 1000 frame/sec color video camera. Liquid crystal responses to these changing-shear environments were then recorded with the same video system, documenting color-play response times equal to, or faster than, the time interval between sequential frames (i.e., 1 millisecond). For the flow-direction experiments, a planar test surface was exposed to equal-magnitude and known-direction surface shear stresses generated by both normal and tangential subsonic jet-impingement flows. Under shear, the sense of the angular displacement of the liquid crystal dispersed (reflected) spectrum was found to be a function of the instantaneous direction of the applied shear. This technique thus renders dynamic flow reversals or flow divergences visible over entire test surfaces at image recording rates up to 1 KHz. Extensions of the technique to visualize relatively small changes in surface shear stress direction appear feasible.

Reda, Daniel C.↗
NASA NTRS1992

Vortex-induced disturbance field in a compressible shear layer

The disturbance field induced by a small isolated vortex in a compressible shear layer is studied using direct simulation in a convected frame. The convective Mach number, M(sub c), is varied from 0.1 to 1.25. The vorticity perturbation is rapidly sheared by the mean velocity gradient. The resulting disturbance pressure field is observed to decrease both in magnitude and extent with increasing M(sub c), becoming a narrow transverse zone for M(sub c) greater than 0.8. A similar trend is seen for the perturbation velocity magnitude and for the Reynolds shear stress. By varying the vortex size, we verified that the decrease in perturbation levels is due to the mean-flow Mach number and not the Mach number across the vortex. At high M(sub c), the vortex still communicates with the edges of the shear layer, although communication in the mean-flow direction is strongly inhibited. The growth rate of perturbation kinetic energy declines with M(sub c) primarily due to the reduction in shear stress. For M(sub c) greater than or equal to 0.6, the pressure dilatation also contributes to the decrease of growth rates. Calculation of the perturbation field induced by a vortex doublet revealed the same trends as in the single-vortex case, illustrating the insensitivity of the Mach-number effect to the specific form of initial conditions.

Papamoschou, D.↗
NASA NTRS1994

Ignition dynamics of a laminar diffusion flame in the field of a vortex embedded in a shear flow

The role of streamwise-spanwise vorticity interactions that occur in turbulent shear flows on flame/vortex interactions is examined by means of asymptotic analysis and numerical simulation in the limit of small Mach number. An idealized model is employed to describe the interaction process. The model consists of a one-step, irreversible Arrhenius reaction between initially unmixed species occupying adjacent half-planes which are then allowed to mix and react in the presence of a streamwise vortex embedded in a shear flow. It is found that the interaction of the streamwise vortex with shear gives rise to small-scale velocity oscillations which increase in magnitude with shear strength. These oscillations give rise to regions of strong temperature gradients via viscous heating, which can lead to multiple ignition points and substantially decrease ignition times. The evolution in time of the temperature and mass-fraction fields is followed, and emphasis is placed on the ignition time and structure as a function of vortex and shear strength.

Macaraeg, Michele G.↗
NASA NTRS1994

Nonlinear Instability of a Uni-directional Transversely Sheared Mean Flow

It is well known that the presence of a weak cross flow in an otherwise two-dimensional shear flow results in a spanwise variation in the mean streamwise velocity profile that can lead to an amplification of certain three-dimensional disturbances through a kind of resonant-interaction mechanism (Goldstein and Wundrow 1994). The spatial evolution of an initially linear, finite-growth-rate, instability wave in such a spanwise-varying shear flow is considered, The base flow, which is governed by the three-dimensional parabolized Navier-Stokes equations, is initiated by imposing a spanwise- periodic cross-flow velocity on an otherwise two-dimensional shear flow at some fixed streamwise location. The resulting mean-flow distortion initially grows with increasing streamwise distance, reaches a maximum and eventually decays through the action of viscosity. This decay, which coincides with the viscous spread of of the shear layer, means that the local growth rate of the instability wave will eventually decrease as the wave propagates downstream. Nonlinear effects can then become important within a thin spanwise-modulated critical layer once the local instability-wave amplitude and growth rate become sufficiently large and small, respectively. The amplitude equation that describes this stage of evolution is shown to be a generalization of the one obtained by Goldstein and Choi (1989) who considered the related problem of the interaction of two oblique modes in a two-dimensional shear layer.

Wundrow, David W.↗
NASA NTRS1995

Method for determining shear direction using liquid crystal coatings

A method is provided for determining shear direction wherein a beam of white light is directed onto the surface of a liquid crystal coating to cause the white light to be dispersed (reflected) from the surface in a spectrum having bands of different colors in a fixed spatial 2 (angular) sequence. The system is calibrated by locating an observer, e.g., a video and movie camera, such that a particular color band (preferably at or near the center of the reflected spectrum) is observed to thereby provide a reference color band. Because the application of shear causes either clockwise or counterclockwise rotation of the reflected spectrum dependent on the direction of the shear, a determination is then made of the reflected color band observed by the observer when the surface of the liquid crystal is subjected to shear to thereby determine the direction of the shear based on the directional (rotation) relation of the observed color band with respect to the reference color band in the spatial sequence of color bands.

Reda, Daniel C.↗
NASA NTRS1995

Effect of mesh distortion on the accuracy of transverse shear stresses and their sensitivity coefficients in multilayered composites

A study is made of the effect of mesh distortion on the accuracy of transverse shear stresses and their first-order and second-order sensitivity coefficients in multilayered composite panels subjected to mechanical and thermal loads. The panels are discretized by using a two-field degenerate solid element, with the fundamental unknowns consisting of both displacement and strain components, and the displacement components having a linear variation throughout the thickness of the laminate. A two-step computational procedure is used for evaluating the transverse shear stresses. In the first step, the in-plane stresses in the different layers are calculated at the numerical quadrature points for each element. In the second step, the transverse shear stresses are evaluated by using piecewise integration, in the thickness direction, of the three-dimensional equilibrium equations. The same procedure is used for evaluating the sensitivity coefficients of transverse shear stresses. Numerical results are presented showing no noticeable degradation in the accuracy of the in-plane stresses and their sensitivity coefficients with mesh distortion. However, such degradation is observed for the transverse shear stresses and their sensitivity coefficients. The standard of comparison is taken to be the exact solution of the three-dimensional thermoelasticity equations of the panel.

Noor, Ahmed K.↗
NASA NTRS1994

Selective excitation of tropical atmospheric waves in wave-CISK: The effect of vertical wind shear

The growth of waves and the generation of potential energy in wave-CISK require unstable waves to tilt with height oppositely to their direction of propagation. This makes the structures and instability properties of these waves very sensitive to the presence of vertical shear in the basic flow. Equatorial Kelvin and Rossby-gravity waves have opposite phase tilt with height to what they have in the stratosphere, and their growth is selectively favored by basic flows with westward vertical shear and eastward vertical shear, respectively. Similar calculations are also made for gravity waves and Rossby waves. It is shown that eastward vertical shear of the basic flow promotes CISK for westward propagating Rossby-gravity, Rossby, and gravity waves and suppresses CISK for eastward propagating Kelvin and gravity waves, while westward shear of the basic flow has the reverse effects.

Zhang, Minghua↗
NASA NTRS1994

Coupling of microprocesses and macroprocesses due to velocity shear: An application to the low-altitude ionosphere

Recent observations indicate that low-altitude (below 1500 km) ion energization and thermal ion upwelling are colocated in the convective flow reversal region. In this region the convective velocity V(sub perpendicular) is generally small but spatial gradients in V(sub perpendicular) can be large. As a result, Joule heating is small. The observed high level of ion heating (few electron volts or more) cannot be explained by classical Joule heating alone but requires additional heating sources such as plasma waves. At these lower altitudes, sources of free energy are not obvious and hence the nature of ion energization remains ill understood. The high degree of correlation of ion heating with shear in the convective velocity (Tsunoda et al., 1989) is suggestive of an important role of velocity shear in this phenomenon. We provide more recent evidence for this correlation and show that even a small amount of velocity shear in the transverse flow is sufficient to excite a large-scale Kelvin-Helmholtz mode, which can nonlinearly steepen and give rise to highly stressed regions of strongly sheared flows. Futhermore, these stressed regions of strongly sheared flows may seed plasma waves in the range of ion cyclotron to lower hybrid frequencies, which are potential sources for ion heating. This novle two-step mechanism for ion energization is applied to typical observations of low-altitude thermal ion upwelling events.

Ganguli, G.↗
NASA NTRS1995

Disruption of a helmet streamer by photospheric shear

Helmet streamers on the Sun have been observed to be the site of coronal mass ejections, dynamic events that eject coronal plasma and magnetic fields into the solar wind. We develop a two-dimensional (azimuthally symmetric) helmet streamer configuration by computing solutions of the time-dependent magnetohydrodynamic (MHD) equations, and we investigate the evolution of the configuration when photospheric shearing motions are imposed. We find that the configuration disrupts when a critical shear is exceeded, ejecting a plasmoid into the solar wind. The results are similar to the case of a sheared dipole magnetic field in a hydrostatic atmosphere (Mikic & Linker 1994). However, the presence of the outflowing solar wind makes the disruption significantly more energetic when a helmet streamer is sheared. Our resutls suggest that shearing of helmet streamers may initiate coronal mass ejections.

Linker, Jon A.↗
NASA NTRS1995

Large-Amplitude Long-Wave Instability of a Supersonic Shear Layer

For sufficiently high Mach numbers, small disturbances on a supersonic vortex sheet are known to grow in amplitude because of slow nonlinear wave steepening. Under the same external conditions, linear theory predicts slow growth of long-wave disturbances to a thin supersonic shear layer. An asymptotic formulation is given here which adds nonzero shear-layer thickness to the weakly nonlinear formulation for a vortex sheet. Spatial evolution is considered, for a spatially periodic disturbance having amplitude of the same order, in Reynolds number, as the shear-layer thickness. A quasi-equilibrium inviscid nonlinear critical layer is found, with effects of diffusion and slow growth appearing through nonsecularity condition. Other limiting cases are also considered, in an attempt to determine a relationship between the vortex-sheet limit and the long-wave limit for a thin shear layer; there appear to be three special limits, corresponding to disturbances of different amplitudes at different locations along the shear layer.

Messiter, A. F.↗
NASA NTRS1996

Ames Research Center Shear Tests of SLA-561V Heat Shield Material for Mars-Pathfinder

This report describes the results of arc-jet testing at Ames Research Center on behalf of Jet Propulsion Laboratory (JPL) for the development of the Mars-Pathfinder heat shield. The current test series evaluated the performance of the ablating SLA-561V heat shield material under shear conditions. In addition, the effectiveness of several methods of repairing damage to the heat shield were evaluated. A total of 26 tests were performed in March 1994 in the 2 in. X 9 in. arc-heated turbulent Duct Facility, including runs to calibrate the facility to obtain the desired shear stress conditions. A total of eleven models were tested. Three different conditions of shear and heating were used. The non-ablating surface shear stresses and the corresponding, approximate, non-ablating surface heating rates were as follows: Condition 1, 170 N/m(exp 2) and 22 W/cm(exp 2); Condition 2, 240 N/m(exp 2) and 40 W/cm(exp 2); Condition 3, 390 N/m(exp 2) and 51 W/cm(exp 2). The peak shear stress encountered in flight is represented approximately by Condition 1; however, the heating rate was much less than the peak flight value. The peak heating rate that was available in the facility (at Condition 3) was about 30 percent less than the maximum value encountered during flight. Seven standard ablation models were tested, of which three models were instrumented with thermocouples to obtain in-depth temperature profiles and temperature contours. An additional four models contained a variety of repair plugs, gaps, and seams. These models were used to evaluated different repair materials and techniques, and the effect of gaps and construction seams. Mass loss and surface recession measurements were made on all models. The models were visually inspected and photographed before and after each test. The SLA-561 V performed well; even at test Condition 3, the char remained intact. Most of the resins used for repairs and gap fillers performed poorly. However, repair plugs made of SLA-561V performed well. Approximately 70 percent of the thermocouples yielded good data.

Tauber, Michael↗
NASA NTRS1997

Buckling Behavior of Long Symmetrically Laminated Plates Subjected to Shear and Linearly Varying Axial Edge Loads

A parametric study of the buckling behavior of infinitely long symmetrically laminated anisotropic plates that are subjected to linearly varying edge loads, uniform shear loads, or combinations of these loads is presented. The study focuses on the effects of the shape of linearly varying edge load distribution, plate orthotropy, and plate flexural anisotropy on plate buckling behavior. In addition, the study exmines the interaction of linearly varying edge loads and uniform shear loads with plate flexural anisotropy and orthotropy. Results obtained by using a special purpose nondimensional analysis that is well suited for parametric studies of clamped and simply supported plates are presented for [+/- theta](sub s), thin graphite-epoxy laminates that are representative of spacecraft structural components. Also, numerous generic buckling-design charts are presented for a wide range of nondimensional parameters that are applicable to a broad class of laminate constructions. These charts show explicitly the effects of flexural orthotropy and flexural anisotropy on plate buckling behavior for linearly varying edge loads, uniform shear loads, or combinations of these loads. The most important finding of the present study is that specially orthotropic and flexurally anisotropic plates that are subjected to an axial edge load distribution that is tension dominated can support shear loads that are larger in magnitude than the shear buckling load.

Nemeth, Michael P.↗
NASA NTRS1994

Particle Distribution in Suspension Shear Flow

An earlier proposed constitutive relation for normal stresses originated by random particle fluctuations is used to describe a joint effect of thermal and shear-induced fluctuations on concentrational distributions in suspension flow. Averaged products of components of the fluctuation velocity are evaluated on a basis of the rational mechanics approach combined with a simple kinematic consideration. The equation of momentum conservation of the dispersed phase of a suspension closed with this constitutive relation is applied to unidirectional shear flow in the gravity field and to rotational Couette flow. Coupling of the thermal and shear-induced fluctuations results in that the ability of shear flow to suspend particles has a minimum at a certain particle size, all other things being equal. The developed model provides also for a reasonable explanation of particle distributions observed in Couette flow. The approach based on the consideration of momentum balance for the dispersed phase is proved to lead to an effective equation of convective diffusion of the suspended particles. Coefficients of mutual diffusion due to both thermal and shear-induced fluctuations are drastically different from corresponding self-diffusivities as regards both their scaling and their concentrational dependence.

Buyevich, Yu A.↗
NASA NTRS2002

Shear Instabilities as a Probe of Jupiter's Atmosphere

Linear wave patterns in Jupiter clouds with wavelengths strongly clustered around 300 km are commonly observed in the planet's equatorial atmosphere. We propose that the preferred wavelength is related to the thickness of an unstable shear layer within the clouds. We numerically analyze the linear stability of wavelike disturbances that have nonzero horizontal phase speeds in Jupiter's atmosphere and find that. if the static stability in the strongly clustered around 300 km are commonly observed in the planet's equatorial atmosphere. We propose that the preferred wavelength is related to the thickness of an unstable shear layer within the clouds. We numerically analyze the linear stability of wavelike disturbances that have nonzero horizontal phase speeds in Jupiter's atmosphere and find that. if the static stability in the shear layer is very low (but still nonnegative), a deep vertical shear layer like the one measured by the Galileo probe can generate the instabilities. The fastest growing waves grow exponentially within an hour, and their wavelengths match the observations. Close to zero values of static stability that permit the growth of instabilities are within the range of values measured by the Galileo probe in a hot spot. Our model probes Jupiter's equatorial atmosphere below the cloud deck and suggests that thick regions of wind shear and low static stability exist outside hot spots.

Bosak, Tanja↗
NASA NTRS2003

Measurements of Shear Lift Force on a Bubble in Channel Flow in Microgravity

Under microgravity conditions, the shear lift force acting on bubbles, droplets or solid particles in multiphase flows becomes important because under normal gravity, this hydrodynamic force is masked by buoyancy. This force plays an important role in furnishing the detachment process of bubbles in a setting where a bubble suspension is needed in microgravity. In this work, measurements of the shear lift force acting on a bubble in channel flow are performed. The shear lift force is deduced from the bubble kinematics using scaling and then compared with predictions from models in literature that address different asymptotic and numerical solutions. Basic trajectory calculations are then performed and the results are compared with experimental data of position of the bubble in the channel. A direct comparison of the lateral velocity of the bubbles is also made with the lateral velocity prediction from investigators, whose work addressed the shear lift on a sphere in different two-dimensional shear flows including Poiseuille flow.

Nahra, Henry K.↗
NASA NTRS2003

DNS and LES of a Shear-Free Mixing Layer

The purpose of this work is twofold. First, given the computational resources available today, it is possible to reach, using DNS, higher Reynolds numbers than in Briggs et al.. In the present study, the microscale Reynolds numbers reached in the low- and high-energy homogeneous regions are, respectively, 32 and 69. The results reported earlier can thus be complemented and their robustness in the presence of increased turbulence studied. The second aim of this work is to perform a detailed and documented LES of the shear-free mixing layer. In that respect, the creation of a DNS database at higher Reynolds number is necessary in order to make meaningful LES assessments. From the point of view of LES, the shear-free mixing-layer is interesting since it allows one to test how traditional LES models perform in the presence of an inhomogeneity without having to deal with difficult numerical issues. Indeed, as argued in Briggs et al., it is possible to use a spectral code to study the shear-free mixing layer and one can thus focus on the accuracy of the modelling while avoiding contamination of the results by commutation errors etc. This paper is organized as follows. First we detail the initialization procedure used in the simulation. Since the flow is not statistically stationary, this initialization procedure has a fairly strong influence on the evolution. Although we will focus here on the shear-free mixing layer, the method proposed in the present work can easily be used for other flows with one inhomogeneous direction. The next section of the article is devoted to the description of the DNS. All the relevant parameters are listed and comparison with the Veeravalli & Warhaft experiment is performed. The section on the LES of the shear-free mixing layer follows. A detailed comparison between the filtered DNS data and the LES predictions is presented. It is shown that simple eddy viscosity models perform very well for the present test case, most probably because the flow seems to be almost isotropic in the small-scale range that is not resolved by the LES.

Knaepen, B.↗
NASA NTRS2004

A Rotary Flow Channel for Shear Stress Sensor Calibration

A proposed shear sensor calibrator consists of a rotating wheel with the sensor mounted tangential to the rim and positioned in close proximity to the rim. The shear stress generated by the flow at the sensor position is simply tau(sub omega) = (mu)r(omega)/h, where mu is the viscosity of the ambient gas, r the wheel radius, omega the angular velocity of the wheel, and h the width of the gap between the wheel rim and the sensor. With numerical values of mu = 31 (mu)Pa s (neon at room temperature), r = 0.5 m, omega = 754 /s (7200 rpm), and h = 50.8 m, a shear stress of tau(sub omega) = 231 Pa can be generated. An analysis based on one-dimensional flow, with the flow velocity having only an angular component as a function of the axial and radial coordinates, yields corrections to the above simple formula for the curvature of the wheel, flatness of the sensor, and finite width of the wheel. It is assumed that the sensor mount contains a trough (sidewalls) to render a velocity release boundary condition at the edges of the rim. The Taylor number under maximum flow conditions is found to be 62.3, sufficiently low to obviate flow instability. The fact that the parameters entering into the evaluation of the shear stress can be measured to high accuracy with well-defined uncertainties makes the proposed calibrator suitable for a physical standard for shear stress calibration.

Zuckerwar, Allan J.↗
NASA NTRS2004

The Need for a Shear Stress Calibration Standard

By surveying current research of various micro-electro mechanical systems (MEMS) shear stress sensor development efforts we illustrate the wide variety of methods used to test and characterize these sensors. The different methods of testing these sensors make comparison of results difficult in some cases, and also this comparison is further complicated by the different formats used in reporting the results of these tests. The fact that making these comparisons can be so difficult at times clearly illustrates a need for standardized testing and reporting methodologies. This need indicates that the development of a national or international standard for the calibration of MEMS shear stress sensors should be undertaken. As a first step towards the development of this standard, two types of devices are compared and contrasted. The first type device is a laminar flow channel with two different versions considered: the first built with standard manufacturing techniques and the second with advanced precision manufacturing techniques. The second type of device is a new concept for creating a known shear stress consisting of a rotating wheel with the sensor mounted tangentially to the rim and positioned in close proximity to the rim. The shear stress generated by the flow at the sensor position is simply tau = (mu)r(omega)/h, where mu is the viscosity of the ambient gas, r the wheel radius, omega the angular velocity of the wheel, and h the width of the gap between the wheel rim and the sensor. Additionally, issues related to the development of a standard for shear stress calibration are identified and discussed.

Scott, Michael A.↗