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At least 145 records · Page 8
All sourcesNASA NTRSDOE OSTI
NASA NTRS1996

A Numerical Experiment on the Role of Surface Shear Stress in the Generation of Sound

The sound generated due to a localized flow over an infinite flat surface is considered. It is known that the unsteady surface pressure, while appearing in a formal solution to the Lighthill equation, does not constitute a source of sound but rather represents the effect of image quadrupoles. The question of whether a similar surface shear stress term constitutes a true source of dipole sound is less settled. Some have boldly assumed it is a true source while others have argued that, like the surface pressure, it depends on the sound field (via an acoustic boundary layer) and is therefore not a true source. A numerical experiment based on the viscous, compressible Navier-Stokes equations was undertaken to investigate the issue. A small region of a wall was oscillated tangentially. The directly computed sound field was found to to agree with an acoustic analogy based calculation which regards the surface shear as an acoustically compact dipole source of sound.

Shariff, Karim↗
NASA NTRS1990

Turbulence and shear stress measurements in hypersonic flow

An assessment has been made of the potential for hot wire and laser anemometer measurements of turbulent fluctuations in hypersonic flow fields. The results of laser velocimeter experiments conducted in two hypersonic wind tunnels are reported and comparisons made with previous hot wire turbulence measurements. A new concept for the measurement of the compressible shear stress terms which combines the attributes of both hot wire and laser anemometry is presented.

Owen, F. K.↗
NASA NTRS1997

Effect of simulated microgravity and shear stress on microcin B17 production by Escherichia coli and on its excretion into the medium

Production of the antibacterial polypeptide microcin B17 (MccB17) by Escherichia coli ZK650 was inhibited by simulated microgravity. The site of MccB17 accumulation was found to be different, depending on whether the organism was grown in shaking flasks or in rotating bioreactors designed to establish a simulated microgravity environment. In flasks, the accumulation was cellular, but in the reactors, virtually all the microcin was found in the medium. The change from a cellular site to an extracellular one was apparently not a function of gravity, since extracellular production occurred in these bioreactors, irrespective of whether they were operated in the simulated microgravity or normal gravity mode. More probably, excretion is due to the much lower degree of shear stress in the bioreactors. Addition of even a single glass bead to the 50-ml medium volume in the bioreactor created enough shear to change the site of MccB17 accumulation from the medium to the cells.

NASA Center JSC↗
NASA NTRS1997

The Effects of Composition and gamma'/gamma Lattice Parameter Mismatch on the Critical Resolved Shear Stresses for Octahedral and Cube Slip in NiAlCrX Alloys

Prototypical single-crystal NiAlCrX superalloys were studied to examine the effects of the common major alloying elements, Co, Mo, Nb, Ta, Ti, and W, on yielding behavior. The alloys contained about 10 at. pct Cr, 60 vol pct of the gamma' phase, and about 3 at. pct of X in the gamma'. The critical resolved shear stresses (CRSSs) for octahedral and primary cube slip were measured at 760 C, which is about the peak strength temperature. The CRSS(sub oct) and CRSS(sub cube) are discussed in relation to those of Ni, (Al, X) gamma' alloys taken from the literature and the gamma'/gamma lattice mismatch. The CRSS(sub oct) of the gamma + gamma' alloys reflected a similar compositional dependence to that of both the CRSS(sub cube) of the gamma' phase and the gamma'/gamma lattice parameter mismatch. The CRSS(sub cube) of the gamma + gamma' alloys also reflected the compositional dependence of the gamma'/gamma mismatch, but bore no similarity to that of CRSS(sub cube) for gamma' alloys since it is controlled by the gamma matrix. The ratio of CRSS(sub cube)/CRSS(sub oct) was decreased by all alloying elements except Co, which increased the ratio. The decrease in CRSS(sub cube)/CRSS(sub oct) was related to the degree in which elements partition to the gamma' rather than the gamma phase.

Miner, R. V.↗
NASA NTRS1986

Shear stress cleaning for surface departiculation

A cleaning technique widely used by the nuclear utility industry for removal of radioactive surface contamination has proven effective at removing non-hazardous contaminant particles as small as 0.1 micrometer. The process employs a controlled high velocity liquid spray inside a vapor containment enclosure to remove particles from a surface. The viscous drag force generated by the cleaning fluid applies a shear stress greater than the adhesion force that holds small particles to a substrate. Fluid mechanics and field tests indicate general cleaning parameters.

Musselman, R. P.↗
DOE OSTI2024

PpIX‐enabled fluorescence‐based detection and photodynamic priming of platinum‐resistant ovarian cancer cells under fluid shear stress

Over 75% percent of ovarian cancer patients are diagnosed with advanced-stage disease characterized by unresectable intraperitoneal dissemination and the presence of ascites, or excessive fluid build-up within the abdomen. Conventional treatments include cytoreductive surgery followed by multi-line platinum and taxane chemotherapy regimens. Despite an initial response to treatment, over 75% of patients with advanced-stage ovarian cancer will relapse and succumb to platinum-resistant disease. Recent evidence suggests that fluid shear stress (FSS), which results from the movement of fluid such as ascites, induces epithelial-to-mesenchymal transition and confers resistance to carboplatin in ovarian cancer cells. This study demonstrates, for the first time, that FSS-induced platinum resistance correlates with increased cellular protoporphyrin IX (PpIX), the penultimate downstream product of heme biosynthesis, the production of which can be enhanced using the clinically approved pro-drug aminolevulinic acid (ALA). These data suggest that, with further investigation, PpIX could serve as a fluorescence-based biomarker of FSS-induced platinum resistance. Additionally, this study investigates the efficacy of PpIX-enabled photodynamic therapy (PDT) and the secretion of extracellular vesicles under static and FSS conditions in Caov-3 and NIH:OVCAR-3 cells, two representative cell lines for high-grade serous ovarian carcinoma (HGSOC), the most lethal form of the disease. FSS induces resistance to ALA-PpIX-mediated PDT, along with a significant increase in the number of EVs. Finally, the ability of PpIX-mediated photodynamic priming (PDP) to enhance carboplatin efficacy under FSS conditions is quantified. These preliminary findings in monolayer cultures necessitate additional studies to determine the feasibility of PpIX as a fluorescence-based indicator, and mediator of PDP, to target chemoresistance in the context of FSS.

59 BASIC BIOLOGICAL SCIENCES↗
NASA NTRS1993

Method of determining shear stress employing a monomer-polymer laminate structure

A laminate structure attached to the test surface of an article is presented. The laminate structure is comprised of a liquid crystal polymer substrate. A light absorbing coating is applied to the substrate and is thin enough to permit bonding steric interaction between the liquid crystal polymer substrate and an overlying liquid crystal monomer thin film. Light is directed through and reflected by the liquid crystal monomer thin film and unreflected light is absorbed by the underlying coating. The wavelength of the reflected light is indicative of the shear stress experienced by the test surface.

Singh, Jag J.↗
NASA NTRS1986

Development of buried wire gages for measurement of wall shear stress in Blastane experiments

Buried Wire Gages operated from a Constant Temperature Anemometer System are among the special types of instrumentation to be used in the Boundary Layer Apparatus for Subsonic and Transonic flow Affected by Noise Environment (BLASTANE). These Gages are of a new type and need to be adapted for specific applications. Methods were developed to fabricate Gage inserts and mount those in the BLASTANE Instrumentation Plugs. A large number of Gages were prepared and operated from a Constant Temperature Anemometer System to derive some of the calibration constants for application to fluid-flow wall shear-stress measurements. The final stage of the calibration was defined, but could not be accomplished because of non-availability of a suitable flow simulating apparatus. This report provides a description of the Buried Wire Gage technique, an explanation of the method evolved for making proper Gages and the calibration constants, namely Temperature Coefficient of Resistance and Conduction Loss Factor.

Murthy, S. V.↗
DOE OSTI2025

High y + Shear-Stress Turbulence Implementation for High Flux Isotope Reactor Narrow Channel Flows

The research objective of this work was to improve the engineering predictions of the turbulence characteristics of flows in curved narrow channels. Such channel flows are commonly encountered in nuclear research and test reactors, with one of them being the high-flux isotope reactor (HFIR). Research reactors bear high heat fluxes, and the proper computing of turbulence is paramount for safe and reliable reactor operation. The study builds on the results of a previous direct numerical simulation of turbulence to inform a well-known Reynolds-averaged Navier–Stokes shear-stress turbulence model and improves its accuracy in simulating parallel channel flows. A new formulation of the loss term in the dissipation conservation equation is suggested. Combined with high wall distance computational grids, the new implementation provides a fast-running flow solution, suitable for engineering purposes. Model generalization for parallel channel flows, in a broader range of frictional Reynolds numbers, is suggested by introducing a new form of the model constants.

CFD↗
DOE OSTI2021

Frequency–Magnitude Statistics of Laboratory Foreshocks Vary With Shear Velocity, Fault Slip Rate, and Shear Stress

Understanding the nucleation phase of earthquakes is key for advancing earthquake hazard assessment and improving earthquake early warning systems. However, little progress has been made in this area due to a poor understanding of nucleation processes and incomplete seismic and fault zone measurements. The ability to integrate measured fault zone properties with seismic data could significantly improve our understanding of how earthquakes begin and whether there are systematic variations in seismic properties preceding failure. In this work, we use high-resolution laboratory measurements of fault zone properties along with acoustic emission data to document temporal variations of foreshock properties. Our data show that foreshock size increases with shear stress, loading rate, and fault slip rate. We propose that the preseismic fault slip rate and fault zone thickness (i.e., porosity) work in concert to modulate foreshock properties.

15 GEOTHERMAL ENERGY↗
NASA NTRS1980

Interaction between a normal shock wave and a turbulent boundary layer at high transonic speeds. Part 1: Pressure distribution. Part 2: Wall shear stress. Part 3: Simplified formulas for the prediction of surface pressures and skin friction

An asymptotic description is derived for the interaction between a shock wave and a turbulent boundary layer in transonic flow, for a particular limiting case. The dimensionless difference between the external flow velocity and critical sound speed is taken to be much smaller than one, but large in comparison with the dimensionless friction velocity. The basic results are derived for a flat plate, and corrections for longitudinal wall curvature and for flow in a circular pipe are also shown. Solutions are given for the wall pressure distribution and the shape of the shock wave. Solutions for the wall shear stress are obtained, and a criterion for incipient separation is derived. Simplified solutions for both the wall pressure and skin friction distributions in the interaction region are given. These results are presented in a form suitable for use in computer programs.

Adamson, T. C., Jr.↗
NASA NTRS1987

Reynolds shear stress development in pressure-driven three-dimensional turbulent boundary-layers

The development of the Reynolds stresses has been examined experimentally in an initially two-dimensional boundary layer which is driven to three dimensionality by a spanwise pressure gradient. The pressure field was imposed by an upstream-facing wedge. Two different wedge angles were used in order to vary the level of boundary layer skewing. Bradshaw's Al parameter was found to decrease with the rate of decrease being dependent on the level of skewing between the freestream and the wall flow. It was also concluded that the ratio of the cross-stream to streamwise shear stress components was governed by the rate of freestream turning.

Anderson, S. D.↗
DOE OSTI2021

Machine Learning Predicts the Timing and Shear Stress Evolution of Lab Earthquakes Using Active Seismic Monitoring of Fault Zone Processes

Abstract Machine learning (ML) techniques have become increasingly important in seismology and earthquake science. Lab‐based studies have used acoustic emission data to predict time‐to‐failure and stress state, and in a few cases, the same approach has been used for field data. However, the underlying physical mechanisms that allow lab earthquake prediction and seismic forecasting remain poorly resolved. Here, we address this knowledge gap by coupling active‐source seismic data, which probe asperity‐scale processes, with ML methods. We show that elastic waves passing through the lab fault zone contain information that can predict the full spectrum of labquakes from slow slip instabilities to highly aperiodic events. The ML methods utilize systematic changes in P‐wave amplitude and velocity to accurately predict the timing and shear stress during labquakes. The ML predictions improve in accuracy closer to fault failure, demonstrating that the predictive power of the ultrasonic signals improves as the fault approaches failure. Our results demonstrate that the relationship between the ultrasonic parameters and fault slip rate, and in turn, the systematically evolving real area of contact and asperity stiffness allow the gradient boosting algorithm to “learn” about the state of the fault and its proximity to failure. Broadly, our results demonstrate the utility of physics‐informed ML in forecasting the imminence of fault slip at the laboratory scale, which may have important implications for earthquake mechanics in nature.

58 GEOSCIENCES↗
NASA NTRS2011

Sensor for Direct Measurement of the Boundary Shear Stress in Fluid Flow

The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear and normal stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear and normal stress and their fluctuations are attractive alternatives. However, this approach is a challenging one especially for high spatial resolution and high fidelity measurements. The authors designed and fabricated a prototype miniature shear stress sensor including an EDM machined floating plate and a high-resolution laser optical encoder. Tests were performed both in air as well as operation in water with controlled flow. The sensor sensitivity, stability and signal-to-noise level were measured and evaluated. The detailed test results and a discussion of future work will be presented in this paper.

shear stress↗
NASA NTRS2011

Sensor for Direct Measurement of the Boundary Shear Stress in Fluid Flow

The formation of scour patterns at bridge piers is driven by the forces at the boundary of the water flow. In most experimental scour studies, indirect processes have been applied to estimate the shear and normal stress using measured velocity profiles. The estimations are based on theoretical models and associated assumptions. However, the turbulence flow fields and boundary layer in the pier-scour region are very complex. In addition, available turbulence models cannot account accurately for the bed roughness effect. Direct measurement of the boundary shear and normal stress and their fluctuations are attractive alternatives. However, this approach is a challenging one especially for high spatial resolution and high fidelity measurements. The authors designed and fabricated a prototype miniature shear stress sensor including an EDM machined floating plate and a high-resolution laser optical encoder. Tests were performed both in air as well as operation in water with controlled flow. The sensor sensitivity, stability and signal-to-noise level were measured and evaluated. The detailed test results and a discussion of future work will be presented in this paper.

sheer stress↗