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At least 307 records · Page 17
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NASA NTRS1995

Technology Transfer of Plate Wave NDE to Ultrasonic Rotary Actuation

Plate waves have been the subject of NDE research and applications. These waves, also known as guided waves of Lamb waves, are formed in two distinct modes--symmetric and antisymmetric --depending on their vibration characteristics in relation to the plate geometry. Experiments have corroborated the predictions for various plate wave modes, allowing the elastic properties of composite materials and adhesive bonded joints to be determined.

joints↗
NASA NTRSDate not supplied

Development of Physics-Based Transition Models for Unstructured-Mesh CFD Codes Using Deep Learning Models

Predicting transition locations over a vehicle surface is of fundamental importance for many engineering applications. With the transition information, the Reynolds-averaged Navier-Stokes (RANS) computations can turn on the turbulence model at the right locations so that drag, lift and other aerodynamic quantities can be accurately predicted. In contrast to the popularity of RANS-based transition modeling in which transition onset is governed by the turbulence equations, physics-based transition models that account for instability waves within the boundary layer, thus more compliant to flow physics, only gained more attention in recent years. This paper describes the development of a new physics-based transition model based on either the linear stability theory (LST) or parabolized stability equations (PSE). The model is designed to communicate with a structured or unstructured-mesh RANS solver back and forth in order to more accurately compute transition fronts over a three-dimensional body. In the developed model, the Python suite of interface codes in conjunction with the LASTRAC software can be executed autonomously to produce transition onset locations for a given laminar or RANS-computed transitional state. In addition, as a proof of concept, the tool set consists of a deep learning neural network model that has been designed and trained to predict instability wave evolutions inside the boundary layer for various instability wave mechanisms across a selected speed range. A machine-learned intelligent profile interpolation model has also been devised to enable reliable instability-wave spectra predictions with just a few points in the mean flow profiles.

Transition Modeling↗
NASA NTRS1995

Protostellar Collapse with a Shock

We reexamine both numerically and analytically the collapse of the singular isothermal sphere in the context of low-mass star formation. We consider the case where the onset of collapse is initiated by some arbitrary process which is accompanied by a central output of either heat or kinetic energy. We find two classes of numerical solutions describing this manner of collapse. The first approaches in time the expansion wave solution of Shu, while the second class is characterized by an ever-decreasing central accretion rate and the presence of an outwardly propagating weak shock. The collapse solution which represents the dividing case between these two classes is determined analytically by a similarity analysis. This solution shares with the expansion wave solution the properties that the gas remains stationary with an r(exp -2) density profile at large radius and that, at small radius, the gas free-falls onto a nascent core at a constant rate which depends only on the isothermal sound speed. This accretion rate is a factor of approx. 0.1 that predicted by the expansion wave solution. This reduction is due in part to the presence of a weak shock which propagates outward at 1.26 times the sound speed. Gas in the postshock region first moves out subsonically but is then decelerated and begins to collapse. The existence of two classes of numerical collapse solutions is explained in terms of the instability to radial perturbations of the analytic solution. Collapse occurring in the manner described by some of our solutions would eventually unbind a finite-sized core. However, this does not constitute a violation of the instability properties of the singular isothermal sphere which is unstable both to collapse and to expansion. To emphasize this, we consider a purely expanding solution for isothermal spheres. This solution is found to be self-similar and results in a uniform density core in the central regions of the gas. Our solutions may be relevant to the 'luminosity' problem of protostellar cores since the predicted central accretion rates are significantly reduced relative to that of the expansion wave solution. Furthermore, our calculations indicate that star-forming cloud cores are not very tightly bound and that modest disturbances can easily result in both termination of infall and dispersal of unaccreted material.

Tsai, John C.↗
NASA NTRS1995

Protostellar Collapse with a Shock

We reexamine both numerically and analytically the collapse of the singular isothermal sphere in the context of low-mass star formation. We consider the case where the onset of collapse is initiated by some arbitrary process which is accompanied by a central output of either heat or kinetic energy. We find two classes of numerical solutions describing this manner of collapse. The first approaches in time the expansion wave solution of Shu, while the second class is characterized by an ever-decreasing central accretion rate and the presence of an outwardly propagating weak shock. The collapse solution which represents the dividing case between these two classes is determined analytically by a similarity analysis. This solution shares with the expansion wave solution the properties that the gas remains stationary with an r(sup -2) density profile at large radius and that, at small radius, the gas free-falls onto a nascent core at a constant rate which depends only on the isothermal sound speed. This accretion rate is a factor of approx. 0.1 that predicted by the expansion wave solution. This reduction is due in part to the presence of a weak shock which propagates outward at 1.26 times the sound speed. Gas in the postshock region first moves out subsonically but is then decelerated and begins to collapse. The existence of two classes of numerical collapse solutions is explained in terms of the instability to radial perturbations of the analytic solution. Collapse occurring in the manner described by some of our solutions would eventually unbind a finite-sized core. However, this does not constitute a violation of the instability properties of the singular isothermal sphere which is unstable both to collapse and to expansion. To emphasize this, we consider a purely expanding solution for isothermal spheres. This solution is found to be self-similar and results in a uniform density core in the central regions of the gas. Our solutions may be relevant to the 'luminosity' problem of protostellar cores since the predicted central accretion rates are significantly reduced relative to that of the expansion wave solution. Furthermore, our calculations indicate that star-forming cloud cores are not very tightly bound and that modest disturbances can easily result in both termination of infall and dispersal of unaccreted material.

Tsai, John C.↗
NASA NTRS1989

Laboratory evaluation and application of microwave absorption properties under simulated conditions for planetary atmospheres

Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments and earth-based radio astronomical observations can be used to infer abundances of microwave absorbing atmospheric constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. Work performed has shown that laboratory measurements of the millimeter-wave opacity of ammonia between 7.5 mm and 9.3 mm and also at the 3.2 mm wavelength require a different lineshape to be used in the theoretical prediction for millimeter-wave ammonia opacity than was previously used. The recognition of the need to make such laboratory measurements of simulated planetary atmospheres over a range of temperatures and pressures which correspond to the altitudes probed by both radio occultation experiments and radio astronomical observations, and over a range of frequencies which correspond to those used in both radio occultation experiments and radio astronomical observations, has led to the development of a facility at Georgia Tech which is capable of making such measurements. It has been the goal of this investigation to conduct such measurements and to apply the results to a wide range of planetary observations, both spacecraft and earth-based, in order to determine the identity and abundance profiles of constituents in those planetary atmospheres.

Steffes, Paul G.↗
NASA NTRS1976

The leaking mode problem in atmospheric acoustic-gravity wave propagation

The problem of predicting the transient acoustic pressure pulse at long horizontal distances from large explosions in the atmosphere is examined. Account is taken of poles off the real axis and of branch line integrals in the general integral governing the transient waveform. Perturbation techniques are described for the computation of the imaginary ordinate of the poles and numerical studies are described for a model atmosphere terminated by a halfspace with c = 478 m/sec above 125 km. For frequencies less than 0.0125 rad/sec, the GR sub 1 mode, for example, is found to have a frequency dependent amplitude decay of the order of 0.0001 nepers/km. Examples of numerically synthesized transient waveforms are exhibited with and without the inclusion of leaking modes. The inclusion of leaking modes results in waveforms with a more marked beginning rather than a low frequency oscillating precursor of gradually increasing amplitude. Also, the revised computations indicate that waveforms invariably begin with a pressure rise, a result supported by other theoretical considerations and by experimental data.

Kinney, W. A.↗
NASA NTRS1994

Numerical Calculations of Shock-Wave/Boundary-Layer Flow Interactions

The paper presents results of calculations for 2-D supersonic turbulent compression corner flows. The results seem to indicate that the newer, improved kappa-epsilon models offer limited advantages over the standard kappa-epsilon model in predicting the shock-wave/boundary-layer flows in the 2-D compression corner over a wide range of corner angles and flow conditions.

Huang, P. G.↗
NASA NTRS2012

Calibration of PCB-132 Sensors in a Shock Tube

While PCB-132 sensors have proven useful for measuring second-mode instability waves in many hypersonic wind tunnels, they are currently limited by their calibration. Until now, the factory calibration has been all that was available, which is a single-point calibration at an amplitude three orders of magnitude higher than a second-mode wave. In addition, little information has been available about the frequency response or spatial resolution of the sensors, which is important for measuring high-frequency instability waves. These shortcomings make it difficult to compare measurements at different conditions and between different sensors. If accurate quantitative measurements could be performed, comparisons of the growth and breakdown of instability waves could be made in different facilities, possibly leading to a method of predicting the amplitude at which the waves break down into turbulence, improving transition prediction. A method for calibrating the sensors is proposed using a newly-built shock tube at Purdue University. This shock tube, essentially a half-scale version of the 6-Inch shock tube at the Graduate Aerospace Laboratories at Caltech, has been designed to attain a moderate vacuum in the driven section. Low driven pressures should allow the creation of very weak, yet still relatively thin shock waves. It is expected that static pressure rises within the range of second-mode amplitudes should be possible. The shock tube has been designed to create clean, planar shock waves with a laminar boundary layer to allow for accurate calibrations. Stronger shock waves can be used to identify the frequency response of the sensors out to hundreds of kilohertz.

Berridge, Dennis C.↗
NASA NTRS1981

Application of millimeter-wave remote sensing to the investigation of comets

It is pointed out the despite years of investigation, the solid, quasi-permanent component of comets, the nucleus, remains largely a mystery. The composition and thermal properties of the nucleus determine the evolution of the more familiar and often spectacular cometary features, the coma and tail. Under certain circumstances, the nucleus may be obscured by a dust cloud of much higher temperature. The most appropriate technology for the investigation of the surface and subsurface layers of the nucleus is considered to be millimeter-wave sensing from an interplanetary spacecraft. Simple radiative transfer models, adapted from methods employed for the interpretation of remote-sensing data on terrestrial ice and snow fields, are used to predict the millimeter-wave spectra of representative model nuclei. The spectra determine the choice of the minimum set of observing frequencies that is required. An instrument configuration driven by these requirements and guided by available technology and the restraints of a proposed NASA spacecraft is then derived.

Hobbs, R. W.↗
NASA NTRS2019

The Evolution of a Pitch-Angle "Bite-Out" Scattering Signature Caused by EMIC Wave Activity: A Case Study

Electromagnetic ion cyclotron (EMIC) waves are understood to be one of the dominant drivers of relativistic electron loss from Earth's radiation belts. Theory predicts that the associated gyroresonant wave‐particle interaction results in a distinct energy‐dependent “bite‐out” signature in the normalized flux distribution of electrons as they are scattered into the loss cone. We identify such signatures along with the responsible EMIC waves captured in situ by the Van Allen Probes on 15–16 February 2017. Using the cold plasma approximation, we predict the pitch‐angle cutoffs for the scattering signature for the captured EMIC wave and find it in good agreement with the observed electron bite‐out scattering signature. Employing the close conjunction between the Van Allen Probes and THEMIS during this time, we explore the temporal and spatial evolution of the scattering signature, as well as the surrounding wave activity, and find that the scattering signature formed during continued wave activity over a period less than a day. These results are consistent with wave‐particle interaction theory and support the hypothesis that EMIC waves are an important mechanism for rapid relativistic electron loss from the radiation belts.

Bingley, L.↗
NASA NTRS1996

Heat and Momentum Transfer Studies in High Reynolds Number Wavy Films at Normal and Reduced Gravity Conditions

We examined the effect of the gas flow on the liquid film when the gas flows in the countercurrent direction in a vertical pipe at normal gravity conditions. The most dramatic effect of the simultaneous flow of gas and liquid in pipes is the greatly increased transport rates of heat, mass, and momentum. In practical situations this enhancement can be a benefit or it can result in serious operational problems. For example, gas-liquid flow always results in substantially higher pressure drop and this is usually undesirable. However, much higher heat transfer coefficients can be expected and this can obviously be of benefit for purposes of design. Unfortunately, designers know so little of the behavior of such two phase systems and as a result these advantages are not utilized. Due to the complexity of the second order boundary model as well as the fact that the pressure variation across the film is small compared to the imposed gas phase pressure, the countercurrent gas flow affect was studied for the standard boundary layer model. A different stream function that can compensate the shear stress affect was developed and this stream function also can predict periodic solutions. The discretized model equations were transformed to a traveling wave coordinate system. A stability analysis of these sets of equations showed the presence of a Hopf bifurcation for certain values of the traveling wave velocity and the shear stress. The Hopf celerity was increased due to the countercurrent shear. For low flow rate the increases of celerity are more than for the high flow rate, which was also observed in experiments. Numerical integration of a traveling wave simplification of the model also predicts the existence of chaotic large amplitude, nonperiodic waves as observed in the experiments. The film thickness was increased by the shear.

Balakotaiah, V.↗
NASA NTRS1972

A model for predicting aortic dynamic response to -G sub z impact acceleration.

A steady state dynamic response model for the radial motion of the aorta is developed from in vivo pressure-displacement and nerve stimulation experiments on canines. The model represented by a modified Van der Pol wave motion oscillator closely predicts steady state and perturbed response results. The applicability of the steady state canine aortic model to tailward acting impact forces is studied by means of the perturbed phase plane of the oscillator. The backflow through the aortic arch resulting from a specified acceleration-time profile is computed and an analysis for predicting the forced motion aortic response is presented.

Advani, S. H.↗
NASA NTRS1992

Connection between ambient density fluctuations and clumpy Langmuir waves in type III radio sources

A recent stochastic-growth theory of clumpy Langmuir waves in type III sources is shown to imply that the clumps will have the same size distribution as the ambient low-frequency density fluctuations in the solar wind. Spectral analysis of Langmuir-wave time series from the ISEE 3 plasma wave instrument confirms this prediction to within the uncertainties in the spectra. The smallest Langmuir clump size is inferred to be in the range 0.4-30 km in general, and 2-30 km for beam-resonant waves, and it is concluded that the diffusion of waves in the source is anomalous.

Robinson, P. A.↗
NASA NTRS1995

Implementation of algebraic stress models in a general 3-D Navier-Stokes method (PAB3D)

A three-dimensional multiblock Navier-Stokes code, PAB3D, which was developed for propulsion integration and general aerodynamic analysis, has been used extensively by NASA Langley and other organizations to perform both internal (exhaust) and external flow analysis of complex aircraft configurations. This code was designed to solve the simplified Reynolds Averaged Navier-Stokes equations. A two-equation k-epsilon turbulence model has been used with considerable success, especially for attached flows. Accurate predicting of transonic shock wave location and pressure recovery in separated flow regions has been more difficult. Two algebraic Reynolds stress models (ASM) have been recently implemented in the code that greatly improved the code's ability to predict these difficult flow conditions. Good agreement with Direct Numerical Simulation (DNS) for a subsonic flat plate was achieved with ASM's developed by Shih, Zhu, and Lumley and Gatski and Speziale. Good predictions were also achieved at subsonic and transonic Mach numbers for shock location and trailing edge boattail pressure recovery on a single-engine afterbody/nozzle model.

Abdol-Hamid, Khaled S.↗
NASA NTRS1994

Sonic boom propagation through turbulence: A ray theory approach

A ray theory approach is used to examine the propagation of sonic booms through a turbulent ground layer, and to make predictions about the received waveform. The rays are not propagated one at a time, as is typical in ray theory; instead, sufficient rays to represent a continuous wave front are propagated together. New rays are interpolated as needed to maintain the continuity of the wave front. In order to predict the received boom signature, the wave front is searched for eigenrays after it has propagated to the receiver. The Comte-Bellot turbulence model is used to generate an instantaneous 'snapshot' of the turbulent field. The transient acoustic wave is assumed to be sufficiently short in duration such that the time-dependacde of the turbulent field may be neglected.

Robinson, Leick D.↗
NASA NTRS1976

Wave propagation in fiber composite laminates, part 2

An experimental investigation was conducted to determine the wave propagation characteristics, transient strains and residual properties in unidirectional and angle-ply boron/epoxy and graphite/epoxy laminates impacted with silicone rubber projectiles at velocities up to 250 MS-1. The predominant wave is flexural, propagating at different velocities in different directions. In general, measured wave velocities were higher than theoretically predicted values. The amplitude of the in-plane wave is less than ten percent of that of the flexural wave. Peak strains and strain rates in the transverse to the (outer) fiber direction are much higher than those in the direction of the fibers. The dynamics of impact were also studied with high speed photography.

Daniel, I. M.↗
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.↗