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At least 1,063 records · Page 59

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NASA NTRS1989

Flow-induced morphological instabilities due to temporally-modulated stagnation-point flow

The influence of periodically-modulated planar stagnation-point flow on the morphological stability of a directionally-solidifying interface is presently considered with a view to the effect of unsteady nonparallel flows on single-crystal growth. The modeling of the system assumes that the viscous boundary layer thickness is much greater than that of the solute boundary layer, and that the modulation frequency is much smaller than the strength of plane stagnation-point flow. The solidifying interface is either stabilized or destabilized depending on the ratio of the period of modulation to the solute-diffusion time.

Merchant, G. J.↗

NASA NTRS1989

Effects of a temperature-dependent rheology on large-scale continental extension

The effects of a temperature-dependent rheology on large-scale continental extension are investigated using a thin viscous sheet model. A vertically-averaged rheology is used that is consistent with laboratory experiments on power-law creep of olivine and that depends exponentially on temperature. Results of the calculations depend principally on two parameters: the Peclet number, which describes the relative rates of advection and diffusion of heat, and a dimensionless activation energy, which controls the temperature dependence of the rheology. At short times following the beginning of extension, deformation occurs with negligible change in temperature, so that only small changes in lithospheric strength occur due to attenuation of the lithosphere. However, after a certain critical time interval, thermal diffusion lowers temperatures in the lithosphere, strongly increasing lithospheric strength and slowing the rate of extension. This critical time depends principally on the Peclet number and is short compared with the thermal time constant of the lithosphere. The strength changes cause the locus of high extensional strain rates to shift with time from regions of high strain to regions of low strain. Results of the calculations are compared with observations from the Aegean, where maximum extensional strains are found in the south, near Crete, but maximum present-day strain rates are largest about 300 km further north.

Sonder, Leslie J.↗

NASA NTRS1992

Wall functions for the kappa-epsilon turbulence model in generalized nonorthogonal curvilinear coordinates

A k-epsilon turbulence model suitable for compressible flow, including the new wall function formulation, has been incorporated into an existing compressible Reynolds-averaged Navier-Stokes code, F3D. The low Reynolds number k-epsilon model of Chien (1982) was added for comparison with the present method. A number of features were added to the F3D code including improved far-field boundary conditions and viscous terms in the streamwise direction. A series of computations of increasing complexity was run to test the effectiveness of the new formulation. Flow over a flat plate was computed by using both orthogonal and nonorthogonal grids, and the friction coefficients and velocity profiles compared with a semi-empirical equation. Flow over a body of revolution at zero angle of attack was then computed to test the method's ability to handle flow over a curved surface. Friction coefficients and velocity profiles were compared to test data. All models gave good results on a relatively fine grid, but only the wall function formulation was effective with coarser grids. Finally, in order to demonstrate the method's ability to handle complex flow fields, separated flow over a prolate spheroid at angle of attack was computed, and results were compared to test data. The results were also compared to a k-epsilon model by Kim and Patel (1991), in which one equation model patched in at the wall was employed. Both models gave reasonable solutions, but improvement is required for accurate prediction of friction coefficients in the separated regions.

Sondak, D. L.↗

NASA NTRS1993

Venus pancake dome formation: Morphologic effects of a cooling-induced variable viscosity during emplacement

The distinctive steep-sided 'pancake' domes discovered in the Magellan images of Venus have morphologies that suggest formation by a single continuous emplacement of a high viscosity magma. A resemblance of the venusian domes to much smaller terrestrial rhyolite and dacite volcanic domes has prompted some authors to suggest that the domes on Venus also have high silica compositions and thus, high viscosities. However, viscosity is a function of crystallinity as well as silica content in a magma, and thus increases as a result of magmatic cooling. To investigate the effect of a cooling-induced viscosity increase on dome morphology, we have modeled the domes as radial viscous gravity currents that cool during emplacement. Various aspects of the investigation are discussed.

Sakimoto, S. E. H.↗

NASA NTRS2020

Gas-Liquid Flows Through Porous Media in Microgravity: The International Space Station Packed Bed Reactor Experiment

Experimental results on pressure drop and flow patterns for gas-liquid flow through packed beds obtained in the International Space Station with two types of packing are presented and analyzed. It is found that the pressure drop depends on the packing wettability in the viscous-capillary (V-C) regime and this dependence is compared with previously published results developed using short duration low-gravity aircraft tests. Within the V-C regime, the capillary contribution is the dominant force contributing to the pressure drop for the wetting case (glass) versus the viscous contribution dominating for the non-wetting case (Teflon). Outside of the V-C regime, it is also found that hysteresis effects that are often strong in normal gravity gas-liquid flows are greatly diminished in microgravity and pressure drop is nearly independent of packing wettability. A flow pattern transition map from bubble to pulse flow is also compared with the earlier aircraft data.

Viscous-capiliary regime↗

NASA NTRS1992

A note on the effects of viscosity on the stability of a trailing-line vortex

The linear stability of the Batchelor (1964) vortex is examined with emphasis on new viscous modes recently found numerically by Khorrami (1991). Unlike the previously reported inviscid modes of instability, these modes are destabilized by viscosity and exhibit small growth rates at large Reynolds numbers. The analysis presented here uses a combination of asymptotic and numerical techniques. The results confirm the existence of the additional modes of instability due to viscosity.

Duck, Peter W.↗

DOE OSTI2022

Nonequilibrium effects and transverse spherocity in ultrarelativistic proton-nucleus collisions

We investigate the effects of nonequilibrium dynamics in small colliding systems by comparing a nonequilibrium transport approach, Parton-Hadron-String Dynamics (PHSD), with a (2+1)-dimensional viscous hydrodynamic model, VISHNew. Focusing on p + Pb collisions at energies available at the Large Hadron Collider, we extract the initial conditions for the hydrodynamic model from PHSD, in order to reduce the impact of the early out-of-equilibrium dynamics and focus on the traces of nonequilibrium in the ensuing medium evolution. We find that in the transport approach quantities like energy density and bulk viscous pressure are highly inhomogeneous on the transverse plane during the whole evolution, whereas the hydrodynamic simulations dissolve more efficiently the initial spatial irregularities, still keeping a high degree of inhomogeneity due to the smaller size and lifetime of the medium produced in p + Pb collisions with respect to heavy-ion reactions. As a first step that will help to identify the impact of these nonequilibrium effects on final observables in proton-nucleus collisions, we perform an analysis of the transverse spherocity, an event-shape observable able to distinguish between jetty and isotropic configurations of transverse momenta. We found that the spherocity distribution in PHSD is slightly shifted towards the isotropic limit with respect to the hydrodynamic result. Even though this dissimilarity is partially due to the difference in the final charged particle production, it mainly comes from the different description within the two frameworks of the medium produced in small colliding systems. This finding supports the idea that multidifferential measurements, such as those based on event categorization according to multiplicity and spherocity, are useful to study final-state observables in ultrarelativistic proton-nucleus collisions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

NASA NTRS1982

Analyses of processing variables in propellant burn rate and modulus. I - Mixing times

The effect of the final mixing time on quality of smokeless AP/HTPB propellant with bimodal AP was examined. Identical ingredients were mixed for various intervals, cast in a circular tube, then fired in a static test. Mechanical properties were measured and initial moduli were calculated from the initial slope of the S-N curves. A theoretical model was developed using the data to quantitatively configure a quality control procedure. A minimum time for mixing fine and coarse AP particles was identified and was approximated to within a factor of two or three using a viscous dispersion time and the surface areas of the particles. The model could be improved by considering settling time and particle size distribution effects.

Ramohalli, K.↗

NASA NTRS1989

Compressible studies on dynamic stall

The purpose of this work is to investigate the effects of compressibility on the dynamic stall phenomenon by numerical simulation of the unsteady flow. The full two-dimensional unsteady compressible Navier-Stokes equations are solved for flows over oscillating airfoils and airfoils pitching rapidly to high angles of attack. The free-stream speeds vary from low subsonic with mild compressibility effects, to moderate subsonic where strong compressibility effects appear close to the leading edge at high angles of attack. An Alternating Direction Implicit scheme is implemented for the numerical solution with the viscous terms retained in both directions. The numerical results are compared with available experimental data for a Sikorsky airfoil for compressible high Reynolds number flows. There is good agreement between the computed and measured unsteady lift and pitching moment coefficient time histories. The computed high-speed subsonic unsteady results give a good picture of the entire flow field, and the dynamic stall progression in the compressible flow regime. It was observed that compressibility effects are more severe close to the leading edge at moderate angles of attack, and that the dynamic stall vortex appears at lower angles of attack as the free-stream speed increases.

Ekaterinaris, J. A.↗

DOE OSTI2021

Suppression of electroconvective and morphological instabilities by an imposed cross flow of the electrolyte

Electroconvective (EC) instability and its influence on surface morphological perturbations (Morph) are important in many applications, including electrodialysis, batteries, and fuel cells. In this work, we study the effects of a two-dimensional channel flow on the EC and Morph instabilities using two approaches. In the bulk analysis, we derive the asymptotic solutions for small and large wave numbers by neglecting the space charge layer and imposing a second kind electroosmosis slip velocity boundary condition on the electroneutral bulk region. In the full analysis, the instability of the entire region of the liquid electrolyte is analyzed using the ultraspherical spectral method. Both studies show that the flow significantly affects the EC instability. The imposed flow distorts the concentration field, causing a sheltering effect which hinders the ion transport from low- to high-concentration regions, and therefore suppresses the EC instability below a certain wave number. In combination with the viscous stabilization of the high wave number modes by the space charge layer, a sufficiently strong imposed flow can fully suppress the EC instability. The increment in the critical voltage for the instability onset is roughly proportional to the square root of the product of imposed velocity and double layer thickness. The imposed flow has a smaller effect on the Morph instability, except that it may remove the morphological modes resulting from the EC instability and thereby change the wave number of the most unstable mode.

42 ENGINEERING↗

NASA NTRS1995

Effects of highly intermittent magnetic fields and plasma flows on solar wind dynamics

Filamentary magnetic fields and intermittent mass flows with highly variant physical parameters as observed in coronal holes provide, from a theoretical point of view, natural conditions for strongly nonlinear dynamics. The presence of sheared mass flows along fine scale magnetic structures results in strong nonlinear instability, most important of which is the explosive instability. We specify the physical conditions for several different manifestations of the onset of explosive instability and its further evolution: (1) fully developed explosive instability - explosive release of the energy; (2) shock formation - stabilization of instability by small scale spatial inhomogeneities leads to formation of subsequent shocks having a number of peculiarities that is determined by the interplay of thermal and viscous losses (for example, in predominance of thermal losses the isothermal jump occurs); and (3) solitary waves - stabilization of explosive instability by nonlinear dispersion effect leads to formation of a 'gas' of solitons which are later either damped away with characteristic time and energy input or evolve to solitons with explosively growing amplitudes. Each scenario is completely determined by the physical parameters of the medium, thus producing a quite uneven distribution of energy in a coronal hole and, respectively, an uneven outward propagation of the energy flux.

Ryutova, M.↗

NASA NTRS2007

The Effect of Acoustic Disturbances on the Operation of the Space Shuttle Main Engine Fuel Flowmeter

The Space Shuttle Main Engine (SSME) uses a turbine fuel flowmeter (FFM) in its Low Pressure Fuel Duct (LPFD) to measure liquid hydrogen flowrates during engine operation. The flowmeter is required to provide accurate and robust measurements of flow rates ranging from 10000 to 18000 GPM in an environment contaminated by duct vibration and duct internal acoustic disturbances. Errors exceeding 0.5% can have a significant impact on engine operation and mission completion. The accuracy of each sensor is monitored during hot-fire engine tests on the ground. Flow meters which do not meet requirements are not flown. Among other parameters, the device is screened for a specific behavior in which a small shift in the flow rate reading is registered during a period in which the actual fuel flow as measured by a facility meter does not change. Such behavior has been observed over the years for specific builds of the FFM and must be avoided or limited in magnitude in flight. Various analyses of the recorded data have been made prior to this report in an effort to understand the cause of the phenomenon; however, no conclusive cause for the shift in the instrument behavior has been found. The present report proposes an explanation of the phenomenon based on interactions between acoustic pressure disturbances in the duct and the wakes produced by the FFM flow straightener. Physical insight into the effects of acoustic plane wave disturbances was obtained using a simple analytical model. Based on that model, a series of three-dimensional unsteady viscous flow computational fluid dynamics (CFD) simulations were performed using the MSFC PHANTOM turbomachinery code. The code was customized to allow the FFM rotor speed to change at every time step according to the instantaneous fluid forces on the rotor, that, in turn, are affected by acoustic plane pressure waves propagating through the device. The results of the simulations show the variation in the rotation rate of the flowmeter due to the interaction of the flow straightener wakes and the upstream propagating acoustic waves. A detailed analysis of the acoustic disturbance effects is presented along with an assessment of the impact on measurement accuracy.

Marcu, Bogdan↗

DOE OSTI2021

Dissipative magnetohydrodynamics for nonresistive relativistic plasmas: An implicit second-order flux-conservative formulation with stiff relaxation

Based on a 14-moment closure for nonresistive (general-) relativistic viscous plasmas, here we describe a new numerical scheme that is able to handle all first-order dissipative effects (heat conduction, bulk and shear viscosities), as well the anisotropies induced by the presence of magnetic fields. The latter is parametrized in terms of a thermal gyrofrequency or, equivalently, a thermal Larmor radius and allows to correctly capture the thermal Hall effect. By solving an extended Israel-Stewart-like system for the dissipative quantities that enforces algebraic constraints via stiff-relaxation, we are able to cast all first-order dissipative terms in flux-divergence form. This allows us to apply traditional high-resolution shock capturing methods to the equations, making the system suitable for the numerical study of highly turbulent flows. We present several numerical tests to assess the robustness of our numerical scheme in flat spacetime. The 14-moment closure can seamlessly interpolate between the highly collisional limit found in neutron star mergers, and the highly anisotropic limit of relativistic Braginskii magnetohydrodynamics appropriate for weakly collisional plasmas in black-hole accretion problems. We believe that this new formulation and numerical scheme will be useful for a broad class of relativistic magnetized flows.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

NASA NTRS1975

Estimation of transonic aircraft aerodynamics to high angles of attack

A recently developed method for estimating transonic aircraft aerodynamics to high angles of attack which is especially suited to computerized aircraft design and optimization studies is described. Following theoretical and empirical guidelines, explicit nonlinear equations are formulated for subsonic, transonic, and supersonic speeds; new algorithms are used for assessing compressibility effects and for analyzing transonic, shock-dominated flows which adhere to Laitone's limit Mach-number criterion. Viscous dominated flows are not analyzed. The transonic influence of viscosity is indirectly accommodated by assigning designated inputs rather than extracted solutions for the chordwise locations of shock and separation. The method is extended to complete aircraft configurations by including accounts for nose lift, the wing downwash field, and tail lift. Several comparisons of experiment and estimate are included.

Axelson, J. A.↗

NASA NTRS1975

Magnetospheres of earth and Jupiter after Pioneer 10

Possible reasons are discussed for the marked differences observed between the magnetospheres of earth and Jupiter, and a model of Jupiter's magnetosphere is proposed which can explain the observations of the Pioneer 10 mission. It is shown that the corotating plasma in Jupiter's plasmasphere is in the form of a flattened disk due to inertial forces and that Jupiter's magnetosphere is, in part, a rigidly rotating warped skew 'magnetodisk'. According to the proposed model, the inner part of the magnetosphere consists of a warped magnetodisk, the dipole field lines are modified by a ring current, and the equatorial plasma density increases with distance in the region beyond the synchronous orbit radius. Considerable attention is given to the effects resulting from spiraling of the magnetic-field lines, the tilt in the magnetic dipole, and the strong viscous interaction of the solar wind on the dawn side of the magnetosphere.

Prakash, A.↗

NASA NTRS1982

Finite element analysis of nonisothermal polymer processing operations

A finite element formulation for the analysis of polymer processing is presented and its use in some typical situation including entry flow, transient Couette flow, and the Graetz (forced convection) problem is illustrated. The element formulations are constructed on the premise that momentum convection can be neglected (polymer melt flows typically have very low Reynolds' numbers), but that convective heat transfer may be significant (high Peclet numbers). Nonisothermal effects are considered important in polymer processing, due in part to the significant heating which may occur due to viscous dissipation, and also to the very strong influence of temperature on fluid viscosity. The flow is treated as Newtonian with the flow field being coupled to the heat transfer equation only through the viscous heat generation.

Douglas, C.↗

NASA NTRS1989

Experimental and computational studies of dynamic stall

A review of dynamic stall research in progress under the Navy-NASA Joint Institute of Aeronautics is presented. This effort, which includes both experimental and computational studies of the dynamic stall process, is directed toward better understanding and modeling of the fluid flow that occurs on helicopters and aircraft flying in conditions that induce dynamic stall. The results of research now in progress are presented, with discussion of the experimental program on compressibility effects on dynamic stall, related CFD studies of the stall process based on Navier-Stokes modeling, and viscous-inviscid flow modeling of the incipient stall process.

Carr, L. W.↗

NASA NTRS1999

A Generalized Wall Function

The asymptotic solutions, described by Tennekes and Lumley (1972), for surface flows in a channel, pipe or boundary layer at large Reynolds numbers are revisited. These solutions can be extended to more complex flows such as the flows with various pressure gradients, zero wall stress and rough surfaces, etc. In computational fluid dynamics (CFD), these solutions can be used as the boundary conditions to bridge the near-wall region of turbulent flows so that there is no need to have the fine grids near the wall unless the near-wall flow structures are required to resolve. These solutions are referred to as the wall functions. Furthermore, a generalized and unified law of the wall which is valid for whole surface layer (including viscous sublayer, buffer layer and inertial sublayer) is analytically constructed. The generalized law of the wall shows that the effect of both adverse and favorable pressure gradients on the surface flow is very significant. Such as unified wall function will be useful not only in deriving analytic expressions for surface flow properties but also bringing a great convenience for CFD methods to place accurate boundary conditions at any location away from the wall. The extended wall functions introduced in this paper can be used for complex flows with acceleration, deceleration, separation, recirculation and rough surfaces.

Shih, Tsan-Hsing↗