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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.

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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Solid Surface Combustion at Reduced Gravity

The spread of a flame in the gas over the surface of a solid combustible involves in an essential way the transfer of heat from the flame to the solid fuel immediately ahead of it. This heat transfer is affected by the character of the gas phase flame, and so the phenomenon of flame spreading under reduced gravity, in which the flow is generated by gasification of the solid combustible, is apt to be different from what occurs under the Earth's normal gravitational acceleration where the flow is largely buoyancy driven. An experiment is being designed for the Middeck of the Space Shuttle to aid us in understanding the process of flame spreading in the absence of a buoyancy driven flow. A chamber approximately 0.35 cu.m. in volume is to contain either a thin sample of a cellulosic material or a thick sample of polymethyl-methacrylate and an oxidizing environment of O2 and N2. Samples will be ignited at one end, and the ensuing flame spread will be filmed. The spread rate can be determined from the films, and surface and gas-phase temperatures just above the surface will also be recorded. These data will help to clarify the mechanism of forward heat transfer in the low gravity flames.

Altenkirch, R. A.↗
NASA NTRS1992

Effect of hindlimb unweighting on tissue blood flow in the rat

This study characterized distribution of blood flow in the rat during hindlimb unweighting (HU), and post-HU standing and exercise. The relationship between reduced hindlimb blood flow and the previously observed elevation in anaerobic metabolism observed with contractile activity in the atrophied soleus muscle was examined (Witzmann et al., 1992). Blood flow was measured during unweighting, normal standing, and running on a treadmill (15 m/min), after 15 days of HU or cage control. For another group blood flow was measured during preexercise treadmill standing and treadmill running. During unweighting, PE standing, and running no difference in soleus blood flow was observed between groups. Muscles composed mainly of fast twitch glycolytic fibers received greater blood flow during chronic unweighting. With exercise blood flow to visceral organs was reduced in control animals, a similar change was not seen in 15 day HU rats. These changes suggest a reduction in the ability of the sympathetic nervous system to distribute cardiac output after chronic HU. A reduction in blood flow to the soleus during exercise was not observed after HU and so does not explain the increased dependence of the atrophied soleus on anerobic energy production during contractile activity.

Mcdonald, K. S.↗
NASA NTRS1992

A numerical study of three-dimensional surface tension driven convection with fre surface deformation

The steady three-dimensional thermocapillary motion with a deformable free surface is studied numerically in both normal and zero gravity environments. Flow configurations consist of a square cavity heated from the side. In the analysis, the free surface is allowed to deform and the grid distribution is adapted to the surface deformation. The divergence-free condition is satisfied by using a dual time-stepping approach in the numerical scheme. Convective flux derivatives are evaluated using a third-order accurate upwind-biased flux-split differencing technique. The numerical solutions at the midplane of the square cavity are compared with the results from two-dimensional calculations. In addition, numerial results for cases under zero and normal gravity conditions are compared. Significantly different flow structures and surface deformation have been observed. The comparison of calculated results will be compared with experimental data in the updated version of this paper.

Hsieh, Kwang-Chung↗
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 NTRS1983

Reattaching turbulent shear layers with perturbed structure

The reattachment of a fully turbulent, two-dimensional shear layer downstream of a backward-facing step has been studied experimentally. The work examines the effect of modifications in shear layer structure upstream of reattachment. A series of experiments were conducted in a low-speed wind tunnel using specialized instrumentation suited to the highly turbulent, reversing flow near reattachment. Accurate characterization of the time-mean features of the reattaching flows was possible. Assuming linear scaling normalized on distance from reattachment, distributions of the normalized pressure coefficients, the time-averaged skin friction coefficient, and the forward flow fraction appear universal for two-dimensional reattachment, independent of initial conditions and steep height, for given duct geometry (area ratio) and for high step height Reynolds numbers. The results suggest universal flow structure in the reattachment zone.

Westphal, R. V.↗
NASA NTRS1985

Normal mode Rossby waves observed in the upper stratosphere

In recent years, observational evidence has been obtained for westward traveling planetary waves in the middle atmosphere with the aid of global data from satellites. There is no doubt that the fair portion of the observed traveling waves can be understood as the manifestation of the normal mode Rossby waves which are theoretically derived from the tidal theory. Some observational aspects of the structure and behavior of the normal model Rossby waves in the upper stratosphere are reported. The data used are the global stratospheric geopotential thickness and height analyses which are derived mainly from the Stratospheric Sounding Units (SSUs) on board TIROS-N and NOAA satellites. A clear example of the influence of the normal mode Rossby wave on the mean flow is reported. The mechanism considered is interference between the normal mode Rossby wave and the quasi-stationary wave.

Hirooka, T.↗
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 NTRS1993

Techniques for estimating Space Station aerodynamic characteristics

A method was devised and calculations were performed to determine the effects of reflected molecules on the aerodynamic force and moment coefficients for a body in free molecule flow. A procedure was developed for determining the velocity and temperature distributions of molecules reflected from a surface of arbitrary momentum and energy accommodation. A system of equations, based on momentum and energy balances for the surface, incident, and reflected molecules, was solved by a numerical optimization technique. The minimization of a 'cost' function, developed from the set of equations, resulted in the determination of the defining properties of the flow reflected from the arbitrary surface. The properties used to define both the incident and reflected flows were: average temperature of the molecules in the flow, angle of the flow with respect to a vector normal to the surface, and the molecular speed ratio. The properties of the reflected flow were used to calculate the contribution of multiply reflected molecules to the force and moments on a test body in the flow. The test configuration consisted of two flat plates joined along one edge at a right angle to each other. When force and moment coefficients of this 90 deg concave wedge were compared to results that did not include multiple reflections, it was found that multiple reflections could nearly double lift and drag coefficients, with nearly a 50 percent increase in pitching moment for cases with specular or nearly specular accommodation. The cases of diffuse or nearly diffuse accommodation often had minor reductions in axial and normal forces when multiple reflections were included. There were several cases of intermediate accommodation where the addition of multiple reflection effects more than tripled the lift coefficient over the convex technique.

Thomas, Richard E.↗
NASA NTRS1992

Active control of asymmetric vortical flows around cones using injection and heating

The effectiveness of certain active-control methods for asymmetric flows around circular cones is investigated by using computational solution of the unsteady, compressible full Navier-Stokes equations. Two main methods of active control which include flow injection and surface heating are used. For the flow-injection-control method, flow injection is used either in the normal direction to the surface or in the tangential direction to the surface. For the surface-heating-control method, the temperature of the cone surface is increased. The effectiveness of a hybrid method of flow control which combines normal injection with surface heating has also been studied. The Navier-Stokes equations, subjected to various surface boundary conditions, are solved by using an implicit, upwind, flux-difference splitting, finite-volume scheme for locally-conical flow solutions.

Kandil, Osama A.↗
NASA NTRS1975

A Model for the Vortex Pair Associated with a Jet in a Cross Flow

A model is presented for the contrarotating vortex pair that is formed by a round, turbulent, subsonic jet directed normally into a uniform, subsonic cross flow. The model consists of a set of algebraic equations that describe the properties of the vortex pair as a function of their location in the jet plume. The parameters of the model are physical characteristics of the vortices such as the vortex strength, spacing, and core size. These parameters are determined by velocity measurements at selective points in the jet plume.

Sellers, William L.↗
NASA NTRS1976

Observations on autoregulation in skeletal muscle - The effects of arterial hypoxia

An experimental study was carried out on 25 mongrel dogs of both sexes to re-evaluate autoregulation of blood flow in skeletal muscle, with particular reference to the steady-state resistance and transient response in muscle blood flow following a square wave increase in arterial perfusion pressure and to the examination of the effect of arterial hypoxia on this transient response. The data emphasize the importance of considering the transient changes in blood flow in evaluating the autoregulatory response in skeletal muscle. For quantification purposes, a parameter termed alpha is introduced which represents the ratio between the increase in blood flow from baseline to peak and the return of blood flow from the peak to the new steady-state. Such a quantification of the transient response in flow with step increases in perfusion pressure demonstrates substantial transient responses under conditions of normal oxygenation and progressive attenuation of flow transients with increasing hypoxia.

Pohost, G. M.↗
NASA NTRS1981

Simulation of ideal-gas flow by nitrogen and other selected gases at cryogenic temperatures

The real gas behavior of nitrogen, the gas normally used in transonic cryogenic tunnels, is reported for the following flow processes: isentropic expansion, normal shocks, boundary layers, and interactions between shock waves and boundary layers. The only difference in predicted pressure ratio between nitrogen and an ideal gas which may limit the minimum operating temperature of transonic cryogenic wind tunnels occur at total pressures approaching 9 atm and total temperatures 10 K below the corresponding saturation temperature. These pressure differences approach 1 percent for both isentropic expansions and normal shocks. Alternative cryogenic test gases were also analyzed. Differences between air and an ideal diatomic gas are similar in magnitude to those for nitrogen and should present no difficulty. However, differences for helium and hydrogen are over an order of magnitude greater than those for nitrogen or air. It is concluded that helium and cryogenic hydrogen would not approximate the compressible flow of an ideal diatomic gas.

Hall, R. M.↗
DOE OSTI2025

Experimental and theoretical evidence of universality in superfluid vortex reconnections

The minimum separation between reconnecting vortices in fluids and superfluids obeys a universal scaling law with respect to time. The prereconnection and the postreconnection prefactors of this scaling law are different, a property related to irreversibility and to energy transfer and dissipation mechanisms. In the present work, we determine the temperature dependence of these prefactors in superfluid helium from experiments and a numeric model which fully accounts for the coupled dynamics of the superfluid vortex lines and the thermal normal fluid component. At all temperatures, we observe a pre- and postreconnection asymmetry similar to that observed in other superfluids and in classical viscous fluids, indicating that vortex reconnections display a universal behavior independent of the small-scale regularizing dynamics. We also numerically show that each vortex reconnection event represents a sudden injection of energy in the normal fluid. Finally we argue that in a turbulent flow, these punctuated energy injections can sustain the normal fluid in a perturbed state, provided that the density of superfluid vortices is large enough.

Reconnections↗
NASA NTRS2001

Numerical Simulation of an Enclosed Laminar Jet Diffusion Flame in Microgravity Environment: Comparison with ELF Data

Enclosed diffusion flames are commonly found in practical combustion systems, such as the power-plant combustor, gas turbine combustor, and jet engine after-burner. In these systems, fuel is injected into a duct with a co-flowing or cross-flowing air stream. The diffusion flame is found at the surface where the fuel jet and oxygen meet, react, and consume each other. In combustors, this flame is anchored at the burner (i.e., fuel jet inlet) unless adverse conditions cause the flame to lift off or blow out. Investigations of burner stability study the lift off, reattachment, and blow out of the flame. Flame stability is strongly dependent on the fuel jet velocity. When the fuel jet velocity is sufficiently low, the diffusion flame anchors at the burner rim. When the fuel jet velocity is increased, the flame base gradually moves downstream. However, when the fuel jet velocity increases beyond a critical value, the flame base abruptly jumps downstream. When this "jump" occurs, the flame is said to have reached its lift-off condition and the critical fuel jet velocity is called the lift-off velocity. While lifted, the flame is not attached to the burner and it appears to float in mid-air. Flow conditions are such that the flame cannot be maintained at the burner rim despite the presence of both fuel and oxygen. When the fuel jet velocity is further increased, the flame will eventually extinguish at its blowout condition. In contrast, if the fuel jet velocity of a lifted flame is reduced, the flame base moves upstream and abruptly returns to anchor at the burner rim. The fuel jet velocity at reattachment can be much lower than that at lift off, illustrating the hysteresis effect present in flame stability. Although there have been numerous studies of flame stability, the controlling mechanisms are not well understood. This uncertainty is described by Pitts in his review of various competing theories of lift off and blow out in turbulent jet diffusion flames. There has been some research on the stability of laminar flames, but most studies have focused on turbulent flames. It is also well known that the airflow around the fuel jet can significantly alter the lift off, reattachment and blow out of the jet diffusion flame. Buoyant convection is sufficiently strong in 1-g flames that it can dominate the flow-field, even at the burner rim. In normal-gravity testing, it is very difficult to delineate the effects of the forced airflow from those of the buoyancy-induced flow. Comparison of normal-gravity and microgravity flames provides clear indication of the influence of forced and buoyant flows on the flame stability. The overall goal of the Enclosed Laminar Flames (ELF) investigation (STS-87/USMP-4 Space Shuttle mission, November to December 1997) is to improve our understanding of the effects of buoyant convection on the structure and stability of co-flow diffusion flame, e.g., see http://zeta.lerc.nasa.gov/expr/elf.htm. The ELF hardware meets the experiment hardware limit of the 35-liter interior volume of the glovebox working area, and the 180x220-mm dimensions of the main door. The ELF experiment module is a miniature, fan-driven wind tunnel, equipped with a gas supply system. A 1.5-mm diameter nozzle is located on the duct's flow axis. The cross section of the duct is nominally a 76-mm square with rounded corners. The forced air velocity can be varied from about 0.2 to 0.9 m/s. The fuel flow can be set as high as 3 std. cubic centimeter (cc) per second, which corresponds to a nozzle exit velocity of up to 1.70 m/s. The ELF hardware and experimental procedure are discussed in detail in Brooker et al. The 1-g test results are repeated in several experiments following the STS-87 Mission. The ELF study is also relevant to practical systems because the momentum-dominated behavior of turbulent flames can be achieved in laminar flames in microgravity. The specific objectives of this paper are to evaluate the use reduced model for simulation of flame lift-off and blowout.

Jia, Kezhong↗
NASA NTRS1985

Quasi non-intrusive sampling and analysis of gases associated with the boundary layer on the tethered satellite and similar supersonic and hypersonic research vehicles

The effect that one candidate inlet, an assembly of capillary openings in a thin glass plate (a multichannel plate), might have on the overall sampling process is considered. The flow characteristics of the plate, under a variety of conditions of external pressure and mass flow, is evaluated. A review of capillary flow theory is presented with some development of the pertinent equations. The predicted mass flow will be compared to that determined perimentally to assess the effect that mass flow through one capillary might have upon a neighboring capillary. Mass spectrometric measurements of mixtures flowing through the multichannel plate (MCP) is also considered. In the first part of the experiments, the flow was in a direction normal to the surface of the plate. The experimental gases were Argon and mixtures of carbon dioxide in air. Ongoing experiments are discussed which are conducted with the flow parallel to the surface of the plate, a simulation of the kind of flow that a flight vehicle would experience.

Fishel, C.↗
NASA NTRS1991

Compressible stability of growing boundary layers using parabolized stability equations

The parabolized stability equation (PSE) approach is employed to study linear and nonlinear compressible stability with an eye to providing a capability for boundary-layer transition prediction in both 'quiet' and 'disturbed' environments. The governing compressible stability equations are solved by a rational parabolizing approximation in the streamwise direction. Nonparallel flow effects are studied for both the first- and second-mode disturbances. For oblique waves of the first-mode type, the departure from the parallel results is more pronounced as compared to that for the two-dimensional waves. Results for the Mach 4.5 case show that flow nonparallelism has more influence on the first mode than on the second. The disturbance growth rate is shown to be a strong function of the wall-normal distance due to either flow nonparallelism or nonlinear interactions. The subharmonic and fundamental types of breakdown are found to be similar to the ones in incompressible boundary layers.

Chang, Chau-Lyan↗
NASA NTRS1999

Bubble Generation in a Continuous Liquid Flow Under Reduced Gravity Conditions

The present work reports a study of bubble generation under reduced gravity conditions for both co-flow and cross-flow configurations. Experiments were performed aboard the DC-9 Reduced Gravity Aircraft at NASA Glenn Research Center, using an air-water system. Three different flow tube diameters were used: 1.27, 1.9, and 2.54 cm. Two different ratios of air injection nozzle to tube diameters were considered: 0.1 and 0.2. Gas and liquid volumetric flow rates were varied from 10 to 200 ml/s. It was experimentally observed that with increasing superficial liquid velocity, the bubbles generated decreased in size. The bubble diameter was shown to increase with increasing air injection nozzle diameters. As the tube diameter was increased, the size of the detached bubbles increased. Likewise, as the superficial liquid velocity was increased, the frequency of bubble formation increased and thus the time to detach forming bubbles decreased. Independent of the flow configuration (for either single nozzle or multiple nozzle gas injection), void fraction and hence flow regime transition can be controlled in a somewhat precise manner by solely varying the gas and liquid volumetric flow rates. On the other hand, it is observed that uniformity of bubble size can be controlled more accurately by using single nozzle gas injection than by using multiple port injection, since this latter system gives rise to unpredictable coalescence of adjacent bubbles. A theoretical model, based on an overall force balance, is employed to study single bubble generation in the dynamic and bubbly flow regime. Under conditions of reduced gravity, the gas momentum flux enhances bubble detachment; however, the surface tension forces at the nozzle tip inhibits bubble detachment. Liquid drag and inertia can act either as attaching or detaching force, depending on the relative velocity of the bubble with respect to the surrounding liquid. Predictions of the theoretical model compare well with performed experiments. However, at higher superficial,liquid velocities, the bubble neck length begins to significantly deviate from the value of the air injection nozzle diameter and thus the theory no longer predicts the experiment behavior. Effects of fluid properties, injection geometry and flow conditions on generated bubble size are investigated using the theoretical model. It is shown that bubble diameter is larger in a reduced gravity environment than in a normal gravity environment at similar flow condition and flow geometry.

Pais, Salvatore Cezar↗