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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 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Traction of elastohydrodynamic contacts with thermal shearing flow

The formulation and solution for the shear stress and temperature in heavily loaded sliding elastohydrodynamic contacts is presented. The solutions are presented in dimensionless design charts. Integration over the contact area will yield the traction. Accuracy is expected to be very good over the nearly flat part of the contact area where the majority of the sliding traction is generated. The procedure presented is not appropriate for thick film lubrication, for the inlet region, or for the rolling friction of elastohydrodynamic contacts.

Jakobsen, J.↗
NASA NTRS1974

Analytical solution of the optimal three dimensional reentry problem using Chapman's exact equations

This paper presents the general solution for the optimal three dimensional aerodynamic control of a lifting vehicle entering a planetary atmosphere. A set of dimensionless variables is introduced, and the resulting exact equations of motion have the distinctive advantage that they are completely free of the physical characteristics of the vehicle. Furthermore, a general lift-drag polar is used to define the aerodynamic control. Hence, the results obtained apply to any type of vehicle of arbitrary weight, dimensions and shape, having an arbitrary polar and entering any planetary atmosphere.

Vinh, N. X.↗
NASA NTRS1974

Steady simple shear flow past a circular cylinder at moderate Reynolds numbers - A numerical solution

The two-dimensional steady flow of an incompressible viscous fluid past a circular cylinder, placed symmetrically in a simple shear field, has been studied for both the stationary and the freely rotating case by solving numerically the Navier-Stokes equations for values of the Reynolds number R in the range from 0.047 to 70. At R = 0.047, the results obtained are in substantial agreement with the analytic small-R perturbation solution given by Robertson and Acrivos (1970). Inertia effects were found, however, to play a significant role even at R = 1, and hence the calculated flow pattern for R greater than or equal to 1 differs significantly from that of the creeping-flow solution. Specifically, for the freely rotating case, the region of closed streamlines decreases rapidly in extent with increasing R, two symmetrically placed wakes are formed on either side of the cylinder, and the dimensionless rotational speed of the freely suspended cylinder decreases as the reciprocal of the square root of R.

Kossack, C. A.↗
NASA NTRS1974

Transient analysis of a thermal storage unit involving a phase change material

The transient response of a single cell of a typical phase change material type thermal capacitor has been modeled using numerical conductive heat transfer techniques. The cell consists of a base plate, an insulated top, and two vertical walls (fins) forming a two-dimensional cavity filled with a phase change material. Both explicit and implicit numerical formulations are outlined. A mixed explicit-implicit scheme which treats the fin implicity while treating the phase change material explicitly is discussed. A band algorithmic scheme is used to reduce computer storage requirements for the implicit approach while retaining a relatively fine grid. All formulations are presented in dimensionless form thereby enabling application to geometrically similar problems. Typical parametric results are graphically presented for the case of melting with constant heat input to the base of the cell.

Griggs, E. I.↗
NASA NTRS1975

Propagation of transients in a random medium

The propagation of transient scalar waves in a three-dimensional random medium is considered. The analysis is based on the smoothing method. An integro-differential equation for the coherent (or average) wave is derived and solved for the case of a statistically homogeneous and isotropic medium and a delta-function source. This yields the coherent Green's function of the medium. It is found that the waveform of the coherent wave depends generally on the distance from the source measured in terms of a certain dimensionless parameter. Based on the magnitude of this parameter, three propagation zones, called the near zone, the far zone, and the intermediate zone, are defined.

Wenzel, A. R.↗
NASA NTRS1975

The dynamics of atmospheric dust particles in aircraft auxiliary power radial inflow turbines

The results of analytical and experimental studies of the trajectories that atmospheric dust particles follow as they move through a radial inflow turbine are presented. The study reveals the nature of the impacts that occur within the turbine and indicates which surfaces are expected to experience the most severe erosion. In addition, a dimensionless parameter is derived which can be used during preliminary design analysis to indicate the sizes of the particles that will be most damaging to the turbine.

Clevenger, W. B., Jr.↗
NASA NTRS1975

Quantitative determination of smoke and toxic product potential of materials with the AMINCO (registered) NBS smoke density chamber

Quantitative measurements of smoke obscuration produced by standard area samples under high energy pyrolysis both with and without an ignition source, using a collimated vertical light beam in conjunction with an ultra-linear photomultiplier microphotometer are obtainable with the smoke density chamber. Results are expressed in dimensionless specific optical density permitting usage calculations in accordance with a geometric relationship.

Greenberg, S.↗
NASA NTRS1975

Rough-to-smooth transition of an equilibrium neutral constant stress layer

Purpose of research on rough-to-smooth transition of an equilibrium neutral constant stress layer is to develop a model for low-level atmospheric flow over terrains of abruptly changing roughness, such as those occurring near the windward end of a landing strip, and to use the model to derive functions which define the extent of the region affected by the roughness change and allow adequate prediction of wind and shear stress profiles at all points within the region. A model consisting of two bounding logarithmic layers and an intermediate velocity defect layer is assumed, and dimensionless velocity and stress distribution functions which meet all boundary and matching conditions are hypothesized. The functions are used in an asymptotic form of the equation of motion to derive a relation which governs the growth of the internal boundary layer. The growth relation is used to predict variation of surface shear stress.

Logan, E., Jr.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the initial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given; the equations are used to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1975

Amplitude effects on the dynamic performance of a hydrostatic gas thrust bearing

The Reynolds' equation is applied to a strip gas thrust bearing to analyze amplitude disturbance effects on its dynamic performance. The Reynolds' equation is numerically approximated using finite difference techniques. The time dependent load carrying capacity is represented by a Fourier series up to and including the third harmonics. Design curves for the load capacity and the linear stiffness and damping are presented as a function of inlet location, restrictor coefficient, supply pressure, amplitude of oscillation, and squeeze number. For the range of amplitudes investigated the dimensionless load capacity, stiffness and damping does not exhibit an appreciable change in magnitude; thus, only one design curve is needed to represent each relationship. A design methodology is presented.

Stiffler, A. K.↗
NASA NTRS1976

Elastohydrodynamic lubrication of point contacts

A procedure for the numerical solution of the complete, isothermal, elastohydrodynamic lubrication problem for point contacts is given. This procedure calls for the simultaneous solution of the elasticity and Reynolds equations. By using this theory the influence of the ellipticity parameter and the dimensionless speed, load, and material parameters on the minimum and central film thicknesses was investigated. Thirty-four different cases were used in obtaining the fully flooded minimum- and central-film-thickness formulas. Lubricant starvation was also studied. From the results it was possible to express the minimum film thickness for a starved condition in terms of the minimum film thickness for a fully flooded condition, the speed parameter, and the inlet distance. Fifteen additional cases plus three fully flooded cases were used in obtaining this formula. Contour plots of pressure and film thickness in and around the contact have been presented for both fully flooded and starved lubrication conditions.

Hamrock, B. J.↗
NASA NTRS1976

Isothermal elastohydrodynamic lubrication of point contacts. 4: Starvation results

The influence of lubricant starvation on minimum film thickness was investigated by moving the inlet boundary closer to the contact center. The following expression was derived for the dimensionless inlet distance at the boundary between the fully flooded and starved conditions: m* = 1 + 3.06 ((R/b)(R/b)H) to the power 0.58, where R is the effective radius of curvature, b is the semiminor axis of the contact ellipse, and H is the central film thickness for fully flooded conditions. A corresponding expression was also given based on the minimum film thickness for fully flooded conditions. Therefore, for m m*, starvation occurs and, for m m*, a fully flooded condition exists. Two other expressions were also derived for the central and minimum film thicknesses for a starved condition. Contour plots of the pressure and the film thickness in and around the contact are shown for the fully flooded and starved lubricating conditions, from which the film thickness was observed to decrease substantially as starvation increases.

Hamrock, B. J.↗
NASA NTRS1975

Gravitational collapse of a turbulent vortex with application to star formation

The gravitational collapse of a rotating cloud or vortex is analyzed by expanding the dependent variables in the equations of motion in two-dimensional Taylor series in the space variables. It is shown that the gravitation and rotation terms in the equations are of first order in the space variables, the pressure gradient terms are of second order, and the turbulent viscosity term is of third order. The presence of a turbulent viscosity insures that the initial rotation is solid-body-like near the origin. The effect of pressure on the collapse process is found to depend on the shape of the intial density disturbance at the origin. Dimensionless collapse times, as well as the evolution of density and velocity, are calculated by solving numerically the system of nonlinear ordinary differential equations resulting from the series expansions. The axial inflow plays an important role and allows collapse to occur even when the rotation is large. An approximate solution of the governing partial differential equations is also given, in order to study the spacial distributions of the density and velocity.

Deissler, R. G.↗
NASA NTRS1975

Stability and transient motion of a plain journal mounted in flexible damped supports

Results are presented for an extensive study of the influence of damped flexible supports on the stability-threshold speed of a symmetric rotor mounted in nonlinear fluid-film bearings. A stability analysis is conducted by applying the Routh criteria and calculating the damped eigenvalues of the system. Stability maps are given in dimensionless form for a range of support stiffness, damping, and mass ratios. The effect of rotor imbalance on the stability of rotor systems is examined, and it is shown that time transient orbits of shaft and support motions clearly indicate their dynamic behavior for both stable and unstable operating speeds. Simple design criteria are proposed for choosing the support stiffness and damping on the basis of rotor weight and journal clearance to promote optimum stability. It is concluded that the use of a flexible damped support system may increase the stability threshold of a plain journal bearing and that damper supports should promote smoother and quieter machine operation by suppressing oil whirl and attenuating unbalanced rotational forces.

Kirk, R. G.↗
NASA NTRS1975

Non-equilibrium one-dimensional two-phase flow in variable area channels

A one-dimensional nonequilibrium flow analysis has been formulated for a one component two phase flow. The flow is considered homogeneous and essentially isothermal. Phase change is assumed to occur at heterogeneous nucleation sites and the growth of the vapor bubbles is governed by heat conduction from the liquid to the bubble. The analysis adjusted for friction is applied to liquid nitrogen flow in a venturi and comparison is made with the NASA experimental results of Simoneau. Good agreement with the experiments is obtained when one assumes the effective activation energy for nucleus formation to be small but nonzero. The computed pressure distributions deviate from the experimental results in the throat region of the venturi in a manner consistent with centrifugal effects not accounted for in the one-dimensional theory. The results are shown to depend not only on cavitation number but on additional dimensionless parameters governing the nonequilibrium production and subsequent growth of nuclei.

Rohatgi, U. S.↗
NASA NTRS1976

Depletion of solar wind plasma near a planetary boundary

A mathematical model is presented that describes the squeezing of solar wind plasma out along interplanetary magnetic field lines in the region between the bow shock and the effective planetary boundary (in the case of the earth, the magnetopause). In the absence of local magnetic merging the squeezing process should create a 'depletion layer', a region of very low plasma density just outside the magnetopause. Numerical solutions are obtained for the dimensionless magnetohydrodynamic equations describing this depletion process for the case where the solar wind magnetic field is perpendicular to the solar wind flow direction. For the case of the earth, the theory predicts that the density should be reduced by a factor exceeding 2 in a layer about 700-1300 km thick if the Alfven Mach number in the solar wind, is equal to 8. Scaling of the model calculations to Venus and Mars suggests layer thicknesses about 1/10 and 1/15 those of the earth, respectively, neglecting diffusion and ionospheric effects.

Zwan, B. J.↗
NASA NTRS1976

Conduction of thermal energy in the neighborhood of the earth's bow shock

The Rankine-Hugoniot equations for MHD shocks are generalized by the addition of a term to the energy conservation equation representing a nonzero heat flow in the plasma in the neighborhood of the shock. This generalization is found to be compatible with the assumption of infinite electrical conductivity. The effects of plasma waves in this treatment are of the order of the reciprocal Alfvenic Mach number squared and hence are neglected. The effect of alpha particles in the solar wind is discussed. Seven crossings of the earth's bow shock by Explorer 35 in lunar orbit are analyzed. Sufficient data are available so that the determination of a dimensionless parameter, psi, characterizing the heat-flow difference across the bow shock is possible. The values of psi indicate energy-flux densities due to heat flow which are a nonnegligible fraction of the total energy flux. Two possible interpretations of psi are discussed.

Hohlfeld, R. G.↗
NASA NTRS1976

Nonhomologous contraction and equilibria of self-gravitating, magnetic interstellar clouds embedded in an intercloud medium: Star formation. II - Results

The paper studies the equilibrium of self-gravitating isothermal models of interstellar clouds with a frozen-in magnetic field linked smoothly to the field of a hot tenuous intercloud medium. Equilibrium states are determined which can be reached by clouds contracting nonhomologously from a spherical uniform initial state. Three free parameters characterize the problem: a dimensionless initial radius related to the Jeans length of the cloud, the initial ratio of the magnetic and gas pressures in the cloud, and the initial ratio of the intercloud and cloud pressures. The dependence of the solutions on each of these parameters is investigated. It is found that: (1) the frozen-in field causes the cloud to become oblate with its major axis normal to the field lines; (2) the flattening increases as the magnetic-field strength, gravitational forces, or intercloud pressure increases; (3) increasing intercloud pressure eventually leads to gravitational collapse; and (4) the cloud can reach equilibrium only if its radius does not exceed some critical value. The observed inefficiency of the star-formation process within massive clouds is examined and explained in terms of magnetic phenomena in a collapsing cloud.

Mouschovias, T. C.↗