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At least 163 records · Page 9

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NASA NTRS1978

Fermi acceleration and the structure of interstellar turbulence

Recent observations at length scales of at least about 10 to the 18th cm and about 10 to the 11th cm suggest the possibility of a generally turbulent interstellar gas with a Kolmogorov spectrum. The total r.m.s. fluctuation in fluid properties is of the same order as the mean. The consequences of such a turbulence spectrum for cosmic rays are discussed. If the turbulence is composed of Alfven waves propagating in both directions along the average magnetic field, the rate of Fermi acceleration is found to be significant. Consequences of the turbulence for spatial transport and the general dynamics of the interstellar gas, as well as possible energy sources for the turbulence, are discussed. It is concluded that Fermi acceleration by interstellar turbulence is a possible acceleration mechanism for galactic cosmic rays.

Jokipii, J. R.↗

NASA NTRS1979

Theory of deposition of condensible impurities on surfaces immersed in combustion gases

The components resulting from the deposition of inorganic salts (e.g., Na2S04) and oxides present in the combustion products from gas turbine engines were investigated. Emphasis was placed on the effects of multicomponent vapor transport, thermophoretic transport of vapor and small particles to actively cooled surfaces, variable fluid properties within mass transfer boundary layers, and free stream turbulence.

Rosner, D. E.↗

NASA NTRS1980

Analytical and design techniques for drag reduction studies on wavy surfaces

Numerical models for two dimensional turbulent boundary layers over wavy surfaces were investigated. Computations for wavy wall boundary layers indicate possibilities of overall drag reduction in a parameter range of the geometry of the wall. The correction technique using integral methods for analyzing arbitrary surfaces was found to be unsuitable for some cases of interest in drag reduction; a Navier-Stokes solver for wavy walls was built to test these problems. Test results of the Navier-Stokes solver indicate that the solution techniques are accurate enough to handle complex geometries and steep variations in fluid properties.

Balasubramanian, R.↗

NASA NTRS1980

Photogeological constraints on lunar and planetary vulcanism, part 2

The use of several simple theoretical modelling approaches in simulating lava erosion were evaluated. Both one dimensional and two dimensional models were used with the two dimensional model being superior. The two dimensional model accounts, in a purely phenomenological way, for the effects of turbulence and mechanical erosion in a flow. Criteria for mechanical erosion was related to the basal stress and the effect of entrained eroded material on fluid properties was parameterized.

Source record↗

NASA NTRS1981

Magnetic space-based field measurements

Because the near Earth magnetic field is a complex combination of fields from outside the Earth of fields from its core and of fields from its crust, measurements from space prove to be the only practical way to obtain timely, global surveys. Due to difficulty in making accurate vector measurements, early satellites such as Sputnik and Vanguard measured only the magnitude survey. The attitude accuracy was 20 arc sec. Both the Earth's core fields and the fields arising from its crust were mapped from satellite data. The standard model of the core consists of a scalar potential represented by a spherical harmonics series. Models of the crustal field are relatively new. Mathematical representation is achieved in localized areas by arrays of dipoles appropriately located in the Earth's crust. Measurements of the Earth's field are used in navigation, to map charged particles in the magnetosphere, to study fluid properties in the Earth's core, to infer conductivity of the upper mantels, and to delineate regional scale geological features.

Langel, R. A.↗

NASA NTRS1982

Compressibility effects in turbulence modeling

Numerical turbulence modeling is discussed with attention given to fluid property variations caused by compressibility in an adiabatic flow. The models are considered in terms of integral quantities expressed by ordinary differential equations and by those formulated as partial differential equations. Compressibility corrections for both integral and partial differential methods are reviewed. Eddy-viscosity models are explored for their capability to characterize the mass-weighted Reynolds stress, which can be accounted for with primitive and/or mass-weighted variables. Compressible flow simulations are currently constrained to low Re and zero mean dilation. The effects of compressibility are defined in wave number space by resolving the Fourier transforms of the velocity vectors into components which are perpendicular and parallel to the wave number vector. Statistical correlations then permit obtaining a value for each contribution.

Rubesin, M. W.↗

NASA NTRS1983

Improved Two-Dimensional Kinetics (TDK) computer program

Fluid properties, the boundary layer module, and regenerative cooling are discussed. Chemistry, low density flow effects, test cases, input and output for TDK, and documentation are also discussed.

Nickerson, G. R.↗

NASA NTRS1984

A spectral collocation method for the Navier-Stokes equations

A Fourier-Chebyshev spectral method for the incompressible Navier-Stokes equations is described. It is applicable to a variety of problems including some with fluid properties which vary strongly both in the normal direction and in time. In this fully spectral algorithm, a preconditioned iterative technique is used for solving the implicit equations arising from semi-implicit treatment of pressure, mean advection and vertical diffusion terms. The algorithm is tested by applying it to hydrodynamic stability problems in channel flow and in external boundary layers with both constant and variable viscosity.

Malik, M. R.↗

NASA NTRS1985

Influence of Melt Convection on Solid-Liquid Interface Under Terrestrial and Reduced Gravity Environments

Solidification and crystal growth processes involve thermal and solutal gradients within a molten phase. In the presence of gravity, such gradients result in convective flows which interact with diffusion fields at the solid-liquid interface. Dendritic growth kinetics was studied in transparent model systems which freeze similarily to most metals. Succinonitrile shows a strong influence of convection at supercoolings below about 1K. Fluid flows adjacent to solid-liquid interfaces and the behavior of shear flows in vertical annular geometries are studied. Novel low-frequency eigenstates were discovered and classified as coupled modes, for their involvement with interfacial deformation coupled to the fluid flow, and are unknown in systems without deformable interfaces. The dependence of coupled convection modes on interfacial geometry, gravity, fluid properties, and transformation characteristics studied for several annual flow arrangements with nominally pure solid-liquid systems.

Glicksman, M. E.↗

NASA NTRS1985

A spectral collocation method for the Navier-Stokes equations

Malik, M. R.↗

NASA NTRS1987

Vapor condensation on a turbulent liquid interface

An experimental investigation which seeks the fundamental relationship between the interfacial condensation rate and the parameters which control it when the liquid side is turbulent is discussed. The scaling laws for free-surface condensation are discussed for this case. It is argued that the condensation of cryogenic liquids can, in principle, be simulated in experiments using steam and water. Data are presented for the condensation rate in terms of the dimensionless scaling parameters which involve the fluid properties and the liquid-side turbulence velocity and length scales.

Helmick, M. R.↗

NASA NTRS1988

Boundary layer development as a function of chamber pressure in the NASA Lewis 1030:1 area ratio rocket nozzle

Through the use of theoretical predictions of fluid properties and experimental heat transfer and thrust measurements, the zones of laminar, transitional, and turbulent boundary layer flow were defined for the NASA Lewis 1030:1 area ratio rocket nozzle. Tests were performed on the nozzle at chamber pressures from 350 to 100 psia. For these conditions, the throat diameter Reynolds numbers varied from 300,000 to 1 million. The propellants used were gaseous hydrogen and gaseous oxygen. Thrust measurements and nozzle outer wall temperature measurements were taken during the 3-sec test runs. Comparison of experimental heat transfer and thrust data with the corresponding predictions from the Two-Dimensional Kinetics (TDK) nozzle analysis program indicated laminar flow in the nozzle at a throat diameter Reynolds number of 320,000 or chamber pressure of 360 psia. Comparison of experimental and predicted heat transfer data indicated transitional flow up to and including a chamber pressure of 1000 psia. Predicted values of the axisymmetric acceleration parameter within the convergent and divergent nozzle were consistent with the above results. Based upon an extrapolation of the heat transfer data and predicted distributions of the axisymmetric acceleration parameter, transitional flow was predicted up to a throat diameter Reynolds number of 220,000 or 2600-psia chamber pressure. Above 2600-psia chamber pressure, fully developed turbulent flow was predicted.

Smith, Tamara A.↗

NASA NTRS1988

3-D thermal analysis using finite difference technique with finite element model for improved design of components of rocket engine turbomachines for Space Shuttle Main Engine SSME

Three-dimensional finite element models were generated and transferred into three-dimensional finite difference models to perform transient thermal analyses for the SSME high pressure fuel turbopump's first stage nozzles and rotor blades. STANCOOL was chosen to calculate the heat transfer characteristics (HTCs) around the airfoils, and endwall effects were included at the intersections of the airfoils and platforms for the steady-state boundary conditions. Free and forced convection due to rotation effects were also considered in hollow cores. Transient HTCs were calculated by taking ratios of the steady-state values based on the flow rates and fluid properties calculated at each time slice. Results are presented for both transient plots and three-dimensional color contour isotherm plots; they were also converted into universal files to be used for FEM stress analyses.

Sohn, Kiho D.↗

NASA NTRS1988

Boundary layer development as a function of chamber pressure in the NASA Lewis 1030:1 area ratio rocket nozzle

Through the use of theoretical predictions of fluid properties and experimental heat transfer and thrust measurements, the zones of laminar, transitional, and turbulent boundary layer flow were defined for the NASA Lewis 1039:1 area ratio rocket nozzle. Tests were performed on the nozzle at chamber pressures from 350 to 100 psia. For these conditions, the throat diameter Reynolds numbers varied from 300,000 to 1 million. The propellants used were gaseous hydrogen and gaseous oxygen. Thrust measurements and nozzle outer wall temperature measurements were taken during the 3-sec test runs. Comparison of experimental heat transfer and thrust data with the corresponding predictions from the Two-Dimensional Kinetics (TDK) nozzle analysis program indicated laminar flow in the nozzle at a throat diameter Reynolds number of 320,000 or chamber pressure of 360 psia. Comparison of experimental and predicted heat transfer data indicated transitional flow up to and including a chamber pressure of 1000 psia. Predicted values of the axisymmetric acceleration parameter within the convergent and divergent nozzle were consistent with the above results. Based upon an extrapolation of the heat transfer data and predicted distributions of the axisymmetric acceleration parameter, transitional flow was predicted up to a throat diameter Reynolds number of 220,000 or 2600-psia chamber pressure. Above 2600-psia chamber pressure, fully developed turbulent flow was predicted.

Smith, Tamara A.↗

NASA NTRS1988

Inflow/outflow boundary conditions and global dynamics of spatial mixing layers

The numerical simulation of incompressible spatially-developing shear flows poses a special challenge to computational fluid dynamicists. The Navier-Stokes equations are elliptic and boundary equations need to be specified at the inflow and outflow boundaries in order to compute the fluid properties within the region of interest. It is, however, difficult to choose inflow and outflow conditions corresponding to a given experimental situation. Furthermore the effects that changes in the boundary conditions or in the size of the computational domain may induce on the global dynamics of the flow are presently unknown. These issues are examined in light of recent developments in hydrodynamic stability theory. The particular flow considered is the spatial mixing layer but it was expected that similar phenomena were bound to occur in other cases such as channel flow, the boundary layer, etc. A short summary of local/global and absolute/convective instability concepts is given. The results of numerical simulations are presented which strongly suggest that global resonances may be triggered in domains of finite streamwise extent although the evolution of the perturbation vorticity field is everywhere locally convective. A relationship between finite domains and pressure sources which might help in devising a scheme to eliminate these difficulties is discussed.

Buell, J. C.↗

NASA NTRS1988

The magnetic mirror force in plasma fluid models

In the past decade, there have been several attempts to include the magnetic mirror force in the equation of motion for a plasma in a fluid formalism. In the process, some confusion has been evident regarding when and how this should be done. This problem has been addressed in the literature, but these treatments appear to have been forgotten or misunderstood. The mathematical arguments are summarized so that the physical consequences are readily perceived. It is shown that for an isotropic plasma fluid, in the direction parallel or anti-parallel to a magnetic field, the forces associated with a diverging magnetic field cancel out. Only for anisotropies in the fluid properties does the diverging field influence the plasma dynamics.

Comfort, R. H.↗

NASA NTRS1989

Time analysis of polydisperse sprays in complex turbulent environments

The two-phase flow field of a small swirl-stabilized burner has been investigated, using a phase Doppler particle analyzer to obtain the velocity, drop size, and time of arrival components. Water/kerosene comparisons were made for the case of nonreacting flow. It is found that the spray flow field is strongly influenced by the aerodynamic flow field and by the reaction and fluid properties. It is noted that vortex shedding produces clusters of drops.

Rudoff, R. C.↗

NASA NTRS1989

Progress on a Taylor weak statement finite element algorithm for high-speed aerodynamic flows

A new finite element numerical Computational Fluid Dynamics (CFD) algorithm has matured to the point of efficiently solving two-dimensional high speed real-gas compressible flow problems in generalized coordinates on modern vector computer systems. The algorithm employs a Taylor Weak Statement classical Galerkin formulation, a variably implicit Newton iteration, and a tensor matrix product factorization of the linear algebra Jacobian under a generalized coordinate transformation. Allowing for a general two-dimensional conservation law system, the algorithm has been exercised on the Euler and laminar forms of the Navier-Stokes equations. Real-gas fluid properties are admitted, and numerical results verify solution accuracy, efficiency, and stability over a range of test problem parameters.

Baker, A. J.↗