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At least 1,207 records · Page 67

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NASA NTRS1982

Damping seals for turbomachinery

A rotor seal is proposed that restricts leakage like a labyrinth seal, but extends the stabilizing speed range beyond twice the first critical speed. The dynamic parameters were derived from bulk flow equations without requiring a dominant axial flow. The flow is considered incompressible and turbulent. Damping seals are shown to be feasible for extending the speed range of high performance turbomachinery beyond the limit imposed by conventional seals.

Vonpragenau, G. L.↗

NASA NTRS1982

Correlation of Preston-tube data with laminar skin friction

Preston-tube data have been obtained on a sharp ten-degree cone in the NASA Ames Eleven-Foot Transonic Wind Tunnel. Data were obtained over a Mach number range of 0.30 to 0.95 and unit Reynolds numbers of 9.84, 13.1, and 16.4 million per meter. The portions of these data, that were obtained within laminar boundary layers, have been correlated with the corresponding values of theoretical skin friction. The rms scatter of skin-friction coefficient about the correlation is of the order of one percent, which is comparable to the reported accuracy for calibrations of Preston-tubes in incompressible pipe-flows. In contrast to previous works on Preston-tube/skin-friction correlations, which are based on the physical height of the probe's face, this very satisfactory correlation for compressible boundary-layer flows is achieved by accounting for the effects of a variable 'effective' height of the probe. The coefficients, which appear in the correlation, are dependent on the particular tunnel environment. The general procedure can be used to define correlations for other wind tunnels.

Reed, T. D.↗

NASA NTRS1982

Environmental solid particle effects on compressor cascade performance

The effect of suspended solid particles on the performance of the compressor cascade was investigated experimentally in a specially built cascade tunnel, using quartz sand particles. The cascades were made of NACA 65(10)10 airfoils. Three cascades were tested, one accelerating cascade and two diffusing cascades. The theoretical analysis assumes inviscid and incompressible two dimensional flow. The momentum exchange between the fluid and the particle is accounted for by the interphase force terms in the fluid momentum equation. The modified fluid phase momentum equations and the continuity equation are reduced to the conventional stream function vorticity formulation. The method treats the fluid phase in the Eulerian system and the particle phase in Lagrangian system. The experimental results indicate a small increase in the blade surface static pressures, while the theoretical results indicate a small decrease. The theoretical analysis, also predicts the loss in total pressure associated with the particulate flow through the cascade.

Tabakoff, W.↗

NASA NTRS1984

Review of some vortex relations

The evaluation of the circulation from numerical solutions of the momentum and energy equations is discussed for incompressible and compressible flows. It is shown how artificial damping directly influences the time ratio of change of the circulation.

Krause, E.↗

NASA NTRS1985

A three-dimensional spectral algorithm for simulations of transition and turbulence

A spectral algorithm for simulating three dimensional, incompressible, parallel shear flows is described. It applies to the channel, to the parallel boundary layer, and to other shear flows with one wall bounded and two periodic directions. Representative applications to the channel and to the heated boundary layer are presented.

Zang, T. A.↗

NASA NTRS1985

Effect of ground and/or ceiling planes on thrust of rotors in hover

The thrust produced by a helicopter rotor hovering near ground and/or ceiling planes is investigated experimentally and theoretically. In the experiment, the thrust was measured on a 0.324-m-diam rotor operating between floor and ceiling planes which were located from 6 to 0.08 diam from the rotor disk. In the first theoretical model studied, the incompressible and inviscid flow induced by a sequence of vortex cylinders, located above and below the rotor to simulate the rotor wake and its interaction with the floor and ceiling planes, was considered. Comparison with experiment showed that this model overpredicts the change in thrust caused by the proximity of the walls. Therefore, a second arrangement of vortex cylinders was introduced which provides a more accurate prediction of the ground and ceiling effects on the thrust of the rotor in hover. The applicability of these results to a vented wind tunnel is also discussed.

Rossow, V. J.↗

NASA NTRS1985

Numerical experiments on subcritical transition mechanisms

A spectral algorithm for simulating three-dimensional, incompressible, parallel shear flows is described. It applies to the channel, to the parallel boundary layer, and to other shear flows with one wall-bounded and two periodic directions. Simulations performed for the channel identify the one nonlinear interaction which is crucial to the secondary instability. Moreover, it is shown that subharmonic instability ensues for Reynolds numbers as low as 1500. Simulations performed for the heated water boundary layer reveal the operation of the secondary instability in this controlled flow.

Zang, T. A.↗

NASA NTRS1985

In situ measurements of the plasma bulk velocity near the Io flux tube

The flow around the Io flux tube was studied by analyzing the eleven spectra taken by the Voyager 1 Plasma Science (PLS) experiment in its vicinity. The bulk plasma parameters were determined using a procedure that uses the full response function of the instrument and the data in all four PLS sensors. The mass density of the plasma in the vicinity of Io is found to be 22,500 + or - 2,500 amu/cu cm and its electron density is found to be 1500 + or - 200/cu cm. The Alfven speed was determined using three independent methods; the values obtained are consistent and taken together yield V sub A = 300 + or - 50 km/sec, corresponding to an Alfven Mach number of 0.19 + or - 0.02. For the flow pattern, good agreement was found with the model of Neubauer (1980), and it was concluded that the plasma flows around the flux tube with a pattern similar to the flow of an incompressible fluid around a long cylinder obstacle of radius 1.26 + or - 0.1 R sub Io.

Barnett, A.↗

NASA NTRS1985

Review of some vortex relations

Krause, E.↗

NASA NTRS1986

Influence of friction dampers on torsional blade flutter

This paper deals with the stabilizing effects of dry friction on torsional blade flutter. A lumped parameter model with single degree of freedom per blade has been used to represent the rotor stage. The well-known cascade theories for incompressible and supersonic flows have been used to determine the allowable increase in fluid velocity relative to the blade. It has been found that the effectiveness of friction dampers in controlling flutter can be substantial.

Sinha, A.↗

NASA NTRS1986

In situ measurements of the plasma bulk velocity near the Io flux tube

Data obtained with the Voyager Plasma Science Experiment (PSE) during a flyby of the Jovian moon Io are analyzed to quantify the bulk plasma parameters near the Io flux tube. The PSE contains four modulated grid Faraday cups, three pentagonally shaped and the other circular. The path of the spacecraft around Jupiter and past Io ensured that the maximum flux was passing perpendicular to the spacecraft instruments when Voyager was closest to Io (20,000 km). A total of 11 high resolution (M mode) plasma spectra were obtained, revealing 6 species of ions: H(+), O(+), O(2+), S(+), S(2+) and S(3+). All species had the same bulk velocity and convected maxwellian distributions; the heavy ions species were all the same temperature. The mass density of the plasma near Io was 20,000-25,000 amu/cu cm and the electron density was 1300-1700/cu cm. Calculations of the Alfven speed yielded a value ranging from 250-350 km/sec and an Alfven Mach no. of 0.17-0.21. The data were equivalent to that expected from the flow of an incompressible fluid around a long cylinder with a radius about 1.25 that of Io's.

Barnett, A.↗

NASA NTRS1988

Domain decomposition algorithms and computation fluid dynamics

In the past several years, domain decomposition was a very popular topic, partly motivated by the potential of parallelization. While a large body of theory and algorithms were developed for model elliptic problems, they are only recently starting to be tested on realistic applications. The application of some of these methods to two model problems in computational fluid dynamics are investigated. Some examples are two dimensional convection-diffusion problems and the incompressible driven cavity flow problem. The construction and analysis of efficient preconditioners for the interface operator to be used in the iterative solution of the interface solution is described. For the convection-diffusion problems, the effect of the convection term and its discretization on the performance of some of the preconditioners is discussed. For the driven cavity problem, the effectiveness of a class of boundary probe preconditioners is discussed.

Chan, Tony F.↗

NASA NTRS1988

Domain decomposition algorithms and computational fluid dynamics

Some of the new domain decomposition algorithms are applied to two model problems in computational fluid dynamics: the two-dimensional convection-diffusion problem and the incompressible driven cavity flow problem. First, a brief introduction to the various approaches of domain decomposition is given, and a survey of domain decomposition preconditioners for the operator on the interface separating the subdomains is then presented. For the convection-diffusion problem, the effect of the convection term and its discretization on the performance of some of the preconditioners is discussed. For the driven cavity problem, the effectiveness of a class of boundary probe preconditioners is examined.

Chan, Tony F.↗

NASA NTRS1989

Simultaneous detection of separation and transition in surface shear layers

Flush-mounted hot-film gages have proved effective in detecting boundary-layer transition and in measuring skin friction but with limited success in detecting laminar separation and reattachment. The development of multielement micro hot-film sensors, and the recent discovery of the phase reversal phenomena associated with low-frequency dynamic shear stress signals across regions of laminar separation and turbulent reattachment, have made it possible to simultaneously and unambiguously detect these surface shear layer characteristics. Experiments were conducted on different airfoils at speeds ranging from low subsonic to transonic speeds to establish the technique for incompressible and compressible flow applications. The multielement dynamic shear stress sensor technique was successfully used to detect laminar separation, turbulent reattachment, as well as, shock induced laminar and turbulent separation.

Mangalam, Siva M.↗

NASA NTRS1989

An improved algorithm for the modeling of vapor flow in heat pipes

A heat pipe vapor flow algorithm suitable for use in codes on microcomputers is presented. The incompressible heat pipe vapor flow studies of Busse are extended to incorporate compressibility effects. The Busse velocity profile factor is treated as a function of temperature and pressure. The assumption of a uniform saturated vapor temperature determined by the local pressure at each cross section of the pipe is not made. Instead, a mean vapor temperature, defined by an energy integral, is determined in the course of the solution in addition to the pressure, saturation temperature at the wall, and the Busse velocity profile factor. For alkali metal working fluids, local species equilibrium is assumed. Temperature and pressure profiles are presented for several cases involving sodium heat pipes. An example for a heat pipe with an adiabatic section and two evaporators in sequence illustrates the ability to handle axially varying heat input. A sonic limit plot for a short evaporator falls between curves for the Busse and Levy inviscid sonic limits.

Tower, Leonard K.↗

NASA NTRS1991

Direct Finite-Difference Simulations Of Turbulent Flow

Report discusses use of upwind-biased finite-difference numerical-integration scheme to simulate evolution of small disturbances and fully developed turbulence in three-dimensional flow of viscous, incompressible fluid in channel. Involves use of computational grid sufficiently fine to resolve motion of fluid at all relevant length scales.

Rai, Man Mohan↗

NASA NTRS1990

An isoparametric finite-element Stokes flow test problem

Two-dimensional Stokes flow of an incompressible Newtonian fluid about a horizontal cylinder situated next to a moving wall is presented and suggested as a nontrivial benchmark problem. Both the primitive variable Galerkin finite-element and the exact solutions are generated on a sequence of refined meshes. Error norms and convergence rates are presented for the C(o) bilinear-velocity/C(-1) piecewise constant-pressure and the C(o) biquadratic-velocity/C(-1) linear-pressure quadrilateral elements.

Maslanik, Miriam K.↗

NASA NTRS1990

Nonlinear spatial evolution of externally excited instability waves in free shear layers

The effects of critical-layer nonlinearity on spatially growing instability waves on shear layers between parallel streams are considered. In the two-dimensional incompressible case, the flow in the critical layer is governed by a nonequilibrium, nonlinear vorticity equation. The initial exponential growth of the instability wave is converted into algebraic growth during the streamwise 'aging' of the critical layer into a quasi-equilibrium state. This leads to a next stage of evolution where the instability-wave growth is affected by both mean-flow-divergence and nonlinear effects and is eventually converted to decay. A uniformly valid composite formula for the instability wave amplitude, accounting for both nonparallel and nonlinear effects, is given and compared to experimental results. The inviscid solutions to these equations always end in a singularity at finite downstream distance.

Hultgren, L. S.↗