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At least 631 records · Page 35

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NASA NTRS1995

Vibrational and Electronic Energy Transfer and Dissociation of Diatomic Molecules by Electron Collisions

At high altitudes and velocities equal to or greater than the geosynchronous return velocity (10 kilometers per second), the shock layer of a hypersonic flight will be in thermochemical nonequilibrium and partially ionized. The amount of ionization is determined by the velocity. For a trans atmospheric flight of 10 kilometers per second and at an altitude of 80 kilometers, a maximum of 1% ionization is expected. At a velocity of 12 - 17 kilometer per second, such as a Mars return mission, up to 30% of the atoms and molecules in the flow field will be ionized. Under those circumstances, electrons play an important role in determining the internal states of atoms and molecules in the flow field and hence the amount of radiative heat load and the distance it takes for the flow field to re-establish equilibrium. Electron collisions provide an effective means of transferring energy even when the electron number density is as low as 1%. Because the mass of an electron is 12,760 times smaller than the reduced mass of N2, its average speed, and hence its average collision frequency, is more than 100 times larger. Even in the slightly ionized regime with only 1% electrons, the frequency of electron-molecule collisions is equal to or larger than that of molecule-molecule collisions, an important consideration in the low density part of the atmosphere. Three electron-molecule collision processes relevant to hypersonic flows will be considered: (1) vibrational excitation/de-excitation of a diatomic molecule by electron impact, (2) electronic excitation/de-excitation, and (3) dissociative recombination in electron-diatomic ion collisions. A review of available data, both theory and experiment, will be given. Particular attention will be paid to tailoring the molecular physics to the condition of hypersonic flows. For example, the high rotational temperatures in a hypersonic flow field means that most experimental data carried out under room temperatures are not applicable. Also, the average electron temperature is expected to be between 10,000 and 20,000 K. Thus only data for low energy electrons are relevant to the model.

Huo, Winifred M.↗

NASA NTRS1996

Simulation and modeling of the elliptic streamline flow

Direct numerical simulations are performed for the elliptic streamline flow, which is a homogeneous turbulent flow that combines the effects of solid body rotation and strain. Simulations are run over a range of parameters in order to determine the effect of changing rotation and strain separately. For early times the nonlinear cascade is suppressed, but then is re-established at later times. The growth rate of turbulent kinetic energy agrees at early times with the trends from linear theory, but at later times the flow seems to approach an asymptotic state that is independent of the ratio of mean flow rotation rate to strain rate. A comparison with standard Reynolds stress turbulence models is made. It is found that for strong rotation rates, the models predict decay of the turbulence, while the simulations show exponential growth. Close examination of the simulation results shows that they are affected by excessively low Reynolds numbers. Suggestions for reducing low Reynolds number effects in future simulations is given.

Blaisdell, G. A.↗

NASA NTRS1987

Numerical and analytical study of fluid dynamic forces in seals and bearings

A numerical model based on a transformed, conservative form of the three dimensional Navier-Stokes equation and an analytical model based on lumped fluid parameters are presented and compared with studies of modeled rotor bearing seal systems. The rotor destabilizing factors are related to the rotative character of the flow field. It is shown that these destabilizing factors can be reduced through a decrease in the fluid average circumferential velocity. However, the rotative character of the flow field is a complex three dimensional system with bifurcated secondary flow patterns that significantly alter the fluid circumferential velocity. By transforming the Navier-Stokes equations to those for a rotating observer and using the numerical code PHOENICS-84 with a nonorthogonal body fitted grid, several numerical experiments were carried out to demonstrate the character of this complex flow field. In general, fluid injection and/or preswirl of the flow field opposing the shaft rotation significantly intensified these secondary recirculation zones and thus reduced the average circumferential velocity; injection or preswirl in the direction of rotation significantly weakened these zones.

Tam, L. T.↗

NASA NTRS1985

Incompressible flow in stepped labyrinth seals

A steped labyrinth seal was experimentally investigated to determine the effects of pressure ratio, shaft speed, number of teeth, and tooth/step location upon the leakage through the seal for incompressible flow. The dependence of the flow coefficient upon the number of throttles and pressure ratio are similar to those for straight-through labyrinth seals. It can be noted that the axial location of the throttle with respect to the step had a special effect upon the flow coefficient. That is, the dependency of the flow coefficient upon rotation rate and the number of throttles changes with axial location. It was found that the minimum flow coefficient was obtained when the seal teeth were centered on the step surface. Axial pressure distribution measurements show that when the teeth are centered on the step, the pressure drop from cavity to cavity is almost uniform. It is speculated that the obtaining of this uniform pressure gradient is the cause for the enhanced performance of the stepped labyrinth seal when operated in that configuration.

Morrison, G. L.↗

NASA NTRS1989

A new rotational relaxation model for use in hypersonic computational fluid dynamics

The theoretical basis for the Landau-Teller equation commonly used to model rotational nonequilibrium is reviewed. Several assumptions underlying this model are indicated to be unrealistic for rarefied hypersonic flow. A new rotational nonequilibrium model based on recent measurements up to 2000K of quantum state transition rates is presented. The new model is applied to the continuum study of normal shock wave structure using both the Burnett equations and a simplified nonlinear constitutive relation. Comparisons are made between shock wave temperature profiles generated from the new model and those generated using the Landau-Teller model. Comparisons of shock reciprocal thicknesses between experimental data for nitrogen and continuum solutions using both rotational models are made. The new rotational model agrees well with experiment up to Mach 6, and under predicts shock thickness at higher Mach numbers. The Landau-Teller model agrees well with experimental shock thickness up to Mach 3, and over predicts the thickness at higher Mach numbers. A modification to the rotational collision number in the Landau-Teller model is found to give results which agree with experimental shock thicknesses at all Mach numbers up to 11.

Chapman, Dean E.↗

NASA NTRS2014

The Influence of Slope Breaks on Lava Flow Surface Disruption

Changes in the underlying slope of a lava flow impart a significant fraction of rotational energy beyond the slope break. The eddies, circulation and vortices caused by this rotational energy can disrupt the flow surface, having a significant impact on heat loss and thus the distance the flow can travel. A basic mechanics model is used to compute the rotational energy caused by a slope change. The gain in rotational energy is deposited into an eddy of radius R whose energy is dissipated as it travels downstream. A model of eddy friction with the ambient lava is used to compute the time-rate of energy dissipation. The key parameter of the dissipation rate is shown to be rho R(sup 2/)mu, where ρ is the lava density and mu is the viscosity, which can vary by orders of magnitude for different flows. The potential spatial disruption of the lava flow surface is investigated by introducing steady-state models for the main flow beyond the steepening slope break. One model applies to slow-moving flows with both gravity and pressure as the driving forces. The other model applies to fast-moving, low-viscosity, turbulent flows. These models provide the flow velocity that establishes the downstream transport distance of disrupting eddies before they dissipate. The potential influence of slope breaks is discussed in connection with field studies of lava flows from the 1801 Hualalai and 1823 Keaiwa Kilauea, Hawaii, and 2004 Etna eruptions.

disruption↗

NASA NTRS1973

Thermally driven mass flows in the convection zone of the sun

A formulation of the fluid dynamics of convective regions is developed which leads to an analytical description of the solar rotation, the Evershed flow, and the supergranulation. The starting point of the present formulation is the mixing length picture of convective equilibrium, but the earlier point mass model for convective molecules is replaced here by a model with both inertia and intrinsic moment of inertia. This extension introduces three rotational degrees of freedom into the dynamics of individual convective molecules, which enter into the dynamical equations for a mixing length fluid in the form of a separate vector field which we term the spin field. It is shown that for convective molecules having a spherically symmetric mass distribution, the spin field is proportional to the local vorticity.

Dijkhuis, G. C.↗

NASA NTRS1965

Transition in circular couette flow.

Stability and transition in Couette flow between concentric rotating cylinders including design and instrumentation of experimental machines

COUETTE FLOW↗

NASA NTRS1992

A new determination of the solar rotation rate

We use 'stackplot' displays to compare observations of the photospheric magnetic field during sunspot cycle 21 with simulations based on the flux-transport model. Adopting nominal rates of diffusion, differential rotation, and meridional flow, we obtain slanted patterns similar to those of the observed field, even when the sources of flux are assigned random longitudes in the model. At low latitudes, the slopes of the nearly vertical patterns of simulated field are sensitive to the rotation rate used in the calculation, and insensitive to the rates of diffusion and flow during much of the sunspot cycle. Good agreement between the observed and simulated patterns requires a synodic equatorial rotation period of 26.75 +/- 0.05 days.

Sheeley, N. R., Jr.↗

NASA NTRS1979

Large-scale motion in the Venus stratosphere

To examine a postulate that the horizontal momentum exchanges in the Venusian stratosphere are quasi-nondivergent, the properties of two-dimensional turbulence on a slowly rotating sphere in a high-resolution, one-level numerical model are investigated. It is concluded that the forcing which maintains the stratospheric flow is weak and influences the dynamics far less than the inertial effects. Consequently, the behavior resembles that of vorticity-conserving, two-dimensional flows. On a slowly rotating planet like Venus, such flows are dominated by solid-body rotation and by a planetary wave of unit zonal wavenumber - this wave corresponds to the observed Y-shaped UV feature. Although these largest scales of motion stand out, the dynamic balances of the flow are fundamentally nonlinear, in contrast to the quasi-linear Rossby wave regime on rapidly rotating planets.

Rossow, W. B.↗

NASA NTRS1973

Analysis of internal flow characteristics of a smooth-disk water-brake dynamometer

The principal of absorbing power with an enclosed partially submerged rotating disk through the turbulent viscous shearing of water is discussed. Reference information is used to develop a flow model of the water brake. A method is then presented that uses vector diagrams to relate the effects of rotational flow, through flow, and secondary flow to power absorption. The method is used to describe the operating characteristics of an example 111-cm (43.7-in.) diameter water brake. Correlating performance parameters are developed in a dimensional analysis.

Evans, D. G.↗

DOE OSTI2023

Effects of Part-to-Part Flow Variations on Overall Effectiveness and Life of Rotating Turbine Blades

Abstract As firing temperatures in gas turbine engines continue to increase to achieve high efficiencies, components in the main gas path must be protected with cooling flows to ensure lifing targets are met. Manufacturing variations, however, influence the performance and life characteristics of components with the same nominal design. This study presents blade flow and overall cooling effectiveness measurements for nine true-scale, aero engine turbine blades with realistic manufacturing variations. Flow measurements were made through each blade at a fixed pressure ratio to determine flow variability between holes and between blades. Infrared thermography was used to capture spatially-resolved temperature measurements reported as overall effectiveness on the same nine blades under high-speed rotating conditions at the Steady Thermal Aero Research Turbine Laboratory. Thermal performance was correlated with blade flow performance indicating substantial blade-to-blade variations resulting from manufacturing differences. Measurements also indicated wide variations in cooling jet trajectories as well as overall cooling effectiveness. Finally, the observed blade-to-blade variations in effectiveness were scaled to engine conditions with lifing estimates showing some blades would be expected to last only half as long as others due to manufacturing variability.

Engineering↗

NASA NTRS1994

Active control of vaneless diffuser rotating stall

Experiments were carried out to study the feasibility of active stabilization of vaneless diffuser rotating stalls. Pressure fluctuation at the diffuser inlet was monitored and used to control the rotating stall. AC control flow, which is produced by a load speaker, was introduced into the diffuser at the inlet. It is shown that the rotating stall can be suppressed when the phase of the control flow has a certain relation with the phase of the rotating stall. By considering the energy flux due to the AC control flow, it is shown that the rotating stall is suppressed when the control flow has the phase such that the energy is subtracted out from the diffuser flow. Discussions are made on the relations between the energy flux and the amplitude of the pressure fluctuation due to the rotating stalls.

Yoshida, Yoshiki↗

NASA NTRS1991

Flow interaction and noise from a counter rotating propeller

The aerodynamic interaction between the forward and rear rotors in a counter rotating propeller (CRP) system, has been examined using a conditional sampling technique applied to three-dimensional thermal anemometer data. The technique effectively freezes the rotors in any desired relative position and provides the inter-rotor flow field. Axial, radial and circumferential mean flow between rotors is shown relative to the 'fixed' forward rotor for various 'fixed' aft rotor positions. Acoustic far field noise data have also been collected for the same operating conditions. The acoustic results are presented with emphasis on the blade passing frequencies and interaction tone of the CRP.

Chung, Jin-Deog↗

NASA NTRS1999

Friction Stir Welding of SiC/Aluminum Metal Matrix Composites

Friction Stir Welding (FSW) is a new solid state process for joining metals by plasticizing and consolidating materials around the bond line using thermal energy producing from frictional forces. A feasibility study for FSW of Metal Matrix Composites (MMC) was investigated using aluminum 6092 alloy reinforced with 17% SiC particulates. FSW process consists of a special rotating pin tool that is positioned to plunge into the MMC surface at the bond line. As the tool rotates and move forward along the bond line, the material at the bond line is heated up and forced to flow around the rotating tip to consolidate on the tip's backside to form a solid state joint. FSW has the potential for producing sound welds with MMC because the processing temperature occurs well below the melting point of the metal matrix; thereby eliminating the reinforcement-to-matrix solidification defects, reducing the undesirable chemical reactions and porosity problems.

Lee, Jonathan A.↗

NASA NTRS1992

Navier-Stokes analysis and experimental data comparison of compressible flow within ducts

Many aircraft employ ducts with centerline curvature or changing cross-sectional shape to join the engine with inlet and exhaust components. S-ducts convey air to the engine compressor from the intake and often decelerate the flow to achieve an acceptable Mach number at the engine compressor by increasing the cross-sectional area downstream. Circular-to-rectangular transition ducts are used on aircraft with rectangular exhaust nozzles to connect the engine and nozzle. To achieve maximum engine performance, the ducts should minimize flow total pressure loss and total pressure distortion at the duct exit. Changes in the curvature of the duct centerline or the duct cross-sectional shape give rise to streamline curvature which causes cross stream pressure gradients. Secondary flows can be caused by deflection of the transverse vorticity component of the boundary layer. This vortex tilting results in counter-rotating vortices. Additionally, the adverse streamwise pressure gradient caused by increasing cross-sectional area can lead to flow separation. Vortex pairs have been observed in the exit planes of both duct types. These vortices are due to secondary flows induced by pressure gradients resulting from streamline curvature. Regions of low total pressure are produced when the vortices convect boundary layer fluid into the main flow. The purpose of the present study is to predict the measured flow field in a diffusing S-duct and a circular-to-rectangular transition duct with a full Navier-Stokes computer program, PARC3D, and to compare the numerical predictions with new detailed experimental measurements. The work was undertaken to extend previous studies and to provide additional CFD validation data needed to help model flows with strong secondary flow and boundary layer separation. The S-duct computation extends the study of Smith et al, and Harloff et al, which concluded that the computation might be improved by using a finer grid and more advanced turbulence models. The present study compares results for both the Baldwin-Lomas and k-epsilon turbulence models and is conducted with a refined grid. For the transition duct, two inlet conditions were considered, the first with straight flow and the second with swirling flow. The first case permits examination of the effects of the geometric transition on the flow field, while the second case includes the rotational flow effect characteristic of a gas turbine engine.

Harloff, G. J.↗

NASA NTRS1993

Viscous analysis of three-dimensional rotor flows using a multigrid method

A three-dimensional code for rotating blade-row flow analysis was developed. The space discretization uses a cell-centered scheme with eigenvalues scaling for the artificial dissipation. The computational efficiency of a four-stage Runge-Kutta scheme is enhanced by using variable coefficients, implicit residual smoothing, and a full-multigrid method. An application is presented for the NASA rotor 67 transonic fan. Due to the blade stagger and twist, a zonal, non-periodic H-type grid is used to minimize the mesh skewness. The calculation is validated by comparing it with experiments in the range from the maximum flow rate to a near-stall condition. A detailed study of the flow structure near peak efficiency and near stall is presented by means of pressure distribution and particle traces inside boundary layers.

Arnone, A.↗

NASA NTRS1991

An inverse inviscid method for the design of quasi-three dimensional rotating turbomachinery cascades

A new inverse inviscid method suitable for the design of rotating blade sections lying on an arbitrary axisymmetric stream-surface with varying streamtube width is presented. The geometry of the axisymmetric stream-surface and the streamtube width variation with meridional distance, the number of blades, the inlet flow conditions, the rotational speed and the suction and pressure side velocity distributions as functions of the normalized arc-length are given. The flow is considered irrotational in the absolute frame of reference and compressible. The output of the computation is the blade section that satisfies the above data. The method solves the flow equations on a (phi 1, psi) potential function-streamfunction plane for the velocity modulus, W and the flow angle beta; the blade section shape can then be obtained as part of the physical plane geometry by integrating the flow angle distribution along streamlines. The (phi 1, psi) plane is defined so that the monotonic behavior of the potential function is guaranteed, even in cases with high peripheral velocities. The method is validated on a rotating turbine case and used to design new blades. To obtain a closed blade, a set of closure conditions were developed and referred.

Bonataki, E.↗