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At least 739 records · Page 41

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NASA NTRS1990

A study of microkinetic adjustments required to match shock wave experiments and Monte Carlo Direct Simulation for a wide Mach number range

Shock wave thickness predictions from Monte Carlo Direct Simulations, using differential scattering and the Maitland-Smith-Aziz interatomic potential, underpredict experiments as shock Mach numbers increase above about 4. Examination of several sources of data has indicated that at relatively high energies the repulsive portion of accepted potentials such as the Maitland-Smith-Aziz may be too steep. An Exponential-6 potential due to Ross, based on high energy molecular beam scattering data and shock velocity measurements in liquid argon, has been combined with the lower energy portion of the Maitland-Smith-Aziz potential. When this hybrid potential is used in Monte Carlo Direct Simulations, agreement with experiments is improved over the previous predictions using the pure Maitland-Smith-Aziz form.

Pham-Van-diep, Gerald C.↗

NASA NTRS1990

Direct measurements of skin friction in a scramjet combustor

A direction experimental measurement was made of the skin friction in the turbulent boundary layer of three-dimensional supersonic combustor flows. A floating element configuration was designed to detect two components of the small tangential shear forces due to the flow passing over a nonobtrusive instrument. A cantilevered sensing head extending from a very sensitive piezoresistive displacement transducer simultaneously measured displacement in the axial and transverse directions to the flow. The small overall deflection means that no self-nulling is required. Consistently repeatable output from the gage during testing show the skin friction with supersonic combustion is higher than for a corresponding noncombusting flow. The results are probably accurate to within 10 percent for the mean streamwise component.

Deturris, D. J.↗

NASA NTRS1989

Active vibration control for flexible rotor by optimal direct-output feedback control

Experimental research tests were performed to actively control the rotor vibrations of a flexible rotor mounted on flexible bearing supports. The active control method used in the tests is called optimal direct-output feedback control. This method uses four electrodynamic actuators to apply control forces directly to the bearing housings in order to achieve effective vibration control of the rotor. The force actuators are controlled by an analog controller that accepts rotor displacement as input. The controller is programmed with experimentally determined feedback coefficients; the output is a control signal to the force actuators. The tests showed that this active control method reduced the rotor resonance peaks due to unbalance from approximately 250 microns down to approximately 25 microns (essentially runout level). The tests were conducted over a speed range from 0 to 10,000 rpm; the rotor system had nine critical speeds within this speed range. The method was effective in significantly reducing the rotor vibration for all of the vibration modes and critical speeds.

Nonami, K.↗

NASA NTRS1990

A direct-interface fusible heat sink for astronaut cooling

Astronaut cooling during extravehicular activity is a critical design issue in developing a portable life support system that meets the requirements of a space station mission. Some the requirements are that the cooling device can be easily regenerable and nonventing during operation. In response to this, a direct-interface, fusible heat sink prototpye with freezable quick-disconnects was developed. A proof-of-concept prototype was constructed and tested that consists of an elastic container filled with normal tap water and having two quick-disconnects embedded in a wall. These quick-disconnects are designed so that they may be frozen with the ice and yet still be joined to the cooling system, allowing an immediate flow path. The inherent difficulties in a direct-interface heat sink have been overcome, i.e., (1) establishing an initial flow path; (2) avoiding low-flow freeze-up; and (3) achieving adequate heat-transfer rates at the end of the melting process. The requirements, design, fabrication, and testing are discussed.

Lomax, Curtis↗

NASA NTRS1990

Direct numerical simulation of a three-dimensional turbulent boundary layer

The effects of transverse strain on an initially two-dimensional turbulent boundary layer are studied in a direct numerical simulation of a planar channel flow with impulsively started transverse pressure gradient. Consistent with experiments in three-dimensional boundary layers, the simulation shows a decrease in the Reynolds shear stress with increasing transverse strain. Also, the directions of the Reynolds shear stress vector and the mean velocity gradient vector were found to differ. In addition, the simulation shows a drop in the turbulent kinetic energy. Terms in the Reynolds stress transport equations were computed. The balances indicate that the decrease in turbulent kinetic energy is a result of a decrease in turbulence production, along with an increase in turbulent dissipation. Intuitive reasoning and current turbulence models would predict an increase in kinetic energy along with increases in production and dissipation rates as a result of increased mean-flow strain rate. Later in the evolution of the flow, both turbulence production and dissipation increase.

Moin, P.↗

NASA NTRS1989

Direct simulation of compressible turbulence

Several direct simulations of 3-D homogeneous, compressible turbulence are presented with emphasis on the differences with incompressible turbulent simulations. A fully spectral collocation algorithm, periodic in all directions coupled with a 3rd order Runge-Kutta time discretization scheme is sufficient to produce well-resolved flows at Taylor Reynolds numbers below 40 on grids of 128x128x128. A Helmholtz decomposition of velocity is useful to differentiate between the purely compressible effects and those effects solely due to vorticity production. In the context of homogeneous flows, this decomposition in unique. Time-dependent energy and dissipation spectra of the compressible and solenoidal velocity components indicate the presence of localized small scale structures. These structures are strongly a function of the initial conditions. Researchers concentrate on a regime characterized by very small fluctuating Mach numbers Ma (on the order of 0.03) and density and temperature fluctuations much greater than sq Ma. This leads to a state in which more than 70 percent of the kinetic energy is contained in the so-called compressible component of the velocity. Furthermore, these conditions lead to the formation of curved weak shocks (or shocklets) which travel at approximately the sound speed across the physical domain. Various terms in the vorticity and divergence of velocity production equations are plotted versus time to gain some understanding of how small scales are actually formed. Possible links with Burger turbulence are examined. To visualize better the dynamics of the flow, new graphic visualization techniques have been developed. The 3-D structure of the shocks are visualized with the help of volume rendering algorithms developed in-house. A combination of stereographic projection and animation greatly increase the number of visual cues necessary to properly interpret the complex flow.

Zang, T. A.↗

NASA NTRS1989

Direct simulation of rarefied hypersonic flows

As the capability of the space transportation vehicles (STV's) expand to meet the requirements for future space exploration and utilization, the effects of rarefied hypersonic flows will play a more significant role in defining the aerodynamic and aerothermodynamic performance of STV's. This is particularly true of the low lift/drag aeroassisted STV's where aerobraking occurs at relatively high altitudes and high velocity. Because of the limitations of the continuum description as expressed by the Navier-Stokes equations and the difficulties of solving the Boltzmann equation, the particle of molecular approach has been developed over the last three decades for modeling rarefied gas effects. The direct simulation Monte Carlo (DSMC) method of Bird is the most used method today for simulating rarefied flows. The DSMC method provides a direct physical simulation as opposed to a numerical solution of a set of model equations. This is accomplished by developing phenomenological models of the relevant physical events. The DSMC method accounts for translational, thermal, chemical, and radiative nonequilibrium effects. The general features of the DSMC method, the numerical requirements for obtaining meaningful results, the modeling used to simulate high temperature gas effects, and applications of the method to calculate the flow about an aeroassist flight experiment vehicle (AFE) are reviewed. The AFE simulates a geosynchronous return while entering the Earth's upper atmosphere at approximately 10 km/s. Results obtained using a general 3-D code are presented for the more rarefied portion of the atmospheric encounter (altitudes of 200 to 100 km) emphasizing surface, flowfield, and aerodynamic characteristics of the AFE. Finally, results obtained using axisymmetric and 1-D versions of the code are presented for lower altitude conditions.

Moss, James N.↗

NASA NTRS1991

Modelling directional solidification

The long range goal of this program is to develop an improved understanding of phenomena of importance to directional solidification and to enable explanation and prediction of differences in behavior between solidification on Earth and in space. Current emphasis is on determining the influence of perturbations on directional solidification.

Wilcox, William R.↗

NASA NTRS1990

Flight investigation of the effect of tail boom strakes on helicopter directional control

A joint U.S. Army/NASA flight investigation was conducted utilizing an instrumented Bell 240B single-rotor helicopter to determine the effectiveness of horizontally-mounted tail boom strakes on directional controllability and tail rotor power required during low-speed, crosswind operating conditions. The purpose of the strakes was to separate airflow over the tail boom and change fuselage yawing moments in a direction to improve the yaw control margin and reduce tail rotor power required. Low-speed crosswind data were obtained in 5-knot increments at airspeeds of 0 knots to 35 knots and for 30 deg increments in wind azimuth from 0 to 330 deg. In right sideward flight at the most critical wind azimuth and airspeed (60 deg azimuth measured from the nose of the aircraft and 20 knots airspeed), the strakes improved the pedal margin by about 6 percent of total travel and reduced the tail rotor power required by 17 percent. The increase in yaw control and reduction in tail rotor power offered by the strakes can expand the operating envelope in terms of gross weight and altitude capability. No effects in forward flight were noted.

Kelley, Henry L.↗

NASA NTRS1991

Direct numerical simulation and analysis of shock turbulence interaction

Two kinds of linear analysis, rapid distortion theory (RDT) and linear interaction analysis (LIA), were used to investigate the effects of a shock wave on turbulence. Direct numerical simulations of two-dimensional isotropic turbulence interaction with a normal shock were also performed. The results from RDT and LIA are in good agreement for weak shock waves, where the effects of shock front curvature and shock front unsteadiness are not significant in producing vorticity. The linear analyses predict wavenumber-dependent amplification of the upstream one-dimensional energy spectrum, leading to turbulence scale length scale decrease through the interaction. Instantaneous vorticity fields show that vortical structures are enhanced while they are compressed in the shock normal direction. Entrophy amplfication through the shock wave compares favorably with the results of linear analyses.

Lee, Sangsan↗

NASA NTRS1991

Lagrangian and Eulerian statistics obtained from direct numerical simulations of homogeneous turbulence

Direct numerical simulation is used to study dispersion in decaying isotropic turbulence and homogeneous shear flow. Both Lagrangian and Eulerian data are presented allowing direct comparison, but at fairly low Reynolds number. The quantities presented include properties of the dispersion tensor, isoprobability contours of particle displacement, Lagrangian and Eulerian velocity autocorrelations and time scale ratios, and the eddy diffusivity tensor. The Lagrangian time microscale is found to be consistently larger than the Eulerian microscale, presumably due to the advection of the small scales by the large scales in the Eulerian reference frame.

Squires, Kyle D.↗

NASA NTRS1991

Mars direct - A simple, robust, and cost effective architecture for the Space Exploration Initiative

Both the Martian and lunar forms of implementation of the Mars Direct architecture are discussed. Candidate vehicle designs are presented and the means of performing the required in situ propellant production is explained. The in situ propellant process is also shown to present very high leverage for a Mars Rover Sample Return mission flown as a scaled down precursor version of the manned Mars Direct. Methods of coping with the radiation and zero gravity problems presented by a manned Mars mission are discussed. Prime objectives for surface exploration are outlined and the need for substantial surface mobility is made clear. Combustion powered vehicles utilizing the in situ produced methane/oxygen are proposed as a means for meeting the surface mobility requirement. Nuclear thermal rocket propulsion is suggested as a means to improve mission capability.

Zubrin, Robert M.↗

NASA NTRS1989

LQG and direct rate feedback control with model reduction on a flexible laboratory grid structure

This paper presents experimental and theoretical comparisons of three control laws applied to a complex laboratory structure. A reduced finite element model was generated for designing the control systems and then corrected based on measured mode shapes and frequencies. A standard time-invariant linear quadratic regulator with state estimation was investigated first. Two simple direct rate feedback control laws both guaranteeing stability were also designed using the reduced model. One minimizes the maximum control force and the other minimizes the same quadratic performance index as the linear quadratic regulator. The three control laws have comparable performance indices with the direct rate feedback designs having better spillover properties. Experimental results for all designs were obtained with digital implementation. It was shown that the performance of the control system designed on the basis of the corrected finite element model agreed better with experimental results than the performance of the control system designed on the basis of the uncorrected model.

Schamel, G. C., II↗

NASA NTRS1991

Laser-induced stimulated Raman scattering in the forward direction of a droplet - Comparison of Mie theory with geometrical optics

This paper uses Mie theory to treat electromagnetic scattering and to evaluate field enhancement in the forward direction of a small droplet irradiated by a high-energy beam and compares the results of calculations with the field-enhancement evaluation obtained via geometrical optics treatment. Results of this comparison suggest that the field enhancement located at the critical ring region encircling the axis in the forward direction of the droplet can support laser-induced Raman scattering. The results are supported by experimental observations of the interaction of a 120-micron-diam water droplet with a high-energy Nd:YAG laser beam.

Srivastava, Vandana↗

NASA NTRS1990

Suggested future directions in high-speed transition experimental research

Historical developments in the area of high-speed experimental transition research are outlined, and future directions in this area as determined by the panel membership are listed. The directions include measurement and modeling of initial disturbance fields, both in ground facilities and flight, for all modes; development of advanced high-speed instrumentation for disturbance field measurements, measurements of the details of receptivity in multitudinous flows; further development and use of high-speed quiet tunnels; stability and transition studies for multitudinous flows; detailed studies of the transitional region for boundary layers, free flows, vortices separated flows, corner flows, etc.; and studies of flow-chemistry effects on transition phenomena. Applied research such areas as the physics of perforated-surface suction stabilization and the resolution of anomalies in the existing high-speed database is also suggested.

Bushnell, Dennis↗

NASA NTRS1991

Nonequilibrium gas flows. I - A detailed validation of Monte Carlo direct simulation for monatomic gases

One-dimensional shock wave properties in helium and argon are predicted using Monte Carlo direct simulation. The collision model is based directly on the interatomic potential taking angular scattering into account. The potential is assumed to be of the Maitland-Smith n(r)-6 form. The detailed validity of the simulation is studied by comparing the predicted macroscopic and miroscopic flow properties in shock waves to a wide range of available data.

Erwin, Daniel A.↗

NASA NTRS1991

Ultraviolet interstellar absorption toward HD 156359, a halo star at 11 kiloparsecs in the direction l = 329 deg and b = -15 deg

Results based on high-quality multiple IUE high-dispersion spectra are presented for the HD 156359 sight line in the direction l = 328.7 deg and b = -14.5 deg. The UV stellar spectrum of HD 156359 is consistent with an MK classification of O9.7 Ib-II. The high-ionization UV interstellar absorption line in the star's UV spectrum are very broad with negative velocity extensions. Comparison of apparent column density profiles Na(v) for lines within each doublet indicates that there is no significant unresolved saturated structure in the Si IV or N V profiles. Direct integration of the Na(v) profiles for Si IV, C IV, and N V over velocities from -150 km/s to +40 km/s yields total column densities of log N = 14.10, 14.77, and 14.09, respectively. The definite detection of N V absorption lines with profile shapes similar to the Si IV and C IV profiles favors a common origin for all three species.

Sembach, Kenneth R.↗

NASA NTRS1991

Ionospheric convection response to changing IMF direction

By combining ground-based and satellite-based measurements of ionospheric electric fields, conductivities and magnetic perturbations, it was possible to examine the characteristics of instantaneous, ionospheric convection patterns associated with changing directions of the interplanetary magnetic field (IMF). In response to a rapid southward-to-northward turning of the IMF on July 23, 1983, the ionospheric convection reconfigured over a period of 40 minutes. The configuration changed from a conventional two-cell pattern to a contracted four-cell pattern, with reversed convection cells in the high-latitude dayside, associated with a strong potential drop of about 75 kV. Later, in response to a gradual rotation of the IMF from the +Z through the -Y toward the -Z direction, the nightside cells disappeared and the dawn cell in the reversed pair wrapped around and displaced the dusk cell until a conventional two-cell pattern was reestablished, largely in accord with the qualitative model of Crooker (1988). The results suggest that multiple cells can arise as a result of strong southward to northward transitions in the IMF. They appear to persist for sometime thereafter.

Knipp, D. J.↗