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NASA NTRS1994

Resonant Interaction of a Linear Array of Supersonic Rectangular Jets: an Experimental Study

This paper examines a supersonic multi jet interaction problem that we believe is likely to be important for mixing enhancement and noise reduction in supersonic mixer-ejector nozzles. We demonstrate that it is possible to synchronize the screech instability of four rectangular jets by precisely adjusting the inter jet spacing. Our experimental data agrees with a theory that assumes that the phase-locking of adjacent jets occurs through a coupling at the jet lip. Although the synchronization does not change the frequency of the screech tone, its amplitude is augmented by 10 dB. The synchronized multi jets exhibit higher spreading than the unsynchronized jets, with the single jet spreading the least. We compare the nearfield noise of the four jets with synchronized screech to the noise of the sum of four jets operated individually. Our noise measurements reveal that the more rapid mixing of the synchronized multi jets causes the peak jet noise source to move up stream and to radiate noise at larger angles to the flow direction. Based on our results, we believe that screech synchronization is advantageous for noise reduction internal to a mixer-ejector nozzle, since the noise can now be suppressed by a shorter acoustically lined ejector.

Raman, Ganesh↗

NASA NTRS1992

Relative timing of substorm features in MHD simulations

An investigation of the temporal sequence of substorm phenomena based on three dimensional MHD (magnetohydrodynamic) simulations of magnetic reconnection and plasmoid formation is presented. The investigation utilizes a spatially localized resistivity model which leads to a significantly faster evolution than found in previous investigations. The analysis of the results concentrates on substorm features that have received considerable attention in the past. The formation of magnetic neutral lines, the occurrence of fast flows directed both earthward and tailward, and the magnetic field changes leading to the formation of the substorm current wedge, and to the depolarization of the magnetic field earthward of the reconnection region and its dependence on the spatial distribution of resistivity, are discussed. These phenomena are seen as an integral part of the nonlinear evolution of the three dimensional tearing instability.

Hesse, Michael↗

NASA NTRS1995

Sailplane Glide Performance and Control Using Fixed and Articulating Winglets

An experimental study was conducted to investigate the effects of controllable articulating winglets on glide performance and yawing moments of high performance sailplanes. Testing was conducted in the Texas A&M University 7 x 10 foot Low Speed Wind Tunnel using a full-scale model of the outboard 5.6 feet of a 15 meter class high performance sailplane wing. Different wing tip configurations could be easily mounted to the wing model. A winglet was designed in which the cant and toe angles as well as a rudder on the winglet could be adjusted to a range of positions. Cant angles used in the investigation consisted of 5, 25, and 40 degrees measured from the vertical axis. Toe-out angles ranged from 0 to 22.5 degrees. A rudder on the winglet was used to study the effects of changing the camber of the winglet airfoil on wing performance and wing yawing moments. Rudder deflections consisted of-10, 0, and 10 degrees. Test results for a fixed geometry winglet and a standard wing tip are presented to show the general behavior of winglets on sailplane wings, and the effects of boundary-layer turbulators on the winglets are also presented. By tripping the laminar boundary-layer to turbulent before laminar separation occurs, the wing performance was increased at low Reynolds numbers. The effects on the lift and drag, yawing moment, pitching moment, and wing root bending moment of the model are presented. Oil flows were used on the wing model with the fixed geometry winglet and the standard wing tip to visualize flow directions and areas of boundary layer transition. A cant angle of 25 degrees and a toe-out angle of 2.5 degrees provided an optimal increase in wing performance for the cant and toe angles tested. Maximum performance was obtained when the winglet rudder remained in the neutral position of zero degrees. By varying the cant, toe, and rudder angles from their optimized positions, wing performance decreases. Although the winglet rudder proved to be more effective in increasing the yawing moment compared to varying the cant and toe angles, the amount of increased yawing moment was insignificant when compared to that produced by the vertical tail. A rudder on the winglet was determined to be ineffective for providing additional yaw control.

Colling, James David↗

NASA NTRS1994

By-controlled convection and field-aligned currents near midnight auroral oval for northward interplanetary magnetic field

Using the Dynamics Explorer (DE) 2 magnetic and electric field and plasma data, B(sub y)- controlled convection and field-aligned currents in the midnight sector for northward interplanetary magnetic field (IMF) are examined. The results of an analysis of the electric field data show that when IMF is stable and when its magnitude is large, a coherent B(sub y)-controlled convection exists near the midnight auroral oval in the ionosphere having adequate conductivities. When B(sub y) is negative, the convection consists of a westward (eastward) plasma flow at the lower latitudes and an eastward (westward) plasma flow at the higher latitudes in the midnight sector in the northern (southern) ionosphere. When B(sub y) is positive, the flow directions are reversed. The distribution of the field-aligned currents associated with the B(sub y)-controlled convection, in most cases, shows a three-sheet structure. In accordance with the convection the directions of the three sheets are dependent on the sign of B(sub y). The location of disappearance of the precipitating intense electrons having energies of a few keV is close to the convection reversal surface. However, the more detailed relationship between the electron precipitation boundary and the convection reversal surface depends on the case. In some cases the precipitating electrons extend beyond the convection reversal surface, and in others the poleward boundary terminates at a latitude lower than the reversal surface. Previous studies suggest that the poleward boundary of the electrons having energies of a few keV is not necessarily coincident with an open/closed bounary. Thus the open/closed boundary may be at a latitude higher than the poleward boundary of the electron precipitation, or it may be at a latitude lower than the poleward boundary of the electron precipitation. We discuss relationships between the open/closed boundary and the convection reversal surface. When as a possible choice we adopt a view that the open/closed boundary agrees with the convection reversal surface, we can explain qualitatively the configuration of the B(sub y)-controlled convection on the open and close field line regions by proposing a mapping modified in accordance with IMF B(sub y).

Taguchi, S.↗

NASA NTRS1994

Numerical simulation of large-scale field-aligned current generation from finite-amplitude magnetosonic waves

A two-dimensional numerical simulation of finite-amplitude magnetohydrodynamic (MHD) magnetosonic waves is performed under a finite-velocity background convection condition. Isothermal cases are considered for simplicity. External dissipation is introduced by assuming that the field-aligned currents are generated in proportion to the accumulated charges. The simulation results are as follows: Paired field-aligned currents are found from the simulated waves. The flow directions of these field-aligned currents depend on the angle between the background convection and the wave normal, and hence two pairs of field-aligned currents are found from a bowed wave if we look at the overall structure. The majority of these field-aligned currents are closed within each pair rather than between two wings. These features are not observed under slow background convection. The result could be applied to the cusp current system and the substorm current system.

Yamauchi, M.↗

NASA NTRS1994

Properties of the mantle-like magnetotail boundary layer: GEOTAIL data compared with a mantle model

Near the tail boundary beyond about 100 R(sub e), GEOTAIL often encounters a plasma mantle-like boundary layer in which the plasma flowing tailward transitions smoothly from magnetosheath values of speed and density to much smaller values, more characteristic of the tail lobe. This boundary layer had earlier been recognized on the basis of International Sun Earth Explorer 3 (ISEE 3) measurements. GEOTAIL confirms the existence of this layer and extends the documentation on its behavior. Boundary oscillations sweep the boundary layer over the spacecraft enabling GEOTAIL to 'sound' the layer's profile of plasma parameters. The density-versus-speed correlogram of the mantle-like part -- a useful diagnostic for comparisons -- is reasonably well simulated by a 1-D, MHD slow-mode expansion fan model of the plasma mantle. Flow directions are consistent with an open plasma mantle on the northern, duskside flank of the tail, as expected for the standard magnetic merging model of mantle formation.

Siscoe, G. L.↗

NASA NTRS1994

Irregular, long-period boundary oscillations beyond approximately 100 R(sub e): GEOTAIL plasma observations

Near the tail boundary beyond about 100 Re, GEOTAIL often measures irregular, long-period oscillations in plasma velocity and density. Flow speed and density oscillate between magnetosheath values and values an order of magnitude less. The oscillations can persist for days. A typical oscillation lasts 100 minutes, but the range is large. The oscillations are highly asymmetric in that the increasing phase of the oscillation is an order of magnitude faster than the decreasing phase. This asymmetry shows that they are a distinct class of oscillations, not previously explicitly reported, and that they are not mere consequences of tail flapping in a variable solar wind. The changes in flow direction through an oscillation imply that the oscillation results from a motion of the boundary toward and away from the spacecraft with an amplitude between 5 and 10 R(sub e). A consideration of options suggests that the most plausible cause of these oscillations is the 'breathing' of the magnetotail that attends the substorm cycle.

Siscoe, G. L.↗

NASA NTRS1993

Acceleration of cometary H2O group pickup ions by obliquely propagating nonlinear magnetosonic waves

The observations made during the encounter with comet Giacobini Zinner show that the character of MHD turbulence is governed by the magnetosonic (MS) waves generated by the pickup ions via a resonant cyclotron instability. The interaction of cometary ions in the mass loaded solar wind with MS waves propagating away from the comet and oblique to the interplanetary magnetic field (IMF) is investigated using the test particle approach. Ion trajectories, distribution functions, widths of pitch angle scattering and energy diffusion are obtained. We have also investigated the relationship between pitch angle scattering and the three parameters, namely, alpha, the angle between the solar wind flow direction and the ambient magnetic field, theta(sub BK), the angle between the ambient magnetic field and the wave propagation vector, and the ion injection velocity.

Srivastava, Krishna M.↗

NASA NTRS1995

Fuel Injector With Shear Atomizer

Atomizer for injecting liquid fuel into combustion chamber uses impact and swirl to break incoming stream of fuel into small, more combustible droplets. Slanted holes direct flow of liquid fuel to stepped cylindrical wall. Impact on wall atomizes liquid. Air flowing past vanes entrains droplets of liquid in swirling flow. Fuel injected at pressure lower than customarily needed.

Beal, George W.↗

NASA NTRS1995

Improved Back-Side Purge-Gas Chambers For Plasma Arc Welding

Improved chambers for inert-gas purging of back sides of workpieces during plasma arc welding in keyhole (full-penetration) mode based on concept of directing flows of inert gases toward, and concentrating them on, hot weld zones. Tapered chamber concentrates flow of inert gas on plasma arc plume and surrounding metal.

Ezell, Kenneth G.↗

NASA NTRS1995

Automated Laser Cutting In Three Dimensions

Computer-controlled machine-tool system uses laser beam assisted by directed flow of air to cut refractory materials into complex three-dimensional shapes. Velocity, position, and angle of cut varied. In original application, materials in question were thermally insulating thick blankets and tiles used on space shuttle. System shapes tile to concave or convex contours and cuts beveled edges on blanket, without cutting through outer layer of quartz fabric part of blanket. For safety, system entirely enclosed to prevent escape of laser energy. No dust generated during cutting operation - all material vaporized; larger solid chips dislodged from workpiece easily removed later.

Bird, Lisa T.↗

NASA NTRS1997

The Simulation of Read-time Scalable Coherent Interface

Scalable Coherent Interface (SCI, IEEE/ANSI Std 1596-1992) (SCI1, SCI2) is a high performance interconnect for shared memory multiprocessor systems. In this project we investigate an SCI Real Time Protocols (RTSCI1) using Directed Flow Control Symbols. We studied the issues of efficient generation of control symbols, and created a simulation model of the protocol on a ring-based SCI system. This report presents the results of the study. The project has been implemented using SES/Workbench. The details that follow encompass aspects of both SCI and Flow Control Protocols, as well as the effect of realistic client/server processing delay. The report is organized as follows. Section 2 provides a description of the simulation model. Section 3 describes the protocol implementation details. The next three sections of the report elaborate on the workload, results and conclusions. Appended to the report is a description of the tool, SES/Workbench, used in our simulation, and internal details of our implementation of the protocol.

Li, Qiang↗

NASA NTRS1997

Effect of Flow Misalignment and Multi-Hole Interaction on Boundary-Layer Bleed Hole Flow Coefficient Behavior

The effect of flow misalignment on the flow coefficient behavior of a 20 deg. boundary-layer bleed hole and the effect of the interaction between two 90 deg. bleed holes separated by two hole diameters on flow coefficient behavior has been studied experimentally. Both tests were run at freestream Mach numbers of 0.61, 1.62 and 2.49. The flow misalignment study was conducted over a range of 0 to 30 deg. The results show that neither flow misalignment nor hole interaction has much effect on the flow coefficient for the subsonic case. For the supersonic cases, flow misalignment causes significant degradation in the performance of the slant hole. For the supersonic normal hole interaction cases, depending on the hole orientation, either an increase or decrease in overall flow coefficient was observed. The largest change in flow coefficient, 6% increase at near choke conditions, occurred when the holes were oriented in line with the flow direction.

Davis, David O.↗

NASA NTRS1997

Analytical Studies of Boundary Layer Generated Aircraft Interior Noise

An analysis is made of the "interior noise" produced by high, subsonic turbulent flow over a thin elastic plate partitioned into "panels" by straight edges transverse to the mean flow direction. This configuration models a section of an aircraft fuselage that may be regarded as locally flat. The analytical problem can be solved in closed form to represent the acoustic radiation in terms of prescribed turbulent boundary layer pressure fluctuations. Two cases are considered: (i) the production of sound at an isolated panel edge (i.e., in the approximation in which the correlation between sound and vibrations generated at neighboring edges is neglected), and (ii) the sound generated by a periodic arrangement of identical panels. The latter problem is amenable to exact analytical treatment provided the panel edge conditions are the same for all panels. Detailed predictions of the interior noise depend on a knowledge of the turbulent boundary layer wall pressure spectrum, and are given here in terms of an empirical spectrum proposed by Laganelli and Wolfe. It is expected that these analytical representations of the sound generated by simplified models of fluid-structure interactions can used to validate more general numerical schemes.

Howe, M. S.↗

NASA NTRS1997

Heat Transfer Measurements on Surfaces with Natural Ice Castings and Modeled Roughness

An experimental method is described to measure and compare the convective heat transfer coefficient of natural and simulated ice accretion roughness and to provide a rational means for determining accretion-related enhanced heat transfer coefficients. The natural ice accretion roughness was a sample casting made from accretions at the NASA Lewis Icing Research Tunnel (IRT). One of these castings was modeled using a Spectral Estimation Technique (SET) to produce three roughness elements patterns that simulate the actual accretion. All four samples were tested in a flat-plate boundary layer at angle of attack in a "dry" wind tunnel test. The convective heat transfer coefficient was measured using infrared thermography. It is shown that, dispite some problems in the current data set, the method does show considerable promise in determining roughness-induced heat transfer coefficients, and that, in addition to the roughness height and spacing in the flow direction, the concentration and spacing of elements in the spanwise direction are important parameters.

Breuer, Kenneth S.↗

NASA NTRS1997

The Measurement of Sulfur Oxidation Products and Their Role in Homogeneous Nucleation

The loss rate of H2SO4 vapor onto submicron particles was measured for three different particle substrates. The experimental technique involved direct flow tube measurements of H2SO4 decay rates onto a polydisperse aerosol using chemical ionization mass spectroscopic detection. The aerosols of this study were partially hydrated crystalline salts with diameters in the size range of 20 to 400 nm. The mass accommodation coefficients, a, were calculated from the first-order rate constants for H2SO4 loss to be 0.73 + 0.21 and 0.79 + 0.23 for loss onto (NH4)2SO4 and NaCl, respectively. Measurements of the loss rate of H2SO4 onto a NaCl aerosol coated with stearic acid resulted in lower mass accommodation coefficients with values of 0.31 and 0.19 for aerosol with high and low stearic acid coverage, respectively. The observed decrease in a on an aerosol with a hydrocarbon coating suggests that aerosol composition is a key factor in H2SO4 adsorption on to a particle surface.

Eisele, F. L.↗

NASA NTRS1981

Transonic Flow Past a Symmetrical Airfoil at High Angle of Attack

The results of an experimental investigation of shock-induced stall and leading-edge stall on a 64A010 airfoil section are presented. Advanced nonintrusive techniques - laser velocimetry and holographic interferometry - were used in characterizing the inviscid and viscous flow regions. The measurements include Mach contours of the inviscid now regions, and mean velocity, flow direction, and Reynolds shear stress profiles in the separated regions. The experimental observations of this study are relevant to efforts to improve surface-pressure prediction methods for airfoils at or near stall.

Johnson, D. A.↗

NASA NTRS1994

Uncertainty of Five-Hole Probe Measurements

A new algorithm for five-hole probe calibration and data reduction using a non-nulling technique was developed, verified, and reported earlier (Wendt and Reichert, 1993). The new algorithm's simplicity permits an analytical treatment of the propagation of uncertainty in five-hole probe measurement. The objectives of the uncertainty analysis are to quantify the uncertainty of five-hole probe results (e.g., total pressure, static pressure, and flow direction) and to determine the dependence of the result uncertainty on the uncertainty of all underlying experimental and calibration measurands. This study outlines a general procedure that other researchers may use to determine five-hole probe result uncertainty and provides guidance for improving the measurement technique.

Reichert, Bruce A.↗