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At least 163 records · Page 9

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NASA NTRS1986

On the instability of a three-dimensional attachment-line boundary layer - Weakly nonlinear theory and a numerical approach

The instability of a three-dimensional attachment-line boundary layer is considered in the nonlinear regime. Using weakly nonlinear theory, it is found that, apart from a small interval near the (linear) critical Reynolds number, finite-amplitude solutions bifurcate subcritically from the upper branch of the neutral curve. The time-dependent Navier-Stokes equations for the attachment-line flow have been solved using a Fourier-Chebyshev spectral method and the subcritical instability is found at wavenumbers that correspond to the upper branch. Both the theory and the numerical calculations show the existence of supercritical finite-amplitude (equilibrium) states near the lower branch which explains why the observed flow exhibits a preference for the lower branch modes. The effect of blowing and suction on nonlinear stability of the attachment-line boundary layer is also investigated.

Hall, P.↗

NASA NTRS1987

Lewis inverse design code (LINDES): Users manual

The method of complex characteristics and hodograph transformation for the design of shockless airfoils was introduced by Bauer, Garabedian, and Korn and has been extended by the author to design subcritical and supercritical cascades with high solidities and large inlet angles. This new capability was achieved by introducing a new conformal mapping of the hodograph domain onto an ellipse and expanding the solution in terms of Chebyshev polynomials. A new computer code, the NASA Lewis inverse design code, was developed based on this idea. This new design code is an efficient method for the design of airfoils in cascade. In particular, the design of subcritical cascades of airfoils is a very fast, robust, and versatile process. The inverse design code can be made to interact with a turbulent boundary layer calculation to obtain airfoils with no separated flows at the design condition. This report is intended to serve as a users manual for this design code. Material previously reported by the author is included here for completeness and quick access to the user. The manual contains a description of the method followed by a discussion of the design procedure and examples. The input parameters necessary to run the code are then described and their default values given. Output listings corresponding to six different blade shapes designed with the code are given, as well as the necessary input data to reproduce the computer runs. The examples have been chosen to show that a wide range of applications can be covered with the code, ranging from supercritical propeller sections to wind tunnel turning vanes that can operate with a large inlet flow angle range.

Sanz, Jose M.↗

NASA NTRS1986

Unsteady hybrid vortex technique for transonic vortex flows and flutter applications

The well developed surface panel method is extended by adding a volume integral term allowing calculation of the full effect of compressibility. The full effect of compressibility is calculated by using mixed type finite difference scheme consistent with the mixed nature of transonic flow. The solution is obtained through successive iteration cycles for subcritical flows and for critical flows the solution is obtained through satisfying the Rankine-Hugoniot relations across the captured shock in addition to the captured shock. The method is applied to airfoils in subcritical and critical flows and the results are in good agreement with the experimental data and finite-difference solutions of the full-potential and Euler equations.

Kandil, Osama A.↗

NASA NTRS1987

The interaction of quasiperpendicular shock waves in a collisionless plasma

The interaction between collisionless shocks is examined from a kinetic viewpoint using computer simulations of shock collisions spanning the range of ambient plasma and shock parameters typical of the interplanetary medium. It is found that the interaction between two subcritical, perpendicular shocks is largely fluidlike with no evidence for accelerated particles. The electromagnetic fields associated with each shock, but not the plasmas, pass through each other during the collision. This interaction is due to self-consistently generated electric fields which adjust to the value needed to bring to rest the two incoming streams. The interaction between two supercritical perpendicular collisionless shocks is similar to the subcritical case, in that the electromagnetic fields associated with each shock are transmitted whereas particles are not. However, the interaction is also highly dynamic with substantial acceleration of ions. Increasing the Mach number gives rise to an increasing number of energetic particles.

Cargill, P. J.↗

NASA NTRS1987

Increase of ion kinetic temperature across a collisionless shock. II - A simulation study

The present paper is a continuation of the preceding article by Lee et al., (1986) in which it is suggested that the nonadiabatic motion of the directly transmitted ions in a quasi-perpendicular shock wave can result in an increase of the ion kinetic temperature transverse to the ambient magnetic field in the downstream. A series of computer simulations based on a hybrid code have been carried out to examine the dynamics of the transmitted ions in both the subcritical and supercritical shock waves. It is found that, in both cases, the directly transmitted ions can contribute to the heating process. In the case of a resistiveless supercritical shock, the reflected and transmitted ions can be equally important; whereas for a subcritical shock, the transmitted ions are primarily responsible for the ion heating.

Lee, L. C.↗

NASA NTRS1989

Evaluation of supercritical cryogen storage and transfer systems for future NASA missions

Conceptual designs of Space Transportation Vehicles (STV), and their orbital servicing facilities, that utilize supercritical, single phase, cryogenic propellants were established and compared with conventional subcritical, two phase, STV concepts. The analytical study was motivated by the desire to avoid fluid management problems associated with the storage, acquisition and transfer of subcritical liquid oxygen and hydrogen propellants in the low gravity environment of space. Although feasible, the supercritical concepts suffer from STV weight penalties and propellant resupply system power requirements which make the concepts impractical.

Arif, Hugh↗

NASA NTRS1990

Elevated temperature crack growth in aluminum alloys: Tensile deformation of 2618 and FVS0812 aluminum alloys

Understanding the damage tolerance of aluminum alloys at elevated temperatures is essential for safe applications of advanced materials. The objective of this project is to investigate the time dependent subcritical cracking behavior of powder metallurgy FVS0812 and ingot metallurgy 2618 aluminum alloys at elevated temperatures. The fracture mechanics approach was applied. Sidegrooved compact tension specimens were tested at 175, 250, and 316 C under constant load. Subcritical crack growth occurred in each alloy at applied stress intensity levels (K) of between about 14 and 25 MPa/m, well below K (sub IC). Measured load, crack opening displacement and displacement rate, and crack length and growth rate (da/dt) were analyzed with several continuum fracture parameters including, the C-integral, C (sub t), and K. Elevated temperature growth rate data suggest that K is a controlling parameter during time dependent cracking. For FVS0812, da/dt is highest at 175 C when rates are expressed as a function of K. While crack growth rate is not controlled by C (sub t) at 175 C, da/dt appears to better correlate with C (sub t) at higher temperatures. Creep brittle cracking at intermediate temperatures, and perhaps related to strain aging, is augmented by time dependent transient creep plasticity at higher temperatures. The C (sub t) analysis is, however, complicated by the necessity to measure small differences in the elastic crack growth and creep contributions to the crack opening displacement rate. A microstructural study indicates that 2618 and FVS0812 are likely to be creep brittle materials, consistent with the results obtained from the fracture mechanics study. Time dependent crack growth of 2618 at 175 C is characterized by mixed transgranular and intergranular fracture. Delamination along the ribbon powder particle boundaries occurs in FVS0812 at all temperatures. The fracture mode of FVS0812 changes with temperature. At 175 C, it is characterized as dimpled rupture, and at 316 C as mixed matrix superplastic rupture and matrix-dipersoid debonding.

Leng, Yang↗

NASA NTRS1990

Evaluation of supercritical cryogen storage and transfer systems for future NASA missions

Conceptual designs of Space Transportation Vehicles (STV), and their orbital servicing facilities, that utilize supercritical, single phase, cryogenic propellant were established and compared with conventional subcritical, two phases, STV concepts. The analytical study was motivated by the desire to avoid fluid management problems associated with the storage, acquisition and transfer of subcritical liquid oxygen and hydrogen propellants in the low gravity environment of space. Although feasible, the supercritical concepts suffer from STV weight penalties and propellant resupply system power requirements which make the concepts impractical.

Arif, Hugh↗

NASA NTRS1990

An assessment of full potential and Euler solutions for self-generated rotor blade-vortex interactions

An assessment is made of the accuracy and suitability of the three-dimensional full potential and Euler equations in the modeling of subcritical and supercritical rotor blade-vortex interactions (BVI). The interaction velocity fields are obtained through a nonlinear superposition of the rotor and the vortex wake flow fields. Vortex effects are simulated using the velocity 'transpiration' approach. A modified surface condition is prescribed and enforced at each time step of the computations to satisfy the tangency boundary condition. Potential blade-vortex encounters are identified and tracked in time at equal increments of rotor azimuth using the lifting-line helicopter/rotor trim code CAMRAD. This information is then utilized in interpolation routines within the flow solvers to compute the instantaneous position(s) of the interaction vortex element(s) with respect to the blade for the time-accurate calculations. Results for subcritical self-generated BVI and for supercritical interactions resulting from an upstream-generated vortex are presented.

Hassan, A. A.↗

NASA NTRS1990

The effect of an electric field on the morphological stability of the crystal-melt interface of a binary alloy. III - Weakly nonlinear theory

The effect of a constant electric current on the crystal-melt interface morphology during directional solidification at constant velocity of a binary alloy is considered. A linear temperature field is assumed, and thermoelectric effects and Joule heating are neglected; electromigration and differing electrical conductivities of crystal and melt are taken into account. A two-dimensional weakly nonlinear analysis is carried out to third order in the interface amplitude, resulting in a cubic amplitude equation that describes whether the bifurcation from the planar state is supercritical or subcritical. For wavelengths corresponding to the most dangerous mode of linear theory, the demarcation between supercritical and subcritical behavior is calculated as a function of processing conditions and material parameters. The bifurcation behavior is a sensitive function of the magnitude and direction of the electric current and of the electrical conductivity ratio.

Wheeler, A. A.↗

NASA NTRS1991

Improved thermodynamic modelling of the no-vent fill process and correlation with experimental data

The United States plans to establish a permanent manned presence in space and to explore the Solar System have created the need to efficiently handle large quantities of subcritical cryogenic fluids, particularly propellants such as liquid hydrogen and liquid oxygen, in low- to zero-gravity environments. One of the key technologies to be developed for fluid handling is the ability to transfer the cryogens between storage and spacecraft tanks. The no-vent fill method has been identified as one way to perform this transfer. In order to understand how to apply this method, a model of the no-vent fill process is being developed and correlated with experimental data. The verified models then can be used to design and analyze configurations for tankage and subcritical fluid depots. This paper discusses the development of an improved macroscopic thermodynamic model of the no-vent fill process and correlates the analytical results from the computer program implementation of the model with experimental results for two different test tanks at NASA Lewis Research Center.

Taylor, W. J.↗

NASA NTRS1991

Improved thermodynamic modeling of the no-vent fill process and correlation with experimental data

The United States' plans to establish a permanent manned presence in space and to explore the Solar System created the need to efficiently handle large quantities of subcritical cryogenic fluids, particularly propellants such as liquid hydrogen and liquid oxygen, in low- to zero-gravity environments. One of the key technologies to be developed for fluid handling is the ability to transfer the cryogens between storage and spacecraft tanks. The no-vent fill method was identified as one way to perform this transfer. In order to understand how to apply this method, a model of the no-vent fill process is being developed and correlated with experimental data. The verified models then can be used to design and analyze configurations for tankage and subcritical fluid depots. The development of an improved macroscopic thermodynamic model is discussed of the no-vent fill process and the analytical results from the computer program implementation of the model are correlated with experimental results for two different test tanks.

Taylor, William J.↗

NASA NTRS1992

Digital time series analysis for flutter test data

An application of digital time series analysis to flutter test data processing was conducted. A numerical investigation was used to evaluate the method, as well as its sensitivity to noise and parameter variations. These parameters included those involved with data acquisition, as well as system response characteristics. This digital time series method was then used to predict flutter speed from subcritical response wind tunnel tests. Flutter speeds predicted from forced response, subcritical wind tunnel tests were compared to the experimental flutter speeds.

Batill, S. M.↗

NASA NTRS1992

Combustion of liquid-fuel droplets in supercritical conditions

A comprehensive analysis of liquid-fuel droplet combustion in both subcritical and supercritical environments has been conducted. The formulation is based on the complete conservation equations for both gas and liquid phases, and accommodates variable thermophysical properties, finite-rate chemical kinetics, and a full treatment of liquid-vapor phase equilibrium at the drop surface. The governing equations and associated interfacial boundary conditions are solved numerically using a fully coupled, implicit scheme with the dual time-stepping integration technique. The model is capable of treating the entire droplet history, including the transition from the subcritical to supercritical state. As a specific example, the combustion of n-pentane fuel droplets in air is studied for pressures in the range of 5-140 atm. Results indicate that the ambient gas pressure exerts significant control of droplet gasification and burning processes through its influence on fluid transport, gas-liquid interfacial thermodynamics, and chemical reactions. The droplet gasification rate increases progressively with pressure. However, the data for the overall burnout time exhibit a considerable change in the combustion mechanism at the critical pressure, mainly as a result of reduced mass diffusivity and latent heat of vaporization with increased pressure.

Shuen, J. S.↗

NASA NTRS1993

Conjugate heat transfer from a heated disk to a thin liquid film formed by a controlled impinging jet

An experimental and numerical study of the heat transfer from a heated horizontal disk to a thin film of liquid is described. The liquid was delivered to the disk by a collar arrangement such that the film thickness and radial velocity were known at the outer radius of the collar. This method of delivery is termed as a controlled impinging jet. Flow visualization tests were performed and heat transfer data were collected along the radius of the disk for different volumetric flow rates and inlet temperatures in the supercritical and subcritical regions. The heat transfer coefficient was found to increase with flow rate when both supercritical and subcritical regions were present on the heated surface. A numerical simulation of this free surface problem was performed, which included the effects of conjugate heat transfer within the heated disk and the liquid. The numerical predictions agree with the experimental results and show that conjugate heat transfer has a significant effect on the local wail temperature and heat transfer coefficient.

Faghri, A.↗

NASA NTRS1993

An evaluation of excitation techniques for time domain based flutter data processing

Numerically simulated, wind tunnel and flight test experimental data were used to evaluate various types of system excitation sources for subcritical flutter testing. Emphasis was placed upon the determination of modal frequencies and damping from relatively short time series records. Parameter identification based upon digital time series models was used to predict modal characteristics from subcritical test data. Various types of excitation were considered. These were: 1) impulsive loading and the resulting transient free vibration response, 2) random forced response, in which the 'mechanical' forcing function could be directly measured, 3) sine sweep forced response, in which the 'mechanical' forcing function could be directly measured, and 4) forced response from unsteady aerodynamic excitation or turbulence, in which the excitation could only be indirectly inferred from other measurements.

Pinkelman, J. K.↗

NASA NTRS1951

Force and pressure characteristics for a series of nose inlets at Mach numbers from 1.59 to 1.99 V : analysis and comparison on basis of ram-jet aircraft range and operational characteristics

Performance of four experimentally investigated axially symmetric spike-type nose inlets is compared on basis of ram-jet-engine aircraft range and operational problems. At design conditions, calculated peak engine efficiencies varied 25 percent from the highest value which indicates importance of inlet design. Calculations for a typical supersonic aircraft indicate possible increase in range if engine is flown at moderate angle of attack and result in engine lift utilized. For engines with fixed exhaust nozzle, propulsive thrust increases with increasing heat addition in subcritical flow region in spite of increasing additive drag. For the perforated inlet there is a range of increasing total-temperature ratios in subcritical flow region that does not yield an increase in propulsive thrust. Effects of inlet characteristics on speed stability of a typical aircraft for three types of fuel control is discussed.

ENGINES, RAM JET↗

NASA NTRS1956

Preliminary Report on Experimental Investigation of Engine Dynamics and Controls for a 48-inch Ram-jet Engine

Engine dynamics and controls data are presented for a ram-jet engine which was operated in a free-jet facility at a Mach number of 2.76 and altitudes from 68,000 to 82,000 feet. The predominant engine dynamic characteristics was dead time, with values ranging from 0.018 to 0.053 sec. The control systems were designed to hold a constant ratio of a diffuser static pressure to an inlet-cone reference static pressure. Response times and percent overshoot to fuel-flow disturbances that would have caused subcritical operation without control are included for a wide range of control settings. For all settings, the control response was fast enough (0.08 to 0.5 sec) to prevent subcritical operation.

Vasu, George↗