Search NASA⌕ Search

SEARCH · Search NASA

Results for “Transient simulations”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The analysis and simulation of compressible turbulence

Compressible turbulent flows at low turbulent Mach numbers are considered. Contrary to the general belief that such flows are almost incompressible, (i.e., the divergence of the velocity field remains small for all times), it is shown that even if the divergence of the initial velocity field is negligibly small, it can grow rapidly on a non-dimensional time scale which is the inverse of the fluctuating Mach number. An asymptotic theory which enables one to obtain a description of the flow in terms of its divergence-free and vorticity-free components has been developed to solve the initial-value problem. As a result, the various types of low Mach number turbulent regimes have been classified with respect to the initial conditions. Formulae are derived that accurately predict the level of compressibility after the initial transients have disappeared. These results are verified by extensive direct numerical simulations of isotropic turbulence.

Erlebacher, Gordon↗
NASA NTRS1988

Forward-reverse shock pairs associated with transient disturbances in the solar wind at 1 AU

Using color-coded plots of the ISEE-3 solar wind electron data and magnetic field data from ISEE-3 for the period from August 1978 through February 1980, evidence was obtained on two transient disturbances which contained reverse shocks in addition to forward shocks. These disturbances are considered to be associated with coronal mass ejections (CMEs). In the stronger of the two disturbances, the reverse shock was found within the CME and was separated from the forward shock by about 0.2 AU; the pressure between the two shocks was nearly constant. In the weaker disturbance, the reverse shock propagated entirely through the CME, trailing the forward shock by about 0.3-0.4 AU; the pressure between the shocks declined substantially and monotonically. Each disturbance profile can be compared favorably with one of the simple one-dimensional fluid simulations used by Hundhausen (1985) to illustrate the general principles underlying transient disturbance propagation in the solar wind.

Gosling, J. T.↗
NASA NTRS1978

Magnetohydrodynamic simulation of coronal mass ejections into the solar wind

A state of the art fluid continuum technique to describe the MHD transient respose of the corona below 10 solar radii during two well observed events (flares on 21 August 1973 and 5 September 1973) is reviewed. It is concluded that the computer simulation should be subjected to in situ verification of as many of its initial assumptions as possible. Its ability to provide a rational basis for physical understanding of mass ejections suggests its use as one of the tools used in the planning and analysis of such encounter missions.

Dryer, M.↗
NASA NTRS1982

A variable structure approach to robust control of VTOL aircraft

This paper examines the application of variable structure control theory to the design of a flight control system for the AV-8A Harrier in a hover mode. The objective in variable structure design is to confine the motion to a subspace of the total state space. The motion in this subspace is insensitive to system parameter variations and external disturbances that lie in the range space of the control. A switching type of control law results from the design procedure. The control system was designed to track a vector velocity command defined in the body frame. For comparison purposes, a proportional controller was designed using optimal linear regulator theory. Both control designs were first evaluated for transient response performance using a linearized model, then a nonlinear simulation study of a hovering approach to landing was conducted. Wind turbulence was modeled using a 1052 destroyer class air wake model.

Calise, A. J.↗
NASA NTRS1984

Nonlinear transient finite element analysis of rotor-bearing-stator systems

This paper extends the finite element scheme to handle the highly nonlinear interfacial fields generated in the fluid filled annulli of squeeze film and journal bearings so as to model the transient response of rotor-bearing-stator systems. Since such simulations are highly nonlinear, direct numerical integration schemes are employed to generate the overall response. In this context, the paper gives consideration to such items as (1) numerical efficiency/stability, (2) comparison of implicit and explicit schemes, (3) determines extent of response nonlinearity as well as (4) extensively benchmarks the overall concept/methodologies.

Padovan, J.↗
NASA NTRS1984

A variable structure approach to robust control of VTOL aircraft

This paper examines the application of variable structure control theory to the design of a flight control system for the AV-8A Harrier in a hover mode. The objective in variable structure design is to confine the state trajectories to a subspace of the total state space. The motion in this subspace is insensitive to system parameter variations and external disturbances that lie in the range space of the control. A switching type of control law results from the design procedure. The control system was designed to track a vector-valued velocity command. For comparison, a proportional controller was designed using optimal linear regulator theory. Both controllers were evaluated for their transient response performance using a linear model; then a nonlinear simulation study of a hovering approach to landing was conducted. The variable structure controller outperformed its linear counterpart in the presence of wind disturbances and plant parameter uncertainties afforded by the simulation.

Calise, A. J.↗
NASA NTRS1985

Flame flashback in a premixed dump combustor

A stainless steel, rectangular center-dump, premixed-prevaporized combustor was used at the NASA Lewis Research Center to study flashback in controlled conditions simulating gas turbine combustor operations under steady-state and transient conditions. The combustor allowed visual access to the combustion area, and was operated at inlet air temperatures from 600 to 850 K, premixer wall temperatures from 450 to 900 K, and average premixer velocity from 40 to 80 ft/s. A parametric study revealed a slight decrease in the flashback equivalence ratio as the inlet air temperatures increased. It also indicated that the premixer wall temperature and premixer velocity are not governing parameters of flashback. It is suggested that flashback occurs through the premixer wall boundary layer flow reversal caused by combustion instability.

Proctor, M. P.↗
NASA NTRS1988

Development of a two-dimensional skin friction balance nulling circuit using multivariable control theory

Measurement of planar skin friction forces in aerodynamic testing currently requires installation of two perpendicularly mounted, single-axis balances; consequently, force components must be sensed at two distinct locations. A two-axis instrument developed at the Langley Research Center to overcome this disadvantage allows measurement of a two-dimensional force at one location. This paper describes a feedback-controlled nulling circuit developed for the NASA two-axis balance which, without external compensation, is inherently unstable because of its low friction mechanical design. Linear multivariable control theory is applied to an experimentally validated mathematical model of the balance to synthesize a state-variable feedback control law. Pole placement techniques and computer simulation studies are employed to select eigenvalues which provide ideal transient response with decoupled sensing dynamics.

Tripp, John S.↗
NASA NTRS1990

Heat-Flux-Measuring Facility

Apparatus simulates conditions in turbine engines. Automated facility generates and measures transient and steady-state heat fluxes at flux densities from 0.3 to 6 MW/m(Sup2) and temperatures from 100 to 1,200 K. Positioning arm holds heat-flux gauge at focal point of arc lamp. Arm previously chilled gauge in liquid nitrogen in Dewar flask. Cooling water flows through lamp to heat exchanger. Used to develop heat-flux gauges for turbine blades and to test materials for durability under rapidly changing temperatures.

Liebert, Curt H.↗
NASA NTRS1990

Influence of residual gravity on crystal growth processes

The effects of steady and time-dependent residual accelerations on representative melt and solution crystal growth systems have been investigated using numerical models. For the melt growth system, it was found that the lateral nonuniformity in crystal composition, depending on the particular operating conditions, can be very sensitive to the orientation of the steady component of the residual gravity vector down to 10 to the -6th g. Oscillatory residual accelerations (g-jitter) were found to lead to transients in the dopant distribution. For the solution growth, longtime simulations showed that at 10 to the -5th g, and the cooling rates used in Spacelab 3 experiments, diffusive transport dominates only in the beginning. Convective transport becomes increasingly significant after 3 to 5 hours, depending on the orientation of the crystal in the residual gravity field.

Rosenberger, F.↗
NASA NTRS1991

A computer program to perform flow and thermal analysis during pressurization of the Space Shuttle Solid Rocket Motor field joint

A computational technique for prediction of the flow and thermal environment in the SRM field joint cavities is described. The SRM field joint hardware was tested with a defect in the insulation, and due to this defect, the O-ring gland cavities are pressurized during the early part of the ignition. A computer model is developed to predict the thermal environment and flow through the simulated flaw, during the pressurization of the field joint. The transient mass, momentum, and energy conservation equations in the flow passage together with the thermodynamic equation of state are solved by a fully implicit iterative numerical procedure.

Clayton, J. Louie↗
NASA NTRS1999

Interfacing a General Purpose Fluid Network Flow Program with the SINDA/G Thermal Analysis Program

A general purpose, one dimensional fluid flow code is currently being interfaced with the thermal analysis program Systems Improved Numerical Differencing Analyzer/Gaski (SINDA/G). The flow code, Generalized Fluid System Simulation Program (GFSSP), is capable of analyzing steady state and transient flow in a complex network. The flow code is capable of modeling several physical phenomena including compressibility effects, phase changes, body forces (such as gravity and centrifugal) and mixture thermodynamics for multiple species. The addition of GFSSP to SINDA/G provides a significant improvement in convective heat transfer modeling for SINDA/G. The interface development is conducted in multiple phases. This paper describes the first phase of the interface which allows for steady and quasi-steady (unsteady solid, steady fluid) conjugate heat transfer modeling.

Schallhorn, Paul↗
NASA NTRS1998

Transient cavities in liquids and the nature of the hydrophobic effect

The size distributions of transient cavities in water and organic liquids, obtained from computer simulations, have provided a new means to analyze the nature of the hydrophobic effect and to evaluate the adequacy of different analytical models of this effect. The poor solubility of non-polar solutes in water is attributed to a low probability of finding in water cavities of atomic and molecular size. It has been shown that water applies more force per unit area of cavity surface than do hydrocarbon liquids. Models that successfully capture the main characteristics of the hydrophobic effect must at least include information about the density and the radial distribution of oxygen atoms in liquid water. One such model, quantitatively accurate for molecular solutes of arbitrary shape, is presented.

NASA Center ARC↗
NASA NTRS2005

Lightning NOx Production and Its Consequences for Tropospheric Chemistry

Cloud-resolving case-study simulations of convective transport and lightning NO production have yielded results which are directly applicable to the design of lightning parameterizations for global chemical transport models. In this work we have used cloud-resolving models (the Goddard Cumulus Ensemble Model (GCE) and MMS) to drive an off-line cloud-scale chemical transport model (CSCTM). The CSCTM, in conjunction with aircraft measurements of NO x in thunderstorms and ground-l;>ased lightning observations, has been used to constrain the amount of NO produced per flash. Cloud and chemistry simulations for several case studies of storms in different environments will be presented. Observed lightning flash rates have been incorporated into the CSCTM, and several scenarios of NO production per intracloud (IC) and per cloud-to-ground (CG) flash have been tested for each storm. The resulting NOx mixing ratios are compared with aircraft measurements taken within the storm (typically the anvil region) to determine the most likely NO production scenario. The range of values of NO production per flash (or per meter of lightning channel length) that have been deduced from the model will be shown and compared with values of production in the literature that have been deduced from observed NO spikes and from anvil flux calculations. Results show that on a per flash basis, IC flashes are nearly as productive of NO as CG flashes. This result simplifies the lightning parameterization for global models (ie., an algorithm for estimating the IC/CG ratio is not necessary). Vertical profiles of lightning NOx mass at the end of the 3-D storm simulations have been summarized to yield suggested profiles for use in global models. Estimates of mean NO production per flash vary by a factor of three from one simulated storm to another. When combined with the global flash rate of 44 flashes per second from NASA's Optical Transient Detector (OTD) measurements, these estimates and the results from other techniques yield global NO production rates of2-9 TgN/year. Simulations of the photochemistry over the 24 hours following a storm has been performed to determine the additional ozone production which can be attributed to lightning NO. Convective transport of HOx precursors leads to the generation of a HOx plume which substantially aids the downstream ozone production.

Pickering, Kenneth E.↗
NASA NTRS2012

Transient Side Load Analysis of Out-of-Round Film-Cooled Nozzle Extensions

There was interest in understanding the impact of out-of-round nozzle extension on the nozzle side load during transient startup operations. The out-of-round nozzle extension could be the result of asymmetric internal stresses, deformation induced by previous tests, and asymmetric loads induced by hardware attached to the nozzle. The objective of this study was therefore to computationally investigate the effect of out-of-round nozzle extension on the nozzle side loads during an engine startup transient. The rocket engine studied encompasses a regeneratively cooled chamber and nozzle, along with a film cooled nozzle extension. The computational methodology is based on an unstructured-grid, pressure-based computational fluid dynamics formulation, and transient inlet boundary flow properties derived from an engine system simulation. Six three-dimensional cases were performed with the out-of-roundness achieved by three different degrees of ovalization, elongated on lateral y and z axes: one slightly out-of-round, one more out-of-round, and one significantly out-of-round. The results show that the separation line jump was the primary source of the peak side loads. Comparing to the peak side load of the perfectly round nozzle, the peak side loads increased for the slightly and more ovalized nozzle extensions, and either increased or decreased for the two significantly ovalized nozzle extensions. A theory based on the counteraction of the flow destabilizing effect of an exacerbated asymmetrical flow caused by a lower degree of ovalization, and the flow stabilizing effect of a more symmetrical flow, created also by ovalization, is presented to explain the observations obtained in this effort.

Wang, Ten-See↗
NASA NTRS1975

Turbofan compressor dynamics during afterburner transients

The effects of afterburner light-off and shut-down transients on compressor stability were investigated. Experimental results are based on detailed high-response pressure and temperature measurements on the Tf30-p-3 turbofan engine. The tests were performed in an altitude test chamber simulating high-altitude engine operation. It is shown that during both types of transients, flow breaks down in the forward part of the fan-bypass duct. At a sufficiently low engine inlet pressure this resulted in a compressor stall. Complete flow breakdown within the compressor was preceded by a rotating stall. At some locations in the compressor, rotating stall cells initially extended only through part of the blade span. For the shutdown transient, the time between first and last detected occurrence of rotating stall is related to the flow Reynolds number. An attempt was made to deduce the number and speed of propagation of rotating stall cells.

Kurkov, A. P.↗
NASA NTRS1999

Equiaxed Dendritic Solidification Experiment (EDSE)

The objective of the research is to quantitatively determine and understand the fundamental mechanisms that control the microstructural evolution during solidification of an assemblage of equiaxed dendritic crystals. A microgravity experiment will be conducted to obtain benchmark data on the transient growth and interaction of up to four equiaxed crystals of a pure and transparent metal analog (succinonitrile, SCN) under strictly diffusion dominated conditions. Of interest in the experiment are the transient evolution of the primary and secondary dendrite tip speeds, the dendrite morphology (i.e., tip radii, branch spacings, etc.) and solid fraction, the tip selection criterion, and the temperature field in the melt for a range of initial supercoolings and, thus, interaction "strengths" between the crystals. The experiment thus extends the microgravity measurements of Glicksman and coworkers for steady growth of a single dendrite [Isothermal Dendritic Growth Experiment (IDGE), first flown on USMP-2] to a case where growth transients are introduced due to thermal interactions between neighboring dendrites - a situation more close to actual casting conditions. Corresponding earth-based experiments will be conducted to ascertain the influence of melt convection. The experiments are supported by a variety of analytical models and numerical simulations. The data will primarily be used to develop and test theories of transient dendritic growth and the solidification of multiple interacting equiaxed crystals in a supercooled melt.

Beckermann, C.↗