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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.

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At least 865 records · Page 48
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Computation of unsteady transonic flows through rotating and stationary cascades. 1: Method of analysis

A numerical method of solution of the inviscid, compressible, two-dimensional unsteady flow on a blade-to-blade stream surface through a stage (rotor and stator) or a single blade row of an axial flow compressor or fan is described. A cyclic procedure has been developed for representation of adjacent blade-to-blade passages which asymptotically achieves the correct phase between all passages of a stage. A shock-capturing finite difference method is employed in the interior of the passage, and a method of characteristics technique is used at the boundaries. The blade slipstreams form two of the passage boundaries and are treated as moving contact surfaces capable of supporting jumps in entropy and tangential velocity. The Kutta condition is imposed by requiring the slipstreams to originate at the trailing edges, which are assumed to be sharp. Results are presented for several transonic fan rotors and compared with available experimental data, consisting of holographic observations of shock structure and pressure contour maps. A subcritical stator solution is also compared with results from a relaxation method. Finally, a periodic solution for a stage consisting of 44 rotor blades and 46 stator blades is discussed.

Erdos, J. I.↗
NASA NTRS1996

Multigrid Acceleration of Time-Accurate Navier-Stokes Calculations

A numerical scheme to solve the unsteady Navier-Stokes equations is described. The scheme is fully implicit in time and is unconditionally stable (at least for first- and second-order discretizations of the physical time derivatives). With unconditional stability, the choice of the time step is based on the physical phenomena to be resolved rather than limited by numerical stability. This is especially important for high Reynolds number viscous flows, where the spatial variation of grid cell size can be as much as six orders of magnitude. A multigrid-multiblock, steady-state, three-dimensional Navier-Stokes solver, TLNS3D, was modified to iteratively invert the equations at each physical time step. The implementation of this procedure in TLNS3D is discussed. The implications of applying several popular turbulence models to unsteady flow are also considered. Numerical results are presented to show the application of the scheme to various two-dimensional turbulent flows. The results of a three-dimensional laminar flow calculation are also given.

Melson, N. Duane↗
NASA NTRS1993

Transition prediction and control in subsonic flow over a hump

The influence of a surface roughness element in the form of a two-dimensional hump on the transition location in a two-dimensional subsonic flow with a free-stream Mach number up to 0.8 is evaluated. Linear stability theory, coupled with the N-factor transition criterion, is used in the evaluation. The mean flow over the hump is calculated by solving the interacting boundary-layer equations; the viscous-inviscid coupling is taken into consideration, and the flow is solved within the separation bubble. The effects of hump height, length, location, and shape; unit Reynolds number; free-stream Mach number, continuous suction level; location of a suction strip; continuous cooling level; and location of a heating strip on the transition location are evaluated. The N-factor criterion predictions agree well with the experimental correlation of Fage; in addition, the N-factor criterion is more general and powerful than experimental correlations. The theoretically predicted effects of the hump's parameters and flow conditions on transition location are consistent and in agreement with both wind-tunnel and flight observations.

Masad, Jamal A.↗
NASA NTRS2005

Vortobots

The term vortobots denotes proposed swimming robots that would have dimensions as small as micrometers or even nanometers and that would move in swarms through fluids by generating and exploiting vortices in a cooperative manner. Vortobots were conceived as means of exploring confined or otherwise inaccessible fluid environments: they are expected to be especially attractive for biomedical uses like examining the interiors of blood vessels. The main advantage of the vortobot concept, relative to other concepts for swimming microscopic robots, is that the mechanisms for locomotion would be relatively simple and, therefore, could be miniaturized more easily. For example, only a simple spinning paddle would be required to generate a vortex around a vortobot (see Figure 1). The difficulty is that a smart swarming and cooperative control algorithm would be necessary for purposeful locomotion. This necessity arises because, as a consequence of basic principles of vortex dynamics, an isolated single vortex cannot move by itself because its induced flow at the center is zero; however, a vortex can move other vortices by the induced flow. By cleverly adjusting the strength and sign of each member in a group of vortices, the group can achieve net translational motion in the preferred direction through cooperation. Figure 2 presents two simple examples that serve to illustrate the principle of cooperative motion of vortobots. For the sake of simplicity, these examples are based on an idealized two-dimensional potential flow of an inviscid, incompressible liquid. The example of the upper part of the figure is of two vortices of equal magnitude and opposite sign. The centers of the vortices would move along parallel paths. The example of the lower part of the figure is of two vortices of the same magnitude and sign. In this case, both vortices would move in a circle in diametrically opposite positions. More complex motions can be obtained by introducing more vortices (or pairs of vortices) and choosing different vortex strengths and orientations.

Park, Han↗
NASA NTRS1984

Transitions in shallow convection - An explanation for lateral cell expansion

The generalized, seven-coefficient model presented for two-dimensional Rayleigh-Benard convection successfully simulates one way in which lateral cell expansion can occur as the value of the imposed vertical temperature difference is changed. The triad of interacting wavenumbers is varied as the vertical heating rate is varied and only the energetically active components are retained, so that transitional behavior within two-dimensional convective flow can be both simulated and physically interpreted as the representation of the cell expansion process via successive secondary branching. It is found that while a two-dimensional cell broadening mechanism is likely to operate for small Prandtl number values, a three-dimensional mechanism is expected at values greater than about 0.7.

Chang, H.-R.↗
NASA NTRS1977

Unsteady transonic flow computations

The numerical procedures previously developed for computing nonlinear and time-linearized small-perturbation unsteady transonic flows are briefly reviewed, and the effects of unsteady modes of motion on two-dimensional transonic flows are evaluated. The numerical procedure used comprises an alternating-direction implicit scheme and treats shock waves as discontinuities in the flow. Comparison of the time-linearized results with fully nonlinear calculations delineates their range of applicability. The unsteady behavior due to harmonic pitching and flap oscillations of an NACA airfoil is also examined.

Seebass, A. R.↗
NASA NTRS1990

Comparisons of one- and two- interface methods for tunnel wall interference calculation

Tunnel wall interference in two-dimensional subsonic flow can be analytically calculated on the basis of the one- or two-interface method. The application of numerical integration by means of matrix multiplication to both methods has been found to significantly reduce CPU time in on-line operation. An investigation of the sensitivity of these methods to random velocity-measurement disturbances shows the one-interface method to be less sensitive to measurement errors, and therefore to be preferable for practical applications.

Lo, C. F.↗
NASA NTRS1997

A Three-Dimensional Linearized Unsteady Euler Analysis for Turbomachinery Blade Rows

A three-dimensional, linearized, Euler analysis is being developed to provide an efficient unsteady aerodynamic analysis that can be used to predict the aeroelastic and aeroacoustic responses of axial-flow turbo-machinery blading.The field equations and boundary conditions needed to describe nonlinear and linearized inviscid unsteady flows through a blade row operating within a cylindrical annular duct are presented. A numerical model for linearized inviscid unsteady flows, which couples a near-field, implicit, wave-split, finite volume analysis to a far-field eigenanalysis, is also described. The linearized aerodynamic and numerical models have been implemented into a three-dimensional linearized unsteady flow code, called LINFLUX. This code has been applied to selected, benchmark, unsteady, subsonic flows to establish its accuracy and to demonstrate its current capabilities. The unsteady flows considered, have been chosen to allow convenient comparisons between the LINFLUX results and those of well-known, two-dimensional, unsteady flow codes. Detailed numerical results for a helical fan and a three-dimensional version of the 10th Standard Cascade indicate that important progress has been made towards the development of a reliable and useful, three-dimensional, prediction capability that can be used in aeroelastic and aeroacoustic design studies.

Montgomery, Matthew D.↗
DOE OSTI2017

Newton-Krylov method in solving drift-flux two-phase flow problems for both pre-CHF and post-CHF conditions at steady state

Furthermore, this paper presents numerical simulations of two-phase flow heat transfer for both pre-CHF and post-CHF conditions typical in light water nuclear reactors. The conjugate heat transfer problem is modeled with one-dimensional drift-flux two-phase flow model for coolant flow and two-dimensional heat conduction equation in solid walls. Fully implicit time integration schemes were employed, and the resulted non-linear equations were solved with a Newton-Krylov method. For the drift-flux two-phase flow model, the EPRI drift-flux closure correlation is used to model the relative motion between the two phases. Additional closure correlations were adopted from REALP5-3D to determine two-phase flow regime, wall boiling heat and mass transfer, interfacial heat and mass transfer, and two-phase flow frictional pressure drop. Three sets of experiments were used for code validation, including Bartolomei subcooled flow boiling in vertical pipes, FRIGG boiling tests in vertical bundles, and Bennett heated tube tests. RELAP5-3D simulation results were also provided to perform code-to-code benchmark. Overall, numerical results of this work showed good agreements with both experimental data and RELAP5-3D simulation results. Discrepancy between them were also identified in post-CHF region, which suggests necessary further improvement.

42 ENGINEERING↗
NASA NTRS1986

A space-marching method for viscous incompressible internal flows

A numerical algorithm for calculating steady subsonic two-dimensional incompressible channel flows in ducts is developed and extended to three-dimensional flows. A space-marching method similar to that developed by Schiff and Steger (1980) for supersonic flows is applied, and numerical results for developing laminar flows in a two-dimensional channel, in a straight square duct, and in a mildly curved square duct are presented graphically. Good agreement with experimental data and analytical results is found.

Pouagare, M.↗
NASA NTRS1977

Unsteady aerodynamic modeling for arbitrary motions

A study is presented on the unsteady aerodynamic loads due to arbitrary motions of a thin wing and their adaptation for the calculation of response and true stability of aeroelastic modes. In an Appendix, the use of Laplace transform techniques and the generalized Theodorsen function for two-dimensional incompressible flow is reviewed. New applications of the same approach are shown also to yield airloads valid for quite general small motions. Numerical results are given for the two-dimensional supersonic case. Previously proposed approximate methods, starting from simple harmonic unsteady theory, are evaluated by comparison with exact results obtained by the present approach. The Laplace inversion integral is employed to separate the loads into 'rational' and 'nonrational' parts, of which only the former are involved in aeroelastic stability of the wing. Among other suggestions for further work, it is explained how existing aerodynamic computer programs may be adapted in a fairly straightforward fashion to deal with arbitrary transients.

Edwards, J. W.↗
NASA NTRS1975

Inviscid transonic flow computations with shock fitting

First-and second-order numerical procedures are presented for calculating two-dimensional transonic flows that treat shock waves as discontinuities. Their application to a simple but nontrivial problem for which there are limited theoretical results is discussed.

Yu, N. J.↗
NASA NTRS1980

Numerical simulation of flow over wavy walls

Spectral methods are used to solve the Navier-Stokes equations numerically for two-dimensional incompressible flow over wavy surfaces. Results obtained for high Reynolds number laminar flows and turbulent flows are compared to experiment and analytical theory. Overall, good agreement has been achieved.

Balasubramanian, R.↗
NASA NTRS1982

Development of low-frequency kernel-function aerodynamics for comparison with time-dependent finite-difference methods

Finite difference methods for unsteady transonic flow frequency use simplified equations in which certain of the time dependent terms are omitted from the governing equations. Kernel functions are derived for two dimensional subsonic flow, and provide accurate solutions of the linearized potential equation with the same time dependent terms omitted. These solutions make possible a direct evaluation of the finite difference codes for the linear problem. Calculations with two of these low frequency kernel functions verify the accuracy of the LTRAN2 and HYTRAN2 finite difference codes. Comparisons of the low frequency kernel function results with the Possio kernel function solution of the complete linear equations indicate the adequacy of the HYTRAN approximation for frequencies in the range of interest for flutter calculations.

Bland, S. R.↗
NASA NTRS2001

Measurement of Separated Flow Structures Using a Multiple-Camera DPIV System

A novel multiple-camera system for the recording of digital particle image velocimetry (DPIV) images acquired in a two-dimensional separating/reattaching flow is described. The measurements were performed in the NASA Langley Subsonic Basic Research Tunnel as part of an overall series of experiments involving the simultaneous acquisition of dynamic surface pressures and off-body velocities. The DPIV system utilized two frequency-doubled Nd:YAG lasers to generate two coplanar, orthogonally polarized light sheets directed upstream along the horizontal centerline of the test model. A recording system containing two pairs of matched high resolution, 8-bit cameras was used to separate and capture images of illuminated tracer particles embedded in the flow field. Background image subtraction was used to reduce undesirable flare light emanating from the surface of the model, and custom pixel alignment algorithms were employed to provide accurate registration among the various cameras. Spatial cross correlation analysis with median filter validation was used to determine the instantaneous velocity structure in the separating/reattaching flow region illuminated by the laser light sheets. In operation the DPIV system exhibited a good ability to resolve large-scale separated flow structures with acceptable accuracy over the extended field of view of the cameras. The recording system design provided enhanced performance versus traditional DPIV systems by allowing a variety of standard and non-standard cameras to be easily incorporated into the system.

Humphreys, William M., Jr.↗
NASA NTRS1940

Application of the methods of gas dynamics to water flows with free surface I : flows with no energy dissipation

The application is treated in sufficient detail to facilitate as much as possible its application by the engineer who is less familiar with the subject. The present work was undertaken with two objects in view. In the first place, it is considered as a contribution to the water analogy of gas flows, and secondly, a large portion is devoted to the general theory of the two-dimensional supersonic flows.

Preiswerk, Ernst↗