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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

An accurate and efficient method for evaluating the kernel of the integral equation relating pressure to normalwash in unsteady potential flow

This paper describes an accurate economical method for generating approximations to the kernel of the integral equation relating unsteady pressure to normalwash in nonplanar flow. The method is capable of generating approximations of arbitrary accuracy. It is based on approximating the algebraic part of the non elementary integrals in the kernel by exponential approximations and then integrating termwise. The exponent spacing in the approximation is a geometric sequence. The coefficients and exponent multiplier of the exponential approximation are computed by least squares so the method is completely automated. Exponential approximates generated in this manner are two orders of magnitude more accurate than the exponential approximation that is currently most often used for this purpose. Coefficients for 8, 12, 24, and 72 term approximations are tabulated in the report. Also, since the method is automated, it can be used to generate approximations to attain any desired trade-off between accuracy and computing cost.

Desmarais, R. N.↗
NASA NTRS1982

Numerical solution of three-dimensional unsteady transonic flow over wings including inviscid/viscous interactions

A method is presented for computing the unsteady three-dimensional transonic flowfield about twin wings of arbitrary planform and section geometry including the effects of inviscid/viscous interaction. Dominant effects of the shock-boundary-layer interaction are accounted for by a simple empirically defined model, while viscous regions adjacent to the wing surface and along the trailing wake are described by a set of integral equations appropriate for two-dimensional turbulent flows. The validity of the proposed method is verified by comparison with steady three-dimensional experimental data and a solution of more exact equations for a typical transport wing configuration.

Rizzetta, D. P.↗
NASA NTRS1992

Unsteady aerodynamic interaction effects on turbomachinery blade life and performance

This paper is an attempt to address the impact of a class of unsteady flows on the life and performance of turbomachinery blading. These class of flows to be investigated are those whose characteristic frequency is an integral multiple of rotor shaft speed. Analysis of data recorded downstream of a compressor and turbine rotor will reveal that this class of flows can be highly three-dimensional and may lead to the generation of secondary flows within downstream blading. By explicitly accounting for these unsteady flows in the design of turbomachinery blading for multistage applications, it may be possible to bring about gains in performance and blade life.

Adamczyk, John J.↗
NASA NTRS1985

Status and prospects of computational fluid dynamics for unsteady transonic flow

Applications of computational aerodynamics to aeronautical research, design, and analysis have increased rapidly over the past decade, and these applications offer significant benefits to aeroelasticians. The past developments are traced by means of a number of specific examples, and the trends are projected over the next several years. The crucial factors that limit the present capabilities for unsteady analyses are identified; they include computer speed and memory, algorithm and solution methods, grid generation, turbulence modeling, vortex modeling, data processing, and coupling of the aerodynamic and structural dynamic analyses. The prospects for overcoming these limitations are presented, and many improvements appear to be readily attainable. If so, a complete and reliable numerical simulation of the unsteady, transonic viscous flow around a realistic fighter aircraft configuration could become possible within the next decade. The possibilities of using artificial intelligence concepts to hasten the achievement of this goal are also discussed.

Mccroskey, W. J.↗
NASA NTRS1966

Distributed Parameter Analysis of Pressure and Flow Disturbances in Rocket Propellant Feed Systems

A digital distributed parameter model for computing the dynamic response of propellant feed systems is formulated. The analytical approach used is an application of the wave-plan method of analyzing unsteady flow. Nonlinear effects are included. The model takes into account locally high compliances at the pump inlet and at the injector dome region. Examples of the calculated transient and steady-state periodic responses of a simple hypothetical propellant feed system to several types of disturbances are presented. Included are flow disturbances originating from longitudinal structural motion, gimbaling, throttling, and combustion-chamber coupling. The analytical method can be employed for analyzing developmental hardware and offers a flexible tool for the calculation of unsteady flow in these systems.

UNSTEADY FLOW↗
NASA NTRS1977

Computation of unsteady transonic flows by the indicial method

The indicial method is investigated for the computation of unsteady transonic force and moment coefficients for use in flutter analyses. This approach has the advantage that solutions for all reduced frequencies for a given mode of motion can be obtained from a single finite-difference flowfield computation. Comparisons of indicial and time-integration computations for oscillating airfoil and flap motions help define limits on the motion amplitude for the applicability of the indicial method to transonic flows. Within these limits, solutions for various motion modes can be superposed to obtain solutions for multiple-degree-of-freedom aeroelastic systems. Also, a simple aeroelastic problem is solved by an alternative approach in which the structural motion and flowfield equations are integrated simultaneously using a time-integration finite-difference procedure.

Ballhaus, W. F.↗
NASA NTRS2002

Verification Assessment of Flow Boundary Conditions for CFD Analysis of Supersonic Inlet Flows

Boundary conditions for subsonic inflow, bleed, and subsonic outflow as implemented into the WIND CFD code are assessed with respect to verification for steady and unsteady flows associated with supersonic inlets. Verification procedures include grid convergence studies and comparisons to analytical data. The objective is to examine errors, limitations, capabilities, and behavior of the boundary conditions. Computational studies were performed on configurations derived from a "parameterized" supersonic inlet. These include steady supersonic flows with normal and oblique shocks, steady subsonic flow in a diffuser, and unsteady flow with the propagation and reflection of an acoustic disturbance.

Slater, John W.↗
NASA NTRS1975

A theoretical evaluation of rigid baffles in suppression of combustion instability

An analytical technique for the prediction of the effects of rigid baffles on the stability of liquid propellant combustors is presented. This analysis employs both two and three dimensional combustor models characterized by concentrated combustion sources at the chamber injector and a constant Mach number nozzle. An eigenfunction-matching method is used to solve the linearized partial differential equations describing the unsteady flow field for both models. Boundary layer corrections to this unsteady flow are used in a mechanical energy dissipation model to evaluate viscous and turbulence effects within the flow. An integral stability relationship is then employed to predict the decay rate of the oscillations. Results of this analysis agree qualitatively with experimental observations and show that sufficient dissipation exists to indicate that the proper mechanism of baffle damping is a fluid dynamic loss.

Combustion instability↗
NASA NTRS1976

Theoretical evaluation of rigid baffles in the suppression of combustion instability

An analytical technique for the prediction of the effects of rigid baffles on the stability of liquid propellant combustors is presented. This analysis employs both two and three dimensional combustor models characterized by concentrated combustion sources at the chamber injector and a constant Mach number nozzle. An eigenfunction-matching method is used to solve the linearized partial differential equations describing the unsteady flow field for both models. Boundary layer corrections to this unsteady flow are in a mechanical energy dissipation model to evaluate viscous and turbulence effects within the flow. An integral instability relationship is then employed to predict the decay rate of the oscillations. Results of this analysis agree qualitatively with experimental observations and show that sufficient dissipation exists to indicate that the proper mechanism of baffle damping is a fluid dynamic loss. The response of the dissipation model to varying baffle blade length, mean flow Mach number, oscillation amplitude, baffle configuration, and oscillation mode is examined.

Combustion instability↗
NASA NTRS1992

Unsteady airfoil flow solutions on moving zonal grids

Euler and Navier-Stokes solutions for airfoil flows on zonal grids are presented. The governing equations are solved with an implicit, iterative, factorized numerical scheme. The inviscid fluxes are examined with a third-order accurate upwind method. Zonal grid solutions are compared with experimental measurements for flows over airfoils at fixed angles of incidence. The computed unsteady solutions for rapidly pitching and oscillating airfoils are in good agreement with experiments.

Cricelli, Antonio S.↗
NASA NTRS2007

Synthetic Jets in Quiescent Air

An oscillatory jet with zero net mass flow is generated by a cavity-pumping actuator. Among the three test cases selected for the Langley CFD validation workshop to assess the current CFD capabilities to predict unsteady flow fields, this basic oscillating jet flow field is the least complex and is selected as the first test case. Increasing in complexity, two more cases studied include jet in cross flow boundary layer and unsteady flow induced by suction and oscillatory blowing with separation control geometries. In this experiment, velocity measurements from three different techniques, hot-wire anemometry, Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV), documented the synthetic jet flow field. To provide boundary conditions for computations, the experiment also monitored the actuator operating parameters including diaphragm displacement, internal cavity pressure, and internal cavity temperature.

Yao, C. S.↗
NASA NTRS1986

Unsteady transonic flow over cascade blades

An attempt is made to develop an efficient staggered cascade blade unsteady aerodynamics model for the neighborhood of March 1, representing the blade row by a rectilinear two-dimensional cascade of thin, flat plate airfoils. The equations of motion are derived on the basis of linearized transonic small perturbation theory, and an analytical solution is obtained by means of the Wiener-Hopf procedure. Making use of the transonic similarity law, the results obtained are compared with those of other linearized cascade analyses. A parametric study is conducted to find the effects of reduced frequency, stagger angle, solidity, and the location of the pitching axis on cascade stability.

Surampudi, S. P.↗
NASA NTRS1990

Numerical simulation of the incompressible internal flow through a tilting disk valve

A numerical simulation of the incompressible viscous flow through a prosthetic tilting disk heart valve is presented in order to demonstrate the current capability to model unsteady flows with moving boundaries. Both steady and unsteady flow calculations are performed by solving the incompressible Navier-Stokes equations in three-dimensional generalized curvilinear coordinates. In order to handle the moving boundary problems, the chimera grid embedding scheme which decomposes a complex computational domain into several simple subdomains is used. An algebraic turbulence model for internal flows is incorporated to reach the physiological values of Reynolds number. Good agreement is obtained between the numerical results and experimental measurements. It is found that the tilting disk valve causes large regions of separated flow, and regions of high shear.

Chang, I-Dee↗
NASA NTRS1993

A viscous-inviscid interaction method for 2-D unsteady, compressible flows

A Navier-Stokes/potentia/flow interactive solution method suitable for the solution of steady-state and unsteady flowfields around airfoils has been developed. The Navier-Stokes equations are solved in the close proximity of an airfoil and in its wake. The inviscid flowfield surrounding the viscous flow regions is assumed to be irrotational and isentropic. In the inviscid flow region, the potential flow equations, which are based on distributed sources and vortices, are solved. The two solutions are strongly coupled through the application of their boundary conditions. The strongly coupled Navier-Stokes/potential flow interactive solution method confines computations to a small domain in the proximity of an airfoil. For steady flow solutions, the computational domain may be confined to a region which extends less than one-fifth of a chord length distance away from the airfoil surface. Computed solutions have the same accuracy as the full domain NavierStokes solutions. Yet, as a result of the significantly reduced computational domain and increased convergence rates, the Navier-Stokes/potential flow interactive solution method is about 40 percent more efficient computationally.

Tuncer, Ismail H.↗
NASA NTRS1978

Estimating maximum instantaneous distortion from inlet total pressure rms measurements

In the present paper, a new mathematical model of inlet turbulence is developed by application of basic fluid dynamics and statistical concepts. The model provides an understanding of the turbulent inlet flow as well as a means of describing the flow in quantitative terms. Specifically, the maximum instantaneous distortion produced by inlet unsteady flow can be estimated by the simple measurement of rms data. Practical application of these techniques leads to a data/acquisition/reduction system that is at least one, and maybe two, orders of magnitude less expensive than conventional methods. Each data point can be reduced in terms of the mean strength of the turbulent vortices. By storing these two parameters (that are representative of the unsteady flow with the steady state information), the maximal instantaneous distortion can be reconstructed for other distortion factors at any time subsequent to the test.

Melick, H. C., Jr.↗
NASA NTRS1994

Thermohydrodynamic analysis of cryogenic liquid turbulent flow fluid film bearings, phase 2

The Phase 2 (1994) Annual Progress Report presents two major report sections describing the thermal analysis of tilting- and flexure-pad hybrid bearings, and the unsteady flow and transient response of a point mass rotor supported on fluid film bearings. A literature review on the subject of two-phase flow in fluid film bearings and part of the proposed work for 1995 are also included. The programs delivered at the end of 1994 are named hydroflext and hydrotran. Both codes are fully compatible with the hydrosealt (1993) program. The new programs retain the same calculating options of hydrosealt plus the added bearing geometries, and unsteady flow and transient forced response. Refer to the hydroflext & hydrotran User's Manual and Tutorial for basic information on the analysis and instructions to run the programs. The Examples Handbook contains the test bearing cases along with comparisons with experimental data or published analytical values. The following major tasks were completed in 1994 (Phase 2): (1) extension of the thermohydrodynamic analysis and development of computer program hydroflext to model various bearing geometries, namely, tilting-pad hydrodynamic journal bearings, flexure-pad cylindrical bearings (hydrostatic and hydrodynamic), and cylindrical pad bearings with a simple elastic matrix (ideal foil bearings); (2) improved thermal model including radial heat transfer through the bearing stator; (3) calculation of the unsteady bulk-flow field in fluid film bearings and the transient response of a point mass rotor supported on bearings; and (4) a literature review on the subject of two-phase flows and homogeneous-mixture flows in thin-film geometries.

Sanandres, Luis↗
NASA NTRS1985

Finite difference methods for the solution of unsteady potential flows

A brief review is presented of various problems which are confronted in the development of an unsteady finite difference potential code. This review is conducted mainly in the context of what is done for a typical small disturbance and full potential methods. The issues discussed include choice of equation, linearization and conservation, differencing schemes, and algorithm development. A number of applications including unsteady three-dimensional rotor calculation, are demonstrated.

Caradonna, F. X.↗