Engineering topics

Orszag, S. A.

Publications and source records attributed to Orszag, S. A..

At least 37 records · Page 2

NASA NTRS1981

Studies of compressible shear flows and turbulent drag reduction

Compressible shear flows and drag reduction were examined and three methods are addressed: (1) the analytical and numerical aspects of conformal mapping were summarized and a new method for computation of these maps is presented; (2) the computer code SPECFD for solution of the three dimensional time dependent Navier-Stokes equations for compressible flow on the CYBER 203 computer is described; (3) results of two equation turbulence modeling of turbulent flow over wavy walls are presented. A modified Jones-Launder model is used in two dimensional spectral code for flow in general wavy geometries.

Orszag, S. A.↗

NASA NTRS1981

Advanced stability analysis for laminar flow control

Five classes of problems are addressed: (1) the extension of the SALLY stability analysis code to the full eighth order compressible stability equations for three dimensional boundary layer; (2) a comparison of methods for prediction of transition using SALLY for incompressible flows; (3) a study of instability and transition in rotating disk flows in which the effects of Coriolis forces and streamline curvature are included; (4) a new linear three dimensional instability mechanism that predicts Reynolds numbers for transition to turbulence in planar shear flows in good agreement with experiment; and (5) a study of the stability of finite amplitude disturbances in axisymmetric pipe flow showing the stability of this flow to all nonlinear axisymmetric disturbances.

Orszag, S. A.↗

NASA NTRS1981

Three-dimensional instability of plane channel flows at subcritical Reynolds numbers

A three-dimensional instability that predicts subcritical transition in plane channel flows in good agreement with experiment is discussed. Both full simulations of the Navier-Stokes equations and a three-dimensional linear analysis about two-dimensional finite amplitude states confirm that two-dimensional secondary flows are strongly unstable to small three-dimensional perturbations.

Orszag, S. A.↗

NASA NTRS1981

Subcritical transition to turbulence in planar shear flows

The two-dimensional steady and time dependent properties of plane Poiseuille and plane Couette flows are analyzed using iterative techniques and full numerical simulation of the Navier-Stokes equations. It is shown that the finite-amplitude two-dimensional states investigated are strongly unstable to very small three-dimensional perturbations. It is also shown, through full numerical simulation, that this explosive secondary instability can explain the subcritical transitions that occur in real flows. Finally, it is shown that the three-dimensional instability can be analyzed by a linear stability analysis of a two-dimensional flow consisting of the basic parallel flow and a steady (or quasi-steady) finite-amplitude two-dimensional cellular motion.

Orszag, S. A.↗

NASA NTRS1980

Subcritical transition to turbulence in plane channel flows

A linear three dimensional mechanism for the transition of plane Poiseuille flows to turbulence is presented which provides good agreement with experimental observations. The mechanism is based on the evolution of states within a band of quasi-equilibria which slowly approach the stable upper branch solutions for the evolution of flow energy but which are strongly unstable to infinitesimal three-dimensional disturbances. Numerical simulation has shown that if two-dimensional flow persists long enough for the three-dimensional perturbations to attain finite amplitude, the resulting three dimensional flow quickly develops a turbulent character with nonperiodic behavior, and thus transition can be predicted from knowledge of the initial two- and three-dimensional energies and time scales. The mechanism predicts transition to turbulence at Reynolds numbers greater than 1000, as observed in experiments, and implies higher threshold three-dimensional energies in plane Couette flow.

Orszag, S. A.↗

NASA NTRS1980

Disturbance amplification rates

Program computes incompressible linear stability characteristics for swept and tapered wings. Amplification rates of boundary-layer disturbances also are calculated. Program is useful in designing tapered, laminar-flow control wings incorporating suction to prevent boundary layer separation. Program is written in FORTRAN IV and Assembler for batch execution on CYBER 70-series.

Srokowski, A. J.↗

NASA NTRS1980

Drag reduction effects in turbulent boundary layers over wavy walls

Two dimensional incompressible flow over wavy surfaces are analyzed numerically by spectral methods. Algorithms for periodic flows (Fourier modes in the periodic flow direction and Chebycheff modes in the normal direction), and inflow-outflow boundary conditions (Chebycheff modes used in both directions) are described. Results obtained using both codes are reported for laminar flows. Comparisons with known theoretical and experimental results are made.

Balasubramanian, R.↗

NASA NTRS1980

Spectral methods for problems in complex geometries

The properties of spectral methods are surveyed and their extension to solve problems in complex geometries is developed. A new iteration procedure is introduced to solve efficiently the full matrix equations resulting from spectral approximations to nonconstant coefficient boundary-value problems in complex geometries. It is shown that the work required to solve these spectral equations exceeds that of solving the lowest-order finite-difference approximation to the same problem by only O(N log N).

Orszag, S. A.↗

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 NTRS1980

Comparison of methods for prediction of transition by stability analysis

The paper compares several methods of transition prediction of linear stability analysis. The spectral stability analysis code SALLY is used to analyze flows over laminar flow control wings. It is shown that transition prediction by the envelope method and a new modified wave packet method are comparable in reliability, but that the envelope method is more efficient computationally. This is based on the results which show that the wave packet method provides N factors which are at best as consistent as those of the envelope method; since the wave packet method is at least 3 times as expensive as the envelope method, the latter is recommended for engineering design calculations.

Malik, M. R.↗

NASA NTRS1980

Stability analysis for laminar flow control, part 2

Topics covered include: (1) optimization of the numerics of the SALLY stability analysis code; (2) relation between temporal and spatial stability theory; (3) compressible flow stability calculations; (4) spectral methods for the boundary layer equations; and (5) numerical study of nonlinear, nonparallel stability of incompressible flows.

Orszag, S. A.↗

NASA NTRS1980

Numerical simulation of shear flows over wavy boundaries

The full time-dependent two dimensional Navier-Stokes equations are solved using spectral methods to achieve high spatial accuracy and high order time-splitting methods together with conformal mapping methods to allow simulation of flow over steep waves, including general turbulent Reynolds stresses. The programs are used to study the properties of laminar and turbulent flow over a flat plate and in various wavy geometries. Results are compared with both available theory and experiment. For laminar flow, good agreement is achieved. For turbulent flow, the agreement is less satisfactory and may be explained by the simplified turbulent modelling use in the calculations.

Orszag, S. A.↗

NASA NTRS1980

Advanced stability theory analyses for laminar flow control

Recent developments of the SALLY computer code for stability analysis of laminar flow control wings are summarized. Extensions of SALLY to study three dimensional compressible flows, nonparallel and nonlinear effects are discussed.

Orszag, S. A.↗

NASA NTRS1979

Prediction of compliant wall drag reduction, part 2

A numerical model of turbulent boundary layer flows over compliant walls was investigated. The model is based on Burton's observation that outer flow structures in turbulent boundary layers produce large scale pressure fluctuations near the wall. The results of calculations indicate that certain small wavelength wall motions can have a significant effect upon the stability of turbulent boundary layers.

Orszag, S. A.↗

NASA NTRS1979

Spectral methods for problems in complex geometries

Techniques that permit the efficient application of spectral methods to solve problems in nearly arbitrary geometries are presented. These methods were found to be viable alternatives to finite difference and finite element processes. The spectral methods applied are extensions of the standard techniques of separation of variables to the solution of arbitrarily complicated problems.

Orszag, S. A.↗

NASA NTRS1977

Prediction of compliant wall drag reduction, part 1

Computer codes developed to test Bushnell's compliant wall drag reduction model are discussed. One code computes the evolution of mean velocity profiles during the period between bursts as forced by an imposed large-scale pressure pulse due to earlier bursts. Another code computes the local stability characteristics of these computed profiles. The programs use Chebyshev polynomials to resolve the normal boundary layer (y) direction and a staggered grid of mesh points to resolve the x direction. Typically, 257 grid points and 33 Chebyshev polynomials are used in the computations.

Orszag, S. A.↗

NASA NTRS1977

Stability analysis for laminar flow control, part 1

The basic equations for the stability analysis of flow over three dimensional swept wings are developed and numerical methods for their solution are surveyed. The equations for nonlinear stability analysis of three dimensional disturbances in compressible, three dimensional, nonparallel flows are given. Efficient and accurate numerical methods for the solution of the equations of stability theory were surveyed and analyzed.

Benney, D. J.↗

NASA NTRS1977

Mass flow requirements for LFC wing design

The problem of determining optimum suction mass flow requirements for LFC wings is addressed. Some previous methods for predicting the extent of laminar flow over swept wings with suction are briefly reviewed. These range from the purely empirical to those utilizing tabulated linear stability computations. The present method is described. This method solves the linear, incompressible stability equations by spectral techniques. The maximum temporal amplification of boundary layer crossflow and 2-D disturbances is determined for waves of a given frequency. Group velocities are used to integrate these amplification rates along the wing to yield the logarithmic amplitude ratio or 'N factor' of the disturbance. The 'N factor' calibration of a computer code utilizing this method is described, using experimentally determined transition data. The method is shown to be as consistent as previously used 'fixed wavelength' methods.

Srokowski, A. J.↗