Engineering topics

Seddougui, Sharon O.

Publications and source records attributed to Seddougui, Sharon O..

10 records

NASA NTRS1991

The effects of suction on the nonlinear stability of the three-dimensional boundary layer above a rotating disc

There exist two types of stationary instability of the flow over a rotating disc corresponding to the upper branch, inviscid mode and the lower branch mode, which has a triple deck structure, of the neutral stability curve. A theoretical study of the linear problem and an account of the weakly nonlinear properties of the lower branch modes have been undertaken by Hall and MacKerrell respectively. Motivated by recent reports of experimental sightings of the lower branch mode and an examination of the role of suction on the linear stability properties of the flow here, the effects are studied of suction on the nonlinear disturbance described by MacKerrell. The additional analysis required in order to incorporate suction is relatively straightforward and enables the derivation of an amplitude equation which describes the evolution of the mode. For each value of the suction, a threshold value of the disturbance amplitude is obtained; modes of size greater than this threshold grow without limit as they develop away from the point of neutral stability.

Bassom, Andrew P.↗

NASA NTRS1991

The inviscid compressible Goertler problem

The growth rate is studied of Goertler vortices in a compressible flow in the inviscid limit of large Goertler number. Numerical solutions are obtained for 0(1) wavenumbers. The further limits of large Mach number and large wavenumber with 0(1) Mach number are considered. It is shown that two different types of disturbance modes can appear in this problem. The first is a wall layer mode, so named as it has its eigenfunctions trapped in a thin layer away from the wall and termed a trapped layer mode for large wavenumbers and an adjustment layer mode for large Mach numbers, since then this mode has its eigenfunctions concentrated in the temperature adjustment layer. The near crossing of the modes which occurs in each of the limits mentioned is investigated.

Dando, Andrew↗

NASA NTRS1991

Nonlinear Instability of Hypersonic Flow past a Wedge

The nonlinear stability of a compressible flow past a wedge is investigated in the hypersonic limit. The analysis follows the ideas of a weakly nonlinear approach. Interest is focussed on Tollmien-Schlichting waves governed by a triple deck structure and it is found that the attached shock can profoundly affect the stability characteristics of the flow. In particular, it is shown that nonlinearity tends to have a stabilizing influence. The nonlinear evolution of the Tollmien-Schlichting mode is described in a number of asymptotic limits.

Seddougui, Sharon O.↗

NASA NTRS1990

The onset of three-dimensionality and time-dependence in Goertler vortices - Neutrally stable wavy modes

Recently Hall and Seddougui (1989) considered the secondary instability of large-amplitude Goertler vortices in a growing boundary layer into a three-dimensional flow with wavy vortex boundaries. A pair of coupled, linear ordinary differential equations was obtained for this instability which constituted an eigenproblem for the wavelength and frequency of this wavy mode. In the course of investigating the nonlinear version of this problem (Seddougui and Bassom 1990), it has been found that the numerical work of Hall and Seddougui (1989) is incomplete; this deficiency is rectified here. In particular, it is found that many neutrally stable modes are possible; the properties of such modes are derived in a high-wavenumber limit, and it is shown that the combination of the results of Hall and Seddougui and the present modifications lead to conclusions which are consistent with the available experimental observations.

Bassom, Andrew P.↗

NASA NTRS1990

A nonlinear investigation of the stationary modes of instability of the three-dimensional compressible boundary layer due to a rotating disc

The effects of compressibility on a stationary mode of instability of the 3D boundary layer due to a rotating disk are investigated. The aim is to determine whether this mode will be important in the finite amplitude destabilization of the boundary layer. This stationary mode is characterized by the effective velocity profile having zero shear stress at the wall. Triple-deck solutions are presented for an adiabatic wall and an isothermal wall. It is found that this stationary mode is only possible over a finite range of Mach numbers. Asymptotic solutions are obtained which describe the structure of the wavenumber and the orientation of these modes as functions of the local Mach number. The effects of nonlinearity are investigated allowing the finite amplitude growth of a disturbance close to the neutral location to be described.

Seddougui, Sharon O.↗

NASA NTRS1990

Wave interactions in a three-dimensional attachment-line boundary layer

The three-dimensional boundary layer on a swept wing can support different types of hydrodynamic instability. Here attention is focused on the so-called 'spanwise instability' problem which occurs when the attachment-line boundary layer on the leading edge becomes unstable to Tollmien-Schlichting waves. In order to gain insight into the interactions that are important in that problem a simplified basic state is considered. This simplified flow corresponds to the swept attachemnt-line boundary layer on an infinite flat plate. The basic flow here is an exact solution of the Navier-Stokes equations and its stability to two-dimensional waves propagating along the attachment line can be considered exactly at finite Reynolds number. The corresponding problem is studied for oblique waves, and their interaction with two-dimensional waves is investigated. It is shown that there are two types of oblique wave which, if excited, cause the destabilization of the two-dimensional mode and the breakdown of the disturbed flow at a finite distance from the leading edge.

Hall, Philip↗

NASA NTRS1990

Surface-cooling effects on compressible boundary-layer instability

The influence of surface cooling on compressible boundary layer instability is discussed theoretically for both viscous and inviscid modes, at high Reynolds numbers. The cooling enhances the surface heat transfer and shear stress, creating a high heat transfer sublayer. This has the effect of distorting and accentuating the viscous Tollmien-Schlichting modes to such an extent that their spatial growth rates become comparable with, and can even exceed, the growth rates of inviscid modes, including those found previously. This is for moderate cooling, and it applies at any Mach number. In addition, the moderate cooling destabilizes otherwise stable viscous or inviscid modes, in particular triggering outward-traveling waves at the edge of the boundary layer in the supersonic regime. Severe cooling is also discussed as it brings compressible dynamics directly into play within the viscous sublayer. All the new cooled modes found involve the heat transfer sublayer quite actively, and they are often multi-structured in form and may be distinct from those observed in previous computational and experimental investigations. The corresponding nonlinear processes are also pointed out with regard to transition in the cooled compressible boundary layer. Finally, comparisons with Lysenko and Maslov's (1984) experiments on surface cooling are presented.

Seddougui, Sharon O.↗

NASA NTRS1990

On the instability of Goertler vortices to nonlinear travelling waves

Recent theoretical work by Hall and Seddougui (1989) has shown that strongly nonlinear, high wavenumber Goertler vortices developing within a boundary layer flow are susceptible to a secondary instability which takes the form of travelling waves confined to a thin region centered at the outer edge of the vortex. The case is considered in which the secondary mode could be satisfactorily described by a linear stability theory and herein the objective is to extend this investigation of Hall and Seddougui (1989) into the nonlinear regime. It was found that at this stage not only does the secondary mode become nonlinear but it also interacts with itself so as to modify the governing equations for the primary Goertler vortex. In this case then, the vortex and the travelling wave drive each other and, indeed, the whole flow structure is described by an infinite set of coupled, nonlinear differential equations. A Stuart-Watson type of weakly nonlinear analysis of these equations is undertaken and concluded, in particular, that on this basis there exist stable flow configurations in which the travelling mode is of finite amplitude. Implications of the findings for practical situations are discussed and it is shown that the theoretical conclusions drawn here are in good qualitative agreement with available experimental observations.

Seddougui, Sharon O.↗

NASA NTRS1989

A note concerning the onset of three dimensionality and time dependence in Goertler vortices

Recently Hall and Seddougui (1989) considered the secondary instability of large amplitude Goertler vortices in a growing boundary layer evolving into a three-dimensional flow with wavy vortex boundaries. They obtained a pair of coupled, linear ordinary differential equations for this instability which constituted an eigenproblem for the wavelength and frequency of this wavy mode. Investigations into the nonlinear version of this problem by Seddougui and Bassom have revealed several omissions in the numerical work of Hall and Seddougui. These issues are addressed in this note. In particular, it is found that many neutrally stable modes are possible. The properties of such modes are derived in a high wavenumber limit and it is shown that the combination of the results of Hall and Seddougui and the modifications made here lead to conclusions which are consistent with the available experimental observations.

Bassom, Andrew P.↗

NASA NTRS1989

A nonlinear investigation of the stationary modes of instability of the three-dimensional compressible boundary layer due to a rotating disc

The effects of compressibility on a stationary mode of instability of the 3-D boundary layer due to a rotating disc are investigated. The aim is to determine whether this mode will be important in the finite amplitude destabilization of the boundary layer. This stationary mode is characterized by the effective velocity profile having zero shear stress at the wall. Triple-deck solutions are presented for an adiabatic wall and an isothermal wall. It is found that this stationary mode is only possible over a finite range of Mach numbers. Asymptotic solutions are obtained which describe the structure of the wavenumber and the orientation of these modes as functions of the local Mach number. The effects of nonlinearity are investigated allowing the finite amplitude growth of a disturbance close to the neutral location to be described.

Seddougui, Sharon O.↗