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Title: A numerical study of the stability of the swept attachment line boundary layer
Author: Theofilis, Vassilios
Awarding Body: Manchester Metropolitan University
Current Institution: University of Manchester
Date of Award: 1991
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A number of numerical schemes are employed in order to gam insight into the stability of the infinite swept attachment line boundary layer. The basic flow is taken to be of the Hiemenz class, i.e. a two-dimensional stagnation line flow with an added crossflow giving rise to a constant thickness boundary layer along the attachment line. For the perturbation flow quantities the assumption is made that the chordwise velocity components is linearly dependent 011 the chordwise coordinate x in 2-D, while the spanwise and normal velocity components are taken to be independent of x. In all numerical schemes, a second-order-accurate finite-differencing scheme is tlsf'd in the normal to the wall direction, a pseudo-spectral approach is emplo)wl in the other directions; temporally, an implicit Crank-Nicolson scheme is used and the resulting system of equations is solved using an initial-value-problem approach. Extensive use of the efficient Fast Fourier Transform (FFT) algorithm has been made in order to transform the solution betw( > en real and Fourier-transform spaces; the FFT was chosen because of the substantial savings in computing-cost which result from its implementation. The linear two-dimensional results (i.e. results obtained for the perturbation flow quantities when small amplitude two-dimensional perturbation waves are introduced into the basic flow) of previous investigations were accurately reproduced using this initial-value-prohlem approach, thus departing from normal-mode analyses which has invariably been the tool for earlier work. A time-periodic scheme is also employed, producing identical results to those of the time-marching approach in the case of the time-periodic forcings. The two-dimensional work was extended to study non-linear effects in the perturbation (Le. non-small perturbation of the basic flow), although the cost of this study generally proved prohibitively high. However, some (very expensive) nonlinear results obtained suggest that the grid used for the linear calculations (which resulted in very large code sizes) has to be further refined in the spanwise direction in order to account for the growth of the non-linear terms. This result is, as expected, Reynolds number dependent and should be taken into consideration when an investigation of the subcritical regime is undertaken. The small-time behaviour of the flow, i.e. the flow development shortly after small-amplitude perturbations have been introduced into the basic flow, has been studied analytically using the matchf'd asymptot.ics expansions method. The boundary layer is taken to consist of two flow regions, an 'inner' and an 'outer' one. The parameter upon which perturbation qnantities are expanded is in both cases the normal coordinate, taken to be small in the inner region and of order one in the outer region. The stability of the flow is studied at stat.ions off the attachment-line, when three-dimensional perturbations are introduced into the Hiemenz flow; in this case assumptions analogous to those of parallel-flow are used, namely that there exist two chordwise length-scales, a slow one corresponding to the slow boundary layer acceleration and a fast one upon which perturbation quantities are dependent. An important result obtained is that, at a station off the attachment line, the critical for the onset of instability Reynolds numher is an order of magnitude smaller than the corresponding two-dimensional critical Reynolds number at the attachment line. Difficulties were experienced when trying to determine a lower branch of the neutral loop in spanwise wavenumber - Reynolds number space; a lower branch was not found for wavenumbers as low as the accuracy of the numerical method employed, a result suggesting that a lower branch of the neutral curve may not exist.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available