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Title: The effects of curvature and divergence on turbulent mixing layers
Author: Johnson, A. E.
ISNI:       0000 0001 3590 9706
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 1990
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Single-point measurements were made in three single-stream axisymmetric mixing layer flows: 1) undistorted. 2) subjected to simultaneous strong divergence and strong stabilizing curvature. 3) subjected to simultaneous moderate divergence and moderate stabilizing curvature, followed by a region of destabilizing curvature and divergence. Curvature and divergence were imposed by means of deflecting surfaces in the irrotational region of the flow. The contraction boundary layer, found to be contaminated with Taylor- Gortler vortices, was bled off and a new one formed which was turbulent at separation. The data in 1) were found to be broadly in agreement with other workers, but approach to an asymptotic state is clearly non-monotonic contrary to popular belief. No evidence could be found to support Hussain & Zedan's (1978) suggestion that the higher order moments take longer (than moments of a lower order) to reach self-preservation. In the stabilised region of case 2) the large scale structures were suppressed but not destroyed. The intensifying effect of the divergence is such that it initially serves to resist the suppression of the large scale structures, whilst later in the flow it produces a more rapid recovery than was observed for curvature alone (Castro & Bradshaw (1976)). The unwanted effect of the deflecting surfaces on the turbulent structure became significant towards the end of the mixing layer as the curvature approached zero. In the early part of case 3) the effects of the divergence and stabilizing curvature were seen to approximately "cancel" each other, in contradiction of Smits & Joubert (1982), where the divergence was found, surprisingly to reinforce the curvature's stabilizing effect. The Reynolds stresses and triple products appear to vary qualitatively as the lagged strains, although quantitatively the destabilizing effects are unlikely to have been absent in the lagged region of curvature. Investigation of a purely divergent layer (and also a plane layer) was initiated, but measurements were not satisfactorily completed. Some plane layer results are, however, presented to enhance the discussion of the undistorted axisymmetric layer.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Navier-Stokes equations