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Title: Boundary layer transition
Author: Hall, David John
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 1968
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The work presented here is concerned with the prediction of naturally occurring transition under the influence of pressure gradients and free stream turbulence, and with the effects of two and three dimensional tripping devices. Measurements have been made in zero, favourable and adverse pressure gradients. A number of transition prediction methods have been published that attempt to account for the effects of stream turbulence and pressure gradients, but they do not, in general, give similar results under particular conditions. They have been compared with each other and with some of the available experimental results, including some collected here, in an attempt to check on their limitations. In flows with low stream turbulence, it is apparently necessary to take account of the history of the laminar boundary layer's development up to transition. The method of Granville appears to be the most successful. In flows with high stream turbulence there is insufficient experimental information to indicate the best way 01 predicting transition, particularly in favourable pressure gradients. It is possible that a method using purely local criteria, rather than one assessing the boundary layer's history, is acceptable. The present measurements of the characteristics of two dimensional tripping devices (wires) have been concerned largely with measuring drag coefficients and the size of the separation bubble behind the wire. The drag coefficient of the wire varies continuously with changes in only one of the two governing non-dimensional variables. Its value is generally lower than has been assumed previously. The separation bubble can be very large and varies rapidly in size with changes in the two governing non-dimensional variables. Pressure gradients have little effect upon drag coefficients but some effect upon separation bubble sizes. Consideration has also been given to the problem of critical sizes for trip wires. The previous suggestion of a constant value of Ul:/v of 826 for I transition at the wire I, in zero pressure gradient appears to be valid only at high Reynolds numbers, the value decreasing as the Reynolds number falls. Further information has also been obtained on the effect of pressure gradients upon critical wire sizes. Measurements of the characteristics of three dimensional tripping devices (spheres) have given the critical sizes for 'transition at the element', and the movement of transition behind the element. The start of transition moves instantaneously to the element once its critical size is reached, but the end of transition approaches the element at a finite rate and progressively more gradually, until it stops a finite distance from the element. Critical element sizes for movement of the start and end of transition to the element are different. Pressure gradients affect both critical element sizes and the rate of approach of transition to the element.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available