Mechanical Engineering - Theses
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ItemSome aspects of turbulent boundary layers( 1969)A detailed experimental programme of two dimensional rough wall turbulent boundary layers developing in zero and arbitrary adverse pressure gradients is used to investigate aspects of turbulent boundary layer development and surface roughness. It is shown that two types of roughness can be clearly distinguished on the basis of the flow variables involved. The more common ‘k’ or ‘sand grain’ type follows the well documented ‘Nikuradse-Clauser’ correlation scheme where the effect of roughness on the flow depends on the size or scale of the roughness elements. The second type of roughness, typified by a smooth wall containing a pattern of narrow cavities, is independent of the scale of the roughness and does not follow• the ‘Nikuradse-Clauser’ correlation scheme. It is shown that previous pipe flow experiments have involved this second type of roughness and these results are used to show that the dependent variable is pipe diameter. This roughness has therefore been named ‘d’ type in this thesis. No length scale associated with the boundary layer could be found to replace pipe diameter except for zero pressure gradient layers. However, it is found that the distance below the crests of the roughness from where the logarithmic distribution of velocity is measured will correlate both types of roughness action. It is shown that a zero pressure gradient turbulent boundary layer developing on a ‘d’ type rough wall conforms to Rotta's condition of precise self preserving flow. The results are used to illustrate several theoretical consequences of this type of flow. Wall shear stresses are determined by measuring in detail, the pressures on the faces of the roughness elements and thereby calculating their form drag. Similarity laws for these pressure patterns are developed for the ‘d’ type results and explicit expressions for the functions are proposed for the zero pressure gradient case. Pressure patterns around 'k' type roughness elements cannot be described by the similarity laws developed here. Theories proposed by several authors to describe the velocity profiles in regions above the logarithmic distribution are compared in detail and critically examined. Some new work related to these theories is introduced. The predictions of mean velocity distribution are tested against an extensive range of experimental data including the results of this thesis. It is shown that all the theories have important shortcomings in their present form and a recommendation for a basis for future work is offered. The problem of the transition of a turbulent boundary layer from a rough (‘d' type) to smooth wall in an adverse pressure gradient is investigated experimentally for two boundary layers. It is found that the outer regions of the boundary layer appear to be unaffected by this change in wall condition whereas the inner flow makes a rapid adjustment to it. This result is at variance to the published work on flow in conduits and for zero pressure gradient boundary layers. An explanation of this is offered. Literature surveys introduce the work in each topic.