Mechanical Engineering - Theses
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ItemCharacteristics of energetic motions in turbulent boundary layers( 2019)In this dissertation, we present the first measurements of two-dimensional (2-D) energy spectra of the streamwise velocity component (u) in high Reynolds number turbulent boundary layers. The measurements in the logarithmic region of turbulent boundary layers give new evidence supporting the self-similarity arguments that are based on Townsend’s (1976) attached eddy hypothesis. The 2-D spectrum is found to be able to isolate the range of self-similar scales from the broadband turbulence, which is not possible with the measurement of a 1-D energy spectrum alone. High Reynolds number flows are characterized by large separation of scales. Therefore, to obtain converged 2-D statistics while resolving the broad spectrum of length and time scales, a novel experimental technique is required. To this end, we devise a technique employing multiple hot-wire probes to measure the 2-D energy spectra of u. Taylor’s frozen turbulence hypothesis is used to convert temporal-spanwise information into a 2-D spatial spectrum which shows the contribution of streamwise (λx) and spanwise (λy) length scales to the streamwise variance at a given wall height (z). The validation of the measurement technique is performed at low Reynolds number by comparing against the direct numerical simulation (DNS) data of Sillero et al. (2014). Based on these comparisons, a correction is introduced to account for the spatial resolution associated with the initial separation of the hot-wires. The proposed measurement technique is used to measure the 2-D spectra in the logarithmic region for friction Reynolds numbers ranging from 2400 to 26000. At low Reynolds numbers, the shape of the 2-D spectra at a constant energy level shows λy/z ∼ (λx/z)1/2 behaviour at large scales, which is in agreement with the existing literature. However, at high Reynolds numbers, it is observed that the square-root relationship tends towards a linear relationship (λy ∼ λx) as required for self-similarity and predicted by the attached eddy hypothesis. Finally, we present a model for the logarithmic region of turbulent boundary layers, which is based on the attached eddy framework and driven by the scaling of experimental 2-D spectra of u. The conventional attached eddy model (AEM), which comprises self-similar wall-attached eddies (Type A) alone, represent the large scale motions at high Reynolds numbers reasonably well. However, the scales that are not represented by the conventional AEM are observed to carry a significant proportion of the total kinetic energy. Therefore, in the present study we propose an extended AEM, where in addition to Type A eddies, we also incorporate Type CA and Type SS eddies. These represent the self-similar but wall-detached low-Reynolds number features and the non-self-similar wall-attached superstructures, respectively. The extended AEM is observed to predict a greater range of energetic length scales and capture the low- and high-Reynolds number scaling trends in the 2-D spectra of all three velocity components.
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ItemLow-Reynolds-number turbulent boundary layers( 1988-12)This thesis documents an extensive experimental investigation into low-Reynolds-number turbulent boundary layers flowing over a smooth flat surface in nominally zero pressure gradients. The way in which these layers are affected by variations in R(theta), i.e. the Reynolds number based on the boundary-layer momentum thickness, type of tripping device used and variations in freestream velocity, each considered independently, are investigated.
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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.