Behaviour of Energetic Coherent Structures in Turbulent Pipe Flow at High Reynolds Number

This thesis presents a new applied temporal-spatial analysis using time-resolved PIV measurements to investigate coherent structures' co-exist towards pipe axis in fully developed turbulent pipe flow at high Reynolds numbers. The new method facilitates the detection of coherent structures directly through the new streamwise fluctuating velocity field plots. The detection was done without involving the common usable Taylor hypothesis compared to the recognised spectral analysis. For streamwise (u) and wall-normal (v) velocity components, this methodology identifies the shear layer signature created by the encounters of bursting events caused due to the intensive presence of coherent structures and their interaction in the near-wall region. Nevertheless, spectra and co-spectra, as well as proper orthogonal decomposition (POD) analyses, are also applied in this study to identify the contribution of both large (LSMs) and very-large-scale motions (VLSMs) to turbulent kinetic energy and Reynolds shear stress. The spectral analysis utilises high-speed PIV measurements to revisit other earlier studies done via a single hot-wire anemometer. In the present study, hot-wire measurements were conducted at a very high Reynolds number range exceeding one million equivalent to Re_tau= 19 000 with high spatial resolution. The obtained results indicate the dominance of VLSMs in the logarithmic and outer layers. However, the outputs are deduced from their higher contribution to total energy production in streamwise and wall-normal directions at high Reynolds numbers. Furthermore, new observation explored from the temporal-spatial plots stated that VLSMs could be created from the alignment of LSMs.