Shallow flow turbulence: an experimental study
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameMaster of Civil Engineering
A particle tracking velocimetry (PTV) system is used to investigate the turbulent properties at the free surface of shallow shear flows and a shallow vortex street (VS) wake flow. The resolution of the PTV system enables information to be gathered regarding the large-scale turbulent structure of these flows, and also enables analysis to proceed in both the temporal and spatial domains. Statistical tools such as the probability density function (PDF), autocorrelation and power spectral density (PSD) are utilised to characterise the turbulent properties at the flow surface. Two supercritical flows and one subcritical shallow shear flow are analysed. Taylor's frozen turbulence hypothesis is shown to be valid for these flows, and the integral length scales indicate that 2D isotropic structures with scales larger than the flow depth are present at the free surface. Such large-scale structures at the free surface are consistent with observations from dye visualisation experiments and with "spiral eddies" identified by Kumar, et al (1998). The longitudinal extent of near and intermediate wake fields for the shallow VS wake flow is well defined by the integral wake length scale specified by v.Carmer (2005). The near wake region is characterised by high rates of exchange between the mean flow and large-scale 2D coherent structures (2DCS). In the intermediate field, the rate of decay of the turbulent stress components greatly diminishes as the 2DCS are stabilised and dissipated under the action of bed friction. Multiple peaks are observed in the power spectral density of the turbulent fluctuations. The periodic shedding of 2DCS behind the circular cylinder is characterised by an energy peak at a Strouhal number of 0.21, and further energy peaks are observed in the near-wake region. The PSD estimates are consistent with the findings of v.Carmer (2005) in which a -5/3 decay law to high frequencies is observed, and no evidence of an inverse energy cascade is present.