Large-scale eddies measured with large scale particle image velocimetry

Large-scale eddies are turbulent features of open channels that advect along the direction of the mean flow in a streamwise rolling motion and comprise the entire flow depth. In order to advance the understanding of large-scale eddies, it is increasingly important to make measurements on a large scale test section or in a river reach. The objective of this research was to measure characteristics of large-scale eddies using large scale particle image velocimetry (LSPIV) for the first time. A gravel-bed flume was chosen in order that the LSPIV technology could be applied for an environment where the eddies were known to exist. LSPIV was used to make free-surface velocity measurements in the flume for quasi-steady, uniform flow. After removing erroneous LSPIV measurements due to light reflections and inadequate seed concentrations, turbulence decomposition via the triple decomposition theorem was used to isolate the large-scale eddies from the small-scale bursting derived eddies. Visual inspection of the decomposed data, coupled with quantitative analyses of the statistical moments of the decomposed velocity and evaluation of the velocity spectrum, were used to objectively choose a decomposition time-step. The decomposed data was visualized and characteristics of the large-scale eddies were measured. LSPIV data showed that the eddy length of the large-scale eddies was approximately 4 H, agreeing with past research. The large-scale eddies were not fully organized within the flow. The eddies propagated downstream as long-lived stable structures throughout the test section. Future research of the large-scale eddies is needed under a wide range of conditions, and LSPIV is expected to help serve this need in both large scale laboratory investigations and in the field under a range of bed conditions and flow conditions.