High-resolution large field-of-view experimental investigation of turbulent convection velocities in a turbulent boundary layer

Turbulent convection velocities in a turbulent boundary layer at Req = 2250 are examined via the use of a high repetition rate particle image velocimetry measurement undertaken in a water tunnel. Multiple cameras are used to improve the spatial dynamic range of the measurement and reduce the bias towards large-scale structures while simultaneously capturing a wall-normal domain of 0.06d to 1.7d. The impact of measurement noise is minimized via careful temporal and spatial filtering of the velocity fields as guided by the comparison of temporal and spatial velocity power spectra with spatially filtered direct numerical simulation data, enabling an estimation of the effective noise-limited spatial and temporal dynamic range of the present experimental measurement. Space-time correlations and phasespectra are used to estimate the mean and streamwise wavenumber dependent convection velocities at various heights above the wall. Results reveal convection velocities greater than the local mean velocity in the lower log layer, decreasing to a level 3.5% lower than the mean velocity in the upper log and wake regions, with smaller convection velocities observed at smaller scales.