Experimental investigation of the spatial distribution of uniform momentum zones in wall-bounded flow

Experiments are carried out in a water tunnel to measure the velocity field in the streamwise-wall-normal plane by particle image velocimetry with a large field of view, in order to investigate the spatial distribution of the uniform momentum zones in the turbulent boundary layer. Through calculating the probability density function of the measured streamwise velocity, the temporal and spatial distribution of the uniform momentum zones are achieved. Then the lasting distance and the appearing frequency are analyzed for the uniform momentum zones with different numbers. For different uniform momentum zone numbers, there are apparent differences for both the corresponding lasting distance and appearing frequency. When the number of uniform momentum zones is close to their mean value, the uniform momentum zones can last for long distance along the streamwise direction and have small streamwise separation, appearing more frequently. On the contrary, when the number of uniform momentum zones is far from the mean value, the corresponding uniform momentum zones last for smaller distance along the streamwise direction and have a larger streamwise separation, appearing less frequently.