On the detection of turbulence-generating events

The structure of turbulence, especially in the near-wall region, was studied with the variable-interval time-averaging (VITA) technique and the uv -quadrant method. Both methods were applied to the same set of data, measured in a fully developed turbulent channel flow, to detect events associated with turbulence production. A close correspondence was found between VITA events and ejection type of events detected with the uv -quadrant method. Conditional averages of the fluctuating component of the streamwise velocity ( u ) and the component normal to the wall ( v ), as well as of the product uv , were constructed with both methods, and the cause for some of the apparent differences was investigated. In contrast to previous findings it was concluded that the uv -pattern obtained with the VITA technique has only one peak, and hence is quite similar to that obtained with the uv -quadrant method. It was shown that large peaks in the uv -signal (with u 0) imply large instantaneous outflow angles. For typical VITA events the outflow angle was often found to exceed 10°. Some events with large uv -peaks did not correspond to any strong activity in the u -component, but could be detected by applying the VITA technique to the v -signal.