On the detection of turbulence-generating events

Abstract

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, v > 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.