Local wall pressure gradient events in turbulent wall-bounded flow

The wall pressure field from a direct numerical simulation of turbulent channel flow was analyzed to detect significant wall pressure gradient events. Both favorable and adverse pressure gradient events were detected using conditional sampling. The recovered events were ensemble averaged to provide a space and time record of a typical event. Both favorable and adverse pressure gradients occur in conjunction with sharp minima and maxima in the wall pressure. The events extend about 100 wall units in the streamwise direction and are about 50 wall units wide in the spanwise direction. Both types of events begin as a traveling wave packet. An adjacent minimum and maximum in the wave packet grow as the wave packet advects with the sharp gradient between them becoming the detected sharp pressure gradient. The growth and decay of a sharp pressure gradient event is similar to the growth and decay of a composite event made up of a negative and a positive wall pressure peak event. At the instant of detection a characteristic wall pressure signature can be related to the lifting up of a horseshoe vortex and the shear layer of the burst cycle.