MEAN FLOW AND TURBULENCE FIELDS OVER 2-DIMENSIONAL BED FORMS

Detailed laser-Doppler velocity and Reynolds stress measurements over fixed two-dimensional bed forms are used to investigate the coupling b etween the mean flow and turbulence and to examine effects that play a role in producing the bed form instability and finite amplitude stabi lity. The coupling between the mean flow and the turbulence is explore d in both a spatially averaged sense, by determining the structure of spatially averaged velocity and Reynolds stress profiles, and a local sense, through computation of eddy viscosities and length scales. The measurements show that there is significant interaction between the in ternal boundary layer and the overlying wake turbulence produced by se paration at the bed form crest. The interaction produces relatively lo w correlation coefficients in the internal boundary layer, which sugge sts that using local bottom stress to predict bed load flux may not on ly be erroneous, it may also disregard the essence of the bed form ins tability mechanism. The measurements also indicate that topographicall y induced acceleration over the bed form stoss slope has a more signif icant effect in damping the turbulence over bed forms than was previou sly supposed, which is hypothesized to play a role in the stabilizatio n of fully developed bed forms.