The flow over V-groove and U-groove riblets is examined computationall
y using a time dependent model of the viscous wall region. The how at
the upper bound of the computational domain (y(+)similar or equal to 4
0) is described using a streamwise eddy model. Both drag neutral and d
rag reducing riblets are considered. Results suggest that the riblets
limit the lateral spread of inrushes towards the wall and retain low m
omentum fluid in the riblet valleys, thereby effectively isolating muc
h of the wall from such inrushes. The generation of intermittent secon
dary vortices within the riblet valleys also occurs; however, these ap
pear to be both quite weak and fairly short-lived.