Suction effects on turbulence flows over a dune bed

This experimental study investigates how suction affects turbulence flow over an immobile, two-dimensional triangular dune. A Laser Doppler Anemometer is used to measure the streamwise and vertical components of velocity over one dune length for three different suction rates at 0, 0.29% and 0.45%, respectively. The response of the mean flow and turbulence velocities to bed suction is explored in both a local sense, through comparison of the time-average velocity, rms values of velocity fluctuations and Reynolds shear stress profiles at five selected sections; and a spatial sense, by producing a set of contour plots over the dune length for all mean and turbulence parameters. The measurements reveal that suction causes the flow to adhere to the bed resulting in reduction of the separation length. The streamwise velocity profile becomes more uniform when compared to that without suction while suction always produces smaller or more negative vertical velocities in the near-bed region. The measurements also show that both the near-bed turbulence intensity and Reynolds shear stresses decrease with suction, which confirms that suction causes the flow within the boundary layer to be less turbulent. An interesting observation is that the flow does not respond to suction in the circulation zone just downstream of the dune crest.