The effect of slip on the flow over a gap in a channel

The effect of slip applied along a Gaussian shaped gap, located about a fixed position on the
lower wall of a two-dimensional channel, is numerically investigated. Two specific gap shapes have been modelled, whose features are sufficient to generate localised pockets of reversed flow when the channel walls are fully no-slip. The gaps in consideration have the same depth d, but different widths σ. The aspect ratios, defined as η = d/σ, are 0.4 and 0.8, respectively. The wider gap (i.e., η = 0.4), is found to infer a higher intensity of separated flow within the gap concavity. The effect of a Robin-type slip boundary condition is investigated on the regions of reversed flow. The Reynolds number, based on the channel half width, is 4000 for this investigation. It is shown that surface slip (i) reduces the intensity of separation as it diminishes the minimum of the streamwise velocity within the gap region and (ii) reduces the thickness of the separation pocket inside the gap. Ultimately, slip inhibits flow separation in the gaps concavity for a slip length λ equal to 0.15 and 0.1, when η is 0.4 and 0.8, respectively.