Direct numerical simulations of riblets to constrain the growth of turbulent spots

A spectral direct numerical simulation (DNS) code was used to study the growth and spreading of turbulent spots in a nominally laminar, zero-pressure-gradient boundary layer. In addition to the flat-plate case, the interaction of these spots with riblets was investigated. The flat plate, riblets and initial spot perturbation were simulated via an immersed boundary method, and a 'suction wall' allowed the available channel code to model a boundary layer. In both flat-wall and riblet cases, self-similar arrowhead-shaped spots formed. The 22 variable of Jeong & Hussain (1995) was used to visualize the vortical structures within a spot, and a spot was seen to consist primarily of a multitude of entwined hairpin vortices. The range of scales of the hairpin vortices was found to increase as the spot matures. Ensemble averaging was used to obtain more accurate results for the spot spreading angle, both for the flat-wall case and the riblet case. The spreading angle for the flat-wall spot was 6.3, in reasonably good agreement with prior DNS work. The spreading angle for the spot over riblets was 5.4 degrees, a decrease of 14 % compared with the flat-wall.