Near-wall Motions Research Articles

Particle image velocimetry investigation of a turbulent boundary layer manipulated by spanwise wall oscillations

Particle image velocimetry has been applied to the study of a canonical turbulent boundary layer and to a turbulent boundary layer forced by transversal wall oscillations. This work is part of the research programme at the Politecnico di Torino aerodynamic laboratory with the objective of investigating the response of near-wall turbulence to external perturbations. Results are presented for the optimum oscillation period of 100 viscous time units and for an oscillation amplitude of 320 viscous units. As expected, turbulent velocity fluctuations are considerably reduced by the wall oscillations. Particle image velocimetry has allowed comparisons between the canonical and forced flows in an attempt to find the physical mechanisms by which the wall oscillation influences the near-wall organized motions.

Laser Doppler anemometer measurements of turbulent boundary layer over a riblet surface

Laser Doppler anemometer measurements in a turbulent boundary layer downstream of a change from a smooth wall to a V-grooved riblet surface provide some insight into the way the near-wall turbulence structure adjusts to the new boundary condition. Depending on the riblet dimensions, this adjustment results either in drag reduction or drag increase. In the former case (s+ 25). Although near-wall secondary motions occur in both cases, the motions are much weaker when the riblets act to reduce the drag. Nomenclature bjj = Reynolds stress anisotropy tensor C = additive constant in logarithmic law d = average diameter of the quasi stream wise vortices h = riblet height R9 = Reynolds number based on U and 9 s = riblet spacing