The near wake of discrete roughness elements on swept wings

This work presents the first experimental characterization of flow field in vicinity periodically spaced discrete roughness elements (DRE) a swept wing boundary layer. The time-averaged velocity fields are acquired volumetric domain by high-resolution dual-pulse tomographic particle tracking velocimetry. Investigation stationary topology indicates that near-element region is dominated high- and low-speed streaks. layer spectral content inferred spatial fast Fourier transform (FFT) analysis spanwise signal, characterizing chordwise behaviour individual disturbance modes. two signature features transient growth, namely algebraic growth exponential decay, identified evolution energy associated with higher harmonics primary mode. A decay process instead near-wake just aft each DRE, similar to wake relaxation effect previously observed two-dimensional flows. regime found condition onset initial amplitude modal crossflow instabilities. Within critical DRE range (i.e. affecting transition without causing tripping) disturbances strongly receptive spacing diameter elements, which drive distribution within spectra. In super-critical forcing development high-speed fluctuating closely aligned wake. Therefore, elevated shears unsteady affect development. Combined harmonic modes these instabilities initiate laminar streak structure breakdown bypass process.