Near-wall physics of a shear-driven three-dimensional turbulent boundary layer with varying crossflow

Abstract

The near-wall physics and turbulence structure of a planar shear-driven three-dimensional turbulent boundary layer (3DTBL) with varying strengths of crossflow are examined in a specialized facility. Spanwise shear modifies the near-wall turbulence structure. Flow visualization reveals a reduction of mean streak length by as much as 50%, while streak spanwise spacing remains constant. Power spectra of velocity confirm this shift towards higher temporal frequencies, corresponding to decreased streamwise length scales. Particle image velocimetry (PIV) measurements indicate that the spanwise shear increases the number and strength of flow structures that interact in the inner region of the boundary layer. This leads to increased momentum transfer between low- and high-speed fluids, resulting in a thickening of the inner region of the boundary layer. In addition, the organized two-dimensional boundary layer spanwise vorticity layer along the wall surface is disrupted and forms strong vortical structures that are lifted off the wall surface and diffuse into the boundary layer with streamwise distance. The transverse vorticity experiences a similar alteration. Streamwise velocity profiles exhibit an increasing velocity deficit with increased crossflow, consistent with the thickening of the inner region of the boundary layer. Increases in the normal Reynolds stresses are associated with interaction of the secondary flow structures and modifications to vortical structures with increasing three-dimensionality and higher relative wall speed. Significant increases are observed for the most highly sheared cases with translating wall velocities 2.0 and 2.75 times the free-stream velocity. This increase in the normal stresses leads to an increase in the turbulent kinetic energy over the belt surface, which diffuses into the inner region of the boundary layer with streamwise distance. The Reynolds shear stresses also exhibit a significant increase in magnitude over the belt surface owing to enhanced turbulence production over the translating wall section. In general, three-dimensional effects are confined to the area close to the wall and the crossflow increases the interaction of secondary flow structures leading to modifications of the vorticity in the inner region of the boundary layer. These flow distortions disrupt the near-wall streak structure and result in increased levels of turbulent kinetic energy and Reynolds stresses, particularly over the translating wall section.