Experimental investigation of turbulent boundary layers over transversal moving surfaces

Abstract

The influence of a spanwise traveling transversal surface wave on the near-wall flow field of turbulent boundary layers is investigated by particle-image velocimetry (PIV) and micro-particle tracking velocimetry (μ-PTV). The experimental setup consists of a flat plate equipped with an insert to generate a transversal spanwise traveling wave of an aluminum surface. PIV and μ-PTV measurements are conducted for three Reynolds numbers based on the freestream velocity and momentum thickness immediately downstream of the actuated surface Re θ = 1200, 1660, and 2080. The transversal traveling wave is generated by a newly developed electromagnetic actuator system underneath the aluminum surface. Three amplitudes A = 0.25, 0.30, and 0.375 mm at a wave length of $$\lambda \, = \,160\,{\text{mm}}$$ and a frequency of f = 81 Hz are investigated. The detailed analysis of the velocity profile shows the transversal traveling surface motion to redistribute the velocity in the viscous sublayer and in the logarithmic region of the turbulent boundary layer. The streamwise and wall-normal velocity fluctuations in the outer boundary layer are increased and the streamwise momentum in the near-wall regime is lowered. The drag reduction ratio (DR) due to the actuation is determined by the velocity gradient in the viscous sublayer. At the lowest Reynolds number the drag-reducing impact is proportional to the amplitude of the wave. That is, the higher the amplitude, the more pronounced the drag reduction resulting in a friction drag reduction up to 3.4 % compared to the non-actuated configuration.