Turbulence Modulated by a Coherent Shear Wave in a Wall Boundary Layer

Summary

The turbulence in a wall boundary layer is viewed as a dynamic system which is composed of various elements (coherent structures) which act together and constitute a dynamic equilibrium. The general aim of our investigations is to study this system by introducing small external perturbations and observing the resulting time-dependent departure from the equilibrium. The coherent shear wave used as perturbation throughout our experiments may either be excited by a sound wave propagating in a duct carrying turbulent flow, or may be directly excited by longitudinal oscillations of the wall. The results obtained by different measurement methods indicate that the shear wave causes the turbulence to deviate temporally from its state of dynamic equilibrium. So a modulation of the Reynolds’ shear stress is observed. Our latest experiments carried out in a recirculating oil-tunnel yield that the modulation of the normal stress (-ρu′u′), which has been measured by phase avering, results from a modulation of the probability of the occurrence of the so-called bursts. There are good arguments that the modulation of the shear stress (-ρu′v′) originates from this effect, too.