Experimental Investigation Of Decay And Evolution For Artificial Periodic Disturbance In Turbulent Boundary Layer

An experiment is conducted to study development and evolution of artificial periodic disturbance introduced in a turbulent boundary layer with different frequencies by hot-wires anemometry in a wind tunnel. The propagation and decay characteristics of artificial periodic blow-suction disturbance along longitudinal direction in a turbulent boundary layer are investigated. An artificial periodic local blowing/suction forcing with different frequency is produced by a woof and is introduced into the downstream turbulent boundary layer from a 5mm thin latitudinal slot on the flat plate by wall turbulence.Time sequence signals of longitudinal velocity component and normal velocity component at different longitudinal and normal positions in turbulent boundary layer have been finely measured before/after introducing periodic disturbance by IFA300 constant temperature anemometer and double-sensor hot-wire probe with sampling resolution higher than the frequency that corresponds to the smallest time scale of turbulence. The period-phase-average technique is applied to extract the period-phase-average waveforms of periodic disturbance wave from instantaneous time sequence signals of turbulence velocity. The decay and distortion of artificial periodic disturbance wave along longitudinal direction and normal direction in a turbulent boundary layer is researched. The longitudinal and normal disturbing velocity component amplitude of periodic disturbance wave is measured at different longitudinal and normal positions in turbulent boundary layer. From the point of view of hydrodynamic instability theory, the magnitude damp rate of periodic disturbance wave is calculated along longitudinal direction and normal direction in a turbulent boundary layer .The experimental results are compared with the theoretical and numerical results.From the experiment investigation, it is found that the periodic disturbance wave introduced in the near wall region can cause responding wave in the outer region. The responding wave amplitude in the outer region is much larger than that of the disturbing wave in the near wall region. The coherent structure burst in the near wall region is related receptively with the flow state in the outer region. The occurrence of burst in the near wall region is tied up with the flow state in the outer region. The turbulence is a stable state and there is not global instability in turbulent boundary layer. The global evolution tendency of the disturbing wave with all frequencies is generally damping along the longitudinal direction while the decaying tendency becomes more and more weak. But the disturbing wave amplitude does not always decrease monotonously in the downstream direction, although in some cases there is a local momentary increase at particular vertical positions. The decay of 16Hz and 32Hz disturbance is weak in the near wall region near the disturbance source while damp tendency is more obvious in the outer region beyond logarithm-law layer, although the decaying tendency becomes more and more slow. The disturbance that corresponding to the natural frequency in turbulent boundary layer plays a dominate role in the interaction between the disturbance and turbulent boundary layer and decays most rapidly in turbulent boundary layer. Compared with M. Landahl's wave-guide theoretical model and calculated results, the measured damping rateαi of the disturbing wave amplitude is in agreement in tendency with the theoretical and numerical results.