Simulation For The Thermal Effect Of Micro Sensor And PIV Experimental Investigation Of Intermittency In A Turbulent Flow

It has been widely accepted that the coherent structures are very important structures in shear turbulence. The active control of coherent structures needs applying micro-transdusers and understanding statistical properties of turbulence. This thesis is composed by two parts: The numerical simulation for the thermal effect of MEMS wall shear stress sensors and PIV experimental investigation of multi-scale coherent structures'intermittency in turbulent flow.(1) numerical simulation for the thermal effect of MEMS wall shear stress sensorsThe wall shear stress is an essential quantity to compute and measure in a wall-bounded turbulent flow, and the development of the MEMS has provided a powerful tool on the measure of wall shear stress and the active control of turbulence.The thermal effects of MEMS wall shear stress sensors have been numerically simulated and analyzed with three-dimensional model by using the Computational Fluid Dynamics(CFD) commercial software FLUENT. The simulating results demonstrate that it is very necessary to add a vacuum cavity or an air cavity beneath the hot-film of MEMS wall shear stress sensors. The result of calculation in water shows that the difference between the effects of the vacuum cavity and the air cavity is very small in the temperature distribution and the heat transfer rate to the water, and the cavity can signally reduce the heat lost irretrievably to the substrate which is very necessary to improve the sensitivity of wall shear stress sensors. In additional, the geometry of sensors also affects the heat transfer rate of wall shear stress sensors and this conclusion provides a certain basis on design of the wall shear stress sensors. Moreover, preliminary design and process craft of wall shear stress sensors have been introduced.(2) PIV experimental investigation of multi-scale coherent structures'intermittency in turbulent flowThe statistical properties of multi-scale coherent structures are studied experimentally through PIV measurements in streamwise-wall-normal planes of the turbulent boundary layers. The probability density function(PDF) of wavelet coefficients at different vertical locations and scale are calculated by a continuous wavelet transform, and the results indicate a more intermittent process in the buffer region. Furthermore, the extended self-similarity(ESS) is applied to compute the scaling exponents of the wavelet coefficients structure function by using the method with a quantitative analysis of intermittency. In this way, after extracting multi-scale coherent structures, intermittency anomalies are no longer observed and the scaling exponents accord with the Kolmogorov linear scaling prediction, which demonstrate that multi-scale coherent structures are responsible for the anomalous scaling law in turbulent boundary layer. Moreover, local energy is showed using wavelet coefficients in streamwise-wall-normal planes, which provides a method to identification coherent structure from wavelet analysis of PIV data.