Measuring the wall shear stress in wall-bounded turbulence
thesisposted on 29.11.2016, 04:24 by Amili, Omid
In wall-bounded turbulent flows, determination of the wall shear stress is an important task. A primary objective of the present research is to develop a sensor which is capable of measuring the surface shear stress over an extended region applicable to turbulent flows. The sensor, as a direct method for measuring the wall shear stress, enables us to conduct measurements in a relatively large domain with high spatial resolution in a non-intrusive way. The sensor consists of mounting a thin flexible film on the solid surface and is made of a homogeneous, isotropic, and incompressible material. The geometry and mechanical properties of the film are measured, and particles are embedded on the filmâ€™s surface to act as markers. An optical technique is used to measure the film deformation caused by the flow field. For this purpose, the film is imaged at the flow-off and flow-on states and then multi-grid cross correlation digital particle image velocimetry is used to measure the tangential deformation with high sub-pixel accuracy. The film has typically deflection of less than 2% of the material thickness under maximum loading. The sensor sensitivity can be adjusted by changing the thickness of the layer or the shear modulus of the filmâ€™s material. The thesis provides detailed information for the experimental methodology developed in the Laboratory for Turbulence Research in Aerospace and Combustion (LTRAC) at Monash University. It presents the sensor concept, governing equations, sensor fabrication, static calibration, precise dynamic response evaluation, followed by the accuracy assessment, and a description of capabilities and limitations. The static calibration confirms that fabricated films are linear elastic solid. The assessment of the dynamic response of the sensor shows low pass filter behaviour for most of the created films. Measurements of the wall shear stress have been performed in a fully developed turbulent channel flow with Reynolds numbers in the range of 2,100-2,900 based on the friction velocity and the half channel height. The results are compared with alternative wall shear stress measurement methods and also with the wall shear stress data available in the literature. For comparison of the mean wall shear stress and skin friction, the logarithmic law of the mean streamwise velocity profile obtained from the PIV experiments has been utilised. In addition, oil-film interferometry as a reliable and accurate technique to measure the local mean wall shear stress has been employed. The comparisons represent favourable agreement between the mentioned measuring approaches. In addition, a recent DNS turbulent channel flow at the Reynolds number of 2,300 has been used for comparison of the wall shear stress statistics and flow topological analysis. For dynamic wall shear stress measurements, a film with the shear modulus of 80 Pa and 2 mm in thickness has been used. The spatial resolution is 9-12 wall units and the measurement domain is approximately 1200Ã—1500 l+ for the highest Reynolds number case. The distribution of the fluctuating wall shear stress reveals the co-occurrence of low- and high-shear regions aligned in the streamwise direction. It indicates the imprint of existing streaky structures in the near-wall region. The conditionally averaged field of low-shear stress regions exhibits the counter rotating vortex pattern elongated in the streamwise direction. It suggests the signature of long quasi-streamwise vortices or stretched legs of hairpins as the dominant structures existing in the immediate vicinity of the wall. In brief, the thesis presents detailed information on further progress of an in-house film-based shear stress method and its first application to a wall bounded turbulent flow to measure the instantaneous wall shear stress distribution.