This thesis is an experimental study of turbulence, covering three topics. Firstly, a carbon nanotube (CNT) sensor has been developed to measure the mean and fluctuating wall shear stress (WSS) in a macroscopic turbulent boundary layer (TBL). The CNT WSS sensor is based on the thermal-principle and featured by a high spatial and temporal resolution, a low power consumption, and a compact fabrication process. It has been observed for the first time in a macroscopic flow that the sensor output power is approximately proportional to the 1/3-powered WSS. Secondly, the active control of a TBL has been experimentally investigated with a view to reduce the skin-friction drag. A piezo-ceramic (PZT) actuator array, aligned spanwise and flush-mounted with the wall surface, was employed to generate wall-normal oscillations and, given a phase shift between two adjacent actuators, a transverse travelling wave along the wall. A number of control parameters were examined, including the wavelength, oscillation amplitude and frequency. Local skin-friction drag exhibits a strong dependence on the control parameters. A maximum skin-friction drag reduction of 50% has been achieved at 17 wall units downstream of the actuator tip, given the wavelength, oscillation amplitude and frequency at 416, 1.94 and 0.39, respectively, all in wall units. The near-wall flow structures with and without perturbation were measured extensively and compared with each other. All the results point to a pronounced change in the coherent structures in the boundary layer under control. Thirdly, the turbulent wake of two staggered square cylinders has been studied at Re = 300 ~ 1.3 x 104. The configurations examined cover a cylinder-center spacing of 1.5 ~ 5.0 d, where d is the cylinder width, and an incident angle of 0° ~ 90°. Four distinct flow regimes were identified based on the Strouhal number maps and the downstream evolution of the flow structures. Initial conditions, i.e. interactions between the four shear layers around the two cylinders, are connected with different flow regimes and are discussed in detail. Time-averaged and fluctuating drag and lift forces acting on the two square cylinders were measured and discussed for all the configurations.
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