Time-resolved PIV measurement of a developing zero pressure gradient turbulent boundary layer

Time-resolved two-dimensional particle image velocimetr y (TRPIV) is employed to measure the instantaneous velocity field s in an evolving zero-pressure-gradient (ZPG) turbulent bou ndary layer. The spatially developing boundary layer is formed on a flat plate which is towed past a stationary time-resolved PIV sys tem such that each pass yields streamwise/wall-normal planes o f data with Reynolds numbers in the range 500 . Reτ . 2500. This provides a unique view of a spatially and temporally evolvin g turbulent boundary layer from the tripwire up to high Reynol ds numbers. In this frame of reference an evolving large-scale structure, with a convection velocity close to the freestream, ap pe rs nominally stationary within the field of view. This system is effectively equivalent to moving an entire PIV system with t he flow, chasing the large-scale structures as they advect down stream in a wind tunnel or a water channel facility [8]. Compl ete instantaneous velocity vector fields of such a developing tu rbulent boundary layer provide clearer insight into the mechan isms associated with large-scale coherent structures in the out er region of turbulent boundary layers. The mean flow parameters of the experimental data are assessed to evaluate the quality of th e turbulent boundary layers formed on the towed-plate. These res ults indicate that the developing turbulent boundary layer in th is study can be considered ‘ canonical’ with a nominally zero-pressuregradient. An analysis of instantaneous convection velocit ies associated with lowand high-speed structures reveals diffe rences in the trajectory and local convection velocity of these fea tures as the turbulent boundary layer develops along the flat plate . Facility and Experimental Setup Experiments are performed in the tow tank facility located i n the Michell hydrodynamics laboratory at the University of M elbourne. The tow tank has dimensions of 60 × 2× 2 m (length× width× height) and is filled with water. The tow tank facility has a computer controlled traversing carriage that can tow mode ls at various speeds of up to 2.5 m/s. A 5.0 m long and 1.2 m wide flat plate is suspended horizontally beneath the traversing car riage by four aerofoil shaped supports. The height of the supports ca n be individually adjusted so that the pressure gradient along t he plate can be controlled. The bottom side of the plate is positioned 10 mm below the water surface. The test surface of the flat plate consists of black anodised aluminium sheets. The leading ed ge of the plate consists of an 8:1 ratio semi-elliptical nose to prevent a flow separation. A schematic of the tow tank facility and the flat plate setup are shown in figure 2. A 1.0 mm diameter trip wire is located immediately downstream of the leading edge t o stimulate the boundary layers formed on the bottom surface o f the plate. The Reynolds number of the trip based on the diamet er of the wire isRed = dU∞/ν ≈ 870 (whereU∞ is plate velocity,d is trip wire diameter andν is kinematic viscosity) at a given plate velocity of approximately 1.0 m/s. This Red is comparable with recommendations from previous studies [7]. The carriage is fully automated such that multiple passes can be performed to obta in converged statistics of the developing turbulent boundary layers. Figure 1 shows variation of the background turbulence inten si y as a function of time between each pass of the towed plate. The result reveals that at least 8 ∼ 10 minutes are required to allow the time (minutes) √ u 2 /U ∞ (% ) 0 5 10 15 20 0 0.2 0.4 0.6 0.8 1 Figure 1. Variation of background turbulence intensity in t ime between measurements. The broken line indicates the time required b etween measurements