Tow-tank investigation of the developing zero-pressure-gradient turbulent boundary layer

This study details experiments conducted using image-base d measurement techniques to visualise and analyse the develo pment of a zero-pressure-gradient turbulent boundary layer from the trip to a high Reynolds number state ( Reτ ≈ 6000). The spatially developing boundary layer is formed on a flat plate tow ed through a water tank at speeds of up to 1 m/s. Time-resolved images of the boundary layer demonstrate the streamwise evo lution of large-scale structures that originate from the in ner region and extend to the edge of the outer region. Also, instantaneous scalar concentration fields from planar laser induc ed fluorescence(LIF) in streamwise-wall-normal planes revea l the growth of the developing turbulent boundary layer by tracki ng the wall-normal interface of the scalar. Introduction Flow visualisation is an important tool in fluid mechanics th at has been used in numerous studies of boundary layer structur es in wall-turbulence [5, 9, 10]. These have mostly provided qu alitative visual access to the outer edge of the boundary layer , permitting a better understanding of large scale motions in the outer layer. Head & Bandyopadhyay [9] visually demonstrate d that the turbulent boundary layer consists of large-scale h irpin structures which are inclined with a characteristic inc lination angle of 45 ∼ 50 to the surface. More recently, studies have suggested that individual large scale vortices tend to form into groups and align in the streamwise direction. It has als o been shown that smaller packets, which initially originate d from the inner region move outward to the outer region [2, 6]. This result is consistent with the study of Head & Bandyopadhyay [9]. Throughout the studies, results have revealed that the coherent large-scale structures have a significant contribut ion to Reynolds shear stress and to the turbulence production with in the turbulent boundary layer [1]. In the past, towing tank facilities have been utilise to inve stigate flow structures around a testing model. Mohamed et al. [12] showed turbulent spots evolving in a laminar boundary layer formed on a towed flat plate. They used a flow visualisation technique to investigate the growth and entrainment of turb ulent spots while Guiet al. [8] used a moving PIV system to investigate the nominal-wake plane of a model-scale ship by measuring the mean velocity and Reynolds stresses. Colella et al. [3] used an array of eight wall shear stress sensors to measure velocity and wall shear stress fluctuations in a turbule nt boundary layer developed over a towed flat plate. In the present study, a 5.0 m long nominally flat plate towed through water and past a stationary imaging system has been used to measure the developing turbulent boundary layer formed on the plate. The aim of the study is to provide visual access to a developing turbulent boundary layer from th e trip to a high Reynolds number state through the unique frame of-reference. Flow visualisation results reveal the growt h rate of the developing boundary layer and provide some insight in to the boundary layer interface. Figure 1: Image of flat plate attached on the carriage