Simultaneous measurements of wall shear, pressure, and 3D flow in high-Reynolds-number turbulent boundary layers

Time and spatially resolved, simultaneous measurements of wall shear stresses, 3D flow and pressure distributions in high-Reynolds-number turbulent boundary layers remain a major challenge. This study combines a suite of instruments aimed at achieving this goal. The 3D flow is measured using high-speed (15-25 kHz) tomographic particle tracking velocimetry (TPTV), and the pressure distribution is calculated by spatial integration of the material acceleration. The associated optical setup is integrated with a microscopic Mach–Zehnder Interferometry (MZI) system that measures the deformation of a series of surface-mounted, Nanoscale Interferometric Sensors (NIS). Each sensor consists of a 0.2-1 mm diameter, flush-mounted, viscoelastic insert made of PDMS+gel, with a ~100 nm-high transparent bump made of SiO2 deposited on its top surface. As the soft inserts, which have a Young Modulus varying between 25 to 100 kPa, deform under shear, the MZI system measures the displacement of the bump. In addition, wall-normal compression of the insert can be used for determining the local pressure. The ongoing experiments are performed in a refractive-index-matched water tunnel at friction Reynolds numbers (Re_τ) in the 2,000 to 6,000 range. The NIS displacements are calibrated vs. the mean wall shear stress determined from the velocity gradients in the viscous sublayer and fits to the log layer velocity profiles. The wall-normal compression of the NISs are compared to the TPTV based, 3D pressure distributions.