Time-resolved tomographic particle image velocimetry (PIV) measurements (acquisition rate 250 Hz) were performed in a turbulent boundary layer on the side wall of an open channel, water flow facility (cross section 60 × 60 cm2, W × H), 3.5m downstream of the inlet at a bulk flow velocity of Ub = 0.17m/s (Reb = UbH/ν = 97, 679, δ0.99 = 45.0 mm, Reθ = 752). The measurement volume was a horizontal slab (60 × 15 × 60mm3, l × w × h) extending from the wall, 30 cm above the bottom. The setup comprised four high-speed ImagerPro HS cameras (2016×2016 pixels), a high-speed laser (Nd:YLF, Darwin Duo 80M, Quantronix), optics/prisms, and data acquisition/processing software (LaVision, DaVis 8.2). Data were acquired with and without a stationary held sphere that had a diameter, D = 6mm (D+ = 51, “+” denotes inner wall scaling), and was positioned at x3 = 5.4 and 37.6mm ( = 43 and 306) from the wall (measured from the sphere’s center). Sphere Reynolds numbers based on D and the average streamwise velocity at the sphere’s center were 692 and 959, respectively. The mean streamwise velocity profiles of the undisturbed boundary layer clearly exhibit a canonical shape. Introducing the sphere strongly affected log layer and buffer layer mean velocity and Reynolds stress profiles. Recovery to the undisturbed boundary layer characteristics is faster with the sphere positioned closest to the wall. When positioned at h+ = 306, near-wall, uplifted, coherent vortical structures extend from the wall up to the sphere’s wake with which they interact.@en

In many applications, finite-sized particles are immersed in a turbulent boundary layer (TBL) and it is of interest to study wall effects on the instantaneous shedding of turbulence structures and associated mean velocity and Reynolds stress distributions. Here, 3D flow field dynamics in the wake of a prototypical, small sphere (D+=50, 692<ReD<959) placed in the TBL's outer, logarithmic, and buffer layer, were measured using time-resolved tomo-PIV. Increasing wall proximity increasingly tilted the mean recirculating wake away from the wall implying a negative lift force. Mean velocity deficit recovery scaled with the mean wake length with minor effects of wall proximity. Farthest from the wall, streamwise Reynolds normal stresses encircled the mean wake as an axisymmetric tubular "shell," while transverse and wall-normal stresses extended off its tip as axisymmetric tapered cones. Wall proximity removed axisymmetry and attenuated values near the wall. Reynolds shear stresses were distributed as antisymmetric lobes extending off the mean wake displaying increasing values with reducing sphere-wall gap. Instantaneous snapshots revealed a wake densely populated by "archlike" vortices with shedding frequencies lower than for a sphere in uniform flow except in the buffer layer. Tilting of the wake away from the wall resulted from self-induced motion of shed hairpinlike vortices whose symmetry plane was increasingly wall-normal oriented with reduced sphere-wall gap.

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