Understanding The Effect Of Turbulent Structures On Three-Dimensional Sphere Motions In Boundary Layers

Abstract

This paper investigates the effect of turbulent activity on the wall-normal motion of spheres with specific gravities of 1.006 (P1) and 1.152 (P3) a t Re_x = 670 and 1300. These spheres extend into the logarithmic region with d+ = 56 and 116. Both sphere and fluid motions were tracked simultaneously using separate techniques; 3D particle tracking was used to track the individual spheres over a streamwise distance of 5 boundary layer thicknesses while stereoscopic particle image velocimetry was implemented to track the fluid motion surrounding the spheres over streamwise-spanwise planes at multiple streamwise and wallnormal locations. Upon release, sphere P1 accelerated strongly and lifted off of the wall due to strong mean shear before descending back towards the wall at both Re_x. Then, the sphere either ascended again without returning to the wall or else contacted the wall and slid before lifting off again. This sphere did not develop any significant rotations throughout its trajectory. The subsequent lift-offs observed, which were of similar or larger magnitude to the initial lift-offs, were prompted by fluid upwash and/or temporary increases in shear l ift due to passing high momentum zones. While ejection events were found to be important to sphere lift-offs, we did not observe any distinct fluid structures or sweep motions associated with sphere descents. These descents were likely dominated by gravity after the positive lift on the sphere decreased following its detachment from the wall. The upward impulse from Q2 type events was limited, and once the sphere moved away from the wall, the upward shearing lift was insufficient to keep the sphere suspended. The denser sphere P3, by contrast, did not lift off upon release and initially slid along the wall while lagging the fluid significantly. After it had propagated approximately 1.5𝛿 downstream, it began rolling forward (while slipping) and accelerated again. The forward rotation induced sufficient Magnus lift to generate small lift-off events of magnitude ≤ 0.2d repeating at relatively high frequency independent of the larger turbulence structures around the sphere.