Tomographic particle image velocimetry (TPIV) measurement with six high-resolution charge-coupled device (CCD) cameras is conducted to investigate flow structures over a finite circular cylinder with an aspect ratio of 2 ($h/d=2$). This short wall-mounted cylinder is fully immersed in a thick turbulent boundary layer ($\unicode[STIX]{x1D6FF}/h=1.025$). Focus is placed on the three-dimensional instantaneous vortex structures and their dynamic characteristics in the wake flow fields. Based on the present results, a refined topological model of the mean wake field behind the finite circular cylinder is proposed, where the spatial locations of the typical vortex structures and their interactions are described in more detail. Among the reported typical vortex structures (i.e. the horseshoe, tip, base, trailing and arch vortex), emphasis is laid on discussion of the tip and arch vortex. The instantaneous 3D M-shape arch vortex and an alternating large-scale streamwise vortex are first found in the present experiment, and their developments are also discussed. Therefore, it is suggested that the instantaneous finite-cylinder wake is dominated by the arch vortex system and the large-scale streamwise vortices. Moreover, in the instantaneous volumetric flow fields, both the antisymmetric and the symmetric wake behaviours are observed. With proper orthogonal decomposition (POD) analysis, the dynamic characteristics of the wake field are clarified. Different from the flow around an infinite cylinder without control, the third and fourth POD modes are characterized by low-frequency symmetric shedding. The low-frequency feature shown in the second mode pair is observed and associated with the occurrence of instantaneous symmetric 3D wake behaviour triggered by the low-aspect-ratio effect and the extension of the separated shear layer. The low frequency seems be attributed to the flapping phenomenon, i.e. oscillation of the recirculation in the backward-facing step flow. It is found that the flapping motion has a modulating effect on the occurrence of the antisymmetric shedding vortex and thus the large-scale streamwise vortex.

No abstract

Abstract

We investigate the motions of polydisperse droplets in homogeneous and isotropic turbulence at Reynolds numbers ${Re}_\lambda=200$--$300$. The emphasize is put on the parameter dependences of spatial velocity correlations (SVCs) and relative velocities (RVs) of droplets, which are relevant to particle transport and dispersion in turbulence and have been less studied in experiments. The Kolmogorov-scale Stokes number is ${St}_{p}={10}^{-1}$--${10}^1$, and the settling parameter, i.e., the ratio of particle settling velocity to fluid velocity fluctuations, is ${Sv}_{L}=0.5$--$2.0$. The droplet SVCs are smaller than turbulence for all scales, and decrease with both ${ St_p} $ and $ {Sv}_{L}$. At large scales, the droplet RVs, smaller than those of turbulence due to the inertial filtering effect, also decrease with $ {St}_{p}$ and $ {Sv}_{L}$. At small scales, the path-history effect leads to larger droplet RVs than fluid RVs. Interestingly, we find RVs present a non-monotonic trend with $ {St}_{p}$ and reach a valley at $ {St}_{p}\approx1.0$. It may originate from particle clustering and preferential sweeping effects, which both prevail at $ {St}_{p}\approx1.0$. It is also found that droplet motions are less intermittent than turbulence. This is in contrast to the previous observations by simulations with gravity effect being ignored. The intermittency of droplet RVs decreases with $ {Sv}_{L}$ due to the diminished droplet--turbulence interactions, and it presents opposite trends with $ {St}_{p}$ for small and large scales. Finally, the balance between the effects of path histories and turbulent structures makes the velocity statistics of droplets quasi independent from the scale in the dissipative range.