Abstract
The effect of a freestream isotropic turbulence on the drag of a particle and its wake structure is studied through direct numerical simulations. We present here six different cases where the particle diameter is varied from 1.5 to 10 times the Kolmogorov scale, and the particle Reynolds number from about 50 to 600. The DNS results show that turbulence has no substantial effect on the time-averaged mean drag which can be reasonably well predicted by standard drag correlations, such as the Schiller-Neumann law. The accuracy of prediction of the instantaneous drag however decreases with increasing particle size. The inclusion of the added-mass force may worsen the prediction, while the Basset history force is observed to be very small. The mean wake in a turbulent flow shows a shortened wakelength compared to the uniform flow result at the same Reynolds number. The instantaneous wake structures for the different cases are studied and the salient features are presented. For the smallest particle, the wake is simply oscillating without any shedding. For the particle of intermediate size, freestream turbulence creates unsteady wake, and Λ-shaped vortices are formed. For the largest particle, the wake is characterized by strong unsteady shedding but unlike in a uniform flow the vortices do not maintain their structural integrity and they quickly dissipate downstream.