Considerations for Modeling Particle Entrainment into the Wake of a Circular Cylinder

Abstract

The objective of this work is to evaluate the performance of the steady state Reynolds Averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) models for estimating concentration of low Stokes number aerosols (Stk = O(10 −4 )) in the wake of a bluff body. These simulations are compared with experimental data. In the simulations and experiments, particles are released upstream of the body and convected downstream, where some are entrained into the wake. The air velocity is computed using a steady state renormalized group k ∼ ε model. Lagrangian particle trajectory simulations are performed in conjunction with each airflow model to calculate concentrations. The experiments are performed in an aerosol wind tunnel in which phase Doppler velocimetry measurements are obtained the velocity field and for aerosol concentration.The RANS model yields a wake concentration deficit that extends downstream past x/D = 10, while the experiments produce elevated concentrations immediately downstream of the near wake. It is postulated that the concentration peak is at least in part attributed to particle interaction with the boundary layer by the following mechanism. Particles are transported into the boundary layer by turbulent diffusion, turbophoresis, and/or inertial forces. Particles then separate from the cylinder with the airflow and travel in a sheath around the periphery of the near wake to converge at the downstream edge of the near wake. Underestimation of the wake concentration by the RANS model is potentially due to inadequacy in the boundary layer approximation used in the model.

Numerical Simulation of Particle Dispersion in the Wake of a Circular Cylinder

“…However, they did not provide the relationship between particle motion and flow field so the mechanism of particle dispersion in the wake has not been fully discovered. Richmond-Bryant et al (2006) estimated concentration of particles with low St in the wake of circular cylinder using the steady state Reynolds Averaged Navier-Stokes (RANS) renormalized k ∼ ε model. They found the mechanism of particle interaction with the boundary layer.…”

Vortex dynamics and scalar transport in the wake of a bluff body driven through a steady recirculating flow

“…Numerical simulations have been used as a primary, but limited, tool of investigation, due to the challenges of large scales involved, and combination of three-dimensional transient flow features and flow excitations emanating from moving boundaries . The recent availability of high-performance computing resources has allowed using the LES technique over RANS for such flows, because RANS models have difficulties in predicting separation behind bluff bodies (Lakehal and Rodi 1997;Lübcke et al 2001;Richmond-Bryant et al 2006). However, RANS models are still predominant among human exposure studies in the indoor air community as they are readily available in commercial CFD packages and computationally inexpensive.…”

Probability of noise-induced separatrix crossing for inertial particles in flows

“…This is why recirculation cells are, in practice, separated from the rest of the flow by a stochastic zone rather than a well-defined dividing streamline, even in quasi-steady flows. These uncertainties are observed also for any finite-size particle that would be carried by the flow, like aerosols entrained in the wake of a cylinder [1], micron-sized particles in the context of microfluidic [2], Brownian particles settling in the vicinity of Stommel cells [3,4], or pollutants in the wake of large obstacles [5] where turbulence is often treated as a random force, to quote but a few examples. In general, to predict the proportion of particles which cross the separatrix, one has to perform a large number of simulations involving a large number of particles, to obtain reliable statistical data.…”