Structure-dependent drag in gas-solid flows studied with direct numerical simulation

Quantification of drag F is critical to the simulation of gas-solid flows in both discrete particle models and two-fluid models. It is commonly accepted that for homogeneous flow the drag is a function of solid volume fraction ϕ and particle Reynolds number Rep (based on the mean slip velocity and particle radius). However, its adequacy for heterogeneous flows encountered more frequently is in debate yet. In this work, we reveal the strong structural dependence of the drag in both a simple case of two particles and a typical case with stepwise heterogeneity, demonstrating the necessity for a structure-dependent drag description. To quantify such dependence, flow past idealized static suspensions with linear heterogeneity is studied first, which confirms a general form F(Rep,ϕ,|∇ϕ|,θ) suggested previously, where θ is the angle between the gradient ∇ϕ and the mean slip velocity. In the studied range of 5