Numerical Experiments on Downstream Boundary of Flow Past Cylinder

Abstract

The influence of the location of the downstream boundary on unsteady incompressible flow solutions is investigated in a series of numerical experiments performed for flow past a circular cylinder at Reynolds number 100. The governing equations are the velocity‐pressure formulation of the Navier‐Stokes equations, and at the downstream boundary the traction‐free condition is imposed. Temporally periodic flow fields obtained by using computational domains with various lengths are compared. It is observed that as far as the near‐field solution, the Strouhal number, and the lift and drag coefficients are concerned, the downstream boundary can be placed as close as 14.5 diameters from the center of the cylinder with virtually no difference in the solution. Furthermore, only third‐digit variations in the Strouhal number and the lift and drag coefficients and very minor changes in the near‐field solution are observed when the downstream boundary is brought as close as 6.5 diameters from the center of the cylinder. Bringing the downstream boundary closer than this seems to result in more significant changes in the solution. In particular, if the distance is 2.6 diameters or closer, the solution becomes symmetric and steady.