Time stepping

The investigations cover two- and three-dimensional simulations of the HQ17 airfoil with Gurney-flap at a Reynolds number of $Re=10^6$ and incompressible flow conditions. From initial numerical investigations of the clean airfoil, it was known that the flow becomes unsteady at high angles of attack and vortex shedding occurs. Therefore all computations are run in unsteady mode. A separate study of the influence of time stepping indicated that a typical time step of $\Delta t=0.002 c/u_0$ is sufficient to obtain results independent of the temporal resolution. All computations presented here are obtained using $\Delta t=0.001 c/u_0$. For detailed information about the flow around the HQ17 reference profile see Schatz [11].

Due to the large difference between the global length-scales of the flow (represented by the chord-length $c$) and the typical size of vortices (given by the Gurney height $h=0.01c$) the computation requires a large number of time steps to reach a quasi-steady state. For each computation, about 10,000 time steps are required corresponding to about 120 cycles.