ENGINEERING AND CFD

In many engineering applications, fluids or gases flow over curved surfaces. Some examples are the flow over wings and over wind and gas turbine blades. The streamlines of these flows are curved and this streamline curvature has a large impact on the flow and flow drag.

For developing and designing efficient engineering applications like pumps and turbines it is important to have models that can accurately predict flows over curved surfaces. However, engineering flow models often do not correctly capture streamline curvature on flows and therefore cannot accurately predict the flow and drag over such curved surfaces. The underlying reason is that the influence of streamline curvature on flows is complex and not yet well understood.

In the project, large-scale so-called direct numerical simulations are carried out of turbulent flows in mildly to strongly curved channels. The turbulent fluid motions are fully resolved in time and space on a very fine mesh with up to 3 billion grid points and small time steps with a high-order numerical algorithm. These large simulations were performed on the HPC system JUWELS at JSC and are highly accurate since the fundamental governing equations for fluid motions are solved exactly without using any modelling approximations.

The performed simulations give us a much better physical understanding of the influence of streamline curvature on turbulent flows over curved surfaces. The simulations show that the turbulent eddies are much larger and strongly amplified on the outer concave side of the curved channel, whereas the turbulent eddies on the inner convex channel side are smaller and much less intense. As a consequence, the flow drag is much larger on the concave than on the convex side of the channel and this difference grows when the curvature of the channel increases. In the simulation of the strongest curved channel the streamline curvature effects are so large that the turbulence ceases to exist locally on the convex side and the flow there becomes laminar and more regular. The large turbulent eddies or vortices on the concave side of the curved channel form very elongated roll cells parallel with the flow direction. Figure 1 shows a visualization of the flow field in one of the simulations at one time instant in a plane parallel to the curved wall on the concave channel side. The imprint of the large roll cells is obvious in the visualization.