Turbulent secondary flows are described as the occurrence of vortex structures transverse to the main flow direction, i.e. the formation of a vortex strength in the flow direction. Secondary flows thus represent an additional flow perpendicular to the main flow, whose intensity, however, is clearly smaller than this. Nevertheless, secondary flows influence the main flow properties significantly and lead to an additional mixing of the flow, and their understanding and prediction are of great interest for both geophysical and technical flows.

For example, in open channel flows secondary flows are caused by inhomogeneous erosion distribution in the spanwise direction, which in turn causes sediment transport transverse to the flow direction and thereby increases the inhomogeneities. In atmospheric boundary layer flows over complex terrain and topography, large-scale secondary flows influence the distribution of temperature, humidity and aerosols and can thus have a decisive effect on atmospheric exchange processes. In technical flows, such as damaged turbine blades, irregularities or soot deposits on the surface can contribute to the formation of secondary flows and thus significantly influence the heat transfer properties and performance of the turbines.

Figure: Visualization of the instantaneous velocity in streamwise direction above inhomogeneous spanwise distributed Lego ridges. Main flow direction is pointing in image plane.

Within the framework of the project, numerical experiments are carried out using direct numerical simulations (DNS), which completely resolve turbulence and thus provide a valuable database for physical investigation. Different surface structures are investigated in order to identify the relevant mechanisms for the formation of secondary flows as well as the most important parameters for its generation and manipulation. These results can be used in the future for approaches for specific flow control to influence the heat transfer in technical applications and thus contribute to lower energy consumption. On the other hand, a better understanding of the influence of secondary flows on the propagation of temperature and humidity forms the basis for contributing to the improvement of existing weather and climate models.

Flow configuration:

• Fully developed turbulent channel flow with passive scalar (e.g. temperature)

Numerical approach:

• DNS with Immersed Boundary Method (IBM) to resolve surface structures

Figure: Visualization of flow structures (by means of q-criterion) in turbulent channel flow above Lego ridges.