# Best Practice Advice

## Key Physics

The inflow condition is of major relevance for the vortex system. The horseshoe vortex dynamics are driven by the downflow in front of the cylinder. This downward directed flow is caused by a vertical pressure gradient, which in turn depends on the shape of the approaching inflow profile. Therefore, one has to know the flow profile approaching the cylinder in order to be able to interpret the results and compare it to other studies. In the presented study, we took special care to have a fully developed turbulent open-channel flow approaching the cylinder in both experiment and simulation.

The wall distance of the horseshoe vortex is about ${\displaystyle 0.06D}$, the thickness of the wall jet between the cylinder and the horseshoe vortex center is smaller than ${\displaystyle 0.01D}$. This means that the key physics take place in a very thin layer in front of the cylinder. A simulation or a measurement not resolving this layer might strongly miss some key features such as the levels of the wall shear stress and the large levels of the streamwise velocity fluctuations and their bimodal character under the horseshoe vortex.

It has been demonstrated that Reynolds shear stresses are small in the upstream-directed wall jet between the cylinder and the horseshoe vortex. This means that the wall shear stress in this region might not be captured by conventional wall models, such as the law of the wall for turbulent boundary layers.

## Numerical Modelling Issues

A high spatial resolution is required to capture the wall shear stress, the streamwise fluctuations and the horseshoe vortex dynamics. For the Reynolds number considered, a wall-normal resolution of ${\displaystyle D/1000}$ was necessary to obtain converged wall shear stresses. This high resolution was necessary because the wall jet does not follow the law of the wall for turbulent boundary layers and therefore needs to be fully resolved for wall shear stress calculations. A realistic representation of the junction vortex also requires a fine resolution in the horizontal directions.

From the individual terms contributing to the dissipation of TKE the following observations can be done. The dissipation is far from an isotropic state. Around the horseshoe vortex, the gradients of fluctuations in the main flow direction contribute to dissipation in the same way as gradients in the vertical direction. This means that strongly anisotropic grids are probably not suited to capture the physics in a sufficiently accurate way.

For numerical methods based on the Reynolds Averaged Navier Stokes (RANS) equations several challenges appear important. The production term is negative in the wall jet due to the strong local acceleration and the transport by turbulent fluctuations and by pressure dominate the TKE balance in the wall jet under the horseshoe vortex. Realistic levels of turbulent kinetic energy appear to depend strongly on those terms. Under the horseshoe vortex extremely large streamwise fluctuations show up while Reynolds shear stress remain very small. The turbulent viscosity might be strongly overestimated by standard two equation models.

## Measurement issues

The horseshoe vortex system is a complex three-dimensional flow configuration. Therefore, the two-dimensional data acquisition method is a limitation as the out-of-plane velocity component leads to a corresponding loss of particles. The number of valid samples suffered from this issue in combination of the low seeding density resulting from the large size of the flume. To overcome this issue, we additionally evaluated the PIV images with a ${\displaystyle 32\times 32\mathrm {px} }$ grid. Whenever the instantaneous velocity fields based on a ${\displaystyle 16\times 16\mathrm {px} }$ grid revealed a missing vector, the corresponding vector of the coarser evaluation was taken as a substitute, if possible. In this way, we could improve the number of valid samples and still keep the spatial resolution high. However, the spatial resolution of the PIV data was too coarse to resolve the velocity gradient correctly. Therefore, a single pixel evaluation is recommended, in order to capture the wall-shear stress correctly.

## Application Uncertainties

When simulating this flow configuration, we adress the largest uncertainties to the inflow conditions of the approach flow and to the representation of the water surface. Both, numerical and experimental approaches, face the challenge in generating a fully developed turbulent boundary layer. Even though we intended to reproduce identical flow conditions and validated both of our methods (PIV and LES) by comparison to results in the literature, we observed differences in our results concerning the size and location of the horseshoe vortex for example (see Fig. 6), which we attribute to the uncertainties in the structure of secondary flows or in modelling the water surface.

## Recommendations for Future Work

Performing a converged Direct Numerical Simulation would end all discussions about models and is - in our opinion - not far out of reach to date (2019). Further, considering surface roughness might give additional insight into the interaction of the wall jet with the wall.

The experiments could be improved by stereoscopic or tomographic PIV to acquire three dimensional data sets. Furthermore, the temporal resolution could be increased, in order to analyse the time scales of the horseshoe vortex system. The experimental setup can be improved by providing the light sheet from below passing through the transparent bottom plate, while the PIV camera(s) are mounted at the side outside of the flume.

Contributed by: Ulrich Jenssen, Wolfgang Schanderl, Michael Manhart — Technical University Munich