Figure 1: Comparison between smooth surface and Riblets (click to enlarge and show video)
As can be seen in the comparison of the three used geometries for CFD simulations, the trapezoidal riblets are being compared to two smooth plates, one at the height of the valley of the grooves and the other one at the height of the tip. Those “ideal” riblets have a fixed ratio for riblet height to tip distance of 0.5.
For the simulation set-up, water is used as a fluid and the flow is driven by a pressure difference between inlet and outlet. As can be seen in the scheme, the flow is supposed to be aligned with the grooves in the riblet structure. The structures are applied at the bottom of a channel with periodic side walls and symmetry at the top. Additionally both Inlet and Outlet are periodic as well, so the fluid enters the channel at the beginning after exiting the outlet.
All Meshes are generated with refinements close to the bottom wall, leading to more detailed flow resolution close to that wall. This is also necessary, as the vortex size decreases when moving towards the wall.
Figure 2: Boundary conditions for the simulations.
Figure 3: Mesh setup comparison
When comparing the drag change of the simulated trapezoidal riblets to curves for different geometries found in literature, the value for operating point 2 seems to agree rather well with literature, as the middle curve represents trapezoidal riblets with a different angle, the one at the top sawtooth geometry and the one at the bottom blade riblets. The different operating points can be approximated via a polynomial of degree four, which yields a curve very similar to the ones available in literature for trapezoidal riblets.
When comparing the simulation results, the spots of high wall shear from the two smooth cases disappear while leaving the wall shear in the grooves lower than the minimal value of the smooth plates. There are also slightly less vortex structures than for the smooth geometries. The vortex structures close to the surface also change, since they are too big to enter the grooves.
Figure 4: Experimental data compared to literature values in a drag change over dimensionelss tip distance diagram
Figure 4 Comparison of the drag force fluctuations of the three different configurations
In the diagram above can the drag per time step be seen for all three simulation cases. The red cure with the lowest values represents the riblet structure, while green shows smooth valley with a more uniform curve than the blue one for smooth on tip height.