Exploring how Rough Surfaces cause Fluid Flows to Become Turbulent

Tolerance levels are a key consideration for surface finish of various components of gas turbine engines. Such surfaces ought to be sufficiently smooth for efficient performance, and are normally considered to be ‘aerodynamically smooth’. As expected, during its working life, various damage mechanisms progressively degrade the surfaces and lead to a decline in performance of the engine, that ultimately translates into higher fuel consumption, excessive temperatures in localized zones, and, under extreme cases, the loss of stall margin. During design, the aforementioned penalties are usually overlooked. As of now, the effect of roughness on fully turbulent flows has been addressed by researchers, but few have addressed the onset of turbulence — which is called laminar to turbulent ‘transition’.

Recently, Dr. Nagabhushana Rao Vadlamani and Professor Paul G. Tucker at University of Cambridge in collaboration with Professor Paul Durbin at Iowa State University investigated the effect of distributed roughness on subsonic boundary layers typically observed in turbomachines. Ultimately, applications might be to turbine blades, with loading and a distribution of pressure gradients. But, to elucidate the more basic phenomenon, the Vadlamani et al. study focused on the much more fundamental configuration a flat plate with a pattern of asperites. Their work is currently published in the research journal, Flow, Turbulence and Combustion.

The research method was high fidelity computer simulation. It commenced with a thorough description and review of the computational framework, numerical algorithm, and a grid sensitivity study. Then, the researchers explored the effect of surface roughness on transition, validating their simulations by comparisons to correlations that are available from lab experiments. They then engaged in an in-depth investigation of the transition mechanisms. Lastly, they assessed the resultant roughness effects on the spatial development turbulent boundary layers.

The authors observed that the roughness elements that were inside the boundary layer created an elevated shear layer. Alternating high and low speed streaks were observed underneath the shear layer. They noted that secondary, sinuous instabilities on the streaks destabilized the shear layer, promoting transition to turbulence. Moreover, for the roughness topology considered, it was observed that the instability wavelengths were governed by the streamwise and spanwise spacing between the roughness elements.

In conclusion, the Vadlamani et al. study presented a detailed numerical investigation of the transition of a subsonic boundary layer on a flat plate in the presence of roughness elements, distributed over the entire surface, using a series of eddy resolving simulations. In general, the underlying transition mechanisms were shown to change significantly with an increasing roughness height. Roughness elements that were higher than the boundary layer were seen to create turbulent wakes in their lee. In that case the scale of instability is much shorter and transition occurs due to the shedding from the obstacles.