Exploring how Rough Surfaces cause Fluid Flows to Become Turbulent

Significance 

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.

About the author

Dr. Nagabhushana Rao Vadlamani is an assistant professor in the Department of Aerospace Engineering at Indian Institute of Technology (IIT), Madras. He received his B.Tech, M.Tech and PhD degrees from NIT Warangal, IIT Madras and University of Cambridge respectively. In 2015, he was awarded Bowring research fellowship from St. Catharine’s college, Cambridge and worked as a Senior Research associate in the Cambridge University Engineering department. Prior to his PhD, Rao worked with the Digital Mockup team in Airbus India.He and his colleagues were the recipients of the 2013 ASME IGTI Turbomachinery Committee Best Paper Award for their work on transition in low pressure turbines.

Rao’s research interests include Computational Fluid Dynamics, transition to turbulence, Turbomachinery, and High performance computing.

About the author

Dr. Paul Tucker is Rank Professor of Engineering at the University of Cambridge. Prior to joining the University of Cambridge, he was a Professor in the Civil and Computational Engineering Centre at the University of Swansea leading aerospace. Before this he was a Senior Lecturer in the Department of Engineering at the University of Warwick.

He is an Associate Editor of the AIAA Journal and a Professorial Fellow at Murray Edwards College. Professor Tucker has supervised 14 PhD students and 2 MPhil to completion. He was a recipient of NCR Award for Outstanding Technical Achievement and ASME Best Paper Award 2013 from the Turbomachinery committee. He has authored three books on computational aerodynamics.

Paul’s key research interest is the computation of unsteady, turbulent, complex geometry aerodynamic flows. His current research heavily focuses on improving Computational Fluid Dynamics (CFD) in aerospace – especially turbomachinery using Large Eddy Simulation (LES) techniques.

About the author

Dr. Paul Durbin is a professor in aerospace engineering at Iowa State University. He received his BSE in Aerospace and Mechanical Sciences from Princeton University and his PhD from the Department of Applied Mathematics and Theoretical Physics (DAMTP) at Cambridge University. After a post-doc at Cambridge, he became a research engineer at NASA Lewis (now NASA Glenn). He spent two years as a visiting associate professor at the University of Arizona, then went to the Center for Turbulence Research, at Stanford, as a senior fellow. After that he became a Professor in the Mechanical Engineering Department at Stanford. Ten years ago, he moved to Iowa State University, as the Martin C. Jischke professor. He is a fellow of the American Physical Society. He is the author of two books on fluid dynamics and turbulence and regularly serves as a consultant on turbulence modeling.

Professor Durbin’s research is in theory, analytical modeling and simulation of turbulence and transition, especially on bypass transition. He has developed various models for Reynolds averaged prediction of turbulence and transition, and for detached eddy simulation of turbulent flow.

Reference

Nagabhushana Rao Vadlamani, Paul G. Tucker, Paul Durbin. Distributed Roughness Effects on Transitional and Turbulent Boundary Layers. Flow Turbulence Combust (2018) volume 100: page 627–649.

Go To Flow Turbulence Combust

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