A common factor present in both applications is that the viscous forces are small compared to the inertial forces, i. e. high Reynolds number O(107), and the flow fluctuations are relatively large compared to the average flow speed (10% turbulent intensity). Another important common property is that the contribution of the friction drag is significant to the total drag (can be as high as 50%). Our analytical calculations suggest that compliant walls may allow a total friction drag reduction of around 5%. We may conclude that developing this flow control mechanism for the shipping and tidal energy industry can lead to a significant increase of energy efficiency. The use of compliant walls is extremely promising for a number of reasons: (i) the potential drag reduction increases with the level of turbulence of the boundary layer; (ii) it has the additional effect of suppressing vibrations and flow-induced noise; (iii) it is a passive flow control and therefore it is resilient to the hostile marine environment.
The goal of the project is to provide a proof of concept that well-designed compliant coatings can allow drag reduction and underpin future commercial exploitation for the shipping and tidal energy industry. To this end, we perform direct numerical simulations (DNS) of a well-known test case, the fully turbulent channel flow, where we can understand the complex interaction between a compliant wall and the fluid flow. Our coating is designed based on efficient controls which decrease the turbulent kinetic energy production in the buffer layer (Figure 1) through weakening the near wall coherent structures (Figure 2). Later on, the investigations will be extended to the zero pressure gradient flat plate, which is physically closer to the aforementioned applications. A possible outcome of the project is a theoretical compliant coating which could provide a solid basis for the practical design, and at the end, the production.
- T. I. Józsa, I. M. Viola, E. Balaras, M. Kashtalyan, ‘On the drag reduction mechanism of in-plane wall velocity controls’, submitted to Physical Review Fluids 27/09/2017
- T. I. Józsa, I. M. Viola, E. Balaras, M. Kashtalyan, B. Kidd, ‘Passive in-plane wall deformations for turbulent drag reduction’, 6th Annual Conference of the Energy Technology Partnership (ETP), Edinburght, United Kingdom, 10/10/2017
- T. I. Józsa, I. M. Viola, E. Balaras, M. Kashtalyan, B. Kidd, ‘Smart coatings for drag reduction in yachts’, 4thInternational Conference on Innovation in High Performance Sailing Yachts (INNOV`SAIL), Lorient, France, 28-30/06/2017
- T. I. Józsa, I. M. Viola, E. Balaras, M. Kashtalyan, ‘Drag reduction by tangential wall velocity control’, 29th Scottish Fluid Dynamics Meeting, Edinburght, United Kingdom, 20/05/2017
- T. I. Józsa, I. M. Viola, E. Balaras, ‘Streamwise shear stress driven compliant wall for drag reduction’, 68thDivision of Fluid Dynamics Meeting of the American Physical Society (APS-DFD), Boston, United States of America 22–24/11/2015