Department Seminar of Oliver Buxton- ENTRAINMENT ACROSS TURBULENT/TURBULENT INTERFACES: POINTS OF DISTINCTION FROM TURBULENT/NON-TURBULENT INTERFACES
ENTRAINMENT ACROSS TURBULENT/TURBULENT INTERFACES: POINTS OF DISTINCTION FROM TURBULENT/NON-TURBULENT INTERFACES
Monday August 17th 2025 at 14:00
Wolfson Building of Mechanical Engineering, Room 206
Abstract:
A turbulent/non-turbulent interface (top) and turbulent/turbulent interface (bottom) for a cylinder wake.
Turbulent flows are known to grow with downstream distance; think of a volcanic plume broadening as streamwise distance from the volcanic crater increases. This spreading occurs due to the transport, and mixing, of background fluid into the turbulent flow across the sharp interface demarcating the turbulent flow from the background in a process known as entrainment. In the special case where the background is non-turbulent this interface is known as the turbulent/non-turbulent interface and entrainment is known to be driven by viscous diffusion of the turbulent fluid into the background. This was first postulated by Corrsin and Kistler [1] and arises since the turbulent/nonturbulent interface is, in-effect, an isosurface of zero vorticity-magnitude to account for the fact that the background is irrotational whilst the vorticity is, by definition, non-zero in the turbulent portion of the flow. Accordingly the only non-zero source term at the turbulent/non-turbulent interface in the vorticity-magnitude transport equation is viscous diffusion. However, many (most) industrial and environmental flows exist within a turbulent background, for example wind-turbine wakes are exposed to atmospheric turbulence and gas-turbine blades are exposed to the turbulent outflow of the combustor. In such cases the intuition of Corrsin and Kistler [1] breaks down. Indeed, in the review paper of da Silva et al. [2] it was even suggested that when two streams of turbulence with comparable turbulence intensity are adjacent to one another the interface between them breaks down meaning that there is no discernible interface demarcating the adjacent streams of turbulence. In this seminar we prove the existence of a turbulent/turbulent interface [3] for a wake exposed to various degrees of freestream turbulence, including cases where the intensity of the freestream turbulence is greater than that within the wake. We will then explore the physics of the turbulent/turbulent interface which are different than those for turbulent/non-turbulent interfaces [4]. Finally, we will then examine how the presence of freestream turbulence affects the entrainment rate into the wake, considering the spatial evolution of the entrainment of mass, streamwise momentum, and kinetic energy [5, 6]. Understanding these physics is important to being able to more accurately predict the spreading of turbulent flows exposed to freestream turbulence which is important for e.g. designing layouts for more effcient future wind farms.
Bio:
Oliver Buxton (OB) is a Professor of fluid mechanics in the Department of Aeronautics at Imperial College London. He joined the same department as a lecturer in 2013 having previously worked as a post-doctoral research fellow in the Department of Aerospace Engineering and Engineering Mechanics at the University of Texas at Austin, U.S.A. 2011 - 2013. Prior to that he graduated with a B.A./M.Eng. from the University of Cambridge in 2007 and a Ph.D. from Imperial College London in 2011 under the supervision of Prof. Bharathram Ganapathisubramani; in fact he was his supervisor’s first Ph.D. student. During his Ph.D. work he won the 2010 ERCOFTAC da Vinci award for his work on fine-scale turbulence in the far field of turbulent mixing layers. His research is primarily experimental, and in particular he makes use of laser diagnostic techniques such as PIV (both two-dimensional and three-dimensional) and PLIF to interrogate turbulent flow fields, although he has also worked with several different DNS codes though not as a developer. Recently, he has focused on turbulent entrainment and multi-scale generated turbulence. He is currently both an EPSRC fellow seeking to apply his advances in both to the understanding and modelling of turbulent wind-turbine wakes for wind-farm optimisation and an ERC Consolidator grant holder for the project “Turbulence intermittency for cloud physics” (TITCHY). He also has research interests in Martian aerodynamics and hydrogen-powered aircraft.
In terms of service to the community OB is an advisory board member for the U.K. Turbulence Consortium and the incoming lead for the U.K. Pilot Centre of the European Research Community on Flow, Turbulence and Combustion (ERCOFTAC). He has previously been a visiting researcher at Laboratoire PRISME at the University of Orléans in France, is a regular collaborator with the Turbulence, Wind-Energy, and Stochastics research group at the University of Oldenburg in Germany and with Instituto Superior Técnico Lisbon in Portugal, and has delivered an invited lecture series on the application of PIV and PLIF to turbulence research at the Indian Institute of Technology in Kharagpur, West Bengal. He has published over 50 archived journal papers and has supervised/cosupervised 21 postdocs and Ph.D. students who have achieved considerable success of their own.
