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SUMMARY:Entrainment across a turbulent/turbulent interface - Oliver Buxton
 \, Imperial College London
DTSTART:20240614T150000Z
DTEND:20240614T160000Z
UID:TALK216283@talks.cam.ac.uk
CONTACT:Professor Grae Worster
DESCRIPTION:Turbulent flows are known to grow with downstream distance\; t
 hink of a volcanic plume broadening as streamwise distance from the volcan
 ic crater increases. This spreading occurs due to the transport\, and mixi
 ng\, of background fluid into the turbulent flow across the sharp interfac
 e demarcating the turbulent flow from the background in a process known as
  entrainment. In the special case where the background is non-turbulent th
 is interface is known as the turbulent/non-turbulent interface and entrain
 ment is known to be driven by viscous diffusion of the turbulent fluid int
 o the background. This was first postulated by Corrsin and Kistler [1] and
  arises since the turbulent/non- turbulent interface is\, in-effect\, an i
 sosurface of zero vorticity-magnitude to account for the fact that the bac
 kground is irrotational whilst the vorticity is\, by definition\, non-zero
  in the turbulent portion of the flow. Accordingly the only non-zero sourc
 e 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 case
 s the intuition of Corrsin and Kistler [1] breaks down. Indeed\, in the re
 view paper of da Silva et al. [2] it was even suggested that when two stre
 ams of turbulence with comparable turbulence intensity are adjacent to one
  another the interface between them breaks down meaning that there is no d
 iscernible interface demarcating the adjacent streams of turbulence. In th
 is seminar we prove the existence of a turbulent/turbulent interface [3] f
 or a wake exposed to various degrees of freestream turbulence\, including 
 cases where the intensity of the freestream turbulence is greater than tha
 t within the wake. We will then explore the physics of the turbulent/turbu
 lent interface which are different than those for turbulent/non-turbulent 
 interfaces [4]. Finally\, we will then examine how the presence of freestr
 eam turbulence affects the entrainment rate into the wake\, considering th
 e spatial evolution of the entrainment of mass\, streamwise momentum\, and
  kinetic energy [5]. Understanding these physics is important to being abl
 e to more accurately predict the spreading of turbulent flows exposed to f
 reestream turbulence which is important for e.g. designing layouts for mor
 e efficient future wind farms.\n\nReferences\n\n[1] S. Corrsin and A. L. 
 Kistler. Free-stream boundaries of turbulent flows. Technical Report NACA 
 Tech. Rep. TN-1244\, 1955.\n\n[2] C. B. da Silva\, J. C. R. Hunt\, I. Eame
 s\, and J. Westerweel. Interfacial layers between regions of different tur
 bulence intensity. Annual Review of Fluid Mechanics\, 46(1):567–590\, 20
 14.\n\n[3] K. S. Kankanwadi and O. R. H. Buxton. Turbulent entrainment int
 o a cylinder wake from a turbulent back- ground. Journal of Fluid Mechanic
 s\, 905:A35\, 2020.\n\n[4] K. S. Kankanwadi and O. R. H. Buxton. On the ph
 ysical nature of the turbulent/turbulent interface. Journal of Fluid Mecha
 nics\, 942:A31\, 2022.\n\n[5] O. R. H. Buxton and J. Chen. The relative e
 fficiencies of the entrainment of mass\, momentum\, and kinetic energy fr
 om a turbulent background. Journal of Fluid Mechanics\, 977:R2\, 2023.\n\n
 \n
LOCATION:MR2
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