THE NATURE OF THE TURBULENT/TURBULENT INTERFACE AND TURBULENT/TURBULENT ENTRAINMENT
|Heure||14h00 - 15h00|
8 rue léonard de Vinci
Turbulent flows are known to grow with downstream distance; think of a plume of smoke becoming wider as distance away from a chimney 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  and arises since the turbulent/non-turbulent 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 vorticitymagnitude 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 gasturbine blades are exposed to the turbulent outflow of the combustor. In such cases the intuition of Corrsin and Kistler  breaks down. Indeed, in the review paper of da Silva et al.  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 such thing as a turbulent/turbulent interface. In this seminar we prove the existence of a turbulent/turbulent interface  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 for turbulent/non-turbulent interfaces. Finally, we will then examine how the presence of freestream turbulence affects the entrainment rate into the wake. 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 efficient wind farms.
 Stanley Corrsin and Alan L Kistler. Free-stream boundaries of turbulent flows. NACA Tech. Rep. TN-1244.
NACA Washington, DC, 1955.
 Carlos B. da Silva, Julian C.R. Hunt, Ian Eames, and Jerry Westerweel. Interfacial layers between regions of
different turbulence intensity. Annual Review of Fluid Mechanics, 46(1):567–590, 2014.
 K.S. Kankanwadi and O.R.H. Buxton. Turbulent entrainment into a cylinder wake from a turbulent background.
J. Fluid Mech., 95:A35, 2020.