Statistical analysis of coherent vortices near a free surface in a fully developed turbulence

• Published in: Physics of fluids (1994) 2003-02, Vol.15 (2), p.375-394
• Main Authors: Nagaosa, Ryuichi, Handler, Robert A.
• Format: Article
• Language: English
• ISSN: 1070-6631; 1089-7666
• DOI: 10.1063/1.1533071

The dynamics of coherent vortices, their interactions with an unsheared gas–liquid interface, i.e., free surface, and their contribution to turbulent heat transfer has been investigated in a fully developed turbulence using the results from a direct numerical simulation. Fully resolved free surface turbulence simulations were performed at Reynolds numbers of 150 and 300 based on the wall shear velocity and water depth. Passive heat transfer at a Prandtl number of 1 is enforced by imposing a constant temperature difference between the bottom no-slip boundary and free surface. Instantaneous turbulent flow realizations are stored and used to establish a database from which the statistical properties of the flow can be established. The three-dimensional two-point correlations between the total heat flux at the free surface and the subsurface hydrodynamics are evaluated to determine the spatial extent of the coherent vortices which contribute to the enhancement of heat transport at the free surface. A conditional averaging technique is also used to explore the structure of the typical coherent vortices which promote heat transfer at the interface. The two-point correlation technique reveals ring-like coherent vortices in the subsurface region, which are comprised of a vortex pair and spanwise vortices. The conditional averaging strategy is also applied to an intense ejection (second quadrant) event to examine dynamics of coherent vortices and their development. The results of the statistical analysis near the second quadrant event reveals a hairpin-like vortex, known as a bursting eddy, which is generated in the near-wall region and advects toward the free surface. The eddy then changes its shape to a ring-like structure as it approaches very near to the free surface. Backward tracking of the ring-like vortex shows that its origin is clearly a near-wall hairpin-like bursting vortex associated with the ejection of low-speed fluid in the direction normal to the wall. The interaction of the ring-like vortex with the free surface produces a splat event, which involves fluid impingement onto the free surface from the turbulent bulk, an event which intensifies the local temperature gradient at the free surface. It is also found that turbulent shear layers, which can be identified as regions of intense spanwise vorticity, impact onto the free surface, accompanying the interaction of the ring-like vortex with the free surface.