Direct numerical simulation of near-interface turbulence in coupled gas-liquid flow

Abstract

Turbulence structures near the interface between two flowing fluids have been resolved by direct numerical simulation. As a first step the interface has been kept flat, corresponding closely to the recent gas-liquid flow experiments of Rashidi and Banerjee [Phys. Fluids A 2, 1827 (1990)], with the fluids coupled through continuity of velocity and shear stress boundary conditions. For density ratios between the fluids typical of air and water, the turbulence characteristics on the gas side are quite similar to that in wall regions. The liquid side shows larger velocity fluctuations close to the interface and ejections originate closer to the interface. The mean velocity distribution, turbulence intensities, Reynolds stress and various other statistical measures are significantly altered compared to those in the wall region of channel flows. Quasi-streamwise vortices form in the areas between high and low shear stress on both sides of the interface. At any given instant, about a fifth of these appear to be coupled across the interface. Whether the others are, but the coupling is too weak for the detection technique used, or were coupled previously remains an open question. In any case, sweeps usually occur on the high shear stress side of these vortices and ejections on the low shear stress side. Significant coupling exists across the interface with over 60% of the Reynolds stress in the region close to the interface being associated with coupled events –the main coupling coming through gas ejection-liquid ejection events over low shear stress regions, with a lesser but significant number of gas sweep-liquid sweep events over high shear stress regions.

Turbulence structure and interaction with steep breaking waves

“…Alternatively, there have been relatively few DNS-based studies of turbulence in problems involving two-phase flows, if one excludes earlier research focusing on freesurface turbulence with and without wind shear (Lam & Banerjee 1992;Komori et al 1993b;Lombardi, DeAngelis & Banerjee 1996). Because of its relative simplicity, stratified gas-liquid flow has been the configuration best suited to investigating the underlying physics of turbulence at the interface.…”

“…The authors found that the lighter phase might look at the interface like a solid surface. Fulgosi et al (2003) extended the research of Lombardi et al (1996) to non-flat, sheared interfaces by considering stratified flow with a freely deformable interface in the gravity-capillary wave regime. With regard to wave breaking, multiple contributions have been made using LES (Christensen 2006;Lubin et al 2006).…”

“…Because of its relative simplicity, stratified gas-liquid flow has been the configuration best suited to investigating the underlying physics of turbulence at the interface. The DNS investigation of Lombardi et al (1996) concentrated on a flat interface configuration, where gas and liquid were coupled through continuity of velocity and stress jump conditions at the interface. The authors found that the lighter phase might look at the interface like a solid surface.…”

Direct numerical simulation of the near-field dynamics of annular gas-liquid two-phase jets

“…Unsteady RANS and LES approaches were used by Wegner et al 21,22 to simulate unconfined swirling flow and spiral/helical vortical structures were obtained. DNS has been used to simulate the interface changes and turbulence in two-phase environments [23][24][25] but the two phases were divided into two single-phase subdomains while the gas flow was considered to be incompressible. Klein 26 performed DNS of a liquid sheet exhausting into a gaseous incompressible atmosphere under moderate Reynolds number.…”

On the interfacial roughness scale in turbulent stratified two-phase flow: 3D lattice Boltzmann numerical simulations with forced turbulence and surfactant

“…The development of a better understanding of turbulence near deformable interfaces has been a long standing research challenge, and experiments and numerical simulations have already revealed many important aspects of the structure of the interface and the adjacent turbulence (e.g., Lugt and Ohring, 1992;Tsai, 1996;Lombardi et al, 1996;Fulgosi et al, 2003;Guo and Chen, 2010). The interfacial fluctuations respond to the turbulent kinetic energy, length scales and vortex structures in the fluids adjacent to the interface (e.g., Rein, 1998;Smolentsev and Miraghaie, 2005) and the turbulence is affected by the interfacial properties (elasticity, viscosity and energy), posing a twoway interface-turbulence coupling.…”