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In the PHOENICS 2-phase IPSA model (ONEPHS=F), the turbulent stresses in the phase-momentum equations are represented by a Boussinesq turbulent-viscosity model. The turbulent viscosity ENUT may be calculated from any of the turbulence models embodied in PHOENICS on the understanding that the turbulence is assumed to be a property of phase 1 and that ENUT is shared by both phases. The turbulent dynamic viscosity of each phase is then calculated by multiplying ENUT by the phase density RHO1 or RHO2.

The transport equations of all variables except volume fractions contain within-phase diffusion terms which represent molecular and turbulent mixing, as described in Section 2 below. All transport equations contain phase-mass diffusion terms, which originate from the appearance of such terms in the volume-fraction equations. These terms, which are described below in Section 3, represent turbulent transport of phase mass due to correlations of velocity fluctuations with those of volume fraction.

For disperse turbulent 2-phase flows, such as those involving solid particles, gas bubbles or liquid droplets, phase 1 should be selected as the continuous (carrier) phase because ENUT is calculated from phase-1 variables. For non-disperse gas-liquid and liquid-liquid flows, the user is advised to follow the simple expedient of assigning the most dominant turbulence-producing phase to phase 1. However, it is possible for the user to introduce GROUND coding in which the turbulent viscosity is calculated as a weighted average of the individual-phase eddy viscosities.

As was mentioned already, ENUT may be calculated from any of the turbulence models currently available in PHOENICS for single-phase flows. However, the presence of dispersed solid or fluid particles can cause a significant change in the turbulence structure of the carrier phase. For example, small particles are known to attenuate the turbulence; whereas large particles may increase the turbulence. The single-phase turbulence models of course neglect these effects.

The way in which the KE-EP turbulence model is used in 2-phase simulations is described below in Section 4. Provision is made in PHOENICS for enhancements to this and other models so as to account for the presence of fluid and solid particles. Modifications for bubble-induced turbulence are described in Section 5, whereas enhancements for turbulence modulation due to particles are discussed in Section 6.