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### 7.1.Introduction

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.

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