Experimental and Numerical Study of Swirling ... - Solid Mechanics

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
6.1.3 Near Wall Treatment
Chapter 6
The near wall region is defined by adopting the wall function approach. For
all the simulations the two-layer-zone model ‘Non-equilibrium wall
function’ is used. In this model, the flow is divided into viscosity affected and
turbulent regions. The turbulent kinetic energy budget is computed in the
wall neighboring cells (ANSYS, 2009). This model needs less grid points
compared to low-Reynolds number models (Najafi et al., 2005). It also
accounts for the effect of pressure gradients on the distortion of the velocity
profiles i.e. in cases where the assumption of local equilibrium, when the
production of the turbulent kinetic energy is equal to the rate of its
destruction, is no longer valid (ANSYS, 2009).
6.1.4 Solution Methods
For all the simulation cases same solution methods are used. For pressurevelocity
coupling SIMPLE algorithm is used. PRESTO scheme is used for
spatial discretization of pressure equation. For momentum, turbulent kinetic
energy, turbulent dissipation rate and Reynolds stresses second order upwind
scheme is used. For temporal discretization, first order implicit integration
method is used. The time step t for fully open case is defined as 1e-04
seconds.
6.2 Results
The results presented in this section are from transient or URANS
simulations only. For the case of fully open port case, the maximum y+ value
for all the simulation cases are between 126.5 to 130. The cell courant
number range from 0.018 to 1.67.
6.2.1 Tangential Velocity
The comparison of normalized tangential velocity profiles are given in
figures 6.3-6.8 for positions z - z . It can be observed that, in general, for all
1 6
the positions the results of both RNG k model and RSM are not
predicting the (free vortex type) tangential velocity profile in the annular
region. The RNG k model, for both inlet turbulent intensities (T.I = 1%
and 10%), show a tendency towards predicting a forced vortex profile for the
tangential velocity. At z , all the models predict a larger size of the vortex
1
core (Figure 6.3). However, RNG k model with T.I 1% and RSM with
T.I 10%, give a good prediction of the magnitude of the peak tangential
velocity whereas the RNG k model with T.I 10% under predicts the
value. At further downstream positions, all the models show a smaller decay
in the swirl intensity and thus over predict the peak tangential velocity value
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Numerical Modeling