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

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
Chapter 6
shear stress components are under predicted compared to experimental data
in figures 4.29 & 4.32 respectively.
6.3 Discussion
The results of CFD simulations presented in this chapter do not show a
satisfactory agreement with the experimental data. However, there are some
qualitative features like profiles of velocity and modeled Reynolds stress
components that, to some extent, have reasonable agreements. The factors
affecting the performance of the CFD models possibly lie both in the
treatment of turbulence and the numerical aspects.
The performance of RANS based models for swirling flows have already been
discussed in Section 2.4. Numerically, defining the boundary conditions at
the inlet of the computational domain has very significant impact. In the
current simulations, a constant value of the parameters like velocity
components and turbulence parameters like k and have been given at
the inlet. Considering the real experimental setup, this approach may not be
good because the inlet in the computational domain is placed after the guide
vanes and close to the contraction section. The profile of velocity
components may not be uniform at that radial position. The possible reasons
are the wake that is generated behind the guide vanes and the upstream
effect of in-cylinder swirling flow (PVC and instability waves). Both of these
factors can make the velocity profiles and magnitude, at the selected inlet
position (a radial distance of 200 mm from cylinder axis in this case), nonuniform
and unsteady respectively. Similar effect can be on the turbulence
parameters like k , and Reynolds stress components. This requires
experimental measurements to be conducted in order to define the realistic
boundary conditions. (Dong et al, 1993) also studied the effect of different
inlet profiles for the velocity components in a swirling flow and suggested
experimentally measured profiles for better simulation results. In case of
outlet, since the outlet pipe length is very long, therefore, the effect of outlet
boundary condition on the in-cylinder swirling flow may not be very
significant. (Xia et al., 1997) have found significant effect of outlet boundary
conditions but at regions close to outlet i.e. only to a small upstream distance
from the outlet.
The other aspect is the RANS modeling approach itself. In the RANS based
models, the turbulent scales are not fully or partly resolved. Instead the
whole range of turbulent scales is modeled and the simulation results
represent the influence of all the turbulent scales. Thus RANS have a strong
damping influence on any resolved turbulence or unsteady structures, which
is desirable for steady state flows (Gyllenram et al., 2008). In case of unsteady
flows, the time dependent features that unsteady RANS (URANS) may
resolve is restricted only to the coherent periodic motions and not the wide
range of frequencies of broadband turbulence (Spencer et al., 2009). The
unsteady behavior requires the turbulence model to be able to distinguish
between resolvable and unresolvable scales and the RANS based equations
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Numerical Modeling