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 5
more mass into the wake region. This change in velocity profile begins from
cross-sectional positions near the cylinder outlet and moves to upstream
positions as the piston gradually closes the port. However, in case of piston
position with 50% port closure, at cross-sectional positions close to cylinder
inlet, the tangential velocity profiles resembled more towards a wall-jet like
profile rather than the forced vortex. The axial velocity in the whole cylinder
exhibits a jet-like profile with a broad peak. For the cross-sectional positions
close to the intake port, the partial closure of the intake port introduces
asymmetry and variation in the mean tangential velocity profile. At 75% port
closure the tangential velocity in the cylinder has a forced vortex like profile
indicating high velocities in the near wall region. At z 1 , the axial velocity still
has a jet like profile but with a sharp peak. This jet-like mean axial velocity
profile changes back to wake-like at the adjacent downstream cross-sectional
position z 2 . This indicates a vortex breakdown like characteristic. The mean
axial velocity profile then continues to have the wake-like profile at the
remaining downstream positions. The wake effect however is small
compared to when the port was fully open.
The mean axial vorticity of the mean velocity field has a Gaussian like profile
when the intake port is fully open. However with the decay in the swirl
downstream as well as the partial closure of the intake port, the vorticity
confined in the vortex core region is transmitted to the outer regions. This
results in a comparatively uniform mean axial vorticity distribution
throughout the cylinder for 75% port closure.
The increase in port closure increases the magnitude of Reynolds stress
components as well as the anisotropy. The distribution of normal Reynolds
stress components can be understood and summarized from the profiles of
axial and tangential velocity components. In case of tangential velocity
having the burgers vortex like profile, the u and v velocity components have
high fluctuations in the vortex core region and the near wall region. For a
forced vortex like profile, uu and vv attain higher values in the region
close to vortex center than to region with higher tangential velocities. In case
of axial velocity with wake like profile, the fluctuations w is high in the
vortex core region and near wall region. As the axial velocity profile attains a
jet like profile, the ww in the jet center seems to be very low and increase
as the radial distance from the jet center increases. In general, the turbulent
kinetic energy increases with the partial closure of the intake port.
For the incylinder flow at all piston positions, the Reynolds shear stresses
have lower magnitude compared to normal components. Figure 5.37 give
profile of Reynolds shear stress components (in polar coordinates) at z and 1
Re for each piston position. An important thing to be noted is the rise in the
A
magnitude of vrvz with percentage of piston closure especially from the
region at X/R=± 0.6. At 75% port closure, increase in values of vrvz and
vrvz show that the covariance of axial velocity with both tangential and
radial velocity components is large.
145
Effect of Piston Position