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

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
downstream the flow. At position z 7 (z/D=4.116) and cylinder lengths (8D
and 6D), the magnitude of radial velocity becomes very close to zero and
then slowly starts increasing again.
The mean axial vorticity has a Gaussian like profile at z 1 (z/D=0.963) i.e. for
the regions at larger radial distance from the cylinder axis, except the near
wall region, the vorticity is very weak and then in the vortex core region it
gets very strong. With swirl decay downstream the flow, the vortex core loses
its vortical strength by transferring it in outward radial directions. The rate of
this vorticity transfer is higher in case of high Reynolds number thus leading
to quicker decay of the weak vortical ‘annular region’ in the tangential
velocity profile.
The detection of vortex core position in the instantaneous PIV measurements
at z 1 (z/D=0.963) show that the vortex core position is not stationary and
moves in an area of r/R= ±0.2. This indicates a precessing vortex core (PVC)
and LDA measurements show that the precession frequencies at Reynolds
number of 65000 and 32500 are 5.8 Hz and 3.15 Hz. The precession
frequency is found to increase linearly as a function of flow rate. The mean
vortex core position is not aligned with the cylinder axis at all measuring
positions indicating an asymmetric swirling flow with core following a
helical path. The radius of the helical path is not same for all measuring
positions and initially the radius of the helical path is smaller but then
gradually increases. The helix rotation is in a clockwise direction similar to
the swirl which is also in clockwise direction. This together with a Burgers
vortex like tangential velocity profile and wake-like axial velocity profile
indicate a left-handed helical vortex and having a negative pitch. For all the
cylinder lengths the helical vortex core does not complete one revolution. In
case of cylinder length 8D, the helical vortex path at the downstream
positions, instead of rotating around the cylinder axis, re-twists at one side of
the X-axis. In addition, at those far downstream positions the tangential
velocity has forced vortex profile, the axial velocity and mean axial vorticity
profiles become uniform showing an infinite pitch of the helical path. This
indicates a transition of left handed to right handed helical vortex called as
‘L-transition’ and is one of characteristics of vortex breakdown.
In the presence of a precessing vortex core (PVC), the measured values of
Reynolds stresses are actually a combination of turbulence and vortex core
oscillation. The Reynolds stress normal components are high in the vortex
core and near wall regions. Considering the maximum values, for normal
stresses (Cartesian coordinates) at z (z/D=0.963) ww uu vv .
1
Regarding the components in polar coordinates, v v and vrv r are larger
near the wall and in the vortex core regions at z (z/D=0.963). The radial
1
distance, from the vortex center, where vrv r increases towards the vortex
center, is larger than the v v and follows the same for both high and low
Reynolds numbers. As the swirl decays downstream, the magnitude of radial
and tangential fluctuations decrease but their spatial distribution increase to
larger radial distances of the cylinder cross-section.
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Summary & Conclusions