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

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
5.1.4 Reynolds Shear Stresses
25% Port Closure
Chapter 5
The shear stress components of Reynolds stresses are given in figure 5.25 for
z 1 . It can be seen that the covariance of in-plane velocity components u and
v is small compared to uw and vw components. Shear stress component
uv is weak in the region around cylinder axis and large values are observed
in two pairs of distinct regions/ zones. These regions lie adjacent to each
other and have opposite signs, however, with almost same absolute value of
their magnitude. Shear stresses uw and vw are high in the cylinder axis
and near wall region. In the cylinder axis region, uw and vw have high
values in two small adjacent zones with opposite signs. The zones in the near
wall region also exhibit opposite signs for two ends of X-axis and Y-axis for
uw and vw respectively.
At z , the Reynolds shear stresses are reduced by a factor of 10 (Figure 5.26).
6
The spatial distribution of uv component still has the zone pairs as
discussed for z . However, their shape is more distorted showing small
1
gradients of uv in the cross-sectional plane. For uw and vw the two
adjacent zones around the cylinder axis have grown in size and cover large
portion of the cross-sectional plane. The zones in the near wall region have
high values but seem to have radially shrunk toward cylinder wall. In general
it can be said that with the decay in the swirl along the cylinder, the
magnitude of Reynolds shear stresses decreases and their spatial distribution
increase. For z , the region in between the cylinder axis and the near wall
6
exhibits large covariance values among the velocity components.
For 25% port closure, the covariance between v and v r is large at larger
radial distances. The Reynolds stress component v v r at z decreases slowly
1
towards the cylinder axis and increase again to a small extent in the vortex
core region. v v z is low in the near wall region and increases to its peak
value at a radial distance of r/R=0.15 and then decreases again in the vortex
center. vrv z has largest values in the near wall region and decreases towards
the vortex core region. At z , the distribution of vv 6 z and vrv z spread to
distances around the cylinder axis. Both the components attain high values at
a comparatively larger radial distance from the cylinder axis and decreases in
the vortex center.
132
Effect of Piston Position