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

Experimental and Numerical Study of Swirling Flow in Scavenging Process for 2-Stroke
Marine Diesel Engines
4.2.5 Reynolds Normal Stresses
Chapter 4
The Reynolds stresses are expressed in Cartesian coordinates rather than
cylindrical coordinates. The major reasons are (i) the particle image
Velocimetry (PIV) measurement equipment gives data in Cartesian
coordinate system and (ii) transforming the velocity components to the
corresponding components in cylindrical coordinate is very difficult in
instantaneous velocity filed due to the lack of a clear and single defined
vortex core. In the current experiment only at z1, there exist some
instantaneous snapshots of the flow, among all snap shots for that particular
measurement, where a single vortex core can reasonably be observed. (iii) to
the knowledge of author, most of the major CFD codes are written for
Cartesian coordinates and the model validation requires the experimental
data to be in the same coordinate system.
The contour plots of velocity components in Cartesian coordinates can still
provide some good information about the corresponding components in
polar (cylindrical) coordinates. The only difference is to understand the
relation of u and v components to V and Vr because the w and Vz are
the same in both the coordinate system. In case of swirling flow, the
interpretation of Cartesian components in to polar components is easily
understood. Along X-axis, the u component represents Vr and v
component represents V . In case of Y-axis, it is vice versa. Figure 4.14 show
contour plots of u and v components and can thus be used as an example of
understanding the distribution of tangential and radial velocities from
contour plots of u and v components (signs denote the direction of a given
vector component).
The contours of u along Y-axis indicate that the tangential velocity increases
from cylinder axis to its peak value at a region r/R=0.17 (approximately) and
then decreases again towards large radial positions. Whereas along X-axis, the
value of u shows that radial velocity has a very low magnitude. The contour
of v gives similar information about the distribution of tangential and radial
velocity components but along opposite axis lines i.e. X-axis, as discussed
before. The distribution of fluctuating/ turbulent part of the individual
velocity components, for polar coordinates, can also be understood in the
same manner.
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Swirling Flow in a Pipe