Fig.9 Normalized turbulent intensities distribution in all directions at
P3
The turbulent shear stresses for five observing regions were plotted in
Fig.10, from which we can see the whole picture describing the evolution
of turbulence field across the swirl chamber. A comparison of the
magnitude of each shear stress component indicates a decay tendency from
P1 to P5. For instance, the non-dimensional Reynolds stress u’v’ evolves
from 1 at P1, to around 0.4 at P2 and finally decreases to 0.04 at P5.
The decay of Reynolds stress is ascribed to the decay of the overall
swirl intensity resulting from wall friction[21] and also related to
the historical effect of the swirl flow development from the swirl vane
to the recovery vane. As discussed in the part of swirl flow structure,
the velocity distribution especially represented by the radial velocity
experiences drastic change from P1 to P3. Since the swirl vane disturbs
the incoming flow, a very stable swirl flow is not formed until P3. The
instability can also be observed by the air core, the part of which
between P1 and P3 oscillates more violently. Similar decay
characteristics can also be found for other two Reynolds shear stresses.
One more prominent feature of the shear stress is the anisotropic
behavior. Amounts of studies on the turbulence field of swirl flow shows
that the Reynolds stress tensor is anisotropic. As summarized by
Gomez[19], the two shear stress components with the tangential
velocity included is always larger than the third component. However,
this is not the case for the observing planes concerned herein. From
Fig.10 we can see that the magnitude of Reynolds stress component u’v’
denoting the radial and axial fluctuations and Reynolds stress component
u’w’ denoting the radial and tangential fluctuations are two times
higher than that of Reynolds stress component v’w’ at P1 and P2. After
that the u’w’ and v’w’ is always higher, about five times of the
component u’v’. The change of Reynolds stresses represented by the
anisotropic variation is closely related to the evolution of the
averaged velocity profiles, namely the changing of the swirl flow
structure. It can be simply interpreted that the radial fluctuating
component dominates the P1 and P2 section and the tangential fluctuating
component dominates the rest part. The complexity of the Reynolds shear
stress challenges the turbulence modeling and a full anisotropic
turbulence model like the RSM is needed for numerical
simulation[2,22]. Last but not the least is the distribution style
of the turbulence field distribution. at P1 and P2, the distribution of
each stress component presents a single peak pattern, implying almost
all the fluctuations is concentrated in the central zone, while the
pattern of double peak comes into being at P3, P4 and P5. It is also the
change of averaged velocity profiles that accounts for the pattern
variation. In the discussion of swirl flow structure part, we observed
from the tangential velocity profiles that an inner counter-rotating
vortex turns up from P3. A comparison of the double peak in space
indicates that the peak is just located at the boundaries of two
counter-rotating vortexes. Since at the boundary area, huge velocity
gradient is formed due to the change of the tangential velocity sign,
thus producing high level of Reynolds stresses.