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.