Mechanical Engineering - Theses
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ItemThe quiescent core of turbulent channel and pipe flows( 2016)A new conceptual view of turbulent channel and pipe flows is presented via the proposition of the `quiescent core', which is analogous to the free-stream in turbulent boundary layers. The quiescent core is detected as a zone of roughly uniform streamwise momentum, which happens to be relatively quiescent compared to the rest of flow, residing in the central region of channel and pipe flows. It occupies a large proportion of the flow and oscillates with the large-scale wavelengths in a predominantly anti-symmetric manner. Within the intermittent region of wall-turbulence, it is observed that the oscillation of the turbulent/non-turbulent interface or quiescent core can contaminate the fluctuating velocity components under the traditional Reynolds decomposition. A new method of decomposing the total velocity is proposed to remove this contamination. The use of this new decomposition method, along with the zone-averaging technique, enables the examination of `true' turbulent structure and scale purely inside the `turbulent shear flow' region (below the free-stream or outside the quiescent core) of both internal (channel and pipe) and external (boundary layer) flows. The results are compared in internal and external flows to reveal that the structure and scale of turbulence in those flows are indeed much more similar than previous studies have concluded. It is shown that the geometry of the pipe core can be well-represented by a few dominant azimuthal Fourier modes. The dominant azimuthal modes of the pipe core are associated with streamwise streaks and roll-modes in axisymmetric arrangements and they often maintain a high degree of spatial coherence along the streamwise direction (for over a pipe radius). The investigation of temporal progression of the pipe core reveals that it simply convects downstream without azimuthal rotation and rapid evolution. The most energetic modes of channel flow are extracted and investigated by means of proper orthogonal decomposition. It is observed that the large-scale wall-normal eigenfunctions appear in pairs which carry comparable amounts of turbulent kinetic energy and Reynolds shear stress. The turbulent kinetic energy and Reynolds shear stress are mostly concentrated in the large-scale modes (with the first 20 modes carrying about a half of these quantities). The most energetic modes represent the large-scale inclined flow structures with symmetric or anti-symmetric arrangements between the top and bottom channel walls, which can efficiently replicate the associated large-scale geometry of the channel core.