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.
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ItemExperiments in smooth wall turbulent channel and pipe flows( 2011)An experimental investigation of smooth wall, fully developed turbulence was undertaken in a high aspect ratio channel flow facility and a circular pipe flow facility. High fidelity hot-wire measurements of streamwise velocity with carefully matched experimental and anemometer parameters were made in order to facilitate the direct comparison of the two internal flows over a range of Reynolds numbers. Measurements where the inner scaled hot-wire sensor length was held constant at 22 with length-to-diameter ratio greater than 200 revealed that channels and pipes have similar streamwise mean velocity distributions in the near wall and logarithmic regions, with pipe flow mean velocity profiles showing a larger deviation from the log law in the outer region compared to the channel flow. The turbulence intensity profiles were found to match well throughout the flow, and importantly, the near wall peak in turbulence intensity was shown to increase with friction Reynolds numbers up to 3000. Further, the centreline turbulence intensity in both flows was also shown to increase with increasing Reynolds number in a manner consistent with data presented in the literature. The one-dimensional pre-multiplied energy spectra revealed that the near wall peak in turbulence intensity grows due to the increasing energy contribution of the large scales of turbulent motion, however, whilst the spectral energy distribution is similar in channels and pipe for the near wall and overlap regions, the channel flow maintains energy at longer wavelengths than the pipe flow at the centreline, indicating that there are some subtle yet important difference between the core regions of these two flows. Measurements where hot-wire sensor length and aspect ratio were systematically varied reveal that increasing sensor lengths lead to attenuation of an increasingly large range of small scales, whereas an insufficient aspect ratio causes the measured energy to be attenuated over a very broad range of scales. It was demonstrated that the competing effects of spatial filtering and increasing Reynolds number may be the cause for the conflicting reports regarding the Reynolds number dependency of the near wall peak in turbulence intensity. Spatial filtering effects were found to be no different in channels and pipes for the range of sensor lengths and Reynolds numbers investigated. While the effect of spatial filtering diminished when moving away from the wall such that little to no attenuation was observed in the outer region, an insufficient sensor aspect ratio appeared to attenuate energy over a broad range of scales in both the inner and outer regions of the flow. It was determined that a sensor aspect ratio greater than 200 is sufficient for the pure platinum hot-wires used in this study which is consistent with the findings of Ligrani & Bradshaw (1987). In a further set of experiments, an array of multiple wall skin friction sensors was simultaneously employed with a wall normal traversing hot-wire to capture the ‘footprint’ of large scale motions in a turbulent channel flow. These experiments were carried out at a friction Reynolds number of 1000. Conditional averaging revealed that in the presence of low skin friction, there exists an elongated low speed region flanked by high speed regions consistent with `Superstructure' type events. The turbulence intensity within these low speed regions was modulated, with increased activity in the presence of local relative streamwise deceleration and decreased activity in the presence of local relative streamwise acceleration. These observations are consistent with the large scale counter rotating roll modes proposed by Hutchins & Marusic (2007b) and fit both the hairpin packet model of Adrian et al. (2000) as well as the cluster-wake model of del Alamo et al. (2006).