Direct numerical simulations (DNS) have been performed for turbulent pipe flows using a high-order spectral element method code Nek5000. This is one of the first DNS studies of spatially developing turbulent pipe ow with a 90° bend. A turbulent inflow condition was implemented using a recycling technique. To study the unsteady oscillations of the Dean vortices, i.e., swirl switching phenomenon, the pressure force acting on the pipe wall around the bend is analysed. The lateral pressure force exhibits spatio-temporal oscillations. Conditional averaging is performed for large (positive/negative) force oscillation events to find the flow structure responsible for the force oscillation. It is found that the quasi-periodic force oscillation is closely associated with the swirl switching phenomenon. It is also observed that the mass flow rate oscillates with the swirl switching, and the frequency of this oscillation is found to be about St 0:5. 3D conditional averaging analysis reveals that the travelling wave-like three-dimensional ow structures are responsible for the spatio-temporal oscillations of the pressure force. Effects of the bend curvature ratio and Reynolds number on the flow recovery downstream of the bend and the swirl switching phenomenon are investigated. Both mild and sharp pipe bends were considered. Clear dependence on the curvature ratio are observed for various key flow properties, including pressure drop, friction factor, mean velocity profile, velocity fluctuations, etc. It is found that turbulence recovers faster downstream of a sharper bend. Conditional averaging for different curvature ratios shows that 90° bend pipe flow features an underlying travelling wave-like characteristic. The swirl switching frequency is observed to be higher when flow separation is present, whereas it appears to be not affected by the Reynolds number. The first DNS has been performed for a temporally accelerating turbulent pipe flow. Transient behaviour of turbulence statistics is analysed through ensemble averaging. The effect of acceleration rate on the response of turbulence is also investigated. The wall shear stress exhibits a distinctive four-stage development in the transient pipe flow. The pipe core region is found to be frozen during the initial transient stages. Turbulence starts to respond in the near-wall region first, and then propagates radially towards the pipe centre. The propagation speed is observed to be very similar among the three velocity components, and it is largely independent of the acceleration rate. During acceleration, the streamwise velocity component always responds first, followed by the other two components with a longer delay.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:737747 |
| Date | January 2017 |
| Creators | Wang, Zhixin |
| Publisher | University of Warwick |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://wrap.warwick.ac.uk/100281/ |
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