Hochaufgelöste numerische Simulation von turbulenzinduzierten Sekundärströmungen in offenen Kanälen mit Seitenwänden
Zusammenfassung der Projektergebnisse
Fluid flow in a straight duct with rectangular cross-section exhibits turbulence-induced secondary motion of small amplitude, but with large consequences for momentum, heat and mass transport. The corresponding open duct flow (featuring a free surface – here modelled through a symmetry plane) is characterized by a distinct secondary flow pattern, leading to such practically important effects as the so-called “dip phenomenon” (referring to the fact that the maximum of the mean streamwise velocity is not found at the fluid surface). In the present project we have investigated the mechanism of secondary flow formation in open channel flow, where rigid/rigid and mixed (rigid/free-surface) corners exist. High-fidelity flow data has been generated by means of pseudo-spectral direct numerical simulation over a parameter range with bulk Reynolds numbers up to 7000 and aspect ratios up to 8. The data has been analyzed with particular emphasis on scaling aspects. We were able to confirm the scaling with bulk flow scales of the intensity of the secondary motion in open duct flow, whereas we have found that the same quantity in the closed duct follows a mixed scaling. The origin of this result could be traced back to the respective scaling of the secondary Reynolds stress. Varying the geometrical aspect ratio has allowed us to determine a maximum aspect ratio for the dip phenomenon to occur, and to analyze the decay with distance from the side-walls of the secondary flow intensity. Through extensive coherent structure eduction we have established that the secondary motion in the open duct configuration is a statistical footprint of the turbulent dynamics of these structures, analogous to closed duct flow. However, the specific secondary flow pattern in the open duct is very distinct, featuring one persistent streamwise vortex in the vicinity of each mixed corner. The existence of these inner secondary flow cells has been linked to the self-induced motion of an elongated vortex with its axis oriented parallel to a symmetry plane. Furthermore, detailed vortex tracking in time has allowed us to investigate many aspects of the dynamics of the coherent structures in open duct flow, such as their lifetime distribution as a function of position in the cross-plane. It came as quite a surprise to find that the intensity of the secondary motion in the closed duct geometry does not scale with bulk velocity as initially believed. It was a also unexpected that we had to spend much more time than initially planned on the development of efficiency aspects of the numerical method.
Projektbezogene Publikationen (Auswahl)
- High-resolution numerical analysis of turbulent flow in straight open ducts with rectangular cross-section. Proc. 15th EUROMECH Eur. Turb. Conf., Delft, The Netherlands, 2015
Y. Sakai, M. Uhlmann, and G. Kawahara
- Coherent structures and secondary motions in open duct flows. PhD Thesis, Karlsruhe Institute of Technology, 2016
Y. Sakai
(Siehe online unter https://dx.doi.org/10.5445/IR/1000061617) - High-resolution numerical analysis of turbulent flow in straight ducts with rectangular cross-section. In W.E. Nagel, D.B. Kröner, and M.M. Resch, editors, High Performance Computing in Science and Engineering ’15, Transactions of the High Performance Computing Center, Stuttgart (HLRS), pages 301–313. Springer, 2016
Y. Sakai and M. Uhlmann
(Siehe online unter https://dx.doi.org/10.1007/978-3-319-24633-8 20)
