Unsteady Turbulence in a Shock: Physical and Numerical Modelling in Tidal Bores and Hydraulic Jumps

A turbulent flow is characterised by an unpredictable behaviour, a broad spectrum of length and time scales, and its strong mixing properties. Turbulent flows have a great mixing potential involving a wide range of vortical length scales. In steady flows, the turbulence measurements must be conducted at high frequency to resolve the small eddies and the viscous dissipation process. They must also be performed over a period significantly larger than the characteristic time of the largest vortical structures. In a highly unsteady flow, such as a shock, the experimental technique must be adapted, and this is detailed herein for positive surges, tidal bores and hydraulic jumps. A review of detailed turbulence measurements in tidal bores is conducted, and a number of laboratory experimental techniques are compared together with twoand three-dimensional large eddy simulation (LES) calculations. The experimental results demonstrate that the propagation of tidal bores induces some substantial turbulent mixing in natural estuaries. The passage of a tidal bore is associated with some large water depth fluctuations. Both the instantaneous and ensemble-averaged turbulent velocity data highlight some seminal features of the flow field in tidal bores. The instantaneous velocity measurements and the numerical data show a marked effect of the tidal bore front passage. The streamwise • Corresponding author: Email: h. chanson@uq. edu. au Url: http://www. uq. edu. au/-e2hchans/, Ph. : + 61 7 3365 3516Fax: +61 7 3365 4599 114 Hubert Chanson, Pierre Lubin and Stephane Glockner velocities are always characterised by a rapid flow deceleration at all vertical elevations, and large fluctuations of all velocity components are recorded beneath the surge and whelps. Both physical and numerical studies document the production of large coherent structures in tidal bores. The existence of such energetic turbulent events beneath and shortly after the tidal bore front implies the generation of vorticity during the bore propagation. Some experimental results show further that the variable interval time averaged (VITA) data based upon a single run present some non-negligible differences with the ensemble-averaged (EA) median results in terms of all velocity components. Both the EA and VITA methods showed some comparable long-term trends superposed to some rapid turbulent fluctuations, as well as close results in terms of the turbulent Reynolds stress components.