Markus Uhlmann

Direct Numerical Simulation of Sediment Transport in Turbulent Open Channel Flow

Direct numerical simulations of interface-resolved sediment transport in horizontal open-channel flow are currently being performed on the XC-4000. The channel bottom boundary is roughened with a fixed layer of spheres and about 9000 particles are allowed to move within the computational domain. The density ratio of the solid and fluid phase is 1.7 and the bulk Reynolds number of the flow is 2880. In the present configuration, the particles tend to accumulate near the bed because of gravity, but due to the turbulent motions, a cycle of resuspension and deposition is produced. This leads to a particle concentration profile which decreases with the distance from the bed. The preliminary results show that the presence of particles strongly modifies the mean fluid velocity and turbulent fluctuation profiles. The dispersed phase lags the carrier phase on average across the whole channel height. Both observations confirm previous experimental evidence. The different observations suggest that particle inertia, finite-size and finite-Reynolds effects together with gravity play an important role in this flow configuration. Several potential mechanisms of turbulence-particle interaction are discussed.

AN APPROXIMATE RIEMANN SOLVER TO COMPUTE COMPRESSIBLE FLOWS USING SECOND-MOMENT CLOSURES

An approximate Riemann solver is presented in this contribution, which enables to compute turbulent compressible flows using an objective realisable second order closure. The main features of the continuous model are first recalled. An entropy inequality is exhibited, and the structure of waves associated with the non conservative hyperbolic convective system is briefly described. Using a linear path to connect states through shocks, a result of existence and uniqueness of the one dimensional Riemann problem is then given. This result enables to construct exact or approximate Riemann type solvers. An approximate Riemann solver, which is based on Gallouets recent proposal is eventually presented. Some computations of shock tube experiments are then discussed.

Travelling waves in a straight square duct

Isothermal, incompressible flow in a straight duct with square cross-section is known to be linearly stable [1]. Direct numerical simulation, on the other hand, has revealed that turbulence in this geometry is self-sustained above a Reynolds number value of approximately 1100, based on the bulk velocity and the duct half-width [2].

High-Resolution Numerical Analysis of Turbulent Flow in Straight Ducts with Rectangular Cross-Section

Turbulent secondary motion of straight open duct flows with a rectangular cross-section was studied by means of direct numerical simulations, and the unique mean flow patterns were analysed with the aid of instantaneous coherent structure analysis for their Reynolds number dependence. Similar to the closed duct counterparts, it was found that the mean streamwise vorticity pattern is the statistical footprint of the most probable locations of the quasi-streamwise vortices. Furthermore, the existence of tightly-concentrated vortices with preferable rotational directions inside the mixed corners formed by no-/free-slip boundaries was observed. Such flow structures correspond to the side-wall high-speed streaks located directly on the free-slip plane, independently of Reynolds number, as well as the following low-speed streaks reside approximately 50 wall units away from the free-slip plane for friction Reynolds number larger than 200.

Coherent Structures in Marginally Turbulent Square Duct Flow

Direct numerical simulation of fully developed turbulent flow in a straight square duct was performed in order to determine the minimal requirements for self-sustaining turbulence. It was found that turbulence can be maintained for values of the bulk Reynolds number above approximately 1100, corresponding to a friction-velocity-based Reynolds number of 80. The minimum value for the streamwise period of the computational domain measures around 190 wall units, roughly independently of the Reynolds number. Furthermore, we present a characterization of the marginal state, where coherent structures are found to have significant relevance to the appearance of secondary flow of Prandtl’s second kind.

Characterisation of Marginally Turbulent Square Duct Flow

We have performed direct numerical simulation of fully developed turbulent flow in a straight duct with square cross-section. The main objective of the present study is to determine the minimal requirements for maintaining turbulence in duct flows [1]. A detailed analysis of this limit regime allows to elucidate the dominant mechanisms governing the marginally turbulent state, where the self-sustaining coherent structures, i.e. streamwise vortices and streaks, have a cross-streamwise length scale comparable with the duct width. It is therefore expected that the coherent structures are of direct relevance to the appearance of secondary flow of Prandtl’s second kind.

Second Order Entropy Consistent Modelling of Turbulent Compressible Flows

We examine herein the suitability of some second order closures to describe turbulent compressible flows with shocks, applying the standard Favre averaging technique. The basic set of equations reads: n n

Investigating turbulent particulate channel flow with interface-resolved DNS

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