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DIRECT NUMERICAL SIMULATIONS OF A PASSIVE SCALAR IN A TURBULENT CHANNEL WITH LOCAL FORCING AT WALLS

Guillermo Araya
Department of Mechanical, Aeronautical and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY, 12180, US

Stefano Leonardi
The University of Texas at Dallas

Luciano Castillo
Department of Mechanical Engineering, Aeronautical Engineering & Mechanics Rensselaer Polytechnic Institute Troy, NY 12180 USA; Dept. Mech. Eng. Texas Tech University Lubbock, TX 79409-1021, USA

Abstract

The influence of local forcing on a turbulent channel flow is numerically investigated. The high level of information provided by Direct Numerical Simulations (DNS) allows a better understanding of the physical mechanism responsible for local drag reduction and heat transfer enhancement. The molecular Prandtl number is 0.71 and the Reynolds number based on the wall friction velocity and the channel half width is 394 for the unforced case. A phase-averaging procedure is employed to discriminate between coherent and incoherent structures. It is shown that coherent thermal fluctuations reach peak values near the forcing slot, sharply decay and almost disappear in a short distance downstream. The incoherent thermal fluctuations also show peak values next to the source; however, they decay downstream to look similar as incoherent structures of the unperturbed channel. Budgets for the passive scalar variance and turbulent heat fluxes at a dimensionless forcing frequency of 0.64 are compared with the unforced case budgets. In general, it is observed an increase of all terms when the flow is locally perturbed, especially, in a zone very close to the wall, namely o<y+<70 Local increases on molecular heat fluxes up to 50% are accomplished at a dimensionless frequency of 0.64. Furthermore, wall-normal turbulent heat fluxes experience a significant augmentation (~21%) at this forcing frequency.