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Bernard J. Geurts
Multiscale Modeling and Simulation, J.M. Burgers Center, Faculty EEMCS, University of Twente, 7500 AE Enschede, The Netherlands Anisotropic Turbulence, Fluid Dynamics Laboratory, Department of Applied Physics, Eindhoven University of Technology

Rudie Kunnen
Fluid Dynamics Laboratory, Faculty of Applied Physics Eindhoven University of Technology P.O Box 513, 5600 MB Eindhoven, The Netherlands


We present results of direct numerical simulation of turbulent flow in a rotating cylindrical Rayleigh-Benard system of unit aspect ratio in which the rotation frequency is modulated in time. Associated with the time-dependent rotation rate the Euler force arises in addition to the Coriolis and centrifugal forces in the governing equations. This Euler force acts in the circumferential direction and has a marked effect on the turbulent flow structures and the associated heat transfer efficiency. We observe a regime with strong Euler forces in case the modulation of the rotation rate is sufficiently rapid. Sheared velocity structures dominate the vertical boundary layers near the cylinder walls, while a strong thermal column of a characteristic size arises in the core of the system, along the axis of rotation. The Nusselt number is observed to become rapidly oscillating at frequencies associated with the modulation frequency. Compared to the case of constant rotation rate the Nusselt number is increased considerably. The heat transfer displays a striking dynamics; after relatively long periods of strongly increasing Nusselt number, events of sudden collapse of the thermal structure in the core of the system occur. A detailed illustration of such a collapse in the Nusselt number shows the almost complete disappearance of the thermal column and associated reduction of the capacity to transfer heat in the system.