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Axially homogeneous buoyancy driven turbulence – free convection analog of fully developed turbulent pipe flow

Event Type: 
Date and Time: 
Friday, August 12, 2016 - 16:15
CTR Conference Room 103
Event Sponsor: 
Parviz Moin, Director of Center for Turbulence Research
Professor Jaywant H. Arakeri, Indian Institute of Science, Bangalore

Turbulent convection driven by an unstable density difference created across the ends of long vertical tube has many interesting features. Away from the two tube ends, the flow, driven by a linear density gradient, is axially homogeneous. The mean velocities, and the mean Reynolds shear stresses are zero; thus the turbulent production due to shear is zero and is solely due to buoyancy. Near the tube walls, we have shear free turbulence, but which does not decay with time. This flow may be considered to be a free convection analog of pressure driven pipe flow. Both are axially homogeneous, one is driven by a linear density gradient and the other by a linear pressure gradient.  Tube convection (TC) has several features, which make it different from the extensively studied Rayleigh-Benard convection (RBC). The scaling for RBC predicted by Kraichnan (1962) for the Nusselt number, Nu~Ra1/2 for very high Rayleigh numbers (Ra) and constant Prandtl number (also known as the ‘ultimate regime’), which has not been observed so far in experiments, is easily achieved in tube convection (TC) at relatively lower Ra. This scaling implies that the buoyancy flux is independent of viscosity and thermal diffusivity. Also, compared to RBC, orders of magnitude higher fluxes and turbulence Reynolds numbers are obtained in TC.  Results from two types of experiments will be presented. The density difference in one set of experiments is created by brine and fresh water, and in another set by using heat. Besides the Nu~Ra1/2 scaling, we show that below a critical value of Grashof number a different scaling, Nu~Ra0.29, is observed at which is similar to the scaling observed in turbulent RBC. The scalar spectra are found to follow the Bolgiano-Obukhov (BO) scaling while the energy spectra of lateral and longitudinal velocity show Kolmogorov-Obukhov (KO) scaling.  Also presented are some results from experiments of light propagation through the convective turbulence. Light propagation through convective turbulence is encountered in many situations, one example being stellar scintillation.

Professor Jaywant H. Arakeri has been a faculty member of the India Institute of Science since 1988. He is currently a Professor in Mechanical Engineering, and Centre for Product Design and Manufacture. His research interests are in Fluid Mechanics and Heat Transfer, in particular stability, transition and turbulence, unsteady flows, flows with curvature, turbulent natural convection, and fluid mechanical phenomena associated with plants. His group has extensively studied the near-wall dynamics in turbulent Rayleigh-Benard convection and the axially homogeneous turbulent convection in a vertical tube. Studies of unsteady flows include instability, transition to turbulence and boundary-layer separation in decelerating flows, and pulsatile flows in highly curved tubes. The work related to plants is concerned with heat and moisture loss from leaves and from soil like porous media. Professor Arakeri received his Btech from IIT, Madras; ME from IISc, Bangalore; and PhD from Caltech, Pasadena, all in Aeronautical engineering.