Flux enhancement by Shear Free Surfaces in Turbulent Convection

Experimental investigations into the effects of introducing solid inserts termed 'Shear Free Surfaces' (SFS) into an axially homogenous, buoyancy driven zero mean shear turbulent convection in a vertical pipe are reported. These SFSes attenuate the lateral momentum transports due to kinematic wall blocking and modify the large scales of the flow. There is a concurrent increase in the tangential transports. In this work the analogous effects of these SFSes on the scalar transports is studied. The control flow considered for this purpose is inherently shear-free, thus rendering any stationary surface in the flow as a SFS. The suppression of the lateral scalar transport in the presence of the SFS, leads to an increase in the axial transports. Preliminary evidence for this is seen from the enhancement of the salt flux across the pipe. The placement of cylindrical rod inserts acting as SFSes into the flow resulted in flux enhancements disproportionate to the area blocked. Flow field measurements using combined PIV-PLIF techniques revealed that the attenuation of the lateral transports by the SFS, results in reduced mixing which disrupts the decoherence of flow structures, ultimately leading to an increased axial transport and flux enhancement. This phenomenon manifests in the form of reduced lateral velocity fluctuations and increased axial velocity and scalar fluctuations, an increase in the axial spatio-temporal scales of the flow and buoyancy production and decrease of the lateral transports.