Cylinder In Proximity Research Articles

Influence of Buoyancy on Vortex Shedding and Heat Transfer from a Square Cylinder in Proximity to a Wall

The flow and heat transfer past a heated cylinder of square cross section mounted horizontally above a plane wall and subjected to a uniform shear flow is considered. The flow field is considered for a moderate range of Reynolds number (based on the incident stream at the cylinder upstream face and the height of the cylinder) and Grashof number at cylinder-to-wall gap height 0.5 times the cylinder height. The flow field and heat transfer are computed through a pressure-correction-based iterative algorithm with the QUICK scheme in convective terms. The code is tested for accuracy by comparing with published results for certain values of the flow parameters. We examine the combined effects of buoyancy and shear flow on the vortex shedding behind the cylinder in close proximity to a plane wall. A boundary layer develops along the wall, and this interacts with the shear layer formed along the two sides of the cylinder. The role of the thermally induced baroclinic vorticity production term on the vortex shedding is examined. Our results show that the breakdown of vortex shedding takes place beyond certain values of Richardson number (which depends on the Reynolds number). The dependence of the average rate of heat transfer from the surface of the cylinder on Reynolds number and Grashof number is also investigated.

An examination of the effect of boundary layer thickness on vortex shedding from a square cylinder near a wall

Abstract A computational study has been undertaken to examine the effect of boundary layer thickness δ/D on vortex shedding from a square cylinder in proximity to a solid wall. The computations were carried out in a second-moment turbulence modeling framework using a finite-volume technique. The computed results show that, in general, thickening of the wall boundary layer causes wake periodicity to persist for increasingly smaller cylinder-to-wall gap widths, S/D. The nature of the periodic motion changes as S/D approaches the critical value for complete suppression of vortex shedding. Similar to experimental observations, the location at which coupled shear layer motion is first observed shifts downstream of the base region. This shift is characterized by a rise in the shedding frequency and a drop in the time-averaged drag and lift on the cylinder. In addition, the pressure distribution along the lower wall is seen to change significantly due to the reduced size of the recirculation region in the cylinder wake.

A Comparison of Second-Moment Closure Models in the Prediction of Vortex Shedding From a Square Cylinder Near a Wall

A computational study is presented that examines the capability of various second-moment closure models in the prediction of two-dimensional, nonstationary flow around a square cylinder in proximity to a wall. The linear return-to-isotropy/isotropization-of-production model RTI+IP and the nonlinear SSG pressure-strain models were combined with the DH and modified LUM diffusion models in the computations. In terms of global activity, the drag is well-predicted in terms of both magnitude and variation with cylinder-to-wall gap width S/D. The Strouhal number St was reasonably well-predicted in terms of magnitude, but the predicted trend with decreasing S/D was incorrect for all model combinations. The lift was not well-predicted in terms of magnitude or trend. Prediction of the detailed flow structure in the vicinity of the cylinder and in the wake was favourable, though the magnitudes of some velocity and Reynolds-stress components were over-predicted. It was argued that the large differences between the results at the intermediate gap width may be due to the difference between the measured and predicted critical gap widths. On the basis of the predicted global and detailed activity, the modified LUM model combined with the nonlinear SSG model was suggested as being the most viable combination for future studies.

Quantum-coherent phenomena in mesoscopic proximity structures

We have investigated the magnetic response of a Ag cylinder in proximity with Nb at 300 μK≤T≤9K and in the very clean limit. Below a temperature at which the proximity decay length ξN=ℏvF/2πkBT becomes of the order of the perimeterL (σN∼-0.2L), we observe a reentrance of the induced diamagnetic susceptibility χd. The reentrant susceptibility χr follows χr=Aexp(-T/T* withT*=ℏvF/2πkBL. AtT=T*, where σN=L, the logarithmic slope of χr becomes steeper by a factor of 2 so that forT<T*, a new, stronger mesoscopic regime is reached with χr=Bexp(-2T/T*).

Societies and Academies

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