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Schlieren study of circular and square cylinder wakes: Effect of buoyancy and

oscillation

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Experimental apparatus

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Photographs

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Flow geometry

Oscillation geometry

Without Circular Square Circular Square load inline inline transverse transverse

Calibration of actuators

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Influence of Buoyancy

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Suppression of vortex shedding (Circular cylinder)

Re = 110

Flow

Cylinder

Tw = 40oC 60oC 75oC 77oC 79oC 80oC 82oC

40oC : Vortex shedding from opposed shear layers.

60oC and 75oC : More distinct Shedding. Fringes inside the vortices.

77oC : Detached shear layer is elongated like slenders.

79oC and 80oC : Thin plume slowly oscillates in transverse direction.

82oC : Plume becomes steady depicting suppression.

T∞ =21oC

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Tw = 40oC 45oC 55oC 58oC 60oC 63oC 70oC

Suppression of vortex shedding (Square cylinder)T∞ =23oC

Cylinder

Flow

Re = 109

40oC and 45oC : Regular Vortex shedding.

55oC : Distinct vortex Shedding with more no. of fringes in the wake.

58oC : Fringes inside the vortices means temperature distribution.

60 oC : Detached shear layer is more elongated.

63oC : Mild unsteadiness of the shear layer.

70oC : Single steady plume at the centre of the cylinder.

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Influence of buoyancyand

transverse oscillation

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Regular alternate vortex shedding at lower Ri. With increase in Ri, length of heated zone increases with higher number of interference fringes and finally at critical Ri, the wake degenerates into a thin elongated steady plume at the centre of the cylinder depicting suppression of vortex shedding.

One vortex is alternatively shed from each shear layer in one oscillation cycle. Inclination of the vortices with wake centreline is greater resulting in enhanced interactions compared to the stationary cylinder. At higher Ri, large size vortices are formed. At critical Ri, the vortex shedding reappears for the oscillating cylinder.

Tw = 37 oC 51 oC 68 oC 76 oC

Tw = 35 oC 55 oC 68 oC 86 oC

a/d=0.08

Circular cylinder

StationaryCylinder

Effect of excitation frequencySquare cylinder

(fe/fs=0)

Fundamental

oscillation (fe/fs=1)

T∞ =24oC T∞ =25oCRe = 105

Re = 116

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Sub-harmonicoscillation

Tw = 37 oC 51 oC 68 oC 76 oC

Tw = 35 oC 55 oC 68oC 86 oC

a/d=0.08

(fe/fs=0.5)

Non-harmonicoscillation

(fe/fs=1.5)

Circular cylinder

T∞ =24oCRe = 105

Square cylinder

T∞ =25oCRe = 116

Effect of excitation frequency

Two vortices from each shear layer are shed in one oscillation cycle except at critical Ri for both the cylinders. At critical Ri, only one vortex from each side shear layer is shed in one oscillation cycle.

At lowest Ri, two vortices from each shear layer are shed for every three oscillation cycles for the circular cylinder and one vortex from each shear layer is shed in one oscillation cycle for the square cylinder. With increase in Ri, vortex formation is periodic with fe. However, shed vortices initially show irregularity and loss of coherency.

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Tw = 37 oC 51 oC 68 oC 76 oC

Tw = 35 oC 55 oC 68 oC 86oC

a/d=0.08

Super-harmonicoscillation

(fe/fs = 2)

Super-harmonicoscillation (fe/fs = 3)

Square cylinder

T∞ =25oCRe = 116

Circular cylinder

T∞ =24oCRe = 105

effect of excitation frequency

At lowest Ri, one vortex from each shear layer is shed for every two oscillation cycles for the circular cylinder and one vortex from each shear layer is shed in one oscillation cycle for the square cylinder. With increase in Ri, vortex formation is periodic with fe. However, shed vortices show small intermittency which disappears at the highest Ri.

At lowest Ri, one vortex from each shear layer is shed for every three oscillation cycles for the circular cylinder and one vortex from each shear layer is shed in one oscillation cycle for the square cylinder. With increase in Ri, vortex formation is periodic with fe. Much difference in the shape of the vortices are seen between the two cylinders.

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Influence of buoyancyand

Inline oscillations

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fe/fs=1

Unperturbed

flow

Regular vortex shedding at lower Ri and at elevated temperature the wake degenerates into a steady plume.

Mode switching at lower Ri (forced convection regime). Symmetric vortex shedding at higher Ri. Steady plume transformed into symmetric vortex structures at suppression.

fe/fs=1.5

Shedding is alternate but size, shape and period of vortex structures are different than that of a stationary cylinder. Two vortices are shed during each cycle of cylinder oscillation.

Effects of oscillation frequency

The alternate vortex shedding is transformed into symmetric vortex shedding for all the Richardson number considered.

Tw = 34 oC 45 oC 63 oC 77 oC

fe/fs=2

Tw = 34 oC 45 oC 63 oC 77 oC

Antisymmetric Symmetric

Circular cylinder

Re=104

a/d=0.08

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