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

Schlieren study of circular and square cylinder wakes: Effect of buoyancy and oscillation. 1. Experimental apparatus. 2. Photographs. 3. Calibration of actuators. Flow geometry. Without Circular Square Circular Square

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

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  1. Schlieren study of circular and square cylinder wakes: Effect of buoyancy and oscillation 1

  2. Experimental apparatus 2

  3. Photographs 3

  4. Calibration of actuators Flow geometry Without Circular Square Circular Square load inline inline transverse transverse Oscillation geometry 4

  5. Influence of Buoyancy 5

  6. Suppression of vortex shedding (Circular cylinder) T∞ =21oC 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. Cylinder Flow Re = 110 6

  7. Suppression of vortex shedding (Square cylinder) T∞ =23oC Tw = 40oC 45oC 55oC 58oC 60oC 63oC 70oC 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. Cylinder Flow Re = 109 7

  8. Influence of buoyancy and transverse oscillation 8

  9. Effect of excitation frequency Circular cylinder Re = 105 T∞ =24oC Square cylinder Re = 116 T∞ =25oC Tw = 37oC 51 oC 68 oC 76 oC Tw = 35oC 55 oC 68 oC 86 oC Stationary Cylinder (fe/fs=0) 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. a/d=0.08 Fundamental oscillation (fe/fs=1) 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. 9

  10. Effect of excitation frequency Circular cylinder Re = 105 T∞ =24oC Square cylinder Re = 116 T∞ =25oC Tw = 37oC 51 oC 68 oC 76 oC Tw = 35oC 55 oC 68oC 86 oC a/d=0.08 Sub-harmonic oscillation (fe/fs=0.5) 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. Non-harmonic oscillation (fe/fs=1.5) 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. 10

  11. effect of excitation frequency Circular cylinder Re = 105 T∞ =24oC Square cylinder Re = 116 T∞ =25oC Tw = 37oC 51 oC 68 oC 76 oC Tw = 35oC 55 oC 68 oC 86oC a/d=0.08 Super-harmonic oscillation (fe/fs = 2) 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. Super-harmonic oscillation (fe/fs = 3) 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. 11

  12. Influence of buoyancy and Inline oscillations 12

  13. Effects of oscillation frequency Tw = 34 oC 45 oC 63 oC 77 oC Circular cylinder Tw = 34 oC 45 oC 63 oC 77 oC Re=104 a/d=0.08 Unperturbed flow fe/fs=1 Antisymmetric Symmetric 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 fe/fs=2 The alternate vortex shedding is transformed into symmetric vortex shedding for all the Richardson number considered. 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. 13

  14. THANK YOU 14

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