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

2 Experimental apparatus

3 Photographs

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

5 Influence of Buoyancy

6 Suppression of vortex shedding (Circular cylinder) Re = 110 Flow Cylinder T w = 40 o C 60 o C 75 o C 77 o C 79 o C 80 o C 82 o C 40 o C : Vortex shedding from opposed shear layers. 60 o C and 75 o C : More distinct Shedding. Fringes inside the vortices. 77 o C : Detached shear layer is elongated like slenders. 79 o C and 80 o C : Thin plume slowly oscillates in transverse direction. 82 o C : Plume becomes steady depicting suppression. T ∞ =21 o C

7 T w = 40 o C 45 o C 55 o C 58 o C 60 o C 63 o C 70 o C Suppression of vortex shedding (Square cylinder) T ∞ =23 o C Cylinder Flow Re = o C and 45 o C : Regular Vortex shedding. 55 o C : Distinct vortex Shedding with more no. of fringes in the wake. 58 o C : Fringes inside the vortices means temperature distribution. 60 o C : Detached shear layer is more elongated. 63 o C : Mild unsteadiness of the shear layer. 70 o C : Single steady plume at the centre of the cylinder.

8 Influence of buoyancy and transverse oscillation

9 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. T w = 37 o C 51 o C 68 o C 76 o C T w = 35 o C 55 o C 68 o C 86 o C a/d=0.08 Circular cylinder Stationary Cylinder Effect of excitation frequency Square cylinder (f e /f s =0) Fundamental oscillation (f e /f s =1) T ∞ =24 o CT ∞ =25 o CRe = 105Re = 116

10 Sub-harmonic oscillation T w = 37 o C 51 o C 68 o C 76 o C T w = 35 o C 55 o C 68 o C 86 o C a/d=0.08 (f e /f s =0.5) Non-harmonic oscillation (f e /f s =1.5) Circular cylinderT ∞ =24 o CRe = 105Square cylinderT ∞ =25 o CRe = 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 f e. However, shed vortices initially show irregularity and loss of coherency.

11 T w = 37 o C 51 o C 68 o C 76 o C T w = 35 o C 55 o C 68 o C 86 o C a/d=0.08 Super-harmonic oscillation (f e /f s = 2) Super-harmonic oscillation (f e /f s = 3) Square cylinderT ∞ =25 o CRe = 116Circular cylinderT ∞ =24 o CRe = 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 f e. 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 f e. Much difference in the shape of the vortices are seen between the two cylinders.

12 Influence of buoyancy and Inline oscillations

13 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. T w = 34 o C 45 o C 63 o C 77 o C fe/fs=2 T w = 34 o C 45 o C 63 o C 77 o C Antisymmetric Symmetric Circular cylinder Re=104 a/d=0.08

14 THANK YOU