Fig. 1 Unstructured meshes used in the simulation

Fig. 2 Sketch of experimental aparatus and model arrangement .

Fig. 3 Comparisons between numerical and experimental results of the wakes without and with a strip, Re=1 200, A/D=1.25, f_e D/V_∞=0.22 ||||(a) numerical vorticity field, without element;(b) experimental flow field, without element; (c) numerical vorticity field, the strip at X/D=0.8,Y/D=1.2; (d) experimental flow field, the strip at X/D=0.8,Y/D=1.2; (e) numerical vorticity field, the strip at X/D=1.0,Y/D=1.2; (f) experimental flow field, the strip at X/D=1.0,Y/D=1.2

Fig.4 Comparisons between experimental and numerical spectra of fluctuating velocities in the wakes without and with a strip element.||||Re=1 200, A/D=1.25, f_e D/V_∞=0.22 (a) Numerical spectrum, no strip; (b) Experimental spectrum, no strip; (c)Experimental spectrum, the strip at X/D=0.8,Y/D=1.2; (d) Numerical spectrum, the strip at X/D=0.8,Y/D=1.2

Fig. 5 Various modes of vortex shedding in the wake with an element on the lower side of the wake centerline, Re=1 200, A/D=1.25, f_e D/V_∞=0.22, b/D=0.32 ||||(a) P+S mode, upper side vortex-pair and lower side anti-clockwise single vortex, (b) P+S mode, upper side clockwise single vortex and lower side vortex-pair, (c) 2S mode, (d) P mode, (e) 2S+2S mode, (f) 2P mode, (g) P+T mode, (h) S+T mode, (i) 3S mode, (j) P+S+P mode

Fig. 6 The influence of single element：zones of vortex shedding modes on the plane of element position, Re=1 200, A/D=1.25, f_e D/V_∞=0.22, element width b/D=0.32

Fig. 7 Maximum spectrum value of fluctuating velocity v.s sampling position y/D on line x/D=4.0, Re=1 200, A/D=1.25, f_e D/V_∞=0.22,b/D=0.32

Fig. 8 Contours of peak spectrum ratio P/P_0 on the plane of element position (X/D,Y/D), where P and P_0 are respectively the maximum values in 8-points averaged power spectrum of fluctuating velocity in the wakes with and without an element, Re=1 200, A/D=1.25, f_e D/V_∞=0.22,b/D=0.32

Fig. 9 Changes of the characteristic wake width with element position Y/D,Re=1 200, A/D=1.25, f_e D/V_∞=0.22,b/D=0.32

Fig. 10 Sketch of 2P mode vortex shedding generation in the wake without strip element ||||(a) Clock-wise vortex V_A on the upper side, (b) Anti-clock-wise vortex V_B on the upper side, (c) Anti-clock-wise vortex V_D on the lower side, (d) Clock-wise vortex V_E on the lower side

Fig. 11 Sketch of P+S+P mode vortex shedding generation in the strip disturbed wake ||||(a) The single vortex S=V_C in the middle, (b) Anti-clockwise vortex V_D on the lower side, (c) Clockwise vortex V_E on the lower side, (d) Generation of the vortices in a cycle

Fig. 12 Sketch of S+T mode vortex generation in the strip disturbed wake ||||(a) The anti-clock-wise vortex V_B and V_D on the lower side, (b) The clock-wise vortex V_E on the lower side, (c) The clock-wise vortex V_A on the upper side and the earlier formed vortices on the lower side

Fig. 13 Generation of the vortices of mode 2S+2S in the strip disturbed wake ||||(a) Lower side clockwise vortex, (b) Lower side anti-clockwise vortex, (c) Upper side clockwise vortex, (d) Upper side anti-clockwise vortex

Fig. 14 The formation of P-mode vortex shedding on the upper side

Fig.15 The effect of the strip on 2S mode vortex generation ||||(a) The strip blocks the downstream movement of anti-clock-wise vortex VD, and the induction of VD delays the formation of vortex VE, (b) The formation of 2S mode vortex shedding on the upper side, and the vanishing of vortex VD on the lower side

Fig. 16 The strip effect on P+S mode vortex generation ||||(a) Stream-wise shear flow enhances the anti-clock-wise vortex and weakens the clock-wise vortex on the lower side, (b) The formation of the vortex-pair on the upper side and the formation of single vortex on the lower side