Study Of Wake Characteristics And Coherent Structure With The Addition Of Dimples And Bumps To Cylindrical Surfaces

The marine riser is the key equipment connecting the floating platform with the seabed wellhead,and the vortex induced vibration(VIV)is the main cause of its fatigue damage,which contains complex flow phenomena such as boundary layer separation and vortex shedding.In this paper,the effects of adding covering(dimpled and raised)on the velocity fluctuation,turbulent energy,spectral characteristics,velocity profile,velocity circulation region,Reynolds stress,instantaneous velocity and vortex volume are investigated using heat-shrinkable tubes and cylindrical brushes respectively.Meanwhile,the vortex coherence structure and vortex excitation processes of smooth cylinders and cylindrical surfaces with covering are compared based on the intrinsic orthogonal decomposition(POD)to understand the characteristics of adding covering on cylindrical surfaces to change the periodic vortex structure.Firstly,the differences between the three decomposition methods(Orthogonal decomposition,Dynamic modal decomposition and one-dimensional discrete orthogonal wavelet decomposition)in flow field analysis are compared to determine the range of applicability of the three methods.The comparison reveals that POD is suitable for extracting spatially coherent structures,DMD is ideal for extracting coherent structures at specific frequencies,and wavelet decomposition is proper for extracting coherent structures at selected frequencies.Then,at Re=6000,the effect of adding dimpled coverings such as screens and heatshrinkable tubes to the cylindrical surface on the vortex excitation and wake characteristics was investigated.The results show that the dimpled covering reduces the velocity fluctuation,vortex fluctuation,vortex shedding frequency,velocity gradient,velocity circulation region and Reynolds stress during cylindrical bypassing;Increasing the dimpled diameter enhances the intensity of velocity pulsation,turbulence energy and vortex shedding frequency on the wake,and the heat-shrinkable cloth has the lowest pulsation intensity,velocity gradient,velocity circulation region and Reynolds stress.In the multi-scale mode,it is found that the dimpled covering reduces the periodic oscillation frequency of the primary structure and the secondary structure.In the medium-scale coherent structure,the heatshrink fabric was found to suppress the vortex curl.In the small-scale coherent structure,the heat-shrink fabric was found to stretch and compress the vortex street.The mechanism of passive control of wake flow differs between adding bumps on the surface and adding grooves;therefore,the wake structure with bumps on the surface is further investigated.Finally,the effect of adding bumps to the cylindrical surface on the vortex excitation and wake characteristics was investigated at Re=9000.The results show that: bumps reduce the velocity fluctuation,vortex fluctuation and Reynolds stress during cylindrical bypassing;however,bumps increase the velocity gradient and velocity circulation region during bypassing;increasing the length of bumps leads to a more significant velocity gradient.It is found that the turbulence energy of long bumps is reduced by 100% compared to smooth cylinders,and brushes are also found to suppress vortex shear and rotation.In large-scale coherent structures,it is found that long bumps increase the width of the velocity microclusters on both sides of the cylinder.In the large-scale coherent structure,long bumps were found to increase the width of the velocity microclusters on the top and bottom sides of the cylinder.In the medium-scale coherent structure,the addition of brushes was found to reduce the degree of wake velocity shear.In the small-scale coherent structure,the number of vortices after brushing increased.