Influence Of The Microbubble Flow On The Coherent Structure Of The Planar Turbulence Boundary Layer

Microbubble drag reduction is a promising method of drag reduction,which aims to achieve drag reduction by injecting a large number of tiny diameter bubbles into the turbulent boundary layer.However,there is still no clear conclusion about the mechanism of micro-bubble drag reduction.With the deepening of the research,it is found that the existence of microbubbles can change the coherent structure in the turbulent boundary layer,and the coherent structure is an important mechanism for the generation and maintenance of turbulence,and the reason of high frictional resistance.In this paper,the internal mechanism of microbubble drag reduction is explored by coupling analysis of microbubbles and turbulent boundary layer.Firstly,a new turbulence boundary layer generation method is proposed to solve the problem that the inlet condition of the turbulent boundary layer is difficult to be given.The plate turbulence boundary layer is calculated in this way and the causes of high frictional resistance are analyzed.It is found that the calculated results can accurately identify the low velocity band and the vortex structure.The boundary velocity distribution and the pulsating intensity distribution are in good agreement with the theoretical value.By analyzing the vortex structure,it is found that the high frictional resistance of the turbulence is due to the downward sweep caused by the vortex and the "burst".Secondly,the Euler-Lagrange and Euler-Euler models are calculated and evaluated respectively.It is found that the Euler-Lagrange model occupies the advantage in calculating the micro-bubbles with small size and multi-size distribution,which can get the trajectory,time and velocity of each microbubble.The Euler-Euler model has a good performance in calculating the large size,deformation,fusion and rupture of microbubbles,and the flow field in microbubbles can be obtained to calculate the deformation,fusion and fracture phenomena.The reason for the difference between the two methods lies in the nature and quantity of the equation.Finally,the Euler-Lagrange model is used to calculate the micro-bubble into the turbulent boundary layer of the plate.The mechanism of drag reduction is discussed.It is found that the "burst" phenomenon is the core of the whole process.Through the "burst" The movement is transmitted to the pulsating motion,and the turbulence gains energy to be produced and maintained.The underlying and outer layers of the fluid are exchanged by the "burst" and the high frictional resistance occurs due to the high-speed sweep movement at the time of interchange.The occurrence of microbubbles reduces the frequency of "burst",so that the strength of the vortex core region is weakened and the turbulent kinetic energy is reduced.Turbulence is obtained by reducing the energy and turbulence is suppressed.The fluid exchange at the bottom and the outer layer is weakened,reducing the frequency and intensity of the impact of the outer layer of highspeed downstream of the outer layer,and the statistical result is the reduction of the frictional resistance.