Abstract
Microbubble drag reduction has good application prospects. It operates by injecting a large number of bubbles with tiny diameters into a turbulent boundary layer. However, its mechanism is not yet fully understood. In this paper, the mechanisms of microbubble drag reduction in a fully developed turbulent boundary layer over a flat-plate is investigated using a two-way coupled Euler-Lagrange approach based on large eddy simulation. The results show good agreement with theoretical values in the velocity distribution and the distribution of fluctuation intensities. As the results show, the presence of bubbles reduces the frequency of bursts associated with the sweep events from 637.8 Hz to 611.2 Hz, indicating that the sweep events, namely the impacting of high-speed fluids on the wall surface, are suppressed and the streamwise velocity near the wall is decreased, hence reducing the velocity gradient at the wall and consequently lessening the skin friction. The suppression on burst frequency also, with the fluid fluctuation reduced in degree, decreases the intensity of vortices near the wall, leading to reduced production of turbulent kinetic energy.
Highlights
In microbubble drag reduction (MDR), a large number of microbubbles are introduced into a turbulent boundary layer (TBL) to form a mixed layer of gas and liquid
Because bursts can cause strong fluid fluctuation and energy transport, to further explore the changes caused by microbubbles, this study examines the transport of turbulent kinetic energy
This paper focused on bursts to investigate the microscopic mechanisms of MDR and found that the reduction of burst frequency lessens the skin friction
Summary
In microbubble drag reduction (MDR), a large number of microbubbles are introduced into a turbulent boundary layer (TBL) to form a mixed layer of gas and liquid. Unlike the use of layers of air, or super-cavities, which reduce drag by insulating the object from contact with water [7], the mechanism of MDR is more complex because the bubbles do not directly come into contact with the wall of the object, appearing in the turbulent boundary layers. It is generally believed that the presence of microbubbles changes the structure of the boundary layer [8]. The excellent prospects for application and complex mechanism of drag reduction in microbubbles have attracted researchers’ attention to their use, which has made it a research hotspot in recent years
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