Study on grooved wall flow under rarefied condition using the Lattice Boltzmann Method

Decreasing energy consumption is a key issue for stratospheric airship to maintain its high altitude long-endurance feature. The airship envelope covered micro-grooves is considered as a feasible way to reduce resistance in order to save energy. The key to using grooves as the drag reduction technique is to evaluate its drag reduction ability under the condition of rarefied gas which is different from the original continuum atmospheric state. In this paper, the drag reduction ability of V-shape and rectangular micro-grooves under both continuum and rarefied condition is evaluated using the Lattice Boltzmann Method (LBM). This method modified for rarefied flow has been validated by the analytical solution in this paper. The local vortices are obviously observed within the grooves in the continuum and slippery flow regimes. The mechanism of drag reduction with grooves is elucidated at first and the ability of drag reduction under the rarefied condition is then evaluated by comparison of that under continuum condition. In the end, both the V-shape and rectangular grooved wall flow with different height and width are simulated. The relationships of drag reduction ability and the groove geometry are also determined and evaluated. Our results reveal that the grooves can reduce the resistance with a drag reduction rate of about 6% in the continuum. The drag reduction ability of the same grooves, however, decreases to 2.5% or less in the slippery regime. The grooves with appropriate size which make the vortices full within grooves can reduce shear force between fluid and wall. The results obtained in this paper are good references to the design of stratospheric airship envelope and the drag reduction technique.