Experiment and simulation on air layer drag reduction of high-speed underwater axisymmetric projectile

Injection of air could lead to creation of a bubbly mixture or air layer near the surface that can significantly adjust the flow within the turbulent boundary layer. In this paper, stress wave propagation techniques and Split Hopkinson Pressure Bar (SHPB) are used in underwater launching experiment. Simulation with volume of fluid (VOF) method and modified renormalization group (RNG) k-epsilon model is also performed to study the physical process of drag reduction of axisymmetric body. Comparison between numerical and experimental air layer length shows good correlation. Results indicate that air layer has good effect on drag reduction. Friction drag reduction mechanism is analyzed from two aspects due to different air volume fraction alpha. At area with high alpha, fluid is heterogeneous and layered. Drag reduction is from decrease of velocity gradient and dynamic viscosity at the wall. At area with small alpha, the mixture is homogeneous. Empirical equation of turbulent boundary layer shear stress is applied to describe drag reduction mechanism. The unsteady evolution of drag with injection of air is also studied at last. (C) 2015 Elsevier Masson SAS. All rights reserved.