Direct Numerical Simulation Of Gas-Liquid Turbulent Flow

Turbulent drag reduction technology is of great practical value for saving energy,increasing throughput of crude oil pipeline.However,the research on the drag reduction of gas-liquid turbulence is not mature.The existing experimental methods are difficult to accurately measure all the information in flow field,so that mechanism of drag reduction can't be analyzed deeply.Direct numerical simulation as a powerful tool for mechanism research hasn't been reported in the field of gas-liquid turbulence drag reduction.We combine the efficient interface-tracking method VOSET with direct numerical simulation to calculate gas-liquid turbulence flow,by combining the constitutive relation Giesekus for viscoelastic drag-reducing fluid to establish governing equations of viscoelastic gas-liquid flow.It is found from the first-step study that method VOSET can accurately capture the complex phase interfacial changes in gas-liquid turbulent flow,the existence of the drag-reducing additives(DRAs)causes the numerical stability deteriorate compared with no agent case.The main reason is large viscoelastic conformation gradient introduced around the gas-liquid interface,enhancing the instability of the constitutive equation.As a result,computable lid-driven velocity is limited in a narrow range(1 m/s~2 m/s),affecting the study on the characteristics of viscoelastic gas-liquid turbulent flow.To overcome this drawback,we use a smooth Heaviside function to interpolate the viscoelastic conformation near the gas-liquid interface.This improvement greatly enhances the numerical stability and extends the lid-driven velocity up to 50 m/s.Direct numerical simulation in a wider range of Reynolds number is implemented using the improved method.Numerical results confirm that drag-reducing agents largely depress turbulent fluctuations of liquid phase so that drag reduction occurs for the whole gasliquid turbulent flow.Mean velocity of the liquid phase increases about 37%~87%.