Dynamic mode decomposition of mixing characteristics in a T-junction with a rotation impeller

Coherent structures of turbulent mixing between cold and hot waters in a T-junction with a rotation impeller with varying blade numbers (Np = 2-4) are studied using the dynamic mode decomposition method. Velocity and temperature fields are obtained using large eddy simulation at the deflecting flow pattern classified by momentum ratio (MR = 0.49). Relevant dynamic features of a flow with low frequency mainly appear near the impeller and those with high frequency appear at the upper and bottom walls. The maximum velocity fluctuation increases with the blade number. Vortex shedding occurs at the blade tips. Spatial structures of the temperature field are mainly distributed at the mixing interface of the two fluids and impeller region. At Np = 2 and 3, energetic coherent structures appear at the bottom wall, while at Np = 4, they appear at the upper and bottom walls. The maximum temperature fluctuation first increases and then decreases with increasing blade number. The frequencies of velocity and temperature modes comprise multiple blade passing frequency (BPF) and rotation frequency, and the effect of rotation frequency on the velocity and temperature modes disappears with increasing the blade number. Furthermore, the frequency of the temperature mode is higher than that of velocity at Np ≤ 3 except for the identical BPF; Np = 4, the two frequencies tend to be more synchronous.