Flow structure and characteristics of fluid undergoing a sudden contraction to an annular gap under dynamic boundary

Pipeline transport serves as an effective means to alleviate traffic congestion and reduce carbon emissions from transportation. The hydraulic delivery system, which employs pipeline cars as carriers, addresses the limitations of existing systems. However, its transportation efficiency is affected by variations in the flow structure within the pipelines. During the acceleration of the pipeline car, the sudden contraction flow field from circular to annular gap formed in the vicinity of the end face under dynamic boundary conditions. This study utilized particle image velocimetry (PIV) to visualize and measure the sudden contraction flow field. Based on the obtained experimental results, it investigated the impact of dynamic boundary velocity on the flow structure, velocity characteristics, and energy dissipation of the annular gap. The acceleration process of the dynamic boundary is the conversion of flow energy into the kinetic energy of the annular gap flow and the kinetic energy of the pipeline car. This process is accompanied by phenomena of velocity slip and velocity overshoot. As the velocity of the pipeline car increases, the recirculating vortex within the annular gap dissipates and eventually disappears. The velocity slip gradually decreases, the location of the overshoot point shifts radially, and the magnitude of the overshoot diminishes before ultimately vanishing. From static to steady, the probability density distribution of the slipstream face transitions from a distribution with high skewness and low peak value to a normal distribution with high peak value and low skewness. The irreversible losses that arise in a sudden contraction flow field can be quantified by the increase in entropy. Due to the similarity of the solving processes of large Eddy simulation and PIV, a combined sub-grid stress model is used to solve the flow losses in the flow field. The turbulent dissipation occurs mainly in the recirculation region, shear layer, and high-speed shear regions near the wall.