Micro-hydrodynamic mechanism of rebound damping force fluctuations in automotive twin-tube hydraulic shock absorbers

High-frequency damping force fluctuations frequently occur during stroke reversal in automotive twin-tube hydraulic shock absorbers, but the underlying valve-scale fluid–structure interaction mechanism remains insufficiently understood. This study investigated the transient mechanism underlying the damping force fluctuation during the initial stage of the rebound stroke by combining shock absorber bench tests with a progressive fluid–structure interaction (FSI) framework. First, quasi-steady one-way FSI simulations under multiple operating conditions were performed to establish the baseline flow-field characteristics and shim-deformation responses of the rebound and compensation valves. Transient two-way FSI simulations were then performed over the experimentally identified fluctuation interval to resolve the sub-millisecond evolution of the local flow field and flexible-shim deformation. The results show that the rebound valve exhibits only small and stable deformation during the fluctuation interval, whereas the low-stiffness compensation shim undergoes a rapid increase in deformation from 0.21 to 0.35 mm in less than 1 ms. This rapid boundary motion causes a step-like increase in the effective flow area, a sharp decrease in local flow velocity from 1.36 to 0.015 m/s, and the formation of a localised low-pressure region. These coupled structural and hydrodynamic changes trigger the macroscopic damping force fluctuation. Based on this mechanism, a rigid limiting ring was introduced to restrict excessive motion of the compensation shim. Bench tests showed that the root-mean-square (RMS) values of the damping force fluctuations decreased by 27.3% on average and by up to 40.9% at a nominal peak piston velocity of 0.15 m/s, while the peak rebound damping force decreased by only 2.14%–7.37%. The proposed framework provides a mechanism-based approach for analysing and mitigating transient FSI-induced force instability in thin-shim hydraulic valve systems.