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Counterbalance valves are widely used in hydraulic deck machinery to balance the overrunning loads. However, as is well known, counterbalance circuit designed with poor choice of counterbalance valve tends to introduce instability to the system. This paper investigates the dynamic behavior of a pilot operated counterbalance valve which can operate at a flow rate about 2000L/min. A linearized stability analysis of such a hydraulic circuit which consists of a slip in cartridge, a pilot counterbalance valve and a hydraulic winch is presented. Pole-zero plots are employed to reveal the effect of the volume of control cavity, the hydraulic resistance on pilot line and counterbalance valve pilot area ratio on the stability of the system. The analysis results indicate that such a system will be unstable within the normal range of each parameter. An alternative approach that guarantees system stability by adding an accumulator on the pilot line is put forward. The approach stabilizes the pilot pressure by reducing the hydro-stiffness of pilot control cavity, thus the system can reach its stability condition. Finally, a numerical optimization method is putted forward, with the optimized parameters, the dynamic performance of considered system become better.

Trawl nets are mostly flexible structures working in the water. In order to investigate the effect of the fluid-structure interaction on the trawl net's numerical model, we modeled the trawl net and the flow field based on lumped mass method and finite volume method, separately; then we adopted a hybrid volume method (HVM) to model the fluid-structure interaction between the net and surrounding water. Since the gridding of trawl net is independent of its shape within the proposed HVM, large mesh can be used for the calculation of fluid-structure coupling model for higher efficiency. First, existing flume tank experimental data were used to verify the accuracy of the HVM. Then, trawl net states were analyzed based on the HVM by taking the fluid-structure coupling into consideration. The simulation reveals that difference between the trawl net model with and without fluid-structure interaction is about 6%, the main reason for that is the relative velocity of water flow around the codend is only 1/5 of the towing speed because of the flow blocking by the trawl mesh upstream. The above results indicate that fluid-structure interaction is very important for the analysis of trawl net which should not be ignored when numerical modelling.

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