January 11th, 2019
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Robust in nature, resistant to heat, and able to withstand extreme environments, a hydraulic motor is a mechanical rotary actuator that converts hydraulic pressure or fluid energy into mechanical power. From conveyors to bulldozers, mixers and rolling mills, the durability and relative simplicity of hydraulic motors, along with the fact that they do not require electricity to run, make them uniquely suitable for a wide range of applications.
Key to the running of a hydraulic motor is the ability to regulate the flow and pressure of the fluid within the hydraulic system and direct the mechanical power created efficiently. This is controlled by a system of valves that are essential to the motor’s performance, and therefore the design of this system needs to carefully considered. The key risk to this system is cavitation—the formation of bubbles or cavities in the fluid due to areas of relatively low pressure. As these cavities implode or collapse, they cause shock waves within the hydraulic system, which can cause significant damage.
As a developer of these valve systems, Diinef, founded in 2013 in southern Norway, is a company that is particularly interested in reducing pressure drop and, in doing so, lowering the risk of cavitation formation. The company began as a spin-off of ChapDrive, a developer of high-efficiency hydraulic drivetrains for wind turbines. Their high-tech approach has since seen them specialize in the design and manufacture of digital technology for high torque low speed (HTLS) motors.
Diinef came to SimScale with a challenge, how could they use simulation to monitor pressure drop, and therefore reduce the frequency and severity of cavitation damage in their valve systems.
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In Diinef’s case, the operation of their hydraulic motor valve system involves opening and closing under fluctuating pressure conditions (up to several hundred bars). This was expected to lead to areas of pressure drop. Where there is pressure drop, cavitation is often likely, which poses the risk of damaging the valve during prolonged use. These strong fluctuations in pressure made simulating the valve’s movement and conditions in CAE (specifically utilizing computational fluid dynamics) an ideal solution, allowing the team to test and analyze any potential problem areas before they arose from an expensive physical prototype.
The team set two targets for their analysis:
The first step in the analysis was to run a compressible flow simulation of the valve as it underwent the early stages of the opening process. The geometry consisted of a 3D model replicating the basic shape of the valve. This ensured Diinef could obtain the initial results rapidly, and use this information to quickly take their first engineering decisions.
From these results, it was clear that a more complex simulation (for example consisting of the entire valve configuration) would not be necessary, as the initial test had highlighted the opening gap of the valve as a focus area for further analysis. This allowed Diinef and SimScale to quickly iterate and test different design versions.
To speed up testing even further, the development process was shifted to a semi-2D model.
This straightforward testing approach allowed the Diinef team to rapidly iterate, analyze, and improve their designs. In the final iteration, there was a reduction in the pressure drop on the slightly open valve by 80%, which put it within the desired operating range. The simulations also indicated a redirection of the flow, moving the zone of potential cavitation several millimeters away from the wall.
From these optimized designs, physical tests of the valve could commence and they confirmed the reduction in back-pressure drop. Despite this drop in pressure, cavitation was still observed and it was difficult to judge whether the frequency or tendency was reduced. To combat this, the Diinef team used the knowledge gained from their simulations, and included extra material in the observed risk zones, mitigating the effect of any possible cavitation damage.
By using CFD simulation in the early stage of their design process, the team at Diinef was able to effectively analyze, plan and optimize their designs against pressure drop, while avoiding the slow and costly testing of physical prototypes before it was absolutely necessary.
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