Behavior of fluids in hydraulic circuits: how their properties affect system performance
In hydraulic systems, fluid is not simply a means of transmitting power, but an active element that directly influences the efficiency, reliability, and durability of components.
Its physical properties—such as viscosity, density, compressibility, and lubrication—vary depending on operating conditions, temperature, and pressure, thereby modifying the dynamic behavior of the circuit.
Understanding how these parameters evolve during operation is essential for:
- predicting performance losses;
- preventing undesirable phenomena, such as cavitation;
- ensuring the correct response of the actuators.
In the SmartFluidPower simulation library, the fluid is described as a two-phase mixture composed of oil and air, where the oil can be partly in a liquid state and partly in a gaseous state.
This choice stems from the need to realistically represent what happens in real hydraulic circuits, where dissolved air, temperature variations, and pressure fluctuations significantly influence the system’s response.
In this way, we are able to satisfy the best physical models in academic literature, obtaining results with very high accuracy.
Behavior of fluids in in their different phases
Depending on the pressure, the fluid can be in one of four main states of behavior, each representing a different combination of liquid, gas, and dissolved components.
In most hydraulic applications, the fluid operates in a state of partial dissolution of air in the oil: in this phase, the oil is predominantly liquid, but contains a portion of semi-dissolved air that influences its compressibility and dynamic response. This is the typical condition of real circuits and the one in which the most accurate representation of the system’s behavior is obtained.
As the pressure decreases, the oil enters a phase of partial evaporation, in which the fluid simultaneously contains liquid oil, oil in the vapor phase, and air. This transition is crucial for describing the phenomena of incipient cavitation and for assessing the stability of components subjected to strong pressure variations.
As the pressure drops further, total cavitation is reached, where the liquid component disappears completely and the fluid consists only of gas and vapor. Although this phase is abnormal for the correct functioning of the circuit, it is essential for understanding the effects of degradation and performance losses due to extreme cavitation.
Finally, at higher pressures, the fluid may be in a state of total air dissolution in the oil, where the air is completely dissolved and the behavior is similar to that of a perfectly homogeneous fluid. This is a less frequent condition, which only occurs in very high pressure circuits.
Calculating fluid behavior
Each phase of the fluid—liquid, gas, or mixed—is characterized by specific physical properties, depending on temperature and pressure. Our library calculates all these properties individually (density, kinematic viscosity, compressibility modulus) and then weights them according to the relative concentration of the phases, thus obtaining the actual values of the “single” fluid used by the model.
Another strength of the model is the possibility of using up to three distinct fluids within the same simulation. This allows complex circuits in which different mixtures coexist (e.g., oils with different degrees of viscosity or fluids at different temperatures) to be represented, while maintaining an integrated and consistent simulation.
The thermodynamic transformations are considered isothermal, but thanks to the presence of several distinct fluids, it is possible to model areas with different temperatures within the same circuit, for example, hot and cold sections of the same circulating oil.
In this way, each component of the circuit receives the local properties of the fluid in real time, updated according to its operating pressure. This allows the analysis of complex phenomena such as cavitation, variations in density or viscosity along the circuit, and the direct effect that these behaviors have on the overall dynamic performance of the hydraulic system.
Find out if simulation is right for your company
SmartFluidPower is an innovative SME founded in 2018 as a spin-off of the University of Modena and Reggio Emilia; our team combines academic experience and engineering expertise to bring innovation to hydraulic design.
Very often, companies involved in fluid power encounter simulation software that is too complex, expensive, or unsuitable for their real needs.
We decided to change the rules of the game.
We work directly with industry leaders to develop tailor-made products and services designed to simplify modeling, reduce development times, and improve the performance of hydraulic systems.
We create systems that stand the test of time and communicate with each other.
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