Gas accumulator initial pressure

Hi,

I'm running a model with a gas accumulator which is connected to several other hydraulic block (pipeline, hydraulic cylinder...). In all those blocks the initial condition you have to define is the pressure. But for the gas accumulator the initial condition you need to define is the volume. So my 1st question is:

• Does anyone know why is that?

I've seen that a lot of people are having issues with model failing to solve initial conditions with those block, defining initial pressure instead of volume for the accumulator would solve that.

I'm aware of the fix which is putting an orifice in between components. My model is running but I really need to get the same initial pressure for all my component and I can't manage to get it for the accumulator. I've been using the formula from the help (and all sort of variation of it) to calculate the initial volume from my initial pressure. So my second question is:

• Does anyone had the same issue before and know how to fix that?

I'm running: ------------------------------------------------------------------------------------------------------- MATLAB Version: 8.1.0.604 (R2013a) MATLAB License Number: 592455 Operating System: Microsoft Windows XP x64 Version 5.2 (Build 3790: Service Pack 2) Java Version: Java 1.6.0_17-b04 with Sun Microsystems Inc. Java HotSpot™ 64-Bit Server VM mixed mode ------------------------------------------------------------------------------------------------------- MATLAB Version 8.1 (R2013a) Simulink Version 8.1 (R2013a) Parallel Computing Toolbox Version 6.2 (R2013a) SimHydraulics Version 1.12 (R2013a) Simscape Version 3.9 (R2013a) Simulink 3D Animation Version 6.3 (R2013a)

Answer 1

[Yifeng Tang]

The volume in the initial condition (Variables tab) is supposed to be the volume of the liquid. Not sure about R2013a, but in more recent versions, the I.C. is named "Volume of Liquid".

There are also two pressures: one in the parameter, which you have to specify; and one in the variables, which you may choose to specify. The former, pre-charge pressure, refers to "pressure in the gas chamber when the fluid chamber is empty" (see Help). Together with the "total accumulator volume" parameter, it defines the amount of air. There is also an initial pressure of the liquid, which should be the same as the initial air pressure, in the Variables (I.C.) tab. This defines how much the air is compressed from the precharge state. Since the amount of the air is fixed, setting the initial air pressure also implies a certain amount of liquid inside the chamber. As such, one should not set volume of fluid and pressure of fluids at the same time to High priority.