[rohin eapen]

i am rohin eapen, doing B.E mechanical engg in India.
i am trying to simulate how the pressure in the brake line varies if there is air trapped inside the brake line.can anybody help me ,what boundary condition i should give at the inlet,and which turbulence model i hve to choose.

[abdul099]

I assume, you don't need to model the actuator movement as well, right? Pressure will rise after the actuator has already pressed the friction pad to the bake disk, so there should be no significant movement anymore.

In this case, I would just mesh the brake line somewhere after the master brake cylinder and apply a (maybe time varying) velocity inlet.

Don't worry too much about the turbulence model. I suspect the flow to be laminar since the Reynolds number should be pretty low due to high viscosity of the brake fluid, low brake line diameter and relative low velocities.
And also bad boundary conditions or a bad mesh can affect the solution much more than the "wrong" turbulence model.

[rohin eapen]

i am considering only a small part of the brake line and thought of applying sme pressure at the inlet and see how it varies with respect to time if there is any air trapped inside.

My dought is that actually when we apply the pressure there will not be any movement of the fluid but the pressure is just transferd from inlet to outlet.
so if i give my inlet as 'velocity inlet' how i can give the pressure in terms of velocity at the inlet.

There is no option called 'pressure inlet' in starccm.

[abdul099]

Does your brake line have an outlet? When the brake liner attaches at the brake disk, the system is closed at one end.
In reality, you wouldn't notice the trapped air as long as you're just pushing the brake pedal while the liquid can drop out of the open end of the brake line. You will notice the air when the brake line is closed at the end and the liner hits the brake disk, so the pressure can start to rise. In this case, you can feel the pedal be be pushed further than it would be without trapped air.
But there should be still not outlet. That's why I would apply a velocity inlet or a pressure outlet as inlet without any outlet. The velocity inlet represents the fluid displaced by the master brake cylinder. A pressure outlet (can also be used as inlet) will represent a force of the brake pedal.