Initial pressure field from velocity field (incompressible)

dhein · January 17, 2022, 10:32

Hey there!

I defined an initial velocity field for my simulation. As I am using the incompressible model, I can't use like a random pressure field, the pressure field should be given by the given velocity field.

Initially I thought I could define the initial pressure by hitting a checkbox like "Compute from initial velocity", but obviously I can't.

Unfortunately I still need to specify the pressure field by e.g. a user defined field function as well.

Am I able to compute the initial pressure from the given initial velocity by specifying the Pressure Poisson Equation somewhere?

LuckyTran · January 17, 2022, 16:01

Are we talking about initial conditions in a transient problem where you actually want to set an initial velocity field at t=0 and pressure field at t=0 or is this just the initial guess? Either way, you need to provide your own initial guess for the pressure field. Even if this magical checkbox existed, you would still need to provide an initial guess for the pressure field because every iterative solver needs an initial guess.

Computing the pressure field from the velocity field is the same as solving the entire N-S equations (except that you have fixed the velocity field). And solving a poisson equation for pressure is the basis for the segregated solver. Using a segregated solver does what you are describing philosophically. I hope this makes it clearer why "just solve the Poisson equation" is not trivial. Some general statements in mathematics, even when they're true (and they are in this case), are not that trivial to actually do.

dhein · January 18, 2022, 08:46

Yes, I was talking about a transient simulation, starting with a given velocity and pressure field.

I mean actually I'm not that interested in the way of getting to the stationary state.

I guess the easiest way to perform a simulation - e.g. if I want to investigate the behaviour of the flow if I add some disturbances - is to perfom a stationary simulation and taking the velocity and pressure field of that simulation to perform my transient simulation, right?

LuckyTran · January 18, 2022, 09:12

Yes. You should apply the boundary conditions that would get you the desired velocity and pressure field that you expect and iterate/solve your way towards that field. And then you can reset your solution history(but do not re-initialize) so it starts at t=0 if you want.

January 17, 2022, 10:32	Initial pressure field from velocity field (incompressible)	#1
dhein New Member Join Date: Apr 2021 Location: Ulm, Germany Posts: 11 Rep Power: 5	Hey there! I defined an initial velocity field for my simulation. As I am using the incompressible model, I can't use like a random pressure field, the pressure field should be given by the given velocity field. Initially I thought I could define the initial pressure by hitting a checkbox like "Compute from initial velocity", but obviously I can't. Unfortunately I still need to specify the pressure field by e.g. a user defined field function as well. Am I able to compute the initial pressure from the given initial velocity by specifying the Pressure Poisson Equation somewhere?

January 18, 2022, 09:12		#4
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Yes. You should apply the boundary conditions that would get you the desired velocity and pressure field that you expect and iterate/solve your way towards that field. And then you can reset your solution history(but do not re-initialize) so it starts at t=0 if you want. dhein likes this.

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January 17, 2022, 16:01		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Are we talking about initial conditions in a transient problem where you actually want to set an initial velocity field at t=0 and pressure field at t=0 or is this just the initial guess? Either way, you need to provide your own initial guess for the pressure field. Even if this magical checkbox existed, you would still need to provide an initial guess for the pressure field because every iterative solver needs an initial guess. Computing the pressure field from the velocity field is the same as solving the entire N-S equations (except that you have fixed the velocity field). And solving a poisson equation for pressure is the basis for the segregated solver. Using a segregated solver does what you are describing philosophically. I hope this makes it clearer why "just solve the Poisson equation" is not trivial. Some general statements in mathematics, even when they're true (and they are in this case), are not that trivial to actually do.

January 18, 2022, 08:46		#3
dhein New Member Join Date: Apr 2021 Location: Ulm, Germany Posts: 11 Rep Power: 5	Yes, I was talking about a transient simulation, starting with a given velocity and pressure field. I mean actually I'm not that interested in the way of getting to the stationary state. I guess the easiest way to perform a simulation - e.g. if I want to investigate the behaviour of the flow if I add some disturbances - is to perfom a stationary simulation and taking the velocity and pressure field of that simulation to perform my transient simulation, right?