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## "Failed to find consistent initial values"

Posted 12 dec. 2014 12:43 GMT-05:00 Fluid Version 5.0 23 Replies

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Again, the most basic form of the problem is 2D incompressible flow on a fully periodic domain, failing with a uniform flow initial value.

Thanks.

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Genuinely seeking to understand and find possible ways around these issues.

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I can now confirm that the problem is version related, having tested on two independent machines, one windows 64 bit, the other linux 32 bit. All periodic flow problems I tried succeed for version 4.3a and fail for 4.3b and 4.4. Unfortunately I cannot test version 5 due to license constraints.

In absence of anything resembling a bug tracking system I will document my findings here hoping that somebody will pick it up. Meanwhile I will instruct my students to downgrade their systems.

STEPS TO REPRODUCE

- In model wizard select 2D, Laminar flow, Time dependent.

- Under Geometry add a 1x1 square.

- Under Laminar flow select incompressible, and set Fluid properties to rho=1, mu=1.

- Add a periodic flow condition on boundary 1 and 4, and another on 2 and 3.

- Set initial conditions u=1, v=0, p=0.

Leave the rest at default. Computation succeeds for Comsol 4.3a, but fails for 4.3b and 4.4 indicating "Failed to find consistent initial values".

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--

Steven Conrad, MD PhD

LSU Health

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I believe the problem is that there is no unique value for the pressure. If you set a Pressure Point Constraint to zero at any point it works.

Nagi Elabbasi

Veryst Engineering

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Try setting the initial time step to something small. This can be found under Time-Dependent Solver > Initial Step and check the box and specify an initial time step.

Hi Edwin, thanks for the advice. Unfortunately the error did not go away with an initial step as low as 1e-10. Could you perhaps explain the rationale behind this suggestion?

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Post or send a copy of your model, and I will look at it. I have 4.4 and 5.0 installed.

Hi Steve, thanks for the offer. Please find attached a model file corresponding to "steps to reproduce".

Attachments:

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I believe the problem is that there is no unique value for the pressure. If you set a Pressure Point Constraint to zero at any point it works.

Hi Nagi, the pressure singularity should be an issue for stationary problems only; for time dependent problems the pressure is constrained at t=0 by the initial condition.

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Not if you have an incompressible fluid formulation. A uniform increase in pressure at any time is still a solution even in the time dependent problem.

Nagi Elabbasi

Veryst Engineering

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Not if you have an incompressible fluid formulation. A uniform increase in pressure at any time is still a solution even in the time dependent problem.

Well that's embarrassing. I humbly stand corrected, and thank you very much for pointing this out. The reason I got away with the setup earlier was the use of a Krylov solver, but indeed the systems were always singular. Mystery solved. Thanks to all who offered to help out!

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Hello,

I got a quite similar problem. Could you explain better how do you solve it?

How did you change the setting in the solver?

Thanks!

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Nagi's post of 12/22/14 gives the solution: you don't change any setting on the solver, but rather you constrain the pressure at some point to eliminate the singularity.

Jeff

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How to do that?

perhaps by mean a pointwise constraint?

Thanks!

Andrea

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Jeff

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I'm solving the problem with the PDE coefficient form toolbox, so I don't think that there is a possibility to set a pressure constraint...

Again, do I have to do that with a pointwise constraint or something? How ca I do with PDE Coefficient for tool?

Thanks.

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Jeff

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Let me explain better, maybe I have been a bit hermetic.

I'm trying to solve a coupled problem in which the two set of equation are:

-The navier stokes equation in which the forcing terms is coupled with another field that has another own (2nd) equation. Finally there is the incompressibility constraint (div(v)=0)

The geometry is a circle domain with dirichlet boundary condition on v and Neumann boundary condition for the other field.

I followed the following procedures:

1I Wrote the equation in the PDE coefficient form in which also the hydrostatic pressure is a dependent variable. The equation for it in the coefficient form will be everything equal to zero a part of the forcing term equal to the divergence of v.

I tried to solve the problem with the initial condition (compatible with the boundary condition) exactly equal to the solution that I had in the time independent solver.

2.Write the equation in the coefficient form in which the hydrostatic pressure is taken into account as a Lagrangian multiplier with a weak constraint on the divergence of v and repeat the same strategy with the initial condition.

In both case the code find a correct let me say "static" solution but has a problem at the initial time step

("Failed to find consistent initial values") with the time dependent solver.

3. Same strategy but pointwise constraint, the code doesn't find a solution in a reasonable time.

I can't attach the code...

Thank you in advance.

Andrea

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Note that you can clear all solutions from your .mph files (By using the button on the Study ribbon) to make them small enough to post here.

Best,

Jeff

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I am facing a same issue .

"Failed to find consistent initial values.

Segregated group 1

System matrix is zero.

Last time step is not converged."

I am trying to apply a series of voltage values on certain time steps .But i am stuck at this error.

Alas, without knowing what equations you are solving and precisely how you are implementing them in COMSOL, users of this Forum cannot offer much useful help - at best you'll get wild guesses that may send you in the wrong direction.

Note that you can clear all solutions from your .mph files (By using the button on the Study ribbon) to make them small enough to post here.

Best,

Jeff

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Thanks for replying...

Let me explain better, maybe I have been a bit hermetic.

I'm trying to solve a coupled problem in which the two set of equation are:

-The navier stokes equation in which the forcing terms is coupled with another field that has another own (2nd) equation. Finally there is the incompressibility constraint (div(v)=0)

The geometry is a circle domain with dirichlet boundary condition on v and Neumann boundary condition for the other field.

I followed the following procedures:

1I Wrote the equation in the PDE coefficient form in which also the hydrostatic pressure is a dependent variable. The equation for it in the coefficient form will be everything equal to zero a part of the forcing term equal to the divergence of v.

I tried to solve the problem with the initial condition (compatible with the boundary condition) exactly equal to the solution that I had in the time independent solver.

2.Write the equation in the coefficient form in which the hydrostatic pressure is taken into account as a Lagrangian multiplier with a weak constraint on the divergence of v and repeat the same strategy with the initial condition.

In both case the code find a correct let me say "static" solution but has a problem at the initial time step

("Failed to find consistent initial values") with the time dependent solver.

3. Same strategy but pointwise constraint, the code doesn't find a solution in a reasonable time.

I can't attach the code...

Thank you in advance.

Andrea

Hi Andrea,

I have met a similar notification telling that "Failed to find consistent initial values" with PDE equations;

Have you solved it ? and could you please give me a hint?

Thank you in advance.

Best Regards,

Bill

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Not if you have an incompressible fluid formulation. A uniform increase in pressure at any time is still a solution even in the time dependent problem.

Well that's embarrassing. I humbly stand corrected, and thank you very much for pointing this out. The reason I got away with the setup earlier was the use of a Krylov solver, but indeed the systems were always singular. Mystery solved. Thanks to all who offered to help out!

Dear Gertjan,

I added a pressure point constraint in your model, there is no error occuring, but without any solutions.

Any suggestions would be appreciated.

Thank you in advance.

Best Regards,

Bill

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