[Solved] 3D developed square channel flow + force December 15, 2017 07:47PM |
Registered: 4 years ago Posts: 2 |

Hello users!

I am currently having a problem where the solution converges to a steady-state, but__artificial velocities__ have been generated in the cross-section of the 3D channel. There should be no flow in that plane.

The geometry is 34 x 34 x 3.

The walls are no slip for velocity.

The force is applied in*z*-direction: **F**=(0,0,Fz)

The Neumann condition of zero is used in the flow direction (*df/dz=0*) making it really a 2D case.

It gives the exact (*err<1%*) velocity profile as compared to the analytical solution.

However, it also gives artificial velocities in the*xy*-plane.

The density variations are on the same order as the artificial velocity, and the streamlines show the closed circulations in the plane.

The artificial velocities are proportional to the force squared:*velocities ~ force*^{2}.

They are linearly proportional to the viscosity:*velocities ~ viscosity*.

They are proportional to the grid size cubed:*velocities ~ channel width*^{3}.

I have tried two collision schemes: SRT and TRT.

I have tried two forcing schemes: He-Shan-Doolen and Luo's.

I have tried two boundary condition methods: bounce-back, half-way bounce-back.

All the combinations give me the artificial velocities!

The questions are:

If you replicated the problem, would you also get those artificial velocities?

Where are they coming from?

Can I get rid of them?

If yes, How to get rid of them?

I hope somebody can clear this mystery for me :)

Regards,

Ivars

Edited 2 time(s). Last edit at 03/19/2018 03:54PM by ivars.

I am currently having a problem where the solution converges to a steady-state, but

The geometry is 34 x 34 x 3.

The walls are no slip for velocity.

The force is applied in

The Neumann condition of zero is used in the flow direction (

It gives the exact (

However, it also gives artificial velocities in the

The density variations are on the same order as the artificial velocity, and the streamlines show the closed circulations in the plane.

The artificial velocities are proportional to the force squared:

They are linearly proportional to the viscosity:

They are proportional to the grid size cubed:

I have tried two collision schemes: SRT and TRT.

I have tried two forcing schemes: He-Shan-Doolen and Luo's.

I have tried two boundary condition methods: bounce-back, half-way bounce-back.

All the combinations give me the artificial velocities!

The questions are:

If you replicated the problem, would you also get those artificial velocities?

Where are they coming from?

Can I get rid of them?

If yes, How to get rid of them?

I hope somebody can clear this mystery for me :)

Regards,

Ivars

Edited 2 time(s). Last edit at 03/19/2018 03:54PM by ivars.

Re: 3D developed square channel flow + force March 19, 2018 03:53PM |
Registered: 4 years ago Posts: 2 |

I came across a paper "Truncation errors and the rotational invariance of three-dimensional lattice models in the lattice Boltzmann method" by Goncalo Silva and Viriato Semiao dated at 2014 that explains the problems I was experiencing.

Lattice generated spurious currents in the Poiseuille flow at Re = 10*(Figure from the paper above)*

In short, it is to do with isotropy of non-linear truncation errors, and for reduced 3D lattices, such as D3Q15 and D3Q19 (used in my simulations) those errors are anisotropic.

To answer the above questions:

I hope this this can be helpful.

Regards,

Ivars

Lattice generated spurious currents in the Poiseuille flow at Re = 10

In short, it is to do with isotropy of non-linear truncation errors, and for reduced 3D lattices, such as D3Q15 and D3Q19 (used in my simulations) those errors are anisotropic.

To answer the above questions:

The spurious currents would appear in D3Q15 and D3Q19 models, but not in D3Q27.QuoteIf you replicated the problem, would you also get those artificial velocities?

They come from the non-linear truncation errors in reduced 3D models.QuoteWhere are they coming from?

Apparently, yes.QuoteCan I get rid of them?

Apparently, by using the complete 3D cubic lattice D3Q27.QuoteHow to get rid of them?

I hope this this can be helpful.

Regards,

Ivars

Re: [Solved] 3D developed square channel flow + force March 21, 2018 02:09AM |
Registered: 1 year ago Posts: 1 |

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