instability problem

[jepson]

Dear developer,rnrnI am trying to solve the single fluid flow through a parallel fracture. rnrnThe physical problem is: rnwidth b = 0.5mm;rnlength l =1.25m;rnviscosity=1.3 e-6 m^2/s;rnpressure gradient is applied along the horizontal direction.rnrnReynold’s number is around 1000 cos the umax=2.46m/s when the gradient is 16.rnrnI have read the paper on how to chose the unit in LB simulation, however, when the Reynold’s number get higher, there is instability. I cannot extract the result properly. rnrnmany thanksrnrnJepsonrn

[mathias]

Dear Jepson,rnrn What are the disrcetisation parameters dx and uMax (cf. log file)? Have you tried to choose a smaller uMax and refine your grid? Is it more stable then?rnrnMathias

[jepson]

Hi Mathias,rnrnThanks a lot, it works.rnrnDoes the current code can simulate the non-Darcy flow in parallel fracture when the velocity get high? What I do now is simply increasing the Reynolds number, perhaps it is wrong. any suggestions?rnrnThanks a lotrnrnJepsonrn

[mathias]

Hi Jepson,rnrn A more natural way is to fix the Reynolds number and slowly increase the inflow velocity as done in “”example/cylinder3d”” in the function “”reIniGeometry””.rnrnMathias

[jepson]

Hi Mathias, rnrnIt is possible to increase the inflow velocity to get the turbulent flow. However, in my opinion, the Reynolds number is identical in both physical field and the lattice field, is it right to fix the Reynolds number?rnrnwhat I want to do is simulate the fluid flow through single fracture with variable aperture and gradient. As in the “”example/forcedPoiseuille””, the body force is applied by f(umax,ix, Nu), how should I do to simulate both the laminar and turbulent flow in such a boundary condition? Is it proper to only increase the body force ?rnrnmany thanksrn Jepson

[mathias]

Dear Jepson,rnrn You actually fix the viscosity. Then, the Reynolds number just depends on a characteristic length and speed you define. I recommend having a look at “”example/cylinder3d”” to get an idea of how to model a similar situation. If you want, you can send me your problem set-up by email.rnrnMathiasrn

[jepson]

Hi Mathias,rnrnWhat I want to do is verify the experimental results of fluid flow through a single fracture.rnrnThe geometry is:rnaperture B = 0.5mm;rnlength L =1.25m;rnrnviscosity=1.3 e-6 m^2/s;rnrnBy increasing the pressure gradient and change the aperture , there will be a relationship between the hydraulic gradient and flow quantity-the typical Cubic law.rnrnHowever, I only can simulate the laminar flow no matter what the Reynolds number is. I am confused, I have tried to fix the viscosity and simulate the flow in different Reynolds number by change the character length and speed. In addition, it is possible to gradually increase the body force as the “”reIniGeometry””.rnrnBy the way, the pressure is applied as the example “”forced-Poiseuillel””. rnrnDoes the flow property not depend on the Reynolds number in such a simulation?rnrnThanks a lot!rnrnJepsonrn

[jepson]

Dear Mathias, rnrnHow to guarantee the equivalent of the physical field and lattice world if the Reynolds number is different in these two field. Because, if I increase the velocity field or pressure field by re-initial geometry, the Reynolds number will change in the physical field. I am confused when get start to learn the versatile codes. This is most important step to proceed for me. rnrnThanks for your valuable suggestions.rnrnJepson rn

[jepson]

Dear Mathias, rnrnProblems solved ! “”Reynolds number just depends on a characteristic length and speed you defined”” that’s the point, During the simulation, once the reference length and time is determined, Reynolds number is fixed during the simulation, what should to do is just translate the simulation result to dimensionless and then to the physical system. I have read the unit choice of Lattice Boltzmann simulation and refer to your suggestions, that maybe the reason why I confused. rnrnThe Reynolds number is not exactly the physical system Reynolds number only depend on the reference you chose.Am I right now?rnrnmany thanks rnrnJepson

[jepson]

Dear Mathias,rnrnOne more question, during the flow simulation in single fracture with pressure gradient increased, why the data can be exacted in the first 10000 steps when the pressure increase to some value, later on, data shown in the file is “”non””, so as the “”.gif”” file. Could you explain that for me?rnrnJepson

[mathias]

Quote:

Quote from jepson on February 17, 2012, 02:12rnDear Mathias, rnrnProblems solved ! “”Reynolds number just depends on a characteristic length and speed you defined”” that’s the point, During the simulation, once the reference length and time is determined, Reynolds number is fixed during the simulation, what should to do is just translate the simulation result to dimensionless and then to the physical system. I have read the unit choice of Lattice Boltzmann simulation and refer to your suggestions, that maybe the reason why I confused. rnrnThe Reynolds number is not exactly the physical system Reynolds number only depend on the reference you chose.Am I right now?rnAnswer: That is right! [Mathias]rnrnmany thanks rnrnJepson

[mathias]

Quote:

Quote from mathias on February 19, 2012, 08:37rn

Quote:

Quote from jepson on February 17, 2012, 02:12rnDear Mathias, rnrnProblems solved ! “”Reynolds number just depends on a characteristic length and speed you defined”” that’s the point, During the simulation, once the reference length and time is determined, Reynolds number is fixed during the simulation, what should to do is just translate the simulation result to dimensionless and then to the physical system. I have read the unit choice of Lattice Boltzmann simulation and refer to your suggestions, that maybe the reason why I confused. rnrnThe Reynolds number is not exactly the physical system Reynolds number only depend on the reference you chose.Am I right now?rn[MATHIAS]: That is right! rnrnmany thanks rnrnJepson

[mathias]

Dear Jepson,rnrn It seems to me that the simulation have become unstable after 10000 steps.rnrnMathias

[jepson]

Dear Mathias, rnrnWhat should I do to deal with such a situation. I have chosen the “”MRTdynamic”” which should reflect the turbulent property. Is it possible to chose a smaller Umax or higher resolution in order to get the stable velocity distribution when the flow get into the turbulent zone.rnrnThe distribution of Posseuille flow before unstable is bimodal distribution which is not the exactly turbulent flow velocity distribution.rnrnthanks a lot rnrnJepson

[mathias]

Yes, I propose to chose a higher resolution of the mesh and smaller uMax.rnMathias

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