Dear Mohidman,
I'm still having something wrong:
runtime error M6201: MATH sqrt: DOMAIN error.
I'm getting desperated
J.C.
runtime error M6201

 MOHID Developer
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 MOHID Beginner
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 Joined: Thu Jan 01, 1970 00:00
You're having stability problems. The square root error means that the program is calculating the square root of a negative value. In other words, the model made a big boom somewhere and yielded a negative value for a quantity that should be positive (water depth for instance). Conclusion: your domain is far too refined. You've got a Courant number which is about 600 and ideally it should be around 1 for stability. This is only an indicative value, because in practice only trial and error can give us the right condition for stability. Using a Courant number that is under 10 usually causes me no problem. Anyways, in your case, reducing time step only won't do, you will need to coarsen the grid as well.
Compare DX/DT (DXminimal grid spacing, DTtime step) with the velocity of propagation of phenomena that you expect to observe. Let's take the case of a wave propagating in shallow waters at a depth of 10m. It moves at about 10m/s. With DT = 1s, if you grid spacing is 10m, you're ok. If it's 1m, you're having trouble because DX/DT is ten times smaller than the velocity of propagation of your wave. Intuitively seen, by the time you made one iteration, your wave has shifted by 10 cells, so the grid does not \"catch\" the wave. Some would say that all of this comes to nothing if you use totally implicit methods. But then again, implicit methods yield enormous linear systems, usually solved by iterative calculations, that also have stabillity conditions, which resemble strangely to those obtained for explicit methods.
There is another way to make your system stable: by adding diffusion. So go to the turbulence module and check value for horizontal viscosity.
Tip: use vh = 0.2 * DX * U / L
vh  horizontal viscosity, U horizontal velocities (0.1 to 1 m/s), L horizontal length scale(take size of your domain for instance)
Finally, try to work in a progressive way: first step should be a very coarse grid with only hydrodynamics involved. Then you refine your grid untill you're happy with the velocity field definition. Then add all the stuff you enjoy: discharges of dirty water, sun radiation over undefense small creatures, a diver smooking at the bottom of the sea...
Well, that's all folks,
Cheers
Compare DX/DT (DXminimal grid spacing, DTtime step) with the velocity of propagation of phenomena that you expect to observe. Let's take the case of a wave propagating in shallow waters at a depth of 10m. It moves at about 10m/s. With DT = 1s, if you grid spacing is 10m, you're ok. If it's 1m, you're having trouble because DX/DT is ten times smaller than the velocity of propagation of your wave. Intuitively seen, by the time you made one iteration, your wave has shifted by 10 cells, so the grid does not \"catch\" the wave. Some would say that all of this comes to nothing if you use totally implicit methods. But then again, implicit methods yield enormous linear systems, usually solved by iterative calculations, that also have stabillity conditions, which resemble strangely to those obtained for explicit methods.
There is another way to make your system stable: by adding diffusion. So go to the turbulence module and check value for horizontal viscosity.
Tip: use vh = 0.2 * DX * U / L
vh  horizontal viscosity, U horizontal velocities (0.1 to 1 m/s), L horizontal length scale(take size of your domain for instance)
Finally, try to work in a progressive way: first step should be a very coarse grid with only hydrodynamics involved. Then you refine your grid untill you're happy with the velocity field definition. Then add all the stuff you enjoy: discharges of dirty water, sun radiation over undefense small creatures, a diver smooking at the bottom of the sea...
Well, that's all folks,
Cheers

 MOHID Beginner
 Posts: 6
 Joined: Thu Jan 01, 1970 00:00
runtime error M6201
Hi
I have this problem...(runtime error M6201). Is there something related the island in the domain?
what is the best procedure in this case?
thank
Márcio
I have this problem...(runtime error M6201). Is there something related the island in the domain?
what is the best procedure in this case?
thank
Márcio

 Site Admin
 Posts: 349
 Joined: Thu Jan 01, 1970 00:00
 Location: Instituto Superior Tecnico  Lisbon  Portugal
 Contact:
Hi Márcio,
Normally this problem is related with instability of the model (check the max Courant number in the beginning of the simulation, should be around 4; reduce the time step accordingly). Also check if the model grid was correctly designed. Check also the type of coordinate is correct.
Hope this helps,
Luis
Normally this problem is related with instability of the model (check the max Courant number in the beginning of the simulation, should be around 4; reduce the time step accordingly). Also check if the model grid was correctly designed. Check also the type of coordinate is correct.
Hope this helps,
Luis
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