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Error "Attempt to evaluate negative power of zero"

Posted 2011年5月25日 GMT-4 16:50 Modeling Tools & Definitions, Parameters, Variables, & Functions Version 5.2 13 Replies

Xiaoyi Yu
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The error is as follows:

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate expression.
- Expression: d(d((-(mod1.fct-(Tm-mod1.T)*switch(t/unit_s_cf)*mod1.fc^(2/3)/unit_K_cf)*test(mod1.fc))*(dvol),{test@2}),mod1.fc)

In the expression, the only place that contain the function ^ is fc^(2/3). In the model, the fc can be zero, but why the error is "Attempt to evaluate negative power of zero." ? The power is 2/3, why comsol treat it as negative?
13 Replies Last Post 2016年12月16日 GMT-5 11:39
J Yuan
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Posted: 1 decade ago 2011年6月21日 GMT-4 07:33
i meet the same error when dealing with a discontinuse function
i meet the same error when dealing with a discontinuse function
Alexis Grenier
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Posted: 1 decade ago 2011年7月27日 GMT-4 00:21
I have the same problem.

I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power.

I don't find a way to solve this problem, if you succeed can you say me how you did please?




Alexis
I have the same problem. I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power. I don't find a way to solve this problem, if you succeed can you say me how you did please? Alexis
Michael Rembe Certified Consultant
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Posted: 1 decade ago 2011年7月27日 GMT-4 02:48
May be there is an expression that becomes negative. Check your expressions!

In some cases there are expressions that should be zero. But because of numerical effects they get a very small negative value. That may cause this error. In this cases you can add 'eps', a very small number, to your expression. May be, it helps.

Michael
May be there is an expression that becomes negative. Check your expressions! In some cases there are expressions that should be zero. But because of numerical effects they get a very small negative value. That may cause this error. In this cases you can add 'eps', a very small number, to your expression. May be, it helps. Michael
Nina Wilkens
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Posted: 1 decade ago 2011年7月27日 GMT-4 07:02
Hi,
I always had this error, when the initial values (start field) where too far off from the solution. It had nothing to do with the expression being zero, as I had added a small value to stop it from becoming zero. I managed to avoid this error by first solving a simpler problem (without the power 2/3 maybe?) and then using the solution as the start field.
Hope this helps,
Nina
Hi, I always had this error, when the initial values (start field) where too far off from the solution. It had nothing to do with the expression being zero, as I had added a small value to stop it from becoming zero. I managed to avoid this error by first solving a simpler problem (without the power 2/3 maybe?) and then using the solution as the start field. Hope this helps, Nina
Xiaoyi Yu
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Posted: 1 decade ago 2011年7月27日 GMT-4 08:35

I have the same problem.

I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power.

I don't find a way to solve this problem, if you succeed can you say me how you did please?




Alexis


Your are right. In my model, the initial value of fc is zero, so I just plus an very small number eps, and it worked.
[QUOTE] I have the same problem. I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power. I don't find a way to solve this problem, if you succeed can you say me how you did please? Alexis [/QUOTE] Your are right. In my model, the initial value of fc is zero, so I just plus an very small number eps, and it worked.
Alexis Grenier
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Posted: 1 decade ago 2011年7月27日 GMT-4 22:03
I want to create a plastic law : k*(solid.epe^n) where k,n are constants and solid.epe is the plastic strain.

I have tried with k*((solid.epe+eps)^n), the model computes but when I want to analyse the stress I have the same error message!

Have you any idea?


Alexis
I want to create a plastic law : k*(solid.epe^n) where k,n are constants and solid.epe is the plastic strain. I have tried with k*((solid.epe+eps)^n), the model computes but when I want to analyse the stress I have the same error message! Have you any idea? Alexis
Xiaoyi Yu
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Posted: 1 decade ago 2011年7月28日 GMT-4 09:12

I want to create a plastic law : k*(solid.epe^n) where k,n are constants and solid.epe is the plastic strain.

I have tried with k*((solid.epe+eps)^n), the model computes but when I want to analyse the stress I have the same error message!

Have you any idea?


Alexis


I don't know much about your model. You said your model can run, but during the postprocessing, the error occurred. What's the expression of the stress?
[QUOTE] I want to create a plastic law : k*(solid.epe^n) where k,n are constants and solid.epe is the plastic strain. I have tried with k*((solid.epe+eps)^n), the model computes but when I want to analyse the stress I have the same error message! Have you any idea? Alexis [/QUOTE] I don't know much about your model. You said your model can run, but during the postprocessing, the error occurred. What's the expression of the stress?
Alexis Grenier
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Posted: 1 decade ago 2011年8月9日 GMT-4 20:37
Thanks for your answer, I have succeed to create my plastic law by studying the COMSOL model: elastoplastic_plate.

Cheers,

Alexis
Thanks for your answer, I have succeed to create my plastic law by studying the COMSOL model: elastoplastic_plate. Cheers, Alexis
Quang Tuyen Tran
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Posted: 1 decade ago 2011年9月18日 GMT-4 03:24
Hello everyone,

I had the same problem, you can see my problem:

Failed to find consistent initial values.
Attempt to evaluate negative power of zero.
Function: ^
Failed to evaluate expression.
Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k)
Last time step is not converged.

The initial of mu is not zero, it is the density of the material Air, and PE.
I don't find a way to solve this problem, if you succeed can you say me how you did please?


Hello everyone, I had the same problem, you can see my problem: Failed to find consistent initial values. Attempt to evaluate negative power of zero. Function: ^ Failed to evaluate expression. Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k) Last time step is not converged. The initial of mu is not zero, it is the density of the material Air, and PE. I don't find a way to solve this problem, if you succeed can you say me how you did please?
Xiaoyi Yu
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Posted: 1 decade ago 2011年9月18日 GMT-4 11:13

Hello everyone,

I had the same problem, you can see my problem:

Failed to find consistent initial values.
Attempt to evaluate negative power of zero.
Function: ^
Failed to evaluate expression.
Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k)
Last time step is not converged.

The initial of mu is not zero, it is the density of the material Air, and PE.
I don't find a way to solve this problem, if you succeed can you say me how you did please?


There is only one ^ in your expression, try "mu+eps" to see whether it works or not.
[QUOTE] Hello everyone, I had the same problem, you can see my problem: Failed to find consistent initial values. Attempt to evaluate negative power of zero. Function: ^ Failed to evaluate expression. Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k) Last time step is not converged. The initial of mu is not zero, it is the density of the material Air, and PE. I don't find a way to solve this problem, if you succeed can you say me how you did please? [/QUOTE] There is only one ^ in your expression, try "mu+eps" to see whether it works or not.
Chaney Long
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Posted: 1 decade ago 2013年7月19日 GMT-4 16:27


Hello everyone,

I had the same problem, you can see my problem:

Failed to find consistent initial values.
Attempt to evaluate negative power of zero.
Function: ^
Failed to evaluate expression.
Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k)
Last time step is not converged.

The initial of mu is not zero, it is the density of the material Air, and PE.
I don't find a way to solve this problem, if you succeed can you say me how you did please?


There is only one ^ in your expression, try "mu+eps" to see whether it works or not.




Hi,

Have you guys solved this typical problem? I am always caught this problem...
[QUOTE] [QUOTE] Hello everyone, I had the same problem, you can see my problem: Failed to find consistent initial values. Attempt to evaluate negative power of zero. Function: ^ Failed to evaluate expression. Expression: d(d((up(mod1.nitf.rho)*up(mod1.nitf.Cp)*mod1.nitf.C mu^0.25*sqrt(max(mod1.k,0))*if(down(try catch(mod1.nitf.mu,-1))!=-1,(up(mod1.T)-down(mod1.T))*down(test(mod1.T)),(down(mod1.T)-up(mod1.T))*up(test(mod1.T)))/mod1.nitf.Tplus)*(dvol),{if@21}),mod1.k) Last time step is not converged. The initial of mu is not zero, it is the density of the material Air, and PE. I don't find a way to solve this problem, if you succeed can you say me how you did please? [/QUOTE] There is only one ^ in your expression, try "mu+eps" to see whether it works or not. [/QUOTE] Hi, Have you guys solved this typical problem? I am always caught this problem...
Chaney Long
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Posted: 1 decade ago 2013年7月19日 GMT-4 16:29

Thanks for your answer, I have succeed to create my plastic law by studying the COMSOL model: elastoplastic_plate.

Cheers,

Alexis


Hi Alex,

Thanks a lot for your mentioning this model for the self define plastic model, but have you tried self define flow rule in your own plastic law in comsol?

Thanks.
Chaney

[QUOTE] Thanks for your answer, I have succeed to create my plastic law by studying the COMSOL model: elastoplastic_plate. Cheers, Alexis [/QUOTE] Hi Alex, Thanks a lot for your mentioning this model for the self define plastic model, but have you tried self define flow rule in your own plastic law in comsol? Thanks. Chaney
João Santos
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Posted: 8 years ago 2016年12月16日 GMT-5 11:39


I have the same problem.

I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power.

I don't find a way to solve this problem, if you succeed can you say me how you did please?




Alexis


Your are right. In my model, the initial value of fc is zero, so I just plus an very small number eps, and it worked.


Xiaoyi Yu,

I'm having the same problem to model an adsorption column using Freundlich isotherm. My initial value of u1 is zero. Could you give me some help?

Attempt to evaluate negative power of zero.
- Function: ^
Failed to evaluate expression.
- Expression: dvol*(-(-(((c0*(n*((comp1.u1*c0)^(n-1))))*KF)*(1/q0))))
Failed to evaluate Jacobian of expression.
- Expression: dvol*(-(((comp1.u2*S)+comp1.u2t)-((((comp1.u1*c0)^n)*KF)/q0)))
Failed to evaluate Jacobian of expression.
- Expression: (-(comp1.u2t+S*comp1.u2-KF*(c0*comp1.u1)^n/q0)*test(comp1.u2))*(dvol)
- Feature: Time-Dependent Solver 1 (sol1/t1)

Regards,

João
[QUOTE] [QUOTE] I have the same problem. I think COMSOL wants to derive this equation. If you derive x^(2/3) the results is (2/3)x^(-1/3) so there is a negative power. I don't find a way to solve this problem, if you succeed can you say me how you did please? Alexis [/QUOTE] Your are right. In my model, the initial value of fc is zero, so I just plus an very small number eps, and it worked. [/QUOTE] Xiaoyi Yu, I'm having the same problem to model an adsorption column using Freundlich isotherm. My initial value of u1 is zero. Could you give me some help? Attempt to evaluate negative power of zero. - Function: ^ Failed to evaluate expression. - Expression: dvol*(-(-(((c0*(n*((comp1.u1*c0)^(n-1))))*KF)*(1/q0)))) Failed to evaluate Jacobian of expression. - Expression: dvol*(-(((comp1.u2*S)+comp1.u2t)-((((comp1.u1*c0)^n)*KF)/q0))) Failed to evaluate Jacobian of expression. - Expression: (-(comp1.u2t+S*comp1.u2-KF*(c0*comp1.u1)^n/q0)*test(comp1.u2))*(dvol) - Feature: Time-Dependent Solver 1 (sol1/t1) Regards, João

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