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IF statement errors - help needed
IF statement errors - help needed
Hi,
I am having a problem using an if-statement in a model I am developing to simulate bacteria growth in microfluidic systems.
While I have used if-else statements in Modelica before (actually in other sub-models for this system), I am encountering simulation failures for a current model that I am attempting to develop.
The model code is as follows:
model GrowthVelocity
BiofilmNode i,o,p;
parameter Real dz = 1;
parameter Real biofilm_density = 1;
Real Number;
Real biomass_production;
Real inactivebiomass_prod;
equation
if Number > 1 then
i.uz = (( (2*dz) * (biomass_production + inactivebiomass_prod)) / biofilm_density) + p.uz;
else
o.uz = 0;
i.uz = (( (2*dz) * (biomass_production + inactivebiomass_prod)) / biofilm_density) + p.uz;
end if ;
end GrowthVelocity;
When I attempt to run the simulation, I get the following error:
Error: The language feature if-equations is not supported. Suggested workaround: rewrite equations using if-expressions: equation
if thebiofilm.velocity_char[1].Number > 1.0 then
thebiofilm.velocity_char[1].i.uz = 2.0 * (thebiofilm.velocity_char[1].dz * (thebiofilm.velocity_char[1].biomass_production + thebiofilm.velocity_char[1].inactivebiomass_prod) / thebiofilm.velocity_char[1].biofilm_density) + thebiofilm.velocity_char[1].p.uz;
else
thebiofilm.velocity_char[1].o.uz = 0.0;
thebiofilm.velocity_char[1].i.uz = 2.0 * (thebiofilm.velocity_char[1].dz * (thebiofilm.velocity_char[1].biomass_production + thebiofilm.velocity_char[1].inactivebiomass_prod) / thebiofilm.velocity_char[1].biofilm_density) + thebiofilm.velocity_char[1].p.uz;
end if;
I am confused because it is essentially telling me that I am using the wrong if-statement syntax, and then suggests the exact same syntax as a way to fix the error. Again, I have used this exact same construct before in a similar model such as the following with no problems:
model ActiveBiomass "a characteristic :: similar to biomass model"
BiofilmNode i,o,p;
parameter Real dz = 1;
parameter Real Voxygen = 1;
parameter Real Yboxygen = 1;
parameter Real Beta = 1;
parameter Real Koxygen = 1;
parameter Real B = 1;
Real Number;
Real biomass_production;
equation
biomass_production = B*o.rho_biomass*(o.rho_oxygen / (Koxygen+o.rho_oxygen));
if Number > 1 then
der(o.rho_biomass) = biomass_production - (o.uz * ((i.rho_biomass - p.rho_biomass) / (2*dz))) - (o.rho_biomass * ((i.uz - p.uz) / (2*dz)));
else
o.rho_biomass = 0;
end if;
end ActiveBiomass;
I do not see why one model would work with this syntax and another would not, even though they are identical in structure. Has anyone else encountered a similar problem or have any insight as to why this would not be working?
Any help would be greatly appreciated.
Matt
Thank you
Re: IF statement errors - help needed
No, it's suggesting using if-expressions instead of if-statements (and then showing the old if-statement). It failed when trying to rewrite the statement to if-expressions; our backend cannot actually handle statements.
Anyway, I believe the problem is that you have a mismatch in the number of equations in the if-equations. This is only allowed if the condition expression is parameter (?) or constant because then the statement can be optimized to be only one branch.
- sjoelund.se
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- 1700 Posts
Re: IF statement errors - help needed
Thanks for the response @sjoelund !
Technically, what is the difference between an if-statement and an if-equation? And if the issue is that there is 1 equation in the first if statement and 2 equations in the else statement, how would you suggest working around this problem?
Again, thanks for the help!
Re: IF statement errors - help needed
Well, since Modelica is balanced you must not change the number of equations in a system, or it cannot be simulated.
The main difference between if-statement and if-expression is that you can use things like assertions in statements.
In OpenModelica, a statement like
Code:
if cond then
a = b;
else
d = e;
end if;
is converted into
Code:
0 = if cond then a-b else d-e;
If you really need to have a different number of equations, make the condition a parameter. That way you specify that the number of equations cannot change during the simulation.
- sjoelund.se
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- 1700 Posts
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