MTK -- "infection" from one model to another? #3110
Description
Can one model "inherit" initial values from another❓
I create two models: one with default initial unknown, the other without. The model without default initial unknown gives a warning about under-determined initial values, but gives exactly the same result as the other one. WHY?
using ModelingToolkit, ModelingToolkitStandardLibrary.Blocks
using ModelingToolkit: t_nounits as t, D_nounits as Dt
using OrdinaryDiffEq
using Plots, LaTeXStrings
#
# 1. Parameters
pars = @parameters begin
ρ=1, [description = "Liquid density"]
A=5, [description = "Cross sectional tank area"]
K=5, [description = "Effluent valve constant"]
h_ς=3, [description = "Scaling level in valve model"]
end
# 2. Dependent variables
vars = @variables begin
(m(t)=1.5*ρ*A), [description = "Liquid mass"]
md_i(t), [description = "Influent mass flow rate"]
md_e(t), [description = "Effluent mass flow rate"]
V(t), [description = "Liquid volume"]
h(t), [description = "Liquid level"]
end
# 3. Dependent variables -- without default initial value
vars_0 = @variables begin
m(t), [description = "Liquid mass"]
md_i(t), [description = "Influent mass flow rate"]
md_e(t), [description = "Effluent mass flow rate"]
V(t), [description = "Liquid volume"]
h(t), [description = "Liquid level"]
end
#
eqs = [
Dt(m) ~ md_i-md_e,
m ~ ρ*V,
V ~ A*h,
md_e ~ K*sqrt(h/h_ς)
]
#
@named tank = ODESystem(eqs, t, vars, pars) # Acausal tank model, i.e., without defined input functions
@named tank_0 = ODESystem(eqs, t, vars_0, pars) # Acausal tank model, i.e., without defined input functions
#
@register_symbolic md(t)
#
eqs_i = [md_i ~ md(t)]
@named tank_i = ODESystem(eqs_i, t)
tank_bal = extend(tank,tank_i)
tank_bal_0 = extend(tank_0,tank_i)
tanksys = structural_simplify(tank_bal)
tanksys_0 = structural_simplify(tank_bal_0)
#
tspan = (0,10)
md(t) = 2 + 0.1*sin(2pi*t/4)
prob = ODEProblem(tanksys, [], tspan)Here, creation of prob gives no messages. Next,
julia> prob_0 = ODEProblem(tanksys_0,[],tspan)
┌ Warning: Initialization system is underdetermined. 0 equations for 1 unknowns. Initialization will default to using least squares. To suppress this warning pass warn_initialize_determined = false. To make this warning into an error, pass fully_determined = true
└ @ ModelingToolkit [C:\Users\Bernt_Lie\.julia\packages\ModelingToolkit\OYoU3\src\systems\diffeqs\abstractodesystem.jl:1581](file:///C:/Users/Bernt_Lie/.julia/packages/ModelingToolkit/OYoU3/src/systems/diffeqs/abstractodesystem.jl:1581)
ODEProblem with uType Vector{Float64} and tType Int64. In-place: true
timespan: (0, 10)
u0: 1-element Vector{Float64}:
0.0NOTE that u0 seems to be set to 0.
However, when I solve prob_0 (in spite of warning), I get exactly the same result as when solving prob:
Is the reason for this apparent "infection" of initial value from one problem (prob) to the other (prob_0) that the variable names are in the global namespace, and thus are the same somehow??