WOLFRAM SYSTEM MODELER

# setSmoothState

Return thermodynamic state so that it smoothly approximates: if x > 0 then state_a else state_b # Wolfram Language

In:= `SystemModel["Modelica.Media.Interfaces.PartialMedium.setSmoothState"]`
Out:= # Information

This information is part of the Modelica Standard Library maintained by the Modelica Association.

This function is used to approximate the equation

```    state = if x > 0 then state_a else state_b;
```

by a smooth characteristic, so that the expression is continuous and differentiable:

```   state := smooth(1, if x >  x_small then state_a else
if x < -x_small then state_b else f(state_a, state_b));
```

This is performed by applying function Media.Common.smoothStep(..) on every element of the thermodynamic state record.

If mass fractions X[:] are approximated with this function then this can be performed for all nX mass fractions, instead of applying it for nX-1 mass fractions and computing the last one by the mass fraction constraint sum(X)=1. The reason is that the approximating function has the property that sum(state.X) = 1, provided sum(state_a.X) = sum(state_b.X) = 1. This can be shown by evaluating the approximating function in the abs(x) < x_small region (otherwise state.X is either state_a.X or state_b.X):

```    X  = smoothStep(x, X_a , X_b , x_small);
X  = smoothStep(x, X_a , X_b , x_small);
...
X[nX] = smoothStep(x, X_a[nX], X_b[nX], x_small);
```

or

```    X  = c*(X_a  - X_b)  + (X_a  + X_b)/2
X  = c*(X_a  - X_b)  + (X_a  + X_b)/2;
...
X[nX] = c*(X_a[nX] - X_b[nX]) + (X_a[nX] + X_b[nX])/2;
c     = (x/x_small)*((x/x_small)^2 - 3)/4
```

Summing all mass fractions together results in

```    sum(X) = c*(sum(X_a) - sum(X_b)) + (sum(X_a) + sum(X_b))/2
= c*(1 - 1) + (1 + 1)/2
= 1
```

# Inputs (4)

x Type: Real Description: m_flow or dp Type: ThermodynamicState Description: Thermodynamic state if x > 0 Type: ThermodynamicState Description: Thermodynamic state if x < 0 Type: Real Description: Smooth transition in the region -x_small < x < x_small

# Outputs (1)

state Type: ThermodynamicState Description: Smooth thermodynamic state for all x (continuous and differentiable)

# Extended by (1) setSmoothState Modelica.Media.Incompressible.TableBased Return thermodynamic state so that it smoothly approximates: if x > 0 then state_a else state_b