# Updating BC Expression

**URL:** <https://fenicsproject.discourse.group/t/updating-bc-expression/2401>\
**Category:** variational formulation\
**Created:** [February 10, 2020, 11:58am UTC](https://fenicsproject.discourse.group/t/updating-bc-expression/2401 "2020-02-10T11:58:47Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![jacktat](https://avatars.discourse-cdn.com/v4/letter/j/a9a28c/32.png) [@jacktat](https://fenicsproject.discourse.group/u/jacktat)\
**Post date:** [February 10, 2020, 11:58am UTC](https://fenicsproject.discourse.group/t/updating-bc-expression/2401/1 "2020-02-10T11:58:48Z")

</div>

Hello,

I want to update my boudnary condition and I’m running into an error which, I think is due to a worng definition or expresison, I’m not sure. However, I’m not sure how to solve this problem.

```
pres_oo = Expression('art_pressure', art_pressure=0., degree=1)

# Define boundary conditions
bcu_noslip1 = DirichletBC(V, Constant((0,0)), walls)
bcp_inflow = DirichletBC(Q, Constant(8), inflow)
bcp_outflow = DirichletBC(Q, pres_oo, outflow)

```

…

```
Rp = 1e-6
C = .5 * Rp
Z = (1.1 * Rp) * 0.05
RC = Rp * C 
Q_outlet=0.

P_out_new = Expression('((Z+(Rp*dt)/(RC+dt)-((Z*RC)/(RC+dt))*Q_outlet+((RC)/(RC+dt))*p_n))',Z=Z,Rp=Rp,RC=RC,dt=dt,Q_outlet=Q_outlet,p_n=p_n,degree=1)

```

…

`pres_oo.art_pressure=P_out_new #update BC`  
This update BC part is in the time-stepping part.  
The above snippets are the relevant parts of my code. The idea is that every time step the new P\_out is calculated and applied as BC. However, when running this code i get:

> ```
> Traceback (most recent call last):
> File "tut09.py", line 194, in <module>
> pres_oo.art_pressure=P_out_new #update BC
> File "/usr/local/lib/python3.6/dist-packages/dolfin/function/expression.py", line 439, in __setattr__
> self._parameters[name] = value
> File "/usr/local/lib/python3.6/dist-packages/dolfin/function/expression.py", line 299, in __setitem__
> self._cpp_object.set_property(key, value)
> RuntimeError: No such property 
> 
> ```

I cannot seem to figure out whats going wrong, when I want to print the value of P\_out\_new during the time step it does not give me a value but f\_55 or f\_54.

Thanks in advance!  
Cheers,  
Jack

---

<div class="post-metadata">

**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\
**Post date:** [February 10, 2020, 1:11pm UTC](https://fenicsproject.discourse.group/t/updating-bc-expression/2401/2 "2020-02-10T13:11:55Z")

</div>

Hi @jacktat  
Please consider using `UserExpression`, as illustrated below:

```auto
from dolfin import *
mesh = UnitSquareMesh(10,10)
V = FunctionSpace(mesh, "CG", 1)

class press_oo(UserExpression):
    def __init__ (self, art_pressure, **kwargs):
        super(). __init__ (kwargs)
        self.art_pressure = art_pressure
    def eval(self, values, x):
        values[0] = self.art_pressure(x)
    def value_shape(self):
        return ()

art_pressure = Function(V)
pres_oo = press_oo(art_pressure)
print(assemble(pres_oo*dx(domain=mesh)))
x = SpatialCoordinate(mesh)
pres_oo.art_pressure.assign(project(x[0]*x[1],V))

print(assemble(pres_oo*dx(domain=mesh)))

```

which yields the output

```auto
0.0
0.24999999999999983

```

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<div class="post-metadata">

**Author:** ![jacktat](https://avatars.discourse-cdn.com/v4/letter/j/a9a28c/32.png) [@jacktat](https://fenicsproject.discourse.group/u/jacktat)\
**Post date:** [February 10, 2020, 4:10pm UTC](https://fenicsproject.discourse.group/t/updating-bc-expression/2401/3 "2020-02-10T16:10:57Z")

</div>

Thanks for the response Dokken. How does a userExpression look like for the `P_out_new`? I figured this also has to become a userExpression where I update` Q_outlet` every timestep. But I don’t know how how to get that full formula  
`P_out_new = Expression('((Z+(Rp*dt)/(RC+dt)-((Z*RC)/(RC+dt))*Q_outlet+((RC)/(RC+dt))*p_n))',Z=Z,Rp=Rp,RC=RC,dt=dt,Q_outlet=Q_outlet,p_n=p_n,degree=1)`

as a userExpression.

any tips?

---

<div class="post-metadata">

**Author:** ![dokken](https://yyz2.discourse-cdn.com/free1/user_avatar/fenicsproject.discourse.group/dokken/32/1560_2.png) [@dokken](https://fenicsproject.discourse.group/u/dokken)\
**Post date:** [February 10, 2020, 10:08pm UTC](https://fenicsproject.discourse.group/t/updating-bc-expression/2401/4 "2020-02-10T22:08:30Z")

</div>

It is quite straight-forward:

```auto
from dolfin import *
mesh = UnitSquareMesh(10,10)
V = FunctionSpace(mesh, "CG", 1)

Rp = 1e-6
C = .5 * Rp
Z = (1.1 * Rp) * 0.05
RC = Rp * C
dt = 0.1

class p_out_new(UserExpression):
    def __init__ (self, p_n, q_outlet,**kwargs):
        super(). __init__ (kwargs)
        self.p_n = p_n
        self.q_outlet = q_outlet
    def eval(self, values, x):
        values[0] = (Z + (Rp * dt) / (RC + dt)
                     - (Z * RC)/(RC + dt)*self.q_outlet(x)
                     + RC / (RC + dt) * self.p_n(x))

    def value_shape(self):
        return ()

p_new = p_out_new(Function(V), Function(V))
print(assemble(p_new*dx(domain=mesh)))
x = SpatialCoordinate(mesh)

p_new.p_n.assign(project(x[0]*x[1],V))
p_new.q_outlet.assign(project(x[1],V))
print(assemble(p_new*dx(domain=mesh)))

```