I need a check on the following exercise about weak formulations and finite elements.

Consider the advection diffusion system $$ \begin{cases} -(\mu u')' + \beta u' + \gamma u = f \\ u(a)=0 \\ u(b) = g_b \end{cases}$$

where $\mu,\beta \beta', \gamma \in C^{0}([a,b])$ and $f \in L^2(a,b)$

  • Write the weak formulation, specifying the functional spaces

  • Give sufficient conditions s.t. the bilinear form is coercive.

Here's my attempt:

  • As I have Dirichlet bc's, I choose as functional space for the test function $H_0^1$.

Therefore, integratin by parts I obtain: $$\int_a^b \mu u'v'dx + \int_a^b \beta u' v dx + \int_a^b \gamma u v dx = \int_a^b fv dx$$

Therefore, the weak formulation is "Find $u \in H^1$, with $u(a)=0$ and $u(b)=g_b$ s.t. $$a(u,v)=F(v)$$ for every $v \in H_0^1$"

The functional spaces are actually different: if all the coefficients were constants, then I could use a "lifting" and look for a solution of the problem with homogeneous Dirichlet.


So I consider $\bar{u} = u-R_g$, where $R_g(x)$ is the lifting function such that $R_g(b)=g_b$ and $R_g(0)=0$.

Then, I plug $u = \bar{u} +R_g$ in the weak formulation and obtain:

$$\int_a^b \mu \bar{u}'v'dx + \int_a^b \beta \bar{u}' v dx + \int_a^b \gamma \bar{u} v dx = \int_a^b fv dx - \int_a^b \Bigl[ \mu R_g' v' +\beta R_g' v +\gamma R_g v\Bigr ]dx$$

Then, I can find with FEM the solution $\bar{u}$, and recover $u(x)$ thanks to $$u(x)=\bar{u}(x) +R_g(x)$$

  • By computing explicitly $$a(u,u)= \int_a^b \mu u'^2 dx + \int_a^b \beta u' u dx + \int_a^b \gamma u^2$$

Using Poincarè inequality, and assuming $0<\mu_1<\mu(x)$: $$\geq \frac{\mu_1}{1+C_P^2} ||u||_V^2 + \int_a^b \beta u u'dx + \gamma u^2 dx$$

Therefore, I observe that $$\beta u u' = (\beta \frac{u^2}{2})'- \beta' \frac{u^2}{2}$$

Then, integrating by parts, using the fact that $u(a)=0$ and $u(b)=g_b$: $$a(u,u)\geq \frac{\mu_1}{1+C_P^2} ||u||_V^2 + \beta \frac{g_b^2}{2} + \int_a^b[\gamma - \frac{\beta'}{2}]u^2dx$$

So I assume $$\gamma - \frac{\beta'}{2} > 0$$ and $$\beta>0$$

This implies $a(u,u) \geq \frac{\mu_1}{1+C_P^2} ||u||_V^2$.

Is it okay?


1 Answer 1


Since your trial and test spaces are different, you have to use a different version of Lax-Milgram lemma, see e.g., [1], Theorem 5.1.2

You can still use lifting idea since the PDE is linear. Then you can verify the conditions in standard Lax-Milgram lemma.

To show coercivity, you need the condition $$ \gamma(x) - \frac{1}{2} b'(x) \ge -\eta, \qquad -\infty < \eta < \frac{\mu_0}{C} $$ where $$ \mu_0 = \min_x \mu(x) > 0 $$ and $C$ is the constant in Poincare inequality. For this, see [1], Section 6.1.2

[1] Quarteroni and Valli, Numerical Approximation of PDE.

  • $\begingroup$ I just edited my post with the use of a lifting function, because I can only use the "standard" version of Lax-Milgram lemma. The lifting should be $$R_g(x)=\frac{x}{b} g_b $$ so indeed $$\bar{u}(b)=g_b - g_b = 0$$ @cdflab $\endgroup$
    – andereBen
    Sep 6, 2020 at 7:26
  • $\begingroup$ Also, following Quarteroni and Valli, in the discretization (with internal nodes from $1,\ldots, N$) with the lifting, I have: $$R_g(x)=\sum_{\text{boundary nodes}} g(x_i) \varphi_i(x)$$ Now, boundary nodes are just $x=a$ and $x=b$ and also $g(a)=0$. So it is $$R_g(x)= g_b \varphi_{N+1}(x) $$ where $N+1$ is the index of the boundary node $b$. Now, I should just leave this as it is, right? Should I write explicitely $\varphi_{N+1}(x)=\frac{x-x_N}{x_{N+1}-X_N}$? $\endgroup$
    – andereBen
    Sep 6, 2020 at 7:27
  • $\begingroup$ could you please confirm my edit? $\endgroup$
    – andereBen
    Sep 6, 2020 at 15:06
  • $\begingroup$ This lifting will work. Then you have to update your coercivity proof in $H^1_0$. You wont need the condition $\beta > 0$. $\endgroup$
    – cfdlab
    Sep 8, 2020 at 7:01
  • $\begingroup$ so I will end up with the condition $$\gamma - \frac{\beta'}{2} > 0 $$ in $(0,1)$, which is essentially like yours, right? $\endgroup$
    – andereBen
    Sep 8, 2020 at 7:24

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