# Homework 7, due 08.00, 5/12-2014¶

## Solving the wave equation on a curvilinear grid using MPI¶

Consider the wave equation in two dimensions with a variable coefficient

$$$u_{tt} =(a(x,y)u_x)_x + (a(x,y)u_y)_y + F(x,y,t), \ \ (x,y) \in \Omega, \ \ t > 0,$$$

with Dirichlet boundary conditions

$$$u(x,y,t) = g(x,y,t), \ \ \forall \, (x,y) \in \partial \Omega,$$$

and initial conditions

$$$u(x,y,0) = f(x,y), \ \ u_t(x,y,0) = h(x,y), \ \ \forall \, (x,y) \in \Omega,$$$

In this project we will restrict the geometry $$\Omega$$ to be a logically square shaped domain, as we did in homework 4. That is we assume that there is a smooth mapping $$(x,y)=(x(r,s),y(r,s))$$ from the reference element $$\Omega_R = \{(r,s) \in [-1,1]^2$$ to $$\Omega$$.

In the $$(r,s)$$ coordinate system the wave equation takes the form:

$\begin{eqnarray} J u_{tt} =(J r_x a(x,y) (r_xu_r + s_x u_s) + J r_y a(x,y) (r_y u_r + s_y u_s) )_r \\ + (J s_x a(x,y) (r_xu_r + s_x u_s) + J s_y a(x,y) (r_y u_r + s_y u_s) )_s \\ + J F(x,y,t), \ \ (x,y) \in \Omega, \ \ t > 0. \end{eqnarray}$

Here the metric $$r_x, r_y, s_x, s_y$$ and $$J(r,s) = x_r y_s - x_s y_r$$ can be computed in the same way as in homework 4.

### Parallelization Assignments¶

1. Familiarize yourself with the program homework7.f90 in the repository. Currently the parallelization strategy is to split the computational domain into nprocs slabs by introducing a one dimensional domain decomposition with px_max processors ranging from the processor number px=1 to px=px_max. Here the mapping between the labeling of the processors provided by MPI_COMM_WORLD and the labeling for px is simple, myid = px-1. This one dimensional domain decomposition does not minimize the communication between domains and your first assignment is to improve it by splitting the domain into px_max*py_max processors where px_max and py_max are chosen to minimize abs(nx/px_max-ny/py_max).
2. Once you have found px_max and py_max you should split up the number of gridpoints as was done in the 1D case. It is convenient to create functions that maps the myid indexation to the (px,py) indexation.
3. Now that the computational domain is decomposed in a two dimensional way you will also have to update the communication. Start by assigning (and re-assigning) the left, right, top and bottom neighbors of each process.
4. Check that your domain decomposition and communication is working by computing the derivatives (as is already done in the homework7.f90) file.

### Discretization Assignments¶

1. Before you start to discretize the equations you should implement a function or subroutine that produces a Manufactured Solution.

What does this mean?

Say that we are interested in finding a discrete approximation, $$v$$, to the solution of the initial-boundary-value-problem

\begin{align} &u_{tt}=u_{xx}+f, \ \ t > 0,\, x\in[0,1], \\ & u(x,0) = u_0(x), \ \ u_t(x,0) = u_1(x),\\ & \alpha_0 u(0,t) +\alpha_1 u_x(0,t) = u_l(t), \\ & \beta_0 u(1,t) +\beta_1 u_x(1,t) = u_r(t), \end{align}

by implementing some numerical method on a computer. How can we convince ourselves (and the instructor or TA) that the code is implementing the numerical method and thus approximating the initial-boundary-value-problem correctly?

Assume that the numerical method has an error that converges to zero with some discretization parameter $$h$$ (usually the grid spacing) as $$\mathcal{O}(h^r)$$. Then a good test is to measure the error

$$$\epsilon_p(t)= \left( \int_0^1 |u-v|^p \,dx \right)^{\frac{1}{p}},$$$

for some different values of $$h$$ and make sure that $$\epsilon_p \sim \mathcal{O}(h^r)$$, as advertised. The crux of the matter is that the computation of $$\epsilon_p$$ require the knowledge of $$u$$, which we don’t know!

This is where the method of manufactured solution comes in. If we want the solution to be, say,

$$$\label{mms} u(x,t) = \sin(\omega (t+t_0) -k x),$$$

how should we adjust the initial-boundary-value-problem so that this is a solution? The answer is simple, we just have to choose the forcing, initial- and boundary-conditions we get by plugging in this solution. For example, with $$u$$ as above, we would get:

\begin{align*} & f(x,t)=(k^2-\omega^2) \sin(\omega (t+t_0) -k x),\\ & u(x,0) = \sin(\omega (t_0) -k x),\\ & u_l(t) =\alpha_0 \sin(\omega (t+t_0)) - \alpha_1 k \cos(\omega (t+t_0)),\\ & u_r(t) = \beta_0 \sin(\omega (t+t_0) -k) -\beta_1 k \cos(\omega (t+t_0)-k). \end{align*}

Thus, to use this method you will have to build a library or (at least a function or subroutine) of routines of different exact solutions and their derivatives w.r.t. time and space.

Two solutions that are commonly used for manufactured solutions are the trigonometric solution

$$$u(x,y,z,t)= \sin(\omega (t+t_0) -k_x x) \sin( k_y y)\sin(k_z z),$$$

and the polynomial solution

$$$u(x,y,z,t)= \left(\sum a_i t^i \right) \left(\sum b_j x^j \right)\left(\sum c_k y^k \right)\left(\sum d_l x^l \right).$$$

The polynomial solution is very useful for testing separate routines as the approximated solution will be exact if the degree of the polynomials are low enough (a second order method is often exact for first order polynomials, etc.).

The trigonometric solution, being bounded by 1, is well suited if you need to monitor the error over a long time interval. This can be useful if you need to understand the mechanisms behind some numerical instabilities.

I cannot stress enough how useful it is to use the method of manufactured solution, if you start using it now it will save you countless hours once you start implementing more complicated algorithms.

2. Discretization in time.

To discretize in time you will use a simple centered difference. Precisely, if $$v_{i,j}^n \approx u(x_i,y_j,t_n)$$ you should use the approximation

$$$u_{tt}(x_i,y_j,t_n) \approx \frac{v_{i,j}^{n+1}-2 v_{i,j}^n+v_{i,j}^{n-1}}{(\Delta t)^2}$$$

If you have successfully written your MMS routine you can now start to a assemble your complete code. It could look something like this.

do it = 1,nsteps
t = dt*dble(it-1)

! Set boundary conditions here (if you have boundaries!!!)
call set_bc(u,nxl,nyl,t,bc_type,p_left,p_right,p_up,p_down)

! Communicate to update the ghost points in u
! send to the left recieve from the right
call MPI_Sendrecv(u(1,0:nsl+1),nsl+2,MPI_DOUBLE_PRECISION,p_left,123,&
u(nrl+1,0:nsl+1),nsl+2,MPI_DOUBLE_PRECISION,p_right,123,&
MPI_COMM_WORLD,status,ierr)
! send to the right recieve from the left
call MPI_Sendrecv(u(nrl,0:nsl+1),nsl+2,MPI_DOUBLE_PRECISION,p_right,125,&
u(0,0:nsl+1),nsl+2,MPI_DOUBLE_PRECISION,p_left,125,&
MPI_COMM_WORLD,status,ierr)
! etc....
! Compute forcing
call compute_forcing(Force,nxl,nyl,x,y,t,force_type)
! Compute Laplacian, i.e. (c u_x)_x + (c u_y)_y but on the reference square
call compute_lap(Lap,c,J,rx,sx,ry,sy,nxl,nyl,hr,hs)
! Compute the right hand side, if you are just testing the time stepper
! you can set rhside = u_tt !!!
rhside = Lap + Force

! Update the solution at the next time level.
up(1:nxl,1:nyl) = 2.d0*u(1:nxl,1:nyl)-um(1:nxl,1:nyl) + dt**2*rhside
! Shift the time levels
um = u
u = up

! Compute errors and display them here
!(You might not want to do this every single timestep)
call MPI_Reduce(my_max_err,global_max_err,1,&
MPI_DOUBLE_PRECISION,MPI_MAX,0,MPI_COMM_WORLD,ierr)
if(myid == 0) then
write(*,*) 'The max error at time : ', t+dt, ' is: ', global_max_err
end if
end do


Once you have created skeleton routines for the forcing, boundary conditions and the Laplacian you should test your code to make sure it is second order accurate in time. This may also be a good time to develop some output and plotting routines. The easiest way to output in parallel is to simply have each processof write its own part of the solution and then put it all together is a visualization program (for example in Matlab.) Having each processor write its own file is typically not the most efficient way of doing I/O, it is better to have a smaller subset of the processes do the writing (the size of the smaller subset is hardware dependent), but with our limited timeframe in this course it will have to do.

3. Discretization in space.

Now that you have checked that your time-stepping is working and that you are able to plot the solution it is time to put away your mobile device and concentrate for a little bit, getting this part right will take some effort.

I am assuming here that each process has an array u(0:nxl+1,0:nyl+1) where the physical domain owned by each process corresponds to the indicies 1:nxl,1:nyl. There are eight terms of the type $$(J r_x a(x(r,s),y(r,s)) r_x u_r)_r$$ in the equation above. We can write any of them compactly as:

$$$Q(r,s) u_r)_r, \ \ (Q(r,s) u_s)_s, \ \ (Q(r,s) u_r)_s, \ \ (Q(r,s) u_s)_r.$$$

These should be discretized as follows:

$\begin{eqnarray} &\frac{\partial}{\partial r}(Q u_r) \approx & D_-^r\left(E^r(Q) D_+^r u\right), \\ &\frac{\partial}{\partial s}(Q u_s) \approx & D_-^s\left(E^s(Q) D_+^s u\right), \\ &\frac{\partial}{\partial r}(Q u_s) \approx & D_0^s \left(Q D_0^r u\right),\\ &\frac{\partial}{\partial s}(Q u_r) \approx & D_0^r \left(Q D_0^s u\right). \end{eqnarray}$

Here

$\begin{eqnarray} D_+r u_{i,j} = \frac{u_{i+1,j}-u_{i,j}}{h_r}, \ \ D_-^r u_{i,j} = D_+^r u_{i-1,j},\\ D_+s u_{i,j} = \frac{u_{i,j+1}-u_{i,j}}{h_s}, \ \ D_-^s u_{i,j} = D_+^s u_{i,j-1},\\ D_0^r u_{i,j} = \frac{u_{i+1,j}-u_{i-1,j}}{2h_r},\\ D_0^s u_{i,j} = \frac{u_{i,j+1}-u_{i,j-1}}{2h_s},\\ \end{eqnarray}$

are the standard finite difference operators. The averages are defined as:

$\begin{eqnarray} E^r(Q_{i,j}) = (Q_{i+1,j} + Q_{i,j})/2, \\ E^s(Q_{i,j}) = (Q_{i,j+1} + Q_{i,j})/2, \end{eqnarray}$

so that for example

$\begin{eqnarray} D_-^r\left(E^r(Q_{i,j}) D_+^r u_{i,j}\right) = \frac{(Q_{i+1,j}+Q_{i,j})}{2h_r^2}u_{i+1,j} - \frac{(Q_{i+1,j}+2Q_{i,j}+Q_{i-1,j})}{2h_r^2}u_{i,j} + \frac{(Q_{i,j}+Q_{i-1,j})}{2h_r^2}u_{i-1,j}, \end{eqnarray}$

and so forth.

### Experiments (do at least 8)¶

1. Convergence with MMS.

The first thing you should check once you feel your program is bug-free is that the error decreases at the rate you expect (2). Try some different geometries and different $$c(x,y)$$.

2. Convergence for an analytic solution. Bessel functions in an annular region.

3. Strong and week scaling on Stampede.

4. Excitation of resonant modes in complex shapes. See example 3.6 in this paper

5. Mining for gold.

6. Hybrid parallelization. Simple do the do loops on a node using openMP and MPI for communicating between domains.

7. Extra assignments (ask the instructor.)