xref: /petsc/src/snes/tutorials/ex31.c (revision fcf85c8c01162926ce932c17797a2648ce6ee273)

static char help[] = "A Chebyshev spectral method for the compressible Blasius boundary layer equations.\n\n";

/*
   Include "petscsnes.h" so that we can use SNES solvers.  Note that this
   file automatically includes:
     petscsys.h       - base PETSc routines   petscvec.h - vectors
     petscmat.h - matrices
     petscis.h     - index sets            petscksp.h - Krylov subspace methods
     petscviewer.h - viewers               petscpc.h  - preconditioners
     petscksp.h   - linear solvers
   Include "petscdt.h" so that we can have support for use of Quadrature formulas
*/
/*F
This examples solves the compressible Blasius boundary layer equations
2(\rho\muf'')' + ff'' = 0
(\rho\muh')' + Prfh' + Pr(\gamma-1)Ma^{2}\rho\muf''^{2} = 0
following Howarth-Dorodnitsyn transformation with boundary conditions
f(0) = f'(0) = 0, f'(\infty) = 1, h(\infty) = 1, h = \theta(0). Where \theta = T/T_{\infty}
Note: density (\rho) and viscosity (\mu) are treated as constants in this example
F*/
#include <petscsnes.h>
#include <petscdt.h>

/*
   User-defined routines
*/

extern PetscErrorCode FormFunction(SNES,Vec,Vec,void*);

typedef struct {
   PetscReal Ma, Pr, h_0;
   PetscInt N;
   PetscReal dx_deta;
   PetscReal *x;
   PetscReal gamma;
} Blasius;

int main(int argc,char **argv)
{
  SNES           snes;         /* nonlinear solver context */
  Vec            x,r;          /* solution, residual vectors */
  PetscMPIInt    size;
  Blasius        *blasius;
  PetscReal      L, *weight;   /* L is size of the domain */

  PetscCall(PetscInitialize(&argc,&argv,(char*)0,help));
  PetscCallMPI(MPI_Comm_size(PETSC_COMM_WORLD,&size));
  PetscCheck(size == 1,PETSC_COMM_WORLD,PETSC_ERR_WRONG_MPI_SIZE,"Example is only for sequential runs");

  // Read command-line arguments
  PetscCall(PetscCalloc1(1, &blasius));
  blasius->Ma = 2;            /* Mach number */
  blasius->Pr = 0.7;          /* Prandtl number */
  blasius->h_0 = 2.;          /* relative temperature at the wall */
  blasius->N = 10;            /* Number of Chebyshev terms */
  blasius->gamma = 1.4;       /* specific heat ratio */
  L = 5;
  PetscOptionsBegin(PETSC_COMM_WORLD, NULL, "Compressible Blasius boundary layer equations", "");
  PetscCall(PetscOptionsReal("-mach", "Mach number at freestream", "", blasius->Ma, &blasius->Ma, NULL));
  PetscCall(PetscOptionsReal("-prandtl", "Prandtl number", "", blasius->Pr, &blasius->Pr, NULL));
  PetscCall(PetscOptionsReal("-h_0", "Relative enthalpy at wall", "", blasius->h_0, &blasius->h_0, NULL));
  PetscCall(PetscOptionsReal("-gamma", "Ratio of specific heats", "", blasius->gamma, &blasius->gamma, NULL));
  PetscCall(PetscOptionsInt("-N", "Number of Chebyshev terms for f", "", blasius->N, &blasius->N, NULL));
  PetscCall(PetscOptionsReal("-L", "Extent of the domain", "", L, &L, NULL));
  PetscOptionsEnd();
  blasius->dx_deta = 2 / L;   /* this helps to map [-1,1] to [0,L] */
  PetscCall(PetscMalloc2(blasius->N-3, &blasius->x, blasius->N-3, &weight));
  PetscCall(PetscDTGaussQuadrature(blasius->N-3, -1., 1., blasius->x, weight));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Create nonlinear solver context
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
  PetscCall(SNESCreate(PETSC_COMM_WORLD,&snes));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Create matrix and vector data structures; set corresponding routines
     - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */
  /*
     Create vectors for solution and nonlinear function
  */
  PetscCall(VecCreate(PETSC_COMM_WORLD,&x));
  PetscCall(VecSetSizes(x,PETSC_DECIDE,2*blasius->N-1));
  PetscCall(VecSetFromOptions(x));
  PetscCall(VecDuplicate(x,&r));

   /*
      Set function evaluation routine and vector.
   */
  PetscCall(SNESSetFunction(snes,r,FormFunction,blasius));
  {
     KSP ksp;
     PC pc;
     SNESGetKSP(snes, &ksp);
     KSPSetType(ksp, KSPPREONLY);
     KSPGetPC(ksp, &pc);
     PCSetType(pc, PCLU);
  }
  /*
     Set SNES/KSP/KSP/PC runtime options, e.g.,
         -snes_view -snes_monitor -ksp_type <ksp> -pc_type <pc>
     These options will override those specified above as long as
     SNESSetFromOptions() is called _after_ any other customization
     routines.
  */
  PetscCall(SNESSetFromOptions(snes));

  PetscCall(SNESSolve(snes,NULL,x));
  //PetscCall(VecView(x,PETSC_VIEWER_STDOUT_WORLD));

  /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
     Free work space.  All PETSc objects should be destroyed when they
     are no longer needed.
   - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */

  PetscCall(PetscFree2(blasius->x, weight));
  PetscCall(PetscFree(blasius));
  PetscCall(VecDestroy(&x));
  PetscCall(VecDestroy(&r));
  PetscCall(SNESDestroy(&snes));
  PetscCall(PetscFinalize());
  return 0;
}

/*-------------------------------------------------------------------------------
   Helper function to evaluate Chebyshev polynomials with a set of coefficients
   with all their derivatives represented as a recurrence table
-------------------------------------------------------------------------------*/
static void ChebyshevEval(PetscInt N, const PetscScalar *Tf, PetscReal x, PetscReal dx_deta, PetscScalar *f){
   PetscScalar table[4][3] = {
      {1, x, 2*x*x - 1}, {0, 1, 4*x}, {0, 0, 4}, {0, 0, 0} /* Chebyshev polynomials T_0, T_1, T_2 of the first kind in (-1,1)  */
   };
   for (int i=0; i<4; i++) {
      f[i] = table[i][0] * Tf[0] + table[i][1] * Tf[1] + table[i][2] * Tf[2];  /* i-th derivative of f */
   }
   for (int i=3; i<N; i++) {
      table[0][i%3] = 2 * x * table[0][(i-1) % 3] - table[0][(i-2)%3];  /* T_n(x) = 2xT_{n-1}(x) - T_{n-2}(x) */
      /* Differentiate Chebyshev polynomials with the recurrence relation */
      for (int j=1; j<4; j++) {
         table[j][i%3] = i * (2 * table[j-1][(i-1) % 3] + table[j][(i-2)%3] / (i-2)); /* T'_{n}(x)/n = 2T_{n-1}(x) + T'_{n-2}(x)/n-2 */
      }
      for (int j=0; j<4; j++) {
         f[j] += table[j][i%3] * Tf[i];
      }
   }
   for (int i=1; i<4; i++) {
      for (int j=0; j<i; j++) f[i] *= dx_deta; /* Here happens the physics of the problem */
   }
}

/* ------------------------------------------------------------------- */
/*
   FormFunction - Evaluates nonlinear function, F(x).

   Input Parameters:
.  snes - the SNES context
.  X    - input vector
.  ctx  - optional user-defined context

   Output Parameter:
.  R - function vector
 */
PetscErrorCode FormFunction(SNES snes,Vec X,Vec R,void *ctx)
{
  Blasius *blasius = (Blasius *)ctx;
  const PetscScalar *Tf, *Th;  /* Tf and Th are Chebyshev coefficients */
  PetscScalar       *r, f[4], h[4];
  PetscInt N = blasius->N;
  PetscReal Ma = blasius->Ma, Pr = blasius->Pr;

  /*
   Get pointers to vector data.
      - For default PETSc vectors, VecGetArray() returns a pointer to
        the data array.  Otherwise, the routine is implementation dependent.
      - You MUST call VecRestoreArray() when you no longer need access to
        the array.
   */
  PetscCall(VecGetArrayRead(X,&Tf));
  Th = Tf + N;
  PetscCall(VecGetArray(R,&r));

  /* Compute function */
  ChebyshevEval(N, Tf, -1., blasius->dx_deta, f);
  r[0] = f[0];
  r[1] = f[1];
  ChebyshevEval(N, Tf, 1., blasius->dx_deta, f);
  r[2] = f[1]  - 1; /* Right end boundary condition */
  for (int i=0; i<N - 3; i++) {
     ChebyshevEval(N, Tf, blasius->x[i], blasius->dx_deta, f);
     r[3+i] = 2*f[3] + f[2] * f[0];
     ChebyshevEval(N-1, Th, blasius->x[i], blasius->dx_deta, h);
     r[N+2+i] = h[2] + Pr * f[0] * h[1] + Pr * (blasius->gamma - 1) * PetscSqr(Ma * f[2]);
  }
  ChebyshevEval(N-1, Th, -1., blasius->dx_deta, h);
  r[N] = h[0] - blasius->h_0; /* Left end boundary condition */
  ChebyshevEval(N-1, Th, 1., blasius->dx_deta, h);
  r[N+1] = h[0] - 1; /* Left end boundary condition */

  /* Restore vectors */
  PetscCall(VecRestoreArrayRead(X,&Tf));
  PetscCall(VecRestoreArray(R,&r));
  return 0;
}

/*TEST

   test:
      args: -snes_monitor -pc_type svd
      requires: !single

TEST*/