convert a binary flowfield file (.ff) to an ascii data file (.asc)

  -p        --padding                  save full Nx x Ny x Nz grid rather eliminating padded modes
  -g        --gridpoints               write x.asc, y.asc, z.asc gridpoint files
  <fieldname>      (trailing arg 2)    flowfield input file
  <asciifile>      (trailing arg 1)    ascii output file

  field2ascii u.ff u.asc  

ASCII files are stored as physical values of components of the field at the gridpoints (i.e. not spectral coefficients). Fields with 2/3-style padding for dealiasing are saved on reduced grids (Nx,Nz → 2Nx/3, 2Nz/3), unless the -p option is used.

The gzipped ASCII velocity files store gridpoint values of the field in x,y,z,i order using the following C++ code

os << setprecision(16);
for (int nx=0; nx<Nx ++nx)
  for (int ny=0; ny<Ny ++ny)
    for (int nz=0; nz<Nz ++nz)
      for (int i=0; i<Nd ++i)
        os << u(nx,ny,nz,i) << '\n';

The value u(nx,ny,nz,i) is the ith component of the Nd-dimensional field at the gridpoint (nx,ny,nz). The (nx,ny,nz)th gridpoint has spatial coordinates (nx Lx/Nx, cos(ny pi/(Ny-1)), nz Lz/Nz). For 3D velocity fields, the (u,v,w) components are i=0,1,2.

Geometrical and discretization parameters are stored as ASCII in *.geom files as follows

  32                      %Nx
  35                      %Ny
  32                      %Nz
  3                       %Nd
  5.511566058929462       %Lx
  2.513274122871834       %Lz
  0.8771929824561405      %lx=Lx/(2pi)
  0.4                     %lz=Lz/(2pi)
  1.14                    %alpha=2pi/Lx
  2.5                     %gamma=2pi/Lz

In channelflow, Lx and Lz are the canonical geometry specifications. The .geom files provide lx,lz and alpha,gamma for human convenience.