====== field2ascii ======

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

===== options =====

<code>

  -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
</code>

===== usage =====

<code>
  field2ascii u.ff u.asc  
</code>

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. 


===== ASCII velocity fields =====

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.

===== ASCII geometry and discretization =====

Geometrical and discretization parameters are stored as ASCII in *.geom files as follows
<code>
  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
</code>

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