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====== DNS and DNSFlags ======

The DNS((Direct Numerical Simulation)) class advances velocity fields under the 
Navier-Stokes equations, with a variety of times-stepping algorithms, boundary 
conditions, and external constraints. There are quite a few options to the DNS 
class, so many that there is a special DNSFlags class to group them all together. 

DNSFLags and DNS work together like this. You construct a DNSFlags object and set
its member variables to values that define the flow you want to integrate. Then you
construct a FlowField, which defines the grid and cell sizes. Lastly you construct
a DNS object combining the DNSFlags, the velocity field, the kinematic viscosity, 
and the time step. In C++ code it looks like this

<code c++>
  DNSFlags flags;
  flags.baseflow     = PlaneCouette;
  flags.timestepping = SBDF3;            // 3rd order semi-implicit backwards differentiation
  flags.initstepping = SMRK2;            // 2nd order spalart-moser runge-kutta
  flags.nonlinearity = Rotational;
  flags.dealiasing   = DealiasXZ;
  flags.constraint   = PressureGradient; // enforce constant pressure gradient
  flags.dPdx = 0.0;                      // hold mean pressure gradient at zero

  FlowField u(Nx,Ny,Nz,3,Lx,Lz,a,b);     // velocity
  FlowField p(Nx,Ny,Nz,3,Lx,Lz,a,b);     // pressure

  DNS dns(u, nu, dt, flags);

  for (Real t=T0; t<=T1; t += N*dt) {
     dns.advance(u,p,N);
     ...
  }
</code>

Sections to be written...

===== DNSFlags =====

===== Time-stepping algorithms =====

===== Nonlinearity =====

===== Base flow =====

===== Mean constraint =====

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