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In channelflow the total velocity field is broken into a fixed base flow U(y) and a fluctuating part u(x,y,z,t)

$u_{tot} = U(y) + u(x,y,z,t)$

In previous versions of channelflow you had to explicitly construct a function U(y) and give it to DNS as a parameter. Whether or not u(x,y,z,t) was a fluctuation or the total flow dependend on whether you set U(y) to a nonzero or zero value.

In the new version there is a simplified syntax for the most common cases:

1. $U(y) = 0$ and $u(x,y,z,t) = u_{tot}$ , for a channel flow

   DNSFlags flags;
   flags.baseflow = Zero;
   flags.constraint = PressureGradient;
   flags.dPdx = some real number;
   ...
   FlowField u(Nx,Ny,Nz,Lx,Lz,a,b);
   ...
   DNS dns(u, nu, dt, flags, T0);

2. $U(y) = 1 - y^2$ , a parabolic base flow, and u is the fluctuation, for a channel flow

   DNSFlags flags;
   flags.baseflow = Parabola;
   flags.constraint = PressureGradient;
   flags.dPdx = some real number;
   etc.

3. $U(y) = y$ and $u$ is the fluctuation, for plane Couette flow

   DNSFlags flags;
   flags.baseflow = PlaneCouette;
   flags.constraint = PressureGradient;
   flags.dPdx = 0.0;

Another possibility is to set the mean constraint to be BulkVelocity and give a value for flags.Ubulk.
The old syntax

   ChebyCoeff Ubase(Ny,a,b,Physical);
   for (int ny=0; ny<Ny; ++ny){   
     Ubase[ny] = 4*(1.0 - square(y[ny])); // or some other prescribed function
      ...
   DNS dns(u, Ubase, nu, dt, flags, T0);

Is still available in case you want to set U(y) to something like the mean velocity profile of a turbulent flow.

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