# channelflow.org

### Site Tools

docs:math:baseflow

In channelflow the total velocity field is broken into a fixed base flow U(y) and a fluctuating part 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. and , for a channel flow

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

2. , a parabolic base flow, and u is the fluctuation, for a channel flow

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

3. and is the fluctuation, for plane Couette flow

```   DNSFlags flags;
flags.baseflow = PlaneCouette;
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.