# Boundary conditions in large fluid problem in the document

Hello, I am new to cfd and I just try to understand the boundary condition of the epsilon in the page large fluid problem, the link is `[large fluid problem] (https://doc.freefem.org/tutorials/aLargeFluidProblem.html)`
the code is

89problem ViscosityTurbulence(ep, q)
90 = int2d(Th)(
91 (1.92epp/kp + alpha) * ep * q
93 )
94 + int1d(Th, b1, b2)(
95 T * q * 0.001
96 )
97 + int2d(Th)(
98 prode * q
99 - alpha
convect([Upx, Upy], -dt, epp)q
100 )
101 + on(b5, b6, ep=0.00001)
102 + on(b1, b2, ep=beta
nuep*pow(stress,1.5))
103 ;
in the int1d part, what is the boundary condition of epsilon, that is ep,on b1,b2?

b1 and b2 are the boundaries. I think since the `int1d` and `on` commands both act on boundaries b1 and b2, this is a mixed boundary condition.

Note that strong form of the epsilon equation contains a viscous diffusion term (Laplacian). Hence, the term on line 92 was integrated by parts to get this weak form. Since there is no corresponding boundary integral, this imposes a Neumann condition on `ep` for any boundary without corresponding Dirichlet conditions (i.e. `b3, b4`).

The `int1d` term on line 95 is a boundary term that does not involve `ep`, hence it only appears in the RHS.

So, from what I can tell, we have homogeneous Neumann conditions along `b3,b4` and inhomogeneous Dirichlet conditions on `b1,b2,b5,b6`.