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
92 + muT * grad(ep)’ * grad(q)
93 )
94 + int1d(Th, b1, b2)(
95 T * q * 0.001
96 )
97 + int2d(Th)(
98 prode * q
99 - alphaconvect([Upx, Upy], -dt, epp)q
100 )
101 + on(b5, b6, ep=0.00001)
102 + on(b1, b2, ep=betanuep*pow(stress,1.5))
103 ;
in the int1d part, what is the boundary condition of epsilon, that is ep,on b1,b2?

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.