Let me say somewhere I have the y axis and this y axis. Downward direction is the negative direction. I assign the surface as well as the bottom surface, free surface as well as the bottom surface. They are, both are represented by y is equal to eta extend. Here, it is represented y minus h x. Now, these are the 2 boundaries, free surface boundary on the bottom surface. So, these are the 2 boundaries. So, on these boundaries, what will happen because on the free surface, there are 2 condition come. In one, I call the kinematic condition. The other one, I call the dynamic condition. There are 2 conditions; on the free surface because as I said that on the free surface, eta is not known.
On the other hand, the pressure on the free surface is pressured on the free surface. The pressure p is equal to p atmosphere. On the other hand, eta is not known a priori. So, it is not known in advance. So, here we have phi is not known. We only said that we satisfy the velocity potential phi satisfies. So, what we do here? We look into two things, one is the determination of phi eta and the other is the dimension of phi.