How to create open cavity module

Hi,I am new to fenics,and I want to creat mesh with some simple modules,such as open cavity,so how can I create mesh and define boundaries. Thanks

I suggest you read the tutorials to get started:

I want to know how to define Neumann boundary condition with fenics such as (du/dy=0),are there any examples to refer to,a more easier example?(I didn’t understand the example in fenics-manual-2011).
Thank you for your help!

The Neumann BC goes into your weak form. If you can follow the example of the Poisson equation available on the tutorial website, you will have an idea of how to define a Neumann BC. This specific link follows the steps:

If your problem is defining a specific subdomain for the Neumann boundary condition, I use the package mshr or gmsh. Personally, there is a lot of helpful advice on either of these packages on this forum.