Devendra Badgujar
Forum Moderator

Hello Matthew, 

Thanks for reaching out. For simulations including multiple fluids, all gases in the simulation will be treated as compressible using variable density with the ideal gas law. All liquids (here it seems you are using water) in a simulation with multiple fluids are treated as incompressible, regardless of whether compressibility is turned on.

Do you want to add compressibilty to Water or Air? You can chnage the material to air and see if that helps. 

For more information please refer Compressible Flows in Ansys Discovery help documentation.