I am currently doing my Final Year Research Project and I am researching a green energy mechanical buffer system known as the Gravity Power Module. To give a brief description, there is an internal piston that has been installed inside a major cylinder body to store energy in the form of mechanical energy by utilizing the power of gravity and the piston's weight, and the medium inside the cylinder body is water. Currently, I am using Flow Simulation to study the respond characteristics of the system with correlation to the piston's density, particularly the time taken to allow the system to fully charge and discharge at the optimum rate. My simulation includes in changing the material of the piston to represent different densities. However, even with different materials changed, the results I get from my surface goals (AVG Pressure on the piston face (both sides), AVG Force (Y) (both sides)) are the same even with different materials applied. The only thing that changes is the mass flow rate (I set this as a Global Goal). I was wondering, is there anyway to solve this problem?