I have a tank full of water. How do i model the water for a frequency analysis? The water has no tensile properties. I am not interested in stress values in the water just the mass effects on the natural frequencies. As a first guess I simply increased the density of the metal until the weight of the unfilled vessel matched the weight of the water filled vessel but it really does not match the weight distribution of a water filled vessel (it is not a simple cylinder).

Thanks in advance, Dennis