Simulation Tech Tip: Modeling a Hydrostatic Pressure with Simulation

Distributed pressures and forces can be modeled in a Simulation study with an equation in X, Y, or both. A commonly-understood distributed load is hydrostatic “water pressure”.

If you’ve ever been swimming, maybe snorkeling in the Caribbean, and dived down more than 10 feet or so, you’d be sure to notice the increased pressure on your ears as the weight of the water above you piles up the deeper you go. We engineers call this a hydrostatic load, and it increases linearly with depth.

My favorite way to set this up in Simulation is by first creating a Split Line in SolidWorks to define the waterline and wetted surfaces, and then creating a custom Coordinate System to make the equation-writing more intuitive.

Here’s a half-section of a rectangular bucket showing that done:

Hydrostatic Image1

To model the water pressure, we add a Pressure “External Load” and choose the checkbox option for “Nonuniform Distribution”:

Hydrostatic Image2

  • Set the direction as Normal To Selected Face, since that’s how water pressure acts.
  • Select all the wetted surfaces.
  • For the Pressure Value, just plug in “1” unit. This value multiplies by the equation we will write. I find this approach easier than playing with the coefficients.
  • Toggle Reverse Direction as needed based on the preview so that the water is on the inside or outside of your part (positive or negative).
  • Select your custom Coordinate System.
  • Write in the coefficients for the generic formula given… in our case they are all zero (ignored) except for “Y”.

Any engineering undergrad can tell you the formula for hydrostatic load is “negative rho, g, h”:   P(h) = -ρgh

Rho (ρ) is the mass density, g is gravity, and h is height (or depth if you like). Well I love inchworms but hate slugs, so let’s use weight density instead of mass and therefore we don’t need to multiply by g.

The weight density of water, which also happens to be the default density of a SolidWorks part with no material assigned, is 0.036 lbs per cubic inch. So that’s our “Y” coefficient. Pounds per cubic inch of density, times inches of  depth, gives us pounds per square inch pressure, which is how we roll in the USA.

When you’re done, SolidWorks Simulation even updates the preview arrows to indicate approximate pressure variation. It should look like this:

Finally, we add some Restraints and Run it and we get this:

Hydrostatic Image4

[Note: This example shown requires Large Displacement solution for correct results. But that’s a topic for another blog post!]

If you’d like to explore on your own, here’s the downloadable part file for you enjoyment!

Bonus points: If you investigate the Reaction Forces at the Restraints, you can find out how many pounds of water are in the container!

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