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In a blog post from February, I enlisted the help of the Bluehead Fairy Wrasse to describe the importance of good meshing practices in SOLIDWORKS Simulation.

Now we’ll check the integrity of her new home, a 29-gallon aquarium, using the nonuniform pressure load available in SOLIDWORKS Simulation to make sure the tank can withstand the stress of the water pressure. As you know, fluid pressure varies with depth, so using the default constant pressure would generate an unrealistic load.

Figure 1: Aquarium Model – Credit: Jeff Thompson (posted on 3DCC)

Within the static study, we exclude the components that aren’t involved in the containment of the water by right-clicking on them in the Simulation tree. Material properties of the 1/8-inch thick tempered glass and ABS poly-carbonate plastic are applied to the tank and edge supports, respectively. For thin walls, shell meshing is appropriate. Restraints are also added to stabilize the model.

Figure 2: Simulation Static Study Tree Showing Included Parts

Figure 3: Static Study Fixtures

Activating the nonuniform pressure distribution is possible within both the force and pressure load property managers. The variation can be applied relative to a Cartesian, cylindrical or spherical coordinate system (CS).

We’ll use the Cartesian CS to vary the pressure according to the water depth per the relationship,

The water pressure is 0 N/m² at a free surface and linearly increases to 4,140 N/m² at the maximum depth of 0.422 meters. To facilitate this relationship in Simulation, a SOLIDWORKS reference CS is first created at the top vertex of the part that represents the water, with the Y-axis pointing down into the water in the direction of increasing depth. Note that the water part is only used for reference here; it has been excluded from the static analysis study.

Figure 5: SOLIDWORKS Reference Coordinate System at Top of “Water” Part

We start a pressure load boundary condition, select all the inside faces of the tank walls and check the Nonuniform Distribution box. The use of consistent units is important to achieve a proper setup; we’ll use the SI (MKS) system. First, select N/m² as the Pressure Value unit. In the Pressure Value field, enter 9812 N/m², which is the product of the mass density of the fluid (1000 kg/m3 for water), the acceleration of gravity (9.812 m/s2) and a unit depth of 1 m. Select meters (m) as the Unit for x, y and z, then pick the Edit Equation button and enter ‘Y’ for the linear variation equation.

Figure 6: Pressure Load Property Manager

Figure 7: Nonuniform Pressure Equation Editor

The constant value in the Pressure Value field is multiplied by the equation, which represents the depth. Therefore, the pressure applied to the tank walls varies as shown below. The boundary condition icons qualitatively confirm that our setup is correct.

Figure 8: Linear Pressure Variation on a Tank Wall

Figure 9: Nonuniform Pressure Load Boundary Condition Icons

After solving the analysis, we find that the highest principal stress (P1) is 20.78 MPa. Note that the default stress plot in Simulation gives the vonMises equivalent stress, which is an appropriate measure for ductile materials only. Glass is brittle and fails abruptly without yielding. For brittle materials, we can compare the P1 stress to the allowable material stress (70 MPa) to calculate a factor of safety of approximately 3.4. Also, the maximum deflection of the tank side walls is 2 mm.

Figure 10: First Principal (P1) Stress Distribution

Figure 11: Maximum Side Displacement of Tank Walls

Finally, don’t forget about the relatively new Simulation Display functionality that allows us to overlay simulation results on the complete assembly model, as shown below.

Figure 12: Simulation Results Displayed on Assembly

The aquarium walls are properly sized. After a check of the corner adhesive joint strength, we can confidently fill the tank and introduce the Bluehead Fairy Wrasse to her new environment!

Kurt Kurtin
Manager, Simulation and Electrical
Computer Aided Technology, Inc