The boundary conditions will differ depending on the launch configuration. For this model, I assumed the CubeSat was placed inside a Poly-PicoSatellite Orbital Deployer (P-POD) with the legs of the satellite aligned with the direction of the acceleration of the launch vehicle. In other words, the long rectangular P-POD is pointing up towards the sky.
The side of the CubeSat that faces the ground should be fully constrained. This will simulate the floor of the P-POD during launch.
Right click Modal in the Outline window, then select Insert>Fixed Support. While holding down Ctrl, select the bottom face of all four -y-side legs. Select Apply next to the Geometry tab.
The P-POD is a spring-loaded deployment system, which can hold up to three 1U CubeSats. This spring-loaded design allows for a very small amount of vertical movement during launch, while the side rails keep the CubeSat rigidly fixed in two directions. Because of the this, the side of the CubeSat facing the sky should be constrained in all directions other than the vertical direction (+y-direction for our model). This will allow the structure to warp slightly during takeoff, and will simulate the rigid sides of the P-POD.
Right click Modal in the Outline window, then select Insert>Displacement. While holding down Ctrl, select the top face of all four +y-side legs. Select Apply next to the Geometry tab.
To simulate the constraint of the side rails, set the X and Y components to Constant (0 m) and the Y component to Free.
As a CubeSat specification requirement, the satellite must have its 1st resonant frequency above 100 Hz. Most major forcing amplitudes in launch vehicles occur below 100Hz. The goal is to design a satellite that will not be destroyed by some forcing near or at a resonant frequency during the launch phase.
Under the Modal tab in the Outline window, left click Analysis Settings. Set the Max Modes to Find to 5. Select Limit Search to Range and click* Yes. Leave *Range Minimum at 0 Hz and set Range Maximum to 1000 Hz. These settings allow us to reduce run time by using some foresight as to what modal frequencies we expect to see and care about.