Squeeze film analysis simulates the effects of fluid in small gaps between fixed surfaces and structures moving perpendicular to the surfaces. Depending on the operating frequencies, the fluid can add stiffening and/or damping to the system. At low frequencies, the fluid can escape before it compresses. Therefore, the fluid only adds damping to the system. At high frequencies, the fluid compresses before it can escape. Therefore, the fluid adds both stiffening and damping to the system.

A static analysis is used to determine the damping effects at low frequencies. A harmonic analysis is used to determine the stiffening and damping effects at high frequencies.

The FLUID136 element is used to model the fluid domain between a fixed surface and a structure moving normally to that surface. The fluid domain is modeled by overlaying FLUID136 elements on the moving surface of the structure (that is, lower surface of the structure adjacent to the fixed wall).

Figure 3.1: Moving Plate above a Fixed Wall

The FLUID138 element is used to model the pressure drop through holes in a moving structure. Microstructures are commonly perforated to reduce the damping effects of the fluid layer. FLUID138 elements are used in conjunction with FLUID136 elements to obtain a consistent pressure distribution, including the effects of holes.

Figure 3.2: Perforated Plate Structure

A squeeze film analysis can be performed on structures with known velocities or unknown velocities. If the velocity profile is known, you can apply it directly to the fluid elements. If the velocity profile is not known (or is too complicated), it can be determined from the mode-frequency response of the structure (model projection method). This section deals with directly applying the velocities. Modal Projection Method for Squeeze Film Analysis covers the modal projection method.