Air film damping plays a significant part in the design of micro electro mechanical devices. If a stationary surface is placed in close proximity to a mass or another surface which is allowed to move normal to its plane, the air in between the mass and the stationary surface will be squeezed when the suspended mass moves towards the stationary surface developing a pressure gradient across the width of the mass. This will push the air out of the gap. Alternately, when the mass moves away from the stationary surface, the pressure in the gap is reduced and air flows into the gap. The work done in this process would reduce the energy of the suspension and thereby the air acts as a damper and the process is called squeeze film damping.
Forces on the moving surface from gas film can be obtained from linearized Reynolds equation. The coefficient of damping force can be calculated from this provided the air gap is known. If free vibration frequency of the microsuspension is known, the damping ratio can be calculated from it for that particular mode of vibration. The natural vibration frequency can be estimated under Vibration > Free vibration. The vibration mode should be such that the movement of the suspended mass would squeeze the air out of the gap.
This design form can be used to estimate the coefficient of damping force for the given air gap and plate design. Damping ratio is calculated for the given vibration frequency. The quality factor is calculated from the damping ratio for slight damping.
The plot shows the variation of damping ratio with the air gap thickness. Using the crosshair tool, the damping ratio for any air gap thickness upto 20µm can be estimated. It can be used to calculate the air gap that would give a damping ratio of 0.7 for optimum damping of a harmonically forced vibration.