Subtracting the structural damping ratio from the total yields the two-phase fluid-damping ratio (Noghrehkar et al., 1995). Total damping includes structural damping, viscous damping and a two-phase component of damping as explained by (Pettigrew et al. 1994). The damping ratio increases as the void fraction increases and peaks at 60% (Carlucci, 1983), then the ratio decrease with a (Figure 20). Damping also decreases as the vibration frequency increases (Pettigrew et al., 1985).

Damping in two-phase is very complicated. It is highly dependent upon void fraction and flow regime. The results for the two-phase component of damping can be normalized to take into account the effect of confinement due to surrounding tubes by using the confinement factor C (Pettigrew et al., 2000). This factor is a reasonable formulation of the confinement due to P / D. As expected, greater confinement due to smaller P / D increase damping. The confinement factor is given by equation below:

[1 + (D /De)3]

[1 — (D /De)2] 2