what is the concept of this Abaqus Doc. paragraph?

hi im modeling submerged cylinder subjected to underwater explosion.

this paragraph is related to example of Abaqus in Documentaion.

what is the meaning and concept of this paragraph (Addess mass): 

 i attached the image of paragraph text.

i appreciate your kind helps :)

 tnx 

 "9.1.3 RESPONSE OF A SUBMERGED CYLINDER TO AN UNDERWATER EXPLOSION

SHOCK WAVE"

 The external fluid ismeshed with 4-node AC3D4 acoustic tetrahedral elements. The outer boundary

of the external fluid is represented by a cylindrical surface with spherical ends. The characteristic radius

of the outer boundary is 0.915 m. The outer boundary must be placed a sufficient distance from the

cylinder so that the added mass associated with the low-frequency beam bending modes of the cylinder

is represented adequately. The beam bending modes correspond to an sinusoidal translation of

the cylinder’s cross-section through the fluid. For evaluating added mass effects when using a simple

plane wave radiation impedance boundary for the external fluid, the outer boundary of the fluid can be

considered rigid (nonradiating). Therefore, an analytical solution for the added mass associated with the

translation of an infinite cylinder of radius located within a fluid-filled infinite cylinder of radius

can be used to determine an appropriate characteristic radius for the external fluid. Results for

the analytical solution presented by Blevins are listed in Table 9.1.3–1. The characteristic radius is

based upon an outer boundary ( ) to cylinder radius ( ) ratio of 6.0, which corresponds to an added

mass error of about 6% for infinite cylinders. When using enhanced surface impedance models (i.e.,

*IMPEDANCE PROPERTY, TYPE=CIRCULAR or SPHERE) the outer fluid boundary location can

be placed at about half of the distance required when using the plane wave radiation impedance model.

However, for this example the ratio was maintained at 6.0 even when using the source-based

surface impedance models for the external fluid. Comparable results for the structural response can

be obtained when the source-based boundaries were located half as far from the structure. For the

low-frequency beam bendingmodes, system losses (damping) caused by hydrodynamic drag and/or fluid

viscosity are not accounted for by acoustic radiation. Therefore, mass-proportional damping applied to

the test cylinder mesh is used to approximate these types of losses.