Added Mass
The Added Mass node is available on different geometrical entity levels, and can be used to supply inertia that is not part of the material itself. Such inertia does not need to be isotropic, in the sense that the inertial effects are not the same in all directions.
The Added Mass node is only available with some COMSOL products (see http://www.comsol.com/products/specifications/).
Shell Properties

Interface Selection

Coordinate System Selection
The added mass values are given with respect to the selected coordinate directions.
Face Defining the local Orientations
This section is only present in the Added Mass node in the Shell interface.
Added Mass
Select a Mass type using Table 4-10 as a guide. The default option is the type for the geometric entity. The added mass matrix can be entered as Isotropic, Diagonal, or Symmetric. For Isotropic the same mass is used in all the diagonal elements of the mass matrix.
pV
pA
pL
Frame Acceleration Forces
Select the Exclude contribution check box to switch off the loads that can be caused by the added mass when the frame is accelerated when using a Gravity or Rotating Frame feature. The setting will also determine whether the node will contribute when computing mass properties; see Computing Mass Properties.
Location in User Interface
Context Menus
Ribbon
Physics tab with Solid Mechanics selected:
Physics tab with Shell or Membrane selected:
Physics tab with Plate selected:
Physics tab with Layered Shell selected:
Physics tab with Beam or Truss selected:
Physics tab with Multibody Dynamics selected: