Dynamic effect of annular flow with finite axial length on the rotor

The dynamic force of the annular flow with finite axial length on the rotor is investigated in this article. A test rig to study the annular flow is built in the CNC machine tool. The rotor of the test rig is connected with the spindle of the machine tool by flange while the stator is located at the machine table. Between the rotor and stator, there is the annulus which is filled with water. The modal frequencies of the test rig are tested for different axial lengths of the annular flow with hammer impulse excitation. The dynamic fluid-structure model consisting of rotor, annular flow and stator is established by the finite element method. Two-dimensional annular flow model which is based on the Bulk-flow theory is utilized to describe the dynamic force of the annular flow. The added mass, damping and stiffness matrix of the annular flow are generated to represent the transient interaction force of the rotor and stator by the annular flow. To study the 3D effect of the annular flow with short axial length, the modification coefficient of the added mass, damping and stiffness matrix is proposed. When the axial length of the annular flow changes, the variation of the modal frequencies of the test rig predicted by the dynamic fluid-structure model is in good agreement with the experimental results. The modification coefficients are almost linear with the axial length of the annular flow for different average gap thickness. For the annular flow with short axial length, the 3D effect is significant and can’t be ignored. The annular flow force with short axial length may have a positive correlation with the square of the annulus axial length