What is the difference between "normal" and "attachment" modes calculated in modal analysis?
How can normal modes calculated in other softwares be compared to those calculated in Bladed?
Blades
For the blades, only normal modes are calculated. The blade root is constrained, and then the eigenvector/value problem is solved.
Support structure (tower)
Attachment mode shapes are calculated by applying loads to the tower top and calculating the resulting nodal displacements. Tower tower is constrained at the proximal node* for this calculation. Such modes are sometimes referred to as “static” modes as they represent static deflection as a result of an applied unit force or moment.
Normal modes are calculated by constraining the nodes at the tower base and the tower top and then solving the eigenvector/value solution to find the “internal” vibration mode shapes.
For the support structure, the interpretation of the definition of "normal" modes can sometimes cause confusion. The conventional ‘normal’ modes of a support structure include free vibration modes where the top of the tower is free to move with no external forces on it. In Bladed, the ‘normal’ modes where the top of the tower is free to move are not calculated. The attachment modes are calculated instead which are more realistic as in reality the tower top will move due to the application of external forcing from the structure above it.
This means that if the free vibration ‘normal modes’ for the tower structure are calculated with only the tower base constrained, they will not match the mode frequencies calculated by Bladed.
Bladed also calculates “coupled” vibration modes in the Campbell diagram, that show how the normal and attachment modes combine into coupled vibration modes at a specific operating point. Typically, these coupled modes correspond well to normal modes calculated in other software, with the tower base constrained and the tower top free to move.
* The proximal node is the component (e.g. blade or tower) structural node to which the modal deflections are calculated relative to. For the blade, this is the blade root. For the support structure, the proximal node is and extra structural structural node at 0,0,0. The support strucure foundations (either rigid or flexible) link the proximal node to the foundation nodes in the support structure.