# COMSOL Multiphysics® 5.5 Release Highlights

For users of the Rotordynamics Module, COMSOL Multiphysics® version 5.5 includes a dynamic coefficients calculation in hydrodynamic bearings, a multi-spool bearing feature, and a squeeze film damper feature. Learn more about the rotordynamics updates below.

### Dynamic Coefficients Calculation in Hydrodynamic Bearings

You can now calculate dynamic coefficients, the equivalent linear stiffness and damping coefficients, about an equilibrium location of the journal. This is useful in bearing design, where you need to limit the cross-coefficients in the bearing to avoid instabilities. Additionally, you can use bearing coefficients directly in rotor simulations to speed up the modeling process. You can see this demonstrated in the Evaluation of Dynamic Coefficients of a Plain Journal Bearing and Damping Coefficients of a Squeeze Film Damper models.

Comparison of the computed bearing stiffness with analytical counterparts.

Comparison of the computed damping coefficients with analytical counterparts.

### Multi-Spool Bearing

Using the new Multi-Spool Bearing feature, you can model vibrations in coaxial rotors running at different speeds. This feature models the intershaft bearing between two coaxial shafts. Such rotors are common in steam turbines used in power plants, where the systems consist of a sequence of high-pressure (HP), intermediate-pressure (IP), and low-pressure (LP) turbines with coaxial rotors running at different speeds. Another example of multi-spool rotors is in turbofan engines, where the LP turbine on the inner shaft drives the fan and the HP turbine on the outer shaft drives the compressor. The Multi-Spool Bearing feature is demonstrated in the Critical Speed of a Dual-Rotor System model.

Geometry of a dual-rotor system (left) and one of the whirl modes at a particular speed (right). In the model in figure, the two rotors are placed at different positions. The fact that they are actually coaxial is handled entirely by the formulation of the multi-spool bearing.

### Squeeze Film Damper

As the name suggests, squeeze film dampers are components that provide additional damping to rotating systems by squeezing the fluid film. Usually, these components are used with rolling element bearings, which do not offer enough damping on their own. However, fluid film dampers are also used with hydrodynamic bearings. There are two ways to model squeeze film dampers. The first is by numerically solving the Reynolds equation for the pressure distribution in the film, thus computing the net reaction forces and moments of the damper. For this type of modeling, the new Squeeze Film Damper feature is provided in the Hydrodynamic Bearing interface.

The second way to model squeeze film dampers is by using an analytical expression for the forces and moments of the damper, obtained using a short-length approximation. For this type of modeling, you can add a subnode Squeeze Film Damper to either the Journal Bearing or Radial Roller Bearing nodes in the Solid Rotor and Beam Rotor interfaces, respectively. You can use the Damping Coefficients of a Squeeze Film Damper model to view this new functionality.

Comparison of the computed damping coefficients of a plain squeeze film damper with analytical values.

### Default Geometry Plot for Beam Rotor

A new default plot for the geometry of the rotor has been added in the Beam Rotor interface. This helps in visualizing the location of various components while analyzing results, as with stress and whirl plots. A disk is shown as a thin circular plate at the corresponding location, and its appearance depends on the known information.

1. Disks having nonzero mass and moment of inertia are drawn with the diameter computed based on the assumption that inertial properties correspond to a circular disk

2. If moment of inertia of the disk is neglected and a disk has only mass, a standard diameter is used for the disk irrespective of its mass

a. The shade of the color is lighter than for disks having complete information

3. If a disk has only nonzero moment of inertia, a circular disk of a standard diameter (larger than the disk with only mass) is drawn

a. The shade of the color is darker than for disks having complete information

A radial bearing is shown as a cone in the radial direction pointing toward the rotor. Axial bearings at the end of the rotor are represented by cones in the axial direction. If the axial bearing is located at the interior of the rotor, a small disk representing the collar of the bearing is shown together with two cones pointing toward the collar from both sides. You can see this new functionality in the Critical Speed of a Dual-Rotor System model.

New default geometry plot of a rotor. Circular plates represent disks, radial cones pointing toward the rotor represent the radial bearings, axial cones at the ends and double cones with circular plates in the interior represent thrust bearings.

Applied mechanical loads are now available as default plots in all structural mechanics physics interfaces. The loads plots are solution dependent, so both arrow directions and colors are updated when a dataset is updated with a new solution. Even abstract loads, such as forces and moments applied to rigid connectors and rigid domains are plotted at their true point of application. A new arrow type, used for plotting applied moments, has been introduced for this functionality. More than 100 models are updated with this new functionality.

Three sets of loads plotted on a model of a tube.

### New Tutorial Models

Version 5.5 brings several new tutorial models.

##### Evaluation of Dynamic Coefficients for a Plain Journal Bearing

Model of a plain journal bearing for evaluating its dynamic coefficients; the graph shows a comparison of the computed bearing stiffness with analytical values.

Application Library Title:
journal_bearing_dynamic_coefficients

##### Damping Coefficients of a Squeeze Film Damper

Model of a squeeze film damper; the graph shows a comparison of the computed damping coefficients with analytical values.

Application Library Title:
squeeze_film_damper_damping_coefficients

##### Critical Speed of a Dual-Rotor System

Model to evaluate the critical speeds of a dual-rotor system. Geometry of the system (left) and one of the whirl modes at a particular speed (right).

Application Library Title:
dual_rotors