Fluids & Structural Mechanics > NCH > Flow Acoustics (FA)

We model the sound radiated by vibrating structures with our ARL developed Boundary Element (BE) modeling technique.

ARL's approach converts surface vibrations into lumped volume velocities, leading to a computationally efficient approach which does not have the 'forbidden frequency' problems that plague many other boundary element approaches.

Our software allows us to compute radiated sound fields at any frequency, like the sound from this loudspeaker.

Fluid Loading

Much of our research is conducted on structures submerged in water, like marine propellers.

The mass loading and radiation damping that water imparts on a structure can be computed over a wide range of frequencies with unlimited frequency resolution.

Compute complex fluid-loaded modes of submerged structures are also computed by combining our BE fluid loading matrices with structural FE models, as in the submerged cylindrical shell modes shown here.

Computationally Efficient BE Modeling

ARL's boundary element approach does not require dense acoustic meshes. When coupling a structural Finite Element model to a BE model, we often condense structural mesh regions into lumped acoustic elements, which saves considerable computer time.

The loudspeaker example shows two meshes. The coarser mesh is suitable for low frequency analysis, and the denser mesh for mid-frequency analysis.

Fluid Film Bearings

We model fluid film bearings by solving the Reynolds equation for hydrodynamic lubrication theory and estimated bearing stiffness and damping characteristics using a standard perturbation approach.

Our approach to appling the bearing coefficients involves a distribution of the coefficients to their physical location such that they produce a physically accurate interaction between rotating and stationary components. The general practice in industry is to represent the bearing coupling as a lumped effect and is applied at a single location (and often for a single frequency) in the model which can lead to large errors for some types of equipment.

The approach is described in ASME - IMECE 2003/NCA-43770 (ADD LINK HERE TO: Distributed Journal Bearing Dynamic Coefficients for Structural Finite Element Models. Proceedings of 2003 ASME International Mechanical Engineering Congress and Exposition, November 15-21, 2003, Washington D.C.). The distributed coefficients are used in finite element and substructure synthesis modeling to couple rotor and stator components of motors, gearboxes, propulsors, and other rotary equipment.

Rolling Element Bearings

Rolling element bearings are modeled with a Hertzian contact model to compute dynamic stiffness coefficients that couple linear and rotational relative motions. Our physics-based modeling approach enables bearing stiffness estimation for any bearing preload condition. The general industry practice ignores preload effects and applies a constant bearing stiffness that is not direction- or preload-dependent.

Bearing stiffness matrices are used in finite element and substructure synthesis modeling to couple rotor and stator components of motors, gearboxes, propulsors, and other rotary equipment.

Finite element models and mode shapes

We model complex propulsors and other structures with Finite Elements (FE), and compute the modes of our models with various commercial FE software packages, like NASTRAN and ABAQUS.

Normal modes are the basis of the noise and vibration analysis capability, combining them with Boundary Element (BE) models of surrounding acoustic spaces, like air and water.

The approach is extremely efficient, allowing for the modeling of very large systems across a wide range of frequencies with precise frequency resolution.

Structural Intensity

When driving FE models with dynamic forces, Structural Intensity can be computed using in-house software, which shows how vibrational energy travels through a structure.

Structural intensity is used to:

• Identify regions where noise control elements, like dampers, are required,
  
• Investigate how intensity can be used to identify potential damage in different parts of the structure.

The structural intensity patterns in an automobile body model are shown here, for a vertical drive on one of the shock towers (where road noise enters a vehicle). Orange colors show high intensity, and the vectors show how structure-borne sound travels through the vehicle. An automobile company used our structural intensity capability to redesign the shock tower in one of their vehicles, reducing road noise.

Component Mode Synthesis

For complex structural systems, Component Mode Synthesis is often used to model assembled dynamic behavior.

A system is broken into logical components, like the motor housing and rotor shown on the left. In this example, the modes of the housing and rotor are computed separately, and coupled via impedances at the forward and aft bearings in the housing.

State of the art tools are used to compute Bearing Impedances of journal and rolling element bearings, and one of our Graduate Students is measuring the impedances distributed around the fluid film in a journal bearing.