Detached Eddy Simulations of aerodynamic sound generation by a bi-directional cooling fan
In the railway industry self-ventilated traction motors, where the cooling fan is mounted directly on the motor shaft, are highly appreciated for their robust, reliable and simple cooling principle. However, this requires bi-directional cooling fans, which typically use straight unsymmetrical blades. This gives a bad aerodynamic design, and generates a significant broadband noise, as compared to unidirectional fans, which are quieter and generate more of a blade-passing-frequency tonal noise. In fact, at high motor speeds, the cooling fan in self-ventilated motors can become the dominant sound source on a train.
The focus of this project is state-of-the-art Computational Aero Acoustics (CAA) for the prediction of aerodynamic noise from low Mach number bi-directional fans. The purpose is to explore the possibility to numerically predict the generated near- and far-field sound. The CAA method chosen is Direct Noise Calculations (DNC), where the flow and sound are retrieved simultaneously in one simulation. This is achieve with compressible Improved Delayed Detached Eddy Simulations (IDDES), carried out with the multipurpose commercial CFD code STAR-CCM+. The motivation for the DNC approach is the complexity of the acoustic near-field problem, with the rotation of the fan and believed strong installation effects, and that the sound is only studied at a short distance from the source.
For the above purpose, a mock-up of a modern railway traction motor with a radial cooling fan is used as a test case. The fan has 16 straight blades, with non-uniform spacing to reduce tonal noise. Air is sucked into the fan at the centre axis, where after it is forced out in the radial direction, and then is turned into an annular channel with cooling vanes, surrounding the motor. The main sound sources are believed to be located inside the fan. Here the highest velocities are found, with a highly complex and unsteady flow field, and, thus, strong wall pressure fluctuations are present, which tend to be the main aerodynamic sound sources at the low Mach numbers found in this type of cooling fan.
The pressure field, illustrating sound waves propagating upstream from the fan.
Wall pressure rms and tangential velocity in a plane cutting through the middle of the geometry.
