Design of Noise Reduced Large Fans for Wind Tunnel Application with CFD-based Optimization – a Case Study
2.6 Case Studies: Acoustics
Aeroacoustic wind tunnels require not only appropriate passive sound attenuation measures but also low noise fans. Due to innovative manufacturing methods large fans may now be equipped with highly skewed rotor blades – a proven method for noise reduction.
In this case study we describe the development steps of a model axial fan stage. The reliable prediction of the acoustic emission of a fan is still computationally extremely expensive. Therefore the low noise target is taken into account by enforcing a substantial skew of the rotor blades. Naive blade skew can introduce 3D-effects with a substantial degradation of efficiency. Therefore, in a first step the rotor blade shape and the guide vanes are optimized aerodynamically via an automated CFD-RANS-based optimization scheme. In addition, the numbers of rotor/stator blades are set to prevent propagation of as many duct modes as possible.
In a second step the fan stage is manufactured and its aerodynamic and acoustic characteristics are measured. A comparison with experimental data from a model scale state-of-the-art wind tunnel fan with unskewed blades reveals the aerodynamic and acoustic differences between the new and the standard design. Eventually, a preliminary prediction of the expected performance of scaled-up versions for wind tunnel applications is given.