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01.01 Fan and Turbomachinery Noise, Part 2

Lighthill’s analogy applied to a automotive turbocharger compressor
Time: 8:40 am

Author: Clemens Freidhager

Abstract ID: 1414

Computing transient CFD simulations of turbocharger compressors is computationally very demanding. It is of fundamental importance to resolve turbulent structures at the location of their generation and to establish a fine enough grid to allow propagation of the resolved structures. This results in high-resolution grids, existing of more than 20 million cells. Applying Lighthill’s analogy, it is possible to only resolve turbulent structures at their location of generation and compute the pressure propagation by using an additional, not that demanding, acoustic grid. This allows using coarser CFD grids in the inlet and outlet section. For transferring Lighthill’s source terms from the CFD to the acoustic grid, advanced interpolation algorithms are used. The simulation results are validated by measurements of a cold gas test rig are considered.

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Computationally efficient source grid selection and source interpolation in computational aeroacoustics applied to an axial fan.
Time: 8:00 am

Author: Andreas Wurzinger

Abstract ID: 1666

The noise generation of an axial fan is mainly caused by flow-induced noise and can therefore be extracted from its aeroacoustics. To do so, a hybrid approach separating flow and acoustics is well suited due to its low Mach number. Such a computationally efficient hybrid workflow requires a robust conservative mesh-to-mesh transformation of the acoustic sources as well as a suitable mesh refinement to guarantee good convergence behavior. This contribution focuses on the mesh-to-mesh transformation, comparing two interpolation algorithms of different complexity towards the applicability to the aeroacoustic computation of an axial fan. The basic cell-centroid approach is generally suited for fine computational acoustic (CA) meshes and low phase shift, while the more complex cut-volume method generally yields better results for coarse acoustic meshes. While the cell-centroid interpolation scheme produces source artifacts inside the propagation domain, a grid study using the grid convergence index shows monotonic convergence behavior for both interpolation methods. By selection of a proper size for the source grid and source interpolation algorithm, the computational effort of the experimentally validated simulation model could be reduced by a factor 4.06.

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Numerical simulations of flow induced noise from a dual rotor cooling fan used in electronic cooling systems
Time: 7:40 am

Author: Sahan Wasala

Abstract ID: 1809

Hard Disk Drive (HDD) system enclosures in a data center require effective cooling systems to avoid HDD overheating. These systems often rely on air cooling because of their cost efficiency and maintainability. Air cooling systems typically consist of an array of axial fans which push or pull the air through the system. These fans emit high level tonal noise particularly at high tip-speed ratios (TSR). High-capacity HDDs, on the other hand, are sensitive to high acoustic noise, which consequently increases the risk of read/write error and deteriorates drive performance. Therefore, cooling fan noise adversely affects the function of the HDD enclosure systems and emphasizes the need to understand the noise sources and develop methods to mitigate HDD noise exposure. This study focuses on understanding the aerodynamic properties and related aeroacoustic behavior of a contra-rotating fan representative of the types used in a modern data center cooling system. A numerical investigation was conducted using high fidelity Large Eddy Simulation (LES) and the Ffowcs Williams and Hawkings (FW-H) acoustic analogy, as well as using experimentally measured acoustic data as a validation. Initial simulation results showed a good agreement with the experimental data and led to a better understanding of noise directivity.

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Two-step computational aeroacoustics approach for underhood cooling fan application
Time: 7:20 pm

Author: Parag Chaudhari

Abstract ID: 2467

Aeroacoustic noise is one of the important characteristics of the fan design. Computational Aeroacoustics (CAA) can provide better design options without relying on physical prototypes and reduce the development time and cost. There are two ways of performing CAA analysis; one-step and two-step approach. In one-step CAA, air flow and acoustic analysis are carried out in a single software. In two-step approach, air flow and acoustic analysis are carried out in separate software. Two-step CAA approach can expedite the calculation process and can be implemented in larger and complex domain problems. For the work presented in this paper, a mockup of an underhood cooling fan was designed. The sound pressure levels were measured for different installation configurations. The sound pressure level for one of the configurations was calculated with two-step approach and compared with test data. The compressible fluid flow field was first computed in a commercially available computational fluid dynamics software. This flow field was imported in a separate software where fan noise sources were computed and further used to predict the sound pressure levels at various microphone locations. The results show an excellent correlation between test and simulation for both tonal and broadband components of the fan noise.

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Design of axial flow fans for reduced noise and improved efficiency
Time: 7:20 am

Author: Erika Quaranta

Abstract ID: 2481

Axial flow fans are used in a wide variety of applications, from cooling systems for electronics to ventilation in buildings.  Whatever the application, there will be competing design constraints which make it difficult to achieve the required pressure-flow performance characteristic, within a specified space envelope, whilst meeting a target aerodynamic efficiency and noise level. This paper describes a design methodology for optimizing aerodynamic performance and noise.  It is based on use of a semi-analytic 2-D design tool for preliminary predictions and design, combined with a 3-D numerical CFD analysis to visualize the flow.  Both models can be extended to the design of multi-stage systems. The 2-D model predicts the flow velocity at the trailing edge of the blades for each point on the fan performance curve, which is then used to estimate self-noise characteristics of the rotor using a classical model of airfoil trailing edge noise.  The CFD analysis provides detailed validation of assumed airfoil characteristics, including the effect of 3D design features such as blade sweep, and confirms the flow and aerodynamic efficiency predictions; it can also used to estimate parameters such as turbulence intensity that is a key driver for the noise level.

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Measurement of loudspeaker mechanical impedance by changing the sound load at the throat of loudspeaker
Time: 7:40 am

Author: Shichun Huang

Abstract ID: 3112

A loudspeaker is a device that converts electrical energy into acoustic energy by coupling between electrical impedance, mechanical impedance, and radiation impedance. The loudspeaker electro-mechanical-acoustic coupling model provides the experimental feasibility to measure the characteristic parameters. In this paper, an economical and practical measurement method of loudspeaker mechanical impedance is proposed. First, the mathematical relationship between loudspeaker electrical impedance and mechanical impedance is obtained based on the loudspeaker electro-mechanical-acoustic coupling model. Second, two electrical impedances with different known radiation impedance are measured by using a developed measurement system. Finally, the real and imaginary parts of the mechanical impedance are obtained according to the mathematical relationship. This method neither assumes that the loudspeaker mechanical impedance is constant in a frequency band nor does it build FEM models based on structural parameters. A loudspeaker is measured by using a developed measurement system. The result shows that the mechanical impedance and the force factor are functions of frequency. Moreover, a radiation impedance measurement is performed to verify the feasibility and accuracy of the proposed method.

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Effects of moisturized inflow on compressor performance and aerodynamic noise
Time: 8:20 am

Author: Changhong Sun

Abstract ID: 3114

The effects of moisturized inflow on turbocharger compressor performance and aerodynamic noise were numerical analyzed in this paper. The gas-liquid two-phase flow method based on Euler-Lagrange model was firstly introduced. The influence of water concentration and water droplet diameter on compressor performance and internal flow characteristics at design speed were studied using the two-phase flow method. The compressor aerodynamic noise was also predicted at design condition under two different inflow conditions, including ideal inflow and moisturized inflow with 0.1% water concentration. The results indicate that moisturized inflow with an appropriate water concentration can reduce the outlet temperature of the compressor and improve the compressor performance, in which the water concentration is a dominant parameter. There is a phase transition process of water in the compressor with moisturized inflow, but moisturized inflow has little effects on the compressor internal flow characteristics. Moreover, the moisturized inflow also has influence on compressor aerodynamic noise.

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