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01.02 Computational Methods in Flow-Induced Noise & Vibration – Part 3

Time: 12:20 pm

Author: Satya Prasad Maddula

Abstract ID: 1108

Wind turbine noise is a critical issue for siting and its operation in offshore and terrestrial conditions. In this work, we analysed trailing edge bluntness vortex shedding noise source for a land based turbine of size 2MW and blade span of 38m using modified BPM noise solver. A regression approach has been implemented to predict the shape function in terms of thickness to chord ratio of aerofoils used for blade.  For trailing edge height of 1 % chord, computations for sound power level were done at wind speed of 8m/s, 17 RPM, and showed that present regression approach predicts the noise peak of 78dBA at f ~ 10 kHz. These results were also validated using experiment data from GE 1.5sle, Siemens 2.3MW turbines with blade lengths of 78 -101m and agreed within 2 % at very high frequencies, f > 5kHz.  In addition, results from present approach agreed with original BPM and modified BPM by Wei et al at high frequencies, f ~ 10kHz where the bluntness noise becomes predominant. The slope of noise curves from present approach, and modified BPM methods are lower when compared with original BPM and show sound level coincidence with peak Strouhal number of ~ 3.3.

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Turbulent model validations with CFD/wind tunnel test and application to statistical energy analysis for wind noise prediction
Time: 7:40 pm


Abstract ID: 1330

Wind noise is becoming to have a higher priority in automotive industry. Several past studies investigated whether SEA can be utilized to predict wind noise by applying a turbulent spectrum model as the input. However, there are many kinds of turbulent models developed and the appropriate model for input to SEA is still unclear. Due to this, this paper focuses on clarifying an appropriate turbulent model for SEA simulation. First, the input turbulent pressure spectrum from five models are validated with wind tunnel tests and CFD. Next, a conventional numerical approach is used to validate models from the aspect of response accuracy. Finally, turbulent models are applied to an SEA model developed for a wind tunnel, and the SEA response is validated with test data. From those input/response validations, an appropriate turbulent model is investigated.

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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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Implementation of Direct Acoustic Simulation using ANSYS Fluent
Time: 8:40 pm

Author: Dennis Huang

Abstract ID: 1787

Direct Acoustic Simulation (DAS) is a powerful Computational Aero Acoustics method that obtains hydrodynamic and acoustic solutions simultaneously by solving compressible Navier-Stokes equation together with state equation of ideal gas. Thus, DAS has advantages for cases with flow acoustic coupling and high Mach numbers (). With an increasing demand of massive-scale calculations, a robust numerical solver for DAS is required. ANSYS Fluent is a suitable CFD platform with proven robustness. However, there is no direct implementation of DAS in the current version of ANSYS Fluent. The present study, therefore, aims to investigate an approach for implementing DAS using ANSYS Fluent. Given the acoustic part of fluctuations is much smaller than the hydrodynamic part in amplitude, a DAS solver requires high accuracy and low dissipation. Based on these needs, proper solution methods, spatial discrete methods and boundary conditions are firstly determined through simple calculations of two dimensional propagating plane waves. Afterwards aeroacoustics of a two-dimensional cavity flow at 0.6 is calculated to verify the capability for solving separating flow with the aforementioned set-up. Finally, aeroacoustics of a cylindrical bluff body at a turbulent regime and 0.2 is calculated in three-dimensions to verify the capability for solving turbulent flow using Monotonically Integrated Large Eddy Simulation.

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A semi-analytical model to predict the flow-induced noise of an open cavity with complex geometry
Time: 6:20 am

Author: Tingsheng Zhong

Abstract ID: 1791

Shear flow past a cavity involves a complex fluid dynamic process. Of vital importance is the occurrence of self-sustained oscillations that give rise to tones and the amplitudes of which may be further amplified if the hydrodynamic mode is coupled with the cavity mode. Extensive efforts have been made to investigate the mechanisms of such a simple yet compelling system as well as to predict the noise generated, while most of them are focused on geometry of rectangular shape. For an irregular shaped cavity, numerical methods are usually used which are computationally expensive. A method is developed to predict the tones generated by the shear flow past an open cavity of a complex geometry. In view of the feedback process involved within the system, a describing-function method decomposing the system into a non-linear part and a linear part is used. The linear description function is established by the patch mobility method where the transfer function between patches is extracted from finite element results, while the nonlinear description function is established based on the vortex sound theory. The proposed method showed a superb computation efficiency over CFD method and its accuracy was justified by comparing with the results of public literature.

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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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Energy fluctuations in recorder pipes during transient sound attacks and steady state sound
Time: 1:00 pm


Abstract ID: 2264

The evaluation of temporal and spatial fluctuations of energy using compressible fluid analysis is proposed as an effective method to clarify the fundamental mechanism of the self-sustained oscilla-tions in a actual recorder. The main factors of the self-sustained oscillations are investigated in more detail by evaluating not only the steady state of the sound where the flow field and the sound field are completely coupled, but also the characteristics at the attack transient of the sound before the coupling is established. By analyzing the large energy fluctuations that occur just below the edge of the labium in the attack transient, it was shown that this phenomenon may be one of the main causes of the self-sustained oscillations. And the characteristics of the energy fluctuations and sound power generation during the steady state of the sound are discussed. It was also focused on the energy variations in another region that is near the exit of the windway.

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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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Prediction of wind turbine blade trailing edge noise under various flow conditions for a passive damage detection system
Time: 12:40 pm

Author: Caleb Traylor

Abstract ID: 2597

Noise generated by turbulent boundary layer over the trailing edge of a wind turbine blade under various flow conditions is predicted and analyzed for structural health monitoring purposes. Wind turbine blade monitoring presents a challenge to wind farm operators, and an in-blade structural health monitoring system would significantly reduce O&M costs. Previous studies into structural health monitoring of blades have demonstrated the feasibility of designing a passive detection system based on monitoring the flow-generated acoustic spectra. A beneficial next step is identifying the robustness of such a system to wind turbine blades under different flow conditions. To examine this, a range of free stream air velocities from 5 m/s to 20 m/s and a range of rotor speeds from 5 rpm to 20 rpm are used in a reduced-order model of the flow-generated sound in the trailing edge turbulent boundary layer. The equivalent lumped acoustics sources are predicted based on the turbulent flow simulations, and acoustic spectra are calculated using acoustic ray tracing. Each case is evaluated based on the changes detected when damage is present. These results can be used to identify wind farms that would most benefit from this monitoring system to increase efficiency in deployment of turbines.

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Numerical and experimental investigation into effects of tip-rake shape of axial-flow fan on its flow and noise performances
Time: 7:00 am

Author: Seo Yoon Ryu

Abstract ID: 2667

As the potential of computational resources dramatically increases, the so-called computer-aided engineering readily replaces experiment-based engineering in related industrial fields. In this study, the virtual fan flow and acoustic performance testers are developed using the RANS solvers and the acoustic analogy. Two types of forward-curved centrifugal fans are selected for numerical and experimental investigations into its flow and acoustic performances. First, to experimentally evaluate the performances of the centrifugal fan units, their P-Q curves and sound power levels are measured using a fan flow performance tester and a semi-anechoic chamber, respectively. Second, the virtual fan flow and acoustic performance testers are constructed using the RANS solvers and the acoustic analogy based on the FW-H equation and CFD method. The validity of the current virtual methods is confirmed by comparing the prediction results with the measured ones. During the validation, the effects of the wall functions, y+ distribution, and turbulence models on predicted flow performance accuracy are closely examined. The effects of the integral surfaces used for the computation of the FW-H equations are also assessed on the predicted spectral levels of sound pressure.

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Investigation into effects of side-window rubber sealer on cabin interi-or noise due to external flow disturbances of vehicle
Time: 8:20 pm

Author: Sangheon Lee

Abstract ID: 2675

Electric vehicles' rapid commercialization increases the relative importance of wind noise, especially for cabin interior noise. In this study, systematic numerical methods are developed to assess the wind noise insulation performance of side-window rubber seals in a design stage. First, the simplified automotive cabin model (SACM) is constructed to test the rubber seals' sound insulation performance due to external flow disturbance generated by jet flow. The pressure signals due to the jet flow are measured inside and outside the SACM. The difference between the two signals is used as sound insulation performance criteria, so-called insertion loss (IL). Second, a numerical methodology is developed to predict the IL. The surface pressure field on the side window due to jet flow is predicted by using the high-accurate Lattice Boltzmann Method. The predicted surface pressure fluctuations are applied as input load causing side-window vibration. The interior sound is then computed by using the calculated window vibration as input. The validity of numerical methods is confirmed by comparing the predicted results with the measured ones. Finally, the present methods' ability as a design tool is confirmed by comparing the IL of the pad-added rubber seal with that of the regular seal.

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Numerical study on the contribution of surface and volume components of flow-induced noise in baffle silencers
Time: 7:00 am

Author: Iván Herrero-Durá

Abstract ID: 3035

Baffle silencers are a well-known solution for noise mitigation in industrial applications. One of the issues concerning these devices is the flow-inducted noise produced when a non-laminar flow of the medium in the duct occurs. These situations occur, for example, in dedusting installations or exhaust systems with the high-speed flow (large Reynolds number of the turbulence and small Mach number). This kind of installation has a complex shape that causes a turbulent flow in the medium. Installing a baffle silencer in these conditions causes additional noise. This noise cannot be predicted by using a standard approach with equations for laminar flow conditions. This paper presents the first step of the research in this field. The first step is to find a relation between CFD simulations’ results and self-noise of the baffle silencer. In this work, we use the formulation proposed by Proudman in 1952 to calculate the sound power generated by the flow. The formulation is based on the turbulent kinetic energy k and dissipation rate ? of the flow, which is calculated by CFD simulations. The resulting sound power level needs to be calibrated. The calibration method is developed and presented. The aim of this research is to design an experimental setup.

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A numerical investigation of flow-induced cavity noise control
Time: 6:00 am

Author: Zhenan Song

Abstract ID: 1410

The influence of the multiple ultrasound transmitting units and the steady injecting water or suck-ing water on the shear layer oscillation and noise by flow-induced cavity is numerically investi-gated in this paper. The ultrasound transmitting units and the steady injecting water or sucking water are located upstream of the leading edge of the cavity. The flow field near the cavity is com-puted based on the large eddy simulation method (LES). The calculation and analysis results show that the peak amplitude of noise can be reduced by the steady injecting water at the leading edge of the opening. And within a specific range of flow rates, the greater the injecting rate is, the more obvious the peak amplitude of noise decreases

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Validation setup for the investigation of aeroacoustic and vibroacoustic sound emission of confined turbulent flows
Time: 6:40 am

Author: Paul Maurerlehner

Abstract ID: 1498

Confined flows induce sound at certain flow conditions, which can be annoying in electric vehicles due to the absence of combustion noise. Noise in internal flow may occur due to unfavorable flow-guiding geometries caused by the complex packaging required in engine compartments of modern vehicles. The flow-induced sound is emitted at duct openings (e.g., ventilation inside the passenger cabin). It also originates from the vibroacoustic emissions of the flow-guiding structure excited by the flow. We propose a modular validation procedure for aeroacoustic simulations of confined flows. The experimental setup includes the vibroacoustic emission of the involved flow-guiding structure. The test rig consists of a sensor system, a high-pressure blower, modular pipe sections, and absorbers, which decouple the system from blower noise and avoid acoustic reflections at the pipe exit. A sufficiently long straight inlet section ensures fully developed flow conditions entering the investigated region. For capturing the vibroacoustic sound radiation of the flow-guiding structure, the measurement object and the surrounding microphones are encapsulated in a wooden box, lined with micro-perforated plates. Measurement results of a straight pipe and a pipe with a half-moon-shaped orifice are presented. Additionally, the sound generation is reproduced by Lighthill’s aeroacoustic analogy applying a hybrid approach.

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