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Velocity distribution at a given time step, and pressure fluctuations for a selected location, result related to CFD#1
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... The phenomenon of the flow-related acoustic wave generation is the subject of many works focused on the identification of basic mechanisms of sound generation. In Ref. [6], a numerical analysis is presented of the operation of a typical-geometry Helmholtz resonator placed in a fluid flow. In Ref. [7], an analysis is conducted of the flow along a cavity with a lid, characterized by a geometry similar to the one analysed herein and operating in conditions corresponding to the Helmholtz resonator. ...
Cavities and gaps are a common design element of a lot of types of machinery and equipment. The fluid flow over a cavity may generate acoustic waves, which may have both negative and positive implications. This paper is focused on the possibility of improving the heat transfer conditions using the phenomenon of the acoustic wave generation. The operating conditions of a system including an acoustic generator are analysed both numerically and experimentally. A measuring stand and the applied measuring techniques are presented. The sound pressure level characteristic is determined and then compared with the results of a numerical simulation performed for different models. Additionally, the Schlieren technique is used to determine flow structure within the cavity.
The second part of the paper concentrates on the analysis of the impact of a change in the acoustic generator scale on the propagation of waves and heat transfer conditions. Changes are observed in the acoustic wave key parameters. Distributions of the heat transfer coefficient values are determined for the generator individual walls. A reduction in scale involves changes in frequency, the sound pressure level and the heat transfer coefficient mean value.
... These limitations have pushed Free Field Technologies (FFT) to implement an alternate method [22] in the finite element code Actran. The method is based on the variational formulation of the Lighthill equation, is designed to be used for exterior or interior problems with or without liners, and has been shown to possess the potential to handle industrial problems [81,21,23]. ...
... The main benchmark used for these investigations is the noise generated by a Helmholtz resonator placed in a duct. Previous tests [21,56,57,60] have already proved the ability of the methodology to properly capture the peak of resonance (narrow-band noise), in terms of frequency and amplitude. Nevertheless, the broad-band far-field noise was rather underpredicted. ...
... 1. The user produces a file with the node coordinates of the subset of the acoustic mesh where the sources must to be accounted for; is consistent with the analytical formula, f th given in Ref. [21] f th = c 0 2π ...
Due to the continuously increasing economical and environmental constraints, the standard industrial CFD methods (mostly Reynolds Averaged Navier-Stokes equations, RANS) are no longer sufficient to answer the design requirements of the industry, in particular when off-design performance and noise need to be predicted. Therefore, scale-resolving simulations, where the full (Direct Numerical Simulation, DNS) or at least a significant portion (Large-Eddy Simulation, LES) of the turbulence spectrum is resolved, are required. However, as these simulations require a nearly flawless representation of very small turbulent structures, current industrial solvers require huge computational resources in order to provide sufficient accuracy. The discontinuous Galerkin method (DGM) could alleviate this to a large extent as it seems to bridge the gap between the flexibility of industrial codes and the accuracy of academic solvers.
During this thesis, the flexibility and the parallel efficiency of a DGM solver has been improved to tackle the large requirements of DNS and LES. The method was subsequently assessed for DNS and LES based on canonical benchmarks. Due to its interesting dissipation and dispersion properties, DGM seems to offer an accuracy similar to pseudo-spectral (PS) solvers for DNS. As the dissipation targets only the smallest scales, the method seems well suited to use an implicit LES approach. This approach has been validated on the simulation of homogeneous isotropic turbulence and on the channel flow at several Reynolds numbers. Finally, the method has been successfully applied on industrial cases, including a low pressure turbine blade, airfoil profiles and a high Mach number jet flow, thereby showing the maturity of the method.
... The method is based on the variational formulation of the Lighthill equation, is designed to be used for exterior or interior problems with or without liners, and has been shown to possess the potential to handle industrial problems. [6][7][8] FFT has recently implemented the Möhring analogy in Actran. As in the Lighthill analogy, Möhring uses a scalar equation directly derived from the Navier-Stokes equations but with the ability to handle higher Mach number flows (M > 0.2), where the convection and refraction effects cannot be neglected. ...
This paper reports on the validation of a hybrid computational aeroacoustics (CAA) method on a benchmark of a simplified HVAC duct. An acoustic analogy is adopted and an in-house unstructured flow solver is coupled to the ACTRAN commercial finite element solver. Two acoustic analogies are compared: the Lighthill and the Möhring analogies. The sensitivity study on the numerical setup results in a set of parameters that improves significantly the agreement between the numerical acoustic results obtained with the Lighthill analogy and the experiment. The same agreement is obtained using the Möhring analogy. Finally, a study on the parameters of the Fourier transform shows that reasonable results can be obtained on this case at a low cost using a short duration CFD or with a sampling in agreement with the highest frequency of interest. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.
... The method is based on the variational formulation of the Lighthill equation, is designed to be used for exterior or interior problems with or without liners, and has been shown to possess the potential to handle industrial problems. [4][5][6] The acoustic sources captured by the CFD have to be transferred to the acoustic mesh, the sources being the divergence of the Lighthill tensor. The standard approach is based on a linear interpolation. ...
... The main benchmark used for these investigations is the noise generated by a Helmholtz resonator placed in a duct. Previous tests 5,7,9,10 have already proved the ability of the methodology to properly capture the peak of resonance (narrow-band noise), in terms of frequency and amplitude. Nevertheless, the broad-band far-field noise was rather under-predicted. ...
... To illustrate and validate the computational aeroacoustics methodology presented in this paper, the simulation of the noise radiated by a Helmholtz resonator placed in a duct is considered (see Fig. 1). This testcase has been studied experimentally in the reverberation chamber of the acoustic laboratory at Behr. 5 The experimental results show that the resonance frequency, f exp , which is independent of the inlet flow velocity, is equal to 358 Hz which is consistent with the analytical formula, f th given in Ref. 5 ...
This contribution reports on investigations to validate a computational aeroacoustics methodology. An acoustic analogy is adopted in which the Cenaero flow solver Argo is coupled to the commercial acoustic solver Actran/LA that uses a variational formulation of the Lighthill analogy. Two approaches to transfer noise sources from the uid dynamics mesh to the acoustic mesh are studied. The first approach is based on a linear interpolation of the acoustic sources. Although this method ensures a second order accuracy, the conservation of the source term integral from one mesh to another is not fulfilled. Previous work shows that this leads to requirements on the acoustic mesh resolution in the source region that are more severe than those to properly propagate the acoustic waves. The second approach, newly developed by Free Field Technologies, is designed to enforce the conservation of the integral of the source term so that no particular mesh refinement is required in the source region, leaving the propagation as the only criterion to design the acoustic mesh. Both approaches are compared for the computation of the noise radiated by a Helmholtz resonator placed in a duct.
... These limitations have pushed Free Field Technologies to implement an alternate method [4] in the finite element code Actran/LA. The method is based on the variational formulation of the Lighthill equation, is designed to be used for exterior or interior problems with or without liners, and has been shown to possess the potential to handle industrial problems [5][6][7]. ...
... To illustrate and validate the computational aeroacoustic methodology presented in this paper, the simulation of the noise radiated by a Helmholtz resonator placed in a duct is considered (see Figs. 1). This testcase has been studied experimentally in the reverberation chamber of the acoustic laboratory at Behr [6]. The experimental results show that the resonance frequency, f exp , which is independent of the inlet flow velocity, is equal to 358 Hz which is consistent with the analytical formula, f th given in Ref. 6 ...
... A modal basis is applied at the duct inlet while infinite elements applied at the oulet boundary enables to model a free field radiation problem. The monitored value that is compared to the experiment [6] is the power radiated through the outlet boundary. Acoustic sources were sampled every 10 time steps, corresponding to a maximum frequency of 2500 Hz, the total sampling time is 0.2 s and gives a minimum frequency of 5 Hz. ...
This paper reports on the improvement and validation of a computational framework developed for the simulation of aeroacoustics problems. An acoustic analogy is adopted and a in-house three-dimensional unstructured flow solver is coupled to the Actran/LA commercial finite element solver that uses a variational formulation of the Lighthill analogy. Numerical investigations are here performed in order to assess the appropriate level of accuracy required for both computational fluid dynamics and acoustics parts of this methodology to produce accurate noise predictions. As a matter of fact, these investigations are here focused on the noise radiated by a Helmholtz resonator placed in a duct.
... This issue has been demonstrated for a pathological case 17 in which a purely incompressible flow model was used to model the equivalent aeroacoustic sources in Curle's analogy. Alternatives may be based on a volume based finiteelement methodology 18 , or on the use of volumetric equivalent aeroacoustic sources (Lighthill's quadrupoles) to be scattered in a BEM method. The latter option would retains the convenience of only having to deal with surface meshes when computing acoustic scattering, a surrogate volume mesh is only required at locations of the dominant quadrupole sources. ...
Computational Fluid Dynamic (CFD) studies of sunroof buffeting on production vehicles demonstrate accurate prediction of the main buffeting frequency and its harmonics. For production vehicles, none to date has illustrated the phenomenon of buffeting intensity maximization over a vehicle speed range, at a frequency related to the volume of the passenger compartment. All assume that the interior surfaces of the vehicle are rigid, potentially overestimating in-cabin noise intensities by failing to account for surface impedance from non-acoustically rigid trims and linings. In this paper, a modelling study of both effects is presented. Advanced LES-type turbulence modelling, in the form of Detached Eddy Simulation (DES), is used. The hybrid approach, linking acoustic source generation in CFD to acoustic pressure wave propagation with Boundary Element Methods (BEM), is adopted. The former is used to predict the surface pressure excitations induced by the flow, the latter to interpret these as equivalent dipoles to be propagated. The results confirm both the necessity of accounting for compressibility effects in the CFD solver when predicting the buffeting intensity maximization, and surface impedance affects on noise levels perceived by the driver and passengers at higher frequencies closer to the peak audibility range.
Large eddy simulation (LES) on complex geometries by way of unstructured grids can be a tricky problem. As far as spatial discretization is concerned, it is well-known that standard Euler or Reynolds averaged Navier-Stokes (RANS) based schemes are too dissipative to perform LES since their numerical stabilization interacts strongly with the subgrid scale model. As, in the present approach, this spurious interaction is avoided, a low dissipation scheme has to be implemented. This scheme is built on a non-dissipative central scheme that conserves the discrete kinetic energy to reach stability. To prevent the generation of spurious numerical noise in underresolved non-turbulent parts of the simulation domain, a controlled amount of high order numerical dissipation is supplemented. As tetrahedra are reported to be suboptimal in regions where the grid stretching is large, discretization on hybrid meshes is also discussed. Finally, the present methodology is validated with DNS & LES applications.
Though some practitioners consider the simulation process for sunroof and side window buffeting to be mature, there remain considerable uncertainties and inefficiencies as how in predictive methodologies to account for interior panel flexibility, vehicle structural stiffness, seals leakages and interior materials surface finish. Automotive OEMs and component suppliers rightly target flow simulation of open sunroofs and passenger windows with a view to reducing the severely uncomfortable low-frequency booming disturbance. The phenomenon is closely related to open cavity noise experienced also in other transportation sectors; for example in Aerospace, landing gear and store release cavities, and in Rail Transportation, cavities for HVAC intakes and the bogie environment. Recent studies published by the author demonstrate that the uncertainties can be correctly quantified by modeling. This publication defines a hierarchy of CFD/CAE based methods which overcome many of the a-posteriori tuning of simulations based on experiment, and considerably improve the predictive nature and efficiency of the simulation process. The methods range from fully deterministic simulations to phenomenological models requiring standard experimental pre-qualifications of the acoustical response of the system. The former involves CAE-coupling of CFD (Computational Fluid Dynamics) to CAA (Computational Aeroacoustics) and to CSM (Computational Structural Mechanics). The latter incorporates new correlation models published here for the first time.
This paper reports on the application of a computational methodology for the simulation of aeroacoustics problems. An acoustic
analogy is adopted and a in-house three-dimensional unstructured flow solver is coupled to theommercial finite element solver
that uses a variational formulation of the Lighthill analogy. Numerical investigations are performed to study the noise radiated
by a Helmholtz resonator placed in a duct.
