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Optimal Sunroof Buffeting Predictions with Compressibility and Surface Impedance Effects
Abstract
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.
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.
Mit Hilfe akustischer Berechnungen ist es möglich, aufwendige Messungen an Fahrzeugprototypen deutlich zu reduzieren. Dieses
Kapitel gibt einen kurzen Überblick über die wichtigsten derzeit verfügbaren Methoden. Dabei wird zunächst auf Elementverfahren,
wie die Finite-Elemente-Methode (FEM) und die Boundary-Elemente-Methode (BEM), näher eingegangen. Während diese vor allem im tieffrequenten Bereich eingesetzt werden, kommen bei höheren Frequenzen vermehrt
Verfahren zum Einsatz, die auf Energieformulierungen beruhen. Exemplarisch wird hier die Funktionsweise und der Einsatz der
Statistischen-Energie-Analyse (SEA) erläutert. Anhand von repräsentativen Beispielen werden die Einsatzmöglichkeiten und Grenzen der verschiedenen Verfahren
aufgezeigt, wobei vor allem auch auf Vergleiche zwischen Rechnung und Messung eingegangen wird.
Aeroacoustics is a growing field in aeronautical and automotive industry, mainly through the application of the aeroacoustical analogy. Curle's analogy is particularly popular in automotive industry. This paper presents some preliminary results of an aeroacoustical study of the sound produced by the leapfrogging of two rectilinear filament vortices within an infinite two-dimensional straight duct. The flow field is obtained from an inviscid kinematic description of the vortical motion. Such incompressible flow description does not contain any acoustical information over the source region. The sound is predicted in frequency domain following two approaches: using a tailored Green's function based on the non-evanescent duct modes, and applying Curle's analogy to the incompressible flow model. The results show that the pressure field obtained applying Curle's analogy differ significantly from the pressure field obtained using the tailored Green's function. This is particularly severe at frequencies for which non-planar duct acoustical modes are excited by the vortex motion. The discrepancies are attributed to the lacking acoustical information in the source description. A hybrid method combining Curle's analogy with a boundary element method to determine a numerical tailored Green's function, is likely to yield a better sound prediction.
A new formulation of Detached-Eddy Simulation (DES) based on the k-ω RANS model of Menter (M-SST model) is presented, the goal being an improvement in separation prediction over the S-A model.
A new numerical scheme adjusted to the hybrid nature of the DES approach and the demands of complex flows is also presented.
The scheme functions as a fourth-order centered differentiation in the LES regions of DES and as an upwind-biased (fifth or
third order) differentiation in the RANS and outer irrotational flow regions. The capabilities of both suggested upgrades
in DES are evaluated on a set of complex separated flows.
A theory is initiated, based on the equations of motion of a gas, for the purpose of estimating the sound radiated from a fluid flow, with rigid boundaries, which as a result of instability contains regular fluctuations or turbulence. The sound field is that which would be produced by a static distribution of acoustic quadrupoles whose instantaneous strength per unit volume is rho vivj + pij - a02rho delta ij, where rho is the density, vi the velocity vector, pij the compressive stress tensor, and a0 the velocity of sound outside the flow. This quadrupole strength density may be approximated in many cases as rho 0vivi. The radiation field is deduced by means of retarded potential solutions. In it, the intensity depends crucially on the frequency as well as on the strength of the quadrupoles, and as a result increases in proportion to a high power, near the eighth, of a typical velocity U in the flow. Physically, the mechanism of conversion of energy from kinetic to acoustic is based on fluctuations in the flow of momentum across fixed surfaces, and it is explained in section 2 how this accounts both for the relative inefficiency of the process and for the increase of efficiency with U. It is shown in section 7 how the efficiency is also increased, particularly for the sound emitted forwards, in the case of fluctuations convected at a not negligible Mach number.
The theory of sound generated aerodynamically is extended by taking into account the statistical properties of turbulent airflows, from which the sound radiated (without the help of solid boundaries) is called aerodynamic noise. The theory is developed with special reference to the noise of jets, for which a detailed comparison with experiment is made (section 7 for subsonic jets, section 8 for supersonic ones). The quadrupole distribution of part I (Lighthill 1952) is shown to behave (see section 3) as if it were concentrated into independent point quadrupoles, one in each 'average eddy volume'. The sound field of each of these is distorted, in favour of downstream emission, by the general downstream motion of the eddy, in accordance with the quadrupole convection theory of part I. This explains, for jet noise, the marked preference for downstream emission, and its increase with jet velocity. For jet velocities considerably greater than the atmospheric speed of sound, the 'Mach number of convection' Mc may exceed 1 in parts of the jet, and then the directional maximum for emission from these parts of the jet is at an angle of sec-1 (Mc) to the axis (section 8). Although turbulence without any mean flow has an acoustic power output, which was calculated to a rough approximation from the expressions of part I by Proudman (1952) (see also section 4 below), nevertheless, turbulence of given intensity can generate more sound in the presence of a large mean shear (section 5). This sound has a directional maximum at 45 degrees (or slightly less, due to the quadrupole convection effect) to the shear layer. These results follow from the fact that the most important term in the rate of change of momentum flux is the product of the pressure and the rate of strain (see figure 2). The higher frequency sound from the heavily sheared mixing region close to the orifice of a jet is found to be of this character. But the lower frequency sound from the fully turbulent core of the jet, farther downstream, can be estimated satisfactorily (section 7) from Proudman's results, which are here reinterpreted (section 5) in terms of sound generated from combined fluctuations of pressure and rate of shear in the turbulence. The acoustic efficiency of the jet is of the order of magnitude 10-4 M5, where M is the orifice Mach number. However, the good agreement, as regards total acoustic power output, with the dimensional considerations of part I, is partly fortuitous. The quadrupole convection effect should produce an increase in the dependence of acoustic power on the jet velocity above the predicted U8 law. The experiments show that (largely cancelling this) some other dependence on velocity is present, tending to reduce the intensity, at the stations where the convection effect would be absent, below the U8 law. At these stations (at 90 degrees to the jet) proportionality to about U6\cdot 5 is more common. A suggested explanation of this, compatible with the existing evidence, is that at higher Mach numbers there may be less turbulence (especially for larger values of nd/U, where n is frequency and d diameter), because in the mixing region, where the turbulence builds up, it is losing energy by sound radiation. This would explain also the slow rate of spread of supersonic mixing regions, and, indeed, is not incompatible with existing rough explanations of that phenomenon. A consideration (section 6) of whether the terms other than momentum flux in the quadrupole strength density might become important in heated jets indicates that they should hardly ever be dominant. Accordingly, the physical explanation (part I) of aerodynamic sound generation still stands. It is re-emphasized, however, that whenever there is a fluctuating force between the fluid and a solid boundary, a dipole radiation will result which may be more efficient than the quadrupole radiation, at least at low Mach numbers.
ABSTRACT Two rectangular cavity configurations at Mach 0.85 are investigated with the objective of assessing the extent to which 3D CFD with advanced,turbulence modeling,is capable of predicting narrowband,and broadband flow noise. A non -linear, two-equation, eddy-viscosity model run in unsteady mode (URANS) is compared with Detached Eddy Simulation (DES) on a cavity with a L/D ratio of 5, representing cavity flow in so -called shear layer mode. Detailed experimental data for this cavity, configured with and without doors, provides a valuable opportunity to compar e predictions of the spectra at many points along the cavity ceiling and band limited amplitude along the cavity length.
An extension is made to Lighthill's general theory of aerodynamic sound, so as to incorporate the influence of solid boundaries upon the sound field. This influence is twofold, namely (i) reflexion and diffraction of the sound waves at the solid boundaries, and (ii) a resultant dipole field at the solid boundaries which are the limits of Lighthill's quadrupole distribution. It is shown that these effects are exactly equivalent to a distribution of dipoles, each representing the force with which unit area of solid boundary acts upon the fluid. A dimensional analysis shows that the intensity of the sound generated by the dipoles should at large distances x be of the general form Ipropto rho 0 U06a0-3 L2x-2, where U0 is a typical velocity of the flow, L is a typical length of the body, a0 is the velocity of sound in fluid at rest and rho 0 is the density of the fluid at rest. Accordingly, these dipoles should be more efficient generators of sound than the quadrupoles of Lighthill's theory if the Mach number is small enough. It is shown that the fundamental frequency of the dipole sound is one half of the frequency of the quadrupole sound.
A method (governing equation set and numerical procedure) suited to the numerical simulation of fluid-resonant oscillation at low Mach numbers is constructed. The new equation set has been derived under the assumption that the compressibility effect is weak. Because the derived equations are essentially the same as the incompressible Navier-Stokes equations, except for an additional term, we can apply almost the same numerical procedure developed for incompressible flow equations without difficulty. With application of a pressure-based method that treats the continuity equation as a constraint equation for pressure, the stiffness problem that arises in solving the usual compressible flow equations under low Mach number conditions has been alleviated. To verify the present method, we apply it to the flows over a three-dimensional open cavity. The results show that strong pressure fluctuations occur at specific flow velocities and that the frequency of the pressure fluctuations is locked in at the Helmholtz resonant frequency of the cavity. Thus, the present method is confirmed to have the capability of predicting fluid-resonant oscillation in low-Mach-number flows.
In the present study, the PISO algorithm described by Issa (1985), has
been implemented in a finite-volume method which employs Euler's
implicit temporal difference scheme and a hybrid upwind/centered spatial
difference scheme. The developed approach was applied to the circulation
of two cases of axisymmetric laminar flow in circular ducts with abrupt
enlargement. The first case involved an incompressible fluid with an
open duct end, while the second was concerned with a compressible flow
and a closed duct end. It is pointed out that the results of the
computations verify the findings of the analysis conducted by Issa
regarding the accuracy and stability of the algorithm.
DES Predictions on the M219 cavity at M=0.85
- R Allen
- F Mendonça
Allen, R., and Mendonça, F., "DES Predictions on the M219 cavity at M=0.85", Colloquium EUROMECH 449, Chamonix,
France, 7-8 th December 2003
Version 3.2, User Guide and Methodology Manuals, CD-adapco
- Star-Cd
STAR-CD Version 3.2, User Guide and Methodology Manuals, CD-adapco, London, UK, 2004.
On-line User's manual
LMS SYSNOISE Rev 5.6: Computational Vibro-Acoustics, On-line User's manual, LMS International NV, Interleuvenlaan
68, Leuven, Belgium, 2002.
Introduction to numerical acoustics
- W Desmet
- P Sas
Desmet, W., Sas, P.. "Introduction to numerical acoustics", 11 th International Seminar on Applied Acoustics, Catholic
University of Leuven, Belgium, 11 th -12 th September 2000.
Flowsimulationusingcontrolvolumesofarbitrarypolyhedralshapes"-ERCOFTACBulletinNo
- M Peric