When sound waves propagate in a duct in the presence of turbulent flow, tur- bulent mixing can cause extra attenuation of the sound waves in addition to that caused by the viscothermal eects. Experiments show that compared to the vis- cothermal eects, turbulent absorption becomes the dominant contribution to the sound attenuation at suciently low frequencies. The mechanism of this turbulent absorption is attributed to the turbulent stress and the turbulent heat transfer act- ing on the coherent perturbations (including to sound waves) near the duct wall, i.e. sound-turbulence interaction. The purpose of the current investigation is to understand the mechanism of the sound-turbulence interaction in low-Mach-number internal flows by means of theoretical modeling and numerical simulation. The turbulence absorption can be modeled through perturbation turbulent Reynolds stresses and perturbation turbu- lent heat flux in the linearized perturbation equations. In this thesis, the linearized perturbation equations are reviewed, and dierent models for the turbulent absorp- tion of the sound waves are investigated. In addition, a new non-equilibrium model for the perturbation turbulent Reynolds stress is proposed. The proposed model is validated by comparing the computed perturbation fields with experimental data from turbulent pipe flow measurements, and large eddy simulations (LES) of turbu- lent channel flow. Good agreements are observed. Besides the theoretical modeling, LES is also carried out as a numerical investi- gation of the sound-turbulence interaction. Some preliminary results from the LES are presented. / Vid ljudutbredning i kanaler med turbulent flöde kan diusion som orsakas av turbulens ge extra dämpning av ljudvågor utöver den som orsakas av viskoter- miska eekter. Experiment visar att vid låga frekvenser ger denna absorption det dominerande bidraget till ljuddämpning. Mekanismen för denna absorption är tur- bulensens inverkan på koherenta störningar, bland annat ljudvågor, dvs ljud - tur- bulensinteraktion. Syftet med denna undersökning är att förstå mekanismen för ljud - turbulensin- teraktion i internströmning vid låga Machtal med hjälp av teoretisk modellering och numeriska simuleringar. Ljudabsorption pga turbulens kan modelleras via mod- ellering av störningar av de turbulenta Reynoldska spänningarna och störningar i den turbulenta värmetransporten i de linjäriserade störningsekvationerna. I denna avhandling går vi igenom de linjäriserade störningsekvationerna, och olika modeller för turbulent absorption av ljudvågor utreds. Dessutom presenteras en ny icke- jämviktsmodell för små störningar av de turbulenta Reynoldska spänningarna. Den föreslagna modellen utvärderas genom att de beräknade störningsfältet jämförs med experimentella data från mätningar i rör med turbulent strömning, samt med Large Eddy Simulations (LES) av turbulent strömning. God överensstämmelse kan visas. Förutom teoretisk modellering, kommer LES också att användas för att numeriskt undersöka ljud - turbulensinteraktion. Några preliminära resultat från LES presen- teras. / <p>QC 20130927</p>

Flow reversal chambers are mainly used to accomplish a compact silencer design needed on a vehicle. Generally in this configuration the inlet and outlet ports are on the same face and the flow direction is reversed. During many years different authors have tried to develop 1D and 3D models for evaluating the acoustic performance of circular and rectangular reversing chambers. Ih [1] categorizes four methods for evaluating the acoustic performance of the reversing chamber. The first involves utilizing analysis techniques for other types of muffler elements having similar acoustic performances [2]. Analysis techniques for extended inlet/outlet expansion chambers may be used to approximate the behavior of a reversing chamber in which the length-to-diameter ratio is large. When the length-to-diameter ratio is small, the reversing chamber approximates the behavior of a short expansion chamber. In this case, exact predictions of the acoustic performances cannot be made and, moreover, the method itself is a trial-and-error one. The second is a mode-matching method at the discontinuities [3-5], but this is tedious to formulate and the transmission matrix for this type of muffler has not been obtained. A simplified version (third method) of this method has been developed for plane wave propagation, in which the sound pressures and particle velocities at the area discontinuities are matched [6, 7]. However, this method is restricted to a very small frequency range below the cut-off frequency of the first asymmetric mode, i.e., the (1, 0) mode, and the peaks of the transmission loss curves are not correctly predicted due to the disregard of the higher order modes. Furthermore, when the length-to-diameter ratio is small, the actual acoustic performance deviates appreciably from the theoretical transmission loss predicted by this one-dimensional analysis method. The fourth method involves using numerical methods such as finite element analysis [8] and the finite difference method [9], or possibly, the boundary element method. These numerical techniques have some merits in the treatment of more complicated geometries, such as that of an elliptic cross-section and/or a chamber with a pass tube [10], but a great many mesh points or mesh elements are required to deal with the high frequency range, so that the execution time for computation is long and the costs are high. It is also difficult to describe the total exhaust system by incorporating the transmission matrix of each silencer element.Lindborg et al. [11] modeled the flow reversal chamber by two port method. The system under study is broken down into a set of linear subcomponents that are described individually and then assembled in a network. Each component is treated as a black box that is defined at the inlet and outlet ports where plane waves are assumed. This is an efficient tool, but for complicated geometries such as the flow reversal chamber the decomposition into subcomponents is not obvious. Three different approaches are used for the two port modeling of a flow reversal [11]; 1- Large quarter wave resonator 2- More detailed representation consisting of cones and quarter wave resonators 3- A simplification of the second approach into a simple Pipe 6 From the results of this study, it can be concluded that the acoustic characteristics of shallow flow reversal chambers can be modeled, with engineering precision, up to cut on frequency of the first higher order mode using simple two-port elements. Good results were achieved modeling the flow reversal chamber as a simple straight duct connecting the inlet and the outlet. Munjal [12] devised a numerical collocation method. This method is easily applicable to rectangular as well as circular expansion chambers, but is limited to integer multiple area expansion ratios due to its inherent concept of discrete geometrical partitioning. Analytical methods have been introduced over the years. These methods fall into two main groups, one-dimensional and three-dimensional models. However as Ih [13] has mentioned, if the length of the chamber is much shorter than its width, then a large number of modes should be counted for calculating transmission loss even for the very low frequency range and this fact, arising mainly from the higher order acoustic modes generated at area discontinuities which do not fully decay before they reach the counterpart port, because the inlet and outlet are very close to each other. This leakage phenomenon means that the one-dimensional models are quite far from the actual performances even in the low frequency region. Three-dimensional models provide a very simple and exact approach to theoretical prediction of acoustical performance of plenum and reversing chambers. A three-dimensional mathematical formulation for mufflers with circular or rectangular cross-section with arbitrary location of inlet/outlet is derived by using the Eigen function expansion technique by Ih [13, 14]. The same problem is solved by the use of Green's function by Kim and Kang [15] for circular chambers and by Venkatesham et al. [16] for rectangular chambers. These methods take into account the effect of higher order modes which is necessary for successful analysis of a flow reversal chamber. The basic idea for these models stems from the fact that these chambers are in general regular in shape, which permits the use of series of orthogonal eigenfunctions. However, mufflers used in industry are not exactly rectangles or cylinders. Usually they are a bit curved at the edges to increase the stiffness. It is of interest for industry to know how this difference can alter the TL curve. This problem can be solved by FEM, however this method would be expensive and time consuming. One purpose of this thesis work is to investigate other methods for predicting TL of such chambers. One method could be to approximate the chamber which is curved at the edges with one which has sharp edges and then use the available theoretical models like the Green's function method to get TL curve. In the present study we want to find out how to do this approximation. The other possible method can be Neural Network. However this method needs some training data to train the neural network. Data for training can be obtained either through experiment or FEM. The effect of mean flow velocity is not studied here; However it has been found to be of negligible effect when Mach number is smaller than about 0.03 [17]. Besides, when the mean flow velocity is smaller than about M = 0.1, the convective contributions can be considered as negligible second order quantities and flow-generated noise may often be neglected. Further, if the mean flow velocity is small, the flow-generated 7 noise as well as pressure losses can be greatly reduced without degradation of the acoustic performance by streamline guidance: i.e., by using special l/O connecting geometries such as bell mouths and perforated bridges with high perforation ratios over 20% [14].

In this work, a flow reversal resonator fitted with a short-circuit duct connecting the inlet and outlet is analysed and used as a tuneable muffler element, aimed to be used use in a semi-active muffler on an IC-engine. The work done can be divided into 3 main parts. 1), a study of what type of valve that could be used to change the acoustical properties of the short-circuit duct. 2), Design of a flow reversal resonator with a controllable valve as the short-circuit. 3), experimental validation in a flow acoustic test rig. The flow reversal resonator with a controllable valve as short-circuit is successfully validated to work as tuneable muffler element during laboratory conditions. The same valve concept is simulated in a full scale concept but not validated experimentally on a running IC-engine. The theory used to describe the acoustics of a flow duct element is also presented together with three simulation techniques and the two microphone technique used to determine the acoustic properties of the investigated flow reversal resonator.

The Swedish government have a goal that wind turbines shall produce 30 TWh by the year 2020, compared to about 3,5 TWh produced in Sweden during 2010. To minimize the disturbance that wind turbines create the Swedish Environmental Protection Agency has stated a guideline value that noise from wind turbines at nearby residents shouldn’t exceed 40 dBA, at 8 m/s wind speed. With advanced calculation models, like Nord 2000, the emitted sound from future wind turbines can be calculated at an early stage to optimize the power production without exceeding the 40 dBA. When the turbines have been built there is a need to verify that they really are within the guideline. To measure the sound pressure level at nearby residents, a sound immission measurement, there is a Swedish method, Elforsk 98:24 (1) with the translated titled “measuring of noise immission from wind turbines”. There is also an international standard IEC 61400-11 (2) focused on how to measure the emitted sound power level, also called sound emission measurement. To verify the sound level at a resident building a sound immission measurement is the preferred method. However this method has some practical limitations, for example the background sound level is often higher than the guideline value of 40 dBA, which makes it hard to estimate the equivalent sound pressure level of the wind turbine. A further development of this method was outlined in a draft translated “Measuring of sound from wind turbines –draft 2005” (3). The method in this draft is based on a sound emission measurement, compatible with the IEC 61400-11, in combination with a simple formula to calculate the sound immission level. The draft also contains a method for measuring the sound attenuation from the wind turbine to the resident, which can be used together with the measured sound power level to calculate the sound immission level. The intention of this thesis is to further develop today’s method for sound immission measurement of wind turbines. To achieve this, theoretical studies have been undertaken to study the elements relevant to the measurement, like sound generation, attenuation and relevant measurement equipment. Swedish and international measurement methods and guideline values have been studied. Measurements based on the draft (3) have been performed and shows that the method of measuring the sound attenuation, outlined in the draft, can provide results where the present method would fail. But the sound attenuation measuring method is still hampered by high background sound levels and has some practical limitations compared to the present method Elforsk 98:24. Conclusions from this thesis is that calculations of sound immission levels, based on measured sound power level, is a method that is and will continue to be a part of the assessment of sound levels from wind turbines. To ensure high quality of these assessments the procedure should be standardized and the use of advanced calculation models should be included. In cases where the sound attenuation path crosses hilly terrain, water or where the guideline values are exceeded, calculations with more advanced method should be advocated.

Micro-perforated panels (MPP) have a good acoustic damping performance and a potential to be used in mufflers [1, 2]. This thesis aims to investigate the acoustic performance of a parallel baffles muffler, which consists of MPP baffles. The work is both experimental and numerical, i.e., a model is developed for a MPP baffles muffler with local reaction. The prototype MPP mufflers tested in the thesis were all produced by the Swedish company Sontech. First, the experiments were based on the procedure for experimental characterization of heat exchangers described in Ref. [5]. The transmission loss and flow generated sound are tested at four flow speeds for seven kinds of parallel baffles. Second, the mathematical modeling is built on the basis of the standard models of a locally reacting liner muffler [13]. The impedance for the Micro-perforated panels is computed using the models described by Maa [1] and Guo [2].

Percussive tools such as chipping hammers, riveting hammers and concrete breakers produce shock-type (impact) vibrations with large amount of vibration energy in the high frequency region (kHz). Reported injuries from shock-type and high frequency vibrations are higher  occurrence of vascular disorders, myelin disruption, edema and reduced motor performance. An Atlas Copco Tools customer has reported an increase of VWF (vibration induced white finger) during the last decade and the chipping hammers have been identified as the main cause. A mean of 1.8 workers/year has been injured with VWF and the cost of one injured worker has been estimate to 0.5 million SEK. One way to eliminate the injuries could be to replace the tools from Atlas Copco with better tools from another manufacturer. A protective handle was designed for the chipping hammers by Atlas Copco, to prevent operators from holding directly on the vibrating parts. However, the protective handle was not vibration isolated and a follow up project, this thesis, was started to redesign and vibration isolate the handle. An isolated protective handle with two rubber bushings was finally developed from theory of vibration isolation together with a subjective evaluation by experienced personnel and with low and high frequency vibration measurements. The final protective handle significantly reduced both the harmful low frequency vibrations (measured according to ISO 5349) and the potentially harmful high frequency vibrations (not measured according to any ISO standard), compared to the non-isolated original handle  developed by Atlas Copco.

No description available.

The unsteady ow around an aerofoil placed in a uniform ow stream with an angle of attack is investigated, under the assumption of inviscid, incompressible, two-dimensional flow. In particular, a function of the velocity jump over the wake is achieved, where this function depends on the horizontal displacement and time. The aerofoil geometry is represented by two arbitrary functions, one for the upper and one for the lower side of the aerofoil. These functions are dependent on time, hence the aerofoil can perform oscillating movement, which is the case when subjected to utter. The governing equations for the ow are the Euler equations. By assuming thin aerofoil, small angle of attack and that the perturbation of the wake is small, the problem is linearised. It is shown that the linearised Euler equations can be rewritten as the Cauchy-Riemann equations, and an analytic function exists where its real part is the horizontal velocity component and its imaginary part is the vertical velocity component with opposite sign. The ow eld is then investigated in the complex plane by making an appropriate branch cut removing all discontinuities, and with restrictions on the analytic function such that the kinematic and boundary conditions are satis ed. By using Cauchy's integral formula an expression for the anti-symmetric part of the analytic function is achieved. A general expression for the velocity jump over the wake is obtained, which is applied to the speci c case of harmonic oscillations for a symmetric aerofoil. In the end three types of utter is investigated; twisting oscillations around the centre of stiness, vertical oscillation, and aileron flutter.

No description available.

This thesis concerns a study undertaken on a railway vehicle cooling fan system. Several design modifications were proposed and tested in the laboratory in order to evaluate different noise control measures. The most effective noise control measure involving combining lots of different measures gave 7 dB(A) noise reduction. Furthermore a modification by replacing the axial fan by a radial fan can even reduce the radiated sound power by 10 dB(A). The transfer functions were also examined for a dipole source to acoustic pressure in the far field to evaluate the acoustic loading created by the cooling fan enclosure. One fan was taken out from the complete unit and mounted between two acoustic test chambers to measure the effect of various parameters on the acoustic source strength (power) under controlled conditions. To predict accurate data of radiated sound power for comparison with the experimental work, two models have been developed: one model using the Farassat formula for a rotating dipole and the other model based on a 3D acoustic-vortex model. The heat exchangers in the cooling unit can have a significant effect on sound emitted from a cooling fan. An acoustic model based on so called equivalent fluid model for an anisotropic medium was derived to evaluate the sound transmission and the absorption through a parallel plate type of heat exchanger.