| 1. |
- Tinnsten, Mats, et al.
(författare)
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Optimization of acoustic response
- 1999
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Ingår i: Structural Optimization. - 0934-4373. ; 18:1, s. 36-47
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Tidskriftsartikel (refereegranskat)abstract
- The expression 'acoustic optimization' can be applied to numerous different disciplines within the field of acoustics. From seismic waves, sound in the atmosphere through bioacoustic, psychoacoustics, and room and theater acoustics over shock and vibration in mechanical structures. This paper deals with the latter. Sound generated by vibrating structures is often called noise which is to be minimized. However, not all vibrating structures produce noise, for example the violin is also a vibrating structure that in most people's opinion produces sound called music. In the case of the violin, great effort is made not to minimize but to optimize the sound, i.e. to get the 'right' sound out of the structure. Acoustic optimization within this discipline involves automatic changes of structural design variables to obtain minimum or specified sound in specified regions inside or outside the structure. Examples of problem formulations and some theoretical considerations in the field of acoustic optimization in connection with vibrating mechanical structures will be pointed out. Four simple test cases are included as numerical examples of the method proposed.
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| 2. |
- Bergström, Christer
(författare)
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Numerical Modelling of Fuel Sprays
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The way the fuel is introduced into the combustion chamber is one of the most important parameters for the power output and the generation of emissions in the combustion of liquid fuels. The interaction beween the turbulent gas flow field and the liquid fuel droplets, the vaporisation of them and the mixing of the gaseous fuel with the ambient air that are vital parameters in the combustion process. The use of numerical calculations is an important tool to better understand these complex interacting phenomena. This thesis reports on the numerical modelling of fuel sprays in non-reacting cases using an own developed spray module. The spray module uses the stochastic parcel method to represent the spray. The module was made in such manner that it could by coupled with different gas flow solver. Results obtained from four different gas flow solver are presented in the thesis, including the use of two different kinds of turbulence models. In the first part the spray module is coupled with a k-epsilon based 2-D cylindrical gas flow solver. A thorough sensitivity analysis was performed on the spray and gas flow solver parameters, such as grid size dependence and sensitivity to initial values of k-epsilon. The results of the spray module were also compared to results from other spray codes, e.g. the well known KIVA code. In the second part of this thesis the spray was injected into a turbulent and fully developed crossflow studied. The spray module was attached to a LES (Large Eddy Simulation) based flow solvers enabling the study of the complex structures and time dependent phenomena involved in spray in crossflows. It was found that the spray performs an oscillatory motion and that the Strouhal number in the wake was about 0.1. Different spray breakup models were evaluated by comparing with experimental results.
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| 3. |
- Gullbrand, Jessica
(författare)
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Large Eddy Simulation of Turbulent Flows in Combustor Related Geometries
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Numerical simulations using Large Eddy Simulation (LES) are applied to turbulent swirling flow fields. The swirling motion is often introduced into combustors to act as flame holders or enhance the mixing between species. Different turbulence models capture the swirling motion more or less accurately. LES is well suited for understanding details of swirling flows. It resolves all the large scales in the flow field and only the small scales have to be modeled. The small unresolved scales are the Subgrid Scales (SGS) and the model must take into account the interaction between the small scales and their influence on the resolved scales. In order to separate the effects from the SGS models and the numerical scheme, the problem must be well resolved and be of high order. SGS models have been applied, investigated and compared in swirling flow fields. Four SGS models are considered: an implicit, a stress similarity, a dynamic divergence and an exact differential model. The implicit model uses no SGS model. For the stress similarity model, similar behaviour between the resolved and unresolved stresses is assumed. The model parameter in the dynamic divergence model are depending upon both space and time and it is recalculated during the whole simulation. If a particular form of differential filter function is applied, an explicit expression of the SGS stress tensor can be received. This is the exact differential model. In the simulations, the stress similarity model is shown to have the largest effect on the results. Otherwise, the SGS models only show minor effects on both mean velocities and turbulence intensities. A high order Cartesian grid method have been proposed and employed in the simulations. Cartesian grids have features that are very suitable for LES. The grid generation is simple and fast, it does not require a lot of computational storage and the discretized governing equations can be easily extended to higher orders. The drawback of Cartesian grids is that it does not represent complex geometries correctly. The boundary conditions can be misplaced by as much as a cell size and this reduces the order of the solution. A high order wall treatment is proposed to handle the low order wall problem and it is incorporated into the Cartesian grid method. The high order Cartesian grid method is shown to maintain the order of the discretization.
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| 4. |
- Göransson, Peter
(författare)
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Numerical Modelling of Dynamics of Light Porous Materials
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Porous materials are among the most commonly used materials for noise and vibration reduction in modern transportation vehicles. To design, industrially relevant, weight and cost effective noise and vibration measures, there is a need for general prediction models capable of representing the elasto-acoustic behaviour of such materials. The objective of the present work, is to contribute to the modelling of the inherent fluid-structure interaction phenomena related to porous materials. The modelling approach chosen allows for solution of problems having multiple layers of materials with complicated geometrical shapes and including effects of different boundary conditions along the interfaces to other fluid and solid materials. To solve general three dimensional dynamic problems involving porous materials, a finite element formulation of Biot's equations, describing the fluid-structure interaction in porous materials is proposed. The resulting discrete equation systems, including coupling matrices to other fluid and solid materials, have symmetric matrices and are thus readily implemented into standard finite element software packages. Effects of viscous dissipation, thermal interaction, solid frame disspation and inertial coupling are taken into account. In addition, a finite element formulation of a simplified equivalent fluid model for low stiffness porous materials is proposed.
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| 5. |
- Held, Jörgen
(författare)
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Large Eddy Simulations of Separated Compressible Flows around Wing Sections
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns numerical calculations of compressible separated flows, with and without shocks, around wing sections using Large Eddy Simulations (LES). The considered geometries are the NACA 0012 wing section and the ONERA AT15A wing section. Compressible separated flows around wing sections are of considerable interest, for engineering and scientific reasons, since they involve transition, near-wall turbulence and separation. At transonic speed the interaction between shock and the viscous boundary layer is of interest. If the shock is strong enough shock induced separation occurs. Using Reynolds Averaged Navier-Stokes Simulations (RANS) one has to know in advance where transition to turbulence occurs. The RANS turbulence models also include a set of model parameters which have to be set a priori. This is not the case for dynamic LES-formulation since the model parameters are computed during the simulation utilising the information in the resolved flow field. An explicit filter is used to extract this information. A filter derived on a mathematical basis is presented as well as a new dynamic model where the divergence of the subgrid-scale (SGS) terms are modelled rather than the SGS-terms themselves. In this way only three model parameters are needed for the momentum equations and still anisotropy effects can be accounted for. A novel idea how to treat filtering close to a shock is presented. In LES the large scale flow field is resolved both in time and space and the computational time is significant. To obtain results in acceptable time computations using Parallel Virtual Machine (PVM) have been conducted. The important role of the numerical viscosity as an implicit model in LES is demonstrated. The development of streamwise vortices is shown to be strongly dependent of the spanwise distance of the computational domain.
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| 6. |
- Lagö, Thomas L
(författare)
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A Novel Signal Model and Estimation Method for Acoustic Doppler Current Meters
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with the process of estimating underwater current from the backscattering signal transmitted from an ultrasonic transducer. The narrowband sinusoidal burst signal is Doppler-shifted due to the current, and this information is converted into current. The traditional mathematical model for this Doppler process is based on the assumption that the backscattering time signal is Gaussian, due to the Rayleigh backscattering amplitude with random phase. This is based on the assumption that the backscattering is due to many randomly distributed bubbles with about equal size. It is reasonable to question whether this assumption holds for real life signals. Therefore, part of the work discussed in this thesis has concentrated on looking at real life data, and has investigated whether the Gaussian assumption holds for the background noise and the Doppler signal received. It has been found that this is not generally the case. Thus, a different mathematical model has been sought which describes the Doppler signal in a more realistic way. Such a model has been developed, and is presented. When studying the work of other researchers, it has been noticed that some of the more recent modeling work relies on elaborate mathematics, but does not contribute much to the understanding of the physics, nor the effects on the estimation process. Again, and this is emphasized, the aim has been to develop a simple signal model and a simple estimation method that performs well. The latter is important since the complexity of the system needs to be low. This is because systems are often battery operated and a long battery life is needed as the systems are positioned out in the water. The new mathematical model for the backscattering signal is subsequently used to test different estimator approaches, and to investigate their behavior for different types of complexity in the Doppler signal. A new estimator, the Symmiktos Method™ estimator, has been developed, which is a non-linear estimation method. The estimator performance is compared with the covariance method, which is based on the Gaussian model assumption. The Symmiktos Method™ is based on a more complex Doppler signal model. The result is that the Symmiktos Method™ is more robust to changes in the signal complexity. Also, the method is quick and easy to implement, an important factor in real-life use. Otherwise the method becomes a theoretical method with very little practical use, and this has not been the intention with this work. The work should be viewed as a practical approach to solving a difficult signal processing application problem, where cost, size, simplicity and performance are very important for the end result. A large data set from four locations, Trubaduren, Almagrundet, Fladen and Ma-Wan, Hongkong has been collected. This data base was used when performing elaborate statistical analyses like ANOVA, higher order moments, histograms and normal probability plots. Classical signal processing has been performed as well as non-linear filtering using Multiple Peak Count Analysis, MPCA. The latter is presented in both a 2D and a 3D format. Different estimation methods are compared, including the Symmiktos Method™.
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| 7. |
- Lundberg, Karl-Ola
(författare)
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Applications of the Complex Modulation Transfer Function on SEA-systems
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The SEA coupling and dissipation loss factors are determined in-situ from Complex Modulation Transfer Functions, CMTF:s, based on measured impulse-responses. A quotient of CMTF:s is least-square curve-fitted to a SEA model and the SEA loss factors are determined from the results of the curve-fit. A two subsystem SEA model with time-varying power is considered. In the frequency domain, power-energy transfer functions can be formed. In a power-energy transfer function in the model with two poles and one zero, the zero and one pole are almost equal, if the coupling is weak. When trying to determine the poles and the zero from the CMTF curve one then runs into problems since the zero is so close to the pole. The situation with one pole is easier to handle. Therefore instead a quotient of two SEA power-energy transfer functions with the same input power was taken. The result is a model with one pole leading to a robust evaluation of the loss factors. The result is the quotient of the two subsystem energies where the input power does not enter and thus need not be known. The virtual boundary condition using this new transfer function model is given by putting the denominator energy equal to zero, meaning that the corresponding subsystem is energy- earthed. Even for SEA models with more than two subsystems, a model with one pole can be derived. In the physical system the power can be time-varied by letting the system excitation signal consist of random noise modulated with a deterministic time-varying function. However, since the ensemble average of the squared response is proportional to the squared impulse-response convolved with the squared modulating function, random excitation is avoided and replaced by impulse-response measurements. The Fourier transform of the low-pass filtered squared impulse- response is the CMTF. A CMTF curve for a short distance between source and observation position does not show simple low-pass character, but has a "floor". A model for the modulation transfer function could therefore consist of a modulation frequency-independent part diminishing with source- distance and originating from the direct field and a low-pass character source-distance independent part originating from the reverberant field. At a certain distance from the source the magnitudes of the two parts are equal. The distance is here called the modulation direct field radius in analogy with the ordinary direct field radius. The modulation direct field radius is monotonically increasing with the modulation frequency. Thus the distance affected by the direct field is larger for the modulation than for the non-modulated, stationary, case. Experiments were carried out on two plates connected at a point by a spring and in two rooms divided by a wall.
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| 8. |
- Traunmüller, Hartmut
(författare)
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Analytical expressions for the tonotopic sensory scale
- 1990
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Ingår i: Journal of the Acoustical Society of America. - : Acoustical Society of America (ASA). - 0001-4966 .- 1520-8524. ; 88:1, s. 97-100
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Tidskriftsartikel (refereegranskat)abstract
- Accuracy and simplicity of analytical expressions for the relations between frequency and critical bandwidth as well as critical-band rate (in Bark) are assessed for the purpose of applications in speech perception research and in speech technology. The equivalent rectangular bandwidth (ERB) is seen as a measure of frequency resolution, while the classical critical-band rate is considered a measure of tonotopic position. For the conversion of frequency to critical-band rate, and vice versa, the inversible formula z=[26.81/(1+1960/f )]–0.53 is proposed. Within the frequency range of the perceptually essential vowel formants (0.2–6.7 kHz), it agrees to within ±0.05 Bark with the Bark scale, originally published in the form of a table.
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| 9. |
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| 10. |
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