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The aim of this master thesis is an experimental investigation of the acoustic characteristics of absorbing materials used in mufflers for trucks and cars. The difference in the composition between different wool type materials consists of difference in material, fibre diameter, length, density and fibre orientation. It is also possible to construct mufflers using micro perforated plates (MPP), either solely or in combination with wool type materials. When a specific material is selected the characteristics of the performance can be altered by compressing the material to different bulk densities. It was investigated how some of these properties change the airflow resistivity of the material. When the airflow resistivity of the material is known this parameter can be used in FEM software to describe how a specific material will react, behave and perform as an absorbent. Two different methods were used to extract the airflow resistivity. The fastest method is from the ISO standard ISO 9053 were the airflow resistivity is measured over a sample with a flow speed down to 0.5 mm/s. The second method is the Transfer Matrix Method (TMM) with which the airflow resistivity is extracted from the acoustic transfer matrix of the sample. Both methods are fully described in the report. The TMM was used within a frequency range of 0‐1600 Hz at no flow conditions. Measurements using both methods were performed at room temperature . Ten different wool type materials and two different kinds of MPP were studied. For the wool type materials, the airflow resistivity was measured with the fibres parallel and perpendicular to the direction of sound and airflow. The material samples had bulk densities of 80‐210 g/l. For the MPP the specific airflow resistance was measured with the static flow perpendicular to the plates. The results from the two methods were compared and the transmission loss, absorption coefficient, reflection coefficient and the complex speed of sound were calculated using the transfer matrix from the TMM. Regarding the TMM these data were also compared to the results that can be calculated when using the measured airflow resistivity together with the empirical expressions from Delany‐Bazley & Miki. Repacking of some materials were done in order to study the differences introduced by the packing process. IV The conclusions after the measurements were: • The agreement between the two methods was very good. • The value of the resistivity was doubled for measurements with the fibres perpendicular to the direction of sound. • When the materials with a high degree of micro strands were oriented with the fibres perpendicular to the direction of sound there was a resonant behaviour in the sample. The onset frequency of this resonance increased with increasing bulk density. This resonance leads to difficulties in predicting the behaviour of real life exhaust systems. • In order to get reliable results, further studies on the micro perforated plates must be made, with even lower flow velocities and sound pressure levels and maybe with other methods.

The present licentiate thesis addresses the use of well-resolved simulations to simulate turbulent boundary layers (TBL) subjected to adverse pressure gradients. Within the thesis a wide variety of analyses are performed, and a method to improve the performance of the simulations is presented. The first aim of the thesis is to assess the effect of adverse pressure gradients and flow history on the development and fundamental characteristics of turbulent boundary layers. With this in mind, well-resolved large-eddy simulations (LES) of the turbulent boundary layers over two wing sections are performed using the spectral-element-method (SEM) code Nek5000. In order to assess the effects of the adverse pressure gradient on turbulent boundary layers, turbulence statistics are computed and time series are collected from the simulations. The turbulence statistics show a significant effect of the adverse pressure gradient on the mean velocity profiles, turbulent fluctuations and turbulent kinetic energy budgets. In addition, the time series are used to compute the power-spectral densities of the turbulent boundary layers and to analyse the effect of the adverse pressure gradient on the turbulent scales across the boundary layer. After having compared both wings at moderate Reynolds number Rec=400,000, the next goal is to perform high-resolution simulations of wings at higher Reynolds numbers in order to study conditions closer to those in reality, and to evaluate the effect of adverse pressure gradient with increasing Reynolds numbers. To achieve this, better and more efficient computational methods are required. In this thesis, the performance of the adaptive mesh refinement method recently implemented in Nek5000 is assessed for the first time on wing simulations. The obtained results show a large potential of this new method (which includes the use of non-conformal meshes) with respect to the previous simulations carried out with conformal meshes. Lastly, we performed a modal decomposition of the TBLs developing around both wing sections. To this end, we consider spectral proper orthogonal decomposition (SPOD), which can be used to identify the most energetic structures of the turbulent boundary layer. / <p>QC 20200131</p>

The main objective of this work was to gain an increasing understanding of the properties of vegetation noise and also to the relative ratios of different natural ambient noises to mask wind turbine sound. A discrete vegetation noise model was developed and compared to an earlier model showing improved estimations, especially at frequencies below 0.5 kHz. Field measurements of sound from deleafed trees are compared to a deleafed tree model with satisfactory agreement. A wind turbulence model (Sandia method) was coupled to the discrete model and thereby time series of fluctuating vegetation noise can be computed. Several measurements including higher wind speeds than reported in earlier literature were compared to predictions of fluctuating vegetation noise with good agreement. Psycho acoustic tests was performed by 36 subjects to determine the signal-to-noise (S/N) ratios when wind turbine noise is inaudible in three different natural ambient noises. The masking threshold varied between -6.5 dBA and -2.7 dBA for coniferous tree noise and sea wave noise respectively. Further tests revealed that at S/N ratios of +3dBA and above the wind turbine noise was considered as the dominant sound source. / QC 20101109

Abstract Consider a 　uid being heated from below. The heating leads to an upward convective force that is counteracted by the viscous forces of the 　uid. If the convective force is large enough in comparison to the viscous forces the 　uid will be put in an unstable state. This means that a small disturbance will give rise to a 　ow driven by a temperature gradient. This 　ow is characterised by a pattern of convection cells. The phenomenon is called Rayleigh-Bénard convection. An example of this can be seen when heating a pot of oil from below. A part of the contribution to the formation of these cells is attributed to the variation of surface tension due to heating. This contribution is of less signicance when the 　uid layer is thicker. In this report the studied 　ow eld lies between two plates where the convective force drives the motion. The in　uence of surface tension is eliminated since the 　uid lacks a free surface in this problem. The boundary between stability and instability is investigated both theoretically, using simplied Navier-Stokes equations, and by simulation using a DNS-code with the program Simson(Chevalier et al., 2007). The simulation also makes it possible to see the shape of the convection cells. The results is presented in stability diagrams that describe how the stability boundary is aected by the wavelength, related to the wave number K, of the applied disturbance and the dimensionless Rayleigh number, Ra. The critical value for the two parameters is found to be Ra= 1708whenK= 3:12 Finally the similarity between the simplied theory and the more realistic simulation is discussed.

COVID-19 pandemic has caused a severely detrimental effect on the physical, psychological, and functional well-being of the older adults by limiting their social and out-of-home activities, which in turn is likely to affect their habitual physical activity level. Physical activity (PA) is vital for healthy aging and the health-related benefits of PA for older adults are well-established. In the current context of COVID-19 pandemic, the changes in PA level, resulting from physical distancing adherence and social isolation, can be a major health concern for the older adults, as they are more prone to physical inactivity than the younger population. Accurate PA assessment at population levels is necessary to understand the trend in PA and sedentary behavior among the older adults during the pandemic. Self-reported assessments can be inexpensive and easy to administer, but they are often subjected to measurement biases such as misinterpretations or deliberate alterations (social desirability bias) and participants having difficulty in remembering activities (recall bias). Accelerometer-based PA monitoring can overcome these limitations of self-reported assessment and objectively measure the amount and intensity of PA in a free-living environment. The objective of this study is to examine the PA levels in the older adults, who were living under the physical distancing guidelines during the COVID-19 pandemic, using an accelerometry-based assessment. The study also investigates how such objectively measured PA levels varied among the older adults based on different sociodemographic factors. In this cross-sectional study, 124 community-dwelling older adults (Age: 60–96 years) were recruited from the region of Central Florida between March 2021 and August 2021. The findings of this study can infer guidelines and/or interventions to promote physical activity and healthy aging among the older adults, particularly those who are susceptible to social isolation and disconnectedness due to COVID-19 pandemic.

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