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.

No description available.

A new experimental setup for studies on wake flow instability and its control, which has been designed and manufactured, is introduced and described. The main body is a dual-sided flat plate with an elliptic leading edge and a blunt trailing edge. Permeable surfaces enable boundary layer suction and/or blowing that introduce the feature of adjusting the inlet condition of the wake created behind the plate. This, in combination with a trailing edge that is easily modified, makes it an ideal experiment for studies of different control methods for the wake flow instability. Additionally, a vortex detection program have been developed in order to detect, analyse and compare small-scale vortical structures in the wake behind the plate for different inlet conditions and control methods applied to the wake flow. Instantaneous velocity fields behind a cylinder subjected to suction or blowing through the entire cylinder surface have been analysed with this program. The results of the analysis show that the major change for different levels of blowing or suction is the location of vortices while the most common vortex size and strength are essentially unchanged.

No description available.

The understanding of acoustic wave propagation in ducts or pipes is essential for all applications involving fluid machines, e.g., fans, pumps, and compressors. Over the years different methods have been developed to simulate wave propagation in duct networks. The existence of accurate simulation methods saves effort and time in the design phase and can prevent noise problems after implementation. One common type of duct network used in buildings and vehicles is Heating-Ventilation and Air-Conditioning or HVAC systems. Traditionally the so-called “Source-Path-Receiver” method is used in such systems. This method is based on analyzing the flow of acoustic power from the source through a system. This approach is valid when a large number of waves are propagating in a duct or for sufficiently high frequencies. The method neglects reflections, starts from the source sound power, subtracts the attenuation of each element and adds its flow generated noise. In an attempt to further developing and improving the “Source-Path-Receiver” method; a power-based two-port method is proposed in this thesis. The proposed method is developed first to comply with the standard “Source-Path-Receiver” methods as described in ASHRAE and VDI standards. The newly proposed method can include the effect of both reflection and transmission for all elements via a scattering matrix. In order to demonstrate when and to what extent this can be important, a study using power-based two-ports is conducted on purely reflective networks and with mixed reflective and dissipative networks. / Förståelsen av akustisk vågutbredning i kanaler eller rör är väsentlig för tillämpningar som involverar olika strömningsmaskiner såsom fläktar, pumpar, kompressorer. Under årens lopp har olika metoder utvecklats för att simulera vågutbredning i sådana system. Tillgången till bra metoder för simulering sparar tid i designfasen och kan förhindra bullerproblem efter genomförandet. En vanlig typ av kanalnätverk som används i byggnader och fordon är Värme-Ventilation och Luftkonditionerings-system. Traditionellt har en så kallade "Source-Path-Receiver" modell använts i sådana system. Denna metod är baserad på att analysera flödet av akustisk effekt från källan genom ett system. Detta tillvägagångssätt är giltigt när många vågor utbreder sig i en kanal eller för tillräckligt höga frekvenser. Metoden försummar reflektioner och utgår från en känd ljudeffekt hos källan, subtraherar dämpningen från kanalelement och adderar flödesgenererat ljud. I ett försök att vidareutveckla och förbättra denna metod föreslås en ny metod baserad på effektbaserade två-ports modeller. Denna nya metod tillämpas först på "Source-Path-Receiver" modeller som beskrivs i standarder för buller i ventilationssystem. Den nya metoden kan inkludera både reflektion och transmission för alla element. För att visa när och i vilken utsträckning detta kan vara viktigt, genomförs en studie på system med enbart reflektiva element samt system med både reflektiva och dissipativa element. / <p>QC 220519</p>

Today Micro Electrical Mechanical Systems (MEMS) microphones are available in a range of electronic consumer products such as smart phones, tablets, smartwatches and surveillance cameras. The MEMS microphones are usually attached to a circuit board with a hole that lets sound propagate through, as well as additional wave-guides which alter the MEMS microphones original frequency response. The MEMS microphone and additional wave-guides are in the same size order as the thermal and viscous boundary layers. These are called non-ideal losses and are usually not considered when dealing with large scale acoustical systems. The only way to predict the impact of these losses is the use of Finite Element software. The objective of the work is to model the thermoviscous losses when the waves propagate through narrow regions. The system of study is the Axis Network Camera P1367 and the study focuses on the acoustic path into the microphone. The first aim is to model the acoustic path along with the MEMS microphone to produce a frequency response that matches the measured frequency response of the different configurations for the sound-guide. A second aim is to find the configuration which produces the most desirable frequency response. Several measurements with different configurations were made, such as varying the length and radius of the sound-guide hole. All measurements were performed in an an-echoic chamber. Thereafter, a FEM model was created of the simplified acoustic path and the different configurations that were performed in the measurement were compared with the simulated results. The simulated frequency responses differ in terms of where the resonance frequency occurs, but the configurations of the sound-guide match the overall behavior when comparing the simulated and measured results. The most optimal configuration of the acoustical path is obtained. The simulated model requires more work in terms of obtaining a better matching frequency response, most importantly the MEMS cavity. The real geometry of the MEMS sensor cavity did not produce the same frequency response as the one in the data sheet for the specific microphone used for this study, thus a fictive cavity was introduced to produce the desired frequency response. The model did succeed in capturing the overall behavior as well as when the configuration was altered.

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 &amp; 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>