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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.

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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.

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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.

In passive sonar systems, knowledge of low-frequency shipping noise is an important factor for target detection performance. However, an accurate model for the shipping noise structure is difficult to obtain, due to the varying distributions of ships and complicated underwater environment. This work characterizes low-frequency distant shipping noise observed in deep water environments as a function of receiver depth and vertical arrival structure for the case of a receiver below the conjugate depth. Surface shipping noise is examined using Monte Carlo simulations using a normal mode propagation model based on random distribution of ships and realistic parameters. The depth dependence of the simulated distant shipping noise is in agreement with published experimental measurements. A Vertical Line Array (VLA) is used to produce vertical beams that isolate the surface interference from nearby targets. Simulation results quantifying the beamformer output as a function of ocean environment, receiver aperture, and frequency are presented for both conventional and adaptive beamformers. The results suggest that conventional beamforming could detect the noisy target from both direct arrival and bottom bounce in the presence of distant shipping interferers and wind noise. However, the beamwidth of conventional beamforming is wider than that of adaptive beamforming. Once the motion effects of nearby ship interferences are considered, the adaptive beamforming using diagonal loading provides better detection performance. Preliminary adaptive beamforming results corresponding to different snapshot times show that motion effects can be minimized by using short observation times.

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