Noise in street canyons with asymmetrical cross sections

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Noise in street canyons with asymmetrical cross sections


Author: Piippo, Kaj Erik
Title: Noise in street canyons with asymmetrical cross sections
Degree: Ph.D.
Year: 2017
Subject: Streets -- Noise.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xix, 219 pages : color illustrations
Language: English
InnoPac Record:
Abstract: The present investigation is focused on the issue of sound propagation in street canyons of asymmetrical cross sections. The typical street canyon is a straight long street with flanking buildings, of equal height, on both sides. Sound propagation in such urban canyons has been researched extensively in the past. Two different street canyon configurations are investigated where the first is with one inclined façade and the second with one façade of different height. The analysis of these asymmetrical canyon configurations is conducted by 1:4 scale model experiments and numerical calculations using the Finite Element Method (FEM). Before the scale model investigation could be performed, a comprehensive study of how to construct and test the performance of a proper line source is carried out. Next, a scale model for the inclined façade case is assembled with the dimensions being, 2 m height, 1 m width, and 4 m length, which corresponds to an aspect ratio of 2 (ar = 2). The previously fashioned line source, which performed best, is used in the experiment. The case when the façades are parallel (θ = 90deg) is used as a reference case which is compared to cases when one façade is gradually inclined with 10 degree intervals, from 90° to 60°. The angle is measured from the ground as the horizontal plane. Primarily, the investigation is focused on the sound pressure level (SPL) distribution along the height of the façade held stationary. Similarly, for the street canyons with different height façades a 1:4 scale model experiment is carried out using the same methodology and dimensions as for the inclined case. The height of one of the façades is gradually decreased, and the case with parallel façades of equal height is used as the reference case. The subsequent numerical investigation is carried out using the same method (FEM) as for the inclined façade investigation. A limitation of the FEM is that when large domains are considered the computation time increases significantly. To simulate the full scale scenario of the street canyon cases would require tens of millions degrees of freedom, which is not a feasible approach. Instead, a 2D approach is used where the cross section of the street canyon cases is modelled. The experimental results are used to validate the 2D simulated data. The experimental data agreed well with the simulated data at low frequencies. The study is expanded to cover additional street canyon aspect ratios. The façade height is kept constant at 8 m, and the complementary cases are with street width of 2 m (ar = 4), and 8 m (ar = 1). After a full set of simulations are carried out for the 1/3 octave band frequencies from 100 Hz to 2500 Hz, for all cases. The data is A-weighted to obtain a single value for the distribution along the façades.
The effect on LpA along the height of the reference façade when the opposite is being inclined, is a drop of about 4 - 9 dB for a 10° inclination, where the largest drop is seen for the narrow street canyon (ar = 4), and the smallest drop for the wide street canyon (ar = 1). When the opposite façade is not present the SPL drops with 7 - 12 dB for the same aspect ratios. The effect on LpA when the aspect ratio changes from ar = 4 to ar = 1 is a drop of 6 dB along the height of the reference façade. When considering a street canyon with different height façades, the drop along the height of the façade is significant on the section of the reference façade that is higher than the case façade. Due to strong standing waves for some of the studied cases the drop in LpA along the height of the reference façade is over 20 dB for ar = 4, and up to 13 dB for ar = 1. The effect on LpA when changing aspect ratio from ar = 4 to ar =1 isup to 14 dB for the SC3Q. It is not straightforward to find a clear trend for the different height façade cases due to strong standing waves at several 1/3 octave band frequencies.

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