The University of Southampton
University of Southampton Institutional Repository

Measurements of the structure of urban-type boundary layers

Measurements of the structure of urban-type boundary layers
Measurements of the structure of urban-type boundary layers
In order to gain a better understanding of the fundamental structure associated with turbulent flows over very rough, urban-type surfaces, laboratory experiments were undertaken in a subsonic wind tunnel facility at the University of Southampton. It was anticipated that undertaking this work would provide better insight into fundamental differences in the flow structure compared to smooth-wall surfaces. After modification of an existing facility to accommodate a longer working section length, testing proceeded over a regular array of cube roughness elements with a 25% area density. Measurements were conducted with hot-wire anemometry, Laser Doppler anemometry, and particle image velocimetry. Development of the particle image velocimetry technique to obtain accurate turbulence statistics over and among the roughness elements was successfully undertaken providing significant new analysis opportunities and results.
Initial testing characterised a large-scale spanwise variation discovered within the boundary layer developing over the cube surface. It was found that mean velocity variation in the span at a height of 50% of the boundary layer thickness could exceed ±5%. Further testing was conducted at locations far enough downstream to minimise the amplitude of the variation. Time-averaged mean velocity and turbulence statistics were collected revealing the averaged flow features. The peak Reynolds shear stress near the cube surface was found to be a strong function of the relative boundary layer thickness compared to the roughness size. Quadrant analysis showed the instantaneous sweep motions found near the rough surface intermittently producing large percentages of the local shear stress. Spatial correlation analysis of the instantaneous field data collected with particle image velocimetry revealed long, streamwise-stretched regions of streamwise mean velocity cross correlation. Correlation analysis also allowed calculation of the structure angle of the streamwise velocity cross correlation and the associated integral length scales of the turbulence structure. Two integral length scales were found in certain locations near the cube surface highlighting the complex nature of the flow and inherent difference compared to smooth wall flows. Comparisons were made with existing direct numerical simulation studies over identical geometries showing many general similarities but also indicating differences associated with the assumptions governing each approach. Together, the experiments and analysis establish a broad picture of the distinct flow structure found in urban-type flows.
Reynolds, Ryan T.
16c859fe-e628-4ac5-a415-9cce14846791
Reynolds, Ryan T.
16c859fe-e628-4ac5-a415-9cce14846791

Reynolds, Ryan T. (2006) Measurements of the structure of urban-type boundary layers. University of Southampton, School of Engineering Sciences, Doctoral Thesis, 215pp.

Record type: Thesis (Doctoral)

Abstract

In order to gain a better understanding of the fundamental structure associated with turbulent flows over very rough, urban-type surfaces, laboratory experiments were undertaken in a subsonic wind tunnel facility at the University of Southampton. It was anticipated that undertaking this work would provide better insight into fundamental differences in the flow structure compared to smooth-wall surfaces. After modification of an existing facility to accommodate a longer working section length, testing proceeded over a regular array of cube roughness elements with a 25% area density. Measurements were conducted with hot-wire anemometry, Laser Doppler anemometry, and particle image velocimetry. Development of the particle image velocimetry technique to obtain accurate turbulence statistics over and among the roughness elements was successfully undertaken providing significant new analysis opportunities and results.
Initial testing characterised a large-scale spanwise variation discovered within the boundary layer developing over the cube surface. It was found that mean velocity variation in the span at a height of 50% of the boundary layer thickness could exceed ±5%. Further testing was conducted at locations far enough downstream to minimise the amplitude of the variation. Time-averaged mean velocity and turbulence statistics were collected revealing the averaged flow features. The peak Reynolds shear stress near the cube surface was found to be a strong function of the relative boundary layer thickness compared to the roughness size. Quadrant analysis showed the instantaneous sweep motions found near the rough surface intermittently producing large percentages of the local shear stress. Spatial correlation analysis of the instantaneous field data collected with particle image velocimetry revealed long, streamwise-stretched regions of streamwise mean velocity cross correlation. Correlation analysis also allowed calculation of the structure angle of the streamwise velocity cross correlation and the associated integral length scales of the turbulence structure. Two integral length scales were found in certain locations near the cube surface highlighting the complex nature of the flow and inherent difference compared to smooth wall flows. Comparisons were made with existing direct numerical simulation studies over identical geometries showing many general similarities but also indicating differences associated with the assumptions governing each approach. Together, the experiments and analysis establish a broad picture of the distinct flow structure found in urban-type flows.

Text
REYNOLDS_Ryan.pdf - Other
Download (102kB)

More information

Published date: 2006
Organisations: University of Southampton

Identifiers

Local EPrints ID: 47099
URI: http://eprints.soton.ac.uk/id/eprint/47099
PURE UUID: 55e28064-608d-4dbc-b0c9-4eb25ed46592

Catalogue record

Date deposited: 16 Aug 2007
Last modified: 13 Mar 2019 21:00

Export record

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×