Wind-Direction Effects on Urban-Type Flows

Jean Claus, O. Coceal, T. Glyn Thomas, S. Branford, S. E. Belcher and Ian P. Castro
Boundary-Layer Meteorology

Abstract

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.

As one would expect from such a hotel, the food was good and the staff attentive. The awards were hosted by Rob Brydon, who managed to strike a good balance of being funny while keeping the audience interested in the proceedings. The after event party was DJ-ed by Keith Chegwin – though we left after a few minutes, as we did not want to get back too late.

We had been shortlisted for Research Project of the year, but were beaten by an exciting piece of technology from the University of Strathclyde.

Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions

S. Branford, O. Coceal, T. G. Thomas, and S. E. Belcher
Boundary-Layer Meteorology

Abstract

The dispersion of a point-source release of a passive scalar in a regular array of cubical, urban-like, obstacles is investigated by means of direct numerical simulations. The simulations are conducted under conditions of neutral stability and fully rough turbulent flow, at a roughness Reynolds number of Re τ = 500. The Navier–Stokes and scalar equations are integrated assuming a constant rate release from a point source close to the ground within the array. We focus on short-range dispersion, when most of the material is still within the building canopy. Mean and fluctuating concentrations are computed for three different pressure gradient directions (0°, 30°, 45°). The results agree well with available experimental data measured in a water channel for a flow angle of 0°. Profiles of mean concentration and the three-dimensional structure of the dispersion pattern are compared for the different forcing angles. A number of processes affecting the plume structure are identified and discussed, including: (i) advection or channelling of scalar down ‘streets’, (ii) lateral dispersion by turbulent fluctuations and topological dispersion induced by dividing streamlines around buildings, (iii) skewing of the plume due to flow turning with height, (iv) detrainment by turbulent dispersion or mean recirculation, (v) entrainment and release of scalar in building wakes, giving rise to ‘secondary sources’, (vi) plume meandering due to unsteady turbulent fluctuations. Finally, results on relative concentration fluctuations are presented and compared with the literature for point source dispersion over flat terrain and urban arrays.

At the beginning of February I started a new job, in the Evolutionary Biology Group, which is in the School of Biological Sciences at the University of Reading. The project I am researching is concerned with the evolution of phoneme use across sets of related languages. This research is in conjunction with researchers at the Santa Fe Institute and to learn more about the project I spent two weeks visiting them.

It was a good trip and I also had opportunity to spend time exploring the local area. While there I did some running – all of which was either up or down hill and at an altitude of over 2000m.

I am currently working on modelling wind flow and dispersion in urban areas. This research is for the Boundary Layer Meteorology group, which is in the Department of Meteorology at the University of Reading.

The image in this post is from a recent experiment, modelling scalar transport.

(And thanks to the hard work of those that allows me to produce such images!)

]]> http://owls-count.net/blog/work/my-job/feed 10 http://owls-count.net/blog/work/graduation http://owls-count.net/blog/work/graduation#comments Sat, 04 Jul 2009 16:47:13 +0000 Simon http://owls-count.net/?p=383 This morning I graduated. I am really glad that the hotter weather of earlier in the week had relented, making the wearing of a suit and robes much more bearable.

The combination of a surname starting with a letter early in the alphabet and my degree meant that I was the first person to be presented for a degree today. There was a small amount of pressure to start the proceedings off correctly.

]]> http://owls-count.net/blog/work/graduation/feed 2 http://owls-count.net/blog/work/presentation-styles http://owls-count.net/blog/work/presentation-styles#comments Wed, 17 Jun 2009 19:19:28 +0000 Simon http://owls-count.net/?p=371 Today I presented at the Reading e-Research Open Meeting. My presentation is on that website, but that set of slides is my notes rather than the presentation slides I displayed. My usual presentation style is to use many slides. Instead, for this fifteen minute presentation I decided to try a different style and I went minimalist. In the presentation I showed two slides [1] – two and ten from the longer presentation – and drew similar images to those on slide three [2].

I found this to work fairly well. Certainly it stopped me from just reading the slides and just stand and talk about my research. I could have used some better flash cards/notes – just to have some reminder of areas I wanted to mention. I certainly feel this needs experimenting with and more trails. And that is good – I am giving a seminar [3] next week!

Notes
[1] I used no title slide either – everyone in the meeting had a schedule with my name on it and I had just been introduced.
[2] The images on slide three are not those drawn in the meeting. Instead, they are images I drew especially to put in the longer version of the slides.
[3] For the research group weekly meeting.

Yesterday I presented Sparse Hybrid Monte Carlo Methods for Matrix Inversion. The presentation went fairly well and I was asked some interesting questions prompting a good discussion. It would have been useful to have some graphs of the results, but the results were produced too late for this.

Now I am back I am busy putting together the final touches for The Sixth IMACS Seminar on Monte Carlo Methods (MCM2007). I must remember to write my presentation in the next few days!

Also while out in China we visited the Forbidden City, the Summer Palace, the Great Wall, some of the Ming Tombs, and some other areas in and around Beijing. The hotel we were staying at was a spa resort, so we were able to relax in the spa pools and get a massage. All in all a fairly good trip, even if the smog did get a bit much by the end.

Tomorrow I am off to Bulgaria!