Experimental study of atmospheric stratification in environmental flows (StratEnFlo)

Poor urban air quality and the threat of terrorist attacks by spreading hazardous substances in cities are a real concern for everyone. In order to prevent health hazards and to plan emergency procedures effectively, we need to be able to predict and simulate how gases and particles spread. A number of mathematical models currently exist that are able to simulate flow and dispersion with reasonable speed and accuracy at the required small scales, however there are still huge gaps in our knowledge and these models do not work well in all conditions. One of the main problems that current models display is in the way they treat atmospheric stratification. The proposed research will tackle this problem and will establish the role of thermal stratification in flow and dispersion in urban areas.

Stratification is common in environmental flows. This is due, for example, to variations in temperature and humidity with height in the atmosphere, or to variations in temperatures and salinity in the oceans. Neutral atmospheric stratification is characterised by an adiabatic profile of potential temperature, meaning that vertical motions of fluid particles are neither amplified nor damped. On the other hand, vertical movements are enhanced in unstable stratification, while stable flows are characterised by attenuated vertical motion.

Although stratification plays a very important role in atmospheric flow and dispersion, the vast majority of studies focus only on neutral flows, mainly because they are simpler to treat either experimentally or numerically. The proposed research aims to start bridging this gap using one of the very few facilities in Europe, or for that matter the world, that is capable of simulating non-neutral atmospheric boundary layer flows.

In meteorology and in mesoscale air quality models, stratification is an important feature, with parametrisations that are usually accurate enough to capture the main behaviour of the flow in different conditions of stability. At smaller scales, however, these relatively simple parametrisations are inadequate. While other small scale features, such as local geometry, may also become more important in determining flow conditions at such scales, stratification plays a significant role.

The prevalence of non-neutral atmospheric stratification (either stable or unstable) is well known, and a number of studies have highlighted the important effects this has on flow and dispersion. Systematic laboratory studies, however, are very rare, due to the complexity of the physical system to be studied and the very few facilities in the world capable of simulating a deep, non-neutral boundary layer. Because of this lack of experimental data-sets, most current mathematical parametrisations that account for this very important effect were developed using data from neutral test cases, sparse and rather uncertain field measurements, and some theoretical reasoning. The capabilities of the EnFlo laboratory offer a unique opportunity to bridge this gap in current models.

The main purpose of the proposed research is to establish the role of thermal stratification, both external and local, on flow and dispersion within an array of building-like obstacles. Experimental methodologies to simulate these issues will also be refined and further developed, as no established procedures and strategies currently exist. The principal outcome of the work will be a better understanding of the physics of this kind of atmospheric flow, focussing mainly in flow and pollutant dispersion within the urban canopy (particularly below roof level). A systematic experimental database on flow and dispersion in non-neutral flows will be produced. The data-set will help develop parametrisations and mathematical models able to predict atmospheric flow and dispersion at small scales more reliably, for example in urban areas or in wind farms.

This project will improve our understanding of the effects of boundary layer stratification on flow and dispersion, with the main focus on urban areas. Despite its importance in environmental science, the subject has but rarely been studied experimentally in the controlled environment of a laboratory. Experimental studies will be key in developing and enhancing our knowledge of the physical processes at play in non-neutral atmospheric flows when they interact with a complex environment such as a dense urban area. There are, of course, parallels in other branches of engineering, such as cooling air flow over circuit boards.

A better understanding of stratified flows in urban areas and the high quality experimental data from this project will help validate current mathematical models and develop new modelling strategies. This will improve the accuracy of air quality and emergency response models. Another useful outcome of the project will be the development of a suitable methodology for simulating stable and unstable boundary layers in a wind tunnel, standardising and improving ad-hoc current practice.

In the medium term, the main beneficiaries of knowledge arising from this research and the improved models will be organisations and individuals involved in air quality management, modelling and monitoring. This group includes Local Authorities, Environment Agencies, DEFRA, the Met Office, BRE, the HPA and the HSE. The added knowledge concerning stratification in urban areas will help improving the resolution and reliability of the mathematical models currently used for urban areas. The design of fixed monitoring networks will also be improved and optimised using the new knowledge. Other areas will also benefit from the improved understanding of non-neutral atmospheric flows, in particular the wind power sector, where stratified flows have a large impact, particularly off-shore.

Although model improvement fits particularly the needs of the air quality modelling and wind power communities, the project outputs apply equally well to the emergency response community. Another group of beneficiaries can therefore be identified that includes some of the above (in particular, the HPA, HSE and Met Office), plus DSTL and the Home Office in the UK, and, for example, FFI internationally. Their benefits will lie largely in the use of improved tools for planning and managing emergency response.

An additional group of direct users can be identified as dispersion model developers, who would benefit from the high quality experimental data on stratified flows and would be able to better validate and improve their models. For example, CERC Ltd. (developer of ADMS and ADMS-Urban models) and, again, DSTL (developer of the UDM model), the Met Office (developer of the NAME-III model) and Ecole Centrale de Lyon (SIRANE model), who have both collaborated with EnFlo in the past in model development work, are potential organisations for exploitation of the outputs of the proposed research.

In the longer term, urban planners will be assisted by the enhanced modelling methods in assessing the impact of newly (re)designed urban areas on air pollution, pedestrian wind comfort and heat island mitigation. The detailed study of local stratification will also assist microclimate evaluations.

As detailed in the Academic Beneficiaries section, there a number of fruitful interactions with other projects and research groups in the UK and elsewhere. The work carried out during this project will lay a good foundation for future research and collaborations, enabling developments in this and related research fields.

Experimental facilities capable of simulating non-neutral flows are rare in the world and those at EnFlo are unique in the UK in this regard. This project will exploit this capability in full and help the UK maintain its leading role within the environmental fluid dynamics and wind engineering international communities.