ASCOMP provides consulting services in wind and environmental engineering, including urban pollution dispersion from cars, industry or even accidental gas releases. Our studies are here based on detailed 3D simulations using TransAT HPC. The use of this version of our CFD platform is motivated by two important elements: (i) rapid meshing of industrial wind engineering and pollution problems using the immersed surface technique with block mesh refinement, and (ii) advanced turbulence modelling based on URANS, VLES or LES. CPU intensive problems could be treated using VLES instead of LES.
Urban pollution dispersion
Pollution dispersion studies are now requested as a to-do task in most of the environment regulatory bodies in the OECD countries. The main objective of the studies is to analyze and track pollutant dispersion over time, emanating from factories, incinerators or subsequent to an accidental release from a chemical or a nuclear power plant. Analyzing the traffic-induced dispersion or the quality of air inside buildings and its relation to external flow conditions, are also part of the discipline. These flow conditions can indeed connect an external source of pollutants (chimneys releasing exhaust gases from centralized heating devices) with fresh-air admission (windows, etc.) which could in turn be contaminated.
Solving the pseudo-transient flow within an interval of time (up to one year in the past) using archived meteorological data, e.g. wind speed at 10m height, thermal stratification and wall shear, results in (1) a pollution map, (2) a thermal map around the buildings, and (3) a comfort & wind acceleration map showing sitting, walking & critical areas.
Viaducts and suspension bridges
The design of bridges is today conducted by both engineers and architects, albeit it’d naturally be governed by engineering considerations only. Bridge deck span lengths have increased greatly over the last 200 years. The dramatic collapse of the first Tacoma Narrows suspension bridge in 1940 under a relatively low 19m/s wind had pointed to the susceptibility of this type of structure to the dynamic effects of wind action. Subsequent investigations revealed that these violent aerodynamic oscillations could not be predicted by pseudo-static analyses. The interaction of oscillating flexible structures and the fluid flow around them can give rise to a number of different aeroelastic phenomena. Thanks to the LES capability, TransAT delivers instantaneous peak wind loads on long-span bridges, besides steady-state forces, including drag, lift and torsional moment.
Wind loading on buildings
Wind effects on urban structures (towers, tall buildings, stadiums, etc.) can be a major design criterion. Low-rise buildings including residential, institutional, and commercial structures are most vulnerable to destructive windstorms. The analysis of flows past bluff bodies itself poses serious challenges, including involvement of unstable and possibly turbulent boundary layers undergoing separation and reattachment, the formation of vortices and the development of a so-called wake composed of trailing vortices. At ASCOMP we explore efficient simulation methods for analyzing wind-induced loading and response of buildings and other geometrically complex bodies and possible interaction effects with structural oscillations.Thanks to the LES and VLES (in case the Reynolds number is too high) capabilities, TransAT delivers instantaneous wind loads and structural response. The wind engineering module in TransAT involves consideration of the atmospheric boundary layer as influenced by thermal stratification, surface roughness, Coriolis and other effects. It also includes the possibility to enter true meteorological data.
Moisture deposition on buildings
The deterioration experienced by buildings and monuments is caused in part by the impact of hydrometeors and subsequent deposition of moisture by rain, snow and fog. In these instances, the deposited moisture can cause mechanical disruptions by freezing within fissures or by actually dissolving the materials. In addition, atmospheric pollutants dissolved or suspended in water droplets can be carried to the surface. Once these pollutants have been deposited on the surface, capillarity can transport the moisture and pollutants into the interior of porous materials. This often results in chemical transformations and deterioration deep within these structures. The example reported here centers around the prediction of wind-driven raindrop trajectories inside a two-dimensional street canyon; the final aim was to evaluate the impacting water rate on the facades. CFD of such flows involving free surfaces is also used to design the reservoir interiors, in particular in anticipating rapid changes in weather causing floods, sediment deposition and removal.
Crane sudden rotation
The focus is on estimating wind forces that may lead to an accidental rotation of a crane placed in the yard of a building site under the action of wind gusts. Such studies now are mandatory for all in-situ urban constructions under EU laws since a couple of years now. Classical approaches to the problem (based on 3D calculations) rely on the re-construction of the wind energy spectra from steady state RANS simulations. This has shown to be rather limited in most cases, suggesting the use of real unsteady simulations, either using the U-RANS approach or the LES. For high Re numbers, a well-resolved LES requires very large grids and small time steps. One way to avoid these constraints in practical applications (when a full wind rose is required) is to resort to less expensive techniques such as the V-LES technique in TransAT.