BMe Research Grant
BMe Research Grant - 2015
General purpose computational fluid dynamical (CFD) solvers are frequently used in small-scale urban pollution dispersion simulations without taking into account a large extent of vertical flow. Vertical flow, however, plays an important role in the formation of local breezes, such as urban heat island induced breezes that have great significance in the ventilation of large cities and consequently, could be an important factor in civic design. The effects of atmospheric stratification, anelasticity and Coriolis force must be taken into account in such simulations. At the Department of Fluid Mechanics we introduce a general method in order to make general purpose CFD solvers capable of simulating and modeling atmospheric scale flows.
Colleagues at Department of Fluid Mechanics has extensive knowledge in the field of the development and application of computational fluid dynamic solvers such as in buiding-aerodynamics, comfort studies of buildings, environmental studies, automotive industry or in the field of pollution transport studies (see e.g. Fig. 1.). In the past years a new research area started to dynamically evolve, namely the modeling and simulation of atmospheric flows.
Figure 1. Pollution transport studies at the Department of Fluid Mechanics
A clear trend can be observed in the development of mesoscale meteorological codes towards the usage of higher resolution numerical models incorporating multiple physical effects in order to better describe the atmosphere, to give higher resolution models for urban environments or to give more reliable forecast. This process is well reflected in the “urbanization” of several mesoscale meteorological models where more and more fine scale physical effects are introduced.
The urban heat island circulation (UHI), which largely affects the ventilation and thermal comfort of large cities is a good example for the urbanization of such codes. Many researchers investigated the phenomena numerically by different general purpose developed solvers and by mesoscale meteorological models. An excellent agreement with temperature measurements can be achieved by using fine tuned models, however, these still do not allow the exact analyses of contaminant transport in an urban atmosphere as local immission levels are strongly dependent on fine flow structures such as urban canyon effects and eddies in the wakes of buildings.
There is another potential approach, however, for going towards fine scale, namely when modern computational fluid dynamical (CFD) solvers are adapted to handle mesoscale effects. General purpose CFD solvers are already widely used in urban studies such as in the modeling of the ventilation of urban areas, pollution transport studies, wind farm design or calculation of wind loads on different human made constructions. These solvers are capable of handling complex topography, building structures and include a big variety of turbulence and physical models, effective numerical techniques and parallelization.
To further extend the wide range of functionalities mentioned above, a model transformation has been developed  for general purpose CFD solvers to make them capable of handling mesoscale effects. The atmospheric stratification, the Coriolis force and baroclinicity are taken into account by using additional source terms in the conservation equations of the solver. The model uses only a single unstructured grid, and a uniform physical description for close- and far-field flow avoiding interpolation errors and model uncertainties due to model nesting. The authors' intended purpose, furthermore, is to create a more general method, which is easy to implement in any CFD solver allowing programmable user defined volume sources in the governing equations.
What are the expected advantages?
Handling of unstructured meshes (building structures and orography characterized with complex geometry)
Wide variety of physical models (multiphase flows, flows with reactions, multiple choices for turbulence models, radiation models, discrete phase models etc.)
The extendability of the solver (through the usage of UDF functions)
Advanced tools for the evaluation of the results
Problems to be solved: CFD softwares can not handle mesoscale flows without further treatment
Former models applied neutral stratifications
Vertical domain size was limited to a few hundred meters
Adiabatic expansion of vertical flows was ignored
Effect of stratification to the turbulence must be solved
Density changes in the atmosphere should be treated.
The goal is the development of a proper transformation that makes CFD solvers capable of handling meteorological problems. The extension of CFD softwares is possible via the usage of user defined function acting on the governing equations. The transformation system described by Kristóf, Rácz and Balogh (2009) was applied to the governing equations. (Fig. 2.)
Figure 2. Governing equations and volume sources of the CFD solver
Main properties of the transformation system:
The energy equation is solved to the perturbation of temperature around the ISA temperature profile. Changes in density are taken into account by compressing the atmosphere into a layer with a well-defined thickness. (Fig. 3.)
Figure 3. Rise of an air parcel to the intersection of the ISA, and adiabatic temperature profiles (left). Transformation of the vertical coordinate (right), compression of the atmosphere into a constant density layer with a well-defined thickness.
A transformation method described by Kristóf, Rácz and Balogh (2009) was implemented in order to prove the applicability of general purpose CFD solvers for modeling stratified atmospheric flows and thermal convection. The source term in the energy equation was investigated by using large eddy simulation of laboratory scale water tank experiments simulating urban heat island circulation phenomena. This is a core element since it is responsible for the treatment of atmospheric stratification. It was shown with temperature profiles and PIV data that this new approach can model thermal circulation phenomena with a reasonable accuracy. The behavior of the dynamical model core was further investigated with the simulation of a nonlinear density current (Fig. 4.) on an atmospheric scale. Using this example, the importance of considering compressibility effects was demonstrated with regard to the model results.
Figure 4. Cloud structure on the top of Kelvin-Helmholtz instabilities due to a cold front. CFD model (left), observation (right)
The transformation method allows CFD tools to capture and investigate internal gravity waves. The modeling of internal gravity waves has great significance in engineering to reduce aviation hazards, and damages caused by severe downslope windstorms or to determine optimal location of future wind-farms. Simulation of gravity wave propagation also allows the investigation of flows characterized by a wide range of hydrostatic and nonlinear states and gives a good opportunity for the evaluation of model performance.
The original formulation essentially handles dry adiabatic processes. However, practical applications often require the modeling of moist dynamics. The built in moisture models in general purpose CFD solvers are either not compatible with the Boussinesq density model used by the transformation method, or they require a density based solver. Therefore, a new method was developed where the advantages of the original transformation method was combined with an efficient continuum microphysical model for describing phase change and moisture transport. The presented technique is novel in the CFD field since it allows both the computation of micro- and mesoscale stratified flows in a single framework with arbitrary stratification and moist dynamics with reasonable computational efforts due to the bulk microphysical approach.
The model performance was demonstrated with the simulation and on-site measurement of temperature, liquid water content and vertical velocity field in a full scale cooling tower plume dispersion study. (Fig. 5.)
Figure 5. CFD simulation of the wet cooling tower plumes of a nuclear power plant.
Using this novel transformation method, two different fields of the fluid dynamical research have been connected making a good basis for the application of engineering tools in the field of atmospheric, meteorological flows. The new method opens up a wide range of possible application areas for engineers using CFD solvers, as follows. In the modeling of local circulations: urban heat island, coastal winds, valley winds. In the field of energetics and environmental investigations: cooling towers, stack towers, energy towers, wind farms. In the investigations of atmospheric problems: evolution of thunderstorms, flows around high mountains. Simulation of disasters: large scale fires e.g. in forest fires or town fires, volcanic plumes.
The research project is extremely actual and challenging. We expect positive response from both national and international scientific community.
(IF – impact factor)