ANSYS-Fluent

Ansys Fluent [143] is a FVM-based, general-purpose CFD platform that provides comprehensive modelling for a wide range of incompressible, compressible, laminar, and turbulent fluid flow problems, under steady or transient conditions. In the software, a wide range of mathematical models for transport phenomena (e.g., heat transfer, momentum, chemical reactions, etc.) is combined with the ability to model complex geometries with high flexibility in grid meshing. Among a wide variety of applications, the platform has been widely used for assessing microclimate conditions in open spaces. In such cases, Fluent has been frequently used to simulate turbulent airflow within urban canopies. To "relax" modelling complexity of fluid flow and related transport phenomena in porous media (i.e., vegetation), various useful features are provided such as porosity functions and others.

Fluent solves for the majority of physical phenomena encountered in urban systems. In addition to those simulated by ENVI-met, it includes:


In general, ANSYS Fluent is the one of the most complete platforms existing in the CFD industry including well-known and the latest developments of fluid flow-related models. In terms of computational requirements, Fluent envisages solutions using multiple parallel processors, thus reducing computational costs. The latter, however, is a matter of the user's desires of resolution level; i.e., if a large urban area with a high level of geometrical detail is considered, then the computational cost can be very high, similarly to the most micro-scale CFD tools. The main limitation of the platform is that, since it is not targeted for specific problems, it requires relatively high expertise on fluid flow and transport phenomena for the user to formulate a specific problem. In this sense, the software does not include evapotranspiration and thermal comfort models, which means that, for microclimate modelling, the user should provide him/herself the models via user-defined functions. Nonetheless, it can be easily used to produce the results of parameters required to compute thermal comfort indicators (wind speed, relative humidity, temperature, turbulence intensity) externally.

Numerous CFD studies of the UHI by using Fluent exist in the scientific literature. For example, Stavrakakis et al. [103] used Fluent for the assessment of thermal and wind comfort of pedestrians in an urban area in Crete, Greece. Special physical models, such as evaporation from water surfaces and evapotranspiration from vegetation as well as thermal comfort indicators, were incorporated and compiled in the CFD platform (through user-defined function) towards the formulation of a holistic model that solves for UHI effect on pedestrians' perception of thermal comfort. The micro-scale model developed was then used to assess the pre-renovation situation and to indicate the optimum interventions including vegetation, shading devices, and cool materials in proper locations of the urban domain. Saneinejad et al. [144] studied the evaporative cooling effect on air temperature and thermal comfort within urban street canyons. They took advantage of Fluent capability to incorporate user-defined physical models and they developed a coupled CFD model that solves for vapour and heat transfer in the air, heat and moisture transfer within the porous building walls, and radiative heat exchange between building walls. The effect of evaporation of building surfaces on temperature was adequately quantified and a substantial impact of this phenomenon on pedestrian thermal comfort was shown. Recently, Fluent was used as a reliable database generator for validating a novel energy balance-based model, undertaking the calculation of spatially averaged air temperature within the urban canopy [145]. In terms of its prediction accuracy regarding urban microclimate assessments in real-scale cases, Antoniou et al. [116] applied CFD unsteady RANS modelling and computed an average absolute difference of 1.35 ◦C, of 0.57 m/s, and of 2.31 ◦C regarding air temperature, wind speed, and surface temperatures, respectively. As demonstrated in the international scientific literature, Ansys Fluent is particularly useful to test and verify UHI mitigation strategies in cityscapes provided that the designer is familiar with urban physics and possesses computer skills.
