LES over RANS in building simulation for outdoor and indoor applications: A foregone conclusion?

Abstract

Large Eddy Simulation (LES) undeniably has the potential to provide more accurate and more reliable results than simulations based on the Reynolds-averaged Navier-Stokes (RANS) approach. However, LES entails a higher simulation complexity and a much higher computational cost. In spite of some claims made in the past decades that LES would render RANS obsolete, RANS remains widely used in both research and engineering practice. This paper attempts to answer the questions why this is the case and whether this is justified, from the viewpoint of building simulation, both for outdoor and indoor applications. First, the governing equations and a brief overview of the history of LES and RANS are presented. Next, relevant highlights from some previous position papers on LES versus RANS are provided. Given their importance, the availability or unavailability of best practice guidelines is outlined. Subsequently, why RANS is still frequently used and whether this is justified or not is illustrated by examples for five application areas in building simulation: pedestrian-level wind comfort, near-field pollutant dispersion, urban thermal environment, natural ventilation of buildings and indoor airflow. It is shown that the answers vary depending on the application area but also depending on other-less obvious-parameters such as the building configuration under study. Finally, a discussion and conclusions including perspectives on the future of LES and RANS in building simulation are provided.

CFD modelling and PIV experimental validation of flow fields in urban environments

“…In the frame of the numerical study, the three-dimensional steady RANS simulations have been carried out using a commercial CFD code, FLUENT v.17, with the realizable version of k-epsilon turbulence model. According to the literature ( [3][4][5]12]) this model is widely used for analysis of flows for different configurations of buildings.…”

“…However, one of the most important aspects of this type of simulations is that the methods and results need to be validated before such predictions can be relied on. Therefore, PIV verification and validation of CFD studies are imperative, as well as detailed sensitivity studies that can provide guidance in the selection of computational parameters for future CFD analysis [5]. There is relatively little verification of CFD which simulate the air flows around objects in different configurations.…”

A numerical study of ventilation strategies for infection risk mitigation in general inpatient wards

“…Numerous studies on turbulent indoor airflow have been carried out through Reynolds-averaged Navier Stokes (RANS) simulations, whereas for some case studies large eddy scale simulations (LES) have been preferred to provide accurate prediction of flow field variables. However, LES demands much higher grid requirements as well as computational time compared to RANS, which makes RANS widely used (Blocken 2018 (Chen 1995;Zhang and Chen 2006;). The diffuser inlets were defined as velocity-inlet, while corridor and exhaust grilles were treated as outflow boundary condition.…”

Modeling transient particle transport by fast fluid dynamics with the Markov chain method

“…This study applied the renormalization group (RNG) k-ε model, which is one of the best Reynolds-averaged Navier-Stokes (RANS) turbulence models for predicting the indoor airflow Blocken 2018), to simulate the turbulence. Again, since the equations for k and ε are both scalar equations, they are solved in the same way with Eq.…”