Wind Influence in Numerical Analysis of NSHEVS Performance

 

Wind Influence in Numerical Analysis of NSHEVS Performance

By Wojciech Węgrzyński and Grzegorz Krajewski
Building Research Institute (ITB), Warsaw, Poland

 

Introduction

In recent years, many studies regarding the influence of wind on the performance of various smoke and heat exhaust ventilation (SHEV) systems have emerged. In many of these studies the complex phenomena of wind was simplified into an artificial velocity boundary, which limits the flow of smoke out of the building. This simplification may be misleading as wind itself is a complex phenomenon. Various parts of the building may be subject to under- and overpressure at the same time, completely changing how systems perform. This paper presents essential resources, which should allow correct introduction of wind into numerical analysis used in fire safety.

The use of computational fluid dynamics (CFD) for wind engineering applications is referred to as computational wind engineering (CWE).2 CWE is used mostly in prediction of wind comfort, pollution, dispersion or loading on buildings.6 In fire safety engineering (FSE) CFD is often used to predict the movement of smoke and heat within buildings. The meeting point between both can be found in complex applications of natural smoke and heat ventilators (NSHEV), which act due to the small difference in the density of hot gasses inside of the building and atmosphere. For such systems, the wind is an important design factor that may determine its performance and as such define the conditions inside in case of the fire.

 

Relevant Good Practice in CWE

Multiple good practice guidelines are available for CWE, out of which the essential are listed below in the reference section:2–4, 6, 5, 10.

Many rules of CWE originate from decades of testing in wind tunnels—especially with regard to the numerical model. The blockage ratio, described as the proportion of cross-section of the building to the cross-section of the domain in a plot perpendicular to the flow, should not be larger than 3 percent,4 Figure 1. The inlet, lateral and top boundary should be at least 5Hmax away from the group of explicitly modeled buildings, where Hmax is the height of the tallest building. The reason for this is to limit the error caused by the modeling technique on the airflow velocity in building proximity; too small domain will cause strong artificial acceleration. The outflow boundary should be at least 1010 to 156 Hmax away from the group of explicitly modeled buildings to allow for full wake flow development. The size of the domain is presented in Figure 1, and a sample of a large-scale numerical model is shown in Figure 2.

 

Figure 1. Blockage ratio (left) and dimensions of numerical domain (right) [6]

 

Not only the size of the domain is important but also the grid quality. Two characteristics of high-quality computational grids are (i) sufficient overall grid resolution and (ii) quality of the computational cells regarding shape (including skewness), orientation and stretching ratio. For the CWE approach, at least 10 cells per cube root of the building volume should be used and at least 10 cells between buildings. In the ground and roof layer, at least five elements shall be placed at the height at which velocity is critical. These requirements are typically met by high-quality fire-oriented mesh. The second essential mesh requirement—quality of the computational cells regarding shape—is also explicitly met in solvers such as FDS, where Cartesian, structured mesh is used (although rotation of the model may introduce errors related to orientation of cells). Unstructured meshes (such as a tetrahedral mesh) will increase truncation errors and cause issues with convergence, and this should be a part of the individual analysis. For unstructured meshes, a growth function with a ratio less than 1.30:1 is necessary to create sufficient domain. A relevant guide on high-level mesh generation for a coupled outdoor and indoor analysis is available in.7

 

 

Figure 2. Aerial photograph of Warsaw (upper picture, source: Google Earth) and the numerical

domain in the model (bottom picture, source: own work)

  

Turbulence Modelling

For detailed information on the turbulence models used in CWE, choice of the model and their validation, please refer to references 1, 2, 5 and 8. In CWE, the common closure models are Steady and Unsteady Reynolds-Averaged Navier-Stokes (RANS and URANS) and Large Eddy Simulation (LES). LES can be considered intrinsically superior regarding physical modelling to both RANS and URANS. It is suitable for simulating three specific characteristics of the turbulent bluff body in urban physics: three-dimensionality of the flow, unsteadiness of the large-scale flow structures and anisotropy of turbulent scalar fluxes. However, 3D steady and unsteady RANS are computationally cheaper and still remain the main approach, with a satisfactory degree of success. An interesting concept is to combine LES and RANS into one model with massively separated flows, in which large vortices are resolved by LES, while small by RANS approach. This is often referred as Detached Eddy Simulation (DES) model.9 Despite the simplification it still requires similar computational power as LES models.1 A summary of validation studies on both RANS and LES modelling can be found in reference 2.

Framework for Assessment of NSHEVS Performance

In reference 12 authors determined three different approaches to numerical modelling that are common in FSE/CWE coupling, shown in Figure 3:

 

a) model simplified to include only the interior of the building, outlets modelled as pressure boundary conditions;

 

b) model simplified to include the interior of the building, and nearest exterior, outlets modelled as an opening in the walls along with their most important features, pressure boundaries at the edges of the domain;

 

c) model suitable for wind engineering, with exterior domain large enough to not influence the flow around the building, outlets modelled in details as physical openings with all of their features, pressure boundaries at the edges of the domain with velocity boundary, including logarithmic wind profile on velocity inlet.

 

The first approach (a) is sufficient only for most basic, preliminary analysis without the wind. The simplification in the modelling of the inlets and outlets will strongly influence the performance of NSHEVs. The second method (b) is valid for NSHEV performance analysis but without any wind interaction. This method can be used for checking the environmental conditions connected to the evacuation process inside the building, but the designer must add a margin of safety to the results; as in wind conditions they may be significantly worse. The introduction of wind as artificial velocity boundary close to the building will lead to increased wind at the walls due to flow compression and may not be considered as a valid wind analysis. The third approach (c) is used for precise evaluation of NSHEV’s performance in wind conditions for different wind angles.

 

Figure 3. Three approaches to modelling NSHEVS in FSE/CWE coupling12

 

Performing all wind-related studies in a large domain and as transient would require immense computational power. The workaround is to use a decoupled approach in two steps: (1) simplified steady-state analysis to evaluate the pressure coefficients for multiple wind attack angles (min. 12 angles) and (2) transient analysis of fire and wind at the worst wind conditions previously assessed. By assumption, the steady state solution rep-resents the solution of the flows in the volume independent of the time. In FSE most of the analysis is carried as transient, as the fire itself is a transient phenomenon. This is why obtaining a converged solution in steady state even for the fire with a constant size is difficult (or impossible). The CFD user must note that with LES turbulence modeling the solution is transient by assumption.

 

Figure 4. Nondimensional pressure coefficient at buildings subject to the wind,

uref = 4 m/s at various wind angles in steady-state approach12

 

Conclusions

This paper, presented at FEMTC2016 Conference, serves as a brief introduction to much broader work. The full conference paper includes more details on how natural ventilators are tested and certified and presents more details and references on CWE methods. Also, authors’ additional resources on how the natural ventilators work in wind conditions can be found in references 11 and 12.

The introduction of high-quality computational wind engineering into a fire safety engineering workflow is a challenging and arduous task, but there are cases in which this extraordinary expense is of worth: the design of breakthrough structures, which use natural ventilation of unrivalled efficiency and have close to no environmental cost. Such systems are the essence of green and sustainable development. With our current knowledge, we are not bound to traditional, simplified, methodologies for the evaluation of such systems’ performance, which opens a niche for highly optimized solutions.

Multiple problems occur when CWE good practice guidelines meet typical FSE workflow: the mesh generation, order of numerical schemes, the size of the domain, angle sensitivity, introduction of the wind as a boundary condition, to name a few. Some of them may be resolved by a skilled engineer that builds the case and the model; others may require systematic approach (especially LES modelling in Cartesian meshes). The community has to take an active part in resolving these issues if we hope to have a reliable tool for CWE/FSE coupled model in short future at the quality of the FSE-oriented CFD models as we do have today.

 

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