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A numerical simulation of airflow characteristics to design cantilever highway sign structures

Accurate calculation of wind forces on cantilever highway sign support structures is significant due to the probable structural failure of these sign structures under strong winds. The main external load a highway sign support structure is subjected
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    1 A numerical simulation of airflow characteristics to design cantilever highway sign structures   Izet Mehmetaj 1 , Miriam Ndini 2 , Erald Saliasi 3   1  Department of Civil Engineering, Epoka University   2  Department of Civil Engineering, Epoka University 3  Department of Civil Engineering, Epoka University   Abstract   Accurate calculation of wind forces on cantilever highway sign support structures is significant due to the probable structural failure of these sign structures under strong winds. The main external load a highway sign support structure is subjected to is wind load. Due to its large area, the largest wind load is concentrated on the signboard. The use of overhead and cantilever sign structures is quite common throughout Albania. The main objective of the proposed study is to use Computational Fluid Dynamics (CFD) tools to define the wind loads by accurate numerical simulations of airflow characteristics around highway sign structures under yearly and one-time wind speeds conditions. Reynolds-Averaged Navier-Stokes (RANS) simulations are used to estimate the actual pressure distribution on the front and back faces of the signboard. The pressure distributions are needed to perform a linear static structural analysis using ETABS. The numerical simulations were achieved using ANSYS-CFX. However, the results, compared with those obtained from the AASHTO provisions, indicate that the developed methodology can be used to design cantilever highway sign supports structures once the design loads and wind conditions and speed data are realistic.  Keywords:  cantilever highway sign structure, CFD, numerical simulation, wind.   1   INTRODUCTION The use of overhead and cantilever sign structures is quite common throughout Albania. These applications include complicated intersections of highways and roads where high visibility is required, locations with difficult terrain or utility conflicts, etc. Overhead structures are sometimes also used to support signals and lights. Cantilevers consist of a mast arm that is extending over the roadway. They are supported by a single roadside column (a single or double pole or a box-truss structure). The vertical columns are referred to as uprights, poles or posts. The horizontal part of the structure is referred to as the mast arm (usually mono-tubes/boxes or trusses). Highway structures such as signals, signposts, and luminaries are susceptible to wind-induced loads that include flutter, buffeting, vortex-shedding, and wind-rain/ice vibrations [1]. Accurate estimation of wind forces especially on large highway sign structures is important   2 due to a possible structural failure of these sign and traffic signal structures under strong wind conditions [2] [3]. 1.1   Paper Format 1.1   Equations describing fluid flows in CFD The flow of most fluids may be analyzed mathematically by the use of two equations. The first often referred to as the Continuity Equation, requires that the mass of fluid entering a fixed control volume either leave that volume or accumulates within it [4] [5]. It is thus a "mass balance" requirement posed in mathematical form and is a scalar equation. The other governing equation is the Momentum Equation, or Navier-Stokes Equation, and may be thought of as a "momentum balance." The Navier-Stokes equations are the fluid dy namic equivalent of Newton’s second law, force equals mass times acceleration [6]. 1.2   Wind flow modelling To model wind flow as a dynamic phenomenon is very complex [7]. Wind is composed of vortexes of an air stream that moves relative to the earth’s surface . These vortexes provide to the wind the gusty or turbulent nature, with forces and speeds that are time and space depended [1] [7]. Modeling the wind flow, the wind vector at a point is taken as the sum of the mean wind vector static component, dynamic component, or turbulent component due to wind speed variations from the mean [7]. According to AASHTO 2009 “Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, these structures should be designed to resist four different limit states of wind loading: galloping, vortex shedding, truck-induced gusts, and natural wind gusts [1]. Eurocode EN 1991-1-4:2004 [8] defines that the wind action on a structure is represented by a simplified set of pressures or forces whose effects are equivalent to the extreme effects of the turbulent wind. Wind actions fluctuate with time and act directly as   Figure 1 Samples of cantilever sign structures throughout Albania   3 pressures on the external surfaces of enclosed structures and, because of porosity of the external surface, also act indirectly on the internal surfaces. They may also act directly on the internal surface of open structures. Pressures act on areas of the surface resulting in forces normal to the surface of the structure [9]. While, in AASHTO 2009 “Standard Specifications for Structural  Supports for Highway Signs, Luminaires, and Traffic Signals” wind load is considered the pressure of the wind acting horizontally on the structure and the wind pressure equation is based on the fundamental fluid-flow theory presented in ASCE 7-97 [1]. Wind actions on a structure are caused due to air deflection around and above it. Structure properties like height or width as seen in the wind direction influence how much passes around the sides and how much flow overhead. Furthermore, at the windward face momentum is lost and positive pressures are developed as a result. While as the wind continues it accelerates around the obstruction by developing negative pressures on the sides or leeward face [2]. As wind impacts a structure, further chaos is added to the wind unstable nature resulting in the separation of flow, the distortion of the mean flow, as well as the formation of vortices. These effects result in large, fluctuating wind pressures on the surface of the structure that depend on the interactions of the flow characteristics (such as wind speed, wind height, ground surface features, air properties) with the building configuration (i.e. its shape, location, and dynamic and physical structural properties) [10]. As a result of these generated pressures, large loads are imposed on the structure. These aerodynamic type loads may cause important structural excitation and vibration. Therefore understanding the action of wind is of significant interest [3]. Turbulence on the degree that a structure will react is generally described as a natural wind gustiness to be designed against [1]. Whenever turbulence is present in a certain flow it appears to be dominant over all other flow phenomena. That is why successful modeling of turbulence greatly increases the quality of numerical simulations. In CFD, turbulence models are empirical mathematical models that describe the turbulence in the flow. RANS has been widely used in designs and research since the 70’s. K  -epsilon (k- ε) turbulence model is the most common RANS model used in Computational Fluid Dynamics (CFD) to simulate turbulent flow conditions [11] [12] [13]. The standard k- ε model proposed by Launder and Spalding in 1974 [14] gives a general description of turbulence by means of two model equations, one for the kinetic turbulent energy k and one for the rate of dissipation of turbulent kinetic energy per unit mass, ε. 1.3   Scope of the study In this study, the finite element analysis software called ANSYS (academic version 18.2, released on 2017) is used for the analysis of the structural response of the selected sign structure. Especially, ANSYS-CFX is used to study the interaction between the sign support structure and a simulated natural wind gust. The utilization of this software allows for a comparison of the results obtained from CFD analysis and AASHTO provisions in ETABS CSI software (2013), another finite element software. The bending moment at critical locations in both types of analysis, such as at the base and the connection between the horizontal and vertical members, are determined and compared. The study follows these main tasks:   4 •  Analyzing the sign support structure by using a CFD software, (ANSYS CFX), to obtain the induced pressure under a simulated natural wind gust. •  Calculation of the pressure on the sign support by using the AASHTO provision for natural wind gust. •  Analyzing the structure using finite element software, ETABS, to calculate the moments due to the pressure from CFD analysis and AASHTO provisions. •  Comparison between the bending moment from the CFD analysis and AASHTO provision at critical locations. Hence, the aim is to validate the usage of CFD to calculate the wind fatigue design load on the sign support structure. 2   METHODOLOGY 2.1   The chosen design example The chosen cantilever support structure (Figure 2) is made of a square hollow section 250x250x8 mm size (galvanized hollow uniform member for both, column and beam), steel grade S355. The column and the beam have a length of 6.5 m and 6.0 m, respectively. The structure supports an aluminum signboard with dimensions 2625 x 2466 mm and thickness of 2, 54 mm. 2.2   Working with ANSYS CFX Main steps of CFD analysis using ANSYS-CFX are: Geometry: The computational domain representing the fluid flow region and structure surrounding environment is added and a box type enclosure is chosen (Figure 3). This domain has one inlet and one outlet both of 12 m width and 9 m height and other wall sides extended on a 19 m length. Figure 2 A typical highway steel sign support structure to be analyzed.   5 Meshing (pre-processing): the computational domain of the design example, as well as the region around it, meshes with an unstructured grid in a tetrahedral cell arrangement of 417796 nodes and 2318361 tetrahedral elements in total (Figure 3). Physics definition (pre-processing): To simulate the natural wind gust flow, 3 scenarios are employed separately in CFX-Pre: S1 (time-independent steady laminar fluid model), S2 (time-dependent laminar fluid model), and S3 (time-independent steady turbulent fluid model). In all the three scenarios, 4 boundary conditions types were inserted: inlet from the wind alongside, the outlet from the wind downside, the wall at the computational domain side faces and the structure wall defined from the structure surface region. A normal speed equal to 4 m/s was imposed as boundary detail for the inlet boundary condition for the S1 and S3 scenario. For the S2 scenario, the velocity at the inlet was assigned as a function of time in form 4 [m/s] * Time/ 1 [s] instead of a constant value. Also, the no-slip boundary condition is imposed on the both wall type boundaries, that is, the fluid particles on the body move with body velocity. In the transient case (S2 scenario) the flow field is simulated over a specified time period of 25 s. The time step was set to be 2 s providing 12.5 time steps to be solved. Solver: In this study, to monitor the solution, in the three scenarios S1, S2, and S3, the min and max coefficient loops were set to be 1 and 100 respectively. The min and max coefficient loops set limits on the number of iterations to use within each time step. In case of S2 scenario, there are 100 iterations for every 2 s. 2.3   Working with ETABS In this study, to perform static analysis for the structure under imposed CFD pressures in the three scenarios S1, S2 and S3 is used ETABS. This software is also used to perform the static analysis for the structure under AASHTO 2015 provisions for natural wind gust load design. In CFD scenarios, the pressure varies in a certain elemental area resulting in forces of different magnitudes. To define these forces in both directions (windward and leeward side) the area of signboard is divided approximately into elemental areas with same pressure distribution (Figure 4). Same approach follows also the meshing procedure on ETABS while assigning the CFD pressure distribution on shell element represented by signboard. The shell element which the signboard is modelled, is meshed by dividing it into 11x10 areas. On each area is then assigned the corresponding CFD pressure as a uniform load type with direction Global Y Pro  j and load case, pressure. The load assignment is made in both wind directions, windward side as well as leeward side. The forces acting on the elemental areas on the leeward side have been assigned as addition to existing loads. A linear static analysis is run and the moment Figure 3 Computational domain of the chosen design example (left); meshing of the computational domain (right)
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