Use of Computational Fluid Dynamics in Civil 1 INTRODUCTION 1.1 Fluid dynamics and their applications

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    Use of Computational Fluid Dynamics in Civil Engineering

    Prof. Dr.-Ing. Casimir Katz, SOFiSTiK AG, Oberschleißheim

    ir. Henk Krüs, Cyclone Fluid Dynamics BV, Waalre

    Zusammenfassung:

    Der Einsatz von CFD im Bauwesen stellt nach wie vor eine Nischenanwendung dar. Die Gründe

    dafür werden beleuchtet und aufgezeigt, warum sich das jetzt ändern wird.

    Summary:

    The use of CFD in the civil engineering community is still a rare event. The reasons for that will be

    discussed and it will be pointed out why that will change now.

    1 INTRODUCTION

    1.1 Fluid dynamics and their applications

    CFD is the acronym for „Computational Fluid Dynamics“. Fluids in civil engineering are mostly

    air and water and the questions to be answered are the forces induced by fluid motion and the

    transport of heat or particles within the fluid. Typical questions are

     Wind loading on bluff bodies

     Wind loading on moving bodies, especially bridges

     Wind comfort and nuisance to cyclists and pedestrians

     Wind energy

     Heating and Ventilation (HVAC)

     Fire safety engineering

     Wave loadings

    The difference to classical static or dynamic analysis is given by a different mathematical treatment

    and thus different mathematical tools [1]. While structural mechanics use the Lagrangian approach

    based on displacements, fluid mechanics prefer the Eulerian approach based on the velocity of the

    fluid. Structural mechanics use the Finite Element Method, most fluid mechanics use the Finite

    Volume Method. The basic principles of equilibrium and conservation of mass, energy and

    momentum are common to both methods.

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    1.2 Software for CFD

    Software for CFD has been developed for a long time now, the most well known products today are

    the big three FLUENT (now Ansys), STAR-CD/STAR-CCM (CD-Adapco) and CFX (now Ansys),

    but there are many more like the CFDDRC (now ESI-Group), CFDesign (FEM by Blue Ridge

    Numerics, now Autodesk) and FDS (by NIST). There are also different techniques available like

    vortex particle and Lattice-Boltzmann methods.

    The first impression for a structural engineer is, that everything is very complex, that there are

    hundreds of features and parameters and that it will cost a fortune to start into this field. Over the

    years the mantra of a technique much too complex for the common engineer and high licence costs

    has established significant barriers. The same happened to structural FE-Software in the late

    seventies. SOFiSTiK had success because we anticipated the wide spread of that technique on

    personal computers in 1980.

    But the CFD-market is changing now, cheaper versions of CFD-software enter the market, and the

    open source software OpenFOAM has gained wide acceptance especially in the academic world.

    However the structural beginner is still overwhelmed by a wide range of features. What is needed is

    a robust, easy to use entry point for this journey.

    SOFiSTiK has gained some experience with an academic multiphysics software PHYSICA and is

    now supporting DOLFYN, an open source CFD-Solver used in practice in many engineering fields,

    which has been fully integrated in the SOFiSTiK environment.

    2 BASICS

    2.1 Fluid dynamics and their solvers

    Two important metrics are the Mach number (v/c) defining the ratio of the fluid velocity v to the

    speed of the sound c and the Reynolds number derived from the fluid velocity u, the kinematic

    viscosity n, and a characteristic dimension of the structure (like the diameter of a cylinder or the

    height of a bridge deck):

    kinematic viscosity Re

    characteristicdimension

    ; . sec sec

    2 2 6 6

    d u d

    m mAir 15 10 Water 1 31 10

     

      

     

       

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    Fluid dynamics cover a broad range of fields, some of them are:

     Potential Flow (Re = ∞ )

     Creeping Flow (hardly any flow)

     Laminar incompressible viscous flow

    (Navier Stokes)

     Turbulent Flow (Re large)

     Compressible Flow (Ma > 0.3)

     Supersonic Flow (Ma > 1.0)

     Thermal Effects

     Combustion & chemical reaction

     Free Surfaces

     Non-Newtonian fluids

     Radiation

    In civil engineering applications the subsonic incompressible turbulent flow is the most common

    phenomenon. Compressible flow is needed for shocks and high temperature effects. The major

    remaining problem is that a direct solution of the Navier-Stokes-equations is only possible for

    Reynolds-numbers up to approximately 20 000, while practical examples are in the range of several

    millions. Thus turbulence has to be modelled by RANS (Reynolds-Average-Navier-Stokes) models

    allowing for very large Reynolds numbers but do not model all effects, that’s why LES (Large Eddy

    Simulation) has gained some popularity, but requires still high computational effort.

    2.2 Materialparameters

    The selection of fluid material parameters is straight forward: There is a density [kg/m³], a dynamic

    viscosity [Pa sec], a compressibility [Pa/m²] and some thermal properties.

    2.3 Boundary Conditions

    Inflow and outflow boundary conditions are in general not complex, but there is a major problem

    with two aspects. There exists a boundary layer at every wall. At the wall itself there is no flow,

    then we have a tiny laminar viscous sublayer, followed by the turbulent boundary layer. The

    treatment of the wall boundary condition is quite difficult. Though there are possibilities for near

    wall models, the common model is a logarithmic wall law describing the complete wall boundary

    behaviour for a cell sufficiently far away from the boundary:

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    However the atmospheric boundary layer has a height larger than one kilometre, thus all civil

    engineering structures are completely encompassed by it. The design codes for wind loadings

    describe the layer with a logarithmic or an exponential law.

    It is not only the velocity but also the turbulence characteristics like the kinetic energy (turbulence

    intensity) and the dissipation rate (Integral length scale) which need to be described. The correct

    formulation requires not only to model the roughness of the ground with the correct value of z0, the

    driving force at the top of the fluid domain, but also the treatment of the analytic solution of the

    turbulence equations as initial conditions, which are fully implemented in DOLFYN:

     

    *

    *

    *

    ( ) ln

    ( )

    ( )

    ABL 0

    0

    2 ABL

    3 ABL

    0

    u z zu z z

    uk z C

    uz z z

     

        

     

     

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    3 MODELLING

    3.1 Windtunnel or CFD ?

    A wind tunnel is just a model of the reality, so is any CFD model. The wind tunnel needs scaling,

    Reynolds number is not the same and there are cases where this does matter. There is a guideline

    from the WTG (Windtechnologische Gesellschaft) describing in detail how to perform reliable

    tests.

    But for flexible structures, the measurement equipment may change the effects considerably. On the

    other side there are known deficiencies of numerical analysis, which do not allow taking all results

    for granted. “The purpose of computing is insight, not numbers.” said R. Hamming in 1962. So the

    question is not which technique to be used but how to combine both methods to their best use.

    3.2 Reality or design case ?

    Is the requested result the mean values of the wind loading for a static analysis or the variation of

    forces in time either to account for dynamic effects or to get reasonable loads at all. For example a

    flat horizontal roof on columns will have a zero pressure as mean value, but the wind load is of

    course not zero!

    What is appropriate for the design of a building for wind loading, a solitaire in an empty

    environment:

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    Or the within the true environment:

    Most design codes describe wind profiles based on the roughness of the terrain, for urban

    environment the value of z0 may be over 2.0 m, which is not suitable as roughness for a CFD wall

    boundary condition in general, although it is possible for an inlet wind profile. The main purpose is

    to define loads just based on the velocity distribution, not to perform a fluid analysis. This can be

    clearly seen by the fact that the velocity does not vanish at the ground.

    For a wind tunnel test normally the whole environment is modelled. So to compare CFD and wind

    tunnel, the model size of the wind tunnel (including the true geometry of the equipment) may be

    analyzed but the analysis of the true scale of the natural model including the environment should be

    more adequate.

    And is our interest in a model matching reality as close as possible or can we agree on a model on

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