V13 - 1
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.
V13 - 2
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
V13 - 3
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:
V13 - 4
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
V13 - 5
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:
V13 - 6
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
