CFD Methodology. Mathematical Modeling. Turbulence, Rheology, Radiation,
Chemical Reaction, Combustion,. Multiphase Flow, Spray, Boundary Condition ...
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users
Hyon Kook MYONG School pf Mechanical Engineering Kookmin University http://cfd.kookmin.ac.kr CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
CFD Methodology
Mathematical Modeling Turbulence, Rheology, Radiation, Chemical Reaction, Combustion, Multiphase Flow, Spray, Boundary Condition etc.
Numerical Solution Technology Domain, Discretisation Method, Velocity- Pressure Coupling, Moving Grid Implementation, Conjugate Heat Transfer Analysis, Solution Methods etc.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
CFD Application
Aerodynamics of aircraft and vehicles: lift and drag Hydrodynamics of ships Power plant: combustion in IC engines and gas turbines Turbomachinery: flows inside rotating passages, diffusers etc. Electrical and electronic engineering: cooling of equipment including microcircuits Chemical process engineering: mixing and separation, polymer moulding External and internal environment of building: wind loading and heating/ventilation Marine engineering: loads on off-shore structures Environmental engineering: distribution of pollutants and effulents Biomedical engineering: blood flows through arteries and veins Meteorology: weather prediction Hydrology and oceanography: flows in rivers, oceans
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
History of CFD Main reason why CFD has lagged behind: Tremendous complexity of the underlying behavior, which precludes a description of fluid flows that is at the same time economical and sufficiently complete. The availability of high performance computing hardware and the introduction of user-friendly interfaces have led to a recent upsurge of interest. 1970년대: 1980년대: 1990년대: 2000년대:
기본 수치해석 알고리즘 개발 상용 CFD 코드 개발 산업체에서 CFD 코드 적용 Design 과정에서 하나의 Routine
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Advantages of CFD
Substantial reduction of lead times and costs of new designs Ability to study systems where controlled experiments are difficult or impossible to perform (e.g. very large systems) Ability to study systems under hazardous conditions at and beyond their normal performance limits (e.g. safety studies and accident scenarios) Practically unlimited level of details of results Very cheap to perform parametric studies to optimize equipment performance CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Constraint to the Spread of CFD
In addition to a substantial investment outlay, an organization needs qualified people to run the codes and communicate their results and briefly consider the modeling skills required by CFD users. The next constraint to the further spread of CFD amongst the industrial community could be a scarcity of suitably trained personnel instead of availability and/or cost of hardware and software.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Popular CFD Codes CFX
NAGARE(DRAG4D)
CFD-ACE
PAM-FLUID
FIDAP
PHOENICS
FINE/TURBO
POLYFLOW
FIRE
SCRYU
FLOTRAN
STAR-CD
FLOW2000
STREAM
FLOWTHERM
TASCFLOW
FLUENT/RAMPAN/NEKTON
SPEED
KIVA
etc.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
How does a CFD code work ? All codes contain three main elements:
Pre-processor: consists of the input of a flow problem to a CFD program by means of an operator-friendly interface and the subsequent transformation of this input into a form suitable for use by the solver
Solver: numerical algorithm
Post-processor: examine the results CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
User Activities at Pre-Processing Stage Definition of the geometry of the region of interest: the computational domain Grid generation-the subdivision of the domain into a number of smaller, non-overlapping subdomains: a grid (or mesh) of cells (or control volumes or elements) Selection of the physical and chemical phenomena that need to be modeled. Definition of fluid properties Specification of appropriate boundary conditions at cells which coincide with or touch the domain boundary CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Numerical Algorithm in Solver
Formal integration of the governing equations of fluid flow over all the (finite) control volumes of the solution domain. Discretization involves the substitution of a variety of finitedifference type approximations for the terms in the integrated equation representing flow processes such as convection, diffusion and sources. This converts the integral equations into a system of algebraic equations. Solution of the algebraic equations by an iterative method.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Post-Processor (data visualization tools) Domain geometry and grid display Vector plots Line and shaded contour plots 2D and 3D surface plots Particle tracking View manipulation (translation, rotation, scaling etc.) Color postscript output CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Problem Solving with CFD
In solving fluid flow problems we need to be aware that the underlying physics is complex and the results generated by a CFD code are at best as good as the physics (and chemistry) embedded in it and at worst as good as its operator.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users-1 The user of a code must have skills in a number of areas. Prior to setting up and running a CFD simulation there is a stage of identification and formulation of the flow problem in terms of the physical and chemical phenomena that need to be considered. Typical decisions: to model a problem in two or three dimensions to exclude the effects of ambient temperature or pressure variations on the density of an air flow to choose to solve the turbulent flow equations to neglect the effects of small air bubbles dissolved in tap water CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users-2
Good modeling skills requires to make the right choices, because in all but the simplest problems we need to make assumptions to reduce the complexity to a manageable level whilst preserving the salient features of the problem in hand. It is the appropriateness of the simplifications introduced at this stage that at least partly governs the quality of the information generated by CFD, so the user must continually stay away of all the assumptions, clear-cut and tacit ones, that have been made.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users -3 A good understanding of the numerical solution algorithm Three mathematical concepts useful in determining the success of such algorithms: Convergence Consistency Stability Three crucial properties of robust methods that produce physically realistic results: Conservativeness Boundedness Transportiveness CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users -4 Specification of the domain geometry and grid design are the main tasks at the input stage. Two aspects that characterize a successful simulation results are convergence of the iterative process and grid independence. Progress towards a converged solution can be greatly assisted by careful selection of the settings of various relaxation factors and acceleration devices. Good initial grid design relies largely on an insight into the expected properties of the flow. The only way to eliminate errors due to the coarseness of a grid is to perform a grid independence study.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users -5 Every numerical algorithm has its own characteristic error patterns. [e.g. numerical (or false) diffusion] The likely error patterns can only be guessed on the basis of a through knowledge of the algorithms. It is impossible to assess the validity of the models of physics and chemistry embedded in a program or the accuracy of its final results by any means other than comparison with experimental test work. Anyone wishing to use CFD in a serious way must realize that it is no substitute for experimentation, but a very powerful additional problem-solving tool.
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users -6 To validate the accuracy of CFD simulation results To produce experimental data of similar scope Or rely on Previous experience Comparisons with analytical solution of similar but simpler flows Comparisons with high quality data from closely related problems reported in the literature. [e.g. Transactions of ASME (in particular J. of Fluid Engineering, J. of Engineering for Gas Turbines and Power and J. of Heat Transfer), AIAA J., J. of Fluid Mechanics and Proc. Of IMechE]
CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
School of Mechanical Engineering
Copyright ⓒ 2009 Hyon Kook MYONG
Modeling Skills required by CFD Users -7 There are guidelines for good operating practice which can assist the user of a CFD code and repeated validation plays a key role as the final quality control mechanism. The main ingredients for success in CFD are experience and a through understanding of the physics of fluid flows and the fundamentals of the numerical algorithms.
, 명현국 저,문운당 , 명현국 저,문운당 To provide all necessary background material for a good understanding of the internal working of a CFD code and its successful operation. CFD & Turbulence Lab. http://cfd.kookmin.ac.kr
