Is CFD Science or Art ?
 

(1). I have been thinking about this "DESIGN" definition for a long time. It started when I was working on the B-1B bomber in early 80's. (2). Now the bomber has been in service for many years, but I still run into this "DESIGN" definition puzzle. The question was " What is DESIGN?". (3). This was raised again recently, because a senior engineering specialist said,"He has asked the project manager to include the Navier-Stokes CFD analysis in the proposed new blade design." (4). So, before I was able to find the true definition of "DESIGN" , I am now facing another puzzle about the definition of "CFD Analysis". (5). A few years back, when I was wroking for a engine company (the name has just been changed),I remember that they used something like "Turbine Components Design and Analysis" for the department name (a large department with several hundred engineers). And, the Navier-Stokes analysis was included as part of the design sequence. (6). At one occasion, a manager made a statement that," his group is not using the Navier-Stokes codes in the design, they use only Denton's code. So, they don't run into the convergence problems associated with Navier-Stokes codes." So, I guess his definition of design does not include the Navier-Stokes analysis. So, this is different from the definition of the previous manager about the definition of design. (7). So, the puzzle is, "Is CFD Science or Art?" I hope it is not a religion.( I mean a company culture.) (8). The reason why this is very important is that, it will have a great impact on the future of CFD. (9). Before we start the discussion, I think, my definition of CFD is," CFD includes problem definition, complex 3-D geometry definition, the appropriate computational mesh, the turbulence modeling and the physical modeling, the numerical solution algorithms, the graphic presentation and animations, and the results interpretation." (10). I have shown ten years ago that these sub-elements of activities can be included in one single code for simple and well defined problem. So, there is no question about the feasibility of includeing the CFD analysis in the design cycle for the well defined simple 2-D problems. (11). So, the question can be refined further as " Is CFD Science or Art, for complex 2-D, 3-D problems ?" (12). Can we say that, the 3-D CAD can always produce the right geometry for CFD analysis? (13). Can we say that, there is a mesh generation code somewhere which always produces a working mesh? (14). Can we say that, we know enough about the turbulence modeling and by simple selection of a particular model we can always obtain a good solution? (15). What about the solution algorithm and the related boundary conditions, can we say that by using the highest order accurate scheme we can always obtain a converged solution? (16). Can we get any information easily anywhere from the flow field? Even for this seemingly straightforward question, I have been frustrated by the inability to locate the highly skewed meshes on the 2-D screen using some commercial codes. (17). Bill Gates is a great leader in the PC world, he also has a great heart. But it seems to me that the future of CFD requires a lot of work, far beyond the operating systems, word processors and spread sheets programs. (18). So, is CFD science of Art? That is a question in addition to my old puzzle, what is the definition of DESIGN?

Re: Is CFD Science or Art ?
 

Hi John,

The question can even be genralized to scientific computing in general. There is a famous book known as

Numerical Recipes or the Art of Scientific Computing.

The fact is that to write a code with all the initial conditions, boundary conditions, testing, etc... is to some extend an art, since one has to make a lot of trials and get the 'feeling' of how the code performs under certain conditions, even without the DESIGN and the rest.

So we have here a question of definition and classification. Personally I do not like to try to put stickers on things but rather just like to observe the world as it is. So I agree that CFD is not just science, but I would not say it is Art or something else.

Patrick

Re: Is CFD Science or Art ?
 

(1). 1+1=2, is (Math.) (2). F=m*a, Newton's second law (Science). (3). Navier-Stokes Equations, is Physics (Science). (4). Turbulence Modeling, is ? (Fuzzy Science). (5). Geometry modeling, is (Math.+ Art). (6). Mesh Generation, is (Math.+ Art). (7). Numerical Algorithm for CFD, is (Math.+ Art). (8). Visualization and Animation, is (Math.+Art). (9). Art is something unique,which can not be duplicated easily. Science is something universal, the theory is always valid. Math. is something reliable,which always produce the same answer. Fuzzy Science is something looks like science, but not yet qualified as universal. (10). By adding these together, the score for CFD is (Art-4, Math-4, and Science-2.5). The post-processor would be (Art-1, Math-1). The geometry and mesh generation would be (Art-2, Math-1).

Re: Is CFD Science or Art ?
 

You make my feeling for art more engineering than entertainment.

Re: Is CFD Science or Art ?
 

Turbulence modeling is Math!! All the theories about the spectra etc.. (Kolmogorov, and the rest) is not fuzzy science. There is a whole theory behind it, if you don't know about that then, maybe for you it is fuzzy, but certainly not for everybody. Chaos is a theory of theoretical physics and mathematics, and chaos is a natural solution of non-linear equations like NS. So NS and chaos looks chaotic to you, but there is a bunch of theories behind them. Geometry is Math too! Even its modling. If you want a good modeling then let a mathematician do the job... And, remember, turbulence is Universal, it is in the whole universe, at all scales (Galaxies, interstellar medium, intergalactic medium, etc..).

Re: Is CFD Science or Art ?
 

(1). I am talking about the "turbulence modeling". (2). The theory and mathematics of turbulence can only provide a series of equations which are not "closed", that is, there is no way to find the answer. (3). For CFD, we need to find the solution, and the "turbulence modeling" is the process to close the governing equations. That process is "Fuzzy Science", sometimes it works and sometines it doesn't. (3).I am not aware of a mathematician who works as a CAD engineer, a person who creates 3-D models using programs such as CATIA, PRO/ENGINEER, AUTOCAD, Microstation, CADDS, or 3-D studio, etc... I am talking about the final geometry model for the CFD , not the mathematical definitions of point, line, curve, surface. a CAD engineer (or designer) must have a very strong feeling about the 3-D objects.

Re: Is CFD Science or Art ?
 

i agree with you on all points but number (7) the development of modern CFD algorithms is math mixed with fuzzy science. it is true that older schemes were 'cooked up' but more modern techniques (TVD, DRP etc) have a solid mathematical base. when CFD schemes were based solely on the (nebulous) context of order of accuracy, which doesn't precisely mean much then you could say they were art but if we look at newer schemes the mathematical development is mostly rigorous. unfortunately most CFD users (or even developers) don't read Journal of Computational Physics etc so their appreciation of the strong physical and mathematical basis behind modern CFD schemes is lacking and so they are prone to think of these concepts as arbitrary

also i think the art in mesh generation can be removed (or at least reduced) by more wide use of techniques like adaptive meshing that take the subjectiveness out of mesh gen

Re: Is CFD Science or Art ?
 

As I see, the most TVD and DRP schemes are derived from the Euler equation. If one uses them for the NS equation, he is then uncertain whether these schemes are still available, because they produce numerical diffusion which can confuse the real physical diffusions such as viscous layer etc. In this sense, "solid mathematical base" is restricted. I hope you would give a comment.

X. Ye

Re: Is CFD Science or Art ?
 

(1). I think you are right. I was not trying to find a middle ground to present it so that everyone can accept it. (2). There is a very interesting paper on "Accuracy of Shock Capturing in Two Spatial Dimensions" by Mark Carpenter from NASA Langley Center, in AIAA Journal,Vol37,No.9,September 1999. In the conclusion, he asid," Results show, contrary to conventional wisdom, that captured 2-D shocks are asymptotically first order, regardless of the design accuracy of the numerical method that is used to obtain them." In showing this results, he has used a linear fourth-order method and a nonlinear third-order method in the study. (3). In other words, it is not possible to capture something which is smaller than the cell size, regardless of the order of accuracy of the method used. (4). One more thing I didn't mention in the message was, the trend to more user-friendly graphic user interface and interactive environment is the trend to more art approach. In the robotic IC assembly line, everything is pre-programmed event. I don't think that the current trend of CFD is heading in that direction.(it would be very efficient to get thing done that way, if possible.)

Re: Is CFD Science or Art ?
 

DRP (dispersion relation preserving) schemes have nothing to do with artificial viscosity they are schemes which allow for the capturing of high wavenumber disturbances with minimum grid points (read Tam and Webb's paper in J comp Phys, 1993 i think). these schemes were developed for Aeroacoustic analyses and they allow for coarser meshes and when combined with an optimised time stepping scheme (Hu et al J Comp Phys 124 pp 177-191) allow for larger time steps as well. these schemes and others similar have been derived specially for Comp Aero Acoustics (CAA) but can be used for CFD and give the same benefits. if you read the papers you'll see that the numerical schemes, the stability limits and more important the accuracy limits are defined quite rigorously. such rigor is characteristic of CAA schemes (and articles published in J. Comp Phys.)

one of the points i mentioned before is that some journals like J. Comp Phys., and SIAM journals, and applied math journals often contain the derivations of the numerical schemes that we use. however many CFD users don't bother to read these they tend to stick to the 'applied' journals hence missing 'details that are presented elsewhere' in doing so they ignore the rigor and get the impression that these scheme were pulled out of someones ass (to use the colloquialism)

as a result we have lots of users (and developers) going around saying i'm using Jameson's scheme or van Albada's limiter (etc) and having a poor understanding of the advantages and drawbacks of the pparticular scheme they're using. yet in most cases comparisons and developments of these methods are available in the 'rigorous' journals but many don't read them because they are 'full of math'. it is the bane of many engineers that so many of us seek to avoid the details of mathematics whenever we can. but God is in the details. many of us wonder why our codes don't work as well as the papers indicate they should but don't have a deep mathematical understanding of what we're doing the we get mad and say CFD is bullshit and don't get anything done.

Re: Is CFD Science or Art ?
 

Thanks for your comment. Really, for an enginner it is not easy to understand the mathematics behind the numerical schemes. But on the other side, the theorists should consider that the engineers are confronted every day with practical problems governed by the NS equation, hence they should derive and test their schemes for NS euqtion. Maybe, the DRP schemes are very good, but could you mention a paper in which the author tests his scheme for NS equation, for flow with boundary layer? Is the artificial viscosity the unique reason for the numerical diffusion? I am not sure. It seems that the theorists avoid always to touch the complexities of the NS euqtion and let the enigeers use CFD as an art. It's a pity and I am not satisfied with this situation.

X. Ye

Re: Is CFD Science or Art ?
 

For theorists, CFD is science, because they treat the problems on a strict mathematical base. But for convenience, they avoid always to face the complexities of practical problems governed by the NS equations, derive and test their numerical methods for simpler cases governed by Euler euqation. Then they let the design engineers to have a grey zone to use CFD as art in treating practical problems governed by NS equation. The life is easy for the theorists but is difficult for the engineers. I think, it is the resposiblity of theorists to touch the complexities of NS equation and to invent generally available numerical methods, so that the engeneers can use CFD not as art but as science.

X. Ye

Re: Is CFD Science or Art ?
 

I'm using John's numbering system again

(1)i'd say your characterisation of the papers i was talking about as being written by 'theorists' is too general while it true that many of these persons (Tam for example) are applied mathematicians we shouldn't cast the nonengineer aspersion on them (they can't help it). you'll find that the people writing these papers are all the people you've heard about in the past as being originators of the schemes we use everyday (Roe, Jameson, van Leer, Patankar, Thompson etc).

(2)the mathematics used in these papers is not terribly advanced. i'm a second year master's student and for the most part the math is not beyond me. mostly your see series expansions and transforms (laplace,fourier etc) that we should be familiar with. maybe you'd not want to replicate the manipulations (why should you) but you can understand from reading them what is going on. so in general i would not say these papers are beyond the math abilities of most CFDers.

(3)in particular the DRP schemes were specially developed for solving CAA problems where the linearised euler equations or the full euler equations are used. however Tam ( J. Comp. phy. 1993) indicated that versions of them could be derived for the NS equations as well. in fact Dr. Philip Morris here at Penn State has used a DRP scheme to solve the (so called) Non linear disturbance equations which are a perturbation form of the NS eqtns. his email is pjm@psu.edu nd i'm certain he can give you the relevant references.

(4) this DRP scheme is for CAA so you may not be too interested however i must say that reading theoretical journals like J. Comp Phys, or J Fluid Mech etc is good because these are where the new schemes and their derivations are presented. the quality of submissions is very high and the characteristics and motivation for the methods are clearly and explicitly laid out, unlike in the 'applied' journals where such descriptions are glossed over. Case in point: no doubt you've read Jameson, schmidt, and Turkel's famous 1981 paper on the first application of the four stage runge kutta scheme with art. vis. for the euler equations. if you read the references though you'll see that Jameson had published a paper earlier where he develops the scheme. no doubt this paper gives the in depth motivation etc for the scheme. now this scheme is probably the world's most popular for solvinf CFD problems yet most of us have not read the original development.

(5)you speak of the NS equations. i think the main difficulty in solving the navier stokes equations in the industrial (applied) setting where turbulence is the rule centers on turbulence modelling. earlier in this thread mr Patrick Godon (who i think is a physicist) said that the theory of turbulence (Kolmogorov etc) is firmly based and i do not disagree. but engineers need to evaluate and here is where the fuzzy science (as John said) of turbulence modeling kicks in. it seems strange to me for example that persons would use the k-e model, which was developed for high Re external flows where viscous effects are limited to shear regions (ie aircraft aerodynamics), to problems involving internal flow of fluids at lowere Re where the flow is fully developed (ie turbulence is every where in the flow). i don't know much about turbulence as i'm taking that class now but i think that is our greatest problem in CFD

Re: Is CFD Science or Art ?
 

I'll try exclude the problems of physics such as turbulence modelling etc. I'll focus on the numerical method. I can describe the poor situation in terms of numerical schemes through a very funny example that I heard from a young professor of the University Düsseldorf: There was once a benchmarking case in Germany, namely the von Karman's vortex street under the Reynolds number of 100. The physics in this case is very simple because it is a purely laminar flow. Numerous universities and research institutions in Germany participated this benchmarking, but none of them could simulated this flow correctly because their numerical schemes were not fully available for this case with viscous layer and governed by the Navier-Stokes equations. From this example, one obtains the impression that the distance between the development of the new numerical schemes and the practical application in the industry is still large. Maybe in some years this leakage will be filled.

X. Ye

Re: Is CFD Science or Art ?
 

It is good to know there are people like mathematics and physics in the engineering world! It is very important to know the theory behind every engineering methods (CFD is one example), but it does not need to worship math. and physics.

To explain my point, let's go back one step to an even simpler question: How do we know the world? If you don't know the answer, you may get a clue from your academic degree. BS., MS. and Ph.D.. The first two are easy to understand: something to do with Science (even we are working on engineering, we got Science degree). The third seems odd, how come we got a degree in Philosophy. Well, that is the way that science starts with.

The word 'Science' was from Greek, which means 'I know'. The word 'Philosophy' was also from Greek means 'I think'. The world and everything within are 'facts' which exist before we 'think' or 'know' it. We must 'think' it first before we can 'know' it. For example, human knowledge starts from Philosophy (ideas, thoughts, imagination, etc.), then the Philosophy needs to be proved true by means of scientific (already proved to be true - 'known') process (like observation, measurements, etc.), then it becomes a new subject of Science. Next people want to understand it so they set up a model to describe it by menas of a series of symbols and relations (Math.) and then find use of it (Engineering). At this point, everyone made contributions got credit of 'invention, or discovery, or something'. After that, it becomes a common knowledge and we can learn it from books. Then we start to argue if it's right or wrong. Think the approach: Facts - ideas - models - apply to facts. which one is more important? Different people have different answers: physicis says ideas are more important, no ideas, no sciences; mathematics says models are more important, no models, no understant; engieering says application to fact is more important, it starts with fact and end with fact, all others are 'tools'.

Take TVD scheme for example, the concept was derived from the mathematic property of hyperbolic equations regardless its physical meaning. The TV (total variation - a human defined term) will not increase as time goes on, in another word, it will be D (diminishing). This is a Math. property of monotonicity. The 'Philosopy' behind the TVD numerical scheme was that if a numerical scheme preseves TVD, it will prevent 'over-shooting' near the discontinuities (like shock waves, etc.). Then there is a rigorous derivations for a large family of TVD schemes (math.), then there are applications of the scheme to engineering problems (CFD is one of them). Do you what is the real problem? the mathematical theory of TVD is known only to scaler non-linear hyperbolic equation, or linear hyperbolic system equations! It is unknown for muliti-dimension, non-linear system equations like Euer/N-S eqs. Do you still think TVD in N-S eqs. is a rigorous science?

How about turbulent model? Does anyone have a clear 'idea' what tubulent is? If there is no valid 'philosopy' model, how can one develop a 'math.' model?, if no valid math model, how can one say it is a rigorously developed science? On the other hand, is the Reynolds averaged N-S equation a correct approcah to the physics (fact)?

Re: Is CFD Science or Art ?....What is missing?
 

(1). Even though I try to use the words "science" and "art" in the over-simplified question, there are good reasons behind it. (2). The question of " Is CFD Science or Art?" is itself a valid question. (3). The question of " Who is doing what " is another issue, which is also very important to CFD as a whole. (4). At least at this point, there are "CFD engineers" and "CFD theorists" in the CFD field. (5). The CFD theorists are professors working in the academic institutions, where one of the major goal is to develop new theories or technologies and publish the technical papers, without the pressure from the project deadline. (6). Then there are CFD engineers who are responsible for brining the CFD answers to the design described in the project plans. (7). In this case, I think, somewhere along the line, there is something missing between the papers published by the CFD professors and the CFD engineers. So, what is missing? (8). I don't know exactly what is missing, but, I think it has something to do with the "Design". In other words, both sides were not able to connect their activities in CFD to the definition of "Design". (9). By definition, design must be carried out iteratively and in the process, the configuration (the geometry) must be changed many times. (10). When a CFD professor developed a shock capturing method and demonstrated the results in one dimensional flow, the method is good for the one dimensional flows. When someone incorporates the method in a 3-D code, the geometry will be very different from the original one dimensional theory. The users will likely run into troubles, because this CFD design (the change in geometry) is outside the original range of theory. But, if the users keep within the original scope of the 1-D theory (the 1-D design limit), then he is likely to get the right answer. (11). The code validation is actually a very backward method,because it is trying to figure out the applicable range of design. (12). My feeling is, if both CFD professor and the CFD engineer ignore the importance of the CFD design issue, then the CFD technology developed is no longer applicable in the design processes. Even though one could still have a paper published and a code used.

Re: Is CFD Science or Art ?
 

i understand your example. this case is simple in a way since turbulence modeling or compressible effects are not relevant. at the same time the problem is (extremely) unsteady and requires very fine mesh in the regions of the vortex separation from the cylinder. also the problem must have a clean spatial differencing scheme because the flow is affected by numerical dispersion.

however i have seen solutions to this and similar problems. one was a Finite element solution i saw in journal way back. i've also seen a solution (a video actually) using adaptive (cartesian i think) meshes by Pratt&whitney/united technologies research center. also i've seen unsteady solutions of axial and radial turbomachinery which resolve vortex shedding phenomena in a compressible flow (nice huh). you can see these in one of the VKI's lecture series notes on turbomachinery CFD. i could probably find the reference if you're interested.

i think for the anecdote you cited what happened was the researchers all had time marching codes set up for steady state analysis. and while in principle it is easy to do both steady analysis and unsteady analysis with the same code. in practice if the code is written from the start for steady analysis the unsteady capabilities are often compromised

Re: Is CFD Science or Art ?
 

on point (3) i fully agree. this is the essence of discretization methods. i've only seen a couple papers where the internal structure of the shock is investigated and both were very interesting.i also agree with your point (4) if we look at the computational structures field, all the cutting edge research is going toward automatic optimisation and automatic design i.e. less interactivity. they took the user of analysis a long time ago with automatic meshing and mesh adaptivity and we here in CFD are stuck with elliptic mesh generators. in CSM i think there is more pressure from the top (research) and bottom (users) to adopt cutting edge techniques in everyday work as they are introduced and validated (well the validation is easier so that helps) butin CFD we like to hold on to old things. in FEM nobody uses direct solvers anymore it's all wavefront solvers or advanced sparse matrix techniques, but in CFD we still have people using maccormack when MacCormack himself probably doesn't use it anymore. i've even seen Papers of Jameson's where he doesn't use his famous 4 step RK scheme (it's an oldie but goodie though). Some readers of my psots may dislike my harping on the CSM field over CFD but those of us who strive for excellence should emulate the sucessful traits of the more advanced

Re: Is CFD Science or Art ?....What is missing?
 

"DESIGN OPTIMIZATION" of the product visualizing the CFD simulation output. This is how to bridge the missing gap between design and CFD simulation.

Re: Is CFD Science or Art ?....What is missing?
 

(1). Yes, it is something like that. (2). But if we try to implement it as the creation of "CFD Design Optimization Engineer" to bring the two activities together, I don't know whether it is going to work. (3). For example, as a result of hurricane Floyd, many places in North Carolina are flooded and houses under water. To solve that problem, I think, pumping stations can be created and waterways built to transport the excess water . But right now, you can't just put a lot of water pumps in the flooded areas to start pumping water. (4). On the other hands, the earthquakes of equal strength hit both Turkey last month and Taiwan this week, but the degrees of damage are different. This was reported to be due to the building codes used in Taiwan. (5). So, I think, to make the "CFD Design Optimization" work, it must be incorporated in the method development phase, as well as the design application phase. (6). If the "CFD professor" (the person who develop methods) can demonstrate that the method can always provide a consistent results through "Design Optimization", then, there is a much bigger chance for the code to produce useful solutions later. This is a relatively new concept, even though it has been done through the use of grid independent solution exercise. This is because once the mesh independent solution is established, the users can optimize the design using the optimum mesh and arrangement. For example, if a 200x200 mesh has been demonstrated to provide mesh independent answer, then the user can weight his mesh size and the solution accuracy to arrive at the best solution in the actual design applications and still be able to get acceptable answer. (7). But, this is only a very small factor in the Design Optimization. How to incorporate the Design Optimization concept at the stage of method development is a wide open issue, something like the building codes used in the Taiwan's earthquake resistant building design. It is probably hard to make a conventional building earthquake safe after it is build. I guess, the same is true for a CFD method. If someone can come up with a mehtod which will solve your yet undefined CFD problems ahead of time, he must have the quality like Albert Einstein. For the rest of us, we must somehow look at the DESIGN issues before the methods are developed and after the application projects are prepared.

Re: Is CFD Science or Art ?....What is missing?
 

I think, we are missing a good bridge between the pioneering theorists (Roe,van Lee, Grodonov ...) and engineers. This bridge should be such theorists who work further on the base of the poineering theorists and touch the complexities of practical problems bravely and then give engineers good and near-practice numerical methods. If every one will be a pioneer, we'll miss this good bridge for ever.

X. Ye

Re: Is CFD Science or Art ?....What is missing?
 

I agree completely. And it seems the gap between the "pioneering theorists" and the engineers keeps growing. Take turbulence modeling as an example. I think that among most of the theorists, second-moment closure has been considered almost passe for a decade now, and new research is looking at more innovative methods. But most folks in industry aren't even using Reynolds stress models because they're "too advanced" for complex engineering problems that tend to get solved with k-eps or the like. There hasn't been a "bridge" between academic model development and engineering application, but maybe some people will start to fill this niche? Maybe they're the ones who will start to turn science into art or art into science?

Keith

Re: Is CFD Science or Art ?....What is missing?
 

(1). I think the basic issue is the lack of systematic testing of the method before it is published or accepted for applications. In a way, both sides are guilty. (2). In the drug development business, it takes years to develop a new drug and it also takes many more years to do testing. Because it could be a life and death issue. (3). The same is true for the building codes. Without it, it can be a very serious problem when earthquakes hit. (4). For CFD, we are begining to see the early symptons of the problem, that is unreliable CFD results from time to time. (5). This is because the systematic testing and optimization of the methods developed,which is necessary to produce reliable CFD answers, are largely missing. (6). One can say that it is part of technology evolution, it going to take time. (7). I would say that the increasing use of CFD technology without systematic testing and optimization is going to make the CFD industries totally un-reliable. Unless one can say that each CFD problem is part of the general case, I would say that each CFD problem is by itself a special case.

Re: Is CFD Science or Art ?
 

(1).Strive for excellence is the major driving force for technology advancement. As a technology gets more advanced and involves in multi-discipline knowledge, people have to strive into different directions in the same subject. (2). In CFD, people working on CFD algorithm development (that was my research area 10 years ago) have different opinon with CFD application engineers (that is now I am doing). (3). CFD developers (not code developer) also have different directions to strive, some try to improve accuracy in terms of mathematic orders, some to improve the efficiency to make it run faster, and some to improve the resolution of capturing discontinuities. With those efforts, people may spend years and millions of dollers only to make the shock waves from 2 grid points to one and half, with a possible cost of 30% more CPU time. (4).CFD users (include code developers) are more interested how to solve engineering problems quickly, accuratly, and easily. (5) Some engineers also developed algorithms. One very famous professor with engineering background developed a scheme for Euler equations for a particular application, he claimed that it was the fastest solver in that area. Indeed his methods was widely used in a industry code. But CFD developers said this method does not mathematically sound and is not considered as a 'scheme'.

Come back to CFD, there are some special difficulties in CFD over CSM. The main difficulties are the advaction terms, which are the source of eddy viscosity, the numerical instability, the needs for numerical dissapation (artificial viscosity), the problem in discontinuity, and other unkown non-linear problems. Those terms are the most concerns for CFD developers. The second difficulties are the boundary conditions. The eqs. are always the same, only by changing B.Cs, we have sooooo many applications. Unlike CSM where there is a well-defined boundary, in fluids everything is so soft what there is virtually no boundary! And even though we have to define a numerical boundary we certainly don't know what exactly happens on the pre-defined boundaries. Those difficulties are the most concerns of CFD code developers. The third but not the least, the viscous effects, boundary layer, wakes, mixing, and their interactions. These factors are usually affected by grid quality and density, a partular dificulty for CFD application engineers.

Most CFD algorithms was developed between later 70's and early 80's. Some high order/high resolution methods (the Godunov-type schemes) were also developed at that time, or a little after, although the concept was dated back to later 50's. Recently, there are no major breakthrough in mathematic models in these area, except for some minor improvements (I don't know if this is the reason for me to change area from development to application). The main efforts are now shifted to 'all-purpose' code development and user-friendly interface, something more like an art. May be this is the time for new ideas which may lead to a brighter future of CFD. The hope is on the new generation who strive for excellence. There is no hope in the 'old folks' (not in terms of age) who only strive for high-salary or high-reputation.

Re: Is CFD Science or Art ?
 

(1). It is a good example to look at the structure analysis side. On the computer aided activities, the structure analysis is closer to the CAD side. By linking the structure analysis directly behind the CAD, it may be possible to achieve the coupled design and analysis loop, thus shorten the design iteration cycle time. In this case, the driving force is the design and the feed back loop is the analysis. (2). And this is probably one of the major difference between the structure and CFD analysis. In CFD analysis, it is the driving force of the design. In other words, the CFD analysis must determine the optimum shape for the CAD design. This is perhaps one of the difficulties involved in implementing the CFD analysis. For example, we don't ask the CAD designer to come up with a blade shape and then run a CFD analysis.(in a way parallel to the structure analysis) The problem here is CAD designer can't come up with the shape of a blade. The blade has to be designed from the CFD side. (3). I am trying to keep the scope of discussion small here. We really have to see the whole process more clearly in order to develop more reliable CFD technologies. In the actual processes, what happens is: First, the simplified 1-D theory is used to derive the shape, Then, that shape is analyzed by more advanced theory. Then a preliminary shape is created by the CAD, and a 3-D advanced analysis is performed on the shape. (4). In this traditional approach, the analysis is finally used to check the shape of design, which is very similar to the wind tunnel testing of many concept models. By testing, it is hoping that the best can be found. It is one way street, from shape to analysis. Ideally, one would like to use the analysis to determine the shape. So, in this case, the CAD should follow the analysis. (5). From here, you can see the basic difference between the structure analysis and the CFD analysis. The current trend of commercial CFD codes, using the CAD as front end to develop the geometry and mesh is not consistent with the traditional blade design practice.(the shape comes from analysis, not from CAD design.)

Re: Is CFD Science or Art ?
 

Research and Development of designs associated with complex continuum systems in fluid dynamics and solid mechanics. Design/Research Objective: The design of industrial processes, systems, or components.... Research Advances to be Applied: ... through the coordinated application of new advances in physical experimentation and computer simulation at all stages of the design process.

Mechanical Engineering Department, University of Utah Sponsored by National Science Foundation

Is it not a good start to bridge the missing gap between design optimization and CFD simulation output!

Re: Is CFD Science or Art ?
 

i disagree with your point (5) the object of CSM and CFD is the same: produce shapes which give good performance. we can only evaluate performance after having done the analysis and the analysis should lead us to new shapes (as you said) but we do need a shape to start the analysis. in structural design we do the same thing as you said for turbine blades (point (3)) you start with a simplified structure (beams, trusses etc) and make preliminary sizing and shape which can be translated in to more 'fleshed out' structure which is analysed at the most precise level. THEN as my statement above we try another shape (ie the analysis leads to new shape). there is not much conceptual difference between the structural and aerodynamic design. the difference is practical: the structural is easier and we can often cut out some of the simplifications because of low analysis cost

Re: Is CFD Science or Art ?
 

on your comments about the TVD scheme (i am not a defender of TVD schemes) i can only say this: many of the concepts we use in CFD are only applicable for systems of linear equations case in point is the von neumann stability analysis that we are fed as young CFDers. so while these concepts are not perfectly applicable for the nonlinear equations we try to solve they do give use guidelines to help our analysis. indeed there are few things we know absolutely accurately about the NS equations (we can't even show they have unique solutions) but our knowledge of the realities of fluid flow (the actual fluid behaviour) and what (semi rigorous) mathematics we can apply is mostly all we have. all i'm saying is that we must be sure we use as much of it as we can else we won't be doing our best or most efficient job.

Re: Is CFD Science or Art ?....What is missing?
 

as Keith said the gap between the developers and the practisers is large. how to bridge it? in this case i think Mohammed (the users) must come to the mountain (academia). this isn't strictly true because i think there are theorists who can and will get their hands dirty. check out J. of Aircraft vol 36 no 1. there is a paper there by Jameson where he outlines the improvement of a commercial aircraft for McDonnell Douglas. the paper is very design oriented and shows how an academician can go into the industrial world and impact the design process significantly. HOWever i doubt seriously that many professors are interested in this (hence my previous statement) after all why become a professor. so it'll be up to the real engineers to go to the ivory tower and bring back the knowledge they need.

Re: Is CFD Science or Art ?
 

(1). I think, we are getting closer to the other core issue of design, or more specifically, the aerodynamic design. It is a much bigger issue. (2). It is not efficient to design the turbine blade or the aircraft wing from the CAD end. (3). In the CAD, the package is general, and every designer can come up with a different way to put together the final goemetry. This is not how the aerodynamic design is carried out. (4). The geometry can take different forms and formats. And the general CAD program can only handle the geometry is a general way, which does not have any aerodynamic significant at all. (5). All I can say is the geometry in turbine blade and aircraft wing takes special relationships with the aerodynamics, and it has very little to do with general CAD. So, the use of geometry in CFD for turbine blade and aircarft wing is very different from that of a general CAD. (6). That special relationship between the blade geometry (or wing geometry) and the aerodynamics is the key issue in aerodynamic design. And CFD is part of this design process. This is the reason why it is very hard to integrate the general CAD into the aerodynamic design. (it is not the right approach.)

Re: Is CFD Science or Art ?....What is missing?
 

(1). There are at least two issues here. One is the integration and the other is the location. (2).In some countries, the research is carried out mainly in universities (they have the money, the PhD level professors, and the facilities). In other countries, the real technology is in the hands of private industries. And in some other countries, the industries work with the universities in the research subjects. So, there are many ways to bridge the gap. (3). In the first case, the CFD technology developed will be more tuned toward student training, or the paper publishing. So, the output of the CFD code( or technology) is linked to the general features of the problem, for training or publication purposes. (4). In the second case, the output of the CFD code will be linked to the efficiency or performance of the product (or design), to impress the potential buyers (private, government, or foreign customers). (5). In the third case, the CFD output will be expressed in terms of the mutual relationship, hopefully a long and good one. (6). The fact is many countries have been working on this issue with different approaches. (7). My personal feeling is: It is the engineer's total responsibility to solve his problem. One can not say that it is someone's geometry problem, mesh generation problem, turbulence modeling problem, algorithm problem, or even the plotting problem. In CFD, one really have to become a superman (or superwoman) to be able to solve the problem. (it can be a group of engineers or a department in a company scale).

Re: Is CFD Science or Art ?....What is missing?
 

(1).The turbulence modelling is the key to the future of CFD. So, it is a very critical issue. (2).If one is using a commercial CFD code which has four or five turbulence models in it, he will be dead if none of the turbulence models listed can provide a good solution. (3). So, it is very important that a CFD engineer must have the skill to do the turbulence modelling for the types of the problems he is interested in, whether using algebraic models or two-equation models. (4). I don't know exactly why the stress models are not being widely used. It could be because most problems do not require high level model to achieve the accuracy. Or it could be because it is hard to understand like a black box. (5). So far the platforms provided between the algebraic Baldwin-Lomax model and the two-equation k-epsilon low Reynolds model is wide enough to cover a wide range of practical problems and at the same time allows the flexibility for modifications. (6). Instead of waiting for someone to invent a new set of coefficients or models, I think, we should train the student to modify the existing models for particular applications. This is the future of CFD. It is not a small market of future CFD. (7). Even though there are other important issues in CFD, such as mesh, resolution, and algorithm, but I think, these issues are mainly related to the hardware limitations,which are disappearing very quickly every year. So, the main issue will be back to the "turbulence modelling". That is a systematic modelling process for a particular problem at hand, not just a selection of existing models or coefficients. We want to solve the problem and the turbulence model must be further "modelled" to achieve the goal. The ability to implement the model implies that the engineer must have the minimum ability to write the code (or modify the existing code). The ability to model the turbulence and solve the problem will be experience related. That is related to specific applications. (8). The 3-D mesh generation will become more and more automatic. It will be of benefit to the automatic mesh code developers, but not to the advantage of the users' experience. (9). In addition to the turbulence modelling issue, the fundamental issue of aerodynamic design (or thermo/fluid design) will require more thinking or smarter approach.

Re: Is CFD Science or Art ?
 

ok i understand you know.

Re: Is CFD Science or Art ?
 

(1).I think, we are heading into the dark age again. It is not because of the Y2K problem which can cause the power disruption, but because we are not focusing on the technical problems. At least from CFD point of view, the same old Baldwin-Lomax model ,and the two-equation models are still being used. They are at lest a quarter of a century old. (2). Running more CFD calculations or computer runs will not improve the quality of the solutions or products. It can only waste more resources. (3). As the computers become more popular and affordable, I wonder why people are not taking advantage of this opportunity to bring new ideas to the CFD solutions.

Re: Is CFD Science or Art ? ...Daytona Beach Pier and the Turbulent Sea
 

(1). This is true, if the world is steady-state. (2). In reality, the world is not a steady-state. (3). This can happen in stokes market, in Asian economy, American defense spending, new policy of the newly merged large company, reorganization, cut-back to improve the financial performance, ...etc... (4). There were successful examples in other country, where a school was created by the company interested in training the needed engineers. In that case, the company remain stable and the relationship between the school and the company has been good. (5). This is not the case in America, where the change in the market can easily affect the survival of the comapny, thus affect the relationship between the two parties. (6). So, I would tend to believe the superman (or superwoman) approach. In this approach, one would focus all the resources on the potential candidates such that he or she can acquire as much experience or knowledge as possible. With the experience and knowledge, comes the leadership. Then the rest of the development is straight forward. (7). So, in principle, the bridge between the industries and the academic institutions is a good idea. But, in reality, the transient nature of the world make it very in-efficient. By the time, one is ready to challenge the world, he is likely to find out that his old sponsor is now out looking for his next job. Such examples are everywhere. (8). I have been following the superman principle to stay in the CFD and computer graphics, regardless of my sources of income. I think, this is one practical way to understand the whole technical issues of CFD world. And the critical step of over-all optimization requires such knowledge and experience base. (9). It is hard to make the distributed expert system work, because of the transient nature of the states, the communication lines, the priorities, ... and the missing of the nodes due to reorganization. (10). If one look at the CFD-online as a system, then it is a system for superman principle. In this system, the person who asks good questions, decodes messages posted is actually accumulating his knowledge and experience base. As this knowledge and experience base mature, he is ready to challenge the real world problem. (11). To share the distributed expert knowledge and experience among experts? and then solve the real problem? here on CFD-online, most of the time, it is hard to figure out the contents of the posted message. (to get the National Science Foundation money first then ask a CFD question here may take a long time. Same is true for me answering questions here, because in real life, very few come to me for answers at work places. Unless, their project has this item in it. Just a joke. )

Re: Is CFD Science or Art ?....What is missing?
 

At the start of my post-graduate studies involving a lot of CFD-work I read through many papers dealing with topics ranging from discretization methods, turbulence models, grid generation etc. My impression was that there are many issues that are still unresolved in the field of CFD, especially with regards to turbulence modeling. After my literature survey I visited a number of commercial CFD-code websites, mainly to decide which code to use for my investigations. The projected impression from these websites was that there is nothing we cannot solve with CFD.

I realize that in order to sell a product you need to emphasize what it can do rather than the opposite, however discussions with people who use the commercial codes for consultation work leaves me with the uneasy feeling that at times, very little consideration is given to the danger areas of CFD. Whether this is deliberate or due to ignorance I do not know, in some cases I believe it is the latter but I fear that most of the times it has to do with time limitations. My point is this: When considering the difficulties associated with dealing with phenomena such as turbulence etc. one can easily arrive at the conclusion that the way in which CFD is used in industry is somewhat premature. I am NOT saying that no useful solutions are obtained but rather that the use of CFD is running ahead of the capabilities of commercial codes. Maybe we are selling a product that still needs more time to develop. In this sense CFD is more art than science, the product of a simulation is after all a host of colorful pictures.

Re: Is CFD Science or Art ?....What is missing?
 

(1). In terms of the degree of difficulty, CFD is on the top of the listing. (2). In terms of the number of problems solved, it is on the bottom of the listing. (3). How many problems are actually solved? I would say that the number is very small. (4). For the simplest flow over a cylinder,can we say that the solution is known and the procedure to obtain such solution is repeatable? (5). The flow over a flat plate is cosidered known, not because of CFD but because of the boundary layer theory. (6). Then is the flow over the backward facing step solved? Are we still getting different sizes of separating bubble when different turbulence models are used? Do you consider those as the same solution or different solutions? (7). For flow in a curved pipe, such as the IC engine inlet manifold, can we say that we know how to predict the flow profiles entering the engine inlet port? (8). Can we say that we know how to predict the flow over an airfoil? How about the real wing on the aircraft with multiple elements? (9). The four strokes IC engine has four valves per cylinder now, so it is more difficult than the two strokes engine, right? Wrong! There have been attempts to simulate the flow through two strokes engine, but no luck, it is too difficult. (10). That is the state of art about the CFD. (11). It seems to me that there is a "quantum jump" in the degree of difficulty related to the CFD solutions. If someone is still debating on the "upwind method" and "numerical viscosity", it is going to take forever to solve a CFD problem. (12). Perhaps, no one is interesting in getting the CFD problems solved at all. As long as one is running a CFD code, the product should be all right, or at least, it is better than doing nothing at all. (if you are paid to run a code, do you care?)