Tell me some problems or goals now CFD is faced to do

I take your question a one on areas of research. In turbulence LES is one of the most recent accepted models that works well in predicting flows in cases like flow and heat transfer past backward facing steps or past a surface with ribs; however it is still a somewhat new method that needs validation for many other situations. These situations are those involving near wall conditions. The science still cannot get to the wall and modelling of the small eddies is necessary. The search for a universal model continues. Apply this to all the aspects of life thinking that fluids are all around us and within us. The goals are then limitless.

Dear friends

Recent my research area is releted to the simulation of two phase(solid-gas, droplet) combustion. When, I start the simulation of new category among the wide range of combustion, I always think that 'how can I obtain the CONVERGED solution ? or 'what is the best strategy with paying less effort ?' even when I use commercial package.

As far as combustion problem was concerned, it was fairly difficult to obtain 'WELL CONVERGED SOLUTION'.

I guess that many of you had spent the entire night to obtain converged solution except when the problem was fairly simple, for example, simple fluid flow with simple geometry. Of course, it's basic problem is originated from the non-linearity of governing equation(Navier-Stokes eq.). However, if we can obtain good solution without paying much effort, for example, paying same degree of effort for LINEAR structrual analysis, we can contribute much to the engineering/design area and science area.

There might be many goal of CFD. As a personal point of view, I think that one of the most important goal of CFD is the development of algorithm which ensure the convergency, at least better than current situation.

I, myself, would like to see many response about this subject, 'GOAL of CFD'.

Sincerely, Jin Wook LEE

(1). From the begining of the computer age, to the modern days of supercomputers, CFD has been used to design the best weapon system. (2). It will remain so into the next century. (3). Relatively speaking, the use of CFD in the areas other than weapon system development is not essential. That is to say, the total befinit is likely to be negative, and expensive. In those areas, the product design can be accomplished without the use of CFD at all. This includes the design of cars and every day household products. (4). You think you will be able to design an efficient combustor ( or a reliable working combustor ) by solving chemical reactions in turbulent flows? (5). So far, CFD is an opportunity to provide an engineer or researcher to do more thinking. In the process, some new ideas and design will emerge as by-products. (6). To put a working scheme into a code and make it available to the public is unlikely to happen, yestereday, today or tomorrow. (7). So, all we can hope is through the use of non-working or non-converging CFD codes, real useful ideas will be discovered. So, in that sense, CFD is providing a good opportunity. (it even provides the CFD code vendors to make some money, the hard way I think.)

Goal of CFD

CFD discussion in this and many other forums sometimes reminds me about missionary work. I have seen papers with comments like "CFD must better be implemented in Industry ..." etc.

Many CFD related research papers are very much dealing with motivation how CFD is beneficial for industry. Ofcourse, this motivation is sometimes necessacy. CFD may be, if properly used, very beneficial. But I still feel bit irritated by meaningless sentences: "CFD provides us detailed insight information....", "CFD can be used as an optimisation tool ......". I quess it is part of "standard style" of research paper. How many of you really have been thinking about what those sentences actually mean ? Do we really need so detailed results that are actually useless if we are not able to interpret them in order to make design improvements ? What is meant by optimization ?

Most products designed in industry have very little potential in fluid mechanical "optimisation". They are very complex units, fluid flow related phenomena are just one part of them. My personal opinion is that most fluid flow problems can be solved most efficiently by the use of simple basic thermodynamics and fluid mechanics theory: heat transfer, turbulence, boundary layer theory,... People studying and using CFD should have solid theoretical background in (analytical) fluid mechanics and thermodynamics. The use of CFD is not a shortcut to any fluid flow problem. When solving flow problems I have been happy with books like "Handbook of Hydraulic Resistance" based on huge amount of experiments. Rapid prototyping combined with advanced measuring techniques may become a serious competitor for CFD.

Design work is always a team effort. There people from marketing, manufacturing, stress analysis and so on. So, fluid flow is not everything.

"Industrial" flows are mostly extremely complex. They include laminar to turbulence transition, multi-phase flows, combustion, swirling flow, heat transfer and many other topics which are very well known to be difficult to solve by CFD.

Many of the CFD users (including me !) are often very enthusiastic about it. It is very challenging and interesting, ofcourse. But we must not forget the common sense !

Okay, this all may sound bit too skeptic but I think it is just reality. And reality is what we are living (I hope so) in. If we are not carefull CFD becomes only virtual reality.

----- Tapio

Dear John

MYM Rameez proposed very interesting topic, 'Goal of CFD'. Francisco Saldarriaga presented an opinion about LES. And my opinion was one of the possible personal point of view about convergence.

Tapio Paattilammi gave us very useful comments which, I think, most CFD researchers/engineers should be careful of. I really assent to, and respect his sincere opinion.

However, in your case, I agree some of your opinion but some comments were very discouraging to me, and perhaps to many CFD researchers/engineers who are not in weapon area.

(1) Of course, some works by CFD are cost ineffective. However, your comment, 'all works by the use of CFD in the areas other than weapon system is not essential', is seemed to be not proper for many CFD works. I think, many works are cost effective even though they are not related to weapon system. As my personal opinion, cost effectiveness is dependent of user's capability such as knowledge about CFD, insight about the system which he is going to analyze and his experience of both CFD and the system. There might be not so many engineers/researchers who have all these ability but, I think that there are many teams which have such capability.

(2) Working and converging codes are better than non-working and non-converging codes. Of course, very useful ideas can be discovered through the use of non-working and non-converging codes as you said. But, in this case, the discovery is only your (his, her) property, but useful converged solution can be shared. I think that more useful idea can be obtained from the post-processing(the deep consideration of the results, not simple graphical display) of the converged solution. As additional personal point of view, what we really should be aware of is, CFD can be very useful if and only if we know its capability and limit and when it is properly used to the possible application. I still hope that convergency of CFD in the future would become better than now because something is better than nothing.

Sincerely,

(1). Yes , I agree with you. (2). The forum is not a place to encourage or discouage the use of CFD. The world does not change because of what I have said here. In your case, right away, you are saying that you don't want to be changed. (3). Simply said, "non-essential" means that the products have been in use for a long time and the creation of the products do not depend on the use of CFD. On the other hand, cfd-online is essential, because without it , there is no cfd forum. So, CFD has been and can become more useful in many areas, but because it is still in the early stage of development ( convergence problem is part of it), it is not an integrated part of the product design. (4). On the other hand, in the weapon system design, the performance is very critical. A slight difference in weapon system effectiveness can determine the outcome of the battle. ( it is a life or death situation. And CFD is essential there.) (5). For example, the current computer market is promoting the under $500 PC, and you are writing this CFD code which requires a large memory, say one giga RAM. Then, the question is : How are you going to convince your boss to market this code? If your code is the only choice in the market ( you control the market), then the users have to meet your requirement. Otherwise, no luck. (6). Another example is, let's say you have developed a very accurate algorithm to solve this Euler equation, and you are trying to get a job in the automotive industries. And eventually you are able to convince the hiring manager to give you the job to do CFD. When you start solving flow problems in the engine cooling systems, under-hood cooling problems, airconditioning systems,.. , the results obtained would be un-reliable. Actually, in the turbine engine field, Euler solution has been used in the design and analysis. Then you probably will ask: How could they do this. Are they getting more correct answer or are they getting more wrong results? You know, I know that the flow in the turbonmachinery is 3-D turbulent, viscous flows. (7). What I am trying to say is " these seemingly accurate solutions are actually delaying the real prograss in many product design." In this case, it is simply not appropriate to use such analysis. But, can you change the world? In the life and death situation, you would change the approach. But in the non-essential case, the wrong tools will be use non-stop. In this case, it can be easily said that the inviscid solution is efficient and is good enough. There is no need to use the viscous flow solution which normally takes many hours or several days to "converge". (8). My feeling about the goal of CFD is that, commercial use of CFD is one way to keep a reserve CFD engineer data base. This can be critical when a country has to face a war and the CFD engineers available can determine the survival of the country. This is just one point of view. The hardware side of the story is also the same. If the country can not hide the budget to continue development of super-computer technology, then it will also have difficulty to support the weapon system development at war time. (9). So, in the bigger picture, CFD is closely linked to the survival of the country through the weapon system, and the commercial use of CFD and computer hardware is simply to make sure that at the war time, the CFD technology, the CFD researchers, the computer hardware will be available to protect the country. (10). CFD is not going away, but at peace time, it is having difficult time to be accepted by commercial sectors. To change this, it will require a CFD war or CFD revolution! And I have been talking about this since a year ago here.

Good Question,

Most of the people in CFD have probably never thought about the larger implications and a large portion will probably fit in the "building a better bomb" group of the military aero, US govt. research etc. Personally I believe that the basic goal of CFD fits in with the basic goals of science and engineering.....to make life more interensing and bearable! Personally, I have worked on projects from improving the ventialtion storage of sugar-beet piles to reducing the environmental impact from combustion.....It feels good to be involved with significant projects that have humanitarian impacts! I saw a really cool one on modelling the onset of arteriosclerosis by a young prof at Western Ontario University ...... that is the stuff I like to see!!!

Regards................Duane