Hi,

I have a big problem about solid kinetic theory. I am using CFX4.2 to simulate gas-solid flow in riser(circulating fluidized bed).And I use k-epsilon in gas phase, solid kinetic theory of Gidaspow in solid phase. But no matter how I change boundary conditions and solving method,the solid temperature equations always diverges,which leads to diverge of the whole system.I have used false time step short as 1.0e-6. Even the transient model diverges too. Could anybody tell me what is the cause of my divergence.

best regards.

(1). I must say that a CFD code is worse than a political system. (2). Some political systems do not have build-in tolerance or correction, so it always run into trouble over and over again. The best you can say is it is consistent. (3). So, the example I have is, either the political system is rigid (the code itself) or the people are not smart enough ( the user of the code) to prevent having good life (a converged solution). (4). The suggestion I have is: Don't try to change the political system ( the code), it can not be changed. You either have to stay in the system (using the code) and suffer, or become smart enough to deal with it ( lie about the results to your boss). (5). The basic mistake ( or decision) you made was you did not study the feasibility about the code and your problem first. ( people normally adopt a political system first without carrying out the necessary feasibility study, so, they have to suffer.) If you have already proved that the solutions diverged, then I don't think I can make it to converge. (perhaps there is a secret code one needs to use in order to open the door. Talk to the vendor of the code to find it out. That would be a good idea. Hope you don't have to get the approval first to ask the question. just a joke!)

(1).It must be very frustrated when one is not able to get the answer he is looking for. In this case, it is related to the use of the commercial CFD codes. (2). At the professional level, some journals have recognized the need to maintain the standard, and have provided some guidelines for the CFD related papers. (3). But, that does not prevent the users from exploring the codes in his own way. (4). So, I will tell you my experience about using a CFD code. Maybe in this way, you will have a better understanding about the commercial CFD codes. (5). About a dozen years ago, when I was rewriting my 3-D Navier-Stokes code, I decided to type my new code in a hurry. And somewhere along the line, I must have made a mistake. So, during the checkout phase, I could not get the consistent results. When the geometry is symmetric, the solution was all right. But when the geometry is non-symmetric, the result was not realistic. This typing error took me three months to fix. In the array subscript, I had typed a J instead of an I. And this was very hard to find by proof reading. (6). And more recently, I have found several errors in the code which has several modifications in it. The original source code was purchased from a university and then modified for in-house use. (7). So, what I am trying to say is, it is quite normal to have bugs in a code, regardless of whether it is developed in-house or purchased from outside. Based on this, it is not unusual to see a commercial CFD code with bugs in it. And this is the reason why the commercial codes are always sold "AS IS". And newer versions are then released with some bugs fixed.(8). Now, with all these in mind, how can a user obtain reliable answers from the code? The answer is nobody knows. It is like taking a trip by air, the best one can do is to select a good wheather to fly in addition to the purchase of additional insurance. (9). In the case of using a commercial code, I think there are several steps one can take to reduce the risk of getting bad experience. (10). The first one : a good support is essential. In other words, in the use of the code, it must include the support to answer the users questions and be able to help fixing it. (11). The second one : the user must be familiar with the operations of the code. the training is a short cut, the real answer is the time and experience. In this case, the user is trying to become familiar with something like typewriter keyboard. It is hard to type, if the user can't find the particular key. (12). The third one: follow exactly the available sample cases. Since there are many options to set, many operations to perform, each CFD problem definitely will require different steps to prepare. It depends on how each commercial code is structured and written. Some commercial CFD codes use modules created by different third party vendors, and thus each module will have different user interface. That is, it is likely that different methods have to be used to perform similar operation in different modules. (13). The fourth step: go through the standard benchmark test cases first. This is essential to make sure that the code can produce useful results. Each user probably has his own area of interest, and thus different benchmark cases must be selected. And hopefully, these benchmark test cases will be very similar to the users future target problem. (14). As you see, to use a commercial CFD code requires a systematic approach in order to make sure that the code is useful to solve user CFD problems. ( The user's background training in CFD also plays an important role in this process.) Remember that most codes are dead codes, you can only recreate the results or similar results the code was originally designed for. When you are outside the original range of applications, you can only hope that the hint from a support engineer will solve your problem. (if he is a very smart support engineer.)

ASCI Program Centers of Excellence

How the Universities Will Support DOE's Accelerated Strategic Computing Initiative

The Center for Integrated Turbulence Simulations (CITS) at at Stanford University

A Computational Facility for Simulating the Dynamic Response of Materials at the

California Institute of Technology

Center for Astrophysical Thermonuclear Flashes at the University of Chicago

The Center for Simulation of Accidental Fires and Explosions at the University of

Utah/Salt Lake

The Center for Simulation of Advanced Rockets at the University of Illinois at

Urbana/Champaign

The Center for Integrated Turbulence Simulations (CITS) at Stanford University will focus on the development of simulation technology suitable for the design of gas turbine engines, used to fly airplanes, drive locomotives, impel boats, and deliver power for many other applications. With this new design paradigm, the design cycle can be shortened, expensive testing can be reduced, and reliability can be improved; the benefits to science-based stockpile stewardship are improved understanding on compressible flow computations, turbulence and transport modeling.

A Computational Facility for Simulating the Dynamic Response of Materials at the California Institute of Technology will investigate the effect of shock waves induced by high explosives on various materials in different phases (i.e., gas, liquid, solid). The work will prove beneficial in a number of civilian practices that employ high explosives. It will also enable advances in material design and have applications in other areas such as geophysics. It will provide lab scientists in the science-based stockpile stewardship program a tool to simulate high explosive detonation and ignition.

Center for Astrophysical Thermonuclear Flashes at the University of Chicago will aim to solve the long-standing problem of astrophysical thermonuclear flashes through simulation and analysis. The astrophysical work will benefit the stockpile stewardship program by providing further understanding of the physical problems of nuclear ignition, detonation, turbulent mixing of complex multi-component fluids and other materials. It will generally add to the body of scientific knowledge of how the universe and, in particular, the Earth, were formed.

The Center for Simulation of Accidental Fires and Explosions at the University of Utah/Salt Lake will work toward providing a set of state-of-the-art, science-based tools for numerical simulation of accidental fires and explosions, especially within the context of handling and storing highly flammable materials. The simulation study will contribute improved understanding of fire safety and accident scenarios, and for civilian benefits -- reduced risk, increased safety and potential remedies in situations such as industrial chemical fires, the handling and transportation of highly flammable materials, terrorists attacks or car crashes.

The Center for Simulation of Advanced Rockets at the University of Illinois at Urbana/Champaign will focus on the detailed, whole-system simulation of solid propellant rockets under both normal and abnormal operating conditions. This work will help contribute to the understanding of the shock physics and quantum chemistry of energetic materials and the aging and damage of components; on the civilian side, in numerous aerospace and defense applications such as the design safety and reliability testing of new solid rockets.

(1). Do you have a special interest in these universities? (2). If so, we would like to hear from you.

Dear John,

I have a MS degree in CFD from University of Utah (Mechanical Engineering Department) and considering for Phd degree from one of these schools. It has been couple of years since I left Utah and now living in Michigan. I am thinking of applying to all these five schools for review and consideration of my credentials. I have to admit that you have a very good intuition power. I am really impressed to see you replying to many CFD analyst in 'On-line CFD Discussion'. Thank you very much for the reply.

If your problem is related to one special cfd code you should first ask the support people at first because

- you pay for support - they get payed for answering your questions - they know the code much better than we - they might learn something from you application or find errors in the code

Normally when I encounter a problem which I cannot solve myself then I phone the support staff and sometimes send them the case. This practice is not only usefull for the users but also for the developers of the code.