Is CFD Science or Art ?

John C. Chien

(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?