What determines CFD accuracy for aircraft design?

Do CFD results depend on the knowledge of the person who is working with software or do the final results just depend on computer power and CFD software?

Does one need to know aerodynamics to do serious CFD work or is everything in the CFD software?

• Assuming that CFD mean Computational Fluid Dynamics, this is really about computer simulations and not about aviation. I could answer it, but not on this site. Consider asking on one of our computer related sites. Also "software" not "softwear". Dec 19, 2020 at 16:47

CFD is a prime example for "garbage in – garbage out".

There is a large range of parameters that needs to be set correctly, from fluid properties to surface roughness. If the persons feeding the software do not understand what they are doing, it is very hard for them to judge the result. Lots can go wrong and will produce colorful but wrong results.

CFD is best suited when you know what to expect and can check the correctness with real-world data. Then it is fair to assume that the results over the whole flowfield are realistic and looking into corners that cannot be observed in an experiment makes CFD supremely useful.

Why else are wind tunnels still solidly booked and running? Today, it is mostly for validating CFD results and making sure the complex software produced realistic results.

Accuracy depends on:

• The code: Potential code cannot model separation, Euler codes cannot model viscosity. Navier-Stokes codes can, but still have trouble with turbulent, separated flows. For more detail, make sure to read the answer of @XRF.
• The mesh geometry: It is hard to get the mesh right so solutions iterate well and results are robust.
• Mesh resolution: A coarse grid (2D) or mesh (3D) will create poorer results than a fine mesh. Of course, computational load goes up with mesh size.
• The domain being investigated: Subsonic, fully attached and steady state flow is easier to get right than separated flow or the exact shock position in transsonic, instationary flow.
• What is menning of garbage in ,garbage out?
– user52248
Dec 19, 2020 at 18:27
• @Сократ In this context it means nonsense. The term "garbage in – garbage out" means that you will get meaningless results as output, if any of the input parameters are nonsensical. Dec 19, 2020 at 19:06
• @Сократ No, not at all. He is saying you need to know what you're doing when running CFD simulations. Wrong inputs can easily result in wrong outputs. The computer doesn't do the thinking for you. But if everything is done right, CFD is very useful. Dec 19, 2020 at 19:56
• The meaning of "garbage in, garbage out" is "If you put bad information into a system, you will get bad information out (even if the system is good)" Dec 20, 2020 at 13:45

One measure that can greatly affect the results of CFD is the choice of turbulence model. Even just looking at the popular RANS (Reynolds Averaged Navier-Stokes) models, different versions exist. One version of the model that works well for one type of aerodynamic scenario may not work so well for another. Models are often picked based off of knowledge of how well the same model has worked for other similar designs, and followed up with wind tunnel verification.

Now the closest you can get to completely automatic CFD is a process called DNS (Direct Numerical Simulation). DNS foregoes the turbulence model entirely by simulating down to the scales where turbulence naturally disipates. This makes DNS incredibly accurate, but requires extreme amounts of processing power. Even then, to use DNS you still need to know what the time and length scales that the turbelence disipates at. These scales are called the Kolmogorov scales and reqire some aerodynamic knowledge to calculate (although some one could probably add automatic calculation to a program, computational power requirements righ now limit the application of DNS to mostly research use where the user is certain to have some understanding of aerodynamics).

• Can CFD predict and solve aeroelastic deformation ,wing can bend and twist during load..?
– user52248
Dec 20, 2020 at 22:21
• @Aeolus Technically CFD only refers to the fluid dynamincs side of things. FEA (Finite Element Anaylsis) is a more general group of techniques which include CFD, some of which can solve for mechanical stresses. There are commercial solvers available that combine the two, using the surface forces generated by CFD to feed a FEA model that determines forces and flexing of the airframe. These can do aeroelasticity.
– XRF
Dec 22, 2020 at 3:02