A reader asked whether CFD is an “ivory tower” tool and, if so, what would it take to make CFD more reliable.
The simple answers are “no” and “validation and verification.”
But nothing is that simple.
CFD Is One Big Tangled Ball of String
Here’s why this reader is concerned.
“From my perspective, it seems there is a lot of uncertainty in applying CFD because the complexity of flow features and how they interact which each other and a geometry. One has Mach number and Reynolds number, which translates to compressibility, shocks, boundary layers, laminar to turbulent transition, separation regions, small and large eddies, and steady and unsteady. Then there are all the different methods to model such flow, laminar, LES, DES, RANS, DNS, incompressible, compressible, low order, high order, boundary conditions, etc. Then there is implicit and explicit and all the algorithm mumbo jumbo which goes along with it. Sometimes, when listening to non academics who use CFD, it seems like it is one big tangled ball of string. And academics, in general, seem to focus on their region of interest and are forgetful of the existence of other regions. I have not seen a handbook (such as Hoerner’s book on drag) for CFD.”
Is CFD an Ivory Tower Tool?
An ivory tower is a world in which intellectuals engage in pursuits that are disconnected from the practical concerns of everyday life. While I bet each of us knows a CFDer or two who fit that description, CFD is used every day to obtain engineering information – that is, information that can make a product better, faster, or cheaper. Therefore, I have to say that CFD is not an ivory tower tool.
In fact, I think we might be asking the wrong question. It’s not the tool that’s in the ivory tower – it’s the people. The keys to successful CFD are not in the tools – they’re in you. For successful CFD you need:
- a firm foundation in fluids,
- an awareness of your CFD solver’s range of applicability,
- a familiarity with the device you’re simulating and
- an understanding of how to properly use the CFD software.
With all this knowledge, you’ll know how to get the best answers out of your CFD solver and how to best interpret them. As Harry Callahan said, “A man’s got to know his limitations.” The same is true for CFD.
Where Does CFD Uncertainty Come From?
The level of uncertainty boils down to whether verification and validation (V&V) has been performed. Verification is the responsibility of the code developer and ensures that the software is coded properly and produces accurate results for what I’ll call generic benchmarks.
On the other hand, validation is your responsibility and quantifies how accurate the software is for your particular application.
Let’s say your area of responsibility is the exhaust nozzle on a transonic fighter aircraft (which, coincidentally, is where my CFD career started). When you’re given a CFD code to use the first action should be to obtain documentation from the code’s developer about all the verification testing. This will include things like flat plate boundary layers and the NACA 0012 airfoil and backward facing steps – generic benchmarks. The second step will be for you to apply the solver to see if you can reproduce the verification results because then you’ll know you’re applying the software correctly. Next you’ll apply the software to a nozzle for which you have data – perhaps something proprietary or maybe an open benchmark. With everything you’ve learned, you’re ready to begin applying the software to new configurations.
Rather than repeat them all here, let me Google a few things for you.
Workshops, Meshing, and Uncertainty
The pursuit of ever increasing reliability in CFD is an ongoing activity exemplified by the Drag Prediction Workshop, High Lift Prediction Workshop, Propulsion Aerodynamics Workshop, and many others. They provide benchmark cases for comparison and through their multi-year efforts take a long-term look at how accuracy is improving over time. They are even pulling us mesh generation folks into the fray and asking us to quantify how good any particular mesh is. But that’s a topic for another post.
*Note: Harry Callahan was quoted directly, hence the use of the masculine. However, as we all know, women are CFDers too.