Computational fluid dynamics

[tartare]

A question regarding wind tunnel testing.
Please forgive my ignorance regarding aerodynamics.
I seem to remember reading that despite CADCAM now being the norm in aircraft design, at some point you always have to test a model in a wind tunnel, because the variables of airflow are so complex that no supercomputer can yet entirely simulate them.
Given the exponentially increasing speed and power of computers - how far away are we from being able to design and test aircraft entirely as designs inside a digital wind tunnel, and go straight from a digital 3D model to full scale flight ready prototype?

[The African Dude]

Any computer model needs to be verified for errors by comparison with hand calculation, test data. Think about cost/benefit ratio, risk assessment processes and how difficult things would become without such model verification?

It is often a trade-off between scale and accuracy as far as a CFD/FEA model is concerned. An overview of the airflow can be obtained through a coarse model, but for really accurate results this model must be simplified to specific areas with certain assumptions made. You can make many smaller, more accurate, models by taking the results of each calculation, insert them as boundary conditions in the model for the next bit of the structure and so forth to enable assessment of the interaction downstream of aircraft components.

However, how do you assess for small errors in the calculation? These would be passed along the model from calculation to calculation.

My answer to your question: it depends on who's funding the project and how accurate they want their results to be... :O

[Mad (Flt) Scientist]

Given the exponentially increasing speed and power of computers - how far away are we from being able to design and test aircraft entirely as designs inside a digital wind tunnel, and go straight from a digital 3D model to full scale flight ready prototype?

Quite a long way.

CFD simply is not capable today to model the complex flow mechanics which are critical for things like stall characteristics. If I'm honest, wind tunnel models aren't always so great at that either. But the difference is years and years of experience in interpreting the implications of wind tunnel data for the real world. Engineering is an inherently conservative endeavour, especially when put against the kind of scrutiny that certification of an aircraft implies. So it'll be a long time before CFD reaches the same capability as wind tunnel testing, and even longer before its trusted to the same extent.

The state of the art today (with improvements which have occurred in tunnel testing, not just CFD) is that you can often get results faster fom a WT test, even accounting for build times etc., than you can for CFD. Sometimes its eve simpler and cheaper to just flight test something!

[18-Wheeler]

The state of the art today (with improvements which have occurred in tunnel testing, not just CFD) is that you can often get results faster fom a WT test, even accounting for build times etc., than you can for CFD.

Perhaps you should talk to some of the Formula One teams - I think it was Renault that showed Airbus how to perform some CFD work about 30 times faster than they could before. And whilst F1 doesn't do a lot of work with stalled aerofoils they do a lot of work on regular airflow and vortex generation and control.

[archae86]

I believe that Scaled Composites (Burt Rutan's shop in the Mojave) has customarily not been purchasing wind tunnel time for some years now. Last time I read a story on their use of CFD, it was running on PC's. Before you laugh, recall that the most ordinary of modern PC's exceeds in computation power the "supercomputers" available to design ALL of the nuclear weapons currently deployed by the USA!

Not to say even they did not see the need for some physical testing once: in resolving an issue that turned up on SpaceShipOne, they famously attached a test shape to the Ford F250 pickup truck of a member of staff, and ran it down a runway to collect the data they felt they need to make/confirm the needed adjustment. They themselves describe the issue as flow separation inadequately modeled by their CFD.

For a brief account from Scaled, see this web page (http://www.scaled.com/projects/tierone/spaceshipone_ground_tests)

[Genghis the Engineer]

The differences there are that F1 or Scaled will go fairly quickly into vehicle/flight testing and get their data there, also that neither is certifying anything to carry fare-paying passengers.

The light aeroplane world also does relatively little either CFD or wind tunnel work - for that matter they still fairly routinely will do only basic hand calculations or single-operator FEA then build and carry out physical load tests.


It really is an economic decision - you'll always need some combination of analysis, model/sample test, and full scale test on both structures and aerodynamic features. The scale however tends to dictate the proportions.

At the microlight/ homebuilt level, a couple of engineers will probably spend a few daysor weeks with Excel or a basic combined FE/CFD package such as Solidworks working through the main cases, then they'll build two, test one structurally to destruction, and start flight testing the second. At that scale, build and test is usually much cheaper and faster than analysis.

At the airliner level, a massive team of engineers with a cutting edge package such as Catia will analyse every tiny component to every possible case - then the most critical cases will be tested to confirm the analysis is correct. At that large and complex scale the thousands of man-years and millions of pounds on computing time are still far cheaper and faster building and testing.

Structurally also the increasing use of composite structures makes structural strength estimation incredibly tough to get right. In tension it's easy enough, but in compression where the strength is a function of the microscopic-level buckling loads of patterns of fibres that just can't be laid with that sort of microscopic precision in the way that you (relatively) can with metal structures, it is really really hard and you have no choice but to do a lot of testing. Another factor very very hard to model is rate of load onset - so for example undercarriage drop tests are still done at every scale of aeroplane, and probably always will be.

Regarding wind tunnel work specifically, the simple stuff - lift and drag can pretty much be done by analysis. The difficult stuff is behaviour around stalled components where the point of boundary layer separation is incredibly hard to model, anything involving vortex street creation (and thus flutter) which is equally tough to get right because it's so dependent upon very complex shapes and Re/viscosity/Mach interaction effects, and the detail of stability derivatives that lead to prediction of flying qualities - or increasingly determination of control laws for the AFCS.

So in a nutshell, I don't think you'll ever get away from testing - but the more large and complex the project, the more you'll see companies trying. And there will always be a point where the cost of the computing means that it's cheaper to build and test something than "just" do it on computers.

G

[Mad (Flt) Scientist]

Perhaps you should talk to some of the Formula One teams - I think it was Renault that showed Airbus how to perform some CFD work about 30 times faster than they could before. And whilst F1 doesn't do a lot of work with stalled aerofoils they do a lot of work on regular airflow and vortex generation and control.

You missed out my subsequent sentence which (typo corrected ;)) was:
Sometimes its even simpler and cheaper to just flight test something!
That's where the F1 and Scaled sit. F1 teams routinely do a lot of wind tunnel tests (which negates the point of the OP - they are definitely not replacing WT by CFD) and Scaled are very much a prototyping company. Almost no-one above a certain size makes prototypes any more - test airliners are really pre-production aircraft. The cost involved is such that if you're forced into any kind of significant aerodynamic design change once you're in flight test the whole programme is badly in trouble.

There's also an FAA Ac 9or maybe draft AC, I'd have to go digging) on the topic of flight testing for antennae etc., really aimed at STCs, which sets a significantly high bar to the use of CFD to replace flight test, such that flight test is more-or-less mandatory. (The Part 25 regulation in question, 25.251, actually specifically requires flight test as the means of compliance with the reg)

[18-Wheeler]

That's where the F1 and Scaled sit. F1 teams routinely do a lot of wind tunnel tests (which negates the point of the OP - they are definitely not replacing WT by CFD)

I didn't miss it, I was just adding a comment that I thought might help.
The F1 boys certainly do run big wind tunnels full-time but only to test models that have extensive testing through CFD. It only takes a short amount of time to change a model with CFD, it takes a lot longer to build a perfect scale model then run multiple tests in a real tunnel.
Both are needed, the CFD cuts down the overall time, effort, and cost by quite a lot.

[tartare]

Thanks guys - very insightful.
Interesting to hear about Scaled - only increases my admiration for them.