How accurate is our "cheap" CFD ?

[Tommy Cookers]

All aircrafts are design for cruise flight.
They dont care about wheels/gear aerodanyamics like F1 do ....
for example airliners dont include rotating fan at turbo fan engine when simulate plane flight...

3 claims - each wrong

and ....
Reynolds number similarity is big issue eg for airliner work (that's not an issue for F1)

[Uwe]

All aircrafts are design for cruise flight.
They dont care about wheels/gear aerodanyamics like F1 do ....
for example airliners dont include rotating fan at turbo fan engine when simulate plane flight...

3 claims - each wrong

and ....
Reynolds number similarity is big issue eg for airliner work (that's not an issue for F1)

all 3 claims 100% correct

Where do you see aerodynamics influence at gear?not even airfoil shape anywhere!
Totaly negleceted in aerodanynamics sense!

[Hoffman900]

All aircrafts are design for cruise flight.
They dont care about wheels/gear aerodanyamics like F1 do ....
for example airliners dont include rotating fan at turbo fan engine when simulate plane flight...

3 claims - each wrong

and ....
Reynolds number similarity is big issue eg for airliner work (that's not an issue for F1)

all 3 claims 100% correct

Where do you see aerodynamics influence at gear?not even airfoil shape anywhere!
Totaly negleceted in aerodanynamics sense!

https://insights.globalspec.com/images/ ... ype-hl.png

You’re entirely missing the point. 18 posts in... not a good look for you here.

[Uwe]

3 claims - each wrong

and ....
Reynolds number similarity is big issue eg for airliner work (that's not an issue for F1)

all 3 claims 100% correct

Where do you see aerodynamics influence at gear?not even airfoil shape anywhere!
Totaly negleceted in aerodanynamics sense!

https://insights.globalspec.com/images/ ... ype-hl.png

You’re entirely missing the point. 18 posts in... not a good look for you here.

once again,aviation dont care about gear aerodynamics..end

[Hoffman900]

all 3 claims 100% correct

Where do you see aerodynamics influence at gear?not even airfoil shape anywhere!
Totaly negleceted in aerodanynamics sense!

https://insights.globalspec.com/images/ ... ype-hl.png

You’re entirely missing the point. 18 posts in... not a good look for you here.

once again,aviation dont care about gear aerodynamics..end

They literally influence the rest of the design... they are deployed when flaps / slats are deployed, at speeds well north of a F1 car, and with them deployed, the plane has to be at its most controllable at landing which is arguably the hardest/most dangerous thing anyone on a plane does / experiences.

Sorry man, but I think you’re out of your league.

[Uwe]

You’re entirely missing the point. 18 posts in... not a good look for you here.

once again,aviation dont care about gear aerodynamics..end

at speeds well north of a F1 car,

Why do you think all the time that speed increase complexity??????

separted flow and turbulance make complexity, not speed..

[hollus]

Fascinating insight that the real cost of CFD might be in its calibration to the real world rather than in its calculation.
But ignoring the calibration for one moment... how many top of the line 2020 PCs are the equivalent to the rather meager (by the rules) TFlop allocation of CFD if an F1 team?

[Hoffman900]

once again,aviation dont care about gear aerodynamics..end

at speeds well north of a F1 car,

Why do you think all the time that speed increase complexity??????

separted flow and turbulance make complexity, not speed..

A plane has all of those things, but unlike a F1 car, they impact the control surface, and the aero sweet spot has to be good over a HUGE speed range, with different air densities, and winds that could reach a hundred miles per hour or more, and winds from all directions.

[hollus]

Why do you think all the time that speed increase complexity??????

separted flow and turbulance make complexity, not speed..

I think having to let go of the assumption that air is incompressible makes calculations more complicated. Isn't it normally assumed that it is OK to assume constant air density all the way to about 300 Km/h but not beyond?

[Shrieker]

I really don't know if I would be confident enough to fly in a plane designed by Ferrari's aero department

There is a famous crash of the XB-70 in 1966 where a F-104 got sucked into the wing tip wake vortex and crashed into it.

Hey I know this one. Think I watched it in one of curious droid's vids.

[Hoffman900]

Why do you think all the time that speed increase complexity??????

separted flow and turbulance make complexity, not speed..

I think having to let go of the assumption that air is incompressible makes calculations more complicated. Isn't it normally assumed that it is OK to assume constant air density all the way to about 300 Km/h but not beyond?

This is what makes engine CFD hard to do really well. A port could see .6 mach (.7 for a really fast one) average in the intake tract at peak (negative velocity near the valve opening and closing), you have pressure waves moving back and forth, fuel and port temperatures changing density, flow is not laminar, reynolds numbers come into play, etc, and it's all happening really fast.

[Hoffman900]

Bringing it back to F1 briefly, the limits on CFD times is going to reward the teams that have the best calibration from the real world > wind tunnel > cfd. RedBull and Ferrari have struggled with this, Mercedes has been very successful with this. If teams use up their CFD time allotment chasing down correlation issues, then they'll always be on the back foot.

I have no doubt that Mercedes (and everyone should for that matter, it's an obvious problem) has been investing heavily in making sure their wind tunnel and the base of their models are good, as they head towards more CFD restrictions.

[Vyssion]

Seem's I'm a little late to the party here!! Let's try and address a few posts I've seen which provoked conversation from others. Again, any more questions, feel free to ask.

How accurate is our "cheap" CFD ,I am talking about ours personal computer power..not super comupters from F1,Boeing,NASA,American cup,car industry etc...
Can we rely on this data and how much men who working with CFD can make wrong job?
Does results form CFD depend on men knowledge about aerodynamics and CFD-job or this is just depend at softwear and computer power?

Simply put, it's a case of "shît in, shît out".

90% of the challenge of doing CFD is to ensure that your CAD is cleaned up as best you can, and that you have simplified geometry in a way in which is logical and allows for you to be conservative with mesh cell count in areas of low interest, and therefore use those cells in areas of higher interest.

When I design or surface something up, I am quite fortunate in that I tend to also be the person who is going to be meshing it; and so that allows me to build the model in ways in which complement where I know that I will be placing cells to capture the aerodynamics that I need to check out. It can be painstaking work at times, but I quite enjoy hand-stitching my surface meshes to the highest quality I can reasonably get, and only then allowing the volume mesher to inflate and fill the domain.

Most of the time with what I would call "coarse CFD", the results I get out are within 5-10% of our testing data. When I create our normal kind of "workhorse mesh" (call it a "medium CFD"), most of the time, its sub-3% away, depending on what I am looking for. However, when I need to correlate to wind tunnel or actual test data, <1% is most definitely always the minimum expectation, and can be done without ridiculous total cell counts.

It is important to note that the final "number" isn't always what I am looking at when I run a sim. For example, if I was comparing how some newly designed winglet functions whilst under sideslip (e.g. when cornering), then I can actually learn quite a lot by running a set of coarse sims across a beta sweep, and then plotting those performance coefficients to see what the trend in the data suggests. Since they are all compared to one another, and therefore have the same "lack of accuracy" innately, I can draw quite well founded conclusions just looking at deltas, rather than the numbers themselves.

The tool only is as good as the person using it. If you use it and have no clue what you are actually doing, it won't yield desirable results.

I think a home computer(albeit a high-end one) is very much capable of doing CFD, and providing correct results. However, you'll have to do concessions on the detail of the model.

Absolutely agree - you can have the best of everything around you, but if you don't know how to really extract the most out of what you need, then you will not be able to get decent results out of it. Just ask some of my colleagues who I train up!! ... Also check this thread to see that some people, like myself, do have quite a beast of a home computer to play around with viewtopic.php?p=895178#p895178

One day I will get around to updating the Perrinn model and running it on this puppy........... @jjn9128 plz no hurt me

That seems optimistic. 1% is for what kind of design? A glider or a F1 car? How do you know what 1% is if you have nothing to go off of?

From an engineering side, just throwing a general value of 1% out there is kind of reckless without qualifying it.

See above, but <1% is very much attainable. Most teams are able to match their CFD to wind tunnel pretty much bang on, and same with track -- though sometimes a constant delta to drag might be applied, but that delta is very well understood and allows for very good predictions to be made regardless.

I think F1 aerodynamics is far more complex than in aviation.
F1 has lots of aerodynamics sick complex parts that aircraft not need..

So... This is kinda an interesting one... Because the two are mutually opposed in what their "goals" are.

F1 cars generally want to design for maximum downforce under cornering, whilst ensuring that the drag on the straights doesn't massively hamper the top speed the engine can propell the car to. And then trading points of downforce up and down depending on track layouts, running order, lap sim targets, etc. So you have this situation where you can continue to add downforce (forgetting for a moment that drag comes with it for the sake of this argument), and continue to go faster.

Aircraft, on the other hand, need to generate enough "negative" downforce (i.e. lift) in order to maintain 1G flight (i.e. weight force of aircraft). Now, there of course is more to it than that (climb, descent, gusts, manoeuvrability, etc.) but for the most part, whilst an F1 car spends most of its time cornering, an aircraft spends most of its time cruising at 1G flight. Meaning that your goal is to generate 1G of lift with as little as possible drag. And it actually is an extremely difficult thing to do past a point hah.... One only needs to look up something called "excrescence drag" and attempt at calculating the Cd0 increase due to a rivet head on an panel, or a slot gap on a door, and you can see how this pursuit of "ultimate efficiency" really comes down to the nano-detailing control of the boundary layer over the wing -- and let's not forget about transonic flow either....

Their goals are different, but I do concede that F1 cars produce a lot more "exciting airflow features" and so in terms of "complexity", there is definitely a lot more just going on, but their goals are very different, and come with their own challenges.

All aircrafts are design for cruise flight.
They dont care about wheels/gear aerodanyamics like F1 do, F1 wheels are critical part how airflow will moving around car and has great influenece of car performance.

Landing gear actually is a huge thing that has to be CFD'd. In fact, LES is used almost exclusively for landing gear simulation due to how much dirty air is thrown off of those things. Again, the goal of aircraft is to minimise drag, and so folding up the landing gear is a no brainer to do that. But when you have to deploy it to land, the vibration from vortex shedding and drag increase is huge - but it is only used for a short amount of time. And so if they were to shroud it, sure... it would be less draggy... but you have to remember that for 90+% of the flight time, they would be carrying the excess weight of that shroud in cruise -- which means more lift produced, so more drag, so less efficient overall. I can assure you that those calculations do happen. Even down to why planes are painted white -- the paint has less pigment in it, and so it is lighter. You could be forgiven for assuming that polishing the aluminium without paint is best, but the maintenance cost of that (having to wash it and keep it polished so it doesn't corrode) vs. the extra fuel that the plane burns by carrying the white paint weight makes it not worthwhile.

I think having to let go of the assumption that air is incompressible makes calculations more complicated. Isn't it normally assumed that it is OK to assume constant air density all the way to about 300 Km/h but not beyond?

For the most part, around 0.3 Mach, air only has ~1-2% compressibility. But up around 0.7 Mach, that increases to ~10%. Compressibility is technically "harder", since its another thing to solve for, but where the tricky bit comes in is where there is local flow acceleration over a wing, whilst travelling at transonic speeds, and then that acceleration bridges the final 0.1 - 0.2 Mach to make it supersonic, and then create a shock front on the wing's surface... that is baaaaaaaaad. And its actually why a lot of airliners use quite a high degree of wing sweep. The sweep means that air closer to the fuselage hits the leading edge stagnation first, and so that creates a higher pressure, which bleeds out to the tips (where the air isn't stagnating yet) and pushes the flow slightly towards the wingtip.

Over enough of a distance (like a wing), you get air travelling at a slight angle over the wing shape due to this "spanwise pressure gradient". What this does, is mean that the air doesn't see the textbook perfect aerofoil profile, but rather a "slice" of the wing that is slightly different, but more importantly, longer in chord. This means that the air has to travel a longer distance before it reaches the trailing edge, which helps to kinda "slow" the air's acceleration down a little, so you alleviate some of the tendancy to reach supersonic speeds.

The downside to this is that because your air is travelling a longer distance, your boundary layer thickens up more than it otherwise would, and thus can become prone to stalling at the wingtip first, which is really bad. It causes a sudden jump in drag force, right out at the wingtip, far from the CoG of the aircraft, so you have basically a giant lever that can yaw the plane around to one side, which then causes the other wing to generate more lift cause its travelling faster, so it climbs vs. the other one, and <insert flight dynamics theory here> eventually leads to a catastrophic flight mode, that will make the plane crash if not corrected for. So work goes into that area to prevent it (not to mention the sharklet designs too).

[jjn9128]

Big boss has entered the chat

[Tommy Cookers]

All aircrafts are design for cruise flight.
They dont care about wheels/gear aerodanyamics like F1 do ....
for example airliners dont include rotating fan at turbo fan engine when simulate plane flight...

3 claims - each wrong

and .... Reynolds number similarity is big issue eg for airliner work (that's not an issue for F1)

all 3 claims 100% correct

(so-called) low-speed wind tunnels are where the real work is done
LS means 'aerodynamically-big' for high Re similarity (necessary for approach and takeoff AoAs)
this costs big money

the EU has funded noise reduction programmes for 20 years (undercarriage/'gear' noise is a big part of approach noise)
ie RAIN, SILENCER, TIMPAN, OPENAIR, & more recently AFLoNext
eg at DNW (speeds to 78 m/s - noise at 6th power of velocity) - viable mesh fairings and blocker plates were identified

and .....
both DNW and ONERA advertise TPS capabilities (turbofan and prop systems incorporated in the WT models)

other 'low-speed' wind tunnels are available .....