Wind Tunnel vs. CFD

So after getting a prototype made up we finally made it to the wind tunnel yesterday up in Charlotte. The A2 wind tunnel is a bit of a commercial wind tunnel, renting out wind tunnel on a time basis. If you read any cycling publications you’ve seen its fans as backdrop for any number of “Aero” tests. It was a really nice facility. It was an open circuit wind tunnel that was sized almost perfectly for bicycles (although maybe a bit small for cars). It was a really professional facility.

We ran our prototype vs a few of the top aero wheels currently in the US with VERY good results, but we can still use a little refinement.

A2 Wind Tunnel, Aero Rim Prototype

Our Uber Lightweight Prototype Rim in the Wind Tunnel

On the way back from the wind tunnel I had some good car time to think about why my CFD was missing some important things that we were seeing in the model. That’s when I stopped at a bike shop and happened to start talking to a bike builder and we got on the topic of CFD and wind tunnels. I mentioned that we did CFD analysis on a bunch of rim designs before heading to the wind tunnel with a prototype, he looked at me incredulously that we would even consider wasting our time at a wind tunnel when we could just model everything in CFD.

I just about threw a chair at him.

Yes computers now are very powerful, and a lot of CFD out there is VERY good at approximating scenarios. But, CFD will never replace a good wind tunnel test, ESPECIALLY with something like bicycle components. There are a few reasons for this.

First of all is Reynolds number similarity in the wind tunnel, which basically means matching accurately simulating flow phenomenon in a wind tunnel. Essentially you want your Reynolds Number and Mach number to be similar between your model in the wind tunnel and real life. For aircraft and rockets, this is very difficult. You cannot put a 1/50 scale aircraft in a wind tunnel and run it at flight speed, that would reduce your Reynolds number by a factor of 50 relative to real world, changing all the flow phenomenon. Your alternative is to increase airspeed by 50 times, this obviously is very difficult to physically do for any flight type air speed, but it will also entirely change the mach regime your model is in, rendering your model useless. Bicycles on the other hand are VERY easy to put in a wind tunnel. You can put full sized models in a relatively small tunnel, then run them at real world air speeds, giving  you essentially real world drag results.

Second is that fluid flow, specifically turbulence is INCREDIBLY complex:

“I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic” - Horice Lamb (1932) British applied Mathematician

 

“Perhaps the biggest fallacy about turbulence is that it can be reliably described (statistically) by a system of equations which is far easier to solve than the full time-dependent three-dimensional Navier-Stokes equation”  - Peter Bradshaw Professor of Engineering @ Standford University

It might seem that bicycles, traveling at a relatively low speed of 20-30 mph are simple things, but the fact of the matter is that there are a ton of very complex things going on in the bike system. Take spokes for instance. They are not simply wires traveling through air, they are rotating, and they are small relative to the rest of the system. This means that they create entirely different types of flows than the rim. There’s also the interface of the rotating wheel and the fork, the stationary ground. Don’t even start on the body of the cyclist (which accounts for 80-90% of all drag anyways).

The solution to this is approximation. In every field of complex engineering we try to break up the model in to manageable parts, then figure out the interactions between these parts. Currently I’m modeling the rim shapes in a 2 dimensional domain. This has some very obvious shortfalls, but to run an accurate model of a 3D rotating wheel (just the front), is beyond the computational power that I, and probably most bike companies, have on hand. It is a light model that allows us to run through a relatively large design space with essentially a souped up gaming computer. That’s one of the reasons why we go to the wind tunnel in the first place, to refine the CFD. Yesterday I found out that my model was predicting the performance of some of the wheels incorrectly, and now that I have other data to reference the CFD against, I can refine the model.

There’s nothing wrong with approximation, it is how design is done. However if you forget that the model within your computer is just a model, you will make missteps or worse. CFD a.k.a Cleverly Forged Data, Colorful Figure Delivery, etc is great for marketing, it shows that you have made the steps to improve your product methodically. However it is very easily altered to suit your needs, I can make a CFD model show that our product is better than brand X easily with the right assumptions and models. You will never have a fully realized fluid model of a bike or any other complex design, in sports or aerospace. In order to close the gap between the simplified CFD world we work in and the real time gains on the road, the wind tunnel will never be replaced.

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GP Saguenay Wrap up

The rest of Saguenay was more of the same. Since there was no Time Trail in the race or other decisive stage every stage came back to a field sprint.

The second stage, on paper, was easier than the first. However with a stiff cross wind just after the climb things were very tough. I was aggressive early, but it was pretty clear that things weren’t going to go easy, especially since Optum wasn’t going to try and defend the yellow jersey. My own race was made slightly more difficult when my front derailleur cable came loose. I dropped back to the car to tighten it up and got back fairly quickly. Unfortunately that was just going into the cross wind section when the field decided to split. And by decided to I really mean there was a large crash that split the field. I was gaped off by another rider ahead of me (didn’t quite get back front enough before we made the turn) and had to chase for a few K’s. When I got back to the group I saw that we had largely missed a 20 rider split, so we chased and brought back the group fairly quickly the next time up the climb. We then missed another smaller counter attack. We through in our hat to the chasing game again to bring the group back for Brecht to sprint. Going into the sprint we sort of had it lined up. I sprinted into the last corner that was at 800 meters to go and got into probably 10th wheel. Unfortunately Brecht had dropped his chain at that point, I sprinted in sitting down.

Saturday was a downtown criterium that was more of a circuit race. A 4 corner ordeal that was 2 km long, it wasn’t the most technical thing on earth, but even on this tame of a course those who could crit were quickly separated from those who couldn’t. There was a bubble up front of everyone racing (Mostly the American teams), a single file string of riders trying to race, then the rest of the guys just riding it in. I got into a big 15 or so rider move with Ryan and Brecht. We were riding pretty hard and got a decent gap, but the group was too big and unwieldy with too many passengers. We tried attacking the break to split things up a little to no avail and we were caught with about half the crit to go. We missed a 2 man move that went up with Eric Marcotte. We chased (again) and helped Optum bring it back. Stephen and I again tried to lead it out for Brecht but we got a little shuffled in the finish. I also took the front with 2 to go, which in retrospect was WAY too early (plus we were gassed from chasing).

The final stage was only 90 miles (so short) but had the most climbing of any of the previous courses, pretty much either climbing or descending for the entirety of the 6 mile course. I bridged across to a 3 man group with two Columbia riders and a Silber rider. We rolled through the first sprint point and lost one Columbia rider. Our 3 man group rode ok together, I may be wrong but I definitely felt like I was investing more energy into the move than the other two (probably a litte too enthusiastic on my part). Regardless we opened up to 2:30 over the field, until the field actually started racing and split, bringing down our gap fairly quickly. We got through the second sprint and I took it, then the following KOM, but we were caught by the top of the lap (KOM was only half way up).

Once we were caught I felt great initially and immediately followed some attacks (poor choice in retrospect), then quickly started fading after that. At that point with my bonus seconds I was sitting 7th overall in the race!!!! I had glimpses of stage race glory. That was until I came totally unglued with 2 laps to go. My cause was hurt by hitting a moderate pothole with a few laps to go that ejected my Garmin to somewhere in Canada (or picked up by another shady Euro team), and jacked up my handlebars into a old school 1980′s setup.

Overall a good experience. I felt like I raced more in this race than any NRC, UCI or whatever high level race I’d done before, on some particularly hard courses. Def a confidence boost. Maybe if I had raced a little smarter I would have actually achieved a result, but at the same time if I wasn’t aggressive I may not have gotten in that final break and gotten those bonus seconds. Better to be aggressive and fail miserably than be anonymous, I learn better that way.

The team also raced really friggin well. Not only were we super aggressive the entire time, but when we missed the moves we owned up and chased. We changed the outcome of the races nearly every day instead of being just passengers, it was awesome.

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Quebec Update

It’s been a few important races since I’ve updated: US Pro, Philly, and the first stage of GP Saguenay.

Pro Nationals wen’t a little worse than expected. Going into it I felt like I was just getting over some allergies or a cold or something, but felt alright at the race start. However as the race wore on and the strange tactics of the day played out I felt progressively worse. A large group got away in literally the first 2 km of the race and got a 6 minute lead in about 6 minutes. The group rode fairly easy, except for the climb, when Garmin blew all their riders setting pace for no one up the climb, then sitting up on the flats again (no idea what this tactic accomplished except disrupting any chase). I made it 3 big laps of the race, which is about the same as last year, however since I felt great last year I’ll call it progress.

Philly was an awesome race, and has the biggest spectator fanfare of nearly any race out there. I was tasked with following early stuff in hopes of a breakaway, but was a little too active and missed the actualbreak some 3 laps in. Fortunately Clay jumped across to it. I was feeling ok but a little blown on the last lap when Brecht got a flat on the final downhill of the race and I had to give him my wheel. There was no hope of chasing back on so I pedaled in pretty easy, having at least 3 beers between Lemon Hill and the Manayunk Wall.LemonHillPhilly

After that it was a flight up to Quebec City and then transfer to Saguenay for the GP Saguenay a UCI 2.2 race. We spent a few days in Quebec City….which is friggin awesome btw.

GP Saggy is a bunch of circuit road races with one crit thrown in. Yesterday was the first RR 11 laps around the town of La Baie. The race started right up the climb, which peaked out at 20%. The first time over I got into a group of 38 riders with Stephen and Matt. It was a little too big of a group and a few too many people just sitting on, and we were brought back by lap 3.

The counter attack over the climb stayed away for a few more laps, but got back with 3 laps to go. Nothing serious got away after that. The 3 of us that were left got it lined up pretty good with 3km to go but we got pinched and separated.photo 1 photo 5

Today’s more  of the same, 21 km laps, about 177km total on tap for today.

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Wilmington Grand Prix

It’s been a little while since I’ve had a post. Speedweek was a whirlwind of racing with our team coming into the second step on the podium (I also genuinely felt shitty most of speedweek due to allergies or a cold or something that did not allow be to breath without turning on a faucet of phlegm).

Astellas Cycling Speedweek

2nd Overall Team at Speedweek 2014

This past weekend was the NCC Teir 1 race: the Wilmington Grand Prix. I had never done the race and never been to Delaware. Katie and I hitched a ride up with Thomas up the Thursday before in hopes of doing the Monkey Hill Time Trial: an 8 minute TT that had a cobbled finishing climb. It sounded awesome, but heavy storms cancelled the race. It was just as well since our 5 our second part of the drive took 8 hours, barely giving us enough time to do our pre-race openers in Wilmington.

This was my first time at Wilmington and it definitely won’t be my last, this was my kind of course: 8 technical corners and two hills per lap really sifted the race out. We only had 4 guys there, Thomas, Brandon, Michael Pincus, and myself. Brandon just won the Collegiate Nationals D1 Crit and Thomas and I were both fresh off Speedweek so we felt pretty good about just being a reduced squad.

I got a call-up for being awesome, which helped out a lot due to the technical nature of the course. It wasn’t a long race so aggression started early. Pincus got into an early split that was brought back after a few laps. There was a counter with 2 riders that dangled for a few laps. I attacked across to the two along with another rider. Eventually our group swelled to 10+ riders including UHC and our meager gap suddenly exploded to 30 seconds. One of the UHC guys dropped back to I guess help chase our group of now 10 back, leaving Brad White to just sit on the back of the group.

Our group was working not great but well enough together that UHC was unable to chase us down. Their efforts were further hampered by their chase train crashing in corner 1 with 8 laps to go.

I was feeling pretty good and thought the group could split, I countered some of Bobby Lea’s attacks but things were brought back (Guys….LET THE UHC dude chase people back, CMON!!!!). Anyway we went into the last lap as a single group and were diddling around to the point where the field was bearing down on us.

On the back side of the course Champ Sys were the only team to lead out since they were the only team with 2 riders. The back side of the course was a gradual downhill leading into the second to last corner (which had a car sized pothole), then lots of traffic furniture, before a brick laden final corner to a long long uphill drag to the finish.

I did a decent job jockeying into the 2nd to last corner but got a little pinched by the Champ Sys lead-out guy swinging off. I got around him before the corner, but the gap had already opened. I dodged some traffic furniture and opened up my sprint. While I have a decent sprint that can bean non-sprinters, my sprint is not enough to beat many real sprinters. The order coming out of the last corner did not change. I kept the same 5 meter gap from Bobby Lea’s rear wheel all the way to the finish line for 4th, a decent result, but no podium.

My consolation prize was getting drug tested by USADA right after the finish. I’d never gotten tested before and it was a fairly interesting experience. I chugged about 8 bottles of water to the point of getting light headed in order to pea. In the end it took me an hour and a half to pee in front of USADA officials and finally head off (they were really super nice, if only those waters were beer!).

After Wilmington the girlfriend and I met up with my old Purdue roommate Dave and headed up to his place in Philly, which was at the top of the Manyunk Wall (Oh em Gee, Philly). Manyunk is a huge bar district so we had to um…scout things out that night:

The next day Dave and I rode parts of the Philly Classic course, I was pretty much just losing my shit the entire time (every time we’d get on Kelly Drive, I’d get on my super deep announcer voice: “Back on Kelly Drive”).

After that it was MegaBus up to NYC to visit my Bro and Parents, my

Best Race Flyer Ever

Best Race Flyer Ever

Bro’s graduating from Columbia’s Journalisim school with a Masters degree. We missed mos to the ceremony for work and what not but it was cool to hang around NYC and see the family.

Litterally the only existing picture of me in the race

My recovery day Monday consisted of renting CitiBikes and doing a lap around Central Park before the 30 minute time limit ran out (we failed miserably).meandtheGF

The Brains and The Brawn

The Brains and The Brawn

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