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.


Amazon Drone Delivery is Vaporware!

Amazon’s Jeff Bezos let on that they’re looking to start delivery by Amazon Drones in the near future.

This is a very cool, futuristic idea. However there are a few problems that get simply skirted over:

GPS signal commonly fails in Cities

Large cities have very crowded radio frequencies, not to mention all the interference with the signals bouncing off buildings. This can be remedied with improved antenna, but it is not a perfect solution.

GPS does not account for buildings.

Even if you have a VERY accurate GPS system to match the user locations, GPS does not know necessarily where building, or other interference may be. He claimed it would be great for city delivery, but think about the process of dropping a package off on a porch, there’s typically no direct vertical path to a porch. A drone would have to drop down from altitude, navigate below obstacles (like a roof) in order to drop off the package.

It would probably require a Pilot

One way around the variability of where the package is delivered is to have a Drone pilot. The US drone fleet for instance uses pilots for many tasks including takeoff and landing (although fully autonomous systems are being used). You could have an autopilot system, but that would require a whole host of sensors and processors, significantly reducing the range of the Drone.

I the technical problems of automating a drone like this is on the same order of having a self driving car. There is the technology out there, but it is currently too heavy for small drones. Bottom line is the delivery area and method is HUGELY variable.

A 10 mile range is dreaming with current technology

Given this is a few years off with the FAA 2015 thing, but at current rate, flight times for quadcopters of this size, especially large Hexa- and Octacopter drones are currently only 30 minutes maximum. 10 Miles (which is 20 round trip) is going to require a much larger battery capacity, especially with a 10 pound payload (copters are very sensitive to weight).

Not to mention your delivery area would be influenced by local wind conditions.

Not to say it’s not doable, but the Amazon Drones would require some pretty big advancements in sensor weight and battery technology before it’s viable.

The 2015 FAA Drone Rules are not yet Known

It is still a huge unknown. Currently drones are technically limited to a 400 ft max altitude and must be flown by line of sight. Once 2015 hits it’s not that it will be drone free for all. The FAA at this point only has an IDEA what it needs to figure out how to regulate drones. Knowing the FAA there’s going to be a whole host of not only airworthy requirements for drones, but Sense and Avoid systems, secured communication hardware, battery redundancy, and flight traffic integration requirements that are not yet known.

Basically the FAA could sink the Amazon Drone system before it even gets off the ground.


Drones will at some point come into our every day lives, the advances in battery technology, sensors and flight controllers are seeing to that. But it’s not as simple of a problem as Bezos makes it seem.


Some Perspective (not bike racing)

Awesome video, narrated by Neil Degrasse Tyson who is a generally awesome dude, about why we still need NASA and space exploration: to gain some perspective

It’s sad that the most visible thing NASA is doing now is transporting retired space shuttles around for cool photo ops:

In related space news Promethious continues to astound with how little Hollywood knows about how space travel actually works. Here’s a trailer…I’m going to warn you it’s pretty intense and scarry:


SpaceX Flight 2

Pretty big day today for space flight

SpaceX just completed the launch of their Dragon capsule into orbit this morning on a re-supply mission.  SpaceX is the first private company with the ability to achieve orbit (arguably, at least not driven by DoD budget)

The capsule still has to dock with the ISS and return to earth, so the mission is far from over, and the hardest parts of the whole endeavor: making space access easier, consistent, and profitable are still to come.


What was I thinking

I haven’t had a blog post in a while, and the past few posts I’ve had have been kinda junk.

I’m not going to say I was too busy or had no ideas for utterly awesome blog posts. Mathmatically optimizing riding a hilly TT is my current project post, there’s Matlab involved so it’s pretty intense (my matlab skills have pretty much withered into nothing in the past few months). I had ideas about making a tumblr-type blog just with crappy pictures of cycling (dime a dozon), an incogneeto blog where I wrote my “true” thoughts about amateur cycling (way to much of a landmine field).

Then I started thinking why I haven’t been writing anything for a while, besides the fact that the past 3-4 days I’ve had cold from hell. I started thinking about all the things I wanted to blog about and all the reasons I shot them down. I nixed them because I thought they’d be too inappropriate, no one would like them, not technically thorough etc. I started going through my old blog posts searching for inspiration and I realized that it wasn’t that I didn’t have any good things to write about it was that I was writing for the wrong person: not me.

As selfish as it may sound, hear me out. I went back to the first few posts and realized I wasn’t trying to convince or prove anything to anyone (it may sound like it but, that was just me spouting off…trust me I know such things). Instead I was just writing to get my thoughts and experiences on some permanent medium. To document them, in you know, a Journal.

Growing up I was obsessed with everything related to space, airplanes, and Star Wars. I went to Purdue for Aerospace Engineering and currently design aircraft engines. I only started mountain biking in middle-school but really didn’t start seriously cycling until college. In those past 6 years I’ve meet every single person that I consider a close or distant friend and cycling has become a permanent part of my life. So Rockets 2 Sprockets IS my life. Not this blog per say, but the entire ideal behind this blog is me trying to bridge the massive canyon between these two passions of mine.

So that is what I will now try and return my focus to: a blog/journal…or blournal. I will put down my thoughts and experiences about cycling and airplane stuff at least once a week. The posts obviously public because I’m happy to share my thoughts, experiences, and knowledge with everyone (except the Russians or Chinese…Export Control and all) in the hopes…no I actually have no hopes of anyone reading anything. If a person reads some posts of mine and finds them helpful or insightful, great, if not, I won’t care….anymore.