What About Wind Tunnels?
How does testing in wind tunnels help in the design of aero bikes, and which tunnels do you use?
We get a lot of questions about how we measure the aerodynamic effectiveness of a bike and what the importance of actual wind tunnels is in an age when software can mimic some conditions we see in the wind tunnel. It's definitely interesting and we've spent more than 15 years developing the protocol we use in the wind tunnel and investigating aerodynamics and computational fluid dynamics (CFD).
Aerodynamics is the study of forces and the resulting motion of objects through the air. In our case, the object refers to the system comprised by the bicycle and accessories plus the rider.
An everyday example of the forces involved happens when we put a hand out the window of a moving car. We can feel the forces dragging our arm backwards, as well as variations in magnitude as we change the orientation and shape of our palm. If you do this at 50 km/hr and imagine your whole body immersed in the flow, you would feel what an elite rider experiences in a time-trial stage. It is clear that the less resistance there is to the motion, the faster the rider will be. Jim Martin wrote the defining paper on the topic and we've referred to it often over the past few years.
Cervélo discovered very early on that an aerodynamically efficient bike yields winning results. For this reason, we have invested heavily to create a very strong in-house aero department comprised of a full-time aerodynamicist and a consultant with many years of experience in the field.
In addition to having the right personnel, part of the investment has involved acquiring the very best tools for aerodynamic research and development. The three basic tools are road testing, CFD, and wind-tunnel testing. Each method has its own advantages and disadvantages.
Common sense should dictate that road testing would yield the most realistic results, since it is tested with real products, real riders, and under real conditions. However, measuring the drag on the road is very difficult given random variations such as rider position, system vibration, varying ambient conditions, varying road conditions, etc. In addition, there is no actual bike to test when the bike itself is being developed.
Consequently, road testing is limited to improving individual rider performance once the bike has been designed. We are fortunate to have direct access to professional athletes who provide constant feedback. That helps us speed up the development time.
The second tool is a relatively recent phenomenon introduced to the design process of aerodynamic bicycles, and we at Cervélo are pioneers in using this technique. It is a branch of science known as CFD, which uses computer power to provide simulations of fluid flow.
The primary benefit of CFD is the ability to improve and modify a bicycle's shape before it is built with the parallel use of computer-aided design (CAD). It is very easy to test several ideas, conservative or innovative, in a virtual environment to determine which one of them justifies rapid prototyping for further investigation in the wind tunnel. The main disadvantage of CFD is that a full model simulation with a complete bike, plus the rider, is computationally expensive. For this reason, we have devised a methodology to use these two tools to their full potential, yielding better bikes and a shorter turnaround time.
The third tool used at Cervélo is the venerable wind tunnel. It allows test conditions to be highly controlled and independent of the external atmospheric conditions, since the test model stays stationary while the air moves relative to it.
The main tunnels used by Cervélo are the San Diego Air & Space Technology Center Low Speed Wind Tunnel (LWST) and Kirsten Wind Tunnel at the University of Washington Aeronautical Laboratory (UWAL), which are closed-circuit and permit full-scale testing. These low-speed tunnels are perfect for our applications. (Low speed wind tunnels are used for operations with very low Mach numbers, with speeds in the test section up to 400 km/h, or mach 0.3.) In fact, these two tunnels are regularly used by other high-end sports applications such as skiing, bobsledding or golf, as well as for ball designs in soccer or volleyball.
In general, air is blown by one or two fans, and the highest speed is reached at the smallest cross section, commonly called the test section. Ahead of the test section there is an inlet contraction that directs the flow smoothly into the test section, with the objective being to obtain uniform velocities and flow characteristics in the test section. The ratio between the inlet area and the test section is called the inlet-contraction ratio. Larger contractions ratios usually result in more uniform free-stream conditions in the test section. The diverging section behind the test section (diffuser) reduces the speed ahead of the fan.
Like automobile aerodynamics, there is a need to simulate the rotation of the wheels in order to mimic true flow-field conditions. For this reason both LSWT and UWAL provide means to rotate both wheels. Another feature of both tunnels is a raised test plate or fixture, which is utilized to minimize the effect of the ground boundary layer, making the flow more realistic.
In addition to testing the bike on its own, we have a second way of testing the aerodynamic performance of a bike: the bike/rider system. The problem with involving real riders is that it is very hard to maintain the same position for the duration of a test. In fact, we can argue that it is practically impossible, since for us a normal wind tunnel trip lasts a full two to four days. We have circumvented this problem by scanning Dave Zabriskie and manufacturing a full scale copy of him in high density rigid foam, which you see above (we call it simply “DZ”). With DZ we have the ability to test bike/rider models from day to day with a high level of accuracy and repeatability.