|Skinsuits and Boundary Layers|
What do Skinsuits have to do with Boundary Layers?
1 Prologue – The Tour
A week ago Saturday, I sat in front of the TV for a solid two plus hours. I had been looking forward to this self-indulgent, yet taken for granted, treat for nearly a year. It wasn’t so long ago that all of us had to wait a week to get any coverage of the Tour de France – even then, it was watered down and tainted with John Tesh-ian keyboard rhythms and Phil Ligget trying really hard to be a wordsmith, rather than a spirited bike race announcer. Who could forget the ‘87 Tour coverage of the Ventoux TT stage – “How far… How high… How fast…” The images of Jean-Francois Bernard suffering up that climb with Phil doing the voiceover haunt me to this day.
Nowadays, we can turn on OLN at any of four times during the day to catch all the tour action. And so it was last week, during the opening prologue that I noticed something peculiar when Ekimov’s ride was briefly covered by the live feed. Something seemed a little off about the skinsuit he was wearing. The shade of blue was a little darker. The same Postal, TREK, and Nike graphics weren’t all there. Then I noticed that the seams along the side of Eki’s torso trailed off into the center of his back. This all seemed a bit odd and I figured Nike had been up to something in the wind tunnel during the off-season. Sure enough, reports from several online media sources, OLN’s Frankie Andreu, and an official Nike Press Release confirmed that some serious wind-tunnel time had indeed been invested in the Emperor’s new time trial clothes.
The new skinsuit that the US Postal Service squad put into use in the opening prologue and yesterday’s ITT isn’t even really a “skinsuit”. According to Nike, the clothing is called a “Swift-Spin Body Suit”. The technology is a carryover from Project Swift, which began in 1998 as an attempt by Nike to improve the performance of runners during the 2000 Summer Olympic Games. Swift body suits were developed at that time to improve the aerodynamics and muscle performance during the track and field sprinting events. Similar suits were again used during the 2002 Winter Olympic Games in the speed skating events. It has been claimed that during the skating events, the suits improved performance by approximately 0.9%.
Now, 0.9% may not seem like very much, but during an hour-long effort like the TT yesterday, this could mean timesavings of nearly 35 seconds. If this magnitude of effect were real, and if Lance would have used this new skinsuit from Nike, he would be in the yellow jersey today. Maybe we will see Lance donning this peculiar new suit in the next ITT.
These what-ifs and hand wavings are nice and all, but what kind of aerodynamic fundamentals is the whole concept of the suit based on. Nike says that the suits benefit the athlete based on the same reasoning that golf balls benefit from dimples. This can mean only one thing – Nike is claiming that cyclists can reduce aerodynamic drag by controlling the boundary layer.
Furthermore, this new clothing technology is a departure from the historical testing and the historical products offered to athletes at the highest level. High tech skinsuits in the past were traditionally smooth surfaced and rubbery in appearance. The track cycling program during the 1984 Olympics had such apparel. Pearl Izumi has also offered smooth surfaced skinsuits in the past, and one can only assume that this design was a result of their participation/sponsorship of the infamous Project 96 program that was a bitter disappointment when the final medal count was tallied.
Which line of thinking is the correct one – smoother, or rougher? After reading the Nike literature, it appears that Nike thinks that “rougher is better”. The focus of the following article will be in discussing some basic fundamentals of aerodynamics and, in particular, boundary layer control and its manifestations in the pro peloton.
2 What is Aerodynamic Drag?
Fundamentally, aerodynamic drag is a function of the geometry of the object and the physical properties of the fluid (air is a fluid) it is traveling through. For example, an object on top of Mount Everest experiences less drag than an identical object at sea level because the air is less dense. Drag is often times defined by the following equation:
Where r is the fluid density, u is the fluid velocity, Cd is the coefficient of drag, and A is the frontal area.
Historically, designers have decreased drag by two methods: decreasing the frontal area or decreasing the drag coefficient by streamlining. For example, the Cd of a round bicycle spoke is approximately 1.2. Efficient streamlining can reduce this value to around 0.3 - 0.5 (for example, the Hed3/Specialized Tri-spoke significantly reduces spoke drag coefficient by tapering its spoke geometry in much the same way as an airplane wing). Optimal boundary layer control might be able to reduce the Cd of a bicycle spoke to nearly 0.6 to 0.7.
3 2-D Cylindrical Flow, 3-D Spherical Flow
The 2-d flow around a cylinder is one of the fundamental flows in fluid dynamics. Much research has been done to investigate this flow since it is simple, yet filled with many unique features. For purposes of this discussion, it can be assumed that the flow around a 2-d cylinder adequately represents the basic features of the flow around a cyclist.
One of the most interesting features of the 2-d cylinder flow is the dramatic drop in drag it experiences when the boundary layer transitions from laminar to turbulent flow (see Figure 1 – the same is true for the flow around a sphere). The point at which this phenomenon occurs is known as the critical Reynolds number, and it stands to reason that it would be desirable to design a skinsuit (or a golf ball) so that the flow resides in the corresponding low drag region above this critical value. When Nike says that their “Swift-Spin” suit is based on the same reasoning as why golf balls have dimples, they are saying that they have induced the low drag flow above the critical Reynolds number. But this is a lot of information to digest without a little background information.
Figure 1. Drag on 2-d cylinder, with the critical Reynolds number located near 2E+05 (based on data from Lindsey, W.F – NACA Rept., 619 – 1938).