why lift and separation is important

I was struck by this Louis Vuitton ad with Michael Phelps and Larisa Latynina - two remarkable athletes in the service of formal clothing and quite a shift from the standard attire as well as depiction of athletes from the original ancient games.  

 

Modern athletes wear a wide range of clothing appropriate to the sport.  Some is protective, some only has minor functionality other than basic modest and offering sponsors billboard space for television, but some athletic clothing offers competitive advantage.

 

Running offers interesting examples.  Run fast enough and you will feel a considerable amount of wind resistance.  You want to reduce that force against you as much as possible.  Basically you are moving the air in front of you as you run.  About a year ago I wrote a post with a quick derivation of the power you have to develop to deal with that force.  The force turns out to be  half of the coefficient of drag for your body * the area you present as you run * the density of the air *  the square of your velocity relative to the air.¹  The important bits that you can control are your velocity, your frontal cross sectional area, and your coefficient of drag.  A cyclist can adapt a posture to minimize her frontal area while running generally makes that impossible.  Cyclists use a lot of aerodynamic tricks with bikes, wheels, riding position and even helmets.  And even runners can play with their clothing.  

 

To understand running attire it is useful to consider golf.

 

Sports balls are fascinating objects.  Apart from ping pong balls, few are smooth and spherical.  The shape gives them unusual properties that makes playing with them more interesting.  Balls tend to evolve with a sport and then, at some point, standardize so everyone can play on even terms.  

 

Early golf balls were mostly spherical hardwood.  It was noticed at the time that some balls were better than others, specifically some that we beaten up a bit would go further on a drive, but other beaten up balls performed worse.  Progression eventually led to a realization that small surface imperfections were important for good performance and the modern dimpled ball emerged.

 

The physics is pretty neat.  Two things are going on - one is caused by the spin on the ball and the other is related to the airstream immediately around it.²  The second is the larger effect, so I'll only consider it for the time being.

 

There are two basic types of airflow - laminar and turbulent.  Laminar flow is smooth (think of laminations), while turbulent flow is rough and choppy.  Laminar flow tends to take place at "low" speeds and turbulence becomes dominant as airspeed increases. 

A smooth ball generates a large wake as it pushes air molecules aside. Immediately behind the ball the air molecules are further apart compared to the air around and in front of the ball. A region of low pressure forms just behind the ball sucking it backwards and slowing it in the process. The air immediately around the ball is the boundary layer and one worries about where it separates from the ball - having it separate further behind the ball produces a smaller amount of total separation and the pressure drop behind the ball is smaller.  

. The dimpled ball separates the air less and has a smaller wake. The air behind the ball does not differ in air pressure as much as with the smooth ball so the dimpled ball has a smaller pressure drag and can travel further.

 

If you look at the airflow very close to the dimpled ball you notice they are roughing up the flow a bit and generating a bit of turbulence.  This makes the air stick a bit more to the ball.  It generate a bit of drag, but that turns out to be completely washed out by the fact the boundary layer sticks around a bit longer.  

 

Sweet   Dimpled balls are just better and have been around for a long time, but the effort was mostly trial and error - these days it has been possible to show what is taking place.  

 

But what does this have to do with running?

 

As it happens we create boundary layers when we run.  Our size and speed mean that if we're wearing something smooth - like a lycra bodysuit - the airflow is mostly laminar and there is an early and largish boundary separation that creates a low pressure area behind us.  One can imagine a diet of beans might help, but only for brief moments and the impact wouldn't be that great anyway, so it makes much more sense to make sure the boundary layer doesn't detach as quickly.

 

It turns out the trick is a tight fitting almost smooth body suit (you don't want a loose fit as the flapping fabric would present extra area creating even more drag) covered with a thin ribbing - about a half millimeter high and just enough to rough up the air and get the boundary layer to hang around a bit longer.  The C_d drops and the runner can put more of her power into moving herself rather than wasting it on moving air.  The effect can be dramatic.  The effect is more important as the airspeed goes up and at the very high speeds seen in the 100 meter event such suits are worth as much as a tenth of a second.³  At slower speeds air resistance is much less important (remember the force goes as the square of the relative airspeed and therefore the necessary power required to overcome it goes as the cube).

 

Hair, it turns out, is less of a problem than you might think.  There was a period when sprinters wore hoods, but the heat generated around their heads became excessive.  The head is only about 7% of your total body area and the impact of air resistance is about 3% of the total effort a sprinter must create at top speed.  Seven percent of three percent is under a hundredth of a second and the impact of heat buildup renders that counterproductive.

 

Talk of the Olympics and specialized suits brings up the Speedo LRZ swimsuits see prior to the 2008 competition.  Of all of Olympic sport, more science has probably been devoted to swimming than anything else.  Lowering the drag from moving through water is a big thing - after all - water is nearly 800 times denser than air and the underlying physics is well understood (although the calculations can be very difficult).  Speedo came up with a seamless and wrinkle-free shell of a suit and its shape stayed smooth throughout the swimming stroke. There was also a thin layer of a foamy material that had very small gas pockets that made the swimmer more buoyant.  They rode higher in the water and produced less drag.  

 

The overall effect was pretty dramatic - a 5% to 8% reduction in overall drag - but the cost was high.  The suits were very expensive ($500), took a half hour to put on and only lasted a race or two.  Since their impact was about the same on the top swimmers the decision was made to ban them and records set using them are noted separately.

 

There are many more examples - it turns out shoes are a fascinating area, but it would take pages and pages to describe the current understanding of shoes vs no barefoot as well as the difference between shoes.  So that's it for today.

__________

¹ The area for this is what only what the oncoming air sees and is often called the frontal cross sectional area.  Also note that the relative velocity is important.  If you are running at 10 mph with a 10 mph tailwind, the effective velocity is 0 - you don't have to move the air that is in your path.  Tailwinds are even better and records have strict limit on how big they can be.

The coefficient of drag, or C_d, is a quantity that represents how easily a shape moves through a fluid.  At a given speed the frontal area times the C_d will tell you the force you have to generate to move the air column before you aside - double the area or C_d and you need twice the force (or power).  C_d range from well under 0.1 for a sailplane, about 0.20 to 0.30 for many sports cars,  0.7 for a cyclist, 1.0 for a runner, 1.4 for the Empire State Building, 1.9 for the Eiffel Tower, and 2.1 for a brick.

² A spinning ball produces lift through the Magnus effect.  I won't go into it, but ping me if you are interested.  I've worked it out for volleyballs and found an interesting result that explains part of the ball's trajectory ..,

³ In his world record 100 meter run Usain Bolt was moving at 12.27 meters per second at the 65 meter point - about 27.3 mph!  He was producing about 3,000 watts of power at the time - about twice what a stove top electric burner consumes.  This is near the human limit for burst power production and can only be maintained for a short amount of time. The body can't sweat out this extra heat immediately and the core body temperature of a sprinter increases rapidly.  If their muscles could be powered a bit longer the increase in body temperature would kill them.

__________

Recipe corner.

First a technique from shesimmers for making sticky rice that doesn't require much kit.  I just tried it and had excellent results.  recommended!

 

There are a variety of tiger salads.  I don't know where this one originated, but we tried it last night and found it deliciousSim as well as easy to make.

 

Simple Tiger Salad 

 

Ingredients

 

° 1 large bunch cilantro, with the thick stems removed, cut into 3-inch lengths (about 5 cups, loosely packed)

° 1 green bell pepper stemmed, seeded and slivered (cucumber would do nicely too)

° 1 fresh chili pepper, stemmed, seeded and slivered

° 2 scallions, slivered

° 2 tsp vegetable oil

° 1/4 teaspoon salt

° sesame oil and white vinegar, to taste

° a handful of chopped peanuts (optional)

 

Technique

 

° In a bowl, combine cilantro, green pepper, chili pepper and scallions. Add vegetable oil and salt; toss lightly.

° Add peanuts if you like and a few drops of sesame oil, vinegar and more salt to taste; toss again. Pile on a plate and serve immediately as it wilts quickly