CFD and the Olympics

When I watched the opening ceremonies of the Olympics, I knew I needed to analyze something related to the Olympics during this special time of year.  At first I thought I would analyze the Olympic Rings.  Not a real scientific CFD analysis, but just something cool and easy to do, by hollowing out some Olympic CAD rings and putting some flow through.  Well, using FloEFD to do this analysis was easy, getting this by our legal team was not so easy.  Term’s like “copyright infringement” were thrown around, and since I didn’t want to worry about job security I decided to look for something else to analyze.

I went to my favorite free CAD website, grabcad.com.  I found some different sporting equipment to analyze, but the one that stood out to me was a badminton shuttlecock (hearby referred to as a birdie to keep this blog semi-professional).  I’m not a badminton fan, but my wife and my cubical neighbor are, so I thought they would get a kick out of this.  Also, compared to all other sports, badminton isn’t played with a ball.  The birdie is designed to be very stable in flight, but also to be very “draggy”.  This meant there would be some interesting aerodynamics at play.

I started investigating the CAD, and noticed the dimensions were very wrong.  It was a skyscraper of a birdie.  So I adjusted the CAD to be the right dimensions based on what I could find online, and started to analyze the results.  When I looked closely at the drag numbers, I noticed they were extremely large.  It seems, that I had missed one important detail, the units.  The values for the dimensions were correct, but my PTC Creo model was set to inches instead of millimeters.  I guess just like the wood shop motto “measure twice, cut once”, I needed to have a CAD-FloEFD motto “measure twice, extrude/revolve/pattern once”.  Or something like that.  Luckily, because FloEFD for Creo is in the CAD tool, all I needed to do was update the units.  The CAD file updated, and FloEFD recognized the CAD change and I could simply run the existing analysis setup on the updated geometry.  A huge time saver.

Now, at this point the Olympics are rocked by the big badminton controversy.  What are the odds the sport I choose to do a FloEFD analysis on became the black mark on the games.  Anyway, I decided to analyze the birdie at 2 speeds.  Online, I found the maximum speed a person has smashed a birdie was 206 mph.  Think about that.  A slapshot goes about 100-110.  A fastball pitch in baseball is about 90.  Even tennis serves aren’t near that speed.  Now, the thing the website didn’t say is was this the average flight speed, or was this a instantaneous speed of the birdie right when it was hit, which then slowed immediately.  I am 99% sure a birdie isn’t flying at 200+ mph across the length of the court, but I thought this would show some cool aerodynamics.

My other flight speed used was 60 mph.  This was my initial guess for a typical speed for a birdie, before doing the above research.  So let’s just call this one the speed of a hit birdie when you are trying for a better semifinal match in your round robin :D

Let’s start by looking at the 206 mph case.  First, I always like to show some results showing FloEFD’s adaptive mesh.  That is what makes analyzing the complex geometry of a CAD model possible.  You can see how not only it refined on the geometry, but also downstream in the wake of the birdie.

FloEFD for Creo Adaptive Mesh and Velocity Results for a Shuttlecock at 206 mph

We can see behind the cork/nose, there is a large recirculation zone.  We can also see how the feathers push the air outward as it flows away from the birdie.  But, if we look at the velocity vectors, we can also see how because the feathers are on sticks/quills, this allows some space where air is sucked into the middle of the birdie.  It is sucked down because of the low pressure from the recirculation zone.  You can also see on the inside of the feathers there is a low pressure area.  I believe this is because of the air being sucked into the middle of the birdie, it has an “angle of attack” compared to the feather, and there is a flow separation.  I think this explains the flight stability of the birdie.  On all sides there is a inwards force on the feathers, keeping them from yawing.  Of course to really analyze this effect, we would need to analyze the birdie in a yawed position, and see the resulting aerodynamic moments.  That is an analysis for another day.

FloEFD for Creo Velocity Results with Velocity Vectors for 206 mph flight speed

FloEFD for Creo Pressure Results with Velocity Vectors for a Flight Speed of 206 mph

FloEFD for Creo Surface Plot of Velocity (slightly offset from surface)

FloEFD for Creo Surface Plot of Pressure

Looking at the drag force on the birdie, we see it has 4.4 N of force on it.  This may seem small, but remember the units of a newton.  1 N is the force to accelerate a 1 kg object at 1 m/s^2.  Now our birdie doesn’t weigh 1 kg, it is more in the 5 g (0.005 kg) range.  Therefore, at 4.4 N, our birdie is experiencing a deceleration of 880 m/s^2, or ~ 90 g’s.  I believe a fighter aircraft can pull about 10 g’s, just for comparison.  This tells me for sure that there is no way the birdie was flying through the air at that speed for any amount of time.  Instantly this drag force would be working to slow the birdie down.  Of course if the birdie is only in the air for a second or two, then there isn’t a lot of time for this force to act, and the birdie would still be moving at a good clip when it reaches the opposing player.

Looking at the 60 mph case, the flow field looks the same except for lower velocity values.  The force is a lot lower too, at 0.144 N.  Still, that equates to a deceleration of 28.8 m/s^2 (~ 3 g’s), but well below that of the world record speed.

In the future, it would be cool to analyze the birdie in a yawed situation to see if there is a righting/stabilizing moment.  Or even more interesting to look into the forces on the birdie right when it has been hit, and is flying backwards.  What are the aerodynamics at play that spin the birdie around to fly in the right orientation.  I’ll have to remember to do that in  4 years at the next summer Olympics.

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Posted August 10th, 2012, by

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Commented on August 16, 2012 at 11:16 am
By Prasad Tota

Really interesting analysis Travis. When I read 206 mph, I immediately calculated the Mach number ( at sea level of course) which turns out to be 0.27 too high for a birdie like you said ! It would be cool to see the results at an angle of attack ( CL vs alpha… remember).

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