I know I promised that the next blog would be a BBQ CFD analysis, but I came across this model and it was too interesting to pass up. A number of years ago, I was flipping through the channel guide and saw a show about choppers. At first, I thought I read copters, and being an aerospace engineer, watching a show on helicopters was something I was interested in. Of course the show was about motorcycles, but the first episode I saw was of a fighter jet inspired chopper. From that point on, I was hooked on all these types of motorcycle shows.
The other day, I came across this CAD model on www.grabcad.com, of a person’s dream chopper. I wondered about the aerodynamics of a chopper, as just a couple weeks earlier I had seen a “Mythbuster” episode where they tested the fuel efficiency of a motorcycle vs a car. The motorcycle killed on fuel efficiency, but was poor on air pollution. Then they made an aerodynamic shell and tested it again to see if they could improve the fuel efficiency enough to overcome the bad air pollution. Interesting stuff for sure.
Of course the motorcycle they used was nothing as cool as this. I’m sure unlike mass manufactured motorcycles, a custom chopper has never been wind tunnel tested or analyzed using CFD…until now
For my analysis, I set the airspeed to be 75 mph, because just like the song, “I can’t drive …. fifty five”! Other than that, the setup was pretty easy. I just needed to figure out the RPM for the wheels at 75 mph, and I took an educated guess at the engine exhaust/intake air flow rates. I created some CAD to represent a two lane road (I couldn’t figure out how to paint a centerline on my pavement texture map), and some grass. I didn’t get to making a CAD person riding the chopper, as then I’m sure most of the drag would have to do with his riding position (leaning forward vs straight up) and really what I was interested in was the bike itself.
Here are some of my results. Now, the bike looks fast even just sitting there, but in this first cut plot showing air speed and the mesh, doesn’t it look like it’s going even faster? If we look at the second contour plot of air speed, this time cropping out any air speed faster than 70 mph, you can see how far downstream the wake of the chopper extends. You can also see at some point, the wake becomes unstable and oscillates, creating that wiggle at the end. But I also like this plot because it shows that the wake also is very compact in the width direction. If you were a person on the grass, you wouldn’t feel the air move much when the motorcycle passes by, unlike when a transport truck passes by and you definitely feel the air move.
If we change the contour to pressure, and zoom in a bit, we can see areas that are affecting the drag on the bike. Anywhere that there is high pressure on the front side, and low pressure on the rear side is creating a pressure drag force. You can clearly see this on the front wheel. You can also see this on the headlight, which surprised me a bit. I was thinking the engine would have a lot of drag, but the headlights? Well it’s not just the headlights in this area; there are also the metal cross members that attach the risers to the body of the motorcycle. These are solid chunks of metal, so no air can get by these as well. Also, the headlights are experiencing fresh high speed air, so that’s creating more pressure. The engine is seeing the air that has been slowed when going around the front wheel, so the pressure isn’t as high.
Next I made an isosurface connecting places with the same velocity values, one at a speed of 45 mph, and another at 70 mph. I added some grid lines to help show the contours of the isosurface. This really helps to show the extent of the wake, with the 45 mph isosurface being the air that we have greatly reduced its speed, and 70 mph isosurface showing the extent of the air we have slightly disturbed. Now because the 70 mph isosurface extends so far, it’s hard to get a sense for it because it’s obscuring the chopper. So I made a cropped isosurface down the chopper centerline so we can see inside the isosurface.
One thing of interest to me was the aerodynamics of the wheels. I know in open wheel racing, the spent a lot of research on the aerodynamics of exposed wheels. When it comes to custom choppers, something that is always done is creating a cool looking wheel. I’m sure every design could be improved aerodynamically. I created a surface plot of velocity on the front wheel to see what I could see. Obviously velocity is zero on a surface, so this plot is offset into the air slightly to get the near wall air speed.
You can see in the tread area of the middle of the wheel there is a low speed zone. You can also see on the spokes near the rim, there is a lot of yellow high speed air, whereas near the hub behind the brake disk the air speed is pretty low. Now these wheels are pretty simple by chopper standards, so it would be interesting in the future to compare the drag and torque results for these wheels verses some other design.
Of course any analysis isn’t complete without some videos. First are some streamlines flowing around the bike. The interesting thing to me, is the air going by the front wheel. Even though the wheel is rotating clockwise, you can see how the air moving over top of the wheel flows downward, because of the engine. You can see some clockwise flow within the wheel, but in general you have competing airflows in this area. Now because the air has a downward trajectory as it goes towards the engine, I think this would reduce the drag effects of the engine because we aren’t getting any stagnation points on the front of the engine. The air is being deflected downward and away from the engine.
Another air flow we maybe interested in is seeing where the exhaust air goes. Every custom chopper design has to have cool pipes, so some designs may put the hot exhaust gases where we don’t want them. Looking at the animation, we can see some exhaust gas gets entrained with the air circulating around the rear wheel, but the exhaust gas cools very quickly. Again, I took an educated guess for the gas flow rate here.
Similarly, we would be interested in seeing how the air gets to our engine intake. We don’t want to put our intake in an area with poor airflow that could starve the engine and reduce the power output. We can see here, that other than a little wiggle to go around the front wheel, the air has a nice straight line into the intake.
Of course we could dissect this model further and looking forces and torques numbers on different components or the entire chopper. But I think I’ve learned a good amount about the aerodynamics of choppers. I hope you did to. This took very little time to analyze as I had the CAD geometry, and FloEFD works within the CAD system and automatically meshes the model. As such, I don’t want to get too deep into dissecting these results. Instead I would like to save some time to work on the BBQ analysis before the summer is over, so we will skip the numbers on this analysis. Until next time…