Ask Sarah: Dimples on cars


Mitsubishi Evo vortex generators roof spoiler
Mitsubishi tested the down force generated by installing vortex generators on the roof of previous generations of the Evo and found they generated a 0.006 reduction in both the drag coefficient (about 2% down) and lift coefficient.  


Drag without vortex generators
The area of low pressure (cooler colors) is much greater behind a car with no vortex generators.


Drag with vortex generators

With vortex generators, the low pressure region is smaller and the car experiences less pressure drag.
In other news, MotoIQ is seeking kids ages 3-8 who like to color outside the lines and have an appetite for Crayons to put these illustrations together.  We pay in life experiences…

But that's not to say the dimpled concept hasn't been applied in real life speed competitions. Fuhgeddabout cars for a minute- let's visit competition bike racing.  Zipp Speed Weaponry makes racing bicycle wheels that have a patented dimpled disc.  The Reynolds number is lower (turbulent flow introduced more quickly) on the dimpled wheels than the smooth discs.  

Zipp also uses dimples on wheel covers and even wheels to keep the airflow attached longer.  This is especially beneficial at higher angles such as taking tight turns on a bicycle race course.  While the dimples produce slightly higher skin friction drag, it is just a fraction compared to the reduction in pressure drag, making the wheel faster throughout the range of conditions you're likely to experience on a race course.   

Zipp dimpled discs
Zipp has tried a variety of shapes for their dimples, adopting something similar to the one Titleist uses – a meniscus.  It's flat on the bottom and sweeps up towards the edges.  Golf ball manufacturers have tried all sorts of shapes from oval to hexagonal to icosahedral (a 20 point polyhedron) and there hasn't yet been a definitive victor.  Hexagons tend to be used the most due to the greater number of faces but producing shapes with too many faces creates issues. The corners become more rounded and it's the corners that trip the airflow to become turbulent.  
Back to cars and more specifically the 2004 Lexus LS430 which received a lot of attention from its Super Bowl ad that turned the LS upside down in a wind tunnel to reveal the dimpled panels on the undercarriage.  Dimpled panels on the underbody are actually becoming quite the norm from the Mercedes Benz CLS to many Porsche, Volkswagen, and Audi models, among others.  These are typically used on an undercarriage where there is a zero pressure gradient geometry, more commonly referred to as flat plate type flow.  The flow in this environment doesn't tend to separate since two dimensional instabilities such as Tollmien-Schlichting (T-S) waves cause the flow to naturally transition to turbulent without needing any type of vortex generators.  T-S waves carry a certain frequency that amplifies in a range of Reynolds numbers.  Environmental disturbances, typically sound, cause wave instability and transitions the flow from laminar to turbulent.  Car manufacturers may put dimples on the underbelly pan, airflow dams, or other parts of the cars to reduce wind noise and drag. Additionally these dimples can also help strengthen the panels.  
 Lexus LS360 golf ball
Lexus took it one step further when they revealed the LS460 at the US Open, paying homage to the sport of golf with an entire dimpled body. The best selling feature about this car is you can park next to the driving range without worrying that a stray slice to the body will dent your car! 
“Wrap it up already!”  Ok, there are dimples on blunt objects like golf balls and vortex generators on streamlined things like car and airplane wings.  In the end, they energize the air flow (turbulent), which accelerates the flow redirecting its energy forward and yadda yadda yadda, keeps the air from separating until later, producing less pressure drag (and overall drag) on the object. 

Leave a Reply

Your email address will not be published. Required fields are marked *