Ask Sarah: Dimples on cars

 

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.  

 

 

“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.