Accumulated drag coefficient over an automotive

AI Thread Summary
The discussion centers on the concept of negative local drag in the context of a CFD simulation of airflow over an automobile. It explains that regions of negative slope in the accumulated drag curve indicate local areas where the drag can be negative, particularly due to faster flow creating below-ambient pressure, such as at the top of the windshield. This phenomenon does not imply that an object can have overall negative drag, but rather that specific sections can contribute negatively to the total drag. The urgency of the inquiry is noted, along with a reminder about forum rules against cross-posting. Understanding these dynamics is crucial for analyzing aerodynamic performance in automotive design.
ryan298
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Hello everyone,

I have a question regarding the accumulated drag curve shown in the figure below for the CFD simulation of external flow over an automobile. (It is written Vehicle Cd in the figure)

It can be seen in several regions that the accumulated drag curve is dropping (ie., having a negative slope, for instance, as in the region between 2500 and 3500 for the x axis). Now this should mean that in those regions, the drag is negative locally, as only this would bring a reduction in the overall drag. But how is this possible,? How can drag be negative locally in the said region for instance?

Correct me if I'm misinterpreting the curve. :)

PS: 1) The figure I've posted here is taken from a source which I've referenced below

"Levin, Ridgal, Aerodynamic analysis of drag reduction devices on the underbody for SAAB 9-3 by using CFD, Master's thesis, Chalmers University of technology, 2011"

2) It is kind of urgent, so if someone could help me out ASAP, it would be great :)
 
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No figure attached.
 
I am sorry. I acutally posted the same question in the automotive engineering forum as well, and the website says that since I attached the figure there, I cannot attach it again here. (not sure why)
Here is the link to the thread in the automotive forum, and the figure can be seen here

https://www.physicsforums.com/showthread.php?t=759991
 
Drag can definitely be negative locally - in that particular instance, I would expect the flow to be faster than freestream in the region near the top of the windshield, which means that area is below ambient pressure (negative gauge pressure). Since the top of the windshield faces forwards, a below-ambient pressure at the top of the windshield results in a negative contribution to overall drag.

(That doesn't mean that an object can have negative drag overall, just that small regions of an object can have negative drag if considered separately from the rest of the object).
 
ryan298 said:
I am sorry. I acutally posted the same question in the automotive engineering forum as well, and the website says that since I attached the figure there, I cannot attach it again here. (not sure why)
Here is the link to the thread in the automotive forum, and the figure can be seen here

https://www.physicsforums.com/showthread.php?t=759991

Please don't post the same thread to different forums. It's against the Rules of PF and can earn you infraction points (or worse in extreme cases).
 
Thank you cjl, i think i get understand it now.
Steamking, I am sorry for posting it in two forums. I wasn't sure which one would be a more appropriate place to ask this question.
 

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