Accumulated drag coefficient over an automotive

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Discussion Overview

The discussion revolves around the concept of accumulated drag coefficients in the context of a CFD simulation for external flow over an automobile. Participants explore the implications of a negative slope in the accumulated drag curve, questioning the possibility of local negative drag in certain regions of the vehicle's flow field.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the interpretation of the accumulated drag curve, specifically regarding the regions where the curve exhibits a negative slope, suggesting that this indicates local negative drag.
  • Another participant proposes that local negative drag can occur in regions where the flow velocity exceeds the freestream velocity, such as near the top of the windshield, leading to below-ambient pressure and a negative contribution to overall drag.
  • There is acknowledgment that while local regions can exhibit negative drag, it does not imply that the overall drag on the object can be negative.
  • Some participants express concern about posting the same question in multiple forums, indicating a potential violation of forum rules.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of the accumulated drag curve, and the discussion includes multiple viewpoints regarding the nature of local negative drag.

Contextual Notes

The discussion references a specific CFD simulation and its associated figure, which is not included in the thread. The lack of visual context may limit the clarity of the arguments presented.

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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