# Accumulated drag coefficient over an automotive

1. Jun 29, 2014

### ryan298

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 :)

2. Jun 29, 2014

### SteamKing

Staff Emeritus
No figure attached.

3. Jun 30, 2014

### ryan298

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

4. Jun 30, 2014

### cjl

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).

5. Jun 30, 2014

### SteamKing

Staff Emeritus
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).

6. Jun 30, 2014

### ryan298

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.