Accumulated drag coefficient over an automotive

In summary: Thank you for informing me.In summary, the drag on an automobile can be negative locally in a certain region, due to the pressure being below ambient.
  • #1
ryan298
5
0
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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  • #2
No figure attached.
 
  • #3
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
 
  • #4
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
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).
 
  • #6
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.
 

1. What is the accumulated drag coefficient over an automotive?

The accumulated drag coefficient over an automotive is a measure of the total drag force acting on a vehicle as it moves through the air. It takes into account the shape and size of the vehicle, as well as other factors such as air pressure and velocity.

2. How is the accumulated drag coefficient calculated?

The accumulated drag coefficient is calculated by multiplying the individual drag coefficients of different parts of the vehicle, such as the body, tires, and mirrors. These individual drag coefficients are determined through wind tunnel testing and computer simulations.

3. Why is the accumulated drag coefficient important?

The accumulated drag coefficient is important because it directly affects a vehicle's performance and fuel efficiency. A higher drag coefficient means the vehicle will experience more resistance, requiring more energy to move, resulting in lower fuel efficiency and slower speeds.

4. How can the accumulated drag coefficient be reduced?

The accumulated drag coefficient can be reduced by designing the vehicle with aerodynamics in mind. This includes streamlining the shape of the vehicle, reducing the number of protruding parts, and using materials that are more streamlined. Additionally, regular maintenance and keeping the vehicle clean can also help reduce drag.

5. How does the accumulated drag coefficient impact electric vehicles?

The accumulated drag coefficient has a significant impact on electric vehicles, as they rely on maximum efficiency to achieve longer driving ranges. By reducing the drag coefficient, electric vehicles can travel further on a single charge, making them more practical for everyday use.

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