Coefficient of Drag Problem

[leekelley]

Fellow engineers,

I'm working through some practice exams (specifically the Lindeburg Practice Exam) and running into some extremely frustrating issues with assumptions at times. This latest issue is about the morning session's #28.

There are several ways of finding the coefficient of drag on an object in a fluid stream, but the solution here finds the Reynolds #, and the L/h ratio, and uses them to find the coefficient of drag "graphically." Ok... Where's that graph? It's not in the Lindeburg MERM. And what's up with needing to estimate a coefficient of drag, and having answers that are nearly within 10% of each other?!

Depending on what online reference you would like to choose, you could have the coefficient of drag differing from their assumption by almost 100%! And keep in mind, the problem gave no description of shape other than its length, width, and height. I would assume it was a rectangular prism, but then I'd miss the problem.

What do you guys use to estimate the coefficient of drag on an object moving through air?

Thanks for any help!

 

[Mike M PE]

My MERM IS at work so please forgive me for not having the page number available. But the graph is in the MERM (Reynolds Number to Coefficient of Drag). In order to calculate you need the Drag Force against the fluid, fluid velocity, face area of object to fluid and mass density of the fluid (for air at that temp).

In order to use the graph you have to calculate the Reynolds Number and also remember you only care about surface area (facing fluid force) so when you have various sizes you need to break each up individually so that you can calculate Cd for each section.

 

[leekelley]

Mike, thanks for taking the time to reply!

Unfortunately, I'm not sure we're on the exact same page. There are indeed graphs in the MERM that you can find the Cd based on the Re and the Area FOR A GIVEN SHAPE. For example, disk, sphere... But, in this problem we are actually looking for the Drag Force (actually looking for the amount of HP it takes to overcome the drag force that you calculate). You are only given that it's a certain size car-top carrier with length, width, and height. You also know the speed of the air, but that is all.

The solution referred to a graph where you would use the Re and the L/h ratio to find the Cd. That graph is nowhere to be found.

 

[Ramnares P.E.]

Can you post more details about the problem? I looked at my Lindeburg PE practice exam and #28 on the morning session deals with static friction. Maybe we have different versions or are just looking at different problems.

Any additional information would help us to help you.

 

[leekelley]

Absolutely! Here's the problem verbatim:

A 150 lbf cartoons carrier measures 51" long by 35" wide by 18" high. If a 3000 lbf car is driven at 65 mph into a 10 mph headwind, the added net motor power required when the carrier is used is most nearly:

I can solve the problem all but an ACCURATE estimate of the Cd, since no shape is given. I assumed a rectangular prism to be the shape, but in the solution... Well, you can read my original post to see what they did.

Thank you for your help!

 

[leekelley]

Absolutely! Here's the problem verbatim:

A 150 lbf cartoons carrier measures 51" long by 35" wide by 18" high. If a 3000 lbf car is driven at 65 mph into a 10 mph headwind, the added net motor power required when the carrier is used is most nearly:

I can solve the problem all but an ACCURATE estimate of the Cd, since no shape is given. I assumed a rectangular prism to be the shape, but in the solution... Well, you can read my original post to see what they did.

Thank you for your help!

 

[P-E]

Is this from the six min solutions book? I remember there being problems that needed tables that weren't in the merm. I ended up burning that book in a campfire in NH.

 

[Ramnares P.E.]

Sorry for the late feedback. I found the problem, it's #24 in the morning session of Lindeburg. I agree with your method of solution.

The graph in the MERM (17.50 I believe) isn't applicable to this problem so I'm not sure why it is referenced. That graph is for elliptical discs etc.

I went back to my college Fluid Mechanics text which provides a graph for Cd based on shape and direction of flow. I would treat this as a rectangular body normal to flow which would give a Cd of 2, almost twice the value Lindeburg uses.

The exam should not have problems with this level of discrepancy and would not have answers within 10%.

I would treat this as an error by Lindeburg and simply move on.

 

[leekelley]

Thank you so much, Ramnares! That's very good to hear! I would've used a Cd of 2 as well, so that makes me feel better. I will move on and not let it bother me.

 

[Ramnares P.E.]

Good luck on the exam. If you have any further questions don't hesitate to post them here.