Drag coefficient w/o terminal velocity

In summary, the conversation discusses the equation for terminal velocity and drag coefficient, and the difficulty in finding one without the other. It is mentioned that when an object reaches terminal velocity, there is no net force acting on it. The example of finding the terminal velocity of a quarter is given, and it is noted that the drag force is of equal magnitude to the object's weight. The conversation then delves into the iterative process of finding the drag coefficient using the Reynolds number. It is also mentioned that drag coefficient should be found experimentally. The conversation ends with a discussion about the different types of drag force and the possibility of the falling quarter exceeding the critical Reynolds number.
  • #1
picklefeet
16
0
I know the equation for terminal velocity and drag coefficient. But I can't find one without the other. It's a real catch-22. I also don't know the Drag Force. HEEELP!
 
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  • #2
Do you have a specific question? Also, remember, when an object reaches terminal velocity, there is no net force acting on the object (as it isn't accelerating). Hence, the drag force is of equal magnitude to the object's weight.
 
  • #3
An example would be finding the terminal velocity of a quarter. the weight is 5.67g. The cross-sectional area is 4.522 cm. (rounded to the nearest thousandth.) and the density is 1.184. I have the equation Fd= 1/2*p*V^2*Cd. I get 5.67=.592*Cd*V^2. (I can't remember if drag weight is measured in grams or kilograms.) You see my problem. I stil need the velocity of the object.
 
  • #4
It will have to be an iterative process. Cd is a function of Reynolds Number. You would need to guess at an original Cd. Use that Cd to calculate the speed based on your equation of motion. Use that speed to calculate Reynolds number. Finally, use that Reynolds Number to check your original guess for the Cd. Eventually it will converge on a solution.
 
  • #5
FredGarvin said:
Cd is a function of Reynolds Number.
This is getting past what Aero I took, but isn't that only for situations where viscous drag changes a lot? Can't we just use this equation there: http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

If your reference conditions aren't vastly different from the conditions you are studying (ie, if you find the Cd at 60mph and your terminal velocity is around 120), it should work out ok, shouldn't it?

Obviously, this doesn't help with the ultimate problem, of course: that drag coefficient is something that really needs to be found experimentally.
 
  • #6
I was thinking in general terms. I think you're right though. The speed isn't going to be changing by that much so it should be pretty simple to get the answer. I guess we won't be covering a few orders of magnitude in Re for this.

I did look at a chart I had for a round plate perpendicular to the flow. It is a constant 1.1 over a very wide range so that makes this a very easy problem.
 
  • #7
...easy if the flat plate (a quarter) falls at a stable position parallel to the ground...

I do believe that Mythbusters actually tested this, though...
 
  • #8
Wouldn't the quarter turn to the stable position in air therefore making the cross sectional area equivalent to the diameter times the width of the quarter?
 
  • #9
There are two basic types of drag force; drag force at high Reynold's number (Re>1000), proportional to velocity squared, and Stokes drag force (Re<1000), linearly proportional to velocity. It is not obvious the the falling quarter will exceed Re=1000. The Reynold's number is proportional to velocity times sqrt(cross section), so a small size object would have to have a very high velocity.
See
http://en.wikipedia.org/wiki/Drag_(physics )
Also see
http://en.wikipedia.org/wiki/Reynolds_number
 
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FAQ: Drag coefficient w/o terminal velocity

1. What is drag coefficient without terminal velocity?

Drag coefficient without terminal velocity refers to the measure of the resistance a moving object experiences due to the air around it, without considering the effects of gravity. It is a dimensionless quantity that is influenced by the shape, size, and speed of the object.

2. How is drag coefficient without terminal velocity calculated?

The drag coefficient without terminal velocity is calculated by dividing the drag force by the product of the dynamic pressure and the reference area of the object. The drag force is determined by measuring the force acting on the object as it moves through a fluid, while the dynamic pressure is calculated by taking into account the density and velocity of the fluid.

3. What factors affect the drag coefficient without terminal velocity?

The drag coefficient without terminal velocity is influenced by several factors including the shape and size of the object, the speed at which it is moving, the density and viscosity of the fluid, and the roughness of the object's surface. These factors can impact the overall drag force experienced by the object.

4. Why is drag coefficient without terminal velocity important?

Understanding the drag coefficient without terminal velocity is important in various fields such as aerodynamics, fluid dynamics, and vehicle design. It helps scientists and engineers predict the behavior of objects moving through fluids, and allows them to design more efficient and streamlined objects to reduce drag and improve performance.

5. Can the drag coefficient without terminal velocity be reduced?

Yes, the drag coefficient without terminal velocity can be reduced through various methods such as changing the shape or size of the object, smoothing out its surface, or increasing its speed. By reducing the drag coefficient, the overall drag force acting on the object can be minimized, resulting in improved performance and efficiency.

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