# Fluid Dynamics - Calculating Coefficient of Drag

• RandomDude123
In summary, the conversation discusses how to calculate the drag on a projectile by comparing theoretical and experimental results. However, the equations used have two unknown variables and the problem is complicated by variable acceleration. The suggestion to use the Magnus Effect in Saltation is helpful, but does not directly answer the main question of finding the drag experienced by the projectile using the gathered information. It is suggested to use empirical data or determine the problem experimentally using calculus and differential equations.
RandomDude123
How would one calculate the drag on a projectile (in this case a 1cm3 cube) that was launched at 0°.

The vertical drop, initial velocity, distance, and time (taken to travel distance) where measured.

I want to say that I could compare these experimental drop (bellow hight that projectile was shot from) to the theoretical SUVAT drop (which assumes no air resistance) and find the drag experienced by the cube that way, however I am unable to find the proper equation / an example of this.

Also, using the drag coefficient of a cube/square won't work because the cube was erratically spinning on multiple (axis).

Any help is appreciated.

(this is purely an extracurricular hobby/experiment)

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RandomDude123 said:
(ignoring fluid resistance) and find the drag that way,
Fluid resistance is "drag." Could you restate your problem.

Bystander said:
Fluid resistance is "drag." Could you restate your problem.
I am aware of that.
I guess i wasn't clear in my wording, sorry.

What i meant is that my theoretical SUVAT values for the drop that would occur are assuming no air resistance. Compared to my experimental results which are effected by air resistance. So, by comparing the two values, the theoretical value (of drop) given no air resistance and the experimental result (of drop) with air resistance, I would think there should be some way to find the drag on the projectile.

So i was wondering what equations or theories i should look for to do calculate the drag on the projectile from the data that i have.

Hope this clears it up.

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Just Googled "magnus effect;" got three hundred and some thousand hits --- first couple pages did not apply it to anything more complex than spheres and cylinders --- you might find cubes --- "+'cube shape' " trims it to
--- and third entry refers to Magnus effect in saltation, J. Fluid Mech., 1977.

RandomDude123
Bystander said:
Just Googled "magnus effect;" got three hundred and some thousand hits --- first couple pages did not apply it to anything more complex than spheres and cylinders --- you might find cubes --- "+'cube shape' " trims it to
--- and third entry refers to Magnus effect in saltation, J. Fluid Mech., 1977.

Although the "Magnus Effect in Saltation" is very helpful in explaining the discrepancy between my theoretical and experimental results, it doesn't answer my main question. How would I go about finding the drag experienced by the projectile using the information I have gathered? (or is it not possible due to lack of necessary data?)

Cheers :)

Edit:

I was hoping I could find a way to to work out the FD. Currently using this equation I have two unknowns. Since the equation for CD is a rearrangement of the the equation above, I do not see any useful substitutions I can make to work it out.

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The problem is that the drag formula you have cited is a purely empirical relationship and it is very rare that ##C_D## can be calculated analytically. Instead, it is typically measured in a number of experiments so that it can be applied later.

Your SUVAT idea is theoretically sound, but the problem is that those equations don't deal in situations that have variable acceleration, which is the case here. The thing is, the SUVAT equations are derived from more general principles, so if you have any familiarity with calculus and, preferably, differential equations, you could go about determining this experimentally.

RandomDude123

## 1. What is the coefficient of drag?

The coefficient of drag, also known as the drag coefficient, is a dimensionless quantity that represents the resistance a fluid exerts on an object moving through it. It is a measure of how streamlined an object is, with lower values indicating a more streamlined shape and higher values indicating a less streamlined shape.

## 2. How is the coefficient of drag calculated?

The coefficient of drag is calculated by dividing the drag force by the product of the fluid density, the object's velocity squared, and its reference area. This formula is represented as Cd = Fd / (0.5 * ρ * V^2 * A), where Cd is the drag coefficient, Fd is the drag force, ρ is the fluid density, V is the object's velocity, and A is its reference area.

## 3. What factors affect the coefficient of drag?

The coefficient of drag is influenced by several factors, including the shape and size of the object, the speed at which it is moving through the fluid, the viscosity of the fluid, and the roughness of the object's surface. Changes in any of these factors can affect the value of the coefficient of drag.

## 4. How is the coefficient of drag used in fluid dynamics?

The coefficient of drag is an important parameter in fluid dynamics as it helps in predicting the amount of drag force an object will experience while moving through a fluid. It is used to analyze and optimize the aerodynamic properties of objects, such as airplanes and cars, and to improve their efficiency and performance.

## 5. What are some methods for reducing the coefficient of drag?

There are several methods for reducing the coefficient of drag, including changing the shape of the object to make it more streamlined, smoothing out any rough surfaces, reducing the object's size, and minimizing the object's cross-sectional area. Additionally, using coatings or materials that reduce air resistance can also help decrease the coefficient of drag.

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