Drag coefficient of a sphere ()

In summary, the reynolds number of the air coming out of an air supply is not always determined by the air supply. It is based on the size of your object and the velocity of the air around it. Assuming that your supply gives a reasonably smooth flow, you can calculate the reynolds number using the equation Re = ρvL/μ.
  • #1
pavelbure9
6
0
While writing a physics report, I obtained a data that
for balls of rough surfaces, there is a higher drag force and thus
the ball can stay stable at a much smaller angle when put up in an airstream.
However, while analyzing this result, I found out that the drag coefficient is not always
bigger for rough spheres : it depends on the reynolds number of the flow.
I would really like to know whether the flow past a sphere
(in my experiment, styrofoam balls) is attached flow (Stokes flow) and steady separated flow, separated unsteady flow, separated unsteady flow with a laminar boundary layer at the upstream side, or post-critical separated flow, with a turbulent boundary layer.
Put simply, what is the reynolds number of the air coming out of an air supply?
For further information, the air supply used in our lab was SF-9216, PASCO.
 
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  • #2
After discussion with an intimate physics professor, we reached contradicting results:
like in the case of golf balls, the rough surface can make air pockets,
or when the air meets a certain condition (some sort of Reynolds number boundaries)
the drag coefficient is bigger for rougher spheres.
Also, we concluded that the air flow from the supply is turbulent.
Could you please help us out? Thank you!
 
  • #3
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  • #4
The reynolds number is not determined by your air supply. It is based on the size of your object and the velocity of the air around it. Assuming that your supply gives a reasonably smooth flow (not always a guarantee), you can calculate the reynolds number using the equation Re = ρvL/μ, where ρ is the density of the fluid (air, in this case), v is the velocity of the fluid, L is a characteristic dimension of your object (for a sphere, this would be the diameter), and μ is the viscosity of the fluid. Density and viscosity can be calculated or looked up on a table based on the temperature and pressure of the air in your lab, and you should be able to measure your flow velocity directly (or if you do not have that capability, it may be specified in the manual of your air supply).

Once you know the reynolds number, then you will have a better idea of what kind of flow conditions your sphere will have around it, and that will determine the effect that dimples will have.

(Wiki even has a fairly nice image showing the relevant flow regimes: http://upload.wikimedia.org/wikipedia/commons/3/3f/Reynolds_behaviors.png)
 
  • #5


Thank you for sharing your findings on the drag coefficient of a sphere. It is interesting to note that the roughness of the surface can affect the drag force and stability of the ball at different angles. However, it is important to consider the Reynolds number of the flow when analyzing this result.

The Reynolds number is a dimensionless quantity that characterizes the type of flow around an object. It takes into account the fluid velocity, density, and viscosity, as well as the characteristic length of the object. In the case of a sphere, the characteristic length would be its diameter.

The type of flow around a sphere can vary depending on the Reynolds number. At low Reynolds numbers, the flow is typically attached and steady, known as Stokes flow. As the Reynolds number increases, the flow can become separated and unsteady, with a laminar or turbulent boundary layer. This is known as post-critical separated flow.

To determine the Reynolds number of the air coming out of an air supply, we would need to know the velocity, density, and viscosity of the air, as well as the diameter of the sphere. The specific air supply used in your experiment, SF-9216 from PASCO, may have this information available. Alternatively, you could measure the air velocity and use the known properties of air to calculate the Reynolds number.

Overall, the Reynolds number is an important factor to consider when studying the flow around a sphere and can help explain the varying drag coefficients observed for rough spheres. I hope this information is helpful in further understanding your results.
 

Related to Drag coefficient of a sphere ()

1. What is the drag coefficient of a sphere?

The drag coefficient of a sphere is a dimensionless quantity that represents the resistance of a sphere moving through a fluid. It is typically denoted as Cd and can range from 0 (perfectly streamlined) to 2 (highly turbulent flow).

2. How is the drag coefficient of a sphere calculated?

The drag coefficient of a sphere can be calculated using the formula Cd = (FD / (ρ * v2 * A)), where FD is the drag force, ρ is the density of the fluid, v is the velocity of the sphere, and A is the cross-sectional area of the sphere.

3. What factors affect the drag coefficient of a sphere?

The drag coefficient of a sphere is affected by several factors, including the shape and size of the sphere, the density and viscosity of the fluid, and the speed at which the sphere is moving through the fluid. Roughness of the surface of the sphere and the presence of any obstacles in the fluid can also impact the drag coefficient.

4. How does the drag coefficient of a sphere change with speed?

The drag coefficient of a sphere typically increases with speed, as the flow around the sphere becomes more turbulent. At low speeds, the drag coefficient is relatively low, but as the speed increases, the drag coefficient also increases until it reaches a maximum value. After this point, the drag coefficient may decrease slightly due to the formation of a turbulent wake behind the sphere.

5. Why is the drag coefficient of a sphere important?

The drag coefficient of a sphere is important because it helps engineers and scientists understand the resistance and forces acting on a sphere moving through a fluid. This information is crucial in a variety of applications, such as designing efficient vehicles, predicting the behavior of objects in fluid flows, and optimizing the performance of sports equipment.

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