Drag coefficient of a sphere ()

[pavelbure9]

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.

 

[333cory333]

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!

 

[rcgldr]

In the case of dimpled golf balls, the flow tends to stay attached longer producing less drag at speeds between 55 mph == 88 kph and 300 mph == 480 kph. Below 55 mph drag is about the same. Above 300 mph a smooth ball has less drag. Links to articles:

http://www.franklygolf.com/golf-ball-aerodynamics.aspx]golf_ball_aerodynamics.aspx[/PLAIN]


wiki_golf_ball_aerodynamics.htm

 

[cjl]

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)

 

[PJC01]

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.