Drag Coefficient: Does Cd Vary in Different Mediums?

In summary, the drag coefficient for an object can vary in different mediums and at different velocities. However, by considering nondimensional parameters such as the Reynolds number and Mach number, it is possible to match the flow behavior and obtain useful data for comparisons. This allows for testing in different environments and at different size scales, providing valuable insights into the behavior of the object.
  • #1
peterg07
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Hi

Is the drag coefficient for a particular object the same in different mediums? Say we have Cd = 1 in air for one object, is Cd the same for this object in water?
 
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  • #2
The drag coefficient isn't even the same at different velocities in the same medium, let alone in different media.
 
  • #3
Thanks for the quick feedback, D H. Thread ended.
 
  • #4
D H said:
The drag coefficient isn't even the same at different velocities in the same medium, let alone in different media.

That depends. This gets into a discussion of nondimensional parameters - in a low speed flow, the drag coefficient will be the same so long as the reynolds number is similar. The reynolds number describes the relative importance of inertia and viscous forces on the fluid behavior. This is a key principle behind a lot of small scale testing of models in wind/water tunnels - similar reynolds number means similar flow behavior, including drag and lift coefficients and generation of turbulence.

In a high speed flow (greater than mach 0.3 or so, typically), you need to also match the mach number. This can still be done in different media, but it is more difficult to match both the reynolds and mach number properly. Depending on the flow parameters of interest, sometimes only one is matched, and the other is considered to be close enough to get useful data (but if this is the case, the drag and lift coefficient and flow behavior will not be identical between the model and the real situation).

In the case of water and air, with the same object in each and a low speed flow, we have a density ratio of about a thousand (air ~1 kg/m3, water ~1000kg/m3), a viscosity ratio of about 50 (air ~2*10-5 Pa*s @ 300K, water ~1*10-3 Pa*s @ 20C), so to match the reynolds number, we need a velocity in water that is 1/20 of the velocity in air. So, the drag coefficient of an object in 20C water at 5 mph should be similar to that of the same object in 300K (27C) air at 100 mph. We can also manipulate the length scale of the object to achieve similar results - a half-scale object in water at 10mph will also have a similar drag coefficient to a full scale in air at 100mph, and a quarter scale at 10mph in water will have a similar flow behavior to a full scale in air at 50mph. You could even manipulate the temperatures of the flows as well (changing the viscosity and density) or the pressure of the air flow (changing the density), and so long as the reynolds number stayed similar (and the flow stayed below mach 0.3 or so), the drag coefficient and flow behavior will be the same.

Basically, what I'm getting at here is that although DH is technically correct, there are parameters you can look at that do allow for comparisons between different media, different velocities, and different size scales, and as a result, you can actually get quite a bit of useful information by testing in a completely different environment than the intended one.
 
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  • #5


The drag coefficient, or Cd, is a dimensionless quantity that represents the resistance an object experiences as it moves through a fluid medium. It is dependent on various factors such as the shape and size of the object, the speed at which it is moving, and the properties of the fluid.

In general, Cd can vary in different mediums due to the varying properties of each medium. For example, the density, viscosity, and compressibility of air and water are different, which can affect the drag coefficient of an object moving through them. This means that the Cd of an object in air may not be the same as its Cd in water.

Additionally, the shape and size of the object can also play a role in the variation of Cd between different mediums. For example, an object with a streamlined shape may have a lower Cd in water compared to air due to the difference in viscosity between the two fluids.

Therefore, it is important to consider the specific medium in which an object is moving when determining its drag coefficient. Conducting experiments or simulations in each medium can help accurately determine the Cd for a particular object.
 

1. What is drag coefficient?

Drag coefficient, denoted as Cd, is a dimensionless quantity that measures the resistance an object experiences when moving through a fluid (such as air or water). It is a crucial parameter in predicting the amount of drag force acting on an object.

2. How is drag coefficient calculated?

Drag coefficient is calculated by dividing the drag force acting on an object by the product of the fluid density, the object's characteristic area, and the square of the fluid velocity. This calculation can be done experimentally or through computational fluid dynamics simulations.

3. Does drag coefficient vary in different mediums?

Yes, drag coefficient can vary in different mediums. This is because the type and properties of the fluid (such as density and viscosity) can affect the drag force acting on an object, thus changing its drag coefficient. For example, a car traveling through air will have a different drag coefficient than the same car traveling through water.

4. How does the shape of an object affect its drag coefficient?

The shape of an object can greatly impact its drag coefficient. Generally, objects with a more streamlined shape (such as a teardrop) will have a lower drag coefficient compared to objects with a more blunt shape (such as a cube). This is because streamlined shapes have less surface area in contact with the fluid, resulting in less drag force.

5. Can drag coefficient be reduced?

Yes, drag coefficient can be reduced through various methods such as changing the shape of an object, adding a streamlined fairing, or using a different fluid with lower density or viscosity. These methods aim to reduce the drag force acting on an object, ultimately reducing its drag coefficient and improving its aerodynamic performance.

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