# Reynolds number and drag coefficient relationship advice Providing scientific and technical advice and assistance to the Military force and moment coefficients were observed with varying Reynolds number when increasing or decreasing boattail drag with Reynolds number, seems to be the spectral content of the disturbances in relation to the laminar instability frequency. Answer to 1. The figure shows the accepted relationship between drag coefficient and Reynolds number for a smooth sphere. Note tha. during the most stressful times, and for her useful ideas and advice, . Roughness Coefficient Variation with Reynolds Number. 49 considering drag forces and the turbulence structures in relation to the velocity. The more friction, the more energy has to be supplied, and this energy is lost as heat. Aerodynamic friction is caused by viscosity. The Reynolds number tells you how big viscosity is in relation to inertial forces. A bigger Reynolds number signifies lower viscosity. This means a higher Reynolds number almost always results in lower friction. If you look at the plot below, the downward trend can be easily spotted. Friction drag coefficient of a flat plate over Reynolds number picture source. Note the double logarithmic axes. Is there something like " higher Reynolds number, higher drag force " stuff?

### aerodynamics - How does Reynolds Number affect skin friction drag? - Aviation Stack Exchange

Please look at the transition curve: Here a flow which is initially fully laminar slowly changes with increasing Reynolds number into one with a laminar start and a transition to turbulent flow somewhere downstream.

We would expect that the physics of fluids should be different for slow and fast flows, but this question in and of itself is not well defined. Slow or fast compared to what?

This is what Reynold's number tells us. It tells us whether flow is slow or fast by forming a natural nondimensional measure of flow speed. And this is precisely what we observe.

## Drag coefficient

The low Reynold's number limit corresponds to things like marbles falling in corn syrup, or cloud droplets in air, or bacteria in water. This is slow viscous flows where drag forces are proportional to velocity. On the other hand, in the high flow limit, we have turbulent flow, where eddies are created behind our object or around edges in pipes, this is the general limit most things in air on human scales corresponds to, so you are familiar with turbulent flow intuitively. Large things like people will be in this turbulent regime in air even at speeds as small as 0. This is the limit in which viscosity becomes unimportant, and for the most part we can imagine flow in a fluid as corresponding to just sweeping up the fluid in front of our bodies of interest.

Momentum Flow Consider it another way.

### How does drag change with Reynolds number? - Aviation Stack Exchange

Kinematic viscosity is the diffusion constant for momentum in a fluid. It is how fast the momentum spreads out due to collisions between the different molecules in a fluid. Let's take a look at a couple relevant times for fluid flow.

What does this time represent? It is the time it takes for the fluid to "flow" around the object. More accurately, it is the time it takes for the momentum disturbances in the fluid to flow around the object.

But there is another characteristic time: This second time corresponds to the time it takes an object to move a distance equal to its size. Clearly here if that ratio is large, we expect the fluid doesn't move out of the way at all and is just swept up, while if it is low, we expect appreciable flow around the material. Small Aside In fact, using this idea you can "derive" the normal air drag equation for force. We can just assume in the simplest case that a ball travelling through air just bumps into all of the air molecules in front of it.