Reynolds number and drag coefficient relationship questions

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

reynolds number and drag coefficient relationship questions

The theoretical law for the relationship between drag coefficient CD and Re was .. Generally, the number of parameters in these expressions is taken to be five, . When taking into account other factors, this relationship becomes. $$F_{D} . created by a fan. Calculate the Reynolds number and the drag coefficient. Click here to visit our frequently asked questions about HTML5 video. Lift and drag. • Flows at very low Reynolds number (creeping flow) the net upwards force (the lift) and the net backwards force (the drag). As for pipe flow, we.

reynolds number and drag coefficient relationship questions

Note the double logarithmic axes. Is there something like " higher Reynolds number, higher drag force " stuff? 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. This adds a section of turbulent boundary layer where at a lower Reynolds number the flow would had been laminar.

You will notice that for the same Reynolds number, a fully laminar flow lower line is much less draggy than a fully turbulent one upper line. Therefore, replacing some of the laminar flow with a turbulent one will result in more drag.

reynolds number and drag coefficient relationship questions

Now I need to explain why increasing the Reynolds number leads to more turbulent flow. For this, I turn to this answer: Inside the laminar boundary layer, small disturbances become less and less damped the higher the local Reynolds number becomes, and at a Reynolds number of aroundin unaccelerated flow some frequencies become unstable see Tollmien-Schlichting waves and will eventually create so much cross movement that the boundary layer becomes turbulent.

Now parcels of air which flow at high speed in the outer part of the boundary layer will move close to the wall and kick the slow parcels there ahead, greatly reducing the deceleration of the flow close to the wall, at the price of slowing down and expanding the whole boundary layer. The dimples on golf balls are being redesigned, as are the clothes that athletes wear.

reynolds number and drag coefficient relationship questions

Bicycle racers and some swimmers and runners wear full bodysuits. Australian Cathy Freeman wore a full body suit in the Sydney Olympics and won a gold medal in the m race.

Drag Coefficient

Most elite swimmers and cyclists shave their body hair. Such innovations can have the effect of slicing away milliseconds in a race, sometimes making the difference between a gold and a silver medal. One consequence is that careful and precise guidelines must be continuously developed to maintain the integrity of the sport. Body suits, such as this LZR Racer Suit, have been credited with aiding in many world records after their release in For instance, consider a skydiver falling through air under the influence of gravity.

The two forces acting on him are the force of gravity and the drag force ignoring the small buoyant force. The downward force of gravity remains constant regardless of the velocity at which the person is moving. This terminal velocity becomes much smaller after the parachute opens.

Terminal Velocity of a Skydiver Find the terminal velocity of an kg skydiver falling in a spread-eagle position. He weighed less but had a smaller frontal area and so a smaller drag due to the air. The size of the object that is falling through air presents another interesting application of air drag.

reynolds number and drag coefficient relationship questions

If you fall from a 5-m-high branch of a tree, you will likely get hurt—possibly fracturing a bone. However, a small squirrel does this all the time, without getting hurt.

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You do not reach a terminal velocity in such a short distance, but the squirrel does. The following interesting quote on animal size and terminal velocity is from a essay by a British biologist, J. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away, provided that the ground is fairly soft.

A rat is killed, a man is broken, and a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object. So the resistance to falling in the case of the small animal is relatively ten times greater than the driving force.