Drag of Blunt Bodies and Streamlined Bodies
The Reynolds number tells you how big viscosity is in relation to inertial Friction drag coefficient of a flat plate over Reynolds number (picture. Homework Help: Reynolds number and the relationship with lift and to calculate the friction coefficient (skin friction drag) acting on the wing. Download/Embed scientific diagram | The relationship between the drag coefficient C D and the Reynolds Number Re. The marks show the different observation.
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? 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. Frictional drag comes from friction between the fluid and the surfaces over which it is flowing. This friction is associated with the development of boundary layers, and it scales with Reynolds number as we have seen above. Pressure drag comes from the eddying motions that are set up in the fluid by the passage of the body. This drag is associated with the formation of a wake, which can be readily seen behind a passing boat, and it is usually less sensitive to Reynolds number than the frictional drag.
- Drag coefficient
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- Drag Coefficient
Formally, both types of drag are due to viscosity if the body was moving through an an inviscid fluid there would be no drag at allbut the distinction is useful because the two types of drag are due to different flow phenomena.
Frictional drag is important for attached flows that is, there is no separationand it is related to the surface area exposed to the flow. Pressure drag is important for separated flows, and it is related to the cross-sectional area of the body. We can see the role played by friction drag sometimes called viscous drag and pressure drag sometimes called form drag or profile drag by considering an airfoil at different angles of attack.
Homework Help: Reynolds number and the relationship with lift and drag.
At small angles of attackthe boundary layers on the top and bottom surface experience only mild pressure gradients, and they remain attached along almost the entire chord length. The wake is very small, and the drag is dominated by the viscous friction inside the boundary layers.
However, as the angle of attack increasesthe pressure gradients on the airfoil increase in magnitude. In particular, the adverse pressure gradient on the top rear portion of the airfoil may become sufficiently strong to produce a separated flow.
This separation will increase the size of the wake, and the pressure losses in the wake due to eddy formation Therefore the pressure drag increases. At a higher angle of attack, a large fraction of the flow over the top surface of the airfoil may be separated, and the airfoil is said to be stalled.
At this stage, the pressure drag is much greater than the viscous drag. When the drag is dominated by viscous drag, we say the body is streamlined, and when it is dominated by pressure drag, we say the body is bluff. Whether the flow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape of the body. A streamlined body looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks like a brick, a cylinder, or an airfoil at large angles of attack.
Drag coefficient - Wikipedia
For streamlined bodies, frictional drag is the dominant source of air resistance. For a bluff body, the dominant source of drag is pressure drag. For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. Drag coefficients of blunt and streamlined bodies.
Cylinders and spheres are considered bluff bodies because at large Reynolds numbers the drag is dominated by the pressure losses in the wake.