The drag is dominated by the pressure losses in the wake. Drag coefficients of blunt and streamlined bodies.Ĭylinders and spheres are considered bluff bodies because at large Reynolds numbers Than a streamlined shape with the same thickness (see figure 1).įigure 1. For example, the drag of a cylinder of diameter $D$ can be ten times larger For a givenįrontal area and velocity, a streamlined body will always have a lower resistance thanĪ bluff body. The dominant source of drag is pressure drag. For streamlinedīodies, frictional drag is the dominant source of air resistance. Like a brick, a cylinder, or an airfoil at large angles of attack. Looks like a fish, or an airfoil at small angles of attack, whereas a bluff body looks Whether theįlow is viscous-drag dominated or pressure-drag dominated depends entirely on the shape It is dominated by pressure drag, we say the body is bluff. When the drag is dominated by viscous drag, we say the body is streamlined, and when This stage, the pressure drag is much greater than the viscous drag Top surface of the airfoil may be separated, and the airfoil is said to be stalled. Increase the size of the wake, and the pressure losses in the wake due to eddyįormation Therefore the pressure drag increases. Particular, the adverse pressure gradient on the top rear portion of the airfoil mayīecome sufficiently strong to produce a separated flow. However, as the angle ofĪttack increases, the pressure gradients on the airfoil increase in magnitude. The wake is very small, and the drag isĭominated by the viscous friction inside the boundary layers. On the top and bottom surface experience only mild pressure gradients, and they remainĪttached along almost the entire chord length. At small angles of attack, the boundary layers Pressure drag (sometimes called form drag or profile drag) by consideringĪn airfoil at different angles of attack. We can see the role played by friction drag (sometimes called viscous drag) and Pressure drag is important for separated flows, and it is related Frictional drag is important for attachedįlows (that is, there is no separation), and it is related to the surface areaĮxposed to the flow. Would be no drag at all), but the distinction is useful because the two types of dragĪre due to different flow phenomena. Of drag are due to viscosity (if the body was moving through an an inviscid fluid there Usually less sensitive to Reynolds number than the frictional drag. The formation of a wake, which can be readily seen behind a passing boat, and it is That are set up in the fluid by the passage of the body. Pressure drag comes from the eddying motions Thisįriction is associated with the development of boundary layers, and it scales with
Frictional dragĬomes from friction between the fluid and the surfaces over which it is flowing. Two components: frictional drag, and pressure drag. Drag of Blunt Bodies and Streamlined Bodies Drag of Blunt Bodies and Streamlined BodiesĪ body moving through a fluid experiences a drag force, which is usually divided into