Both of these forces, in general, have components in the direction of flow, and thus the drag force is due to the combined effects of pressure and wall shear forces in the flow direction. A moving fluid, however, also exerts tangential shear forces on the surface because of the no-slip condition caused by viscous effects. Likewise, it is the drag that makes it possible for people to parachute, for pollens to fly to distant locations, and for us all to enjoy the waves of the oceans and the relaxing movements of the leaves of trees.Ī stationary fluid exerts only normal pressure forces on the surface of a body immersed in it. Friction, for example, is a "life saver" in the brakes of automobiles. But in some cases drag produces a very beneficial effect and it is sought to be maximized. Reduction of drag is closely associated with the reduction of fuel consumption in automobiles, submarines, and aircraft, improved safety and durability of structures subjected to high winds, and reduction of noise and vibration. But, like friction, usually it is sought to be minimized. More sophisticated drag-measuring devices, called drag balances, use flexible beams fitted with strain gages to measure the drag electronically.ĭrag is usually an undesirable effect but can also be desirable, depending on the application. The drag force can be measured directly by simply attaching the body subjected to fluid flow to a calibrated spring and measuring the displacement in the flow direction (just like measuring weight with a spring scale). The force a flowing fluid exerts on a body in the flow direction is called drag. A fluid may exert forces and moments on a body in and about various directions. It is difficult for a person to walk in deep water because of the much greater resistance it offers to motion compared to air. It is a common experience that a body meets some resistance when it is forced to move through a fluid, especially a liquid. The components of the pressure and wall shear forces in the direction normal to the flow tend to move the body in that direction, and their sum is called lift.
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