Tham khảo tài liệu 'advanced vehicle technology episode 3 part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | direction of flow represented by the pulley weight which opposes the movement of the plate. Pressure drag can be reduced by streamlining any solid form exposed to the air flow for instance a round tube Fig. b encourages the air to flow smoothly around the front half and part of the rear before flow separation occurs thereby reducing the resistance by about half that of the flat plate. The resistance of a tube can be further reduced to about 15 of the flat plate by extending the rear of the circulating tube in the form of a curved tapering lobe see Fig. c . Even bigger reductions in resistance can be achieved by proportioning the tube section see Fig. d with a fineness ratio a b of between 2 and 4 with the maximum thickness b set about one-third back from the nose see Fig. e . This gives a flow resistance of roughly one-tenth of a round tube or 5 of a flat plate. Air resistance opposing the motion of a vehicle Fig. The formula for calculating the opposing resistance of a body passing though air can be derived as follows Let us assume that a flat plate body Fig. is held against a flow of air and that the air particles are inelastic and simply drop away from the perpendicular plate surface. The density of air is the mass per unit volume and a cubic metre of air at sea level has an approximate mass of kg therefore the density of air is kg m3. Then let Mass m kg Volume Q m3 Density p kg m Hence or Let p kg m-3 m p nF 3 kg Density of air flow p kg m3 Frontal area of plate A m2 Velocity of air striking surface r m s Volume of air striking plate per second Q vA m3 Mass movement of air per second p Q p X vA since Q vA Momentum of this air mv pvA X r therefore momentum lost by air per second pAv1 From Newton s second law the rate at which the movement of air is changed will give the force exerted on the plate. Air flow Weight Fig. Pressure drag apparatus 592 Hence force on plate p Av1 Newton s However the .
