Question

Q23. Write the equation giving the force required on a fluid control volume to bring about a change in momentum and indicate three types of forces which cause a change in momentum. (a) 4 marks] (b) A jet of water 30 mm in diameter and with speed 12 m/s strikes a plate which is moving at 3 m/s in the opposite direction to the jet. The plate is inclined at 25° to the jet. (i) Assuming there is no friction between the water and the plate surface, calculate the magnitude of the force exerted on the plate by the jet. 10 marks () What direction does this force act in, relative to the plate? 1 mark] A jet-propelled aircraft is flying in still air at 180 m/s (Figure Q23). The mass flow rate of the inlet air is m and the mass ratio of fuel to air is 396. The exhaust gas from the engine exits at 540 m/s relative to the engine. Derive an expression, in terms of m, for the rate of change of momentum of the fluid (in the horizontal direction) across the control volume. (c) 5 marks fuel in et engine air inlet exhaust gas control volume Figure Q23

Answer 1

1. For a control volume of fluid element to change in momentum is due to forces. Those forces are 1. Surface forces 2. Body forces 3.inertia forces.

cl dr cv.ds+ J evv. dA

1 LA d<ed

2 2