Notes on the Deflection of Jets by Insertion of Curved Surfaces, and on the Design of Bends in Wind Tunnels

摘要

It is common knowledge that small jets of water can be deflected by partial insertion of a finger or other curved surface. The finger experiences a force impelling it towards the centre of the jet but if this force is withstood by an equal and opposite force the fluid is consequentially drawn round the surface and comes off at an angle. Similar phenomena are observed with jets of air of high total head surrounded by air of small total head: the former air behaves almost like another (immiscible) fluid. The phenomenon can be explained by classical potential theory. The pressure must be constant on all water-air boundaries, but lower on the surface (so that the force shall be towards the centre). In the sequel various plane flows are built up (in the Helmholtz-Kirchhoff manner) by assuming patterns in the hodograph plane, and these show precisely the characteristics observed in practice. What is true of plane 'jets' is true qualitatively of jets of more normal cross-section. The principle application is to the control of jet-propelled aircraft. But the mathematics has another application. If wind-tunnel bends are designed according to this pattern (with walls replacing the sides of the jet) the velocity will be constant along the walls except in a small limited region where deleterious effects of adverse velocity gradient can be checked by moving walls or by suction slots. Both alternatives are investigated, with the latter being perhaps more satisfactory. A 180-deg. bend with all adverse gradient avoided by use of one suction slot is designed, and drawn out.