Other wind tunnel measurements

As well as measuring the forces and pressures on a wind tunnel model, we may want to investigate more detailed features of the flow, so that we can improve the design of our aircraft. We can do this by measuring the velocity at different points in the flow and by using flow visualisation to help us to see what the flow is doing.

One way we can measure speed is to traverse the flow using a miniaturised version of the pitot-static tube (Fig 2.5 earlier), although we must be careful to line it up with the local flow direction. If we want to measure fluctuations in turbulent flow, however, this will not respond fast enough. For this type of speed measurement, we can use a hot wire anemometer or a laser Doppler anemometer. In the hot wire anemometer a very fine wire is heated electrically. The airflow cools the wire and this changes its electrical resistance, which can be measured electronically. Because the wire is so fine (about 5 micro-metres in diameter) it can measure very rapid fluctuations in airspeed. The laser Doppler anemometer (TDA) works by illuminating a very small volume of air at the crossing point of two laser beams. The beams interfere with each other and produce a series of ‘fringes’ (dark and light stripes). Particles, which are light enough to follow the airstream, are introduced into the flow (seeding) and each reflects a pulse of light as it travels through each light fringe. The fre­quency of the reflected light is then used to measure the velocity of the particle but some very clever signal processing is needed to sort things out when par­ticles overlap in the illuminated area or when very large particles come through, which are too big to follow the flow. Seeding can also be used to measure the velocity in a ‘slice’ of the flow illuminated by a light sheet. Pictures are taken, in quick succession, of particles illuminated by the sheet and their velocities are deduced from their changes in position. This is called ‘Particle Image Velocimetry’ (PIV for short).

Particles in the flow can also be used to make the flow visible. Smoke is often used for this. We can also see what the flow is doing on the surface of a model by using short wool tufts, stuck to the surface at their upstream ends, or by the use of surface oil flow. A common oil flow technique is to mix a

white powder such as titanium dioxide with kerosene and a few drops of oleic acid. After running the wind tunnel the titanium dioxide forms streaks showing the pattern of flow on the surface. A note of caution – oil flow needs considerable experience in interpretation as the flow may differ considerably a small distance from the surface, outside the boundary layer.