Measuring pressure

We can measure the pressure on the model surface by connecting flexible tubes between small holes (pressure tappings) drilled in the surface and a suitable pressure-measuring device. A simple way to measure the pressure is to use a liquid-filled tube U-tube (Fig 2.18). If the pressure in the tube, connected to the hole in the model, is lower than atmospheric, the liquid level will rise; rather like sucking a drink up a straw. If the pressure is higher than atmospheric (as in Fig 2.18), the liquid will be pushed further down the tube, like blowing down the straw.

The idea is quite simple. The column of liquid between A and В is sup­ported by the difference between the test pressure and atmospheric pressure, which produces an upward force on the column equal to the weight of the liquid between A and B. As we saw for the liquid column in Chapter 1, the upward force is equal to the pressure difference multiplied by the cross-sec­tional area of the tube. This balances the weight of the liquid, which is equal to its density, p, multiplied by the gravitational constant, g, the cross-sectional area of the tube and the height of the column, h.

So what pressure difference is the equivalent to a manometer deflection (pressure ‘head’ – see Chapter 1) of lm of water?

Pressure difference = p X g X b So, putting in the values in SI units, we get

Pressure difference = 1000 X 9.81 X 1 = 9810 N/m2