IV. Types of low-speed wind tunnels

The general arrangement of the low – speed wind tunnel in its simplest form consists of a fan with which to blow or suck air through the working section. There is usually a contraction upstream of the working section, and a diffuser downstream.

Figure 1

Figure 1 illustrates such an arrangement diagrammatically. The main function of the contraction is to speed up the flow so that its speed in the other parts of the tunnel may be much lower than in the working section; in doing so, it causes the streamlines to be squeezed together, and improves the uniformity and turbulence level of the flow. The diffuser serves to retard the flow after it leaves the working section, and so to keep down the power losses.

Straight through tunnels, as they are called, may be of two types, blower or suction. A typical blower tunnel is shown diagram­matically in Fig. 2. Here the diffuser serves to slow down the air leaving the fan, so that it flows at low speed through a region called the settling chamber.

In this region, the slowly moving air settles to a fairly uniform and low turbulence condition, with the help of a series of gauze screens or honeycombs. The flow is then contracted to give a fast jet in the working section. Such a construction is simple and cheap. However, it is suitable only for demonstration purposes, or for very approximate measurements, because the turbulence level is not low enough for any degree of precision.

Figure 2

All the kinetic energy of the fast moving air in the working section is wasted, and the consequent high power requirement makes such an arrangement impracticable, except on a fairly small scale.

Figure 3 is a diagram of a straight through tunnel of the suction type, in which the air is sucked through by a fan downstream of the working section.

Figure 3

The tunnel illustrated has an enclosed working section. The entry to the tunnel should be clear of walls and other obstacles in the tunnel room. The settling chamber helps to reduce atmospheric turbulence. There is a large contraction ratio, and a small angle diffuser. Such an arrangement gives reason­ably low turbulence, and constitutes a useful general purpose tunnel. There is still considerable power wastage because the kinetic energy of the air leaving the tunnel is not recovered. But it is clearly better in this respect than the blower tunnel, since the speed at exit is less. In a return circuit tunnel, the air leaving the diffuser is not simply discarded, but collected, and it travels round a closed circuit to be passed through the working section again. In this way, the kinetic energy of the jet is recovered, not wasted, and the power required is reduced. Figure 4 depicts a typical return circuit tunnel with an open working section. It has all the features of the straight through tunnel, with the addition of corners. In this case, the diffuser is not immediately downstream of the working section, though it could easily be so.

Figure 4

Here it serves to reduce the speed of flow in part of the tunnel circuit, and, in particular, in the settling chamber and at the entry to the contraction. Corner vanes are fitted to enable the air to flow smoothly round the corners. However, with the open jet, it is difficult to obtain uniform, steady flow, and the tunnel is best suited to demonstration work and fairly rough measurement. Its main advantage is the ease of access to the working section.

Alternatively, the working section may be closed, as shown in Figure 5, with the arrangement otherwise the same as that of the open jet tunnel in Figure 4. There is a breather slot downstream of the working section which serves to maintain the pressure at or near the atmospheric value. The result of enclosing the working section is much improved uniformity and turbulence level. This is the most common type of tunnel for general research and development work.

However, another possible arrangement for a return circuit tunnel is the annular type, illustrated in Figure 6, which shows a section through the axis of the tunnel, which is a body of revolution. It is difficult to achieve uniform, steady flow without elaborate arrangements of corner vanes, etc. Also, access to the working section is very difficult.

Figure 5

A variable density tunnel generally consists of an annular type, tunnel enclosed in a large pressure vessel. By increasing the pressure, and hence the density, it is possible to achieve relatively high Reynolds numbers. Pressurization also results in power economy. Some such tunnels are also designed to operate at much reduce pressures, hence at low density, and high speeds.

Figure 6

The cross-sectional shape of a wind tunnel working section is not necessarily always the same right round the circuit. The fan section, for obvious reasons, is circular in section. But other components may be different in section.

Closed working sections are usually rectangular in cross-section, with filleted corners, to prevent the accumu­lation of dead air in these regions. The fillets may incorporate tunnel lighting arrangements. They sometimes diverge slightly to allow for thickening boundary layers, and thus maintain constant effective area of cross-section. There are usually turn - tables in the roof and floor of the working section, in which models are mounted for easy adjustment of attitude.

Open working sections are usually circular or elliptical. The working section is surrounded by a mixing region, which spreads outwards slightly. The collector of the jet must therefore be slightly wider than the exit from the contraction. The static pressure in the working section is atmospheric.

The function of the diffuser is to slow down the air with the smallest possible loss of energy, while maintaining maximum uniformity of flow. Because of the adverse pressure gradient, it is difficult to avoid flow separation, with the consequent energy loss and increase in turbulence. Experimental results show that small angle diffusers should be used, with an angle of about 5° between opposite walls. If this is done, the diffuser is necessarily long. There may be more than one part of the tunnel where diffusion takes place. It is possible to incorporate devices which help to prevent separation, such as vortex generators, or boundary layer suction devices.

The efficiency of a diffuser is given by the ratio of the rise in static pressure to the loss in dynamic pressure.

Fine mesh gauze screens are inserted to increase uniformity of flow and reduce turbulence. A pressure drop, and hence a loss of energy, occurs across such a screen. A honeycomb performs the same task rather less effectively, but with a smaller pressure drop. Gauzes are most effective in low speed sections, i.e., at speeds of no more than about 10 m/s, so the speed is always low in the settling chamber, where the gauzes are located. For a given pressure drop, it is less effective to use one high resistance gauze than to use several low resistance gauzes, with adequate intervals between them. A long settling chamber is therefore required.

The function of the contraction is to speed up the flow into the working section, but in doing so it also squeezes the streamlines together and smooths out some of the velocity fluctuations. For this reason, it is often better to slow down the flow leaving the fan and then speed it up again through a contraction, rather than pass it straight through. The ratio of the areas of cross-section at entry and exit respectively is called the contraction ratio. The higher this is, the better; but it should in any case be arranged so as to give speeds of no more than about 10 m/s in the low speed sections of the tunnel. The shape of the contraction should be such that its area decreases steadily, with no stationary points, so that the velocity increases steadily. The ratio of the velocities at exit and entry respectively is clearly equal to the contraction ratio.

Struts, which are of highly cambered aerofoil section, are placed so as to span the tunnel at the corners, and so help the flow to negotiate the corners smoothly. They are called corner vanes. In the absence of such vanes, separation would occur, with consequent non-uniformity, turbulence and power wastage. The vanes require very careful design, especially at corners where the speed is relatively high.

The fan should ideally be situated as far as possible from both ends of the working section, so as not to cause more turbulence than necessary. The size of the fan is a matter of compromise. A small fan is inefficient, but a large fan is liable to give blade flutter and some flow pulsations. It is usually placed in a section whose area of cross-section is some one and a half to two times that of the working section.

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