1.1 Construction details
Construction details of the open-jet wind tunnel are located on the World Wide Web at the following URL http://www.aoe.vt.edu/research/facilities/openjet.php. Further details (some of which may be out of date) can be found in the original article describing the facility, due to Conner (1937).

1.2 Flow through the test section
Flow speed through the wind tunnel test section may be varied from zero to about 30 m/s (note that this speed range is somewhat lower than that implied on the department web page above). The flow quality in the test section, while adequate for the experiments described in this manual is not ideal. Figure 1 shows velocity profiles measured across the empty test section. The velocity is about 5% higher at the edges of the jet than at its center. This non-uniformity results from the shortness of the contraction and the small distance between the final corner and the test section. The small size of the plenum and the lack of sufficient screens and a honeycomb within it mean that some of the turbulence and unsteadiness generated by the final set of turning vanes is felt in the test section. Turbulence intensity (r.m.s. of velocity fluctuations divided by mean velocity) in the empty test section is about 4% - at least an order of magnitude greater than in most wind tunnels.

1.3 Associated equipment
Flow speed in the test section is monitored using a 3-mm diameter Pitot-static probe connected to an inclined manometer. The probe measures the flow speed towards the upstream end of the test section where it is assumed to be unaffected by the presence of a model. Note that placing a large model in the test section may artificially increase the velocity sensed by the probe. The manometer has an accuracy of about ±0.02 inches of water.

Temperature inside and outside the tunnel is monitored by a digital thermometer fixed to the side of the tunnel downstream of the test section. It displays temperatures in Fahrenheit or centigrade to an accuracy of 0.1 degrees. Ambient (atmospheric) pressure is measured by a Mechanism Ltd. Mk. 2 precision aneroid barometer attached to the right hand side of the wind-tunnel control panel.

A traversing gear is available for moving probes horizontally and vertically across the test section. A lead screw, covered with a shroud of airfoil cross-section, is used to move the probe horizontally. A rack, pinion and ratchet system mounted outside the flow allows the probe to be moved vertically in discrete steps of 5.7mm.

A multitube manometer capable of measuring up to 40 pressures simultaneously is available for use with the open jet wind tunnel. The manometer is filled with fluid having the same density as water (1000 kg.m^-3). All the manometer tubes are connected at one end to a reservoir of fluid (figure 2) which is open to atmosphere. At the other end each tube is connected to the pressure to be measured. The manometer is thus sensitive to pressures relative to atmospheric.

A Pitot-static probe is perhaps the simplest device for measuring flow-velocity at a point.

A Pitot probe measures stagnation pressure (the pressure produced by bring the flow to a halt). It consists of a tube connected at one end to a pressure sensing device (such as a manometer or pressure transducer) and open at the other. Stagnation pressure is measured by pointing the open end of the tube towards the oncoming flow.

A static probe measures static pressure (the actual pressure in the flow). It consists of an opening (or 'pressure tap') parallel to the local flow direction. The pressure tap may be located in a tube (as shown in the figure), or in the surface of a model.

A Pitot-static probe is a combination of a Pitot tube and static tube. According to Bernoulli's equation the difference between the stagnation p_o and static pressure p is the dynamic pressure,

Given the flow density a Pitot static probe can thus be used to measure velocity.

The principle sources of error in velocity measurements made with a Pitot-static probe are misalignment and turbulence.

Since the local direction of the flow around a model is not know in advance it is usual to make measurements are made with the Pitot-static probe pointing in the direction of the oncoming free stream. Some misalignment of the Pitot-static probe may therefore occur. Figure 3 shows typical errors in velocity measurements as a function of angle of misalignment. Errors become substantial for angles greater than about 30^o.

A Pitot-static probe is designed only to measure velocities in a steady flow. In a turbulent flow, where the magnitude and direction of the velocity fluctuates with time, the Pitot-static probe measures, approximately, the time-averaged flow velocity. The errors in this measurement depend on the scale of the turbulent eddies encountered by the probe. If the open end of the probe is large in comparison to the turbulent eddies then the eddies stagnate and the end of the probe, artificially increasing the pressure difference it senses. If it is small then turbulent fluctuations in flow direction, produced as the eddies pass the probe, appear as misalignment and artificially decrease the measured pressure difference.

1. Conner, N. W., Construction and Calibration of the V.P.I. Wind Tunnel, Bulletin of the Virginia Polytechnic Institute, vol. 30, no. 9, July 1937, pp. 1-32.

Figure 1. Mean-velocity distribution across the empty test-section of the open-jet wind tunnel.

Figure 3. Errors due to misalignment in velocity measurements made with a Pitot-static probe. V - actual velocity; V_meas - velocity sensed by Pitot-static probe;  - angle of misalignment.