Setting the speed of a wind tunnel appears straightforward as we apply our basic understanding of fluid dynamics. However, it turns our that this basic question absorbs a considerable amount of attention even for experienced aerodynamicists When there is no model in the rest section a of attention even for experienced aerodynamicists. When there is no model in the rest section, a measuring device, most commonly a Pitot-static tube, can be put there to determine the air speed. One cannot, however, insert a Pitot-static rube or other measuring device in the test section to measure dynamic pressure or speed along with an object under test because the test object will cause changes in the flow. These changes are referred to as "induced flow."
Consider Fig. 1, which gives a schematic indication of the settling chamber, contraction and test section of a typical wind tunnel. The tunnel speed is usually determined by measuring either static or total pressure in the settling chamber ahead of the contraction cone as indicated by static or total pressure in the settling chamber ahead of the contraction cone, as indicated by station L, and a static pressure ahead of the test section, as indicated by station S. If
honeycomb or screens are used in the settling chamber, station L will be downstream of these devices. Using the subscripts L for the bellmouth or settling chamber and S for the region before the test section, Bernoulli's equation between the two stations will be
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Calibration of Wind Tunnels
Test Section Speed
Speed Setting
Fig. 1 Typical measurement stations for a wind tunnel “q” system
Test Section Speed
Speed Setting
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
Calibration of Wind Tunnels
Test Section Speed
Speed Setting
A better approach would be to survey a volume of the test section that is occupied by the model. If the Pitot-static tube used has a hemispherical nose, it can be equipped with two yaw heads at 90° to each other, and the distribution of the upflow and cross-flow can be yaw heads at 90 to each other, and the distribution of the upflow and cross flow can be obtained simultaneously.
The survey of the lest section can be done with a simple Pitot-static tube assuming there is a method to position it throughout the test section or a rake of Pitot statics can be used. lf there is no method of remotely positioning the Pitot-static tube, the survey of the test section becomes a very tedious operation. In any event, this is a critical operation and the effects of the manner of holding and supporting any selected instrument must be carefully evaluated. If a volume of the test section is surveyed and there are large variations in the dynamic pressure it may be desirable to use a weighted average of the measured dynamic pressure, it may be desirable to use a weighted average of the measured dynamic pressure. When the dynamic pressure calibration is completed, there is a relation
established between the indicated dynamic pressure (PL- PS) and the dynamic pressure
q
,Calibration of Wind Tunnels
Test Section Speed
Speed Setting
It is desirable to obtain for each test-section configuration the distribution of the total pressure, static pressure, upflow, and cross-flow throughout the region occupied by the model When the tunnel has an air exchanger or heat exchanger the temperature the model. When the tunnel has an air exchanger or heat exchanger, the temperature distribution should also be measured.
The pressure sources for the tunnel speed control should be, ideally, either a ring around a tunnel station or at least a portion of a ring on the two side walls or the ceiling and floor. The possibility of the model pressure field directly affecting the nearer of the pressure sources at station S should always be kept in mind and evaluated if a larger model than normal or an upstream mounting location is to be considered. The static source should consist of either a series of static sources or a
AE 2751
Wind Tunnel Techniques
July-Nov. 2016
sealed tube flush with the surface with many holes evenly spaced along its length. The tubes or multiple static sources of the ring should be manifolded together to yield an average static pressure at the station. If total pressure is used in the contraction cone. It is desirable to have multiple sources also.
Calibration of Wind Tunnels
Test Section Speed
Speed Setting
The two pressure sources in the simplest case can be connected to a U-tube
manometer that is used for setting the tunnel speed. It is more common to use a high- quality differential pressure transducer. Then the electrical output can be transmitted
q y p p
to any location convenient for the tunnel operator and to the data system.
Extreme care must be taken to make sure that there are no leak in the tunnel dynamic pressure system. In large tunnels, the leak checking can be a time-consuming
process. It is highly desirable and strongly suggested that the system should not be disturbed once it has been leak checked. It is often useful to record the pressures at each of the two tunnel stations as separate readings relative to atmospheric ambient pressure. Additional sources for this use should be provided rather than tapping into the tunnel dynamic pressure system.
the tunnel dynamic pressure system.
It is also desirable to have a simple method of periodically checking the system for leaks. One way to do this is to use a calibration wing that can be installed easily. The wing can be run through a pitch series at several dynamic pressures. If the slope of the lift curve does not change, there is no change in the dynamic pressure calibration. This has the further advantage of also checking the tunnel upfIow. If the drag polar does not rotate, the upflow has not changed.