**2.2. Operational problems**

feet for the loose fill. The duct interline is surfaced with 3/16 inch thick perforated plywood and the outside of the silencer is sealed with 1 inch thick, exterior quality waterproof ply‐ wood. Both internal and external surfacing materials are heavily glued, screwed and nailed to substantial connecting framing. The second section of the silencer, which is 16 ft long is a rectangular duct lined on two sides with 6 inch thickness of rockwool batts backed by a 3 inch airspace. The remaining two sides of this duct are 1 inch thick exterior plywood. Other constructional details are similar to those of the first section silencer. The second diffuser section is run in the laboratory ceiling space and is supported from the roof structure on 'Si‐

Initial tests on completion of the tunnel indicated a large direct sound transmission through the walls of the first stage subsonic diffuser. This was found to be caused by high frequency resonance of the 3/16 inch thick flat steel plate walls. The vibration was almost completely eliminated and the noise level reduced by decreasing the spacing of the existing 1 inch x 0.25 inch stiffening bars from approximately 12 inch x 6 inch to 6 inch x 3 inch centres as descri‐

In the final form, the silencer has reduced the noise level in the vicinity of the tunnel to about 75 to 90 dB, for the 100 to 2000 Hz band, depending to some extent upon the operating stagnation pressure. It is estimated that the duct silencer provides an attenuation of about 2

Tunnel stagnation pressure is read on 0.15% accuracy, temperature compensated, abso‐ lute pressure 'Heise' test gauge and recorded by a pressure transducer having 0.1% com‐ bined non-linearity and hysteresis. The transducer output can be displayed directly in psia on an 11 inch 'Honewell' strip chart recorder. The control panel is provided with an electrically actuated pneumatic calibration circuit which connects the stagnation pressure transducer and test gauge in a closed system. This circuit has an electrical override if the

Stagnation temperature instrumentation consists of an exposed-junction 'BLH' micro-minia‐ ture thermocouple connected to an 11 inch strip chart recorder and reading directly in 0F. Bothe stagnation temperature and pressure recorders contain electrically operated chart speed-up facilities which automatically increase the chart speed by a factor of 60:1 when the tunnel run is started. A typical speed change is from 10 inches per hour to 10 inches per mi‐ nute. Both chart recorders are provided with event markers which are connected into the tunnel timing circuit. The circuit operates an electrically actuated second timer which is con‐ trolled from a timer switch in the remote control box on a wandering lead. The box also con‐ tains the tunnel stop switch and a pressure clamp switch. The event markers are automatically actuated at the start and stop of a timing run. The wandering lead control box

enables one man to control the run and monitor Schlieren and instrument read out.

to 3 dB per foot of length in the frequency range 100 to 1000 Hz.

The tunnel stop-start system has been briefly described in section 2.1.4.

lentbloc' vibration isolators.

88 Wind Tunnel Designs and Their Diverse Engineering Applications

bed in section 4.4.

*2.1.6. Instrumentation*

tunnel is started.

The gas dynamics and supersonic tunnel facilities have proved to be simple to operate, relia‐ ble and comparatively trouble-free. However, there have been two operational problems which may of interest, and they are described below.

