DENSITY MEASUREMENTS IN A SUPERSONIC FLOW BY THE X-RAY ABSORPTION METHOD*)

The method of absorption of X-rays was applied to measure the distribution of air density in a free jet at the exit from a nozzle with supersonic velocity and across a shock wave.

The measurements were carried out in an aerodynamic tube measuring \(18 \times 18\) cm at a Mach number \(M = 2.83\).

In the apparatus described, the radiation source is an X-ray tube with a beryllium window, operating at a voltage of 4 kV, which corresponds to a mean effective wavelength of 3.7 Å. The X-ray beam, 1.5 mm in diameter, enters the tube and leaves it through special windows made of cellophane 20 \(\mu\) thick. Measurements of intensity are made by means of two Geiger–Müller counters (type TGsP-1): one of them serves to measure the intensity of the primary X-ray beam, the other to measure the intensity of the attenuated beam.

In Fig. 1 are shown the results of measurements of the density distribution in an undisturbed flow along the central line of the test section of the tube. The dashed line is given for comparison of measurements made by means of a Pitot tube.

Fig. 1.

In Fig. 2 are shown the results of X-ray measurements of the density distribution of the flow along the direction transverse to the shock wave. To produce the latter, a wedge-shaped model is used with a span of 178 mm and a test-section width of 182 mm. In the same figure the above-mentioned model and the region of measurements made across the shock wave are also shown schematically. The general form of the density distribution agrees with shadow patterns obtained from photographs of the air flow. These patterns confirmed the presence of a double maximum, clearly expressed in Fig. 2. The maximum at a distance of 38 mm from the leading edge is explained by the compression of the waves caused by interaction with the last wave from the model holder.

Fig. 2.

When measuring the intensity of the attenuated X-ray beam with an accuracy of \(\pm 1.2\%\), the accuracy of density measurements in an undisturbed

*) E. M. Winkler, Journ. of Appl. Phys. 22, 201 (1951).

in the flow, ±4.5%. The absorption of X-rays at a voltage of 4 kV over a path of 18 mm at 0.1 of normal density reaches about 20%.

The absorption method described can be successfully used for calibrating nozzles over a wide range of densities of an undisturbed flow by selecting the appropriate range of X-ray wavelengths. Obtaining wavelengths of the order of 8 kV and more is limited by absorption in the windows of the X-ray tube.

For studying images of the flow in a shock wave, a narrower beam diameter and a higher intensity are required than was the case in obtaining the orienting measurements shown in Fig. 2.

G. G.