Screech Tone Frequency and Intensity
It is well known that at low supersonic jet Mach numbers, there are two axisymmetric screech modes. They are referred to as the A1 and A2 modes. Earlier, Norum (1983) had compared the frequencies of the A1 and A2 modes measured by a number of investigators. His comparison indicated that the screech frequencies and the Mach number at which transition from one mode to the other takes place (staging) could vary slightly from experiment to experiment. Experimentalists generally agree that
Figure 15.58. Comparison between acoustic wavelengths of simulated screech phenomenon and the experimental measurements of Ponton and Seiner (1992). o, □, Measurements; •, ■, simulations.
the screech phenomenon is extremely sensitive to minor details of the experimental facility and jet operation conditions.
By using this numerical method, it is found that numerical simulation does reproduce both the A1 and the A2 axisymmetric screech modes. Figure 15.58 shows the variation of the computed X/D, where X is the acoustic wavelength of the tone,
Figure 15.59. Intensity of axisymmetric jet screech tones at the nozzle exit plane. (a) r/D = 0.889, (b) r/D = 0.642. Experiment (Ponton and Seiner, 1992): o, A1 mode; □, A2 mode. Numerical simulation: •, A1 mode; ■, A2 mode.
with jet Mach number. Since X/D = aTO/( fD) , where f is the screech frequency, this figure essentially provides the tone frequency Mach number relation. Plotted on this figure also are the measurements of Ponton and Seiner (1992). The data from both the numerical simulation and experiment fall on the same two curves, one for the A1 mode and the other for the A2 mode. This suggests that the computed screech frequencies are in good agreement with experimental measurements. This is so, although the staging Mach number is not the same.
Ponton and Seiner (1992) mounted two pressure transducers at a radial distance of 0.642D and 0.889D, respectively, on the surface of the nozzle lip in their experiment. By means of these transducers, they were able to measure the intensities of screech tones. Their measured values are plotted in Figure 15.59. The transducer of Figure 15.59b is closer to the jet axis and, hence, shows a higher decibel level. Plotted on these figures also are the corresponding tone intensities measured in the numerical simulation. The peak levels of both physical and numerical experiments are nearly equal. Thus, except for the difference in the staging Mach number, the numerical simulation is, indeed, capable of providing accurate screech tone intensity prediction as well.