Mach Disk of Dual Coaxial Axisymmetric Jets

Abstract

1343freestream Reynolds number increases, less excitation would be required to produce the oscillation.As the angle of attack is increased further, a higher frequency oscillation around 27 Hz is identified. This corresponds to a Strouhal number of 0.173 and is recognized as bluff body shedding. This frequency first became distinguishable at a=25 deg and is clearly visible at 28 deg. The angle of attack when the bluff body shedding first occurred in the experiments of Zaman et al. 4 was much lower, about 18 deg, but their Reynolds number for the corresponding data was only 10 5 . Figure 3 presents the spectral intensities measured in the wake of the 12-in. chord aluminum model at several Reynolds numbers from 3xl0 5 to 1.4xl0 6 . Here the spectra at oc=14 deg are shown, but the data at 15 deg are very similar. The frequency of the spectral peak is seen to increase with Reynolds number, although the Strouhal number remains nearly constant. Over the Reynolds number range shown, the magnitude of the peak appears to increase with Reynolds number. The spectra suggest that the low-frequency flow oscillation will continue to be present on this model at even higher Reynolds numbers.A plot of Strouhal number vs Reynolds number for the present data and selected data from Ref. 4 are shown in Fig. 4. The present data taken at 15-deg angle of attack compare well to the earlier data 4 and seem to be a smooth extrapolation to the higher Reynolds numbers. Because of the magnitude of the forces induced by the flow oscillations and structural considerations, the wooden 10-in. chord model could only be used up to 7te=6xl0 5 . The 12-in. chord aluminum model was built to extend the data to the maximum tunnel speed and, therefore, Re, while still maintaining a low tunnel wall interference. Two trends are clearly seen in these data. The Strouhal number of the flow oscillation increases with model angle of attack and with increasing Reynolds number.The increase in Strouhal number in these data is about 0.0034/ 10 6 Reynolds number. However, it should be noted that the phenomenon occurs over a small a range. Stepping in 1-deg increments in a, a clear peak in the u' spectra is only seen at oc= 14 and 15 deg. Data from Ref. 4, shown in Fig. 4, also show a similar trend with angle of attack. The small increase in Strouhal number with Reynolds number seen in the present data was not apparent in the earlier data. 4 Tunnel wall interference appears to affect the measured frequency of flow oscillation. Using exactly the same wooden 10-in. chord model, but in a much smaller tunnel, Zaman et al. 4 measured a Strouhal number of 0.033 at a Reynolds number of 3x 10 5 . The angle of attack at which the flow oscillation occurred was also higher at a = 17 deg. Considering only solid body and wake blockage, the increase in flow velocity at the model would only be about 5%, 7 not nearly enough to account for the discrepancy. An explanation of the discrepancy is not currently available. SummaryA low-frequency flow oscillation on airfoils which occurs ne...

An experimental study of underexpanded sonic, coaxial, swirl jets

“…For NPR pˆ5 .0 and N PR sˆ3 .0, F ig. 13 shows the radial distributions of the impact pressures, where the secondary annular stream of no swirl is indicated by the solid lines and the secondary swirl stream by the dotted line. At x =D eˆ0 :1, the impact pressure distributions for both cases are quite similar, but at x =D eˆ1 :0, the impact pressure distributions are a little different only near the jet axis.…”