Equivalent Source Strength Research Articles

Resonant scatterer configurations near elastic boundaries

Systems of precisely spaced bubbles, air‐filled shells or balloons in water, insonified at frequencies near their intrinsic radial resonance ω0 exhibit true resonant modes. For resonant wavenumber kR (in water) and scatterer radius a, this leads to amplification factors (kRa)−1 above and beyond the similar classical factor due to single scatter, and thus to net pressure amplification of order (kRa)−2 relative to the incident field (i.e., over 120 dB on the intensity scale, on or inside the scatterer). This effect is predicted by formulas for the equivalent source strength B of each scatterer of the system, account being taken of multiple scatter interaction. Whereas for pairs of scatterers, or periodic lattices, B is known to exhibit relatively modest maxima for selected values of x = kl (l being the spacing between scatterers), which had been called resonances [V. Twersky, J. Opt. Soc. Am. 52, 145–171 (1962)], one can show that, under certain conditions, true resonances exist— i.e., in the absence of attenuation, B exhibits real poles. This phenomenon is much enhanced for systems near elastic boundaries for which coupling between scatterers is mediated by surface waves, and doublet or triplet configurations develop spectra of resonant configurations xn at frequencies ωn (the latter being all close to ω0 for a spectrum of scatterer sizes an). [Work supported by ONR.]

On the application of acoustic “mirror”, “telescope” and “polar correlation” techniques to jet noise source location

Jet noise source identification and location is still an important issue since no really conclusive or generally accepted description of the noise generation processes in turbulent jets has been achieved to data. While there is a definite need for a better understanding of jet mixing noise, one may question if far field approaches via acoustic mirror, telescope, or polar correlation techniques can provide the missing information. There are not only limitations in the spatial resolution which restrict these techniques to jet noise at the higher Strouhal and Mach numbers but also more fundamental difficulties in the interpretation of measured “equivalent source strength density distributions”. In particular, if large scale coherent turbulence structures participate in jet noise generation the assumption of a uniform amplitude and phase of the pressure field radiated from compact elements of this distribution is questionable. It is shown that various acoustic interference mechanisms due to mere source coherence and specific source structure effects may cause “mislocation” of sources in a jet. Some of these objections are less valid at the lower Helmholtz numbers but for these, unfortunately, the spatial resolution of all three techniques becomes poor. Since interpretation of source location results depends on a specific model of the jet noise sources it appears to be very difficult to gain from these three techniques a new insight into the problem for the relevant jet flow parameters.

Lift corrections to transonic equivalence rule - Examples

The lift correction to the transonic equivalence rule, with specific reference to its dependence on the wing planform and load distribution, is studied. Calculations of the equivalence-body cross-sectional area, Se(x), are performed for examples from three families of wing planforms; the results are applied to the analysis of a Mach .98 transport, considering alternative wing designs. An increase in Se(x) due to nonlinear lift effects is found to be significant: a rise in Se(x) by 35% above the zero-lift maximum occurs over a distance comparable to the root chord. However, in changing from a symmetrical to an asymmetrical wing arrangement, the maximum of the lift correction to Se(x) can be reduced by as much as a factor of four, with a nearly eight-fold reduction in the correction to the equivalent source strength, dS^/dJC.

Soil Moisture Interface Effects upon Readings of Neutron Moisture Probes

AbstractMeasurements of soil moisture by the neutron scattering method are inaccurate when made in the presence of abrupt moisture changes. The effects of air‐soil interfaces and of wet‐dry soil interfaces were investigated using neutron probes of four different designs. Three of the probes were used to study the effects of wet soil layers within dry soil masses.Readings taken at shallow depths were found to have the greatest errors. These errors were always negative. Absolute error and depth within the soil to which this effect occurs were related to soil moisture content. Some means of correcting such errors should be used. Errors in determining moisture content of soil zones containing wet‐dry interfaces or wet layers were not as serious, and could be neglected in many practical applications. This was especially true when depth reading increments of 6 inches or less were used. These errors were usually negative. Neutron probes tend to underestimate soil moisture content at interfaces or in stratified profiles.Probe depth reading increments of 1, 3, and 6 inches all gave about the same accuracy of moisture determination. Twelve‐inch reading increments gave errors of as much as 6 times greater than those of the smaller increments.The influence of probe geometry upon neutron meter readings is also discussed. When compared on a basis of equivalent source strength, in uniform soils, the probe with end placement of the source was found to be considerably more efficient than the probes with center placement of the source.