Low Frequencies

T his article discusses issues related to indirect lightning on aircraft/rotorcraft. The standard waveforms used for qualifying the vulnerability of a system are introduced, with their frequency spectrum. The identification of the elementary EM coupling phenomenon allows the understanding of the key drivers of the current distribution and field scattering on a complex structure. The system level EM coupling analysis starts with examples of cable-measurements on real aircraft/rotorcraft, from which simple models are derived in order to understand the origin of the resulting waveforms. Various ways of protection are then proposed, ranging from passive to active solutions. Finally, despite limitations, 3D EM modeling is presented as an efficient complement to scale-one tests. The article concludes on the validity of current knowledge on indirect lightning for future aircraft designs.

The dynamics of flexible bodies spinning at rates near or above their first natural frequencies is a notoriously difficult area of analysis. Recently, a method of analysis, tentatively referred to as a method of quadratic modes, has been developed to address this sort of problem. This method restricts consideration to configurations in which all kinematic constraints are automatically satisfied through second order in deformation. Besides providing robustness, this analysis method reduces the problem from one that would otherwise require the reformulation of stiffness matrices at each time step to one of solving only a small number of nonlinear equations at each time step. A test of this method has been performed, examining the vibrations of a rotating, inflated membrane.

The Acoustical Theory for Design of Aircraft Cabins (ATDAC) is a computer program developed to predict interior noise levels inside aircraft and to evaluate the effects of different aircraft configurations on the aircraft acoustical environment. The primary motivation for development of this program is the special interior noise problems associated with advanced turboprop (ATP) aircraft where there is a tonal, low frequency noise problem. Prediction of interior noise levels requires knowledge of the energy sources, the transmission paths, and the relationship between the energy variable and the sound pressure level. The energy sources include engine noise, both airborne and structure-borne; turbulent boundary layer noise; and interior noise sources such as air conditioner noise and auxiliary power unit noise. Since propeller and engine noise prediction programs are widely available, they are not included in ATDAC. Airborne engine noise from any prediction or measurement may be input...

The flow around a circular cylinder has awaken the attention of different researchers since the historic Strouhal's work of 1878. Ever since, many experimental and numeric works have been carried out in order to determine the relationship between the vortex shedding frequency and the flow regime. Recently, a number of studies have been developed using several small modifications in circular cylinder. In this work a circular cylinder modified with a longitudinal concave notch, has been tested in order to determine the relationship between the non-dimensional vortex shedding frequency (Strouhal number) and the Reynolds number has been determined to Reynolds up to 600. Additionally a modified circular cylinder with a longitudinal slit also has been tested in order to determine the Strouhal-Reynolds relationship in several attack angle configurations. The experiments have been carried out in a vertical low turbulence hydrodynamic tunnel with 146x146x500 mm of test section operating ...

The backscattering from partially water-filled cylindrical shells has been studied using finite element (FE) analysis and experimental measurements for low to medium frequencies, corresponding to 2 < ka < 30 (where a is the shell outer radius). For a partially-filled shell, filled to three quarters of the inner diameter, backscattering has been investigated numerically as a function of the elevation angle of the incident wave, and in particular as the wave direction changes from horizontal to vertical. Comparisons with fully air-filled and fully water-filled shells indicate that the shell resembles the former when the wave is incident from above and the latter when the wave is incident horizontally. The experiments were performed in a reservoir, using horizontal incidence and wideband Ricker pulses generated by a parametric array. The short duration of the Ricker pulses made it possible to observe a number of returns after the specular return for a shell with a high filling fr...

: Low (10 - 100 Hz) and very low (1 - 10 Hz) frequency acoustic propagation loss is predicted for a site on the Bermuda Rise near 30 deg N 69 deg W. An extensive survey of the open literature is reported, and a geo- acoustic model of the acoustic environment at the site is developed. The results of a brief numerical investigation of the effect of shear waves and detailed sub-bottom layering on the acoustic propagation-loss predictions are discussed. The acoustic propagation-loss model used is SAFARI, which accounts for shear waves in the sediment but assumes a range-invariant environment. The propagation-loss predictions for 5, 10, 20, and 50 Hz are then presented. Also presented are predictions of the reflection loss in the form of grey-scale plots as a function of frequency and grazing angle. No comparison with experimental data is attempted.

The report overviews shielded open-circuit measurements and presents a comprehensive uncertainty analysis. The authors use a dominant-mode scattering formulation to develop an expression for the reflection coefficient in terms of bead and sample parameters. The formulation developed here eliminates the transformation through the various sections of the sample holder. The authors also extend the formulation to include magnetic measurements. The