Dietrich Heimann

Coupled simulation of meteorological parameters and sound level in a narrow valley

Abstract A meteorological mesoscale model is used to simulate the time dependent distribution of temperature and the corresponding development of slope-wind systems in a narrow two-dimensional valley during the course of a cloud-free day. A numerical sound particle model was designed to cope with both the uneven terrain and the inhomogeneous meteorological environment. The acoustical model takes up the simulated meteorological fields and calculates the propagation of noise which originates from a line source at one of the slopes of this valley. The coupled modeling system ensures consistency of topography, meteorological parameters and the sound field. The temporal behaviour of the sound pressure level across the valley is examined. It is only governed by the time dependent meteorology. The results show remarkable variations of the sound level during the course of a day.

Modelling sound propagation from a wind turbine under various atmospheric conditions

Sound propagation from a wind turbine is investigated with the help of coupled atmospheric and acoustical simulation. Eight instants of time during nine diurnal cycles of idealized large-scale cases result in 72 meteorological situations for which the boundary-layer profiles of wind and temperature, the downstream wake flow of the rotor, and the sound propagation from the wind turbine into upwind and downwind direction were calculated. The resulting sound levels are evaluated relative to a non-refractive atmosphere to focus on the impact of meteorologically induced refraction. Within a range of 1 km from the turbine the sound levels vary stronger in upwind direction than in downwind direction, and the varying background wind speed at hub level causes a higher variability of the relative sound levels than the variations of the surface-layer stability. Surface-layer parameters turn out to be of only limited suitability for a meteorological classification of the wind-turbine noise impact.

Acoustic effects of trees simulated by a finite-difference time-domain model

The simulation of sound propagation in and around forest areas requires a detailed understanding of the acoustic properties of trees. Therefore, the acoustic properties of laser-scanned real trees and idealized trees are studied by means of a three-dimensional finite difference time domain (FDTD) sound propagation model. The aim is to gain functional relationships between backscattered and transmitted sound pressure levels and selected tree characteristics. The choice of the model type enables the simultaneous consideration of multiple reflections and diffraction of sound waves. The spatial resolution of 5 cm admits a frequency range up to 1350 Hz and the resolution of the trunk and major branches. The simulated sound pressure signals are detected by a virtual microphone array in front and behind an infinitely long row of trees, respectively. Regarding the density of tree material within a volume, thin objects reflect less sound energy than dense objects, which in contrast have a noticeable acoustical effect on the transmission loss. The relations are provided by a set of empirical equations.

Meteorologically Induced Variability of Sonic Boom of a Supersonic Aircraft in Cruising or Acceleration Phase

The influence of the meteorological variability on the characteristics of the sonic‐boom from a projected commercial SST aircraft is investigated. The sonic boom is calculated by means of an advanced algorithm, taking into account nonlinear distortion, absorption, refraction including shadow zones, and focusing effects. Real meteorological situations are considered based on a full ten‐year data set in 12 and/or 24 hour resolution. Three different climate regions are studied: a mid‐latitude coastal sea region, a tropical coastal sea area, and a sub‐polar land region. For the acceleration phase, the study is limited to a one year data set above the English Channel for flights from Paris to New York. Frequency distributions of sonic boom characteristics such as wave amplitude, rise time, carpet width, location, strength and geometrical extension of caustics are shown for each area, all seasons and opposing flight directions. While variability turns out to be low for cruising‐flight boom at the ground track, ...