A new UTD formulation for multiple cylindrical diffraction
This paper describes a new formulation of the uniform theory of diffraction (UTD) with application to multiple cylindrical structures. The new formulas present some modified UTD diffraction coefficients in cooperation with a slope diffraction term for higher accuracy in the electromagnetic modeling. The above are then combined with a simple ray tracing algorithm that takes into account multipath propagation between cylindrical structures. Comparisons with existing analytical models that employ other electromagnetic theories have shown that the new UTD solution can give very accurate results while it seems to be superior in terms of computation time and complexity. The solution can be applicable to radiowave paths which are characterized by convex structures and multipath propagation such as mountainous areas or urban areas with special rounded objects
Işın İzleme Tekniğinin Radyo Dalga Yayılım Modellerinde Kullanılması
Daha guvenilir, zaman acisindan verimli ve kesinligi yuksek elektromanyetik dalga siddeti kestirimi yapmak icin bircok yayilim modeli gelistirilmistir. Bu modeller numerik ve isin izleme teknigi tabanli olmak uzere iki kisimdir. Numerik modellerin hesaplama zamani ve kesinligi yuksektir. Buna karsin isin izleme teknigi tabanli modellerin hesaplama suresi daha kisadir. Vericiden cikan ve alicida sonlanan tum isinlarin tespit edilmesi alici uzerindeki bagil yol kaybinin dogru hesaplanmasi icin cok onemlidir. Bu calismada kisaca bazi yayilim modelleri hakkinda bilgi verildikten sonra isin izleme teknigi yazilimi aciklanacaktir ve karsilastirmalar yapilacaktir.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.