A vector fast multipole algorithm for low frequency problems

Instead of the traditional factorization of the scalar Green's function by using scalar addition theorem in the lowfrequency fast multipole algorithm (LF-FMA), we adopt the vector addition theorem (VAT) for the factorization of the dyadic Green's function to realize memory savings for large scale problems. We validate this factorization and use it to develop a low-frequency vector fast multipole algorithm (LF-VFMA) for low-frequency problems. © 2010 IEEE.published_or_final_versionThe URSI International Symposium on Electromagnetic Theory (EMTS 2010), Berlin, Germany, 16-19 August 2010. In Proceedings of the URSI International Symposium on Electromagnetic Theory, 2010, p. 620-62

Large-Eddy Simulation of Flow and Pollutant Transports in and Above Two-Dimensional Idealized Street Canyons

A large-eddy simulation (LES) model, using the one-equation subgrid-scale (SGS) parametrization, was developed to study the flow and pollutant transport in and above urban street canyons. Three identical two-dimensional (2D) street canyons of unity aspect ratio, each consisting of a ground-level area source of constant pollutant concentration, are evenly aligned in a cross-flow in the streamwise direction x. The flow falls into the skimming flow regime. A larger computational domain is adopted to accurately resolve the turbulence above roof level and its influence on the flow characteristics in the street canyons. The LES calculated statistics of wind and pollutant transports agree well with other field, laboratory and modelling results available in the literature. The maximum wind velocity standard deviations σi in the streamwise (σu), spanwise (σv) and vertical (σw) directions are located near the roof-level windward corners. Moreover, a second σw peak is found at z ≈ 1.5h (h is the building height) over the street canyons. Normalizing σi by the local friction velocity u*, it is found that σu/u* ≈ 1.8, σv/u* ≈ 1.3 and σw/u* ≈ 1.25 exhibiting rather uniform values in the urban roughness sublayer. Quadrant analysis of the vertical momentum flux u′′w′′ shows that, while the inward and outward interactions are small, the sweeps and ejections dominate the momentum transport over the street canyons. In the x direction, the two-point correlations of velocity Rv,x and Rw,x drop to zero at a separation larger than h but Ru,x (= 0.2) persists even at a separation of half the domain size. Partitioning the convective transfer coefficient ΩT of pollutant into its removal and re-entry components, an increasing pollutant re-entrainment from 26.3 to 43.3% in the x direction is revealed, suggesting the impact of background pollutant on the air quality in street canyons. © 2011 The Author(s).published_or_final_versionSpringer Open Choice, 21 Feb 201

Flow pattern and pollutant removal behavior for idealized 2D urban street canyons in different thermal stratifications using large-eddy simulation

postprintThe 2011 General Assembly of the European Geosciences Union (EGU), Vienna, Austria, 3-8 April 2011. In Geophysical Research Abstracts, 2011, v. 13, EGU2011-293

Analysis of the momentum and pollutant transport at the roof level of 2D idealized street canyons: a large-eddy simulation solution

To investigate the detailed momentum and pollutant transports between urban street canyons and the shear layer, a large-eddy simulation (LES) model was developed to calculate the flow and pollutant dispersion in isothermal conditions. The computational domain consisted of three identical two-dimensional (2D) idealized street canyons of unity aspect ratio. The flow field was assumed to be periodic in the horizontal domain boundaries. The subgrid-scale (SGS) stress was calculated by solving the SGS turbulent kinetic energy (TKE) conservation. An area pollutant source with constant pollutant concentration was prescribed on the ground of all streets. Zero pollutant concentration and an open boundary were applied at the domain inflow and outflow, respectively. The quadrant and budget analyses were employed to examine the …published_or_final_versionThe 7th General Assembly of the European Geosciences Union (EGU2010), Vienna, Austria, 2-7 May 2010. In Geophysical Research Abstracts, 2010, v. 12, EGU2010-1486-

Large-eddy simulation of turbulent transports in urban street canyons in different thermal stabilities

Three scenarios of large-eddy simulation (LES) were performed to examine the characteristic flow and pollutant dispersion in urban street canyons under neutral, unstable and stable thermal stratifications. Street canyons of unity aspect ratio with ground-heating or –cooling are considered. In the LESs of the thermal stabilities tested, a large primary recirculation is developed in the center core and the turbulence production is dominated at the roof level of the street canyon. The current LES results demonstrate that unstable stratification enhances the mean wind, turbulence and pollutant removal of street canyons. On the other hand, in stable stratification, which has been less investigated in the past, the ground-level mean wind and turbulence are substantially suppressed by the large temperature inversion. Whereas, the weakened recirculating wind in the street canyon results in a larger velocity gradient that increases the turbulence production at the roof level. It also slows down the turbulence being carried from the roof down to the lower street canyon. Therefore, a higher level of turbulent kinetic energy (TKE) is retained at the mid-level of the windward side in the stably stratified street canyon.postprintThe 5th International Symposium on Computational Wind Engineering (CWE2010), Chapel Hill, N.C., 23-27 May 2010

Convergence of low-frequency EFIE-based systems with weighted right-hand-side effect

published_or_final_versio

A memory saving vector fast multipole algorithm for solving the augmented EFIE

An augmented EFIE (A-EFIE)[9], [10] has been proposed to separate the contributions of the vector potential and the scalar potential for avoiding the imbalance at low frequencies. The corresponding low frequency fast multipole algorithm (LFFMA) [11] was also developed for solving the A-EFIE. Instead of the factorization of the scalar Green's function by using scalar addition theorem in the LF-FMA, we adopt the vector addition theorem for the factorization of the dyadic Green's function to realize memory savings. We are to develop a vector fast multipole algorithm for solving the A-EFIE. © 2010 IEEE.published_or_final_versionThe URSI International Symposium on Electromagnetic Theory (EMTS 2010), Berlin, Germany, 16-19 August 2010. In Proceedings of the URSI International Symposium on Electromagnetic Theory, 2010, p. 134-13

Implementation of a simplified form of CMP-EFIE for low-frequency capacitive problems

Session: Well-Conditioned Integral Equation Formulations: paper 106.8In this paper, a simplified form of electric field integral equation with Calder´on multiplicative preconditioner (CMP-EFIE) is presented for solving capacitive problems in the low-frequency regime. The decomposed three-term CMP-EFIE, though very stable, cannot capture the current accurately for a capacitor at low frequencies. By omitting the part of the hypersingular preconditioned term, the electric currents solved from the remaining two terms are of the right frequency order for a capacitor. Hence, the resultant two-term formulation can guarantee the accuracy of results for capacitive problems, and also makes the iterative system more stable at low frequencies. Both theoretical analysis and numerical results are provided to verify the practicality of the proposed method.published_or_final_versio

Large-eddy simulation of street canyon flow and pollutant transport in neutral and unstable stratifications

Session 15 - Urban Dispersion II: 15.1postprintThe 90th Annual Meeting of the American Meteorological Society (AMS), Atlanta, GA., 17-21 January 2010

A vector potential integral equation method for electromagnetic scattering

In this paper, we implement a vector potential integral equation method for electromagnetic scattering problems. The proposed equations are formulated by imposing both the tangential and normal components along the surface of a scatterer. They can be solved by a subspace projection method such as the Galerkin's or moment methods by considering the induced surface currents and the normal component n. A as unknowns. Numerical results validate the effectiveness, stability and accuracy of the proposed formulation.postprin