Turbulence characterization of downbursts using LES

• Large eddy simulation of downbursts impinging on various exposure conditions are performed in order to describe its turbulent characteristics.
• Ground roughness is simulated by fractal surfaces scaled to produce an aerodynamic roughness of the target exposure.
• Large circumferential length scales were obtained reaching up to nine times the jet diameter indicating that the turbulence associated with downbursts is very well correlated circumferentially.
• Vertical length scales are smaller than the circumferential length scales by an order of magnitude.
• Radial length scales are in the order of the jet diameter, which are larger than typical longitudinal length scales for synoptic winds.
• Finally, the significance of the information presented in this study to calculate the gust front factor for structures in general and for transmission lines in particular is discussed.

Loads associated with downbursts represent a significant vulnerability on various structures. Designing the structures to withstand such loads requires the knowledge about the turbulent characteristics of downbursts, which are the focus of the current study. To this effect, large eddy simulations (LES) of downbursts impinging over four different exposures namely open, countryside, suburban and urban, are performed. Ground surface roughness is simulated using fractal surfaces generated by random Fourier modes (RFM) and scaled to match a targeted aerodynamic roughness z0. Simulated wind velocities are averaged spatially and temporally to extract the mean and turbulent components. Properties of both the mean and the turbulent components are discussed. Turbulence length scales, which govern the wide band correlations of the turbulence, are determined in the circumferential, the vertical and the longitudinal directions. It is found that the length scales in the circumferential direction are larger than those in the vertical direction by at least an order of magnitude, indicating that downburst turbulence is more correlated in the circumferential direction. This has a particular importance for long horizontal structures such as transmission lines and long span bridges. Narrow band correlations and the turbulent spectra, which have a particular importance for flexible structures, are also discussed. Applicability of using the resulting turbulent characteristics to estimate the peak forces on structures, e.g. transmission lines, is deduced by employing the gust factor approach.