Analysis of the Vibration Isolation Effect of the Railway Rigid Sandwich Wall

The vibration isolation effect of the rigid sandwich wall was measured and the characteristics of time and frequency domain at low frequency was analyzed on the basis of the environmental isolation engineering of the railway in the Beijing suburb in this paper. The results indicated that environmental vibration significantly decreased after building the rigid sandwich walls. The vibration isolation efficiency of the rigid sandwich wall is respectively 0.32 and 0.46 at a distance of 35 meters and 40 meters away from railway center. It is obvious that a distance of 5 meters behind the vibration isolation barrier shows a more obvious vibration isolation effect than 10 meters. The vibration isolation barrier has an obvious effect in middle-high frequency (10~80 Hz) of the vibration, especially formed by sleeper spacing action rate (22.7 Hz). In addition, the effect is not obvious with a frequency below 10 Hz, such as the low frequency vibration formed by the moving axle load (7 Hz). In this paper, a vibration model of the earth-rigid sandwich wall was established. It also studied the effect of width, depth and position of the barrier on the vibration isolation, and provided the application condition and the setting requirement of key parameters for the rigid sandwich wall. These results provided the technical support for controlling projects of environmental vibration with rigid sandwich walls.

Evaluation of Turbulent Surface Flux Parameterizations over Tall Grass in a Beijing Suburb

Numerical weather and climate prediction systems necessitate accurate land surface–atmosphere fluxes, whose determination typically replies on a suite of parameterization schemes. The authors present a field investigation over tall grass in a Beijing suburb, where the aerodynamic roughness length (z0) and zero-plane displacement height (d) are found to be 0.02 and 0.44 m, respectively (the value of d is close to two-thirds the average grass height during this field experiment). Both z0 and d values are then used as input parameters of an analytic model of flux footprint; the footprint model reveals that eddy-covariance flux measurements are mainly representative of the tall grass surface concerned herein, potential influences from anthropogenic sources in this suburban area notwithstanding. Based on the “fair weather” data (with an energy balance ratio of 0.89), the authors evaluate four parameterizations of turbulent surface fluxes, namely, a total of three traditional “iterative” schemes and one “noniterative” scheme developed recently to reduce computational time. Their performances are intercompared in terms of the estimations of the sensible heat flux and two turbulence components (the friction velocity and temperature scale). In weakly stable to unstable conditions, two schemes are recommended here for their good performance overall; the first scheme stems jointly from the work of Högström and Beljaars and Holtslag, and the second stems from that of Li et al.. For this tall grass surface, the choice of z0/z0h = 100 (z0h is the thermal roughness length) is more appropriate than another choice of 10, because the former produces comparatively accurate sensible heat flux estimations.