دانلود رایگان مقاله اندازه گیری PTV مولفه جهت span حمل و نقل بادی در حالت پایدار

abstract

This paper outlines and validates an improved particle tracking technique (PTV-EPAS) with automated trajectory detection capabilities, and then reports on a novel set of wind tunnel experiments aimed at measuring all three velocity components simultaneously. In order to study a fully adjusted particle cloud, the entire floor of the tunnel was filled with quartz sand (median diameter 550 lm) and the freestream velocity set to 8 ms
1 at an elevation of 0.35 m, above the threshold for particle entrainment at 6.5 ms
1 . This produced a friction velocity (u⁄ ) of 0.38 ms
1 with u⁄ /u⁄ t = 1.3. Measurement of particle trajectories aligned at a spanwise angle (h) relative to the mean airflow along the center-line of the wind tunnel involved incrementally adjusting the light sheet orientation from 0
to 60
. Three replicate experiments were carried out for each of 13 angles. Only 12% of all 2  105 trajectories sampled were strictly aligned with the mean streamwise air flow, while 95% were contained within 45
. As h increases, a greater proportion of the particle transport consists of slow moving ejecta that ascend from and then impact the bed surface at higher angles than observed for saltation.

5. Conclusions

Experimental observation of particles within a fully saturated saltation cloud has historically been a challenging task because of inadequate lighting, difficulties with surface detection, and a high degree of error associated with particle detection and trajectory identification. In this paper the EPAS-PTV method, based upon particle radius comparison, is first optimized to reduce trajectory identification errors and then validated through comparison with measurements obtained using laser Doppler anemometry (LDA). Application of this technology in a novel wind tunnel investigation of the spanwise component of trajectories within a saturated saltation cloud reveals that less than 1/8th of particles travel directly along the path of the mean air flow. However, 95% of the particles sampled are contained within 0
6 h 6 45
. This study provides the first direct measurements of the x, y and z components of particle velocity and their respective probability distributions. The alignment of the flight path is found to systematically alter the total velocity of a given particle, as well as its launch/ impact angle. The observed decline in the proportionate particleborne kinetic energy with increasing spanwise angle, however, is found to be driven primarily by the waning particle counts and not speed. At high angles, the primary mode of transport appears to shift from saltation to reptation. Such observations may have important implications for the parameterization of emerging 3D saltation models, as well as for understanding the inception and growth of small-scale aeolian bedforms in the context of particle diffusion, both of which are beyond the scope of the present paper. An extension of this 2.5D study is presently underway to quantify particle acceleration for the full parabolic trajectories captured and to determine the variation in U and d with elevation above the bed surface.