Wind tunnel Assessment of Wind Environment and Dust Suppression Effect of Windbreak in Port Bulk Storage Yards

. Windbreaks projects are constructed in order to reduce the dusting in port bulk storage yards. The assessment of wind environment and dust suppression effects are important in the planning or preliminary design of such projects. This paper takes a large port bulk storage yard with 853m x 966m area as an example, velocity measurement wind tunnel tests were carried out. The wind velocity reduction ratio and dust suppression rate were used to access the wind environment and the dust suppression effect. The influence of windbreak height was investigated. The comprehensive wind speed reduction rate and dust suppression rate vary with windbreak height nearly linearly. And combined with spraying system, the dust suppression rate can reach beyond 95%, which provides reference and assessment method for relative engineering applications.


Introduction
Dusting in port bulk storage yards is one of the key atmospheric pollution sources for coastal cities. And dusting form bulk piles contribute the most to the port dust pollution [1]. Windbreaks are porous plates that could reduce the wind speed and optimise the wind environment thus suppressing the dusting. Windbreak projects constructed in the bulk storage yards are effective dust suppression facilities, which have been widely applied in engineering practices. However, the selection of windbreak parameters was important especially in the preliminary design process, including the horizontal plan, height and prosperity of the windbreaks.
The assessment of the wind and dust suppression effect should also consider the meteorological conditions and pile conditions. Usually CFD simulation or wind tunnel test should be carried out to assess of the wind and dust suppression effect. CFD simulation on the wind environment takes less effort and fruitful results should be used in the preliminary plan [2,3]. While wind tunnel tests with higher precision, however takes much effort and resources, should be carried out to validate the parametric settings, and provide more evident data to support the detailed construction plans, which is crucial for large and important projects [4,5]. However, some detailed information of key assessment indexes for windbreak engineering application was rarely reported and discussed in literatures, which requires further investigation.
This paper takes a large port bulk storage yard with 853m x 966m area as an example, wind tunnel tests were carried out. The wind velocity reduction ratio was sued to access the wind environment and dust suppression rate were to evaluate the dust suppression effect of the construction of windbreak project. The influence of windbreak height was investigated, which provide reference and assessment method for relative engineering applications.

Test devices and models
The wind tunnel test was carried out in the open circuit wind tunnel with 2.5m x 4.4m section wind tunnel at Tianjin research institute for water transport engineering (TIWTE). The wind tunnel was driven by a 440kW DC motor with maximum simulation wind speed 30m/s. The research object was a large port bulk storage yard with 853m x 966m. The diameter of the yard is about 1.3km with the geometric centre of the yard as the centre. According to the geometric scale ratio of 1:350, the ABS plate is used to make the wind tunnel test model of stacking, and the interference of buildings / structures inside the yard is considered (Fig. 1a). The windbreak was made of ABS with 30% prosperity (see Fig. b). Irwin probe speed measuring point is arranged on the front and side of each pile surface, 65 stacks are arranged in the yard, 5-6 probes are arranged in each stack (see Fig. 1d), 328 Irving probes are arranged in total, each Irwin probe measuring point has both static and total pressure measuring channels(see Fig. 1c), thus, 656 pressure measuring channels in total.

Test conditions and cases
The atmospheric boundary layer (B type in GB50009-2012 [6]) was simulated with power law index 0.15 by roughness elements arranged in the wind tunnel. The simulated wind profile was shown in Fig. 2.
In this wind tunnel test, the geometric scale ratio is 1:350, and the wind speed ratio is 1:1. According to dimensional analysis, the time scale ratio is 1:350, and the sampling time is 15s, which corresponds to the actual wind speed statistical time interval of more than 1 hour, meeting the measurement requirements. In this wind tunnel test, the wind speed of each stacking surface in 16 wind directions under three conditions of no windbreak, windbreak height of 19 ~ 25m was measured by Irwin probe through electronic pressure scanning valve. During the test, Cobra Probe is used to measure the incoming reference wind speed at the stacking height of the incoming flow position. In each case, 16 azimuths were considered, N for 0°, tested in each 22.5°. The test cases were shown in Table 1 and the test photos of each condition were shown in Fig. 3.

Wind velocity reduction ratio
The wind velocities measured by the Irwin probe in each case was processed as non-dimensional wind velocity reduction ratio denoted as λj where Uj0 is the wind velocity on measuring tap j of pile surface without windbreak (from case 1~16). And Uj1 is the wind velocity on measuring tap j of pile surface with windbreaks of different heights (from case 17~80). λj denotes the influence of windbreaks on wind environment. The lower value of λj means the more effective windbreak acts. To average the wind velocity reduction ratio with surface area, the bulk yard wind velocity reduction ratio was calculated as where Aj is the surface area of each measuring tap j. Then λ denotes the comprehensive wind environment situation under a certain windbreak plan.

Dust suppression rate
Using the dusting formula recommended by EPA [7], the dusting coefficient is related to the friction velocity, estimated by where Q is the dusting coefficient, Bj is the material dusting modification coefficient of tap j, kDj is the dusting factor of tap j, wDj is the material mass factor of tap j, PDj is the dusting potential of tap j, u*j is the friction velocity of tap j, u*Djt is the critical friction velocity of tap j, uzj is the wind velocity of at height z of tap j, z0 is the roughness length and κ is the von-Karman constant. The comprehensive dust suppression rate η was calculated by where Qi0 is the dusting coefficient of the ith azimuth without windbreak (from case 1~16). And Qi1 is the dusting coefficient of the ith azimuth with windbreak of different heights (from case 17~80). Ei is the dusting rate of the ith azimuth and fi is the wind frequency of azimuth i obtained form meteorological information.

Wind environment assessment
The bulk yard wind velocity reduction ratio calculated by Eq. (2) was plotted in a rose map shown in Fig. 4. It is indicated from the figure that, as the windbreak height increase, the wind velocity is reduced. When the flow are direct to the windbreak plates (N, E, S, W) the wind velocity reduced more than the oblique flows.
The dusting rate showed opposite feature, demonstrated by Fig.5. Under the flow are direct to the windbreak plates (N, E, S, W), the dusting is effectively suppressed. For the flow from the corner of the bulk yard, dusting still remained to some extent.

Dust suppression effect
The wind velocity reduction ratio and dust suppression rate are plotted with non-dimensional height (the ratio between windbreak height and pile height), shown in Fig.  6. It can be inferred that in the non-dimensional height range 1.25 to 1.7, the variation of wind velocity reduction ratio and dust suppression rate with non-dimensional height is nearly linear. It can be also inferred from figure that, combined with spraying system, when the moisture content reach about 8.0%, the dust suppression rate could reach more than 95%.

Conclusion
In this paper, wind tunnel tests on accessing the wind environment and dust suppression effect of a large port bulk storage yard with 853m x 966m area were carried out. The wind velocity reduction ratio to access the wind environment and dust suppression rate were to evaluate the dust suppression effect of the construction of windbreak project. The results shows that the windbreak shows better effect when the flow comes direct to the plates than oblique flows. The comprehensive wind speed reduction rate and dust suppression rate vary with windbreak height nearly linearly, which provides reference and assessment method for relative engineering applications.