2.3 Dust emission investigation in wind-tunnel
3.1.2 Influence of stockpile orientation on the non-erodible particles effects on dust
3.1.2.2 Influence of cover rate and wind flow velocity on the temporal evo-
The present section analyses the temporal evolution of the emitted mass flux obtained by continuous measurements in wind-tunnel. Previously, section 3.1.1, with focus on the perpendicular orientation, has carried out an equivalent analysis that will be presented herein for 30◦ and 60◦. Indeed, the present discussions concern the possible modifications that may occur on the influence of
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UFigure 3.2: Qualitative comparison of erosion near the surface of an oblong stockpile oriented 60◦: (a) experimental photographs representing a top view of the pile and some highlighted zones and (b) contours ofus/ur values from CFD calculations
erodible particles on the temporal evolution of emitted mass flux for oblique stockpiles. In addition, the influence of cover rate and incoming wind velocity magnitude on the plots of emitted mass flux are likewise subject of interest. The results are divided into three parts for each tested wind flow orientation:
• evolution of non-erodible particles agglomeration over the surface (known as covering of the
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Figure 3.3: Temporal evolution of surface covering in wind-tunnel experiments of a stockpile oriented 30◦, cover rate 10% and wind velocity 7 m/s: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
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Figure 3.4: Influence of the cover rate on the temporal decrease of the emitted mass flux of a stockpile oriented 30◦ and wind velocity equal to 8 m/s: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
erodible surface) together with the decrease of the emitted mass flux for a given configuration:
correlation between the zero mass flux reached in the plot and the last pattern observed by the surface covering,
• analysis of the plot of temporal evolution of emitted mass flux and the non-erodible particles distribution over the sand pile for a same incoming wind velocity and different cover rate values and
• identical analysis of the precedent item, but for three wind velocity magnitudes and one cover rate.
Figure 3.3 exhibits the temporal evolution of the emitted mass flux for an isolated stockpile submitted to a turbulent flow in wind-tunnel. Additionally, a sequence of photographs, which are top views of the stockpile during the experience, is presented.
The decrease of the emitted mass flux, noticed in the plot of Figure 3.3a, may be explained through the growing surface for non-erodible particles which covers more and more the pile surface.
The emitted mass flux presents a rapid decrease during the first four minutes. The photographs in Figure 3.3b show the begin of the covering over the crest line, the most eroded zone over the pile, at 2’30”. The photographs after 2’30” show the evolution of the pavement over the highly eroded regions, namely, the leeward wall (effects of the main vortex) and some zones over the windward wall. The pattern shown at 10’30” remains unchanged if the air still flows over the pile. At this
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Figure 3.5: Influence of the wind velocity on the temporal decrease of the emitted mass flux of a stockpile oriented30◦and cover rate equal to 10%: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
time, the emitted mass flux is undoubtedly zero as shows the plot. Finally, it should be said that the temporal decrease of the emitted mass flux varies accordingly with the pavement of the surface by the non-erodible particles.
The influence of the amount of non-erodible particles, i.e., the cover rate, for the same orientation, is analysed in Figures 3.4a and 3.4b which present, respectively, the plot of temporal decrease of emitted mass flux and the evolution of the agglomeration of non-erodible particles over the stockpile surface for 10 and 20% and a high wind velocity of 8 m/s.
The temporal decrease seems to be slightly faster for the cover rate 20%. The erosion at the level of the stockpile crest is very strong for this orientation (30◦). There are more erodible particles in the cover rate 10%. Thus, those erodible particles are rapidly eroded from the crest and enable the the agglomeration of the emerging non-erodible particles. The pavement occurs faster on the crest zone for this cover rate. The zero seems to be reached slightly faster for 20% as the mass flux is smaller since the beginning and the higher amount of non-erodible particles develops this condition.
The cover rate has lower influence on the different evolution patterns of emitted mass flux decrease than it was observed for the perpendicular orientation.
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Figure 3.6: Temporal evolution of surface covering in wind-tunnel experiments of a stockpile oriented 60◦, cover rate 10% and wind velocity 7 m/s: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
Figure 3.5 shows the temporal evolution and photographs for the two tested velocities and a same cover rate (10%). The photographs taken at the same time instant, indicate a higher covering for 8 m/s. The initial values (first four minutes, approximately) of emitted mass flux are also highly different among the tested velocities: almost two times the value for 8 m/s compared with 7 m/s.
The plot for 8 m/s shows a rapid decrease. As a result of that the zero seems to be reached at the same time for the two configurations.
Identical analysis, as those carried out for the stockpiles oriented30◦ are shown in Figure 3.6, 3.7 and 3.8 for another oblique configuration: 60◦. The zones of erosion over the pile (see Figure 3.1.2.1 for more details about the distribution of wall erosion) shows that the main eroded zones are entirely covered at the instant 8’30” and 10’30”. It is worth to note that, the instant of surface covering happens later for this orientation than for 30◦. This is in-line with previous CFD calculations and
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Figure 3.7: Influence of the cover rate on the temporal decrease of the emitted mass flux of a stockpile oriented 60◦ and wind velocity equal to 8 m/s: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
mathematical quantification of dust emission (Turpin and Harion (2009) [72] and Furieri et al.
(2012) [31]) have already predicted that the oblique orientation 30◦ is the more eroded among the tested configurations.
Furthermore, Figures 3.7 and 3.8, showing respectively the influence of cover rate and wind velocity. The analysis of the influence of cover rate is quite different for60◦. The temporal evolution presents a slower decrease for the tested case presenting more non-erodible particles. The photographs in Figure 3.7 qualitatively check this statement. The evolution of the covering is very similar between the two cover rate values. The very strong erosion in these oblique configurations cause a high movement of the coarse particles over the surface. Thus, for a longer time the erodible surface is free for erosion. The wind velocity promotes equally evolution in time for each tested velocity.
The last part of this section concerns the summary of mass balance (weighing of the sand model stockpile before and after the wind-tunnel experiments) for the three incoming flow orientations tested in this work. Thus, Table 3.1 shows the values of emitted mass. These values are shown in grammes. For each value, an interval is presented, which is the result of the uncertainty calculated by means of repeatability tests carried out for at least one configuration of all tested orientations (those tests are detailed, for the perpendicular configuration, in section 3.1.1). The uncertainty considered for the values of emitted mass flux is the standard deviation of the repeatability tests which is about 100 g/min.m2. The values of emitted mass shown in Table 3.1 reveal a pattern similar to that predicted in the numerical quantification (cf. section 4.1.2 for more details about the dust emissions estimation). There is a generally good agreement between the present experimental mass balance and numerical prediction.
The values of emitted mass for the orientation30◦ are slightly higher (mean values) or even may present the same values of other orientations depending on the uncertainty. For all tested cases, the dust emission is considerably lower for the highest cover rate value (20%). Also, for all orientations
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Figure 3.8: Influence of the wind velocity on the temporal decrease of the emitted mass flux of a stockpile oriented60◦and cover rate equal to 10%: (a) plot of emitted mass flux and (b) experimental photographs of sand stockpile top view in which dashed lines show the evolution of non-erodible particles agglomeration
and cover rate values tested, the dust emission increases with the increase of wind velocity.