Near wall flow topology over a rounded crest of an oblique and

3.3 Non-erodible particles effects on the methodology of dust emission quantification

4.1.3 Analysis of a rounded crest stockpile: influence on dust emissions

4.1.3.2 Near wall flow topology over a rounded crest of an oblique and

Oblique - 30^◦

For30^◦, Figures 4.3a, 4.3b and 4.3c are the results of near wall flow distribution for, respectively, a sharped crest, a rounded with rpile = 1 and a rounded with rpile = 5. The near wall flow distribution is represented with the same scale to enable the visualization of differences between the configurations.

The numerical results have shown differences but an overall near wall flow pattern nearly the same. In Figures 4.3a, 4.3b and 4.3c three main regions have been highlighted:

• regions 1-a, 1-b and 1-c: stockpile crest and the detachment point,

• regions 2-a, 2-b and 2-c: leeward wall,

• regions 3-a, 3-b and 3-c: ineffective zone of the stockpile surface (the zone without effects on dust emissions)

Other regions have less importance on the final distribution: the windward wall and the impinge-ment region (smaller shear over the wall).

In Figure 4.3b, about the rounded crest with rpile = 1, some significant modifications are ob-served. The rounded crest (region 1-b) presents a significant increasing of us/ur levels compared to region 1-a on the sharped crest, from approximately 0.90 to 1.30. Due to the modification of the crest’s shape (from sharped to rounded) the fluid behaviour on the crest line is strongly altered.

For the sharped crest, the elevated levels of us/ur are result of flow acceleration, normally observed on this kind of geometry. In the case of a rounded crest, the flow presents a different pattern of acceleration. The rounding of the crests causes the increasing of zones with strong velocity gradient and consequently higher levels of us/ur.

For rpile = 1, there are no significant modifications of the leeward wall flow pattern or the ineffective zone (respectively, regions 2-b and 3-b). The main vortex is differently formed and as a consequence has lower effects on the leeward wall of the rounded configuration. The ineffective

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Figure 4.3: Numerical simulation results of the near wall flow distribution by means of the ratio us/ur of a stockpile oriented30^◦: (a) sharped crest, (b) rounded crest with radiusrpile= 1and (c) rounded crest with radius r_pile= 5

zone on the leeward wall is slightly augmented and the highest value observed over this zone is not greater than 1.00 while for the sharped crest it overpasses this value.

Among the tested configurations the crest presented in region 1-c (rpile = 5) has the lowest exposure levels. The maximum value ofus/uris about 1.15 near the portion located more upstream the crest. However, the higher radius caused the formation of a crest having a higher surface of contact with the incoming wind flow at the top of the pile. In that case, even the weaker acceleration occurs over a greater zone which may represent in the quantifications a elevated dust emission rate.

Regions 2-c and 3-c have smaller surfaces than those seen in Figure 4.3b but present the same features: ineffective zone and effects of the main downstream vortex.

Oblique -90^◦

Besides the results of the pile oriented 30^◦, the investigation of an another oblique pile (60^◦) and a perpendicular pile is also the scope of the present work . The results corresponding to the perpendicular stockpile will be presented in this chapter before those of 90^◦. For 60^◦ additional informations are given due to their more importance in near wall, air flow and dust quantification.

In that way, Figure 4.4 illustrates the near wall flow topology for the perpendicular stockpile. The main near wall flow features for this configuration are: the highest zones of wall friction values (or near wall velocity) are located on both stockpiles lateral sides (region 1-a) and over the entire crest equally distributed (region 2-a) and two zones of very low levels of velocity near the stockpile surface, close to the ground on the windward wall and on the leeward wall (3-a). The first ineffective zone on the pile is the stagnation region where the incoming flow is deviated in this region towards sides and top of the pile. Secondly, the leeward wall is inside the recirculation bubble formed downstream the pile.

The modification of the crest, firstly consideringr_pile= 1, causes the increase of the maximum velocity ratio value from about 1.00 to approximately 1.15 (region 2-b). These high values are noticed over a significant region along the crest line. The others features, commonly observed on the sharped crest, are also noticed on the rounded crest with r_pile = 1. The featuring of r_pile = 5 shows the formation of an extended zone of high values of us/ur along the crest (regions 1-c and 2-c). However, its maximum value is reduced to approximately 0.95, the lowest among the crests tested.

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Figure 4.4: Numerical simulation results of the near wall flow distribution by means of the ratio us/ur of a stockpile oriented90^◦: (a) sharped crest, (b) rounded crest with radiusr_pile= 1and (c) rounded crest with radius rpile= 5

Oblique -60^◦

The near wall flow distribution of the stockpiles oriented 60^◦ is shown in Figures 4.5a-4.5d.

Modifications of the fluid flow pattern caused by the rounded shape of the crest are more clearly noticed for this wind flow direction. For this configuration, a third value was tested: rpile = 0.5.

The aim is to check the sensibility of the fluid flow pattern due to a slight rounding of the crest.

The sharped pile does not present a clear visualization, because of this scale, of the fluid flow pattern normally observed in other sections. Here, only a local augmentation of the levels ofus/uris observed, the detachment point very upstream the crest line (near to mark 2-a). us/urlevel reaches 1.30 in this region. On region 3-a the main vortex presents its main effects in which high levels of us/urare noticed near the ground at the bottom of the wall. In a comparison between the four cases, regions 1-a to 1-d present very few differences. Only approaching the crest, the most considerable discrepancies are noticed. The zones of us/ur greater than 0.75 slightly increases over regions 3-a to 3-d. The pattern of region 3 is highly modified forr_pile= 5.

Finally, the rounded crest causes a high increase ofus/urvalues for a significant surface over the crest line. The distribution over the crest line, increases from values lower than 1.00 to maximums about 1.80. Figure 4.5b, which represents rpile = 0.5, shows that, even for a small rounding of the crest line the us/ur values are increased. The configuration with r_pile = 1 (Figure 4.5c) also presents the high levels near wall velocity distribution. Figure 4.5d (rpile= 5) shows a maximum of about 1.30 for the values of the near wall velocity ratio. As for the other orientations, the maximum magnitude is not very high, but the surface of impact is the largest. For instance, in the configuration

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Figure 4.5: Numerical simulation results of the near wall flow distribution: ratious/ur of a stockpile oriented60^◦: (a) sharped crest, (b) rounded crest withrpile= 1and (c) rounded crest withrpile= 5

30^◦,rpile= 1 presented a maximum of 1.40 and rpile= 5 a maximum of 1.05.

A more detailed understanding of the way that the fluid flow is modified due to the different shapes of crest, is presented in Figure 4.6. In this figure, for the four tested cases of the 60^◦ wind direction, the fluid flow is shown by means of contours of the longitudinal wind velocity over a plane XZ in the middle of the pile. The longitudinal velocity values are normalized by their values measured at the distance corresponding in the model scale to 10 m height in real scale (Uref = 5.26 m/s).

The fluid flow pattern near the stockpile peak (crest line) is magnified permitting a more detailed observation of the velocity distribution very near the crest.

The longitudinal fluid flow observed for the four tested configurations presented in Figures 4.6a-4.6d has similar characteristics. The most relevant discrepancies are found on: (i) the near wall of the crest line, (ii) the formation of the main vortex and (iii) its effects on the leeward wall. A representation of the main vortex is noticed in this plane by the negative velocity values downstream the pile. The smaller vortex is more attached to the leeward wall of the pile and presents more effects on this surface (highlighted arrow at the right of Figures 4.6). Moreover, on the leeward wall, the ineffective zone is smaller for the rounded crest piles. The rounding of the crest promotes a very less abrupt flow detachment. This situation is clearly observed in Figure 4.6a which is compared to the other Figures 4.6. In fact, the recirculation observed in Figure 4.6a means a flow that has been more accelerated due to the shape of the stockpile crest. Additionally, the colours of the contours near the leeward wall show a more accelerated flow in Figure 4.6d. The length of the main vortex is strongly reduced.

Zone B on the four sub-figures show the evolution of the downstream main coherent structure which correspond to the change in wind exposure downstream of the pile seen in Figure 4.6. For the sharped crest, the flow separation is located on the crest. The separation angle is very close to the side slope. For rpileranging from 0.5 to 5, the separation angle decreases and the separation is extruded downstream from the beginning of curvature of the pile surface.

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Figure 4.6: Contours of longitudinal wind velocity plotted over a XZ plane located at the computa-tional domain center line for the stockpile oriented 60^◦ (at right, detail of separation on the crest):

(a) sharped crest, (b) rounded with radius rpile = 0.5, (c) rounded with radius rpile = 1 and (d) rounded with radiusrpile= 5

Dans le document Thèse de doctorat. Pour obtenir le grade de Docteur de l Université de VALENCIENNES ET DU HAINAUT-CAMBRESIS. et de L ECOLE DES MINES DE DOUAI (Page 164-168)