Concerning the concentration, please note that on the one hand, the solids do not flow with the same velocity as water, because of their large size and of the large density ratio—in other words there is a non-negligible slip velocity. On the other hand, in horizontal parts, among the various regimes that may be observed there exist regimes with a stationary layer of solids at rest at the bottom of the pipe[4,12,13]. There could thus be a great difference between the transport or deliveredconcentration C and the in situ , local or volu-metric concentration s that is the ratio between the area occupied
by the solids and the area of the pipe. Further, the volumetric con-centration is a key parameter in the modelisation, as illustrated in Sections 3.1 and 3.3. This concentration is however not a control parameter but results from the geometry, the physical properties of the phases and the boundary conditions.
Concerning the concentration, please note that on the one hand, the solids do not flow with the same velocity as water, because of their large size and of the large density ratio—in other words there is a non-negligible slip velocity. On the other hand, in horizontal parts, among the various regimes that may be observed there exist regimes with a stationary layer of solids at rest at the bottom of the pipe[4,12,13]. There could thus be a great difference between the transport or deliveredconcentration C and the in situ , local or volu-metric concentration s that is the ratio between the area occupied
by the solids and the area of the pipe. Further, the volumetric con-centration is a key parameter in the modelisation, as illustrated in Sections 3.1 and 3.3. This concentration is however not a control parameter but results from the geometry, the physical properties of the phases and the boundary conditions.
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