Selection of the samples, used as wall material, was made based on the fabrication process of the surface. It would be far easier to work on aluminum than on steel plates, although the action of the particles could cause wearing, but this could be accounted for later on (as a correction factor, maybe). Previous researchers have made similar studies by using plates with a standard finish. There is a table (that you can find on the internet) that lists standard machined finish characteristics, which include the roughness values measured with a profilometer (either mechanical or optical), different values are given in this table (one of these is called RMS, or root mean square) that one could use to establish the roughness of the finish on your plates. There are manufacturing procedures that ensure that the surface would have the corresponding standard finish and, consequently, the corresponding surface roughness.
But the problem with this approach is the lack of consistency in the orientation of the so-called grain. It is an experimental fact that grain (which is the structure above the mean wall surface, such as peaks) plays an influential part in friction. If the flow of the powder is against the grain (picture this as particles finding obstacles as they move along the surface), the wall friction increases. But if the powder flows along the grain (particles move along the peaks, instead of going over the peaks), then wall friction reduces appreciably.
To be continued…
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