Hi gtg,

Any fluid passing between two surfaces with differing velocities will experience some shear. In the set-up you describe i think that more pumping is achieved than solely shear but both at the entrance to the "nip" of the gears as well as the exit the fluid will be shearing as the fluid contact the first gear will want to follow that gear i.e. stay in contact with the gear and the fluid contacting the other gear will likewise do the same. But due to the gears co-rotating a element of velocity is added as the gears are pushing fluid out of the trailing edge of the nip and pulling fluid in at the leading edge. So pumping is achieved to an extent.

Your machinist may be confusing terms because having the gears contact will increase the flow induced by the gear creating a true pumping action. This usually increases the pressure on the outlet side of the pump depending on viscosity of the fluid and the specific shape of the gear teeth. Also a newtonian fluid will typically thin when work is done to it. A gear pump not only moves the fluid but shear losses, friction, and non-isotropic compression decompression cycles all add work and heat to the fluid which typically reduces viscosity, therefore thinning the fluid.

If no pumping is needed a more effective method of adding shear to a fluid would be either counter rotating smooth rollers, or rollers rotating at different co-rotational velocities. Or just a simple stirring mechanism. These all are more efficient at inducing shear because of more distinctly different fluid movement vectors. The gear movement is parallel at the point of intersection of the gears and only chnges as the arcs of the gears move apart. For counter rotating rollers the vectors are opposite (therefore doubled) at the same point.

My thoughts would be that the gear pump is designed to add some shear to the fluid as well as adding pump head (pressure) and maintains a small gap so that the fluid acts as a lubricant preventing wear. Totally meshing gears that contact one another will wear much more quickly. The gap on your gears is probably small enough that fluid friction prevents a loss of pumping power.

-T

Of course I will try to state a little more clearly,

Nip is referring to the point where the gears mesh, if they were simplified to two circles it would be the point where the two circles touch. I use it to speak of any point where there is contact, or the gap is so small that it is nearly contact.

Shear is a little more difficult to explain so bear with me. Fluids by nature "want" to stay in contact and have little to no internal movement. A way to think of this is a bubble or drop of water, the water molecules pull together due to not only surface tension but internal forces as well. Because of this property if something is submerged in a fluid the fluid "sticks" to the item, think of motor oil or cooking oil (trying to think of good household examples). If you dip a finger into one of these very thick fluids and pull it out the fluid to somextent will come up with your finger before it drips back off, and some residue is normally still on your finger. Shear is a force which overcomes this "stay in contact" force. A way this was explained to me was a block of Jello, if you have a square of Jello and you move the top to the right and the bottom to the left it will first become a trapezoid and then will eventuallt tear, leaving you with two pieces of Jello. Shear is essentially breaking the internal forces holding the fluid together. For something like Jello it is a larger force and more easily seen. For a less viscous fluid like water or oil it takes less force.

Another way I like to try and explain it is shear is like cutting, if you think of a fluid as a bunch of large units (like a jar of marbles) and you shear it sufficiently it becomes a bunch of smaller units (like a jar of sand). But with shear as the fluid is allowed to sit still the internal forces return it to larger units.

The gears in your set up I liken more to taking the jello and pulling the top up and the bottom down, where two rollers spinning opposite would be like the first example pulling opposite directions.

Polishing a fluid is not a term I am familiar with, I do not know if your fluid has particles of solids in it in which case the solid particles may be broken down by the gears, or if the particles are rough the working of the fluid may polish the particles, like rocks in a stream bed. But if it refers to polishing the fluid itself that term is new to me. Possibly it could refer to mixing two components more thouroughly or could be another term for shearing a fluid, I am not sure.

Hope I have clarified some of my techno-speak, sometimes I rudely forget that not everyone had to learn how to interpret the babble coming out of my professors' mouths.

-T