Turbulence & Mechanical Stress

This article is rather long, so here is the basic TAKEAWAY:  No pump choice or low-flow scheme eliminates turbulence . . . and the good news is that this doesn't matter with wine (although it does with must).


The related concepts of turbulence & mechanical stress are important to understand for winemaking.  We all know that when pumping must, we should be concerned about mechanical stress (also called "shear" by liquid process engineers).  For this reason, ideally we use a peristaltic pump to move must.


However, when it comes to wine, we need to start with a basic understanding of what turbulence and mechanical stress are.  Keep in mind, when reading below, whether I am discussing must, or wine, as the circumstances are very different when it comes to engineering the best way to move them.


Turbulence

Picture a hose in cross-section, flow is always greater (under winery conditions) in the center of the hose than at the edges. That is because of drag due to the contact of the liquid with the inner surface of the hose.  This creates turbulence--the intermixing of layers due to the fact that they are flowing at different rates.


If you are pumping must, turbulence gets your seeds and skins and solids grinding against each other.  Obviously, this can encourage extraction of elements we don't want.


Mechanical Stress

Mechanical Stress is what happens when one layer is flowing much faster than the other AND there are undissolved solids (and/or unmixables like oil and water) in a mixture. The speed differential can rearrange the ratio of unmixables to each other or the ratio of solid to liquid.


Mechanical stress can cause the mixing of things. This is how you make mayonnaise (you create two zones of differing velocity with a whisk). Mechanical stress can cause the unmixing of things, including the slurry of skins, seeds, solids, and juice we call "must".  Mechanical stress will not cause the unmixing of wine components under anything remotely close to the flow conditions utilized in wine processing.


Wine

It is actually turbulence that is the problem in pumping must, not mechanical stress.  To say that the must experienced "mechanical stress" would merely mean that the ratio of skin, seed, and juice was somehow redistributed.  However, the mechanical stress that causes that redistribution would also cause the skins and seeds and such to rub and grind together, and against the hose walls, and so on.  This extracts harsh elements we don't want.  This is why we try to minimize turbulence when moving must (nobody is prepared to go to the lengths that would be required to eliminate turbulence).


Wine is not particularly sensitive to mechanical stress.  A wine free of seeds and skins, as it must be prior to treatment, simply cannot be "rearranged" by pumping at velocities and pressures anywhere close to what we (or even the much higher pressures of our competitors, to be fair) are working with here, as its structure is at the molecular level.


Laminar Flow Myth

Laminar flow is the opposite of turbulent flow--the liquid along the wall is flowing at the same rate as that in the center.  There is a sharp inflection point (known by a Reynolds Number <2300) between turbulent and laminar.  Beware of claims of "avoiding turbulence" as it is simply neither a problem (for wine), nor is it remotely close to possible under any practical conditions to avoid turbulence.


For example, at 1800 liters/hour (475 gallons/hour) pumping rate, a system would have to have openings of 11 inches in diameter to "avoid turbulence".  This would mean an 11" port on the tank, 11" full port valve, 11" hoses, pipes, valves, housings, etc. on the filtration system.  Ever seen this?  Truth is, every time you move wine it experiences turbulence, and is none the worse for the wear. This is true even if moved by gravity, in fact, gravity induces more turbulence than pumping in some cases.


Pressure Drop

Pressure drop across a surface introduces turbulence.  As liquid flows along the membrane surface, some permeates through. This reduces the flow and pressure.  All relevant stress applied to wine during treatments such as VA reduction, takes place at the membrane surface, not in the pump, hoses, or pipes.


What increases pressure drop across a membrane, and therefore increases mechanical stress?  Two things:

  • high pressure--this maximizes the difference in pressure between the two sides of the membrane &
  • low flow across the membrane--this maximizes the turbulence left in the void as volume crosses to other side of membrane


Everyone is familiar with the dangers of high pressures, but you should also beware of claims that "low crossflow" is good because it avoids turbulence and mechanical stress.  The opposite is actually true!


Minimizing Mechanical Stress

We have designed our systems to apply the least mechanical stress possible.  We do this by:

  • operating at lower pressures than all other membranes in use (regardless of whether they are nanofiltration or reverse osmosis)
  • maintaining sufficient flow to minimize pressure drop across the membrane

At the end of the day, we make choices on the basis of what is best for your wine.
 
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