Other artifacts can also be present, see Figure 4. For instance, in circularly symmetric geometries such as the cone and plate, parallel plate and concentric cylinders, it is possible to set up vortex-like secondary flows that absorb extra energy compared with the primary flow and hence display a higher-than-expected viscosity if not taken into account (see below). These manifest themselves in a number of ways depending on the particular geometry. In all geometries inertia effects can be important. In this case we have shear and extensional flow, plus inertia and time effects present simultaneously, and it is virtually impossible to extract only the shear viscosity as a function of shear stress which is of interest.
The best example of this is the flow cup, where liquids runs out from a cup through a given nozzle under the action of gravity. Figure 3. Schematic representation of the layout of typical controlled-stress and controlled-strain viscometers.Ī number of supposedly simple geometries are used to measure viscosity, but although the geometry seems simple, the flow field is not.
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