Talk:Viscosity

ASTM Cps

• What is ASTM Cps measured used for viscosity???

Mistatement concerning kinematic viscosity

The statement

"In many situations, we are concerned with the ratio of inertial to viscous forces, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity:"

would be correct if modified as follows:

In many situations, we are concerned with the ratio of

                                VISCOUS TO INERTIAL

forces, the latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity:

If no one comes up with a contrary argument in a few days, i'll edit the article elzorro

OK.  Done.   elzorro

Bulk viscousity

If you add pressure to a viscoelastic sample then it takes time for the volume to relax. In the old days they used to model this with a variant of the Maxwell model (See Maxwell_material). This model contains a term named the bulk viscousity. I don't think that bulk viscosity is relevant to this article.

Temperature dependence of viscosity

Because viscosity of a fluid is critically dependent on temperature it is important that its value is quoted for a fixed temperature and it is measured in thermostatically controlled environment (equipment and fluid kept constant within, say ±0.1°C).

For petroleum oils between a limited range of temperatures the change of viscosity with temperature is near logarithmic. ASTM (American Society for Testing and Materials) has produced a chart somewhat better than an ordinary logarithmic chart, with a near linear temperature function, on which the slope of the viscosity-temperature lines of petroleum lubricants can be suitably compared. The smaller the slope of the line representing the viscosity change of the oil, the better it is, as its viscosity decreases less due to temperature rise caused by internal friction.

Eddy Viscosity

Would someone be so kind as to expand this section, and maybe add some references?

Teaching the concept of viscosity to undergraduates

The textbook I'm using as I've seen with other textbooks introduces the concept of viscosity by drawing analogies to solids and shear stress. I'm teaching this concept for the first time and I wanted to ask if there were clever ways of presenting the concept of viscosity to students in their first course in Fluid Mechanics? Any interesting figures or mpegs or anecdotes or examples that students can relate to would be much appreciated. Thanks.

Examples

In the examples peanut butter has a higher viscosity than chocolate. I thought this was strange as one can spread peanut butter on a sandwich but not milk chocolate! I checked the reference, the test used chocolate which was heated to 50ºC and allowed to cool to 40ºC. Assuming that the human hand is at ~37ºC this would mean melted chocolate was used in the test (as chocolate melts in your hand). It looks ambigious as the moment.

--Any test for viscosity should mention the temperature at which it's done. Most viscosity tests are done on liquids. I'd assume that the chocolate was heated to melt it before the test was done.

A More Correct Equation for Viscosities of Mixtures

The equation given for the viscosities of mixtures (using either the fluidity equation or its equivalent in dynamic viscosity) is at best a very rough approximation. A more correct form uses logarythms. In most regressions I have done on real world data, the plots of mixture viscosities are most often fit extremely well by

ln(ln(ʋ_mix))=SUM{x_i·ln(ln(ʋ_i))}

where

ʋ_i = the dynamic viscosity of component i, and x_i = mass fraction of component i.

Granted, theoretical intuition suggests that using mole fraction might be more accurate (than using weight fraction), however I have not found that to be an enhancement that justifies the extra complexity, assumptions, and resulting increased error bars when dealing with complicated real-world substances for which there is a range of molecular weights.

(I am a chemical engineer with 28 years in R&D in the lubricant addivitves industry.)