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 additives industry.)

Newton view of viscosity

This statement in the Dynamic Viscosity section is misleading

Isaac Newton expressed the viscous forces by the differential equation

\tau=\mu \frac{\partial u}{\partial y},

Newton's only discussion of a friction in fluids is one sentence in the Principia quote (translation) here:

"The resistance which arises from the lack of slipperiness of the parts of the liquid, other things being equal, is proportional to the velocity with which the parts of the liquid are separated from one another."

There is no equation and no partial differential equation. The equation in the article is better attributed to Navier, Poisson, Saint Venant or Stokes who all derived it from fundamental considerations. — Preceding unsigned comment added by 67.193.115.212 (talk) 23:55, 24 January 2015 (UTC)

UNITS

In the information box it is stated that the SI unit for viscocity is Pa·s, however ...

The table in the article gives gas viscosities under the label μPa·s

The first mention of μPa·s in the article is to state that: μ = dynamic viscosity (Pa·s or μPa·s)

The wiki article on Drag gives the viscocity of water as 10⁻³ Pa·s

So, this article appears to suggest that the viscocity of air is 1,000 times GREATER than that of water.

Would someone clarify the text(s) so that this is an impossible conclusion. Drag refers to dynamic vscocity but from this article on Viscocity the first use of the word "dynamic" makes the two terms interchangeable.

LookingGlass (talk) 15:10, 24 March 2015 (UTC)

They're two different μ's and both usages are standard. When μ is stuck immediately before a unit it means the unit used is a millionth of the original unit, see International System of Units#Prefixes. Dmcq (talk) 11:37, 25 March 2015 (UTC)

In the article it says "The dynamic viscosity of water is 8.90 × 10−4 Pa·s". Is this one order of magnitude to low? — Preceding unsigned comment added by 128.39.88.40 (talk • contribs)

Seems that way. By definition water should be 1 centipoise at 20^oC, which is 0.001 Pa*s. That means the value reported in the article text off by a power of ten, and yet curiously the table immediately below it uses the correct units. (+)H₃N-Protein\Chemist-CO₂(-) 14:29, 14 April 2015 (UTC)

Never mind, I wasn't really paying attention there. At room temp it's 0.89 mPa*s, thus 8.9e-4 Pa*s. (+)H₃N-Protein\Chemist-CO₂(-) 14:37, 14 April 2015 (UTC)