Doc talk:Manual/Physics/Fluids

I just gotta say that this new feature is totally rad. thx!

bug in template

there's a bug in the mediawiki template that kept example3 from being displayed right-justified on the page. The error even exists in the style guide. I tried both to guess the none was right, and the right|thumb settings, but both are broke.

---

viscocities

There still some things that aren’t correct in this table, I think. Let me put as clear as I can:

• The dynamic viscosity international unit is the Pascal-seconds (Pa.s). There are also Poise (P = 0.1 Pa.s), and centiPoise (cP = 0.001 Pa.s).
• The kinematic viscosity international unit is in m².s^-1.
• The density international unit is in kg.m^-3.

Which implies that a Pascal corresponds to 1 kg.m^-1.s^-2, or else you cannot divide Pa.s by kg.m^-3 to obtain m².s^-1!

So if I take the kinematics values given bellow, and try to get the corresponding dynamic values, I have:

• water: density: about 1000 (kg.m^-3); kinematic viscosity: 1×10^-6 (m².s^-1)

→ dynamic viscosity is 1000 × 1×10^-6 = 1×10^-3 Pa.s, hence 1 cP.

→ COHERENT

• Oil: density: more or less like water, so about 1000; Kinematic viscosity: 5×10^-5

→ dynamic viscosity is 1000 × 5×10^-5 = 1×10^-2 Pa.s, hence 50 cP, and not 500 cP

→ NOT COHERENT, unless Oil SAE 50 is ten times heavier than water!

• Honey: density: about 1250 (kg.m^-3); kinematic viscosity: 2×10^-3

→ dynamic viscosity is 1250 × 2×10^-3 = 2.5 Pa.s, hence 2500 cP, and not 1×10⁴ cP

→ NOT COHERENT, unless honey is five times heavier than water!

• And so on,
  • chocolate syrup density should be of 1×10⁴ kg.m^-3 (ten times water density),
  • ketchup density should be of 1×10³ kg.m^-3 (same as water density, coherent I think),
  • melting glass density should be of 1×10¹² kg.m^-3 (a thousand million times water density, it’s more like black hole!)

In general, there should be six orders of magnitude between (dynamic) values in cP, and (kinematic) values in m².s^-1 (unless for realy heavy things like melting lead, where there should seven orders of magnitude…).

So, either the values in the tables are wrong (one way or the other), or the law to pass from dynamic viscosity to kinematic viscosity is just a “trick”, an approximation, only working with fluids around water viscosity…

I don’t know, I’m not a physicist, but there’s definitively something wrong here, so if someone who knows better about this matter could check and correct it, it would be nice!

--Mont29 13:35, 7 August 2009 (UTC)

PS: here is the table:

Blender Viscosity Unit Conversion

Fluid	dynamic viscosity (in cP)	kinematic viscosity (Blender, in m2.s-1)
Water (20°C)	1.002×100 (1.002)	1.002×10-6 (0.000001002)
Oil SAE 50	5.0×102 (500)	5.0×10-5 (0.00005)
Honey (20°C)	1.0×104 (10,000)	2.0×10-3 (0.002)
Chocolate Syrup	3.0×104 (30,000)	3.0×10-3 (0.003)
Ketchup	1.0×105 (100,000)	1.0×10-1 (0.1)
Melting Glass	1.0×1015	1.0×100 (1.0)

---

Mont29: what can we say? the values in the user manual are the values supplied in Blender, and we cannot change them. I was only trying to give a more "normal" measurement in the dynamic column. Regarding the oil, the oil is SAE 50 which is thicker and denser than oil. I guess the honey is hot, and the chocolate is very sweet and cold, so it is denser. --Roger 18:13, 10 January 2008 (CET)