Question

FLUID MECHANICS

The viscosity of water at 20⁰C (68⁰F) is 1.008 cp (centipoises). (A) Compute the absolute viscosity in lb-sec/ft² . (B) If the specific gravity at 20⁰C is 0.998, compute the kinematic viscosity in ft²/sec.

Use:
1 poise = 1 dyne-sec/cm²
1 lb = 444,800 dynes
1 ft = 30.48 cm

(Please provide a brief step explanation since a solution has been already provided, but it isn't clear how they arrived at the final solution. Thank you!)

Answer 1

A)

The viscosity of water at 200C (680F) is 1.008 cp (centipoises). This is the absolute viscosity

1 P( Poise) = 100 cP (CentiPoise)

or 1 cP = 0.01 P

Therefore, 1.008 cP = 1.008 x 0.01 = 0.01008 P

Now, 1 poise = 1 dyne-sec/cm2

Therefore,  0.01008 P =  0.01008 dyne-sec/cm²

1 lb = 444,800 dynes

Or, 1 dyne = (1/444800) lb

1 ft = 30.48 cm

Or 1 cm = (1/30.48) ft

Therefore ,  

Absolute Viscosity = 0.0000210535 lb-sec/ft²

B)

Specific gravity at 200C is 0.998

Density of water at room Temperature = 1.94 slugs/ft³ (Standard)

Specific Gravity = (Density of water at 200 C) / (Density of water at Room Temperature)

Therefore, 0.998 = (Density of water at 200 C) / (1.94)

Hence, Density of water at 200 C = 1.93612 slugs/ft³

Kinematic Viscosity = Absolute Viscosity / Density

Therefore, Kinematic Viscosity = 0.0000210535 / 1.93612 = 0.00001087406 ft²/ sec