Themes > Science > Physics > Mechanics > Fluid Mechanics > Fluid and their Properties > Fluid Properties Examples
 
  1. Explain why the viscosity of a liquid decreases while that of a gas increases with a temperature rise.
    The following is a table of measurement for a fluid at constant temperature.
    Determine the dynamic viscosity of the fluid.
du/dy (s-1) 0.0 0.2 0.4 0.6 0.8
t (N m-2) 0.0 1.0 1.9 3.1 4.0
  
  Using Newton's law of viscocity
where m is the viscosity. So viscosity is the gradient of a graph of shear stress against vellocity gradient of the above data, or
Plot the data as a graph:

Calculate the gradient for each section of the line
du/dy (s-1) 0.0 0.2 0.4 0.6 0.8
t (N m-2) 0.0 1.0 1.9 3.1 4.0
Gradient - 5.0 4.75 5.17 5.0

Thus the mean gradient = viscosity = 4.98 N s / m2
  1. The density of an oil is 850 kg/m3. Find its relative density and Kinematic viscosity if the dynamic viscosity is 5 ´ 10-3 kg/ms.


    r oil = 850 kg/m3 r water = 1000 kg/m3

    g oil = 850 / 1000 = 0.85

    Dynamic viscosity = m = 5´ 10-3 kg/ms

    Kinematic viscosity = n = m / r


     
  2. The velocity distribution of a viscous liquid (dynamic viscosity m = 0.9 Ns/m2) flowing over a fixed plate is given by u = 0.68y - y2 (u is velocity in m/s and y is the distance from the plate in m).
    What are the shear stresses at the plate surface and at y=0.34m?

    At the plate face y = 0m,

    Calculate the shear stress at the plate face

    At y = 0.34m,

    As the velocity gradient is zero at y=0.34 then the shear stress must also be zero.
     

  3. 5.6m3 of oil weighs 46 800 N. Find its mass density, r, and relative density, g.


    Weight 46 800 = mg

    Mass m = 46 800 / 9.81 = 4770.6 kg

    Mass density r = Mass / volume = 4770.6 / 5.6 = 852 kg/m3

    Relative density 
     

  4. From table of fluid properties the viscosity of water is given as 0.01008 poises.
    What is this value in Ns/m2 and Pa s units?

    m = 0.01008 poise

    1 poise = 0.1 Pa s = 0.1 Ns/m2

    m = 0.001008 Pa s = 0.001008 Ns/m2
     

  5. In a fluid the velocity measured at a distance of 75mm from the boundary is 1.125m/s. The fluid has absolute viscosity 0.048 Pa s and relative density 0.913. What is the velocity gradient and shear stress at the boundary assuming a linear velocity distribution.
m = 0.048 Pa s

g = 0.913


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