What is Viscosity and its types , Kinematic Viscosity and explaination?

 ASSALAM-O-ALAIKUM GUYS!

 What is Viscosity and its types, Kinematic Viscosity and explanation?

                Viscosity is the measure of the resistance of a fluid to shear and angular deformation. Viscosity is the measure of the internal resistance between molecules in a fluid(liquid or gas).

     Viscosity is the resistance of a fluid to change in shape which process means deformation or movement of neighboring portions relative to another. Viscosity is the process that is opposite to the fluid. Fluid is the process of the flow of liquid or gas.

    The process which is reciprocal to the viscosity is named fluidity. Fluidity means the easiness of the flow.

Now we take different examples for the understanding of the process of viscosity.

Firstly we take Motor oil that has a high viscosity and which is resistant to shear and feel sticky and is cohesive while Gasoline has less viscosity.                                                                                                       Molasses has great viscosity than water. Glycerin has more viscous than honey and honey is more viscous than water.

  

                                                                                                                                                       

The Frictional forces in the flowing of fluids are due to the cohesion and the momentum interchanges between the molecules. 

Dependance:

            The viscosity of fluid depends upon the changes in temperature. As the viscosity of liquids decreases with the increase of temperature and the viscosity of gasses increases with the increase of temperature. This is because forces of cohesion that are diminished with the temperature, predominates with liquid, but in the gasses predominating factor is the interchanging of molecules between the layers of different velocities. Thus a rapidly moving gas molecule shift into a slower-moving layer that tends to speed up the latter. And slow-moving gas molecules entering into a faster-moving layer and tends to slow down the speed-moving layer. This molecule's interchanges set up the shear or produce frictional forces between adjacent layers. At higher temperature molecular interchanging activity increase so the viscosity of gases increases with the increase of temperature. At higher temperature liquid flow more easily due to lower viscosity while gasses flow more sluggishly.

   Viscosity is the major factor in determining the forces that must be overcome when the fluid is used in lubrication and transported in pipelines. It controls the liquid flow in such processes as spraying, injection molding, and surface coating.

  Explanation:                                                                                        

                  Consider the classical case of two parallel plates, sufficiently large that we can neglect edge conditions, a small distance Y apart, with fluid filling space between them. The lower plate is stationary and the upper plate moves parallel to it with velocity U due to force F corresponding area A of the moving plate.

       At boundaries, particles of fluid adhere to the walls and so their velocities are zero relative to the wall. This so-called no-slip condition occurs with all viscous fluids. The fluid velocities must be U wherein contact with the plate at the upper boundary and zero to the lower boundary. We call the form of velocity variation with a distance between these two extremes as depicted in a velocity profile. If the separation distance Y is not too great, if the velocity U is not too high,  and if there is a non-net flow of fluid through the space, the velocity profile will be linear. This behavior of fluid is much as if it consisted of a series of thin layers, each of which layer slips a little relative to the next. 

Mathematical Form:

                    𝐹 ∝ 𝐴𝑈/Y

                      𝐹 =μ 𝐴𝑈/Y

        We know that stress (tauu) is the ratio of force per unit area.

                   τ = F/A

                  F/A = μ  U/Y

We see from similar triangles we can replace U/Y with the velocity gradient dU/dY.

                τ = F/A =  μ dU/dY

   dU/dY  is the velocity gradient.

         Now dynamic viscousity muu is 

                    μ =  τ / velocity gradient(dU/dY)

For many fluids, the tangential and shear stress that cause flow is directly proportional to the rate of shear strain or rate of deformation. 

The shear stress is divided by the rate of shear strain is constant for a given fluid at a fixed temperature. This constant is called Dynamic or absolutely viscosity or simply a viscosity. The fluid that behaves in this way is called Newtonian Fluid in honor of Sir Issac Newton who first formulated the mathematical description of viscosity.

Unit of Dynamic Viscosity:

            The  Unit of Dynamic viscosity is  Force per unit area per velocity gradient. The unit is Nsm^-2.

Dimension of Dynamic Viscosity:

                   The dimension of dynamics or simply viscosity is [ML^-1T^-1].

  A widely unit that used in the metric system for viscosity is Poise. (P).The poise (P)=0.10Ns/m^-2.

The centipoise (cP) (= 0.01 P = 1 mN s/𝑚2) is frequently a more convenient unit. It has a further advantage in that the viscosity of water at 20°C is 1 cP. Thus the value of the viscosity in centipoises is an indication of the viscosity of the fluid relative to that of water at 20°C.

Kinematic Viscosity 𝝑 (nu):                 Kinematic viscosity is defined as the ratio of absolute viscosity to density.                                                                    𝝑 =  𝜇 /  𝜌    We usually measure kinematic viscosity  𝝑 in m^2 /s in the SI. Previously, in the metric system is cm^2/s also called stokes (st). After Sir George Stokes, an English physicist and pioneering investigator of viscosity. Many found that centistokes (cst) (0.01 St =10^6 m^2 /s ) a more convenient unit to work with. Kinematic viscosity is the absolute viscosity divided by the mass density.