##### As we have already discussed in earlier topics that whenever a fluid passes over a surface, the surface resists its motion and apply a force to oppose the motion in the similar way when fluid flow a resisting force known as viscous force act.

**When fluid moves over a surface, each layer of fluid slides over the adjacent layer.Due to relative velocity between two layers, the velocity gradient du/dy exists and a shear stress τ = μ du/dy acts on layer.**

**Laminar Flow**

##### Fluid in which layer of flows are not intermixing with other layers is known as laminar flow or in other words, if fluid flow is smooth and there is no disruption in fluid layers then it is laminar flow.

**Circular Pipe**

**Flat Plate**

Where,**ρ** = density**v** = velocity of fluid**D** = Diameter of Pipe**L** = length of Pipe**μ** = dynamic viscosity of fluid

**Turbulent Flow**

##### Fluid in which layer of flows are intermixing with other layers is known as turbulent flow or in other words, if fluid flow is moving randomly instead of moving in streamline path, then the fluid is turbulent flow.

##### Now we will have a comparision between head loss due to friction in a pipe.

##### According to

**Hagen-Poiseuille equation of head loss**

**Darcy Weisbach equation of head loss**

Comparing both of these equations, we get

**here f is known as coefficient of friction in viscous flow.**

## ♦Pro Tip

##### To calculate head loss

##### If the viscosity of a fluid, μ is given then Hagen-Poiseuille will be applied.

##### If the coefficient of friction, f is given then Darcy Weisbach equation will be applied.

**f ‘ = friction factor = 4 * coefficient of friction**