Most of the engineering flow problems are turbulent in nature. So knowledge in turbulence is imperative for an engineer. In this video lesson we will see how to predict and quantify effect of turbulence.
Why Turbulence ?
There is no universally accepted answer for reason behind turbulence. Many scientific searches to find out reason behind turbulence of flow have ended up in vain. Take a look at a famous witticism made by Heisenberg regarding this.
How to distinguish a Turbulent flow ?
All turbulent flows have got following 3 characteristics
- 3 dimensional
- Chaotic - With eddies and vortices
A Daily Life Experience to Predict Turbulence
To understand nature of turbulence we will consider a daily life experience, a tap water problem. Consider following 3 cases, where in each case flow rate of water increases. It is clear that as flow rate increases turbulence of flow also increases. So finding number one turbulence increases with increase in flow velocity.
|Fig.1 Increase in turbulence of flow as flow rate of water is increased|
|Fig.2 Decrease in turbulence of flow as flow as viscosity of fluid is increased|
From above findings it can be summarized that turbulence increases with increase in flow velocity and decrease in fluid viscosity. Flow velocity increases with increase inertial force on the fluid and if fluid viscosity is high viscous force in fluid will also be high. So it can be summarized that turbulence increases with increase in inertial force and decrease in viscous force.
Concept of Reynolds number
Ratio of inertial force to viscous force is know as Reynolds number .
More analysis - Concept of Averaging
Consider a turbulent tap water case with constant flow rate input. If you measure velocity at tap outlet for this case you will find that velocity is highly unsteady as shown in figure below.
|Fig.3 Fluctuating velocity field at outlet of a turbulent flow problem|
|Fig.4 Result of averaging operation in constant flow input flow problem|
Averaging is defined as follows
An engineer always speak about averaged quantities when he comes across a turbulent flow. Because averaged quantities are pretty enough for his purpose. Knowledge of actual fluctuating value of a turbulent flow might be useful in scientific world, but for an engineer it is of no use most of the time. Figure below shows averaging operation in a turbulent-unsteady flow.
|Fig.5 Averaging operation on a turbulent-unsteady problem|
Shear stress in a Turbulent Flow & Turbulence Modeling
Let us consider a turbulent pipe flow case, if you want to determine shear stress near pipe wall, first thing you have to obtain is averaged velocity profile near wall as shown in figure below.
|Fig.6 Average velocity profile and inter layer mixing in a turbulent flow|
Applications Utilizing Effect of Turbulence
Most of the time turbulence has positive effect on engineering devices. It increases convective heat transfer, it increases mixing and reduces drag around a body.
Heat Transfer Enhancement
Convective heat transfer coefficient increases drastically when the flow becomes turbulent, due to effective mixing of different fluid layers in the flow. This behaviour is shown in following figure.So it is a common practice among designers to covert laminar flows into turbulent by introducing suitable vortex generators in the flow.
Coefficient of drag around a body reduces by a huge amount when flow changes from laminar to turbulent.This phenomenon is shown in following figure.This is the reason why golf ball has got lot of dimples on it.This irregularities on surface of the ball will help in transforming laminar flow into turbulent and reduces drag, with low drag ball can travel more distance.
|Fig.7 Increase in heat transfer coefficient due to turbulence|
|Fig.8 Change in drag coefficient over a sphere when flow changes from laminar to turbulent|