TURBULENT FLOW

Introduction

                    

                     

Turbulent Flow in basic terms is basically flow of a more chaotic variety.

As you change factors such as velocity and diameter of piping you affect the flow. Laminar flow is easy and non chaotic. Essentially, it is easier to model and predict. Turbulent flow usually has an increased velocity compared to laminar if you leave all other Reynolds number variables constant(such as pipe diameter). This increased velocity usually causes the flow to become more chaotic and harder to model.

Empirical models are used to deal with turbulent flow. A great example to get the concept of what is happening is a garden hose. You turn on the hose and it comes out smooth and non chaotic(laminar). Once you put your thumb at the exit you cause it to spray in all directions chaotically. Due to you decreasing the diameter of the piping with you thumb, you have changed the Reynolds number to create now turbulent flow.

Some Interesting Fact about Turbulent Flow

Turbulence is a ubiquitous(commonly found) phenomena. Even though the turbulent flow is random, irregular and chaotic in nature(see the fig below),can be completely described by the Naiver-Stokes Equations. We have tried our best to explain few interesting facts in layman terms, which are listed below

1. Dominant inertial disturbing forces(>>viscous damping forces),i.e. Large Reynolds no flow

2.Always Three-dimensional and vorticial(rotational) in nature

3.Chaotic in nature--->Individual realizations of Identical experiments are highly sensitive to the initial conditions(Characteristic of a Non-linear system)

4.Highly diffusive in nature (Smearing of sharp gradients in the flow variables)due to enhanced transport due to fluctuating eddies

5.Not self-sustaining in nature. External agency is needed to supply the energy to maintain turbulence

6.Large scales(eddies) are almost viscosity independent while small scales are highly influenced by viscosity

7.Highly dissipative(at small scales) in nature. Kinetic energy at the small scales usually converted to heat due to viscosity

What would life be like without Turbulent Flow?

Quite scary.

Turbulence is very good at dissipating energy. Think about waves in open water-bodies. These waves are a result of an instability- when wind blows over water, water first moves along with the wind, but that flow is unstable, and the resulting instability is the waves you see. In the atmosphere, there are lots of air-currents in different directions and with different velocities. When these currents interact, their instability produces structures such as tornadoes. The problem is, laminar flow isn't good at killing large structures. And that's where turbulence comes to the rescue- it breaks up these huge structures into smaller and smaller structures, until finally they are small enough to be dissipated into heat. Without turbulence, tornadoes and hurricanes would live much longer than what we usually see- and that's quite scary.

You would rarely be able to smell anything, because diffusion is REALLY slow with laminar flows. Cooking would be a pain, because mixing seasoning into sauces would take a long time. We wouldn't have cars, because combustion of fuel in the engine depends on the fuel mixing with air, and without turbulence, it wouldn't be fast enough to produce energy to move the pistons explosively. Simply put, any process that needs strong mixing would be impossible.

Application of Turbulent Flow

Some of the practical applications of the turbulent flow are:

• Flow near an aeroplane : Aeroplane flies at a very high speed and the flow of air above and below the wings are turbulent. As a result, the drag exerted on the wings are very high. To reduce this to some extent, wings are made aerofoil shaped, i.e. highly streamlined.

          

• Dimples present in a golf ball: Boundary layer separation is a common phenomena and in laminar flow, the separation occurs faster. The drag to the ball is considerably higher in laminar flow and hence travel smaller distances. To make the flow turbulent, making the seperation slower and hence reduce the drag, dimples have been introduced.

• Heat transfer rate is increased when the flow is turbulent in pipes.

Difference between Turbulent Flow and Laminar Flow

In the laminar flow, as you see in the image above, the flow is quite smooth and the particles in the flow do not cross each other and follow their own distinct paths.

Turbulent flow, on the contrary, is quite chaotic, wherein the particles of the flow do not have a specific path and cross each others’ rapidly.

The Reynolds number of the flow should be less than 2000 for laminar conditions to prevail. This will include the characteristic diameter and velocity of the flow, accordingly.

The Reynolds number higher than 4000 shall create turbulent conditions in the flow.

Obviously, the shear stress in laminar flow depends on the viscosity and is independent of the density of fluid.

But, in turbulent conditions, the shear stress is primarily a function of density.

Basically, in laminar flow type, the fluid flow is quite orderly and the adjacent layers don’t mix. In turbulent this condition is not true.

References 

• https://swyde.com/s/Airfoil
• https://www.businessinsider.com.au/why-golf-balls-have-dimples-2016-1
• http://abyss.uoregon.edu/~js/glossary/turbulent_flow.html#:~:text=Turbulent%20flow%20is%20a%20type,in%20both%20magnitude%20and%20direction.
• https://www.britannica.com/science/turbulent-flow

                                                                 Submitted By    Group No 5