1.4. Experimental setup for studying laminar and turbulent flow

So, now, when the flow in the pipes occur, the important question is, whether it is laminar or turbulent flow because that is one of the classification of the flows that we also saw in the open-channel flow. Therefore, what is laminar and turbulent flow? That I am going to explain.

When fluid flows through pipes, it can exhibit two distinct characteristics: laminar flow and turbulent flow. Laminar flow occurs when the fluid moves in smooth, parallel layers, whereas turbulent flow is characterized by chaotic and irregular motion. Understanding these types of flow is critical for hydraulic engineering as they affect flow resistance and pressure drop in pipes.

Imagine a serene river (laminar flow) where water flows smoothly, versus a wild, swirling river (turbulent flow) during a storm. Just like the calm river allows you to easily float downstream, laminar flow enables efficient transport of fluids in pipes.

So, the figure a, represents a pipe in which the water is flowing, water or any liquid for that purpose and we have a set for a dye, this is a dye. What we do is, we drop a little bit of dye here, using this apparatus. You know what dye is? It is a coloured thing, that takes the color of the liquid in which it is. And with the velocity, this dye will also start moving.

In this experimental setup, dye is introduced into the flow to visualize how fluid moves within the pipe. In laminar flow, the dye streak moves along an unbroken line, demonstrating smooth, orderly flow. As the flow transitions to turbulent, the dye begins to disperse, indicating chaotic movement.

Think of adding food coloring to a glass of calm water (laminar). The color spreads evenly. Now, shake that glass (turbulent), and you'll see the color swirl wildly, resembling how turbulence disrupts orderly flow.

So, it has been found out that for laminar flow, the Reynolds number should be less than 2100. This is an important Reynolds number that I expect you to remember.

The Reynolds number (Re) is a dimensionless quantity used to predict flow patterns in different fluid flow situations. For laminar flow, the Reynolds number should be below 2100. In contrast, turbulent flow occurs when the Reynolds number exceeds 4000, and the range between 2100 and 4000 indicates transitional flow where characteristics of both laminar and turbulent speeds may occur.

Consider the speed limits on a freeway. The calm flow of traffic under a certain limit represents laminar flow, while rush hour traffic, where drivers zigzag and change lanes wildly, shows turbulent flow.

You see, this is the u, the velocity in x direction have been plotted, with respect to time. So, for laminar flow, it is going to be a straight line, very straight line, here. For the transitional flow, you see, there are some disturbance, at some point then it becomes straight and then it becomes, so this is transitional. The Reynolds number has risen, but not that highest so that it becomes fully turbulent.

The behavior of velocity over time differs distinctly between laminar and turbulent flows. In a graph plotting velocity against time, laminar flow appears as a straight line, indicating consistent speed. Transitional flow shows disturbances in this straight path, whereas turbulent flow is erratic and fluctuates significantly.

Picture a calm morning drive (laminar) where your speed is constant; then, imagine a bumpy road (transitional) where occasionally you hit potholes, leading to a mixed experience; finally, envision a roller coaster (turbulent) with twists, turns, and unexpected drops.

Therefore, for turbulent flow, if the Reynolds number is greater than 4000, that flow is definitely turbulent and for the range in between 2100 and 4000 Reynolds number, the flow is transitional.

The classification of fluid flow based on Reynolds numbers is critical in fluid dynamics. A lower Reynolds number indicates laminar flow, while values above 4000 designate turbulent flow. The 'transitional' range can exhibit characteristics of both flow types, necessitating varied approaches in engineering and design.

Think of a freeway that changes from clear to jammed: below 2100 is smooth sailing (laminar), between 2100 and 4000 is a mix of slow and fast (transitional), and above 4000 represents complete gridlock and chaos (turbulent).

Now, as the speed has been increased in steps. Initially, the dye is injected through a small diameter tube at the left portion of the screen. So, from here, the dye has been injected. Initially, when the speed is low or we can also say when the Reynolds number was less than 2,100 the flow is laminar.

In experiments, increasing the flow speed results in observable changes in how dye behaves in the fluid. At low Reynolds numbers, the dye remains largely in a straight line, indicating laminar flow. As the flow speed increases, changes in the dye's path reveal a switch from orderly to chaotic flow.

Consider pouring syrup into water. If poured slowly (low speed/lower Reynolds), it flows smoothly. But if poured quickly (high speed/higher Reynolds), it splashes everywhere, mixing with water in an unpredictable manner.

So, you see, in this particular video, you will see, that the dye stream is dispersed, in all the directions. And clearly indicating what we had learned in the laminar and turbulent fluid flow chapter, in this particular course of hydraulic engineering was correct.

Through visual experiments with dye, students can confirm the theories and calculations surrounding laminar and turbulent flow. These experiments demonstrate the dispersion and mixing behavior characteristic of turbulent flow, contrasting heavily with the defined streaks of laminar flow.

Like watching a painter blend colors on a canvas. In laminar flow, colors would sit side-by-side perfectly, creating defined lines; in turbulent flow, colors mix unexpectedly, creating a vibrant swirl.

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