22.4.1. Nikuradse's Experiment

Now coming back to very interesting experiments what it happened in 1930s, okay much before the World War II okay. The Germans the professor used to do a simple experiments okay which one of classical experiment conducted in a pipe flow and which helped to make a this complex flow to a simpler empirical based energy loss quantification, velocity distributions quantification as well what could be this wall shear stress.

Nikuradse's experiment, conducted in the 1930s, played a significant role in understanding pipe flow behaviors. Prior to this, understanding how fluids interacted with various surfaces—especially in terms of energy loss and velocity distribution—was complex. His experiments simplified these concepts by focusing on how rough surfaces (like those created by sand grains) affected flow. This foundational work laid the groundwork for many principles still relevant in fluid mechanics today.

Think of how a road surface can affect a car's speed. A smooth road lets cars travel faster with less energy loss than a rough, bumpy road. Similarly, Nikuradse's work helps us understand how roughness in pipes can slow down fluid flow and cause energy loss.

If you look at this any of the pipe flow we can have a laminar flow, the turbulent flow or the transitional in between the laminar and the turbulent flow. What the experiment is conducted by the Nikuradse is that with having very simple concept that the pipe put it with a roughness, this equivalent roughness is from the sand grains.

Nikuradse designed his experiment using pipes with rough surfaces made from sand grains. This enabled him to study how different roughness levels affected fluid flow—particularly the patterns of laminar and turbulent flow. By manipulating the roughness, he could observe changes in wall shear stress and energy loss, key elements in fluid mechanics.

Imagine pouring water over different surfaces: a smooth glass versus a textured sponge. The water flows easily over the glass but slows down when it hits the sponge. Nikuradse's experiment explored this phenomenon in detail to quantify the effects of surface roughness on fluid flow.

The also we know very well the Reynolds numbers also control the flow behaviors. So that way to know it what is will be the wall stress the tau value, what could be the velocity distributions, what could be the energy losses when you have a pipe length of L with the informations about the roughness, informations about flow Reynolds numbers.

Reynolds number is a dimensionless quantity that helps predict flow patterns in different fluid flow situations. It indicates whether fluid flow is laminar or turbulent, which in turn influences wall shear stress, velocity distribution, and energy losses. By studying how Reynolds numbers interact with roughness, Nikuradse could derive equations that describe these behaviors quantitatively.

Think of your kitchen sink: slow water flow (laminar) vs. fast water flow (turbulent) when you turn the faucet. The Reynolds number helps us understand how the speed and nature of the water flow change drastically based on how much you open the faucet.

Now if you look it that by conducting a series of experiments, by Nikuradse finally bring a the friction factors relations with the Reynolds numbers which is Moody chart and we have been using that for designing all this pipe flow networks for the industry, the water supplier, sewage treatment plant all where we are using the same equations.

Nikuradse's extensive series of experiments resulted in the establishment of the Moody chart, a valuable tool in fluid mechanics that relates friction factors to Reynolds numbers. This chart is widely used in engineering to design piping systems in various applications such as water supply, sewage treatment, and more. It helps engineers and designers understand how different types of flows might behave under specific conditions.

Just like a traffic map helps drivers find the best route based on real-time conditions, the Moody chart helps engineers choose the right pipe design by predicting how fluids will flow under different conditions.

Now if you come back to that what we have been doing at IIT Guwahati, we also do similar sort of creating the roughness in open channel flow. That is what you can see this vegetations are there. Some degree of we are creating the roughness and we try to measure the velocity distribution. Try to quantify the wall shear stress what could be there, bed shear stress could be there.

At IIT Guwahati, researchers are building on Nikuradse's foundational work by experimenting with roughness in open channel flow, such as examining how vegetation affects fluid dynamics. These experiments allow for the measurement of velocity distributions and the quantification of wall shear and bed shear stresses in various flow conditions, helping to bridge knowledge from historical experiments to modern challenges in fluid mechanics.

Consider a riverbank with plants and debris. The plants add roughness to the water flow, affecting how quickly it moves and how much energy it loses. Just as Nikuradse studied these effects in controlled conditions, modern researchers are exploring similar themes in natural settings.

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