Fluid Mechanics

How do Fluids work? We're going to need to learn about fluid mechanics. Fluid mechanics explains how the air moves around your car, how food colouring moves through the water, And it explains what makes quicksand act like quicksand.

While a car is moving, it's going to interact with the fluid that it's moving through which will be air in most cases. As the car interacts with the air, the two can affect one another, and this can lead to what we call transfer. The transfer is when something moves or is moved, from one place to another. There can be a transfer of momentum or a transfer of heat maybe even mass. But if we're looking at moving fluids, then we'll often have transfer of momentum, which can be better understood with Fluid Mechanics. Fluid Mechanics studies how fluids respond to the forces exerted on them.

So how exactly do fluids move and how does a particle or anything for that move within a fluid? To answer these questions, we need to know about stress, strain, and viscosity. Suppose we have fluid between two flat plates. If we were to move the bottom plate, what would happen to the fluid? How would it move? At the top and bottom, where the fluid is in contact with the surface, the individual particles of the fluid will go through something called the no-slip condition. In the no-slip condition, fluid in motion will come to a complete at a solid surface and assume a zero velocity relative to the surface. The particles of the fluid that are touching the solid will stick to its surface, meaning that they won't slip. Because of this, the fluid particles in contact with the bottom plate will move with it, while the fluid particles at the top will stay with the stationary plate.

This is happening due to stress, the force that's applied to a cross-sectional area of an object or substance. If the forces are normal or perpendicular to the surface of the object, then we have normal stress. If it's parallel, then we have shear stress. We can find stress by taking the applied force and dividing it by our cross-sectional area. Once the fluid is stressed, the degree to which it stretches is called strain. The strain is the deformation that stress causes on a system. If the deformation causes something in a system to become either shorter or longer, Then we can find its strain by taking the change in length and dividing it by the initial length. And this is called normal strain. But if the deformation is a change in angle between two segments that had been perpendicular to each other, then we have shear strain. And we can find that by subtracting the change in angle from the original angle, which will either be pi over 2 or 90 degrees, depending on your units. So all of this is what would happen if our bottom plate was moving. But what if neither plate move, and we have a pump driving the flow of the fluid between them?

The no-slip condition would apply. While the fluid moved, its particles at the surface of the two plates would stay stationary. But we need to take into account is viscosity. Viscosity is essentially a measure of a fluid's resistance to flow, and it's often referred to as the thickness of a fluid. Water has a low viscosity and honey has a higher viscosity. The law of viscosity says that Newtonian fluid as fluids with a viscosity that's independent of stress. No matter how much stress you put on the fluids, its viscosity never changes. Non-Newtonian fluids, don't follow the law of viscosity like quicksand and water mix with cornstarch. This means that their strain can change too. If you add stress to quicksand it will thin out and you will sink and if you mix water with cornstarch and add stress it will thicken. Most of the non-Newtonian fluid will thin out. Like how quicksand sucks you in, you step on the surface of quicksand creating stress and making quicksand become less viscous.


Now, there's still the matter of how one fluid moves within another fluid. In 1868, Osborne Reynolds graduates college, he becomes the first professor of engineering at Owens College in Manchester. Reynolds did an experiment on moving water he found when you add a drop of food colouring which has the same density of water. When he adds a drop of food colouring into slow-moving water the flow of the food colouring maintained its place and pattern in the centre of the water we call this Laminar flow. Laminar flow is the flow of fluid when each particle of the fluid follows a smooth path, paths which never interfere with one another.

But when he adds a drop of food colouring into fast-moving water the food colouring spread out and diffused mixing with the water we call this Turbulent flow. Turbulent flow is the fluid motion characterized by chaotic changes in pressure and flow velocity. Those are the main two types of flow that Reynolds found but we also have something called transitional flow. Transitional flow is a mixture of laminar and turbulent flow, with turbulent in the centre of the pipe and laminar flow near the edges. To find out if the fluid is flowing laminar or turbulent we need to use Reynolds number for the flow of a fluid in a pipe by taking the diameter of the pipe and multiplying it by the velocity of the fluid and the density of the fluid, then dividing all of that by the viscosity of the fluid. The value we get for our Reynold number will be dimensionless, meaning there are no units attached to it, but it can tell us a lot about the movement of a fluid. Reynolds number help us how predictable or how chaotic fluid flow will be.

This is because we look at Reynold number as a ratio of inertial force to viscous force. Inertial force means to represent the driving kinetic movement of the fluid, which results in chaotic flow movement, like the swirling motion of eddies and vortices. Viscous force means represent resistance to flow and are move likely to provide slow, steady motion. A low Reynold number represents laminar flow and a high Reynold number represents a turbulent flow. Laminar flow usually has a Reynold number lower than 2100 and turbulent flow usually have a Reynold number high than 4000. Reynold number between these two values typically represents the transitional flow. 

Why is fluid mechanics is important because we need to know-how fluid workaround us and object and how fluid move from one place to another like sewage. Sewage is important because we make a lot of waste and we need to get rid of it this is why fluid mechanics. Also, fluid mechanics help us to develop a better car and others.