17.5. Friction and Pressure Drags

If you look at the drag, it is a very simple definitions as you can understand it that is a force flowing fluid exerted on a body in the flow direction is called the drag.

Drag is defined as the force exerted by a fluid (like air or water) on an object moving through it. This force opposes the motion of the object and acts in the direction of the flowing fluid. Think of it as resistance that slows down the object due to the fluid it travels through.

Imagine riding a bicycle against the wind. The air pushes against you, making it harder to pedal forward. This force exerted by the air is similar to drag. The faster you go, the more wind (fluid) pushes against you, increasing the drag force.

But you have a lift force you have a drag force. The component of the pressures and the wall strips in the direction normal to the flow tend to move the body in that direction.

Lift is the force that acts perpendicular to the flow of the fluid, allowing objects like airplane wings to rise. It results from the pressure difference between the upper and lower surfaces of the wing. Flow separation occurs when the smooth flow of fluid is disrupted, leading to turbulence, which can dramatically affect lift and drag.

Think of a bird flying. As its wings cut through the air, the shape of its wings creates a pressure difference: lower pressure on the top of the wing and higher pressure beneath it. This difference generates lift, enabling the bird to soar. If the bird's wing position caused the airflow to separate, it would lose lift, akin to a car losing traction on a slick road.

So the basics of this coefficient drags we can get it from experiment basically conducting wind tunnel test or we can do numerical experiment.

The coefficient of drag (Cd) is a dimensionless value that helps quantify the drag or resistance experienced by an object in a fluid. It is determined through experimental setups like wind tunnel tests, where the effects of various shapes and sizes are studied to observe how they alter the drag force experienced by an object.

For example, when designing a new car, engineers use wind tunnels to model airflow around the vehicle. By changing the car's shape, they can measure how much drag it experiences. The goal is to design a shape with a low Cd, making the car more fuel-efficient while cruising.

This drag and lift coefficients are primarily functions of shape of the body as I said it it also depends upon the Reynolds numbers and the surface roughness.

The drag and lift coefficients greatly depend on the shape of the object moving through a fluid (like a balloon vs. a flat plate). Objects that are smooth and streamlined, like a teardrop, will have lower drag coefficients. Meanwhile, the Reynolds number identifies the flow characteristics (laminar or turbulent) around the object, and surface roughness (like the texture of a basketball) can affect how smoothly fluid flows around it.

Consider a penguin sliding on ice. Its streamlined body causes it to glide easily with minimal drag, allowing it to move quickly with little resistance. Now picture a bulky box being pushed through water; it likely faces much more drag due to its unwieldy shape and rough surfaces.

As I said, that part of drag directly from the wall shear stress which is called skin friction drag since it is caused by the boundary layer formations.

Friction drag occurs due to the contact between the fluid and the surface of the object. This is influenced by shear stress at the surface. In contrast, pressure drag arises from differences in pressure on opposite sides of an object, depending on its shape and the fluid flow direction. Together, these components help explain the total drag experienced by an object.

Imagine swimming in a pool. When you press your hand against the water (friction drag), the smoothness or roughness of your hand affects how easily you can slice through. If you suddenly change to a wider arm position (increasing pressure drag), the additional water resistance slows you down significantly!

So we can look at this elementary geometry and you can understand it that which one will give a minimum Cd values.

When designing structures (like tall buildings), understanding how to minimize drag forces impacts safety and performance. Engineers can select shapes (square, circular) based on the expected wind speed. Properly selecting these geometric forms can result in lower drag coefficients, thereby improving stability during high winds.

Think about designing a windmill. Its blades need to be streamlined to capture wind effectively while minimizing drag. If the blades are too bulky or oddly shaped, they may not rotate efficiently, reducing energy generation compared to a sleek, well-designed blade.

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