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Chapter 7: Problem 2

What is drag? What causes it? Why do we usually try to minimize it?

Short Answer

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Answer: Drag is a force experienced by an object when it moves through a fluid, such as air or water, and acts opposite to the object's direction of motion. It is caused by friction and pressure. Drag is usually minimized to improve efficiency, increase performance, and reduce noise in various systems, such as vehicles, airplanes, and ships. By minimizing drag, energy consumption can be reduced, higher top speeds can be achieved, and noise levels can be lowered.

Step by step solution

01

Introduction to Drag

Drag is a force experienced by an object when it moves through a fluid, such as air or water. It acts opposite to the object's direction of motion, and can be thought of as the fluid resisting the movement of the object.
02

Causes of Drag

Drag is mainly caused due to two reasons: friction and pressure. 1. Friction: Friction occurs at the interface between the object's surface and the fluid due to the viscosity of the fluid, leading to a force called skin friction drag or viscous drag. This force acts parallel to the fluid flow and opposite to the object's motion. 2. Pressure: As an object moves through the fluid, it causes changes in the fluid's velocity and pressure. The pressure drag, sometimes called form drag, occurs when the fluid pressure around the object's leading and trailing edges is different and creates a net force opposing the object's motion.
03

Effects of Drag

The presence of drag affects the efficiency and performance of various systems, such as vehicles, airplanes, and ships. For example, it directly impacts the fuel consumption and top speed of a vehicle. Drag also has an effect on stability and control. As an object moves through a fluid, it loses energy to overcome drag, which reduces its overall performance.
04

Minimizing Drag

We usually try to minimize drag for various reasons: 1. Improve the efficiency: Reducing drag can lead to a significant reduction in energy consumption, especially in vehicles like cars, trains, and planes. The less energy needed to overcome resistance, the more fuel-efficient the vehicle becomes. 2. Increase performance: By minimizing drag, higher top speeds can be achieved since the energy required to overcome drag is smaller. Increases in performance can also lead to improved stability and control. 3. Noise reduction: Drag causes turbulence which in turn creates noise. Reducing drag can help lower the noise level, which is particularly important in applications such as airplanes and cars. In conclusion, drag is a force acting on objects moving through fluids, it is caused by friction and pressure, and we usually try to minimize it to improve efficiency, performance, and noise reduction.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Friction and Pressure in Fluids
When objects travel through fluids鈥攂e it a car slicing through air or a submarine navigating underwater鈥攖hey encounter a phenomenon called drag. That's just a fancy term for resistance! Imagine you're pushing through a thick crowd of people; everything is slowing you down.
Similarly, as objects push through fluids, they're slowed by two main forces:
  • Friction: This is mainly experienced along the object's surface. It's somewhat like rubbing your hand against a rough table and feeling the drag on your skin.
  • Pressure: It's the battle between the leading and trailing edges of the object. When pressure isn't even, it causes further slowdowns, sort of like wind pushing at you from different angles.
Understanding these forces helps design vehicles and objects for better efficiency and performance.
Skin Friction Drag
Skin friction drag is a type of drag caused by the interaction between an object鈥檚 surface and the fluid it is moving through. Think of it as the fluid sticking slightly to the surface of the object. As the fluid moves past the object, this stickiness results in friction.
This friction is what we call skin friction drag. In essence, the viscosity or 'thickness' of the fluid plays a big role here. For example:
  • In air, skin friction drag can be experienced by aircraft surfaces meandering through the sky.
  • In water, it's encountered by ships or underwater vessels trying to glide smoothly.
Managing skin friction drag is crucial in reducing fuel usage, especially for vehicles that often move through these environments.
Pressure Drag
Pressure drag is the resistance caused by differences in pressure around an object moving through a fluid. It's akin to running against the wind: if the wind pushes harder against your front while the back is cushioned, you'll feel that resistance. Similarly, when an object travels through any fluid:
  • The fluid pressure in front (leading edge) is different from that at the back (trailing edge).
  • This difference creates a backward force known as pressure drag.
Imagine pressure drag like the force a battering ram might feel, only, instead of wood, it's battling air or water. By refining the shape or profile of objects, designers work tirelessly to reduce this form of drag and boost efficiency.
Reducing Drag in Vehicles
Reducing drag is pivotal in making vehicles energy efficient. The less resistance a vehicle has, the less energy it requires to move. Here鈥檚 how designers and engineers tackle this problem:
  • Streamlined shapes: Objects like cars and planes are designed with smoother forms to minimize turbulence and drag.
  • Improved materials: New materials can help reduce drag by having surfaces that produce less friction when interacting with air or water.
  • Add-ons and adjustments: Features such as spoilers on cars or winglets on airplanes are used to adjust airflow and cut down resistance.
These adaptations help vehicles achieve higher speeds, increase fuel efficiency, and often are designed to make less noise. Understanding how to effectively reduce drag can lead to innovations that redefine how we travel.

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Most popular questions from this chapter

Air at \(20^{\circ} \mathrm{C}\) flows over a 4-m-long and 3-m-wide surface of a plate whose temperature is \(80^{\circ} \mathrm{C}\) with a velocity of \(5 \mathrm{~m} / \mathrm{s}\). The rate of heat transfer from the laminar flow region of the surface is (a) \(950 \mathrm{~W}\) (b) \(1037 \mathrm{~W}\) (c) \(2074 \mathrm{~W}\) (d) \(2640 \mathrm{~W}\) (e) \(3075 \mathrm{~W}\) (For air, use \(k=0.02735 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \operatorname{Pr}=0.7228, \nu=1.798 \times\) \(10^{-5} \mathrm{~m}^{2} / \mathrm{s}\) )

Consider a house that is maintained at a constant temperature of \(22^{\circ} \mathrm{C}\). One of the walls of the house has three singlepane glass windows that are \(1.5 \mathrm{~m}\) high and \(1.8 \mathrm{~m}\) long. The glass \((k=0.78 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) is \(0.5 \mathrm{~cm}\) thick, and the heat transfer coefficient on the inner surface of the glass is \(8 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\). Now winds at \(35 \mathrm{~km} / \mathrm{h}\) start to blow parallel to the surface of this wall. If the air temperature outside is \(-2^{\circ} \mathrm{C}\), determine the rate of heat loss through the windows of this wall. Assume radiation heat transfer to be negligible. Evaluate the air properties at a film temperature of \(5^{\circ} \mathrm{C}\) and \(1 \mathrm{~atm}\).

The local atmospheric pressure in Denver, Colorado (elevation \(1610 \mathrm{~m}\) ), is \(83.4 \mathrm{kPa}\). Air at this pressure and at \(30^{\circ} \mathrm{C}\) flows with a velocity of \(6 \mathrm{~m} / \mathrm{s}\) over a \(2.5-\mathrm{m} \times 8-\mathrm{m}\) flat plate whose temperature is \(120^{\circ} \mathrm{C}\). Determine the rate of heat transfer from the plate if the air flows parallel to the (a) 8 -m-long side and \((b)\) the \(2.5 \mathrm{~m}\) side.

What does the friction coefficient represent in flow over a flat plate? How is it related to the drag force acting on the plate?

For laminar flow of a fluid along a flat plate, one would expect the largest local convection heat transfer coefficient for the same Reynolds and Prandl numbers when (a) The same temperature is maintained on the surface (b) The same heat flux is maintained on the surface (c) The plate has an unheated section (d) The plate surface is polished (e) None of the above
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