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Chapter 7: Problem 6

What is the difference between skin friction drag and pressure drag? Which is usually more significant for slender bodies such as airfoils?

Short Answer

Expert verified
Explain briefly. Answer: For slender bodies like airfoils, skin friction drag usually has a more significant effect compared to pressure drag. This is because the streamlined shape of these bodies minimizes pressure drag, making skin friction drag the primary source of resistance to fluid flow. However, the relative significance of skin friction and pressure drag can vary depending on factors such as the body's surface roughness, Reynolds number, and angle of attack.

Step by step solution

01

Define skin friction drag

Skin friction drag is caused by the friction between the surface of a body and the fluid flowing over it. This friction causes a shear force between the fluid layers and the body surface, resulting in a drag force. In terms of fluid dynamics, skin friction drag is associated with the viscous forces in the fluid.
02

Define pressure drag

Pressure drag, also known as form drag, is caused by the difference in pressure between the front and rear surfaces of a body moving through a fluid. This pressure difference results in a net force that opposes the motion of the body. Pressure drag is primarily caused by the separation of the fluid flow around the body, leading to the formation of vortices, turbulence, and wake, which cause a loss of momentum in the fluid.
03

Discuss the effect of slender bodies on skin friction drag and pressure drag

For slender bodies, such as airfoils, the shape of the body is designed to minimize flow separation and pressure drag. The streamlined shape of the airfoil allows the fluid to flow smoothly along the surface, reducing the pressure difference between the front and rear surfaces and reducing the amount of turbulence and wake generated. This minimizes the pressure drag on slender bodies such as airfoils.
04

Determine the more significant type of drag for slender bodies

For slender bodies like airfoils, skin friction drag usually has a more significant effect compared to pressure drag. This is because the streamlined shape of these bodies minimizes pressure drag, making skin friction drag the primary source of resistance to fluid flow. However, it is important to note that this may not be the case for all slender bodies, and the relative significance of skin friction and pressure drag can vary depending on factors such as the body's surface roughness, Reynolds number, and angle of attack.

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Key Concepts

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

Skin Friction Drag
Imagine water flowing over a perfectly smooth plate. The movement between the liquid and the surface creates friction, which is what we refer to as skin friction drag. It's a bit like the resistance you feel when you rub your hand on a surface. This type of drag arises from the viscosity, or thickness, of the fluid moving over the surface.
  • Viscous forces: This refers to the internal friction within the fluid that causes it to resist flow.
  • Shear forces: These are forces that cause layers of fluid to slide past each other at different speeds.
In aircraft design, minimizing skin friction drag is crucial, particularly in the area of airfoil design for maximum efficiency.
Pressure Drag
Pressure drag, or form drag, is quite different from skin friction drag. It comes from how air pressure varies around the object. When air hits the front of a body, it starts to slow down, causing high pressure, while low pressure is created behind it. This mismatch pushes the body backward, creating a drag force.
  • Fluid separation: Happens when the flow of fluid disrupts, leading to vortices and turbulence.
  • Wake formation: The turbulent air that is left behind as the object moves forward.
For example, in car design, automobile companies strive to reduce pressure drag to improve fuel efficiency and speed.
Aerodynamics
Aerodynamics is the broader science that studies how gases interact with moving bodies. It's fundamental in the design of airplanes, cars, and sports equipment. The goal is always to improve performance by reducing resistance or drag.
  • Streamlining: Designing the body's shape to allow smooth and easy flow of air over the surface, reducing both skin friction and pressure drag.
  • Lift and Drag: Aerodynamics involves balancing lift (upward force) and drag effectively to achieve efficient flight or movement through air.
In airplanes, engineers use aerodynamic principles to achieve balance, ensuring that the plane uses just the right amount of energy to remain airborne and move efficiently.
Slender Bodies
Slender bodies like airfoils are specifically designed to account for aerodynamic performance. These bodies usually have a long and narrow shape to allow air to move smoothly around them with minimal disturbance.
  • Minimized flow separation: Helps in reducing pressure differences and drag.
  • Smooth contours: Reduce turbulence and wake creation, minimizing pressure drag.
The most significant drag for these slender shapes often turns out to be skin friction drag, as their design already accounts for minimizing pressure drag. This is why engineers focus on surface smoothness and material choice to further reduce skin friction.
Airfoil Design
Airfoil design is a perfect example of applying aerodynamics to improve flight efficiency. It primarily focuses on maximizing lift while minimizing drag. This involves carefully shaping the wings of an aircraft to optimize airflow.
  • Leading Edge: The front part of the wing that first contacts air, shaped to cut smoothly into the air.
  • Trailing Edge: The back part of the wing where airflow leaves, often shaped to control vortices and turbulence.
Successful airfoil design helps in balancing skin friction and pressure drag, by promoting a stable and even airflow over the wing's surface, allowing aircrafts to fly efficiently with less fuel consumption.

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Most popular questions from this chapter

Ambient air at \(20^{\circ} \mathrm{C}\) flows over a 30-cm-diameter hot spherical object with a velocity of \(2.5 \mathrm{~m} / \mathrm{s}\). If the average surface temperature of the object is \(200^{\circ} \mathrm{C}\), the average convection heat transfer coefficient during this process is \(\begin{array}{ll}\text { (a) } 5.0 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K} & \text { (b) } 6.1 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\end{array}\) (c) \(7.5 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\) (d) \(9.3 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\) (e) \(11.7 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\) (For air, use \(k=0.2514 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \mathrm{Pr}=0.7309, v=1.516 \times\) \(\left.10^{-5} \mathrm{~m}^{2} / \mathrm{s}, \mu_{s}=1.825 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}, \mu_{s}=2.577 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}\right)\)

During a plant visit, it was noticed that a 12-m-long section of a \(10-\mathrm{cm}\)-diameter steam pipe is completely exposed to the ambient air. The temperature measurements indicate that the average temperature of the outer surface of the steam pipe is \(75^{\circ} \mathrm{C}\) when the ambient temperature is \(5^{\circ} \mathrm{C}\). There are also light winds in the area at \(10 \mathrm{~km} / \mathrm{h}\). The emissivity of the outer surface of the pipe is \(0.8\), and the average temperature of the surfaces surrounding the pipe, including the sky, is estimated to be \(0^{\circ} \mathrm{C}\). Determine the amount of heat lost from the steam during a 10 -h-long work day. Steam is supplied by a gas-fired steam generator that has an efficiency of 80 percent, and the plant pays \(\$ 1.05 /\) therm of natural gas. If the pipe is insulated and 90 percent of the heat loss is saved, determine the amount of money this facility will save a year as a result of insulating the steam pipes. Assume the plant operates every day of the year for \(10 \mathrm{~h}\). State your assumptions.

Hot engine oil at \(150^{\circ} \mathrm{C}\) is flowing in parallel over a flat plate at a velocity of \(2 \mathrm{~m} / \mathrm{s}\). Surface temperature of the \(0.5-\mathrm{m}-\) long flat plate is constant at \(50^{\circ} \mathrm{C}\). Determine \((a)\) the local convection heat transfer coefficient at \(0.2 \mathrm{~m}\) from the leading edge and the average convection heat transfer coefficient, and (b) repeat part ( \(a\) ) using the Churchill and Ozoe (1973) relation.

Kitchen water at \(10^{\circ} \mathrm{C}\) flows over a 10 -cm-diameter pipe with a velocity of \(1.1 \mathrm{~m} / \mathrm{s}\). Geothermal water enters the pipe at \(90^{\circ} \mathrm{C}\) at a rate of \(1.25 \mathrm{~kg} / \mathrm{s}\). For calculation purposes, the surface temperature of the pipe may be assumed to be \(70^{\circ} \mathrm{C}\). If the geothermal water is to leave the pipe at \(50^{\circ} \mathrm{C}\), the required length of the pipe is (a) \(1.1 \mathrm{~m}\) (b) \(1.8 \mathrm{~m}\) (c) \(2.9 \mathrm{~m}\) (d) \(4.3 \mathrm{~m}\) (e) \(7.6 \mathrm{~m}\) (For both water streams, use \(k=0.631 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \operatorname{Pr}=4.32\), \(\left.\nu=0.658 \times 10^{-6} \mathrm{~m}^{2} / \mathrm{s}, c_{p}=4179 \mathrm{~J} / \mathrm{kg} \cdot \mathrm{K}\right)\)

What is lift? What causes it? Does wall shear contribute to the lift?
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