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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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Most popular questions from this chapter

A glass \((k=1.1 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K})\) spherical tank is filled with chemicals undergoing exothermic reaction. The reaction keeps the inner surface temperature of the tank at \(80^{\circ} \mathrm{C}\). The tank has an inner radius of \(0.5 \mathrm{~m}\) and its wall thickness is \(10 \mathrm{~mm}\). Situated in surroundings with an ambient temperature of \(15^{\circ} \mathrm{C}\) and a convection heat transfer coefficient of \(70 \mathrm{~W} / \mathrm{m}^{2} \cdot \mathrm{K}\), the tank's outer surface is being cooled by air flowing across it at \(5 \mathrm{~m} / \mathrm{s}\). In order to prevent thermal burn on individuals working around the container, it is necessary to keep the tank's outer surface temperature below \(50^{\circ} \mathrm{C}\). Determine whether or not the tank's outer surface temperature is safe from thermal burn hazards.

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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 \(k=0.2514 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}, \mathrm{Pr}=0.7309, v=1.516 \times\) \(\left.10^{-5} \mathrm{~m}^{2} / \mathrm{s}, \mu_{s}=1.825 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}, \mu_{s}=2.577 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot \mathrm{s}\right)\)

Air at 1 atm is flowing in parallel over a \(3-\mathrm{m}-\) long flat plate with a velocity of \(7 \mathrm{~m} / \mathrm{s}\). The air has a free stream temperature of \(120^{\circ} \mathrm{C}\) and the surface temperature of the plate is maintained at \(20^{\circ} \mathrm{C}\). Determine the distance \(x\) from the leading edge of the plate where the critical Reynolds number \(\left(\operatorname{Re}_{c r}=5 \times 10^{5}\right)\) is reached. Then, using the EES (or other) software, evaluate the local convection heat transfer coefficient along the plate. By varying the location along the plate for \(0.2 \leq x \leq 3 \mathrm{~m}\), plot the local convection heat transfer coefficient as a function of \(x\), and discuss the results.

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