The form and friction drags together contribute to the total drag when flow of air occurs past any object. Two orientations of a finite flat plate are shown in the figure. In Orientation-1, the plate is placed perpendicular to the flow while in Orientation-2, the plate is placed parallel to the flow. If the velocity (\( V \)) of air in both orientations is the same, which one of the following options is TRUE?
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Show Hint
- Orientation-1 has higher form drag and lower friction drag and Orientation-2 has lower form drag and higher friction drag
- Orientation-1 has lower form drag and lower friction drag and Orientation-2 has higher form drag and higher friction drag
- Orientation-1 has lower form drag and higher friction drag and Orientation-2 has higher form drag and lower friction drag
- Orientation-1 has higher form drag and higher friction drag and Orientation-2 has lower form drag and lower friction drag
The Correct Option is A
Solution and Explanation
- Form drag is associated with the pressure difference between the front and rear of the object due to its shape and orientation to the flow.
- Friction drag is the result of the frictional forces between the object's surface and the air as it moves over the object.
Analysis of the two orientations:
- Orientation-1 (perpendicular to the flow): - In this orientation, the plate presents a larger surface area to the flow, leading to a larger separation of flow at the rear of the plate. This creates a high-pressure drag (form drag). However, the contact area for friction is smaller compared to Orientation-2, leading to lower friction drag.
- Orientation-2 (parallel to the flow):
- Here, the plate's surface area exposed to the flow is smaller, resulting in lower form drag. However, the frictional drag increases because the air flows smoothly along the length of the plate, creating more friction between the air and the surface.
Thus, Orientation-1 has higher form drag due to the larger exposed surface area and lower friction drag, while Orientation-2 has lower form drag but higher friction drag because of the larger surface area in contact with the flow. Therefore, the correct answer is (A) Orientation-1 has higher form drag and lower friction drag, and Orientation-2 has lower form drag and higher friction drag. Final Answer: (A) Orientation-1 has higher form drag and lower friction drag and Orientation-2 has lower form drag and higher friction drag
Top Questions on Drag forces
- A spherical metal ball (of density \( \rho_s \) and diameter \( D \)), attached to a string, is exposed to a crossflow (of velocity \( U_\infty \)) of a viscous fluid (of viscosity \( \mu \) and density \( \rho_f \)). Due to the crossflow, the string makes an angle of inclination \( \theta \) with the top surface as shown in the figure. The acceleration due to gravity is denoted by \( g \). For this flow, Reynolds number, \( \text{Re} = \frac{\rho_f U_\infty D}{\mu} \ll 1 \) and buoyancy force in the fluid is negligible compared to viscous force. Assuming the string to be weightless and offering negligible drag, the expression for \( \theta \) is
\includegraphics[width=0.5\linewidth]{imagew11.png}- GATE XE - 2021
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