Multiple choice

The laminar flow takes place between closely spaced parallel plates as shown in figure below. The velocity profile is given by $u = V \frac{y}{h}$. The gap height h is 5 mm and the space is filled with oil (specific gravity = 0.86, viscosity m = 2 × 10-4 N-s/m2). The bottom plate is stationary and the top plate moves with a steady velocity of V=5 cm/s. The area of the plate is 0.25 m2.

The power required to keep the plate in steady motion is

  1. 5 × 10-4 watts

  2. 10-5 watts

  3. 2.5 × 10-5

  4. 5 × 10-5 watts
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Shear stress tau = mu * du/dy = mu * V/h. Force F = tau * Area. Power P = F * V. P = (mu * V/h) * Area * V = (2e-4 * 0.05 / 0.005) * 0.25 * 0.05 = 2.5e-5 Watts.

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