In a turbulent flow, the velocity of the liquid molecules in contact with the walls of the tube is:
Correct answer: A. Zero
- A. Zero
- B. Maximum
- C. Equal to critical velocity
- D. May have any value
Explanation
The no-slip condition requires that the fluid relative velocity at the solid boundary equals the wall velocity. For a stationary wall, the velocity of fluid at the wall is zero even in turbulent flow. This is because the no-slip boundary condition applies to viscous fluids: the fluid velocity at the wall equals the wall velocity even in turbulence. If the wall is stationary, the instantaneous fluid velocity at the wall is zero. (Turbulence causes fluctuating velocities away from the wall, but the instantaneous velocity at the wall still equals the wall velocity).
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Fluid flow is described through speed, pressure and density, including steady flow and the equation of continuity for conserving mass. Bernoulli's equation relates pressure, speed and height, while fluid drag and terminal velocity explain why a falling object eventually moves at constant speed when drag balances its weight.
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