16. An incompressible fluid flows over a flat plate with zero pressure gradient. The boundary layer thickness is 1 mm at a location where the Reynolds number is 1000. If the velocity of the fluid alone is increased by a factor of 4, then the boundary layer thickness at the same location, in mm will be

1. 1. 4

   
   
   

   2. 2

   
   
   

   3. 0.5

   
   
   

   4. 0.25

   
   
   

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   Boundary layer thickness, δ ∝	1
   	√Re

   
   

   Re ∝ V ⇒ δ ∝	1
   	√V

   
   

   Correct Option: C

   Boundary layer thickness, δ ∝	1
   	√Re

   
   

   Re ∝ V ⇒ δ ∝	1
   	√V

   
   

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17. Consider an incompressible laminar boundary layer flow over a flat plate of length L, aligned with the direction of an incoming uniform free stream. If F is the ratio of the drag force on the front half of the plate to the drag force on the rear half, then

1. 1. F < 1/2

   
   
   

   2. F = 1/2

   
   
   

   3. F = 1

   
   
   

   4. F > 1

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: D

   NA

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18. For air flow over a flat plate, velocity (U) and boundary layer thickness (δ) can be expressed respectively, as
    
    If the free stream velocity is 2 m/s, and air has kinematic viscosity of 1.5 × 10^-5 m²/s and density of 1.23 kg/m³, the wall shear stress at x = 1 m, is

1. 1. 2.36 × 10² N/m²

   
   
   

   2. 43.6 × 10^-3 N/m²

   
   
   

   3. 4.36 ×10^-3N /m²

   
   
   

   4. 2.18 × 10^-3 N/m²

   
   
   

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   NA

   Correct Option: C

   NA

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19. If x is the distance measured from the leading edge of a flat plate, then laminar boundary layer thickness varies as

1. 1. 1/x

   
   
   

   2. x^4/5

   
   
   

   3. x²

   
   
   

   4. x^{1 / 2}

   
   
   

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   ρ =	5x	for laminar flow
   	√Re

   
   

   Rex =	Vx
   	υ

   
   

   ∴ ρ =	5√x	for laminar flow
   	√V / υ

   
   ∴ ρ ∝ √x
   

   Correct Option: D

   ρ =	5x	for laminar flow
   	√Re

   
   

   Rex =	Vx
   	υ

   
   

   ∴ ρ =	5√x	for laminar flow
   	√V / υ

   
   ∴ ρ ∝ √x
   

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20. Flow separations flow past a solid object is caused by

1. 1. A reduction of pressure to vapour pressure

   
   
   

   2. A negative pressure gradient

   
   
   

   3. A positive pressure gradient

   
   
   

   4. The boundary layer thickness reducing to zero

   
   
   

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1. View Hint View Answer Discuss in Forum

   NA

   Correct Option: C

   NA

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