SOLUTION:

Applying energy balance at plane 1 and 2

      (Atmospheric pressure)
      Since opening is horizontal
        No fan
 Velocity at both planes 1 and .

(1)

(2)

(3)

 is velocity of air in duct. De is equivalent diameter

 
 

Substituting the values in eq 3
 
.
 

Now we can show that the flow is turbulent.
 ; the flow is turbulent.