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Assuming incompressible flow, when considering streamlines in the vicinity of an aerofoil, which of the following answers is correct?
  • A
    Converging streamlines indicate that the speed of the flow is increasing and that the static pressure is decreasing.
  • B
    Diverging streamlines indicate that the speed of the flow is decreasing and that the static pressure is decreasing.
  • C
    Diverging streamlines indicate that the speed of the flow is increasing and that the static pressure is increasing.
  • D
    Converging streamlines indicate that the speed of the flow is decreasing and that the static pressure is increasing.

When considering streamlines, we can say that converging streamlines indicate an increasing airflow velocity, and diverging lines indicate a decreasing airflow velocity.

Now when considering the pressure, we know that the total pressure always remains constant in subsonic airflow. Next, knowing that the total pressure is equal to the dynamic pressure plus the static pressure, we know that if one increases, the other must decrease to maintain a constant total pressure.

Now what is the dynamic pressure and what is static pressure:
  • Dynamic pressure is equal to: q = ½ rho V2. Therefore we can say that the dynamic pressure is proportional to the velocity of the airflow.
  • Static pressure is the pressure that any object feels even when not moving. This pressure is caused by the force of gravity forcing particles closer together and pushing against each other and any objects around them.
Knowing that dynamic pressure is basically the velocity of the airflow, we can say that if the velocity increase, hence increasing the dynamic pressure, there has to be a reduction in static pressure.
This is known as Bernoulli’s principle!
What this means for our question is that:
  • Converging streamlines indicate a speed increase
  • Diverging streamlines indicated a speed decrease
  • A decrease in static pressure occurs with an increasing speed
  • And an increase in static pressure occurs with a decreasing speed

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