Just started getting into aerodyanamics/fluid dynamics and came across this paragraph.

In it, it explains how layer a exerts a frictional force on layer b due to a velocity gradient but it also says that we have to imagine an area dA that is perpendicular to the y-axis. I was wondering what exactly this meant or whether it matters at all i.e. does it mean that the area is facing in the direction of the y-axis?

I thought that didn't make sense because there would be no friction between the layers as the area of concern is flat so I was wondering if it meant that the direction that the area is facing is perpendicular to the y-axis.

I would appreciate any help! Thanks!

enter image description here

  • $\begingroup$ Yes, what they said is correct. What area did you think the tangential frictional force was acting on? $\endgroup$ Jun 22, 2020 at 11:35
  • $\begingroup$ An area that is perpendicular to the y direction is tangential to the surface of the body. $\endgroup$ Jun 22, 2020 at 11:47
  • $\begingroup$ Oh ok. I thought that the frictional force was acting on the side of the fluid element that is parallel to y but this area was so small that it could be regarded as being tangential to the streamline. $\endgroup$
    – Ewan
    Jun 22, 2020 at 13:56
  • $\begingroup$ y is perpendicular to a streamline, so an area can't be perpendicular to both y and perpendicular to a streamline. $\endgroup$ Jun 22, 2020 at 20:28


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.