4
$\begingroup$

I understand that static pressure is a scalar quantity as it acts equally in all directions, then by the same reasoning dynamic pressure should be a vector quantity as it only can be measured by opposing the flow.

I understand that the act of opposing the flow will instantly convert dynamic to static but I am asking in theory?

Thanks all the answers ( @Nikos M. ) and assistance , However I am still unclear as to why dynamic pressure (at any instant) is not thought of to be a force acting in a specific direction (perhaps vector is not the right term ?)

Improve this question
$\endgroup$
8
  • 2
    $\begingroup$ Do you know why kinetic energy is a scalar, and not a vector? $\endgroup$
    – Bernhard
    Commented Nov 9, 2014 at 10:02
  • $\begingroup$ It is so by its definition. There is a vector quantity, the momentum flux or so, but it is of different dimension - it is not an energy density. $\endgroup$
    – Vladimir Kalitvianski
    Commented Nov 9, 2014 at 11:13
  • $\begingroup$ It doesnt match the definition of vector en.wikipedia.org/wiki/Vector_space $\endgroup$
    – user65081
    Commented Nov 9, 2014 at 11:15
  • 2
    $\begingroup$ possible duplicate of Define Pressure at A point. Why is it a Scalar? $\endgroup$
    – Kyle Kanos
    Commented Nov 9, 2014 at 16:06
  • $\begingroup$ Thanks all the answers (@Nikos M.) and assistance , However I am still unclear as to why dynamic pressure is not thought of to be a force acting in a specific direction (perhaps vector is not the right term ?) $\endgroup$
    – Quentin Chester
    Commented Nov 10, 2014 at 0:49

1 Answer 1

Reset to default
1
$\begingroup$

As mentioned in comments the dynamic pressure is the equivalent of the kinetic energy in fluid dynamics.

It is dynamic since it can change with time (like kinetic energy) and also like kinetic energy it is an invariant under coordinate transformations (e.g rotations) as such it is a scalar and not a vector (just like energy which also depends on velocity in similar manner).

quoting from related answer:

A fluid does not sustain shear, and this is true whether it is still or moving, by the principle of relativity. This means that if you put fluid between two plates, and squeeze, the force per-unit-area with which you squeeze (the local flow of momentum in the direction perpendicular to the plates) is equal to the force per unit area pushing outward at the edge of the plates. The flow of momentum is the same in all directions.

This means it is isotropic, in other words invariant under rotations (and other coordinate transformations), thus it is a scalar quantity (a vector is not invariant under rotations)

Improve this answer
$\endgroup$
2
  • $\begingroup$ Thanks all the answers ( @Vladimir Kalitvianski @) and assistance , However I am still unclear as to why dynamic pressure (at any instant) is not thought of to be a force acting in a specific direction (perhaps vector is not the right term ?) $\endgroup$
    – Quentin Chester
    Commented Nov 10, 2014 at 2:50
  • $\begingroup$ @QuentinChester, really it is just a matter of definition, a quantity of the characteristics you mention can indeed be defined, but dynamic pressure is what is stated above as equivalent to kinetic energy $\endgroup$
    – Nikos M.
    Commented Nov 10, 2014 at 7:38

Not the answer you're looking for? Browse other questions tagged or ask your own question.