3 Replies Latest reply on Mar 5, 2012 6:01 PM by Legt Chevrollier

# Negative value of Velocity?

I am getting a negative value of Velocity in my Flow Simulation analysis. It is not much, something like - 0.5 m/s

This made me puzzled: How can a velocity of a wind have a negative value???

I know that pressure can have a negative and positive value. Positive value of wind will act like a compressional force towards the object. And negative value of pressure will act like a suction force.

But velocity can only take positive value. How can a velocity take a negative value?

Did I just miss some of the physics classes, or is this just Flow Simulation error?

Thank you for the replies.

• ###### Re: Negative value of Velocity?

Velocity is a vector quantity, and therefore can be negative (i.e. flow direction is opposite the coordinate axis positive).  Velocity resultant can not be negative, but individual components of velocity can be.  Are you looking at a resultant, or component of velocity?  And to be picky, pressure can only be negative relative to a reference point, for example gauge pressure vs. absolute pressure.  Hope this helps.

• ###### Re: Negative value of Velocity?

I think you're mixing up the terms velocity and speed. Velocity, as Billy indicates above, is a vector  quantity and therefore can be negative or positive.  Speed is a scalar quantity and because of the way it's defined, is always greater than or equal to zero, since speed is defined as the magnitude of the velocity, that is, the square root of the sum of the squares of each velocity component in a Cartesian coordinate system.

• ###### Re: Negative value of Velocity?

Thank you for the replies, both of you.

Now I understand it:

velocity is a vector quantity, and speed a scalar one.