Flow velocity GudangMovies21 Rebahinxxi LK21

      In continuum mechanics the flow velocity in fluid dynamics, also macroscopic velocity in statistical mechanics, or drift velocity in electromagnetism, is a vector field used to mathematically describe the motion of a continuum. The length of the flow velocity vector is scalar, the flow speed.
      It is also called velocity field; when evaluated along a line, it is called a velocity profile (as in, e.g., law of the wall).


      Definition


      The flow velocity u of a fluid is a vector field





      u

      =

      u

      (

      x

      ,
      t
      )
      ,


      {\displaystyle \mathbf {u} =\mathbf {u} (\mathbf {x} ,t),}


      which gives the velocity of an element of fluid at a position




      x




      {\displaystyle \mathbf {x} \,}

      and time



      t
      .



      {\displaystyle t.\,}


      The flow speed q is the length of the flow velocity vector




      q
      =


      u




      {\displaystyle q=\|\mathbf {u} \|}


      and is a scalar field.


      Uses


      The flow velocity of a fluid effectively describes everything about the motion of a fluid. Many physical properties of a fluid can be expressed mathematically in terms of the flow velocity. Some common examples follow:


      = Steady flow

      =

      The flow of a fluid is said to be steady if




      u



      {\displaystyle \mathbf {u} }

      does not vary with time. That is if









      u




      t



      =
      0.


      {\displaystyle {\frac {\partial \mathbf {u} }{\partial t}}=0.}



      = Incompressible flow

      =

      If a fluid is incompressible the divergence of




      u



      {\displaystyle \mathbf {u} }

      is zero:







      u

      =
      0.


      {\displaystyle \nabla \cdot \mathbf {u} =0.}


      That is, if




      u



      {\displaystyle \mathbf {u} }

      is a solenoidal vector field.


      = Irrotational flow

      =

      A flow is irrotational if the curl of




      u



      {\displaystyle \mathbf {u} }

      is zero:





      ×

      u

      =
      0.


      {\displaystyle \nabla \times \mathbf {u} =0.}


      That is, if




      u



      {\displaystyle \mathbf {u} }

      is an irrotational vector field.
      A flow in a simply-connected domain which is irrotational can be described as a potential flow, through the use of a velocity potential



      Φ
      ,


      {\displaystyle \Phi ,}

      with




      u

      =

      Φ
      .


      {\displaystyle \mathbf {u} =\nabla \Phi .}

      If the flow is both irrotational and incompressible, the Laplacian of the velocity potential must be zero:



      Δ
      Φ
      =
      0.


      {\displaystyle \Delta \Phi =0.}



      = Vorticity

      =

      The vorticity,



      ω


      {\displaystyle \omega }

      , of a flow can be defined in terms of its flow velocity by




      ω
      =

      ×

      u

      .


      {\displaystyle \omega =\nabla \times \mathbf {u} .}


      If the vorticity is zero, the flow is irrotational.


      The velocity potential



      If an irrotational flow occupies a simply-connected fluid region then there exists a scalar field



      ϕ


      {\displaystyle \phi }

      such that





      u

      =


      ϕ

      .


      {\displaystyle \mathbf {u} =\nabla \mathbf {\phi } .}


      The scalar field



      ϕ


      {\displaystyle \phi }

      is called the velocity potential for the flow. (See Irrotational vector field.)


      Bulk velocity


      In many engineering applications the local flow velocity




      u



      {\displaystyle \mathbf {u} }

      vector field is not known in every point and the only accessible velocity is the bulk velocity or average flow velocity






      u
      ¯





      {\displaystyle {\bar {u}}}

      (with the usual dimension of length per time), defined as the quotient between the volume flow rate






      V
      ˙





      {\displaystyle {\dot {V}}}

      (with dimension of cubed length per time) and the cross sectional area



      A


      {\displaystyle A}

      (with dimension of square length):







      u
      ¯



      =




      V
      ˙


      A




      {\displaystyle {\bar {u}}={\frac {\dot {V}}{A}}}

      .


      See also




      References

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