What kind of fluids can be assumed to be continuum and what not?

What kind of fluids can be assumed to be continuum and what not? 


  • kkanadekkanade Forum Coordinator
    The primary idea behind the continuum assumption is that Fluid is continuously divisible and we do not have to concern ourselves with individual molecules. This assumption is valid under normal conditions. However, it very quickly breaks down when the characteristic length scale (dimension) of our problem is on the same order as the mean free path of the fluid molecules. For example, we must abandon the concept of continuum in case of rarefied gases. The validity of continuum assumption can be verified using the Knudsen Number.
  • he Knudsen Number represent the ratio between the mean-free-path of the molecules (lambda) and the problem characteristic length (L):

    Kn = lambda/L

    This number is used in fluid dynamics to define the limits of the continuum approximation. For Kn<<1 a fluid can be assume to be a continuum, while at Kn close to 1 or larger we have free molecular flow. Also, in the range of values 0.01<Kn<0.1 we have the slip regime, where the fluid at the wall has a different velocity than the solid.

  • prajputprajput Member
    Thanks @Kremella
Sign In or Register to comment.