Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Relationship between Mach number and Reynolds number

  1. Nov 27, 2015 #1
    Is there any correlation between Mach number and Reynolds number?
    Both of these nondimensional numbers involve speed, but they don't seem all that related other than that.
    For high speed flow, we have high Mach number. Is it incorrect to say that "generally" high speed flow is turbulent?
     
  2. jcsd
  3. Nov 27, 2015 #2

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    There is no relationship other than both of them feature the velocity.

    It is incorrect to say that generally high speed flows are turbulent. In fact, the effect of compressibility is stabilizing to a boundary layer.
     
  4. Nov 27, 2015 #3
    Also, what if you have high speed flow of a very viscous fluid?
     
  5. Nov 29, 2015 #4
    When we classify how "viscous" a fluid is, do we typically look at the dynamic or kinematic viscosity?

    Water is regarded as more viscous than air. It's dynamic viscosity is higher than air, but it's kinematic viscosity is smaller. So I guess we look at dynamic viscosity?
     
  6. Nov 29, 2015 #5
    When we talk about how viscous a fluid is, we are taking about its dynamic viscosity.

    Chet
     
  7. Nov 29, 2015 #6

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Kinematic viscosity is really more of a mathematical tool than anything else.
     
  8. Nov 29, 2015 #7
    I'd like to know this as well.

    If we're talking about hypersonic flow, what does mean in regards to viscous effects?
     
  9. Nov 29, 2015 #8
    $$Re=\frac{\rho v D}{\mu}=\frac{\rho c D}{\mu}\frac{v}{c}=Ma\frac{\rho c D}{\mu}$$
     
  10. Nov 29, 2015 #9
    Hmmm, but does this relation tells us much?
    Can we directly say that that Reynolds number scales with Mach number?
    I don't think so because there's that extra speed of sound term in the numerator.
     
  11. Nov 29, 2015 #10
    Why don't you just run some calculations for some sample situations, and see what the Mach number and Reynolds number come out to be. Then you won't need to speculate.

    Chet
     
  12. Nov 29, 2015 #11

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    You can say that it is directly proportional to Mach number for a constant speed of sound. Of course this doesn't mean much as it is equivalent to saying that Reynolds number is directly proportional to velocity, which is true by definition.

    What exactly is your question about "what does it mean in regards to viscous effects"? I'm not sure I follow what you are trying to ask so I'm not quite sure how to respond.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Relationship between Mach number and Reynolds number
  1. Physics and number (Replies: 6)

  2. Reynold's Number (Replies: 4)

  3. Nusselt number (Replies: 1)

Loading...