smpod@csd.lerc.nasa.gov (Steve Podleski (Sverdrup)) (06/06/90)
From: "Steve Podleski (Sverdrup)" <smpod@csd.lerc.nasa.gov>
In article <1531@cirrusl.UUCP>, you write...
:Since no professional aerodynamicist has said anything, I'll take a
:stab at it. I warn you, this will get long-winded, as I'm basically
:thinking out aloud.
: Reynolds number (Rn) is a dimensionless number used in establishing
:flow conditions with incompressible fluids, just like Mach number can
:be used to compare flow conditions with compressible fluids (really,
:gases). ..........text deleted....
Your explanation of Reynold's number is very good except that Rn is
also used for compressible flow. Mach number is an ADDITIONAL parameter
needed to describe compressible flow.smpod@csd.lerc.nasa.gov (Steve Podleski (Sverdrup)) (06/06/90)
From: "Steve Podleski (Sverdrup)" <smpod@csd.lerc.nasa.gov> In article <1990May4.021200.15764@uokmax.uucp>, jabishop@uokmax.uucp (Jonathan A Bishop) writes... >In article <1531@cirrusl.UUCP> gopal@sun701 (Gopal Ramachandran) writes: >>If viscosity is very high, the force is going to be very high, and the >>velocity gradient is going to be very low. This is like trying to wade > > Not true. It is misleading to assume that high viscosity necessitates >high forces. As you state later in your article, Rn depends not only on >viscosity but also on velocity. This may be semantics but viscosity is an intrinsic property of the fluid and does not depend on fluid velocity but on fluid temperature. Viscous forces depends on viscosity AND velocity gradients. > > I toured the General Dynamics plant last month and saw their water >tunnel. You can set up tests with the same Rn as an airplane flying in >the air. I do not think you can replicate full-scale Rn in a water tunnel. Eventhough the kinematic viscosity (ratio of viscosity over density) of water is about 7 times less than air, the typical length of a water-tunnel model wing chord is 1 ft. and test speed is about 20 mph. Compare this to a typical full-scale fighter wing chord of about 15 ft. and 200 mph (for high-alpha work) and you get a Rn discrepancy by a factor of 21!