Originally Posted by icecyc1

Sorry to be confusing... Google is correct, and yes, the hole diameter is in the numerator.... BUT... When you are substituting the velocity based on the flow difference between the piston velocity and the velocity of the oil in the hole, the diameter of the hole itself ends up in the denominator.

The key equation that is missing is the conservation of flow. Qp=n*Qh, where Q1 = Vp*Ap and Q2 = Vh*Ah. (Ah=pi/4*Dh^2) Since the Re formula needs the velocity of the oil through the hole (Vh), you must rearrange the flow formula for Vh. It ends up being:
Vh = (Vp*Ap)/(Ah*n).... and that is how the diameter of the hole ends up in the denominator. The Diameter of the piston is just used because you know the velocity of the shock and you are using that flow ratio to calculate the flow velocity through the hole.

I hope that's a little clearer???

Originally Posted by avaldes

That is because he is showing velocity as a function of the ratio of the hole diameters. The significant length in the Reynolds number is always on the top of the ratio, meaning as length increases, Re increases as do the influence of inertial vortex forces.

An important fundamental of Reynolds number is that it is a ratio of inertial vortex forces to viscous forces. Low numbers mean viscosity dominates and high numbers mean vorticity dominates....often referring to the tendency of a fluid to roll up into vorticies instead of staying smooth and laminar/viscous.

It takes a professor who really understands the subject to teach this appropriately in college/grad school so that the student gets a good handle on the fundamentals.

** looks like we were posting at the same time!

Originally Posted by BobW

One thing that needs to be considered besides just the Re No is the orifice discharge coefficient (Cd). For short tube orifices, which we have, the discharge coefficient varies with Re No. and the L/Dh ratio (piston thickness/orifice diameter). The graph below shows the Force vs Piston Velocity (F-V) characteristics for two pistons with the same total orifice area. One has 6 holes and one has 3. As you can see the piston with more smaller holes produces much higher force than the piston with fewer larger holes. This is a result of the Cd coefficient.

Note that this damper model does not account for fluid compressibility. What compressibility will do is significantly reduce the force the damper produces during the first half of the compression stroke. This effect will be much much more pronounced at higher velocities. Pack velocities as they are referred to.

This model is being validated along with Icecyc1's work so I am very confident the effect of hole size is real. We see this effect in the dyno results.