Your point is taken and parallels my original thinking. However, testing has developed a few anomalies and empirical results that raise new questions.cwb wrote:Re is arbitrary by definition but it is an important indicator, especially when trying to figure out whether a particular flow structure is laminar or turbulent.olefud wrote:Calculating a Re is somewhat arbitrary given the large variation in the velocity and temperature of the rotor, Re being directly proportional to the speed and inversely so to the air viscosity, which increases with temperature. Air temperature immediately adjacent the rotor is not measured in conventional dyno testing.
Just from observation, the boundary layer appears to be laminar away from the pad when the rotor is glowing dark red based on the incandescing particles. On the other hand, some of these heavier ?such particles escape immediately following the pad. My working assumption is that a heat stressed rotor has a laminar boundary layer though it may take a while to form after the pad disruption.
My back of the envelope calcs suggest a Re of the order of 250,000 - 400,000 at ambient air temperatures and 50,000 - 80,000 at temperatures of 400 degC, halving again to 30,000 - 40,000 around 800 deg C depending on what assumptions you make about rotor/air speed and dimensional scales. These numbers do show the dramatic effect of the change in density and dynamic viscosity over temperature that you have often mentioned. However they are all well north of the (yes arbitrary) threshhold between laminar and turbulent flow around Re = 5000, to get to this number vehicle speed would have to be reduced by a factor of 6.
I guess I would like to challenge the assumption that the flow patterns you are observing are laminar. I suggest that what you are observing are sections of relatively stable flow structure, due to favourable pressure gradients and relatively small length scales due to the flow being adjacent to the rotor, but it is still very turbulent.
I realise this doesnt actually shed much light on your situation but to me its an important distinction.
With reasonable assumptions, braking at 120 MPH allows about .03 seconds between a rotor point passing the pad and arriving again. Testing has shown that vanes closely trailing the pads are of diminished effectiveness while those leading the pads are quite effective. Assuming –as I do- that the pads effectively disrupt the rotor boundary layer, there are rapid changes in conditions, particularly temperature, during these few tenths of a second. The boundary layer forms, or is forming, and apparently carries along cooler air that acts on the pads. Given, if you will, these dynamic, rapidly changing conditions, and the probable nascent early boundary layer, Re offers a bit too much certainty more appropriate for established, steady-state situations.
Since, in my view, the concept deals with a new mechanism, I’m working to conservatively sort what is known while being open to theories and explanations.