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Man vs. Machine (or CFD code!)

Question asked by Karl Mohr on Dec 18, 2011
Latest reply on Dec 21, 2011 by Rich Bayless

OBJECTIVE: assess the validity of Flow Simulation results for Reynolds numbers COMMON to COMMON transportation!  The range Re=1e6 to 1.5e7, PRECISELY where the tutorials and validation examples do not cover!


The Cd on a continuous CYLINDER, normal to flow, at Re=1e7, in water has been shown as ~0.6 (ref.: R.L.Panton) and in air, with a surface roughness of ~5.6 micrometers, Cd~0.7! (ref. Roshko)


Given that a Cylinder has a Diameter=D and a (cambered) Airfoil a Chord=C and a thickness=t=0.28C where:

  • t=Thickness normal to flow
  • D=C=5m
  • fluid=air
  • ALL runs have roughness set at 5 micrometers (adds to results ~8%!)
  • ALL runs have Gravity applied (reduces cylinder results by ~6%!)
  • and Re=1e7 (constant)
  • CD's set at front=15C upper=15C lower=15C rear=25C (as per CYLINDER DRAG tutorial
  • Initial Mesh= 7
  • Surfaces have Local Initial Mesh (S/F 4,4,4, 0.31756, 1.25)
  • three additional LIM's of generally concentric, eliptical shape, refined accordingly
  • ALL runs use refinement level 2
  • ALL runs use 4 refinements (1,1.25,1.5,1.75 travels)
  • ALL runs are required to use all 4 refinements
  • FINISH is at convergence 'auto' relaxation: +0.2 travels


QUESTION 1: Is there ANY man/woman/child that would BELIEVE the DRAG on the CYLINDER, with it's greater circumference (=surface area=skin friction) and MORE THAN 3 times the thickness, could be LESS than the DRAG on the AIRFOIL with it's far smaller 'frontal area' and smaller circumference (=surface area)???


Flow Simulation PREDICTS the AIRFOIL of less than one third thickness has MORE drag and a WORSE Cd;

  • CYLINDER: Cd=0.0575, DRAG=30.0697kgf
  • AIRFOIL (C128): Cd=0.058245 DRAG=30.4388kgf


Is this merely due to the fact that: THOUGH tested at angle of attack=alpha=0deg, the heavily cambered airfoil, generates more lift?


QUESTION 2: Flow Simulations prediction are out by a HUGE factor of about 1200% (at this Reynolds number!)

Is this as good as it (SW 2012 Flow Simulation) gets???


QUESTION 3: If 'YES' to Q2, shouldn't ALL users be made aware of this in the interest of costs and consequences/safety/liability associated with being UNAWARE of the HUGE errors?


Please help!