The optimum turbulator should avoid laminar separation bubbles without increasing the drag. Unfortunately, on most high performance, low Reynolds number airfoils, the position of the separation bubble tends to move when the lift coefficient changes. Also the Reynolds number is depending on the lift coefficient: circling in a thermal requires high lift coefficients and low Reynolds numbers, whereas cruising at high speed leads to high Reynolds numbers at low lift coefficients.Thus a fixed turbulator will be a compromise: when it is located forward, to avoid bubbles at higher lift coefficients, it will create additional drag at lower lift coefficients, and when it is located more rearwards, it will be ineffective at higher lift coefficients.
Calculated drag coefficient of the MH 32 for 5 different lift
coefficient/Reynolds number combinations. Transition has been fixed at
different x/c locations.
The figure above shows the results of a numerical experiment. For different combinations of Reynolds number and lift coefficient, corresponding to a typical F3B sailplane, the drag coefficient has been calculated. The transition has been forced to occur at different stations x/c, starting at x/c = 5%. Picking the curve for Re = 171'000, Cl = 0.5 shows, that the drag coefficient decreases steadily, while we move the turbulator rearwards, until we reach a point where the curve levels out. This is the location, where the separation bubble starts. Moving the turbulator further towards the trailing edge leads to an increase in Cd: the turbulator is simply in or even behind the bubble and has no effect anymore. A useable compromise would be to place the turbulator somewhere between 60% and 80% of the chord length, say at 70%. If you were very picky, you would have to chose different positions along the span, depending on local Reynolds number and lift coefficient - but that might generally be considered harmfully close to splitting your hair.
In general, it is possible to design airfoils so, that the separation bubble stays at the same location, but this location has to be chosen according to the low Reynolds number case (close to x/c = 50% for the above example), resulting in higher drag at higher Reynolds numbers.
Turbulators are helpful not only at very low Reynolds numbers - even on full scale sailplanes turbulators are widely used to improve their performance. The figure below shows the results of a numerical study to find the optimum turbulator position on the MH 24 pylon racing airfoil, which operates at Reynolds numbers around 1 Million.
Calculated drag coefficient of the MH 24 for different Reynolds
numbers The optimum position seems to be at 84% of the chord. The drag of
the airfoil comes close to the flat plate with completely laminar flow.
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