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# Reynold Number Setting

Dear Sir

Can I ask several questions: I want to calculate the arodynamic power of HAWT with the data: Airfoil: NACA 4412, Chord : 0,2 m (taperless), Legth of Blade: 2.5 m, windspeed: 10 m/s

Based on that data above, the Reynold number is calculated around 136917. After that I run the airfoil analysis for Reynold number 136917 and I got the aerodynamic performance as seen in (figure 1). After that I continue to generate the circular aifoil as seen in (figure 2) then continuing to design the blade section as seen in (figure 3). After that I tried to optimized the balde configuration by clicking the optimize button and input several parameters (TSR, twist, etc) until I got ” the optimized blade” as shown in (figure 4). My questions are:

1. How do we calculate the value of the drag coefficient of the circular airfoil? in figure 2, I randomly fill it with value of 0.2 at Reynold number 136917.
2. Based on figure 4, we can see that the chord and twist angle are changed from the original blade. However in that configuration (optimized blade) is still using the polar data based on the airfoil calculation at Reynold number 136917. Is that correct to use that data in order to get the “new performance of the optimized blade” or we need to regenerate the airfoil simulation using several  Reynold numbers (the chord is varied due to the optimisation process) to create the other polar data?
3. Figure 5 is the aerodynmic simulation between the original blade and the optimized blade without varying the Reynold number

Thank you

Regard

Setiawan

Hi Setiawan,

for a circular airfoil the drag coefficient needs to be specified by the user. A typical (default) value is 1.2. In general the drag of a cylinder is depending on the Reynolds number, There is a lot of literature out there for more info on this.

Whether a small deviation in Reynolds number has a large effect on the lift/drag coefficients depends very much on the Reynolds number range in which you are operating. For small Reynolds numbers (~1e5) this sensitivity is quite large, for larger Reynolds (~1e6) number the sensitivity gets smaller and smaller.

In QBlade you can design a multi-polar blade where you specify multiple polars over a range of Reynolds numbers for each blade section. If you design such a blade during the simulation the algorithm will automatically interpolate between the polars to obtain lift/drag values at the current effective Reynolds number.

BR,

David 