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Cp's polar is always negative
Hello,
I have a project to test the influence of discontinuities on the blades of vertical wind turbines and I have a problem with the generation of the Cp curve and the values of the power generated.
In fact, I calculated my own values for the drag and lift coefficients using another software program for my profile. I manage to import my profile as well as my CL and CD values into QBlade, I extrapolate them with QBlade and when I define my simulation of the vertical wind turbine my Cp is always negative whatever the configuration of the wind turbine. Is my input file wrong? Qblade can’t read my CL and CD values correctly? I don’t understand why my Cp vs TSR curve is always negative. Thank you in advance for any help. Here are my input values:
angle(°) CL CD
-20 -0,843787 0,571439
-19 -0,832606 0,528438
-18 -0,840638 0,480176
-17 -0,869594 0,428469
-16 -0,892332 0,38262
-15 -0,906306 0,34351
-14 -0,906989 0,309978
-13 -0,898806 0,280056
-12 -0,877753 0,253918
-11 -0,839999 0,23091
-10 -0,781378 0,210886
-9 -0,704481 0,193737
-8 -0,614722 0,179119
-7 -0,514059 0,166598
-6 -0,405014 0,155939
-5 -0,287593 0,146952
-4 -0,165586 0,139625
-3 -0,0401634 0,133863
-2 0,0864461 0,129635
-1 0,213216 0,126876
0 0,341241 0,125508
1 0,468479 0,12547
2 0,597435 0,126729
3 0,731932 0,12941
4 0,868334 0,133535
5 1,00568 0,139147
6 1,13997 0,146177
7 1,26886 0,154646
8 1,38885 0,164569
9 1,49827 0,176123
10 1,59043 0,18979
11 1,63322 0,207524
12 1,5118 0,238976
13 1,43421 0,271335
14 1,40788 0,302042
15 1,39877 0,333592
16 1,39951 0,365896
17 1,40187 0,400573
18 1,40731 0,436381
19 1,41367 0,474251
20 1,41983 0,513855
Hello Jeanneh,
the power coefficient of a VAWT design depends on many more parameters than only the polar data.
Two observations:
- The polar data you referenced shows a glide ratio (lift-to-drag ratio) of less than 10, which indicates relatively low aerodynamic efficiency. This, in turn, leads to reduced rotor performance, though not necessarily below zero.
- I would advise against using the double multiple streamtube (DMS) algorithm in QBlade, as its results can vary significantly depending on the rotor’s geometry. For more insights on this topic, you may want to search the relevant discussions in this forum. Instead, I recommend using the lifting line free vortex wake model, which provides more reliable turbine performance predictions through time-domain simulations.
BR,
David
