PSI - Issue 80

Tafara E. Makuni et al. / Procedia Structural Integrity 80 (2026) 105–116 Tafara E. Makuni / Structural Integrity Procedia 00 (2019) 000 – 000

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results in Table 3 show good agreement with the lift values but not drag. This is mainly because lift has a weak dependence on Re , however, drag has a strong dependence on Re in XFOIL. Drag’s strong dependence on Re in XFOIL is primarily because XFOIL models viscous drag through boundary-layer calculations, and the boundary-layer behaviour (skin friction, transition, separation) changes significantly with Re . XFOIL explicitly models these effects through its viscous coupling, and small changes in Re can shift transition or create/eliminate laminar separation bubbles, causing large changes in drag (Drela, 2013). In addition to this, the 3D printed surface is rough, contributing to a higher drag value from theoretical predictions. Further to this, the measurement for drag includes the aluminium base plate annotated in Figure 2. The base plate is flush with the wind tunnel floor hence does not contribute to significantly to the lift and span wise components of force, but does contribute to the drag measurements, especially given the size of the aerofoil in comparison. JavaFoil can also be used calculate aerodynamic parameters and better captures Re effects compared to XFOIL however, XFOIL is generally regarded as more accurate than JavaFoil (Drela, 2013) (Hepperle, 2017). For the Re values given in Table 2 and Table 3, Figure 4 shows the variation of C L and C D , for α ∞ ranging from -20 o to +20 o .

Figure 3: Pressure distribution around the aerofoil calculated using XFOIL for U ∞ = 5 – 25 ms -1 and α ∞ = 0 o .

Table 2: Lift and drag values for aerofoil section calculated using XFOIL for U ∞ = 5 – 25 ms -1 and α ∞ = 0 o .

M ∞

C L

C D

C M

L/D

Lift (N)

Drag (N)

0.0138 0.0292 0.0442 0.0590 0.0744

0.3364 0.3435 0.3491 0.3532 0.3563

0.01017 0.00835 0.00775 0.00744 0.00719

-0.0723 -0.0734 -0.0734 -0.0750 -0.0756

33.07 41.13 45.02 47.49 49.54

0.0988 0.4593 1.0907 1.9958 3.2671

0.0030 0.0112 0.0242 0.0420 0.0659

Table 3: Lift, drag and spanwise forces obtained from load cell measurements for U ∞ = 5 – 25 ms -1 and α ∞ = 0 o . M ∞ α ∞ ( o ) U (m/s) Re (unit L) Re (chord) Lift (N) Drag (N) Span (N) 0.0138 0 o 4.61 315,990 47,399 0.1871 0.0017 0.0505 0.0292 0 o 9.84 674,299 101,140 0.5782 0.1162 0.1181 0.0442 0 o 15.06 1,030,793 154,620 1.3181 0.2877 0.2822 0.0590 0 o 20.25 1,386,231 207,930 2.2355 0.5126 0.5300 0.0744 0 o 25.79 1,765,893 264,880 3.3298 0.8405 0.8471