The work investigates the effect of turbulent intensity (T_u) on the force and wake of a NACA0012 airfoil at chord Reynolds number Re_c = 5.3×10~3 and 2×10~4. Lift and drag coefficients (C_L and C_D) on and flow fields around the airfoil were measured with T_u varied from 0.6% to 6.0%. Four Re_c regimes are identified based on the characteristics of the maximum lift coefficient (C_(L,max)), i.e., ultra-low (Re_c ＜ 10~4), low (10~4 ～ 3×10~5), moderate (3×10~5～5×10~6) and high (＞ 5×10~6). It is noted that at Re_c = 5.3×10~3 (ultra-low Re_c regime) the stall is absent for T_u = 0.6% but occurs for T_u= 2.6% and 6.0%. As Re_c increases to low Re_c regimes, T_u influence weakens and the critical Re_c between the ultra-low and low Re_c regimes decreases. When the airfoil attack angle (α) varies from 0° to 30°, at low T_u (0.6%) the shear layers over the airfoil are mostly laminar at the ultra-low Re_c regime but involve transition at the low Re_c regime. The effect of increasing T_u on the flow bears some similarity to that of increasing Re_c, albeit with a difference. The concept of the effective Reynolds number Re_(c,eff) advocated for the moderate and high Re_c regimes is re-evaluated for the low and ultra-low Re_c regimes. The Re_(c,eff) treats the non-zero T_u effect as an addition of Re_c and is determined based on the presently defined Re_(c,cr). It has been found that all the maximum lift data from both present measurements and previous reports collapses into a single curve in the low and ultra-low Re_c regimes if scaled with Re_(c,eff).
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