This study investigates the aerodynamic performance enhancement of a spiroid winglet utilizing the NACA 2412 airfoil through three-dimensional numerical simulations. Winglets are a well-established technology for improving the aerodynamic efficiency of aircraft wings by reducing induced drag. However, conventional winglet designs can still experience significant vortex shedding and flow separation, limiting their efficacy. The spiroid winglet geometry, inspired by the theoretical minimum induced drag shape, shows promise in mitigating these issues. Computational fluid dynamics (CFD) analyses were conducted using a finite volume method to solve the Reynolds-Averaged Navier-Stokes equations coupled with a Shear Stress Transport turbulence model. The wing-winglet model incorporated a spiroid winglet design based on the NACA 2412 airfoil. Simulations were performed at angles of attack ranging from 0° to 25° to assess the winglet's performance over the operational envelope. Results demonstrate that the spiroid winglet configuration significantly reduces induced drag compared to the baseline wing without a winglet, with drag reductions of up to 23.8% at the design cruise condition of 2° angle of attack. Furthermore, the wingtip vortices are found to be more diffuse with lower peak vorticity magnitudes, indicating mitigation of strong tip vortex formation. The spiroid winglet delays flow separation up to higher angles of attack compared to conventional winglets. Detailed flow visualizations and surface pressure distributions elucidate the fluid dynamic mechanisms behind the performance gains. Overall, the numerical study validates the merits of spiroid winglets and provides insights for optimizing future aerodynamic designs.