What aerodynamic feature can help reduce spanwise airflow around the wings?

The feature that helps reduce spanwise airflow around the wings is fences and vortex generators. These devices are specifically designed to manipulate the airflow over the wing's surface to enhance its aerodynamic performance.

Fences are vertical surfaces mounted on the wing that disrupt the spanwise flow of air, thereby maintaining the wing's flow attachment and delaying flow separation. This improves the overall lift characteristics of the wing and enhances control at various angles of attack. Vortex generators, which are small, fin-like protrusions on the wing’s surface, generate small vortices that energize the boundary layer, helping to keep the airflow attached to the wing longer.

By using fences and vortex generators, aircraft can improve their lift-to-drag ratios, enhance efficiency, and improve stability and control, especially at high angles of attack when the risk of stall is greater.