A duct is configured for a vehicle having a vehicle body arranged along a longitudinal body axis. The vehicle body includes a first vehicle body end configured to face oncoming ambient airflow when the vehicle is in motion, a second vehicle body end opposite of the first vehicle body end. The duct has a fully-enclosed structure in a cross-sectional view perpendicular to the longitudinal body axis. The duct also has a first port positioned to receive a portion of the oncoming airflow and a second port positioned to exhaust the portion of the oncoming airflow from the duct. The first and second ports together with the fully-enclosed structure are configured to generate an aerodynamic downforce on the vehicle body when the vehicle is in motion.