A gas turbine engine includes a fan, a compressor, a combustor, a turbine, a bypass duct downstream of the fan and outward of the compressor, and a cooling system. The cooling system includes an inlet for receiving air from the bypass duct, an outlet for returning air to the bypass duct, a cooling duct, and a heat exchanger and a modulator positioned in the cooling duct. The modulator includes dividers that extend parallel to each other and are arranged across the duct, and each divider includes two flaps that are rotatable about an axis in opposite directions.