Flow measurement for a gas turbine engine

A system configured to determine airflow through a flow machine having a flow passage, the system comprising:

• a plurality of acoustic sensors, the acoustic sensors comprising at least one acoustic transmitter configured to transmit an acoustic waveform through the airflow passing through the flow passage to an acoustic receiver, wherein at least a first acoustic sensor is rotatable relative to a second acoustic sensor;
• and
• a processing system configured to receive signals from the acoustic sensors and determine a flow rate of the airflow through the flow passage, wherein
• the second acoustic sensor is mounted at an intake of the flow machine on a casing of the flow passage, and
• the first acoustic sensor is mounted to a rotatable hub or shaft provided within the casing at the intake of the flow machine.

A method of determining airflow properties of an airflow through a flow passage of a flow machine, the method comprising the steps of:

• providing signal communication with a plurality of acoustic sensors mounted relative to the flow passage, wherein at least a first acoustic sensor of the acoustic sensors is rotatable relative to at least a second acoustic sensor of the acoustic sensors;
• determining a time of flight of an acoustic waveform between said plurality of acoustic sensors;
• and
• using the time of flight between the plurality of sensors to determine an average flow velocity of the airflow through the flow passage, wherein
• the second acoustic sensor is mounted at an intake of the flow machine on a casing of the flow passage, and
• the first acoustic sensor is mounted to a rotatable hub or shaft provided within the casing at the intake of the flow machine.