Measurements of Multiple Pure Tone Propagation From a High Bypass Turbofan Rotor in an Internal Flow Facility

Turbofan engines are a significant contributor to the objectionable aircraft noise around airports. An important noise source in modern high bypass ratio turbofans is from multiple pure tones produced by the turbofan rotor operating at supersonic tip speeds during takeoff. The multiple pure tones are the product of variations in fan blade shocks that propagate upstream and out of the inlet. The tones can dominate the forward radiated noise from turbofan engines and be a significant contributor to the effective perceived noise level (EPNL) from an aircraft.

An experiment conducted on a high bypass turbofan rotor in an internal flow facility provided dynamic pressure measurements to investigate multiple pure tone generation and propagation. The experiment was conducted in the W-8 Single Stage Axial Compressor Facility at NASA Glenn Research Center on a 22 inch (0.56m), 22-blade rotor. At the aerodynamic design point, the rotor had a tip Mach number of 1.26 and a pressure ratio of 1.5. An exhaust throttle valve was utilized to acquire data along the fan's nominal operating line as well as acquire a few speed lines at fixed rotor speeds.

Since multiple pure tones are generated by blade shock variation primarily due to blade stagger angle differences between successive blades, the blade stagger angles were measured with six over-the-rotor dynamic pressure transducers along the axial chord. Negative slope zero crossings in the phase-averaged over-the-rotor dynamic pressure signal indicate the position of each blade. These measurements of blade position are utilized to assess the blade angle variation and blade untwist from approach to takeoff fan conditions.

Traditionally, in-duct acoustic measurements are decomposed into duct modes assuming that only cut-on duct modes are present. Since multiple pure tones are high-amplitude (in excess of 160dB sound pressure level), and modern turbofan inlets are shorter (less than 0.5 length-to-diameter ratio), cut-off duct modes can propagate to the far-field. Measurements of multiple pure tone propagation of the turbofan rotor were made with an array of 31 wall-mounted dynamic pressure transducers. The array spanned from 0.4 to 1.1 diameters upstream of the rotor. These measurements are utilized to analyze the propagation characteristics of multiple pure tones and quantify their sensitivity to inlet length. The measurement of shock propagation down the duct shows evidence of shock merging and an increasing variation in shock spacing. The multiple pure tone propagation characteristics are compared against shock decay and duct mode expectations.

Additionally, the effect of a few fan casing treatments on multiple pure tone levels are assessed. The casing treatments included circumferential grooves installed over-the-rotor with or without acoustic liners installed at the bottoms of these circumferential grooves. Overall, these casing treatments had subtle effects on the resultant multiple pure tone noise levels.