Acoustic signatures of a model fan in the NASA-Lewis anechoic wind tunnel

Abstract

One-third octave band and narrowband llpectra and continuous directivity patterns radiated from an inlet are presented over ranses of fan operating conditions, tunnel velocity, and anale of attack.. Tunnel flow markedly reduced the unsteadiness and level of the blade passa .. tone, revealed the cutoff design faature of tne blade passa .. tone, and exposed a lobular directivity pattern for the second haraDnic tone. The full effects of tunnel flow are shown to be complete above a tunnel velocity of 2r mKters/second. The acoustic signatures are also shown to be strongly affected by fan rotational speed, fan blade loading, and inlet angle of sttack.The inability of ground-static acoustic tests to yield turbofan engine fan acoustic characteristics indicative of thst which would be messured during flignt is well documented. Refs. 1 through 7 all indicate that inflow disturbances present during static tests and absent under flignt conditions interact with the rotor to generate tone and broadband noise not present during flight. This noise resulting from the rotor interacting with inflow disturbances tends to mask the benefits attained through design techniques aimed st redUCing rotor/atator interaction noise levels. Refs. 8 through 14 attest to the number of investigations that have attempted to eliminste the excess noise in ground based facilities. These attempts have realized varying degrees of succeas.An approach to Simulating flight conditions is through the use of an anechoic wind tunnel. To this end the NASA-Lewis 9 by 15 low speed wind tunnel has been modified to achieve near anechoic or free-field properties in the test section. The capabtlities and limitationa of the facility are descriDed in Refs. 1,15, and 16. Ref. 1, in particular, concludea that the anechoic wind tunnel is a useful facility for applied research on aircraft engine fan noise during flight.The purpose of this paper ia to prupnt representative model fan noise data indicating the type of acoustic information that can be obtained in the NASA-Lewis anechoic wind tunnel. The model fan used for the test is designed to have the blade passage tone iUI-off below 107% of the design rotational speed. , 7,18,19 Where applicable, the results of the teat are related to known forward flight effects and to the rotor/stator cut-off theory . 17 One-third octave band and narrowband spectra .. well as continuous directivity patterns radiated from an inlet are presented. Data are shown over a range of conditions. The fan operating conditions varied include the rotational speed from 79 to 116% of the design value and the nozzle exit araa. The ranges of wind tunnel airflow velocities and inlet angle of attack are zero to 43 m/sec and zero to 30~ respectively. ApparatusAnechoic Wind Tunnel Fig. 1 ia a schematic representation of the NASA-Lewis 8-by 6-foot 20 and 9-by l5-foot wind tunnels. 21 The NASA-Lewis anechoic wind tunnel is the terminology used to refer to the combination of the 9-by l5-foot low speed test section, the tunnel drive motors, and enclos...

Wind tunnel measurements of blade/vane ratio and spacing effects on fan noise

“…The tunnel test section is considered to have free-field anechoic properties at frequencies above 1000 Hz. All tunnel-on tests in the present study are Presented as Paper 81-2032 at the AIAA 7th Aeroacoustics Conference, Palo Alto, Calif., Oct. [5][6][7]1981; submitted Oct. 16,1981; revision received April 15, 1982. This paper is declared a work of the U.S. Government and therefore is in the public domain.…”

Effects of simulated flight on fan noise suppression

“…These cil;l.nges at 1.5 and 2.5 BPF (presumably broadband noise) are more pronounced than those observed for the ran operating line of the previous study. 3,4 Any medal properties infercd from 1/3 octave band data must be qualified because of the var.1cd spectral structure that can occur withIn the 1/3-octave bandwidth. The following results obtained from narrow band spectral onalyses of the data illustrates that the decai1£!d spectral struetu::e indeed is compl~x.…”