Engineering topics

Dittmar, J. H.

Publications and source records attributed to Dittmar, J. H..

52 records · Page 3

NASA NTRS1977

Noise of fan designed to reduce stator lift fluctuations

An existing fan stage was redesigned to reduce stator lift fluctuations and was acoustically tested at three nozzle sizes for reduced noise generation. The lift fluctuations on the stator were reduced by increasing the stator cord, adjusting incidence angles, and adjusting the rotor velocity diagrams. Broadband noise levels were signficantly reduced in the middle to high frequencies. Blade passage tone sound power was not lessened, but decreases in the harmonics were observed. Aerodynamic improvements in both performance and efficiency were obtained.

Dittmar, J. H.↗

NASA NTRS1976

Effects of long-chord acoustically treated stator vanes on fan noise. 2: Effect of acoustical treatment

A set of long chord stator vanes was designed to replace the vanes in an existing fan stage. The long chord stator vanes consisted of a turning section and axial extension pieces, all of which incorporated acoustic damping material. The long chord stator vanes were tested in two lengths, with the long version giving more noise reduction than the short, primarily because of the additional lining material. The noise reduction achieved with the acoustically treated long chord stator vanes was compared with the reduction achieved by an acoustically treated exhaust splitter. The long chord stator was at least as good as the splitter as a method for incorporating acoustic lining material. In addition, comparing an acoustic three ring inlet and an acoustic wall-only inlet discloses that the wall-only inlet could be used in an engine where the noise reduction requirements are not too stringent.

Dittmar, J. H.↗

NASA NTRS1976

Noise reduction from the redesign of a fan stage to minimize stator lift fluctuations

An existing fan stage, redesigned to reduce stator lift fluctuations, was acoustically tested for reduced noise generation. The lift fluctuations on the stator were reduced by increasing the stator chord, adjusting incidence angles, and by adjusting the rotor velocity diagrams. The experiments showed significantly reduced broadband noise levels in the middle to high frequencies. Blade passage tone power was not reduced, but decreases in the harmonics were observed. Aerodynamic improvements in both performance and efficiency were obtained.

Dittmar, J. H.↗

NASA NTRS1976

Noise reduction from the redesign of a fan stage to minimize stator lift fluctuations

Dittmar, J. H.↗

NASA NTRS1975

Reduction of noise from a fan stage for a turbofan engine by use of long-chord acoustically-treated stator vanes

A set of acoustically-treated long-chord vanes was designed to replace the vanes in an existing fan stage to investigate the noise reduction possibilities of both increased stator chord length and this method of incorporating acoustic damping material. The vanes were tested with both active and inactive acoustic surfaces. The inactive tests showed significant broadband noise effects with noise reductions in the middle to high frequencies and an increase at low frequencies. No reduction in blade passage tone was observed, but decreases in the overtones were observed. The tests with the active acoustic treatment showed large noise reductions over a wide frequency range.

Dittmar, J. H.↗

NASA NTRS1975

Effects of long-chord acoustically treated stator vanes on fan noise. 1: Effect of long chord (taped stator)

A set of long-chord stator vanes was designed to replace the vanes in an existing fan stage. The long vanes consisted of a turning section and axial extension pieces, both of which incorporated acoustic damping material. The acoustic damping material was made inactive for these tests by covering with metal tape, and the stator vanes were tested in three length configurations. Compared to the values for the original stage, broadband noise was reduced in the middle to high frequencies with the long stator vanes, but a broadband noise increase was observed at the low frequencies. No change was observed in the blade passage tone, but some aft end reduction in the overtones was observed.

Dittmar, J. H.↗

NASA NTRS1975

An experimental study of the effect of treated length on fan inlet noise suppressors

An experimental study of the effect of length of acoustic treatment was performed on a full scale fan using two different inlets. Analysis of the data revealed that measured suppression varied linearly with the length of the ringed linear segments as anticipated. Further analysis indicated that the wall only portion of the treatment was substantially more effective than anticipated suggesting that, if only moderate suppression is needed, wall only treatment may suffice. Contrary to expectation the two inlets behaved similarly. Introduction of a hard exhaust splitter into the configuration resulted in a noise increase at certain frequencies.

Dittmar, J. H.↗

NASA NTRS1975

Reduction of Noise from a Fan Stage for a Turbofan Engine by Use of Long-Chord Acoustically-Treated Stator Vanes

A set of acoustically-treated long-chord vanes was designed to replace the vanes in an existing fan stage to investigate the noise reduction possibilities of both increased stator chord length and a method of incorporating acoustic damping material. The vanes were tested with both active and inactive acoustic surfaces. Results of the inactive tests show significant broadband noise effects with noise reductions in the middle to high frequencies and an increase at low frequencies. No reduction in blade passage tone was observed, but decreases in the overtones were observed. Results of the tests with the active acoustic treatment show large noise reductions over a wide frequency range.

Dittmar, J. H.↗

NASA NTRS1974

Effect of treated length on performance of full scale turbofan inlet noise suppressors

Two inlet noise suppressors containing wall treatment plus three treated rings were tested on a fan in an outdoor noise facility. Sound power attenuations were measured for three treated lengths of each suppressor. The noise reduction from the segment of liner closest to the fan, which contained a segment of wall treatment downstream of the splitter rings, was greater than the reduction from either of the other segments. The decibel attenuations of the ringed liner segments were linear with liner length as predicted by theory. The acoustic attenuation of the wall treatment was considerably greater than expected for available theory. This inordinate effectiveness of the wall treatment strongly suggests the possibility of using no-ring inlet suppressors when the required noise reduction is moderate. The decibel attenuations were higher than predicted above 2000 hertz, and the two suppressors behaved similarly despite the prediction of different behavior.

Dittmar, J. H.↗

NASA NTRS1974

Combined Quadrupole-dipole Model for Inlet Flow Distortion Noise from a Subsonic Fan

A combined quadrupole-dipole model has been developed for the noise generated by inlet flow distortion in a subsonic fan. A formula is derived for the total upstream-radiated acoustic power in each tone as a function of the design parameters of the fan and the properties of the inlet flow distortion. Numerical results are obtained for values of the parameters corresponding to various quiet fans. The analysis is compared with noise measurements taken on a 51-cm (20-in.) diameter research fan as well as with those taken on a number of full-scale fan stages. Fairly good agreement was obtained. It should therefore be possible to use this model to study the noise-reduction potential of the various fan design parameters.

Goldstein, M. E.↗

NASA NTRS1974

An experimental study of the effect of treated length on fan inlet noise suppressors

The effect of length of acoustic treatment was studied on a full scale fan using two different inlets. Analysis of the data revealed that measured suppression varied linearly with the length of the ringed liner segments as anticipated. Further analysis indicated that the wall only portion of the treatment was substantially more effective than anticipated, suggesting that, if only moderate suppression is needed, wall only treatment may suffice. The two inlets behaved similarly. Introduction of a hard exhaust splitter into the configuration resulted in a noise increase at certain frequencies.

Dittmar, J. H.↗

NASA NTRS1972

Methods for reducing blade passing frequency noise generated by rotor-wake - stator interaction

A theoretical model for the generation of blade-passage noise caused by the interaction of rotor wakes with the downstream stator blades in a fan stage has been compiled. This model has combined a description of the rotor wake based on existing experimental correlations with an existing theory for the response of the stator blades to those wakes. This model of the interaction-caused blade-passage noise has been used to examine some effects of blade geometry and flow conditions on the noise generated. A set of significant parameters has been identified to reduce this interaction-caused blade-passage frequency noise. A redesign of an existing fan with the use of these concepts has produced a theoretical noise reduction of more than 7 decibels.

Dittmar, J. H.↗

NASA NTRS1972

Effects of installation caused flow distortion on noise from a fan designed for turbofan engines

Far-field noise measurements were taken for three different installations of essentially the same fan. The installation with the most uniform inlet flow resulted in fan-blade-passage tone sound pressure levels more than 10 dB lower than the installation with more nonuniform inflow. Perceived noise levels were computed for the various installations and compared. Some measurements of inlet flow distortion were made and used in a blade-passage noise generation theory to predict the effects of distortion on noise. Good agreement was obtained between the prediction and the measured effect. Possible origins of the distortion were identified by observation of tuft action in the vicinity of the inlet.

Povinelli, F. P.↗

NASA NTRS1972

Installation caused flow distortion and its effect on noise from a fan designed for turbofan engines.

Use of a ground test stand to obtain acoustic data on a full-scale prototype fan designed for quiet subsonic-aircraft engines. The fan was installed in three different ways in the test stand. In two of the installations the fan was driven by a shaft in the inlet; in the third installation the fan was driven from the rear. These three installations, and the structures associated with them, resulted in various amounts of inlet flow distortion to the fan. The rear-drive installation had less inlet flow distortion than the two front drive installations. Differences in blade passage sound pressure levels of more than 10 dB were measured between the rear-drive and front-drive versions, with the rear-drive installation producing less noise. Perceived noise levels were computed and the influence of the distortion on these levels was determined.

Povinelli, F. P.↗

NASA NTRS1972

Installation caused flow distortion and its effect on noise from a fan designed for turbofan engines

A ground test stand was used to obtain acoustic data on a full scale prototype fan designed for quiet subsonic aircraft engines. The fan was installed in three different ways in the test stand. In two of the installations the fan was driven by a shaft in the inlet; in the third installation the fan was driven from the rear. These three installations, and the structures associated with them, resulted in various amounts of inlet flow distortion to the fan. The rear drive installation had less inlet flow distortion than the two front drive installations. Some measurements of inlet flow distortion were made and used in a blade passage noise generation theory to predict the effects of distortion on noise. Good agreement was obtained when the predicted and measured power level differences between the front drive and rear drive installations were compared.

Povinelli, F. P.↗

NASA NTRS1972

Engine noise technology

The characteristics of aircraft engine noise are discussed. Data are provided to show the noise produced by the following aircraft components: (1) fan noise, (2) noise suppressing structures, (3) sonic inlets, (4) jet mixing noise due to nozzle flow, and (5) thrust reversers. Charts are developed to show the sound pressure level and the frequencies for each type of noise source. The use of laminates and composite materials to dissipate acoustic power is examined.

Luidens, R. W.↗