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At least 325 records · Page 18

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NASA NTRS1982

Utilizing numerical techniques in turbofan inlet acoustic suppressor design

Numerical theories in conjunction with previously published analytical results are used to augment current analytical theories in the acoustic design of a turbofan inlet nacelle. In particular, a finite element-integral theory is used to study the effect of the inlet lip radius on the far field radiation pattern and to determine the optimum impedance in an actual engine environment. For some single mode JT15D data, the numerical theory and experiment are found to be in a good agreement.

Baumeister, K. J.↗

NASA NTRS1982

Effect of steady-state pressure distortion on inlet flow to a high-bypass-ratio turbofan engine

Static pressure and total pressure distributions were measured in the inlet duct upstream of the engine inlet and within the fan and compressor of a YTF34 turbofan engine. In addition, the free stream flow angle was measured between the distortion generator and the engine inlet. Distortions were generated using three screen configurations with extents of 90 deg or 180 deg. The screens were mounted on a rotatable screen assembly. Reynolds number index upstream of the distortion device was maintained at 0.5 or 0.2, and engine fan speed corrected to station 2 temperature was maintained at 80 or 90 percent of rated condition (7005 rpm). Flow angle was nearly constant near the distortion device and increased as flow approached the engine inlet. The largest flow angle occurred in the hub region of the engine inlet. Static pressure distortion along the inlet duct increased exponentially as the flow approached the engine. Both static pressure and total pressure distortions were attenuated between engine inlet and compressor exit.

Soeder, R. H.↗

NASA NTRS1983

Turbofan blade stresses induced by the flow distortion of a VTOL inlet at high angles of attack

A 51-cm-diameter turbofan with a tilt-nacelle VTOL inlet was tested in the Lewis Research Center's 9- by 15-Ft Low Speed Wind Tunnel at velocities up to 72 m/s and angles of attack up to 120 deg. Fan-blade vibratory stress levels were investigated over a full aircraft operating range. These stresses were due to inlet air flow distortion resulting from (1) internal flow separation in the inlet, and (2) ingestion of the exterior nacelle wake. Stress levels are presented, along with an estimated safe operating envelope, based on infinite blade fatigue life.

Williams, R. C.↗

NASA NTRS1983

Acoustical modal analysis of the pressure field in the tailpipe of a turbofan engine

The results of a modal analysis of the pressure field in the tailpipe of a turbofan engine are presented. Modal amplitudes, at the tailpipe inlet and exit, are presented, as a function of frequency, for several operating conditions. The modal amplitudes were obtained using an optimization routine to obtain a best fit between measured cross spectra and an analytical expression for the cross spectra between pressures at circumferentially spaced locations. The measured pressure field was decomposed into a set of five modal amplitudes corresponding to the (0,0), (1,0), (2,0), (3,0), and (4,0) modes. The analysis was limited to frequencies below 1500 Hz where higher order modes are cutoff. The results of the analysis showed that at low frequencies, up to the cuton frequency of the (1,0) mode, the (0,0) mode (plane wave) dominated the pressure field. The frequency range from the cuton of the (1,0) mode to the cuton of the (2,0) mode was dominated by the (1,0) mode. The (2,0) mode dominated from its cuton frequency to the upper limit of the analysis, i.e., 1500 Hz. The contribution of modes other than the dominant mode was usually small.

Krejsa, E. A.↗

NASA NTRS1983

Effects of wind on turbofan engines in outdoor static test stands

Wind can affect measured thrust and can cause turbofan engine speed to fluctuate during outdoor testing. Techniques used at an outdoor test stand at NASA Lewis Research Center to make testing easier and faster and to improve data repeatability include using an inflow control device (ICD) to make fan speed steadier, taking many raw data samples for better averaging, and correcting thrust for wind direction and speed. Data from engine tests are presented to show that the techniques improve repeatability of thrust and airflow measurements under various wind conditions.

Mcardle, J. G.↗

NASA NTRS1983

Comparison of advanced turboprop and turbofan airplanes

Results of a parametric study to determine the effects of design variables and penalties on the fuel efficiency of Mach 0.8, 125-passenger, advanced turboprop airplanes show that propeller-wing interference penalty has a major effect. Propeller tip speed has a minor effect, and could be decreased to alleviate the noise problem without significant effects on fuel efficiency. The anticipated noise levels produced by the propfan will require additional acoustical treatment for the fuselage; this additional weight can have a significant effect on fuel efficiency. The propfan advantage over an equivalent technology turbofan is strongly dependent on the interference penalty and acoustical treatment weight. Lowering the cruise Mach number to around 0.73 would result in greatly increased fuel efficiency.

Johnson, V. S.↗

NASA NTRS1983

Propulsion installation characteristics for turbofan transports

Several investigations have been conducted in the Langley 8-Foot Transonic Pressure Tunnel and the Langley 16-Foot Transonic Tunnel to investigate propulsion integration characteristics for turbofan transport aircraft. These studies have focused on reducing the interference drag for conventional underwing, overwing, upper surface and wing aft-mounted engine nacelle installations. The experimental data presented in this paper will illustrate the difficulties associated with properly integrating nacelles and pylons with advanced supercritical wings. In addition guidelines for integrating the nacelle/pylons to eliminate adverse interference effects and possibly achieve favorable interference are discussed.

Henderson, W. P.↗

NASA NTRS1983

Fluctuating pressure measurements on the fan blades of a turbofan engine during ground and flight tests

As part of a program to investigate fan noise generated by a turbofan engine, miniature pressure transducers were used to measure the fluctuating pressures on the surface of a fan blade on the JT15D engine. Tests were conducted on the ground, with tunnel and in flight. Using narrowband spectra analysis and signal enhancement techniques, comparisons for these various test conditions are presented. Results on the blade pressures are also presented from a flight test in which 41 rods were placed in front of the fan face to excite a known spinning mode in the duct. Results showed that with care, ground tests can be conducted in which the pressure loading on the blades are similar to those in flight. Comparison of pressure transducers on the blade surface also showed that a single transducer would provide much of the information available from this type of measurement.

Schoenster, J. A.↗

NASA NTRS1983

Wave envelope and infinite element schemes for fan noise radiation from turbofan inlets

Finite element models are presented for the calculation of radiated fan noise in the vicinity of turbofan inlets. The models involve the use of conventional axisymmetric finite elements in an inner region close to the inlet. The far field is represented by infinite elements or wave envelope elements. Theory and results are presented for the case with no mean flow. Comparisons of computed data with analytic solutions and measured values establish the utility of both the infinite element and wave envelope element schemes. The wave envelope scheme is shown to be effective also in the far field. Both schemes use meshes which are much sparser than would be required in conventional numerical discretizations and may be applied to real inlet configurations at modest computational cost.

Astley, R. J.↗

NASA NTRS1983

The influence of inlet design on the aeroacoustic performance of a JT15D turbofan engine as measured in the NASA-Ames 40 x 80 foot wind tunnel

The influence of inlet design on the aeroacoustic performance of a JT15D turbofan engine was investigated in the NASA-Ames 40 x 80 ft wind tunnel. Four inlet designs representative of those found in commercial service were evaluated at a forward velocity of 80 kn. The inlet designs considered were selected to evaluate the importance of circumferential static pressure distortion (SPD) for a diffusing sound suppressing inlet and throat Mach number (Mth) for a hardwall inlet on the acoustic characteristics of the JT15D engine. Several speed points were studied simulating the operating conditions spanning approach to take-off. Results demonstrated that the dependency of the forward radiated noise on inlet aerodynamic design parameters was significant (about 5 dB) for the range of SPD (0.1-3.0%) and Mth (0.3-0.6) tested. The performance of the bulk absorber acoustic treatment used in the diffusing sound suppressing inlet is also discussed.

Holm, R. G.↗

NASA NTRS1983

Further development of a method for computing three-dimensional subsonic viscous flows in turbofan lobe mixers

Procedure for computing subsonic, turbulent flow in turbofan lobe mixers was extended to allow consideration of flow fields in which a swirl component of velocity may be present. Additional, an optional k-lambda turbulence model was added to the procedure. The method of specifying the initial flow field was also modified, allowing parametric specification or radial secondary flow velocities, and making it possible to consider initial flow fields which have significant inlet secondary flow vorticity. A series of example calculations was performed which demonstrate the various capabilities of the modified code. These calculations demonstrate the effects of initial secondary flows of various magnitudes, the effects of swirl, and the effects of turbulence model on the mixing process. The results of these calculations indicate that the initial secondary flows, presumed to be generated within the lobes, play a dominant role in the mixing process, and that the predicted results are relatively insensitive to the turbulence model used.

Lin, S. J.↗

NASA NTRS1984

HYTESS: A hypothetical turbofan engine simplified simulation

A users manual for a hypothetical turbofan engine simplified simulation is presented. This digital simulation exists as FORTRAN source code. The program is self-contained and was developed to offer those interested in engine dynamics and controls research an efficient, realistic, and easily used engine simulation. The engine is modeled using a state space formulation. Matrix elements within the linear state space structure are nonlinear functions of various engine variables.

Merrill, W. C.↗

NASA NTRS1984

Low frequency noise in a quiet, clean, general aviation turbofan engine

A quiet, clean, general aviation, turbofan engine was instrumented to measure the fluctuating pressures in the combustor, turbine exit duct, engine nozzle and the far field. Both a separate flow nozzle and an internal mixer nozzle were tested. The fluctuating pressure data are presented in overall pressure and power levels and in spectral plots. The combustor data are compared to recent theory and found to be in excellent agreement. The results indicate that microphone correction procedures for elevated mean pressures are questionable. Ordinary coherence function analysis suggests the presence of an additional low frequency noise source downstream of the turbine that is due to the turbine itself. Low frequency narrowband data and coherence function analysis are presented.

Huff, R. G.↗

NASA NTRS1984

Study of stator-vane fluctuating pressures in a turbofan engine for static and flight tests

As part of a program to study the fan noise generated from turbofan engines, fluctuating surface pressures induced by fan-rotor wakes were measured on core- and bypass-stator outlet guide vanes of a modified JT15D-1 engine. Tests were conducted with the engine operating on an outdoor test stand and in flight. The amplitudes of pressures measured at fan-rotor blade-passage fundamental frequencies were generally higher and appeared less stable for the static tests than for the flight tests. Fluctuating pressures measured at the blade-passage frequency of the high-speed core compressor were interpreted to be acoustic; however, disturbance trace velocities for either the convected rotor wakes or acoustic pressures were difficult to interpret because of the complex environment.

Mueller, A. W.↗

NASA NTRS1984

Development of dynamic simulation of TF34-GE-100 turbofan engine with post-stall capability

This paper describes the development of a hybrid computer simulation of a TF34-GE-100 turbofan engine with post-stall capability. The simulation operates in real-time and will be used to test and evaluate stall recovery control modes for this engine. The simulation calculations are performed by an analog computer with a peripheral multivariable function generation unit used for computing bivariate functions. Tabular listings of a simulation variables are obtained by interfacing to a digital computer and using a custom software package for data collection and display.

Krosel, S. M.↗

NASA NTRS1983

Effects of wind on turbofan engines in outdoor static test stands

Mcardle, J. G.↗

NASA NTRS1984

A flight study of tone radiation patterns generated by inlet rods in a small turbofan engine

This paper presents a flight study of tone radiation patterns from a small turbofan engine and compares results with similar static test stand data and a recently developed radiation theory. An interaction tone was produced by a circumferential array of inlet rods placed just upstream of the fan blades. Overhead and sideline flight directivity patterns showed cut-on of a dominant single mode occurred where predicted and the absence of any other significant circumferential or radial modes. In general, good agreement was found between measured flight and static data, with small differences being attributed to inlet geometry and/or forward speed effects. Good agreement was also obtained between flight data and theory for directivity pattern shape, however, the theory consistently predicted higher values for peak radiation angle over a wide range of frequency.

Preisser, J. S.↗

NASA NTRS1984

Investigation of mixing in a turbofan exhaust duct. II Computer code application and verification

A three-dimensional analysis of turbofan forced mixer nozzle aerodynamics demonstrates that the complex flow structure is dominated by geometrically induced secondary flow rather than by turbulence. The test apparatus consisted of a fixed upstream model section and a rotating shroud. The Mach number of the fan and core streams at the mixing plane (lobe exit) was 0.45, the bypass ratio was about 4, and the Reynolds number based on the shroud radius was 1,100,000. The three velocity components near the exit plane of the lobes were measured using flow angularity probes to provide information about the mixer inflow conditions for turbulent computations. The validity of a previous computer code was demonstrated in a comparison of the nozzle exit temperature data with the computed temperature distributions. The mechanism most responsible for the generation of secondary flow within the lobes is due to the turning of the fan and core streams in opposite radial directions.

Povinelli, L. A.↗