Michelle M. Clem

Investigation of Broadband Shock Noise from a Jet Near a Planar Surface

Many current and future aircraft designs rely on the wing or other aircraft surfaces to shield observers on the ground from the engine noise. However, the available data showing how surfaces interact with a jet to shield and/or enhance the jet noise are currently limited. Therefore, far-field noise data and background-oriented schlieren images were acquired for a round jet, operating in the overexpanded, ideally expanded, and underexpanded supersonic flow regimes, near a planar surface to investigate how airframe surfaces might affect the shock-cell structure in the jet plume and the broadband shock noise produced. These data show that broadband shock noise is produced by the relatively weak shocks far downstream of the nozzle exit; consequently, a surface will be effective at reducing broadband shock noise only if it is long enough to shield the noise produced by shocks. Furthermore, the presence of a surface very near the edge of an underexpanded jet increases the shock-cell spacing, pushing the shock c...

Background Oriented Schlieren Implementation in a Jet-Surface Interaction Test

Many current and future aircraft designs rely on the wing or other aircraft surfaces to shield the engine noise from observers on the ground. However the available data regarding how a planar surface interacts with a jet to shield and/or enhance the jet noise are currently limited. Therefore, the Jet-Surface Interaction Tests supported by NASAs Fundamental Aeronautics Programs Fixed Wing Project were undertaken to supply experimental data covering a wide range of surface geometries and positions interacting with high-speed jet flows in order to support the development of noise prediction methods. Phase 1 of the Test was conducted in the Aero-Acoustic Propulsion Laboratory at NASA Glenn Research Center and consisted of validating noise prediction schemes for a round nozzle interacting with a planar surface. Phased array data and far-field acoustic data were collected for both the shielded and reflected sides of the surface. Phase 1 results showed that the broadband shock noise was greatly reduced by the surface when the jet was operated at the over-expanded condition, however, it was unclear whether this reduction was due a change in the shock cell structure by the surface. In the present study, Background Oriented Schlieren is implemented in Phase 2 of the Jet-Surface Interaction Tests to investigate whether the planar surface interacts with the high-speed jet ow to change the shock cell structure. Background Oriented Schlieren data are acquired for under-expanded, ideally-expanded, and over-expanded ow regimes for multiple axial and radial positions of the surface at three different plate lengths. These data are analyzed with far-field noise measurements to relate the shock cell structure to the broadband shock noise produced by a jet near a surface.

Resonant Interaction of a Rectangular Jet with a Flat-Plate

A resonant interaction between a large aspect ratio rectangular jet and a flat-plate is addressed in this experimental study. The plate is placed parallel to but away from the direct path of the jet. At high subsonic conditions and for certain relative locations of the plate, the resonance accompanied by an audible tone is encountered. The trends of the tone frequency variation exhibit some similarities to, but also marked differences from, corresponding trends of the well-known edgetone phenomenon. Under the resonant condition flow visualization indicates a periodic flapping motion of the jet column. Phase-averaged Mach number data obtained near the plate’s trailing edge illustrate that the jet cross-section goes through large contortions within the period of the tone. Farther downstream a clear ‘axis switching’ takes place. These results suggest that the assumption of two-dimensionality should be viewed with caution in any analysis of the flow.

Noise from a Jet Discharged into a Duct and its Suppression

This study addresses unwanted high intensity noise sometimes encountered in engine test facilities. Model-scale experiments are conducted for a round jet discharged into a cylindrical duct. In most cases, the unwanted noise is found to be due to longitudinal resonance modes of the duct excited by the random turbulence of the jet. When the ‘preferred mode’ frequency of the jet matches a duct resonant frequency there can be a locked-in ‘super-resonance’ accompanied by a high intensity tone or ‘howl’. Various techniques are explored for suppression of the unwanted noise. Tabs placed on the ends of the duct are found ineffective; so are longitudinal fins placed inside the duct. Arod inserted perpendicular to the flow (‘howl stick’) is also found generally ineffective; however, it is effective when there is a super-resonance. By far the most effective suppression is achieved by a wire-mesh screen placed at the end of the duct. The screen not only eliminates the super-resonance but also the duct mode spectral peaks. Apparently the screen works by dampening the velocity fluctuations at the pressure node and thereby weakening the resonant condition.

Background Oriented Schlieren Applied to Study Shock Spacing in a Screeching Circular Jet

Background oriented schlieren (BOS) is a recent development of the schlieren and shadowgraph methods. The BOS technique has the ability to provide visualizations of the density gradient in both the axial and radial directions. The resultant magnitude of the density gradients allows for comparison with shadowgraph images. This paper first compares data obtained by the BOS and shadowgraph techniques at identical conditions in a free jet. The patterns and spacing of the shock trains obtained by the two techniques are found to be consistent with one another. This provides confidence in the shock spacing measurement by the BOS technique. Due to its simpler setup, BOS is then applied to investigate the shock spacing associated with the screech phenomenon, especially during stage jumps. Screech frequencies from a 37.6 mm convergent nozzle, as a function of jet Mach number (M(sub j)), are shown to exhibit various stages. As many as eight stages are identified with the present nozzle over the range 1.0 < M(sub j) <1.7. BOS images are acquired at various screech conditions and the shock spacing is examined as a function of M(sub j).

Improvement in Rayleigh Scattering Measurement Accuracy

(Abstract) Spectroscopic Rayleigh scattering is an established flow diagnostic that has the ability to provide simultaneous velocity, density, and temperature measurements. The Fabry-Perot interferometer or etalon is a commonly employed instrument for resolving the spectrum of molecular Rayleigh scattered light for the purpose of evaluating these flow properties. This paper investigates the use of an acousto-optic frequency shifting device to improve measurement accuracy in Rayleigh scattering experiments at the NASA Glenn Research Center. The frequency shifting device is used as a means of shifting the incident or reference laser frequency by 1100 MHz to avoid overlap of the Rayleigh and reference signal peaks in the interference pattern used to obtain the velocity, density, and temperature measurements, and also to calibrate the free spectral range of the Fabry-Perot etalon. The measurement accuracy improvement is evaluated by comparison of Rayleigh scattering measurements acquired with and without shifting of the reference signal frequency in a 10 mm diameter subsonic nozzle flow.

Variation of shock-spacing during screech stage-jumps

Flow visualization is used to investigate shock-spacings in a supersonic jet during stage-jumps associated with the screech phenomenon. Conventional schlieren and shadowgraphy techniques as well as a new projection focusing schlieren technique are employed. Both time-averaged and instantaneous snapshots of the flow field are analyzed for a 37.6 mm round convergent nozzle over the jet Mach number range of 1.0 < Mj < 1.7. Shock-spacing is studied especially during hysteresis of the stage-jumps where two different screech frequencies could be realized at exactly the same Mj depending on whether the latter variable was increased or decreased. The data follow a monotonic trend without any discontinuity at any Mj. This establishes the fact that the stage-jumps in frequency are not associated with abrupt changes in the shock-spacing.

Mission Performance of High-Power Electromagnetic Thruster Systems

Electromagnetic thrusters such as the Magnetoplasmadynamic (MPD) thruster and Pulsed Inductive Thruster provide the relatively unique capability to process megawatts (MW) of power compactly at specific impulses (Isp) of 2,000 to 10,000 seconds. This capability is well suited to demanding future missions such as cargo and piloted missions to Mars, in which large payload masses or short trip times require MW power levels. These two thrusters have been modeled at both the performance and system mass level, addressing thruster efficiency, Isp, voltage and current, and the mass of thrusters as well as their corresponding heat rejection and power processing subsystems. The resulting data have been assessed for representative Mars exploration missions using detailed low thrust trajectory codes in conjunction with the thruster system models. Analyses indicate that the thruster type and technology levels have less impact on overall mission performance than the total power level. For the 2.5 and 5 MW cases considered, the lower power delivered 50% more payload.Electromagnetic thrusters such as the Magnetoplasmadynamic (MPD) thruster and Pulsed Inductive Thruster provide the relatively unique capability to process megawatts (MW) of power compactly at specific impulses (Isp) of 2,000 to 10,000 seconds. This capability is well suited to demanding future missions such as cargo and piloted missions to Mars, in which large payload masses or short trip times require MW power levels. These two thrusters have been modeled at both the performance and system mass level, addressing thruster efficiency, Isp, voltage and current, and the mass of thrusters as well as their corresponding heat rejection and power processing subsystems. The resulting data have been assessed for representative Mars exploration missions using detailed low thrust trajectory codes in conjunction with the thruster system models. Analyses indicate that the thruster type and technology levels have less impact on overall mission performance than the total power level. For the 2.5 and 5 MW cases consider...