Kevin T. Lowe

Stereoscopic PIV Measurements of Swirl Distortion on a Full-Scale Turbofan Engine Inlet

There is a present need for simulating and measuring the inlet swirl distortion generated by airframe/engine system interactions to identify potential degradation in fan performance and operability in a full-scale, ground testing environment. Efforts are described to address this need by developing and characterizing methods for complex, prescribed distortion patterns. A relevent inlet swirl distortion profile was generated by a novel new method, dubbed the StreamVane method, and measured in a small scale tunnel using stereoscopic particle image velocimetry (PIV) as a precursor for swirl distortion generation and characterization in an operating turbofan research engine. The StreamVane prototype tested was made of ABS plastic using additive manufacturing and was used to generate swirl distortion patterns that mimick boundary layer ingesting engine inlets with a pair of tightly wound vortices and swirl angle magnitudes up to 15°. Diagnostic development efforts for distortion measurements within the research engine paralleled the StreamVane characterization. The system used for research engine PIV measurements is described along with data obtained in the wake of a total pressure distortion screen for engine conditions at idle and 80% corrected fan speed engine power settings. Data reduction algorithms are put forth to reduce spurious velocity vectors and uncertainty estimations specific to the inlet distortion test rig are made. Results indicate that the methods developed may be used to both generate and characterize complex distortion profiles at the aerodynamic interface plane, providing new information about airframe/engine integration.

Experimental Reynolds Stress Spectra in Hot Supersonic Round Jets

The motion and evolution of coherent structures in supersonic jet flows is directly related to the intense noise the flow generates. As a preliminary study to experimentally address these relationships, novel non-intrusive measurements using two-component laser Doppler velocimetry (LDV) have been conducted at exceptionally high data rates to lend insight into the statistical behavior of noise-generating flow structures. A new heated supersonic jet facility has been constructed to provide supersonic flow at total temperatures ratios up to 3. The new LDV used here allows for highly spatially and temporally resolved measurements of velocity in the hot supersonic jet under study. In the present work, the instrumentation is validated via comparison of LDV measurements along the centerline of a screeching cold jet with microphone and high-speed shadowgraph results. LDV results for the local appearance of screech tones and harmonics in the Reynolds stress spectra are presented for an over-expanded case (nozzle pressure ratio of 3.2) of a design Mach number 1.65 nozzle operated cold ( = 1). A preliminary study was then conducted in the nearnozzle shear layer, up to x/d = 4.0, at design nozzle pressure ratio (4.58) and total temperature ratio of 2.0. Results are presented for Reynolds stress time-delay correlations and power spectra at Red = 1.1M for this case. The stream-wise Reynolds normal stress spectra are compared with published spectral behavior reported by other researchers, indicating a similar spectral shape in the downstream stations as previously measured with LDV and hot wire anemometry for cold jets, but which differ in shape from density-based techniques. The results reveal spectral detail of an interesting transition region between the nozzle turbulent boundary layer and the jet shear layer in which flow time scales are exceptionally small and spectral signatures of large scale instabilities are superimposed on the incoming boundary layer turbulence. The results point toward the importance of very high frequency coherent phenomena in this region, particularly evident in the radial and shear stresses time-delay behavior.

Spectral analysis of over-expanded cold jets via 3-component point Doppler velocimetry

Supersonic jet noise is a major health concern in military aviation which may be alleviated with physicsbased control strategies. In order to understand the physics of sound generation from supersonic free jets, highly resolved experimental investigation of the dominant unsteady flow features is necessary. In this study we present a single point Doppler Velocimeter (pDV) based on the Doppler Global Velocimetry principle used for spectral analysis of an over-expanded supersonic cold jet. Using discrete laser beams and high speed photomultiplier tubes allow resolution of the finest scales in the flow. Acousto-optic beam multiplexing enables three component measurement, producing velocity vectors at 100kHz mean rates over extended time windows. This information is used to generate spectral correlations as well as three dimensional mean velocity and turbulence statistics in the core of the supersonic jet (Md = 1.65). Measurements at different nozzle pressure ratios (NPR = 2.2−4.7) and along the streamwise direction (NPR = 3.2) in a cold jet are presented.

The Effect of Particle Lag on Supersonic Turbulent Boundary Layer Statistics

Strong evidence exists that experimental data gathered in high speed flows using particle-based techniques exhibit significant particle lag effects on magnitudes of turbulence quantities. The authors present new data and a linear first-order lag correction scheme based upon Reynolds stress spectral functions which explain discrepancies in stress magnitudes and expected,van Driest-scaled results. It is shown that due to the characteristicsally ‘flat’ spectrum exhibited by the normal-to-wall stress, that term is most susceptible to frequency filtering effects due to particle lag, while the stream-wise stress is affected little, corroborating experimental findings presented by other authors. The proposed correction scheme makes use of scaling relationships from subsonic flows to permit use of spectral data. Mean velocities and Reynolds stresses have been measured for a Mach 2.0 supersonic smooth wall turbulent boundary layer flow using a three-velocity-component laser Doppler velocimeter (LDV). The semi-local scaling for turbulent statics shows better agreement with incompressible flows than the conventional Morkovin’s scaling. Reynolds stresses have been corrected based on the particle lag model and subsonic Reynolds stresses spectra. The particle lag correction affects significantly the magnitude of Reynolds stresses and seems to be the main reason for lower Reynolds stresses reported in previous compressible turbulent boundary layer LDV measurements.

Improvements in laser flare removal for particle image velocimetry using fluorescent dye-doped particles

Laser flare, or scattering of laser light from a surface, can often be a major issue in particle image velocimetry (PIV) involving solid boundaries in the flow or a gas–liquid interface. The use of fluorescent light from dye-doped particles has been demonstrated in water applications, but reproducing the technique in an airflow is more difficult due to particle size constraints and safety concerns. The following work presents fluorescent Kiton Red 620 (KR620)-doped polystyrene latex microspheres as a solution to this issue. The particles are small and narrowly distributed, with a mean diameter of 0.87 and diameter distribution standard deviation of 0.30 . Furthermore, the KR620 dye exhibits much lower toxicity than other common fluorescent dyes, and would be safe to use in large flow facilities. The fluorescent signal from the particles is measured on average to be 320 ± 10 times weaker than the Mie scattering signal from the particles. This reduction in signal is counterbalanced by greatly enhanced contrast via optical rejection of the incident laser wavelength. Fluorescent PIV with these particles is shown to eliminate laser flare near surfaces, allowing for velocity measurements as close as 100 to the surface. In one case, fluorescent PIV led to velocity vector validation rates more than 20 times that of the Mie scattering results in the boundary layer region of an angled surface.

Experimental investigation of the impacts of total temperature non-uniformities on the flow and acoustic fields of a heated supersonic jet

In recent years, the noise produced by tactical aircraft has become a growing concern due to stricter community noise standards and the negative health effects it has on flight support personnel. This study proposes the examination of a new novel noise reduction method involving thermal non-uniformities in heated supersonic jets. Thermal non-uniformities have the advantage of increasing turbulent mixing in jets without the use of additional hardware and can most likely be implemented in afterburning engines with minimal modification. In the course of this study a thermally non-uniform Mach 1.5 heated jet with a centered and offset thermal non-uniformities will be examined. It will be shown that thermal non-uniformities reduce the far-field radiated noise up to ~2dB at peak frequencies and have a measurable impact on the directivity of radiated noise, including a narrowing of the Mach wave emission angle and a non-uniform azimuthal directivity. These changes in the acoustic field are directly related to global changes in the turbulence development observed in the jet. These effects include a shortened potential core, increased shear layer thickness, and a decreased mean flow in regions of peak Reynolds shear stress. These effects are captured by stereoscopic PIV and near and far-field microphone measurements.In recent years, the noise produced by tactical aircraft has become a growing concern due to stricter community noise standards and the negative health effects it has on flight support personnel. This study proposes the examination of a new novel noise reduction method involving thermal non-uniformities in heated supersonic jets. Thermal non-uniformities have the advantage of increasing turbulent mixing in jets without the use of additional hardware and can most likely be implemented in afterburning engines with minimal modification. In the course of this study a thermally non-uniform Mach 1.5 heated jet with a centered and offset thermal non-uniformities will be examined. It will be shown that thermal non-uniformities reduce the far-field radiated noise up to ~2dB at peak frequencies and have a measurable impact on the directivity of radiated noise, including a narrowing of the Mach wave emission angle and a non-uniform azimuthal directivity. These changes in the acoustic field are directly related to gl...