K. Todd Lowe

Eddy Convection in Developing Heated Supersonic Jets

An experimental study of the spatial distribution of integral eddy speeds in heated, supersonic free jets is presented. The physics of such jets are of immediate interest to noise-reduction approaches for tactical aircraft jet noise. In these flows, the noise produced has a strong influence from eddy speeds, particularly for radiation in peak noise directions, due to Mach wave contributions. The measurements obtained using a time-resolved Doppler global velocimetry instrument experimentally indicate a key mechanism of the role of heating in jet noise reduction relative to the classical Lighthill U8 scaling. Measurements are presented in jets issuing from an axisymmetric biconic nozzle at two conditions: total temperatures of 1.6 and 2.0 times that of the ambient and both at approximately 106 diameter Reynolds number and pressure-matched exit conditions. For the higher total temperature case, the scaled streamwise eddy convective velocities within the shear layer were measurably reduced compared to the low...

Velocity Statistics and Spectra in Three-Stream Jets

Velocimetry measurements were obtained in three-stream jets at the NASA Glenn Research Center Nozzle Acoustics Test Rig using the time-resolved Doppler global velocimetry technique. These measurements afford exceptional frequency response, to 125 kHz bandwidth, in order to study the detailed dynamics of turbulence in developing shear flows. Mean stream-wise velocity is compared to measurements acquired using particle image velocimetry for validation. Detailed results for convective velocity distributions throughout an axisymmetric plume and the thick side of a plume with an offset third-stream duct are provided. The convective velocity results exhibit that, as expected, the eddy speeds are reduced on the thick side of the plume compared to the axisymmetric case. The results indicate that the time-resolved Doppler global velocimetry method holds promise for obtaining results valuable to the implementation and refinement of jet noise prediction methods being developed for three-stream jets.

A rapid response 64-channel photomultiplier tube camera for high-speed flow velocimetry

In this technical design note, the development of a rapid response photomultiplier tube camera, leveraging field-programmable gate arrays (FPGA) for high-speed flow velocimetry at up to 10 MHz is described. Technically relevant flows, for example, supersonic inlets and exhaust jets, have time scales on the order of microseconds, and their experimental study requires resolution of these timescales for fundamental insight. The inherent rapid response time attributes of a 64-channel photomultiplier array were coupled with two-stage amplifiers on each anode, and were acquired using a FPGA-based system. Application of FPGA allows high data acquisition rates with many channels as well as on-the-fly preprocessing techniques. Results are presented for optical velocimetry in supersonic free jet flows, demonstrating the value of the technique in the chosen application example for determining supersonic shear layer velocity correlation maps.

Synthesis of Fluorophore-Doped Polystyrene Microspheres: Seed Material for Airflow Sensing

Kiton red 620 (KR620) doped polystyrene latex microspheres (PSLs) were synthesized via soap-free emulsion polymerization to be utilized as a relatively nontoxic, fluorescent seed material for airflow characterization experiments. Poly(styrene-co-styrenesulfonate) was used as the PSL matrix to promote KR620 incorporation. Additionally, a bicarbonate buffer and poly(diallyldimethylammonium chloride), polyD, cationic polymer were added to the reaction solution to stabilize the pH and potentially influence the electrostatic interactions between the PSLs and dye molecules. A design of experiments (DOE) approach was used to efficiently investigate the variation of these materials. Using a 4-factor, 2-level response surface design with a center point, a series of experiments were performed to determine the dependence of these factors on particle diameter, diameter size distribution, fluorescent emission intensity, and KR620 retention. Using statistical analysis, the factors and factor interactions that most significantly affect the outputs were identified. These particles enabled velocity measurements to be made much closer to walls and surfaces than previously. Based on these results, KR620-doped PSLs may be utilized to simultaneously measure the velocity and mixing concentration, among other airflow parameters, in complex flows.

Laser velocimetry with fluorescent dye-doped polystyrene microspheres

Simultaneous Mie scattering and laser-induced fluorescence (LIF) signals are obtained from individual polystyrene latex microspheres dispersed in an air flow. Microspheres less than 1 μm mean diameter were doped with two organic fluorescent dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), intended either to provide improved particle-based flow velocimetry in the vicinity of surfaces or to provide scalar flow information (e.g., marking one of two fluid streams). Both dyes exhibit measureable fluorescence signals that are on the order of 10(-3) to 10(-4) times weaker than the simultaneously measured Mie signals. It is determined that at the conditions measured, 95.5% of RhB LIF signals and 32.2% of DCF signals provide valid laser-Doppler velocimetry measurements compared with the Mie scattering validation rate with 6.5 W of 532 nm excitation, while RhB excited with 1.0 W incident laser power still exhibits 95.4% valid velocimetry signals from the LIF channel. The results suggest that the method is applicable to wind tunnel measurements near walls where laser flare can be a limiting factor and monodisperse particles are essential.

Early Development of Time-Resolved Volumetric Doppler Velocimetry for New Insights in Hot Supersonic Jet Noise

The early development of a novel implementation of Doppler-based flow velocimetry, termed the time-resolved volumetric Doppler velocimetry (TRVDV) technique, is discussed for the specific application of kinematics measurements related to flow-generated noise in hot supersonic jets. By combining new detector technologies with unique uses of advanced laser-based photonics, TRVDV promises to produce many hundreds of velocity vectors distributed throughout volumes of the flow at 100kHz repetition rates over long contiguous durations—information needed in real flows to quantitatively evaluate noise sources. Herein, we describe the instrument currently under development and present some single-point velocity statistics and signals from a two-point prototype instrument in a cold supersonic jet. Spectral analysis for the stream-wise normal stress has been done for the single-point data at the center of the jet near the exit plane via slot correlation, with results out to approximatlely 400kHz presented. Initial results for measuring two points spaced along the collection optics axis exhibit the key attribute required for obtaining volumetric results with the system, namely, sufficient scattering intensity with varying depth of focus. Future development of the instrumentation system for many point measurements are discussed in light of current results. It is planned that the mature TRVDV package will be portable such that the system may be shipped to larger facilities for aeroacoustics experiments with results at levels of detail never before obtained.

Characterization of Fluorescent Polystyrene Microspheres for Advanced Flow Diagnostics

** †† ‡‡ §§ *** ††† Fluorescent dye doped polystyrene latex microspheres (PSLs) are being developed for velocimetry and scalar measurements in variable property flows. Two organic dyes, Rhodamine B (RhB) and dichlorofluorescein (DCF), are examined to assess laser-induced fluorescence (LIF) properties for flow imaging applications and single-shot temperature measurements. A major interest in the current research is the application of safe dyes, thus DCF is of particular interest, while RhB is used as a benchmark. Success is demonstrated for single-point laser Doppler velocimetry (LDV) and also imaging fluorescence, excited via a continuous wave 2 W laser beam, for exposures down to 10 ms. In contrast, when exciting with a pulsed Nd:YAG laser at 200mJ/pulse, no fluorescence was detected, even when integrating tens of pulses. We show that this is due to saturation of the LIF signal at relatively low excitation intensities, 4-5 orders of magnitude lower than the pulsed laser intensity. A two-band LIF technique is applied in a heated jet, indicating that the technique effectively removes interfering inputs such as particle diameter variation. Temperature measurement uncertainties are estimated based upon the variance measured for the two-band LIF intensity ratio and the achievable dye temperature sensitivity, indicating that particles developed to date may provide about ±12.5 °C precision, while future improvements in dye temperature sensitivity and signal quality may enable single-shot temperature measurements to sub-degree precision.

Computational Modeling of Total-Temperature Probes

A sensor for high-temperature applications is the thermocouple; however, this probe is characterized by errors due to convection, conduction, and radiation heat transfer to/from the sensing element...

Particle Image Velocimetry Applications Using Fluorescent Dye-Doped Particles

Polystyrene latex sphere particles are widely used to seed flows for velocimetry techniques such as Particle Image Velocimetry (PIV) and Laser Doppler Velocimetry (LDV). These particles may be doped with fluorescent dyes such that signals spectrally shifted from the incident laser wavelength may be detected via Laser Induced Fluorescence (LIF). An attractive application of the LIF signal is achieving velocimetry in the presence of strong interference from laser scatter, opening up new research possibilities very near solid surfaces or at liquid/gas interfaces. Additionally, LIF signals can be used to tag different fluid streams to study mixing. While fluorescence-based PIV has been performed by many researchers for particles dispersed in water flows, the current work is among the first in applying the technique to micron-scale particles dispersed in a gas. A key requirement for such an application is addressing potential health hazards from fluorescent dyes; successful doping of Kiton Red 620 (KR620) has enabled the use of this relatively safe dye for fluorescence PIV for the first time. In this paper, basic applications proving the concept of PIV using the LIF signal from KR620-doped particles are exhibited for a free jet and a two-phase flow apparatus. Results indicate that while the fluorescence PIV techniques produce a signal roughly 3 orders of magnitude weaker than Mie scattering, they provide a viable method for obtaining data in flow regions previously inaccessible via standard PIV. These techniques have the potential to also complement Mie scattering signals, for example in multi-stream and/or multi-phase experiments.

Time Resolved Filtered Rayleigh Scattering for Temperature and Density Measurements

Early development of a spatio-temporally resolved filtered Rayleigh scattering (STRFRS) instrument to measure temperature and density in compressible flows is discussed. This technique relies on new detector technology and FPGA data acquisition which promises many point, high repetition rate measurements. Results from a small cold jet are presented using similar technology as the full STRFRS technique, showing the robust fundamentals of the technique. Expected ratios of filtered-to-unfiltered Rayleigh scattering agree with theoretical predictions within a few percent. Additionally, numerical results are presented for using two iodine cells to maximize temperature and density measurement sensitivity in transonic jet flows. Future plans are to couple this technique with time resolved velocity measurements in flows of interest to the aeroacoustics community to correlate density and temperature fluctuations with noise production.