Jonas P. R. Gustavsson

Hydrofoil Cavitation Under Strong Thermodynamic Effect

Cavitation was studied for a NACA0015 hydrofoil using a material that simulates cryogenic behavior Several angles of attack and flow speeds up to 8.6 m/s were tested. The material used, 2-trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone, hereafter referred to as fluoroketone, exhibits a strong thermodynamic effect even under ambient conditions. Static pressures were measured at seven chordwise locations along the centerline of the hydrofoil suction side and on the test section wall immediately upstream of the hydrofoil. Frequency analysis of the test section static pressure showed that the amplitude of the oscillations increased as the tunnel speed increased. A gradual transition corresponding to the Type II-I sheet cavitation transition observed in water was found to occur near σ/2α =5 with Strouhal numbers based on chord dropping from 0.5 to 0.1 as the cavitation number was reduced. Flash-exposure high-speed imaging showed the cavity covering a larger portion of the chord for a given cavitation number than in cold water The bubbles appeared significantly smaller in the current study and the pressure data showed increasing rather than constant static pressure in the downstream direction in the cavitating region, in line with observations made in literature for other geometries with fluids exhibiting strong thermodynamic effect.

Inlet/Engine Interactions in an Axisymmetric Pulse Detonation Engine System

The effects of oscillatory backpressure on the air induction system for pulse detonation engines were examined for an axisymmetric, external compression cone guration at a freestream Mach number of 2.1. The pressure perturbations at the diffuser exit were produced by a rotating mechanism simulating a valve, which periodically opened and closed the detonation tubes. Theoscillation frequency wasvaried from 30 to 100 Hzfor each individual tube. Through varying the downstream blockage, the spillage was altered, and different mass e ows wereobtained. In all casestheresults indicated that e uctuations in thestatic and stagnation pressures in the inletwere within 3%, without e ow instability noted near the inlet capture.

Acoustic Measurements of High-Temperature Supersonic Impinging Jets in Multiple Configurations

This study investigated the acoustic characteristics of a Mach 1.5 jet at various temperatures impinging on a flat surface in three geometric configurations: 1) normal impingement, 2) oblique impingement, and 3) jet blast deflector impingement. Near-field and far-field microphone measurements were made at fixed locations relative to the nozzle in all three configurations, and in the jet blast deflector configuration, unsteady pressure measurements were obtained on the deck surface near the base of the jet blast deflector. It was found that discrete impingement tones are generated in the normal impingement configuration at small nozzle-plate distances. For oblique and jet blast deflector impingement, while no tones are present at lower jet temperatures, elevated jet temperatures lead to the generation of tones, suggesting that changes to the impinging flow field as a result of higher jet temperature make it more susceptible to resonant behavior. The maximum unsteady loads on the deck surface occur at large impingement distances that are relevant to the jet blast deflector application. These high unsteady loads are speculated to be a result of (i) the redirected flow from the impingement surface, (ii) flow entrained in the main jet shear layer as the jet spreads, and (iii) the grazing wall jet produced by the interaction between the main jet shear layer and the deck surface. Overall, these results provide valuable information on some aeroacoustic properties of impinging jets in configurations useful from an application as well as fundamental perspective.

Temperature Effect on Acoustics of Supersonic Impinging Jet

Supersonic impinging jets have been of interest both from an applications and a fundamental fluid mechanics point of view for several decades. The vertical hot jet used by aircraft capable of vertical landing or take-off during hover is perhaps the most significant application. When the distance between the ground and the aircraft is small, the impinging jet has been shown to produce lift loss, hot gas ingestion and a highly unsteady flow field producing ground erosion and damaging vibrational loads on aircraft, structures and personnel in the vicinity. Previous tests using an ideally-expanded, axisymmetric Mach 1.5 primary jet heated up electrically to a total temperature of up to 500K have been carried out to establish both baseline acoustic features of this flowfield and noise suppression using microjets. Pressure spectra acquired in this setup showed discrete, high-amplitude acoustic tones (generally known as impinging tones) at frequencies varying with jet temperature. These tones were found to grow more pronounced at elevated versus ambient conditions. In these tests, microjets produced substantial suppression of both tones and broadband. Given this success, the scalability of these techniques was to be assessed in a facility providing test conditions closer to those of the relevant applications. The present paper describes baseline experiments carried out in a larger-scale facility where a 36.2 mm diameter jet heated through ethylene combustion to total temperatures of up to 1030K is allowed to impinge on an instrumented ground plane. The test setup is mounted in an anechoic facility where, in addition to validation and extension of the smaller-scale test results, far-field acoustic measurements have been made. The results show that tones are still present in higher temperatures and Powell’s formula still appears to be valid for predicting the frequency of these. The total amplitude of the ground plane pressure fluctuations near the centerline decreases as the temperature is increased while the amplitude remains constant farther away. The spectral distribution of energy shifts away from the impingement tones to higher frequency broadband noise.

Oxygen/Hydrogen-Planar-Laser-Induced Fluorescence Measurements and Accuracy Investigation in High-Pressure Combustion

DOI: 10.2514/1.39013 Inflow species concentration measurements in reacting flows at high pressures, based on nonintrusive methods, have been acquired so far for isolated conditions in a range of experimental devices and by using a variety of methods. Furthermore, extensive assessments of the uncertainties associated with the measurement techniques are lacking. In general, these methods have been based on calibrations determined from assumptions that were not sufficiently quantified to provide a detailed range of the uncertainties associated with these measurements. This work quantifies the uncertainties associated with OH measurement in an oxygen–hydrogen system produced by a shear, coaxial injector typical of those used in rocket engines. Planar OH distributions are obtained for a range of pressures from 10 to 53 bar, providing instantaneous and averaged distribution in a unified study, using the same experimental setup and maintaining the rest of the parameters the same. The uncertainty of 18 different parameters was evaluated and the overall root mean square error was found as 21.9, 22.8, 22.5, and 22.9% at 10, 27, 37, and 53 bar, respectively.

Fluorescence Spectrum of 2-Trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone

A perfluorinated ketone, 2-trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone, has been investigated to determine several physical and spectroscopic properties. It was found to exhibit fluorescence similar to that of acetone, emitting over the 360–550 nm range with a peak near 420 nm when excited at 355 nm. This compounds emission is nearly unaffected over a wide range of temperature and pressure in an argon bath gas. Its fluorescence efficiency was found to be three times higher than that of acetone. Combined with low reactivity and thermal stability up to 500 8C, this makes the material an excellent tracer for spectroscopic measurement techniques.

Implementing Resonant Enhanced Pulsed Micro-Actuators for the Control of Supersonic Impinging Jets

Recent work at the Advanced Aero-Propulsion Laboratory (AAPL) at Florida State University has produced a micro-actuator design utilizing micro scale cavity resonance phenomena for active control of various high-speed flowfields such as supersonic impinging jets and cavity flows. These micro-scale actuators are capable of producing pulsed supersonic microjets over a wide range of design frequencies which can be chosen depending on the frequencies that are relevant to the application. Pulsed microjet control has the potential to produce improved flow and/or noise control as has been shown with steady microjet control, while introducing less mean momentum into the system by actuating at the natural frequency of the system. This study focuses on the design, characterization, and implementation of these actuators into the STOVL impinging jet flowfield. The results of this implementation are compared to the no control case, steady control, and the first generation implementation of pulsed microjets into our STOVL facility. By operating these pulsed microjet actuators at 6.1 kHz, impinging tones were reduced by up to 23 dB, and overall sound pressure levels were reduced by up to 7 dB as compared to the baseline flow. Compared to steady microjet control, pulsed control showed improved reduction of discrete tones of about 5 dB and overall sound pressure levels within 2 dB of those found in the steady case. Additionally, this second generation implementation does not exhibit the new peaks generated by first generation control, and further reduces OASPL by about 5 dB.

Incipient Cavitation Studied Under Strong Thermodynamic Effect

Incipient cavitation was studied under simulated cryogenic conditions on a NACA0015 hydrofoil in a tunnel filled with the perfluorinated ketone 2-trifluoromethyl-1, 1, 1, 2, 4, 4, 5, 5, 5-nonafluoro-3-pentanone. Through pressure measurements on the hydrofoil, laser-illuminated high-speed photography, and flash-illuminated photography, the extent of cavitation and the characteristic frequencies of its oscillation were studied under varying speeds in the range of 1.7-6.7 m/s and several angles of attack. The results presented in this paper are limited to a 5.1-degree angle of attack. It was found that the vapor formation was much stronger in fluoroketone than in cold-water tests at similar cavitation numbers. The formed bubbles were significantly smaller and there existed an extended speed range over which fluctuation amplitudes grew with no well-defined frequency peaks, as was observed in water.

Forced Oscillations in a Mixed-Compression Inlet at Mach 3.5 for Pulse Detonation Engine Systems

The effects of oscillatory backpressure on the air induction system for pulse detonation engines were examined for a two-dimensional, mixed-compression configuration at a freestream Mach number of 3.5. The pressure perturbations at the diffuser exit were produced by injecting air through four ports located at the corners of the exit cross section. The frequency, coupling of the ports and airflow rates through the ports were varied, simulating the operation of detonation tubes. A terminal normal shock in the diffuser oscillated in the excited inlet, causing large pressure fluctuation amplitudes at some locations. Large injection mass flows resulted in inlet flow oscillations throughout the inlet, increased the spillage, yet did not cause inlet unstart.

Experimental Study of Cryogenic Cavitation Using Fluoroketone

Cavitation under simulated cryogenic conditions was studied on a NACA0015 hydrofoil in a watertunnel filled with the perfluorinated ketone 2-trifluoromethyl-1,1,1,2,4,4,5,5,5nonafluoro-3-pentanone. Through pressure measurements on the hydrofoil, flash high-speed photography and laser-induced fluorescence in the fluoroketone medium, the extent of cavitation and the characteristic frequencies of its oscillation were studied under varying speeds and angle of attack. While Stc=0.56 oscillations were found for a wide range of conditions, these were attributed to effects of dissolved gas. For properly degassed fluoroketone, Mode I-II cavitation transition was observed to occur in a generally similar manner to in water. The size of the formed bubbles was found to be significantly smaller than in water tests under similar conditions.