Advancements in jet turbulence and noise modeling: accurate one-way solutions and empirical evaluation of the nonlinear forcing of wavepackets

Jet noise reduction is an important goal within both commercial and military aviation. Although large-scale numerical simulations are now able to simultaneously compute turbulent jets and their radiated sound, lost-cost, physically-motivated models are needed to guide noise-reduction efforts. A particularly promising modeling approach centers around certain large-scale coherent structures, called wavepackets, that are observed in jets and their radiated sound. The typical approach to modeling wavepackets is to approximate them as linear modal solutions of the Euler or Navier-Stokes equations linearized about the long-time mean of the turbulent flow field. The near-field wavepackets obtained from these models show compelling agreement with those educed from experimental and simulation data for both subsonic and supersonic jets, but the acoustic radiation is severely under-predicted in the subsonic case. This thesis contributes to two aspects of these models. First, two new solution methods are developed that can be used to efficiently compute wavepackets and their acoustic radiation, reducing the computational cost of the model by more than an order of magnitude. The new techniques are spatial integration methods and constitute a well-posed, convergent alternative to the frequently used parabolized stability equations. Using concepts related to well-posed boundary conditions, the methods are formulated for general hyperbolic equations and thus have potential applications in many fields of physics and engineering. Second, the nonlinear and stochastic forcing of wavepackets is investigated with the goal of identifying and characterizing the missing dynamics responsible for the under-prediction of acoustic radiation by linear wavepacket models for subsonic jets. Specifically, we use ensembles of large-eddy-simulation flow and force data along with two data decomposition techniques to educe the actual nonlinear forcing experienced by wavepackets in a Mach 0.9 turbulent jet. Modes with high energy are extracted using proper orthogonal decomposition, while high gain modes are identified using a novel technique called empirical resolvent-mode decomposition. In contrast to the flow and acoustic fields, the forcing field is characterized by a lack of energetic coherent structures. Furthermore, the structures that do exist are largely uncorrelated with the acoustic field. Instead, the forces that most efficiently excite an acoustic response appear to take the form of random turbulent fluctuations, implying that direct feedback from nonlinear interactions amongst wavepackets is not an essential noise source mechanism. This suggests that the essential ingredients of sound generation in high Reynolds number jets are contained within the linearized Navier-Stokes operator rather than in the nonlinear forcing terms, a conclusion that has important implications for jet noise modeling.

Thermodynamic and kinetic behavior of an argon plasma jet. Decomposition of nitric oxide between 1300 - 1750 degrees K in an argon plasma

In the first part of the study, an RF coupled, atmospheric pressure, laminar plasma jet of argon was investigated for thermodynamic equilibrium and some rate processes.

Improved values of transition probabilities for 17 lines of argon I were developed from known values for 7 lines. The effect of inhomogeneity of the source was pointed out.

The temperatures, T, and the electron densities, n_e , were determined spectroscopically from the population densities of the higher excited states assuming the Saha-Boltzmann relationship to be valid for these states. The axial velocities, v_z, were measured by tracing the paths of particles of boron nitride using a three-dimentional mapping technique. The above quantities varied in the following ranges: 10¹² ˂ n_e ˂ 10¹⁵ particles/cm³, 3500 ˂ T ˂ 11000 °K, and 200 ˂ v_z ˂ 1200 cm/sec.

The absence of excitation equilibrium for the lower excitation population including the ground state under certain conditions of T and n_e was established and the departure from equilibrium was examined quantitatively. The ground state was shown to be highly underpopulated for the decaying plasma.

Rates of recombination between electrons and ions were obtained by solving the steady-state equation of continuity for electrons. The observed rates were consistent with a dissociative-molecular ion mechanism with a steady-state assumption for the molecular ions.

In the second part of the study, decomposition of NO was studied in the plasma at lower temperatures. The mole fractions of NO denoted by x_NO were determined gas-chromatographically and varied between 0.0012 ˂ x_NO ˂ 0.0055. The temperatures were measured pyrometrically and varied between 1300 ˂ T ˂ 1750°K. The observed rates of decomposition were orders of magnitude greater than those obtained by the previous workers under purely thermal reaction conditions. The overall activation energy was about 9 kcal/g mol which was considerably lower than the value under thermal conditions. The effect of excess nitrogen was to reduce the rate of decomposition of NO and to increase the order of the reaction with respect to NO from 1.33 to 1.85. The observed rates were consistent with a chain mechanism in which atomic nitrogen and oxygen act as chain carriers. The increased rates of decomposition and the reduced activation energy in the presence of the plasma could be explained on the basis of the observed large amount of atomic nitrogen which was probably formed as the result of reactions between excited atoms and ions of argon and the molecular nitrogen.

Flame imaging of gas-turbine relight

High-altitude relight inside a lean-direct-injection gas-turbine combustor is investigated experimentally by highspeed imaging. Realistic operating conditions are simulated in a ground-based test facility, with two conditions being studied: one inside and one outside the combustor ignition loop. The motion of hot gases during the early stages of relight is recorded using a high-speed camera. An algorithm is developed to track the flame movement and breakup, revealing important characteristics of the flame development process, including stabilization timescales, spatial trajectories, and typical velocities of hot gas motion. Although the observed patterns of ignition failure are in broad agreement with results from laboratory-scale studies, other aspects of relight behavior are not reproduced in laboratory experiments employing simplified flow geometries and operating conditions. For example, when the spark discharge occurs, the air velocity below the igniter in a real combustor is much less strongly correlated to ignition outcome than laboratory studies would suggest. Nevertheless, later flame development and stabilization are largely controlled by the cold flowfield, implying that the location of the igniter may, in the first instance, be selected based on the combustor cold flow. Copyright © 2010.

Performance of choked unsteady ejector-nozzles for use in pressure-gain combustors

If the conventional steady flow combustor of a gas turbine is replaced with a device which achieves a pressure gain during the combustion process then the thermal efficiency of the cycle is raised. All such 'Pressure Gain Combustors' (e.g. PDEs, pulse combustors or wave rotors) are inherently unsteady flow devices. For such a device to be practically installed in a gas turbine it is necessary to design a downstream row of turbine vanes which will both accept the combustors unsteady exit flow and deliver a flow which the turbine rotor can accept. The design requirements of such a vane are that its exit flow both retains the maximum time-mean stagnation pressure gain (the pressure gain produced by the combustor is not lost) and minimises the amplitude of unsteadiness (reduces unsteadiness entering the downstream rotor). In this paper the exit of the pressure gain combustor is simulated with a cold unsteady jet. The first stage vane is simulated by a one-dimensional choked ejector nozzle with no turning. The time-mean and rms stagnation pressure at nozzle exit is measured. A number of geometric configurations are investigated and it is shown that the optimal geometry both maximizes time mean stagnation pressure gain (75% of that in the exit of the unsteady jet) and minimizes the amplitude of unsteadiness (1/3 of that in the primary jet). The structure of the unsteady flow within the ejector nozzle is determined computationally. Copyright © 2009 by J Heffer and R Miller.

Quantative and qualitive identification of the fatty acids in the mullet (Liza aurata), sever juga (Acipenser stellatus) and Persian sturgeon (A. persicus), considering of cold storing effects on them

At the fishing season, in 2000, samples of species persian sturgeon (A. persicus), Severjuga (A. stellatus) and Mullet (L. aurata), were caught from the southern coasts of Caspian Sea and were freezes and preserved in the cold storage for one year They have also become biometery. The tissue's fillet were identified in order to determined the Fatty Acids. This was done during one year, frequently, fresh, two weeks after freezing and then monthly, respectively. So, after the extraction of lipids from the tissues and methylation, was injected to the gas-liquid Chromatography. After calibration, identified Fatty Acids were compared with standards according to their Retention Times. Peroxid value, lipid content and humidity were controlled. The unsaturated Fatty acids had The most amount, and a plenty of Polyunsaturated Fatty acids (PUFA) were observed, so that linoleic (C18:2), a-linolenic (C18:3), Arashidonic (C20:4), EPA (C20:5) and DHA (C22:6) Fatty acids had high amounts. The w-3, PUFA were more in comparison with w-6. The effects of freezing and cold storing on the fish fatty acids , were evaluated by the statistical tests , like SPSS, Tukey, Homogenous and Anova, and showed that in some species, a group of Fatty acids, specially PUFA, had some variation. The peroxide value that indicates the lipid deterioration, increased during toring. So, the best term if preserving in the cold storage, were determined and their Nutrition value and Medical applications due to their consumption were investigated.

Hardfacing using supersonic laser depostion of stellite-6

The capability of manufacturing coatings is of central importance in engineering design. Many components require nowadays the application of additional layers, to enhance mechanical properties and protect against hostile environments. Supersonic Laser Deposition (SLD) is a novel coating method, based upon Cold Spray (CS) principles. In this technique the deposition velocities can be significantly lower than those required for effective bonding in CS applications. The addition of laser heat energy permits a change in the thermodynamic experience of impacting particles, thereby offering a greater opportunity for metallurgical bonding at lower velocities compared to the CS process technology. The work reported in this paper demonstrates the ability of the SLD process to deliver hard facing materials to engineering surfaces. Stellite-6 has been deposited on low carbon steel tubes over a range of process parameters, determining the appropriate target power and traverse speeds for coating deposition. Coating properties and parameters were examined to determine the main properties, micro-structure and processing cost. Their morphology was studied through optical microscopy, SEM and X-Ray Diffraction. The results have shown that SLD is capable of depositing Stellite-6, with enhanced properties compared to laser clad counterparts.

Effect of laser heating temperature on coating characteristics of Stellite 6 deposited by cold spray

Laser-assisted cold spray (LCS) is a new coating and fabrication process which combines some advantages of CS: solid-state deposition, retain their initial composition and high build rate with the ability to deposit materials which are either difficult or impossible to deposit using cold spray alone. Stellite 6 powder is deposited on medium carbon steels by LCS using N 2 as carrier gas pressure. The topography, cross section thickness, structure of the coatings is examined by SEM, optical microscopy, EDX. The results show that thickness and fluctuation of coating are improved with increased deposition site temperature. Porosity of coating is affected by N 2 and deposition site temperature. In this paper, it presents optimal coating using N 2 at a pressure of 3 MPa and temperature of 450°C and deposition site temperature of 1100°C.

Spray flame study using a model gas turbine swirl burner

A model gas turbine burner was employed to investigate spray flames established under globally lean, continuous, swirling conditions. Two types of fuel were used to generate liquid spray flames: palm biodiesel and Jet-A1. The main swirling air flow was preheated to 350°C prior to mixing with airblast-atomized fuel droplets at atmospheric pressure. The global flame structure of flame and flow field were investigated at the fixed power output of 6 kW. Flame chemiluminescence imaging technique was employed to investigate the flame reaction zones, while particle imaging velocimetry (PIV) was utilized to measure the flow field within the combustor. The flow fields of both flames are almost identical despite some differences in the flame reaction zones. © (2013) Trans Tech Publications, Switzerland.

Exhaust gas ignition (EGI)-a new concept for rapid light-off of automotive exhaust catalyst

Increasing pressure on lowering vehicle exhaust emissions to meet stringent California and Federal 1993/1994 TLEV emission standards of 0.125 gpm NMOG, 3.4 gpm CO and 0.4 gpm NOx and future ULEV emission standards of 0.04 gpm NMOG, 1.7 gpm CO and 0.2 gpm NOx has focused specific attention on the cold start characteristics of the vehicle's emission system, especially the catalytic converter. From test data it is evident that the major portion of the total HC and CO emissions occur within the first two minutes of the driving cycle while the catalyst is heating up to operating temperature. The use of an electrically heated catalyst (EHC) has been proposed to alleviate this problem but the cost and weight penalties are high and the durability has yet to be fully demonstrated (1)*. This paper describes a method of reducing the light-off time of the catalytic converter to less than 20 seconds by means of an afterburner. The system uses exhaust gases from the engine calibrated to run rich and additional air injected into the exhaust gas stream to form a combustible mixture. The key feature concerns the method of making this combustible mixture ignitable within 2 seconds from starting the engine when the exhaust gases arriving at the afterburner are cold and essentially non-reacting. © Copyright 1992 Society of Automotive Engineers, Inc.

Absolute and convective instability in gas turbine fuel injectors

Hydrodynamic instabilities in gas turbine fuel injectors help to mix the fuel and air but can sometimes lock into acoustic oscillations and contribute to thermoacoustic instability. This paper describes a linear stability analysis that predicts the frequencies and strengths of hydrodynamic instabilities and identifies the regions of the flow that cause them. It distinguishes between convective instabilities, which grow in time but are convected away by the flow, and absolute instabilities, which grow in time without being convected away. Convectively unstable flows amplify external perturbations, while absolutely unstable flows also oscillate at intrinsic frequencies. As an input, this analysis requires velocity and density fields, either from a steady but unstable solution to the Navier-Stokes equations, or from time-averaged numerical simulations. In the former case, the analysis is a predictive tool. In the latter case, it is a diagnostic tool. This technique is applied to three flows: a swirling wake at Re = 400, a single stream swirling fuel injector at Re - 106, and a lean premixed gas turbine injector with five swirling streams at Re - 106. Its application to the swirling wake demonstrates that this technique can correctly predict the frequency, growth rate and dominant wavemaker region of the flow. It also shows that the zone of absolute instability found from the spatio-temporal analysis is a good approximation to the wavemaker region, which is found by overlapping the direct and adjoint global modes. This approximation is used in the other two flows because it is difficult to calculate their adjoint global modes. Its application to the single stream fuel injector demonstrates that it can identify the regions of the flow that are responsible for generating the hydrodynamic oscillations seen in LES and experimental data. The frequencies predicted by this technique are within a few percent of the measured frequencies. The technique also explains why these oscillations become weaker when a central jet is injected along the centreline. This is because the absolutely unstable region that causes the oscillations becomes convectively unstable. Its application to the lean premixed gas turbine injector reveals that several regions of the flow are hydrodynamically unstable, each with a different frequency and a different strength. For example, it reveals that the central region of confined swirling flow is strongly absolutely unstable and sets up a precessing vortex core, which is likely to aid mixing throughout the injector. It also reveals that the region between the second and third streams is slightly absolutely unstable at a frequency that is likely to coincide with acoustic modes within the combustion chamber. This technique, coupled with knowledge of the acoustic modes in a combustion chamber, is likely to be a useful design tool for the passive control of mixing and combustion instability. Copyright © 2012 by ASME.

A numerical study into the effects of Bio-synthetic Paraffinic Kerosine blends with Jet-A fuel for civil aircraft engine

Growing concerns regarding fluctuating fuel costs and pollution targets for gas emissions, have led the aviation industry to seek alternative technologies to reduce its dependency on crude oil, and its net emissions. Recently blends of bio-fuel with kerosine, have become an alternative solution as they offer "greener" aircraft and reduce demand on crude oil. Interestingly, this technique is able to be implemented in current aircraft as it does not require any modification to the engine. Therefore, the present study investigates the effect of blends of bio-synthetic paraffinic kerosine with Jet-A in a civil aircraft engine, focusing on its performance and exhaust emissions. Two bio-fuels are considered: Jatropha Bio-synthetic Paraffinic Kerosine (JSPK) and Camelina Bio-synthetic Paraffinic Kerosine (CSPK); there are evaluated as pure fuels, and as 10% and 50% blend with Jet-A. Results obtained show improvement in thrust, fuel flow and SFC as composition of bio-fuel in the blend increases. At design point condition, results on engine emissions show reduction in NO x, and CO, but increases of CO is observed at fixed fuel condition, as the composition of bio-fuel in the mixture increases. Copyright © 2012 by ASME.