Effect of Swirl on Turbulent Structures in Supersonic Jets

Direct Numerical Simulation (DNS) is used to study the mechanism of generation and evolution of turbulence structures in a temporally evolving supersonic swirling round jet and also to examine the resulting acoustic radiations. Fourier spectral expansions are used in the streamwise and azimuthal directions and a 1-D b-spline Galerkin representation is used in the radial direction. Spectral-like accuracy is achieved using this numerical scheme. Direct numerical simulations, using the b-spline spectral method, are carried out starting from mean flow initial conditions which are perturbed by the most unstable linear stability eigenfunctions. It is observed that the initial helical instability waves evolve into helical vortices which eventually breakdown into smaller scales of turbulence. 'Rib' structures similar to those seen in incompressible mixing layer flow of Rogers and Moserl are observed. The jet core breakdown stage exhibits increased acoustic radiations

Large Scale Turbulent Structures in Supersonic Jets

Jet noise is a major concern in the design of commercial aircraft. Studies by various researchers suggest that aerodynamic noise is a major contributor to jet noise. Some of these studies indicate that most of the aerodynamic jet noise due to turbulent mixing occurs when there is a rapid variation in turbulent structure, i.e. rapidly growing or decaying vortices. The objective of this research was to simulate a compressible round jet to study the non-linear evolution of vortices and the resulting acoustic radiations. In particular, to understand the effect of turbulence structure on the noise. An ideal technique to study this problem is Direct Numerical Simulations(DNS), because it provides precise control on the initial and boundary conditions that lead to the turbulent structures studied. It also provides complete 3-dimensional time dependent data. Since the dynamics of a temporally evolving jet are not greatly different from those, of a spatially evolving jet, a temporal jet problem was solved, using periodicity ill the direction of the jet axis. This enables the application of Fourier spectral methods in the streamwise direction. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. The DNS jet simulation helps us understand the various turbulent scales and mechanisms of turbulence generation in the evolution of a compressible round jet. These accurate flow solutions will be used in future research to estimate near-field acoustic radiation by computing the total outward flux across a surface and determine how it is related to the evolution of the turbulent solutions. Furthermore, these simulations allow us to investigate the sensitivity of acoustic radiations to inlet/boundary conditions, with possible application to active noise suppression. In addition, the data generated can be used to compute various turbulence quantities such as mean velocities, turbulent stresses, etc. which will aid in turbulence modeling. This report will be presented in two chapters. The first chapter describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. The second chapter is an extensive discussion of numerical work using the spectral method which we use to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which can be solved in O(N) steps. This is a modification of a boundary layer code developed by Robert Moser. A very accurate highly resolved Direct Numerical Simulation (DNS) of a turbulent jet flow is produced

Aeroacoustics of Turbulent High-Speed Jets

Aeroacoustic noise generation in a supersonic round jet is studied to understand in particular the effect of turbulence structure on the noise without numerically compromising the turbulence itself. This means that direct numerical simulations (DNS's) are needed. In order to use DNS at high enough Reynolds numbers to get sufficient turbulence structure we have decided to solve the temporal jet problem, using periodicity in the direction of the jet axis. Physically this means that turbulent structures in the jet are repeated in successive downstream cells instead of being gradually modified downstream into a jet plume. Therefore in order to answer some questions about the turbulence we will partially compromise the overall structure of the jet. The first section of chapter 1 describes some work on the linear stability of a supersonic round jet and the implications of this for the jet noise problem. In the second section we present preliminary work done using a TVD numerical scheme on a CM5. This work is only two-dimensional (plane) but shows very interesting results, including weak shock waves. However this is a nonviscous computation and the method resolves the shocks by adding extra numerical dissipation where the gradients are large. One wonders whether the extra dissipation would influence small turbulent structures like small intense vortices. The second chapter is an extensive discussion of preliminary numerical work using the spectral method to solve the compressible Navier-Stokes equations to study turbulent jet flows. The method uses Fourier expansions in the azimuthal and streamwise direction and a 1-D B-spline basis representation in the radial direction. The B-spline basis is locally supported and this ensures block diagonal matrix equations which are solved in O(N) steps. A very accurate highly resolved DNS of a turbulent jet flow is expected

Effect of Swirl on Turbulent Structures in Supersonic Jets

Direct numerical simulation (DNS) is used to study the mechanism of generation and evolution of turbulence structures in a temporally evolving supersonic swirling round jet and also to examine the resulting acoustic radiations. Fourier spectral expansions are used in the streamwise and azimuthal directions and a 1-D b-spline Galerkin representation is used in the radial direction. Spectral-like accuracy is achieved using this numerical scheme. Direct numerical simulations, using the b-spline spectral method, are carried out starting from mean flow initial conditions which are perturbed by the most unstable linear stability eigenfunctions. It is observed that the initial.helical instability waves evolve into helical vortices which eventually breakdown into smaller scales of turbulence. 'Rib' structures similar to those seen in incompressible mixing layer flow of Rogers and Moser are observed. The jet core breakdown stage exhibits increased acoustic radiations

Modern Privacy Threats and Privacy Preservation Techniques in Data Analytics

Today we are living in a digital rich and technology driven world where extremely large amounts of data get generated every hour in the public domain, which also includes personal data. Applications like social media, e-commerce, smartphone apps, etc. collect a lot of personal data which can harm individual privacy if leaked, and hence ethical code of conduct is required to ensure data privacy. Some of the privacy threats include Digital profiling, cyberstalking, recommendation systems, etc. leading to the disclosure of sensitive data and sharing of data without the consent of the data owner. Data Privacy has gained significant importance in the recent times and it is evident from the privacy legislation passed in more than 100 countries. Firms dealing with data-sensitive applications need to abide by the privacy legislation of respective territorial regions. To overcome these privacy challenges by incorporating privacy regulations, we have designed guidelines for application development, incorporating key features of privacy regulations along with the implementation strategies which will help in developing data-sensitive applications which can offer strong and coherent privacy protection of personal data

Effects of low and high temperature plasma nitriding on electrochemical corrosion of steel

This study concerns plasma nitriding of tool steel at different temperatures and its effects on corrosion resistance. Inside the nitriding reactor steel samples were placed on the sample holder after metallographic polishing and then the vacuum chamber was evacuated to a pressure of 0.5 Pa. At a lower temperature of 450 C and the higher temperature of 550 C nitriding was performed for a fixed duration of 10 h. All the nitrided and the bare steel samples studied under X-ray diffraction (XRD) and scanning electron microscopic analysis/energy dispersive X-ray spectroscopy (EDS). Iron nitride (FexN, x = 2–3, 4) peaks were revealed in the nitrided steels after XRD analyses. EDS revealed the increased amount of nitrogen in the nitrided sample treated at 550 C. For the assessment of corrosion resistance of these steel samples potentiodynamic polarization tests were performed in 3.5% NaCl. On comparison, it was found that the steel nitrided at higher temperature is more effective in enhancing the corrosion resistance

Plasma Nitriding of 90CrMoV8 Tool Steel for the Enhancement of Corrosion Resistance

In the present studies, efforts were made to improve corrosion resistance of 90CrMoV8 tool steel by following plasma nitriding. Plasma nitriding of this steel at 500 oC for6and 8 h significantly improved the corrosion resistance when compared to the as-received steel. X-ray diffraction reveals γ′ (Fe, Cr)4 N) and ε ((Fe, Cr) 2–3 N) phases formed after nitriding. Potentiodynamic polarization tests in 3.5% NaCl reveal that plasma nitriding significantly improved the corrosion resistance as compared to untreated steel. The improvement in corrosion resistance may be attributed to the N solid solution and the presence of Fe-nitrides formed in the compound laye

Electrochemical study on the corrosion resistance of surface modified Cr-Mo-V steel by elevated temperature plasma nitriding

This study presents the effect of plasma nitriding on corrosion resistance of low alloy 90CrMoV8 steel. At a selected temperature of 500 °C, nitriding was performed by varying the exposure time. X-ray diffraction of nitrided steel revealed the presence of Fe, γ′ (Fe, Cr)4Nand ε-(Fe, Cr)2–3N) phases. The cross section of the nitrided steel was analyzed by scanning electron microscope. To understand the effect on corrosion resistance, potentiodynamic polarization along with the electrochemical impedance spectroscopy (EIS) was performed in 3.5% NaCl. The corrosion resistance of the nitrided steel was found to be much higher than the as-received steel. It was also found that the steel nitrided for shorter duration of 6 h has better corrosion resistance than that for 8 h. This enhancement of corrosion resistance may be attributed to the presence of nitrogen in solid solution, γ′-(Fe, Cr)4Nand ε-(Fe, Cr)2–3Nphases