Three-dimensional instabilities in compressible flow over open cavities

Direct numerical simulations are performed to investigate the three-dimensional stability of compressible flow over open cavities. A linear stability analysis is conducted to search for three-dimensional global instabilities of the two-dimensional mean flow for cavities that are homogeneous in the spanwise direction. The presence of such instabilities is reported for a range of flow conditions and cavity aspect ratios. For cavities of aspect ratio (length to depth) of 2 and 4, the three-dimensional mode has a spanwise wavelength of approximately one cavity depth and oscillates with a frequency about one order of magnitude lower than two-dimensional Rossiter (flow/acoustics) instabilities. A steady mode of smaller spanwise wavelength is also identified for square cavities. The linear results indicate that the instability is hydrodynamic (rather than acoustic) in nature and arises from a generic centrifugal instability mechanism associated with the mean recirculating vortical flow in the downstream part of the cavity. These three-dimensional instabilities are related to centrifugal instabilities previously reported in flows over backward-facing steps, lid-driven cavity flows and Couette flows. Results from three-dimensional simulations of the nonlinear compressible Navier–Stokes equations are also reported. The formation of oscillating (and, in some cases, steady) spanwise structures is observed inside the cavity. The spanwise wavelength and oscillation frequency of these structures agree with the linear analysis predictions. When present, the shear-layer (Rossiter) oscillations experience a low-frequency modulation that arises from nonlinear interactions with the three-dimensional mode. The results are consistent with observations of low-frequency modulations and spanwise structures in previous experimental and numerical studies on open cavity flows

Study on Compressible Low-Reynolds-Number Flow over a Sphere

Tohoku University野々村拓課

Effects of wall curvature on the dynamics of an impinging jet and resulting heat transfer

The effects of wall curvature on the dynamics of a round subsonic jet impinging on a concave surface are investigated for the first time by direct numerical solution of the compressible Navier-Stokes equations. Impinging jets on curved surfaces are of interest in several applications, such as the impingement cooling of gas turbine blades. The simulation is performed at Reynolds and Mach numbers respectively equal to 3, 300 and 0.8. The impingement wall is kept at a constant temperature, 80 K higher than that of the jet at the inlet. The nozzle-to-plate distance (measured along the jet axis) is set to 5D, with D the nozzle diameter. In order to highlight the curvature effects, the present results are compared to a previous study of jet impinging on a flat plate. The specific influence of wall curvature is investigated through a frequency analysis based on discrete Fourier transform and dynamic mode decomposition. We found that the peak frequencies of the heat transfer also dominate the dynamics of primary vortices in the free jet region and secondary vortices produced by the interaction of primary vortices and the target plate. These frequencies are approximately 30% lower than those found in the reference study of impinging jet on a flat plate. Imperceptible differences were instead found in the time-averaged integral heat transfer

Numerical Characterization and Modeling of Adiabatic Slot Film Cooling

Film cooling is a technique used to protect critical surfaces in combustors, thrust chambers, turbines and nozzles from hot, chemically reacting gases. Accurately predicting the film's performance is especially challenging in the vicinity of the wall and the film injection plane due to the complex interactions of two highly turbulent, shearing, boundary layer flows. Properly characterizing the streams at the inlet of a numerical simulation and the choice of turbulence model are crucial to accurately predicting the decay of the film. To address these issues, this study employs a RANS solver that is used to model a film cooled wall. Menter's baseline model, and shear-stress transport model and the Spalart-Allmaras model are employed to determine the effect on film cooling predictions. Several methods for prescribing the inlet planes are explored. These numerical studies are compared with experimental data obtained in a UMD film cooling wind tunnel

Transition to chaos in a reduced-order model of a shear layer

The present work studies the non-linear dynamics of a shear layer, driven by a body force and confined between parallel walls, a simplified setting to study transitional and turbulent shear layers. It was introduced by Nogueira \& Cavalieri (J. Fluid Mech. 907, A32, 2021), and is here studied using a reduced-order model based on a Galerkin projection of the Navier-Stokes system. By considering a confined shear layer with free-slip boundary conditions on the walls, periodic boundary conditions in streamwise and spanwise directions may be used, simplifying the system and enabling the use of methods of dynamical systems theory. A basis of eight modes is used in the Galerkin projection, representing the mean flow, Kelvin-Helmholtz vortices, rolls, streaks and oblique waves, structures observed in the cited work, and also present in shear layers and jets. A dynamical system is obtained, and its transition to chaos is studied. Increasing Reynolds number $Re$ leads to pitchfork and Hopf bifurcations, and the latter leads to a limit cycle with amplitude modulation of vortices, as in the DNS by Nogueira \& Cavalieri. Further increase of $Re$ leads to the appearance of a chaotic saddle, followed by the emergence of quasi-periodic and chaotic attractors. The chaotic attractors suffer a merging crisis for higher $Re$, leading to chaotic dynamics with amplitude modulation and phase jumps of vortices. This is reminiscent of observations of coherent structures in turbulent jets, suggesting that the model represents dynamics consistent with features of shear layers and jets.Comment: 28 pages, 18 figure

Modelagem de um jato de gás usando simulações das grandes escalas em um código cfd de baixo momento

Orientador: Sávio Souza Venâncio ViannaDissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia QuímicaResumo: A simulação numérica é de grande importância em diversas áreas da engenharia, tais como otimização e manutenção de processo químico, bem como na indústria do petróleo e segurança do processo. O Fire Dynamics Simulator (FDS) é um código de Fluidodinâmica Computacional com base na simulação das grandes escalas. Este foi desenvolvido pelo Instituto Nacional de Padrões e Tecnologia (NIST). O código FDS foi originalmente projetado para modelar baixo fluxo de velocidade comumente encontrados em cenários de incêndio. Assim, o FDS não é adequado para simulação de casos onde o número de Mach é elevado. Para superar esta limitação, este trabalho propõe um novo modelo dedicado às características próximas da saída do jato a fim de permitir o FDS simular cenários de jatos e dispersão de gás. A abordagem também reduz significativamente o tempo da simulação computacional. A ferramenta proposta é uma alternativa livre e confiável para a modelagem de dispersão de gás. Os resultados são amplamente discutidas e um estudo de caso de uma plataforma é apresentado. A comparação com os resultados experimentais, bem como um pacote CFD comercial mostram boa concordânciaAbstract: The numerical simulation is of great importance in various areas of engineering such as optimization and maintenance of chemical process, petroleum industry and process safety. The Fire Dynamics Simulator (FDS) is a Computational Fluid Dynamics (CFD) code based on Large Eddy Simulation (LES) modeling and developed by National Institute of Standards and Technology (NIST). FDS code was originally designed to model low speed flow commonly found in fire scenarios. Hence, FDS is not suitable for modeling high Mach number cases. To overcome this limitation this work proposes a novel model dedicated to the near field jet characteristics in order to enable FDS to simulate jet scenarios and gas dispersion. The approach also reduces the computational time significantly as far as turbulent jet flows are concerned. The proposed tool is a free and reliable alternative for gas dispersion modeling. Results are extensively discussed and case study for a typical offshore site is presented. Comparison with experimental results as well as commercial CFD package show good agreementMestradoSistemas de Processos Quimicos e InformaticaMestre em Engenharia Químic

Large eddy simulation of highly turbulent under-expanded hydrogen and methane jets for gaseous-fuelled internal combustion engines

Burning hydrogen in conventional internal combustion (IC) engines is associated with zero carbon-based tailpipe exhaust emissions. In order to obtain high volumetric efficiency and eliminate abnormal combustion modes such as preignition and backfire, in-cylinder direct injection (DI) of hydrogen is considered preferable for a future generation of hydrogen IC engines. However, hydrogen's low density requires high injection pressures for fast hydrogen penetration and sufficient in-cylinder mixing. Such pressures lead to chocked flow conditions during the injection process which result in the formation of turbulent under-expanded hydrogen jets. In this context, fundamental understanding of the under-expansion process and turbulent mixing just after the nozzle exit is necessary for the successful design of an efficient hydrogen injection system and associated injection strategies. The current study used large eddy simulation (LES) to investigate the characteristics of hydrogen under-expanded jets with different nozzle pressure ratios (NPR), namely 8.5, 10, 30 and 70. A test case of methane injection with NPR = 8.5 was also simulated for direct comparison with the hydrogen jetting under the same NPR. The near-nozzle shock structure, the geometry of the Mach disk and reflected shock angle, as well as the turbulent shear layer were all captured in very good agreement with data available in the literature. Direct comparison between hydrogen and methane fuelling showed that the ratio of the specific heats had a noticeable effect on the near-nozzle shock structure and dimensions of the Mach disk. It was observed that with methane, mixing did not occur before the Mach disk, whereas with hydrogen high levels of momentum exchange and mixing appeared at the boundary of the intercepting shock. This was believed to be the effect of the high turbulence fluctuations at the nozzle exit of the hydrogen jet which triggered Gortler vortices. Generally, the primary mixing was observed to occur after the location of the Mach disk and particularly close to the jet boundaries where large-scale turbulence played a dominant role. It was also found that NPR had significant effect on the mixture's local fuel richness. Finally, it was noted that applying higher injection pressure did not essentially increase the penetration length of the hydrogen jets and that there could be an optimum NPR that would introduce more enhanced mixing whilst delivering sufficient fuel in less time. Such an optimum NPR could be in the region of 100 based on the geometry and observations of the current study