Large Eddy Simulation Method Research Articles

Transition-based constrained large-eddy simulation method with application to an ultrahigh-lift low-pressure turbine cascade flow

A transition-predictive Reynolds-averaged Navier–Stokes (RANS) model is introduced as the Reynolds controlling condition to constrained large-eddy simulation (TrCLES) to improve the ability of this method to predict wall-bounded laminar–turbulent transition flows. In the TrCLES method, the constraint conditions for total Reynolds stress and heat flux are only imposed to the near-wall region in transitional and turbulent boundary layer. The newly proposed method recovers direct numerical simulation in the laminar boundary region, and retrieves the traditional large-eddy simulation method in the far-wall regions. The TrCLES method is validated in simulations of external flow around the Eppler 387 (E387) airfoil and internal flow past the ultrahigh-lift low-pressure turbine T106C cascade. The improved delayed detached-eddy simulation (IDDES) method, CLES method based on full-turbulence shear stress transport model and RANS method with a three-equation transition model are also evaluated in comparison with the available experimental and numerical data. As expected, the TrCLES method can predict laminar separation bubble and separation-induced transition process in both the E387 and T106C flows pretty well. In contrast, neither IDDES nor the original CLES can provide reasonable prediction for the laminar separation-induced transition phenomenon. The validity and fidelity of the TrCLES method are further verified by simulations of the T106C cascade flows in a wider range of exit Reynolds and Mach numbers. It is shown that the TrCLES method can not only predict the time-averaged aerodynamic quantities such as isentropic Mach number and exit kinetic energy loss very well, but also capture the laminar separation bubble and unsteady flow structures with satisfactory accuracy.

Relationship Between Skin Scales and the Main Flow Field Around the Shortfin Mako Shark Isurus oxyrinchus.

The aim of this study was to reveal potential relationship between the main flow field around a shortfin mako shark and the surface morphology of shark skin. Firstly, a numerical simulation using the large eddy simulation (LES) method was conducted to obtain the main flow field around a smooth shark model. Then, the surface morphology characteristics of a shark (Isurus oxyrinchus) at different positions were characterized by scanning electron microscope (SEM), which showed that the morphology, riblet size, and density of scales at different positions on the shark were significantly different. At positions where the surfaces face into the water flow direction (i.e., nose and leading edge of fins), the scales were flat and round, with a lower density, and the pressure or wall shear stress (WSS) was greater. Scales with three longitudinal riblets ending in three tips were found on the middle and trailing edges of the first dorsal fin and caudal fin, where water flow states progress from transitional to turbulent. The ranges of the ratio of riblet depth to spacing (RD/RS) in the anterior zone, middle zone and posterior zone of the shark were 0.05–0.17, 0.08–0.23, and 0.32–0.33, respectively. The riblet angle generally followed the flow direction, but it varied across different areas of the body. The turbulence intensity increased gradually across the first dorsal fin, pectoral fin, caudal fin, and the shark body overall. In summary, it was found that the microstructure riblets on the shark skin surface, generally thought to be drag reduction structures, were only located in transitional and turbulent regions at the middle and trailing edge of the shark body and fin surfaces, and there were almost no microstructural grooves in the laminar flow regions along the leading edge. These findings can provide design guidance for engineering applications of bionic riblet surfaces. Riblets placed in transitional and fully turbulent regions can be used to effectively reduce drag. The riblet direction should be consistent with the direction of flow.

Study on the dynamic characteristics of the overflow advection and diffusion of trailing suction hopper dredgers based on CFD

Trailing suction hopper dredgers (TSHDs) are the main type of ship used for dredging projects. However, the release of low concentration slurry poses challenges for the development of environmentally focused dredging engineering. Therefore, to effectively reduce the influence of the overflow released by a TSHD, a numerical simulation study based on a combination of the volume of fluid (VOF) and large eddy simulation (LES) methods is carried out to explore the overflow advection and diffusion process near the hull, and the results are agreed well with the experimental results. The results show that the overflow regimes are different under different Richardson numbers (Ri) and velocity ratios (ζ). Surface plumes that are disastrous for sea wildlife mostly occur under a high ζ or low Ri flow. In addition, the influence of the hull tail structure and the propeller produces additional energy disturbances that aggravate the turbulence in the flow field, and the addition of the release frequency causes pulsation, which could destroy the flow state of the main flow. All these factors cause more surface plumes. Furthermore, the effects of different external conditions on the overflow advection and diffusion are more obvious at low towing speeds.

Comparison of Finite-Volume and Methods for Large-Eddy Simulation of Combustor Port Flow

High-order accurate methods provide significant accuracy/cost benefits for well-resolved and geometrically simple scale-resolving turbulent flow simulations. However, the benefit on under-resolved unstructured grids for complex industrial geometries is unclear. The purpose of this work is to contribute to understanding of the benefit of high-order schemes for large-eddy simulations (LES) in practical applications. A crucial requirement is a high-order solver that supports hybrid unstructured grids. In this context, accuracy and cost of a high-order spectral/ Nektar++ solver and a standard second-order finite-volume OpenFoam solver are systematically compared for performing LES (with subgrid scale treatments) on two configurations. The first configuration is Taylor–Green vortex, which shows that the accuracy benefit of fifth-order LES depends upon the mesh type and the level of under-resolution. The second configuration is geometrically more complex and represents the dilution port flows of gas turbine combustors. Specifically, the flow consists of crossflow radial jets impinging onto each other, providing a rich variety of features and thus making the case challenging. It is found that, for a given cost, fifth-order LES reproduces the unsteady flow features significantly better and offers moderate improvements in the mean quantities. Moreover, results from a finer more costly OpenFoam simulation suggest that, for a similar accuracy, fifth-order LES could be cheaper.

Effects of electrically conductive walls on turbulent magnetohydrodynamic flow in a continuous casting mold

In the present study, we have performed a series of numerical simulations of the turbulent liquid metal flow in a laboratory-scale setup of the continuous casting. The liquid metal flow was subjected to an external non-uniform magnetic field reproducing a realistic electromagnetic brake (EMBr) effect. The focus of this research was on the effects of the finite electrical conductivity of Hartmann walls on the flow and turbulence in the mold. To be able to simulate distributions of the electric potential and current in both the fluid and solid wall domains, we applied our recently developed and validated in-house conjugate MHD solver based on the open-source code OpenFOAM. The dynamic Large Eddy Simulation (LES) method was used to simulate the turbulent flow. The results obtained for the neutral (non-MHD) and MHD cases over a range of the imposed EMBr strengths – all for the perfectly electrically insulated walls – were compared with the available Ultrasound Doppler Velocimetry (UDV) measurements. A good agreement between simulations and experiments was obtained for all simulated cases. Next, we completed a series of simulations including a wide range of the finite electric conductivities (ranging from a weakly to perfectly conducting wall conditions) of the Hartmann walls for a fixed value of the imposed EMBr. The obtained results demonstrated a significant influence of the electric wall conductivities on the flow and turbulence reorganization. It is expected that here provided insights can be applicable for the new generation of the laboratory- and real-scale continuous casting setups.

Numerical study of the aerodynamic effects of bio-inspired leading-edge serrations on a heaving wing at a low Reynolds number

A numerical study with large-eddy simulation (LES) method is performed on the aerodynamic effects of bio-inspired leading-edge serrations on a wing in heaving motion at a low Reynolds number of Re=2×104. During the research, rigorous contrastive simulations are performed for the baseline wing and the serrated ones. The baseline wing has a NACA0012 profile and the serrated ones are obtained by stretching/shrinking the leading portion of the baseline sinusoidally. Conclusive results show that leading-edge serrations can help reduce aerodynamic hysteresis and suppress dynamic-stall-induced load oscillations effectively. More importantly, serrated wings can be more efficient in lift production than the baseline, especially for relatively high reduced frequencies. In the currently covered parameter space, the largest lift efficiency improvement achieved by the leading-edge serrations can reach 15.92%, and the least improvement is 2.73%. Along with these good properties, the leading-edge serrations have induced larger drag, which indicates they are undesirable for propulsive motivations. Visualization of the flow fields reveals that a series of counter-rotating streamwise vortex pairs are dynamically induced by the leading-edge serrations. These streamwise vortices can act as momentum transporters and help enhance momentum exchange between low-energy boundary layer and high-energy outer flows, which can affect the flow structures as well as the aerodynamics of a heaving wing greatly. The current results may provide inspirations for improving aerodynamics, economies and stabilities of bionic underwater/aerial micro robots or any other devices which have heaving/flapping parts.

Numerical analysis on the system instability of the cavitating flow around hydrofoil induced by the non-uniform inflow

One common type of system instabilities of partial cavitation, induced by the non-uniform inflow, was numerically investigated in the present study. Large eddy simulation (LES) method coupled with Zwart-Gerber-Belamri cavitation model and volume of fraction (VOF) are used to capture the unsteady cavitating flow around NACA66. Unlike the intrinsic instabilities caused by the cavity itself, the occurrence of system instabilities is usually accompanied by the intrinsic instabilities. Therefore, it is their nonlinear interaction that makes analyzing the system instabilities separately quite difficult. By comparing the effects of the uniform and non-uniform inflow on the cavitating flow as well as the non-cavitating flow, the influences of the system instability induced by the non-uniform inflow on the partial cavitation are analyzed tentatively. The results show that the non-uniform inflow has little effect on the cavity shape evolution, but makes the dominant frequency caused by the shedding behaviors of the cavities higher. In contrast, the non-uniform inflow significantly changes the mean velocity profile in the cavitation regions near the foil wall especially over the front part of the foil (x/C<0.5). Besides, it also makes the maximum values of the Reynolds shear stress <uv> smaller, and its normal locations closer to the foil wall. By the continuous wavelet analysis, it is observed that the large-scale coherent structures containing relatively high energy are mainly generated during the collapse stage of the cavities. The non-uniform carrier fluid may induce more large-scale coherent structures during this stage. By the discrete wavelet transform, the energy of the non-uniform cavitating flow has a more complex distribution at the expense of the energy for the large and moderate scales.

Comparative analysis of drag reduction characteristics of two-type riblets based on numerical simulation

Turbulent drag reduction has always been a hot issue in fluid mechanics. Among various turbulent drag reduction methods, riblets has attracted wide attention due to its simple structure and convenient application. In order to investigate the effect of the riblets shape on the drag reduction, the effect of the triangular and trapezoidal riblets on the turbulent boundary layer flow field and drag reduction characteristics has been studied numerically by adopting the CFD method and based on the large eddy simulation (LES). The numerical simulation of turbulent channel flow was conducted by RANS, DES and LES methods respectively, and their results were compared with the results of direct numerical simulation (DNS) to verify the accuracy and feasibility of the large eddy simulation method. According to the experimental results in references, the flow field parameters such as integral statistics, first-order statistics and second-order statistics of the triangular and trapezoidal riblets on the turbulent boundary layer flow fields were compared and analyzed. The drag reduction characteristics of two-type riblets were investigated by studying the variation in physical parameters such as drag reduction rate, time-averaged velocity, root mean square velocity fluctuation, shear stress, turbulent kinetic energy generation and contour of spanwise velocity fluctuation, and it was concluded that the trapezoidal riblets have better drag reduction effect than the triangular riblets under the same dimensionless condition.

Overlapping Boundary Layers in Coastal Oceans

Abstract Boundary layer turbulence in coastal regions differs from that in deep ocean because of bottom interactions. In this paper, we focus on the merging of surface and bottom boundary layers in a finite-depth coastal ocean by numerically solving the wave-averaged equations using a large-eddy simulation method. The ocean fluid is driven by combined effects of wind stress, surface wave, and a steady current in the presence of stable vertical stratification. The resulting flow consists of two overlapping boundary layers, i.e., surface and bottom boundary layers, separated by an interior stratification. The overlapping boundary layers evolve through three phases, i.e., a rapid deepening, an oscillatory equilibrium and a prompt merger, separated by two transitions. Before the merger, internal waves are observed in the stratified layer, and they are excited mainly by Langmuir turbulence in the surface boundary layer. These waves induce a clear modulation on the bottom-generated turbulence, facilitating the interaction between the surface and bottom boundary layers. After the merger, the Langmuir circulations originally confined to the surface layer are found to grow in size and extend down to the sea bottom (even though the surface waves do not feel the bottom), reminiscent of the well-organized Langmuir supercells. These full-depth Langmuir circulations promote the vertical mixing and enhance the bottom shear, leading to a significant enhancement of turbulence levels in the vertical column.

Numerical investigation of surface roughness effects on non-equilibrium flow in expansion section of rocket nozzle

To further understand the non-equilibrium flow characteristics in a rocket nozzle, we have to consider the rough wall surface caused by nozzle ablation, and explore the relative influence of surface roughness on the flow field, composition field, and wall heat transfer in the nozzle. In this study, first, the data of the inner wall morphology of the real nozzle is obtained through measurement experiments. The parameters used to characterize the inner wall morphology of the expansion section of the nozzle are determined. Then, a computational physical model is built based on these parameters. Finally, the simplified chemical reaction mechanism is used to solve the chemical non-equilibrium flow in the nozzle through the eddy dissipation conceptual model. The numerical calculation method of large eddy simulation is used to obtain the fine structure of the flow field with the shape of the inner wall of the expansion section of the nozzle. The results of the numerical study show that the influence of the wall roughness element of the nozzle expansion section caused by ablation cannot be ignored during the working process of the solid rocket motor. For the non-equilibrium flow in the supersonic nozzle, the roughness element is a crucial factor to promote the flow transition and the generation of hairpin vortices, and it also affects the wall heat transfer and gas composition near the roughness element.

An implicit large eddy simulation method based on all-speed schemes

It remains a challenge for ILES methods to provide physical consistent viscosity in real applications which comprise complex flow structures in both high and low Mach numbers regimes. For these, we analyze an Implicit Large Eddy Simulation (ILES) method based upon all-speed schemes in a framework of the finite volume method in this study. This method can accurately capture both compressible phenomena such as shock waves as well as small turbulent structures. With a detailed analysis of the numerical viscosity and effective eddy viscosities in physical and spectral spaces, the method is verified to be suitable for the simulation of compressible turbulent flows. After it, the derived ILES method is applied to the simulation of a cylinder flow and a transonic cavity flow. Both results are in good agreements with the previous LES or experimental data. These results indicate that it is promising to study the application of the all-speed scheme to the ILES of complex engineering flow problems.

A coupled MMC-LES and sectional kinetic scheme for soot formation in a turbulent flame

A soot kinetic sectional scheme is coupled with the sparse multiple mapping conditioning / large eddy simulation (MMC-LES) method to model soot formation in a turbulent, non-premixed, piloted methane flame (Delft Flame III). The kinetic sectional scheme used here discretises the population balance equation according to soot particle size. The sections (or bins) are treated as lumped species, and the physico-chemical soot formation is modelled through elementary-like reaction pathways with Arrhenius rate expressions. This allows the soot kinetics to be naturally coupled with the gas phase molecular kinetics. Interactions of soot formation and molecular chemistry with turbulence is achieved through the MMC-LES model. This is a Monte Carlo filtered density function approach in which the micromixing is local in an independent reference space permitting use of a computationally efficient sparse distribution of stochastic notional particles. The model results are compared to experimental data for the gas-phase velocity and molecular species in the upstream low-soot region and with soot statistics in the downstream region. The gaseous quantities have very good agreement with the experimental data. Overall the soot predictions are in good qualitative agreement with the data, particularly with respect to the peak value of the mean soot volume fraction, the shape and range of the joint probability density function of soot volume fraction and spatial location, and the rate of soot oxidation and burnout. However, in common with other published attempts at the Delft Flame III using a range of different models, the predicted soot formation begins and peaks well upstream of the experimental measurements. The minimum soot intermittency is underpredicted relative to the data but shows a significant improvement on previous attempts to model it. The analysis also explores the evolution of the mean and instantaneous particle size distributions and some aspects of soot-chemistry-turbulence interactions.

Fluid-particle interaction regimes during the evolution of turbidity currents from a coupled LES/DEM model

In this paper, fluid-particle interactions in lock-exchange turbidity currents (TCs) over a flat bed are investigated using a model combining LES and DEM (Large-Eddy Simulation and Discrete Element Method). The reliability of this model is demonstrated via comparing the numerical solutions with measurements of the front positions, fluid velocity profile, and particle concentration profile of lock-exchange TCs. The following physical understandings are obtained. The vorticity field plays an important role for the current evolution by affecting the fluid lift force (i.e., in the direction normal to the fluid-particle slip velocity) acting on the particles. At the very beginning, a longitudinal positive lift force due to strong positive vorticity promotes longitudinal particle transport. Afterwards, the longitudinal lift force decreases and eventually becomes negative, with a magnitude that even exceeds that of the positive longitudinal drag force, because more and more of the settling particles are affected by the negative vorticity near the bottom wall caused by surface friction. Interestingly, in spite of the complex behavior of the fluid-particle interaction forces and their role in TC evolution, only a very small fraction of the initial particle gravitational potential energy is actually transformed into TC kinetic energy (both particle and fluid).

Noise reduction of bionic coupling blades of horizontal axis wind turbine

Many bio-inspired researches for aerodynamic noise control of wind turbine have been carried out and obtained effective results. Inspired by the silent flight of owl, in this paper, the airfoil of owl's wing and the asymmetric sawtooth structure of the feather have been extracted and reconstructed on the prototype wind turbine blades. The bionic airfoil blade and bionic coupling blade have been designed separately and compared with prototype blade. To obtain the asymmetric sawtooth structure with better acoustic performance, the sawtooth parameters are selected by orthogonal test. The influence of airfoil and asymmetric sawtooth for acoustic performance of blade has been investigated with large eddy simulation method and Ffowcs Williams-Hawkings analogy method. The results show that the bionic coupling blade has a larger surface pressure difference compared with the prototype blade and thus has better aerodynamic performance. The airfoil-structure coupling method can effectively reduce the aerodynamic noise of the blade without affecting the directivity of the acoustic, and optimized blade can reduce the noise by up to 8.42 dB. The blade coupling airfoil has an inhibitory effect on intermediate and high frequency noise. After adding asymmetric sawtooth trailing edge, the noise in the low frequency and intermediate frequency range is further reduced.

Very-Large-Eddy Simulation of Nonreactive Turbulent Flow for Annular Trapped Vortex Combustor

A hybrid Reynolds-averaged Navier-Stokes (RANS) large eddy simulation (LES) method is applied in this work. It called very-large-eddy simulation (VLES) turbulence closure model. The aim of this present study is firstly to validate the accuracy of this method for a specific engineering application (a trapped vortex combustor) and secondly to describe its flow characteristics. The trapped vortex combustor is a new concept that utilizes a large recirculation vortex to stabilize the flame. An accurate prediction of the turbulent flow is meaningful for the trapped vortex combustor. The time-averaged velocity, root-mean-square (rms) velocity, and flow pattern are compared with the experimental data. And the LES model, RANS BSL k-ω model, and RANS k-ɛ model are also applied for the simulation with different mesh resolutions. The results show that the VLES BSL k-ω model provides improved accuracy for velocity prediction. The classical large vortex structure for the trapped vortex combustor is captured qualitatively by the VLES BSL k-ω model also. In addition, the vortex breakdown and processing vortex cone are visualized using the Q-criterion. Furthermore, the VLES BSL k-ω model is not sensitive to the gird resolution. The VLES method is able to predict the turbulent flow of trapped vortex combustor relatively well‎.

Numerical investigation of convection heat transfer characteristics in sloshing corium pools

The convection flow and heat transfer actuated by internal decay heat and the sloshing of vessels are the primary problems in the study of thermal behaviors in volumetrically heated reactor corium pools under ocean conditions. The wall-modeled large eddy simulation (WMLES) method, volume of fluid model, and phase-change model combined with dynamic mesh technology, were used to develop a three-dimensional flow and heat transfer process simulation model of the sloshing corium pool. In addition, the heat conduction behavior in the crust and vessel wall, as well as the pressure-vessel-wall integrity evaluation was considered. The predicted internal heat transfer characteristics (the temperature distribution, the wall heat flux, and the shell thickness) in the corium pool and the evolution feature of the free surface height near the wall were in good agreement with the LIVE-L4 test results and sloshing experimental data, respectively. Furthermore, the coupled heat transfer phenomenon of the melt and vessel was simulated. The simulation results indicated that the time required to reach stable heat transfer was shortened, the structure of thermal stratification was transformed, and the heat flux along the inner surface of the vessel wall of the shell was improved at the resonance state. The comparisons of local Nusselt (Nu) number at the near-wall between static and sloshing corium pool conditions showed that heat transfer characteristics were significantly influenced by sloshing (T = 2.5 s, A = 0.07 rad) motion, and the Nu number of forced convection fraction had a strong correlation with the local Reynolds number in sloshing corium pools. Conclusively, the transient heat flux distribution of the sloshing melting pool differed from that of the static corium pool, increasing the molten range, and molten peak depth of the pressure-vessel-wall.

Numerical investigation of the flow-field characteristics around the suspended pipeline under oblique flows

Abstract The flow field distribution around the suspended pipeline is closely related to the mechanical characteristics and local scour characteristics of the underwater crossing pipelines. To fully investigate the velocity distribution and wake structure characteristics around the suspended pipeline under oblique flows, a flume model test and a numerical simulation based on the LES (Large Eddy Simulation) method were performed. The results showed that the velocity distribution near the suspended pipeline under oblique flows had obvious three-dimensional characteristics. The influence range of pipeline on velocity is 1 d before to 6 d behind the pipeline (d is pipeline diameter). The smaller the flow angle, the more disordered the velocity distribution is. The larger the flow angle, the greater the variation degree of the velocity at each section at the same section is, as well as the higher the velocity deficit value is. In addition, the larger the angle of pipe flow, the more violent the wake structure changes with the flow velocity, and the stronger the regularity of vortex street shedding. This research provides a theoretical reference for the scour prevention design and safe operation of crossing pipelines.

A numerical study of a lifted hbox {H}_2/hbox {N}_2 flame excited by an axial and flapping forcing

The large eddy simulation method combined with the Eulerian stochastic field approach has been used to study excited lifted hydrogen flames in a stream of hot co-flow in a configuration closely corresponding to the so-called Cabra flame. The excitation is obtained by adding to an inlet velocity profile three types of forcing [(1) axial; (2) flapping; (3) combination of both] with the amplitude of 15% of the fuel jet velocity and the frequency corresponding to the Strouhal numbers St=0.30,, 0.45,, 0.60, ,text {and}, ,0.75. It is shown that such a type of forcing significantly changes the lift-off height (L_h) of the flame and its global shape, resulting in the flame occupying a large volume or the flame, which transforms from the circular one into a quasi-planar one. Both the L_h and size of the flames were found to be a function of the type of forcing and its frequency. The minimum value of L_h has been found for the case when the axial and flapping forcing were combined and acted at the forcing frequency close to the preferred one in the non-excited configuration. The impact of the flapping forcing manifested through a widening of the flame in the flapping direction. It was shown that the excitation increases the level of temperature fluctuations caused by an intensified mixing process. The computational results are validated based on the solutions obtained for the non-excited flame for which experimental data are available.

Numerical analysis of non-excited and excited jets issuing from non-circular nozzles

This paper concentrates on the application of passive and active flow control methods to jet flows. The passive control is obtained by shaping a nozzle cross-section (hexagonal, hexagram, square, triangular) and the active one by an external excitation. The research is performed by applying the large eddy simulation method (LES). The obtained results are thoroughly validated by comparisons with theoretical, experimental and numerical data for non-excited jets. The presented results comply well with the reference data, both in terms of the mean velocity profiles, velocity fluctuations profiles and the Strouhal number of the preferred oscillation frequency (Stp). Based on its value, the excitation frequencies (Stf) are chosen such to cover the range of Stp and its subharmonics. The applied excitation is defined as a sinusoidal function, which enforces the axial velocity oscillations at the level of 10% of the bulk velocity. It is shown that the dynamics of the excited non-circular jets is significantly different from the dynamics of their non-excited counterparts. It is demonstrated that by changing Stf one may modify both the instantaneous as well as their time-averaged behaviour, i.e., the centreline velocity decay, turbulence intensity, directional spreading rate and the entrainment rate. It is found that not all jets are prone to excitations to the same extent. In the far-field, the dependence of the velocity decay rate on Stf is the most pronounced in the jets issuing from circular and hexagonal nozzles. In the near-field, regardless of Stf the excitation causes a faster velocity drop, shortening of the potential core and increase of the centreline fluctuations in all jets. However, in the hexagram jet, the fluctuations turn out to be the least sensitive to change of Stf. Apparently, in this case, the complex nozzle shape determines the velocity oscillations so strongly that a relatively low amplitude excitation has only a small influence. In this respect, the jets issuing from square and triangular nozzles show the opposite, as for them the impact of the excitation is the strongest. Depending on Stf three qualitatively different scenarios of the fluctuations growth are identified by either manifesting their smooth increase or the occurrence of local maxima and minima. The entrainment in the square and triangular jets also turned out to be more dependent on the excitation rather than by the nozzle geometry. In the near-field, the excitation with Stp/2<Stf<Stp leads to almost doubling the entrainment maximum found for the non-excited jets.

Three-Dimensional Numerical Analysis of Longitudinal Thermoacoustic Instability in a Single-Element Rocket Combustor

This study numerically investigated the thermoacoustic combustion instability characteristics of a scaled rocket combustor based on a hybrid of the Reynolds-averaged Navier–Stokes and large–eddy simulation method. The turbulence–combustion interactions were treated using flamelet generated manifold approach. An unstable case was simulated with detailed reaction mechanisms (GRI-Mech 3.0). The obtained results agree well with experiment data from Purdue University, in terms of pressure oscillations frequency and power spectral density spectrum. The combustion instability mode was identified to be coupled with the first longitudinal acoustic mode of the combustion chamber by dynamic model decomposition method. According to Rayleigh index analysis, the unstable driving source was found to be located near the combustor step, which was further confirmed by time-averaged flow fields. Detailed three-dimensional vortex ring shedding evolutions at the combustor step were tracked with fine time resolution. Results indicate that the combustion instability arises from periodic vortex ring shedding at the combustor step and interacting with the chamber wall. The unburnt reactants were rolled up by the shedding vortex ring, which would not break up until impact with the chamber wall. Therefore, the mixing performance was significantly enhanced, leading to sudden heat release. Consequently, the thermal energy is added to the acoustic field, and the first longitudinal mode is thus reinforced, giving rise to large amplitude axial velocity oscillations which prompt the generation of the new vortex ring. The results of the present investigation will support the design and development of high-performance rocket engines.