Revealing the details of vortex core precession in cyclones by means of large-eddy simulation

This paper proposes a large eddy simulation approach for the modeling of combusting flow with spray in realistic gas turbine combustors. A one equation subgrid model is used to model the effect of the unresolved subgrid scales on the resolved large scales. Subgrid combustion is modeled by an extended eddy dissipation model in which the filtered reaction rate is controlled by the turbulent mixing rate between the fine structures and the surrounding fluids. An Eulerian-Lagrangian approach is used to model the two-phase spray flow, and spray particles are tracked by a two-way coupling Lagrangian approach. Then the proposed approach is applied to simulate a combusting spray flow in an industrial annular combustor. The objectives of this study are to demonstrate its capability to investigate the complex flow and combustion dynamics in realistic gas turbine combustors. The predicted instantaneous and time averaged fields of velocity, temperature, pressure, fuel mass fraction are investigated. The precessing vortex core caused by the swirling flow as well as pressure oscillations is examined. The predicted results nicely reproduce the flow, spray and combustion dynamics and successfully capture the main features of the studied combustor, such as the processing vortex core. Finally, the predicted exit temperature and the total pressure loss are compared with experimental data and good agreements are obtained.

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Large-eddy simulation of temporally developing double helical vortices

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.910 ◽

2019 ◽

Vol 863 ◽

pp. 79-113 ◽

Cited By ~ 2

Author(s):

J.-B. Chapelier ◽

B. Wasistho ◽

C. Scalo

Keyword(s):

Growth Rate ◽

Large Eddy Simulation ◽

Vortex Core ◽

Large Scale ◽

Axial Direction ◽

Turbulent Wake ◽

Eddy Simulation ◽

Vortex Reconnection ◽

Helical Vortices ◽

Large Eddy

This paper investigates the transient regime and turbulent wake characteristics of temporally developing double helical vortices via high-fidelity large-eddy simulation (LES) for circulation Reynolds numbers in the range $Re_{\unicode[STIX]{x1D6E4}}=7000{-}70\,000$, vortex-core radii between $r_{c}=0.06R$ and $0.2R$ and helical pitches in the range $h=0.36R{-}0.61R$, where $R$ is the initial helix radius. The present study achieves three objectives: (i) assess the influence of $Re_{\unicode[STIX]{x1D6E4}}$, $r_{c}$ and $h$ on the growth rates of the helical vortex instability driven by mutual inductance; (ii) characterize the type of vortex reconnection events that appear during transition; (iii) study the characteristics of turbulence in the far wake, and in particular quantify the anisotropy in the flow. The initial transient dynamics is conveniently described in terms of the non-dimensional time $t^{\star }=t\unicode[STIX]{x1D6E4}/h^{2}$, yielding the dimensionless growth rate of $\unicode[STIX]{x1D6FC}^{\ast }\sim 20$ and collapsing of all the LES data for a given $r_{c}/h$ ratio. The vortex-core displacement growth rate is found to be Reynolds-number independent, and decreases for larger $r_{c}/h$ ratios. Several vortex reconnection events are identified during the transition, mostly initiated by the leap frogging of helical vortices. This phenomenon causes the entanglement of orthogonal vortex filaments, leading to their separation, followed by the creation of elongated threads in the axial direction. The turbulent wake generated by the breakdown of the helical vortices is found to be highly anisotropic with the axial fluctuations being dominant compared to the radial and azimuthal fluctuations (near one-dimensional turbulence). The study of integral length scales shows the presence of a strong large-scale anisotropy, retaining the memory of the initial helical pitch $h$, in particular for the integral scale in the axial direction. The large-scale anisotropy is propagated through the inertial and dissipative ranges, determined from the computation of the moments of velocity gradients in the three directions.

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Large Eddy Simulation of a Submerged Vortex in a Simplified Computational Model

Volume 3B: Fluid Applications and Systems ◽

10.1115/ajkfluids2019-5205 ◽

2019 ◽

Author(s):

Yoshinobu Yamade ◽

Chisachi Kato ◽

Takahide Nagahara ◽

Jun Matsui

Keyword(s):

Large Eddy Simulation ◽

Computational Model ◽

Vortex Core ◽

Static Pressure ◽

Grid Resolution ◽

Simplified Model ◽

Fine Grid ◽

Eddy Simulation ◽

Large Eddy ◽

Pump Sump

Abstract The flow structures of a submerged vortex that appears in a model pump sump were numerically investigated by performing large eddy simulation (LES) of a model vortex in a simplified computational model with a sufficiently fine grid that could resolve the vortex core. The simplified model is designed to simulate the flow under the bellmouth in a model pump sump. The model pump sump is composed of a 2,500 mm-long water channel of rectangular cross section with a width of 300 mm, a water height of 150 mm and a vertical suction pipe with a diameter of 100 mm installed at its downstream end. Our previous large eddy simulations, which used approximately 2 billion grids and were applied to the model pump sump, have fully clarified the origin and formation mechanism of a submerged vortex. In these computations, however, the static pressure in the vortex core decreased only by as much as 4 kPa at a channel velocity of 0.37 m/s. The decrease in the static pressure was far smaller than the one for which one can expect initiation of cavitation in the vortex core. The static pressure drop was most likely to be underpredicted in our previous LES. Insufficient grid resolution was assumed to be one of the reasons for this underprediction. In the present study, LES with a sufficiently fine grid was applied to the simplified computational model that represents the stretch of a submerged vortex under a constant acceleration of the vertical velocity. Case studies for which the grid resolution was varied between 3.25 and 150 micrometres were performed while the size of the vortex core appeared in the simplified model was 500 micrometres. As a result, we confirmed the grid resolution finer than 15 micrometres is needed to resolve the vortex core with a diameter of 500 micrometres. Vertical and tangential velocities obtained by averaging those distributions of a submerged vortex that was computed in our previous LES were prescribed at the bottom wall of the computational domain as the inlet boundary conditions. In the present LES with the grid resolution finer than 15 micrometres, the static pressure decreased by more than 100 kPa. In addition, the parametric studies where the initial swirl numbers were changed have fully clarified the change in the dynamics of a submerged vortex. We found that a strong submerged vortex appears only at a relatively small range of the swirl-number from 1 to 3.

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