Flow, Turbulence and Combustion最新文献

Three-dimensional carrier-phase direct numerical simulations (CP-DNS) of reacting iron particle dust clouds in a turbulent mixing layer are conducted. The simulation approach considers the Eulerian transport equations for the reacting gas phase and resolves all scales of turbulence, whereas the particle boundary layers are modelled employing the Lagrangian point-particle framework for the dispersed phase. The CP-DNS employs an existing sub-model for iron particle combustion that considers the oxidation of iron to FeO and that accounts for both diffusion- and kinetically-limited combustion. At first, the particle sub-model is validated against experimental results for single iron particle combustion considering various particle diameters and ambient oxygen concentrations. Subsequently, the CP-DNS approach is employed to predict iron particle cloud ignition and combustion in a turbulent mixing layer. The upper stream of the mixing layer is initialised with cold particles in air, while the lower stream consists of hot air flowing in the opposite direction. Simulation results show that turbulent mixing induces heating, ignition and combustion of the iron particles. Significant increases in gas temperature and oxygen consumption occur mainly in regions where clusters of iron particles are formed. Over the course of the oxidation, the particles are subjected to different rate-limiting processes. While initially particle oxidation is kinetically-limited it becomes diffusion-limited for higher particle temperatures and peak particle temperatures are observed near the fully-oxidised particle state. Comparing the present non-volatile iron dust flames to general trends in volatile-containing solid fuel flames, non-vanishing particles at late simulation times and a stronger limiting effect of the local oxygen concentration on particle conversion is found for the present iron dust flames in shear-driven turbulence.

Turbulent fountains are widespread natural phenomena with numerous industrial applications. Extensive research has focused on the temporal evolution and maximum height of these fountains, as well as their dependence on Reynolds and Froude numbers. However, the lower boundary of the spreading flow attained by the mixture of the ejected fluid and the surrounding ambient fluid has received little attention. Here, we focus on the dependence of the lower boundary height on the characteristics of the fountain and demonstrate how to control it. Large Eddy simulations were carried out based on a Navier–Stokes solver which implements fully implicit 3D incompressible finite volume method with second-order accuracy in space and time using curvilinear coordinates, and validated with laboratory experiments. Our results present important implications for technological applications of turbulent fountains, particularly in protecting crops from frost. We discuss the potential of our results to improve the efficiency of such applications.

Gasoline direct injection engines can provide higher thermal efficiency and lower emissions than that for engines using conventional combustion techniques. Compositional stratification inside the combustion chamber opens possibilities for ultra-lean and low-temperature combustion. To explore this further, 2D direct numerical simulation (DNS) has been performed to investigate the propagation of syngas flame in an equivalence ratio (ϕ) stratified medium. Several aspects of flame propagation, such as effect of integral scale of mixing (l_ϕ) on the non-monotonic behavior of flame propagation, contribution of each chemical reaction to heat release rate (HRR), and the effect of differential diffusion were analyzed using DNS-data. A spherically expanding flame has been initiated with a hotspot at the center of the square domain of size 2.4 × 2.4 cm². The variations in the degree of stratification were simulated varying l_ϕ and fluctuations ϕ for initial mixture distribution. Further this DNS-data has been used to analyze effects of stratification on flame displacement speed (S_d) and its components, viz. reaction rate (S_r), normal diffusion (S_n), tangential (S_t), and inhomogeneity (S_z). The results reveal that stratification-induced variations in thermal diffusivity resulted in thermal runaways. These thermal runaways influence the extent of burning for simulated cases. The increase in degree of stratification resulted in flame preferably propagating towards leaner ϕ, causing reduction in components of S_d. The preferential propagation of flame also resulted in shifting of peak reaction rate for fuel species (c^*) to a higher reaction progress variable (c). This shifting of c^*, lead to a reduction in the HRR contribution of reactions that attain their peak near the production zone of H and OH species. For unity Le simulations, S_n was observed to be reduced drastically compared to cases with differential diffusion, resulting in an overall reduction in S_d.

In this work, the aerodynamic performances of a reduced scale vehicle characterized by a fully detached flow on the rear end and measured in a wind tunnel, are investigated in order to check the efficiency of active flow control using pulsed jets, implemented on the rear bumper. Here, the pressure increase on the tailgate by the optimum blowing conditions is confirmed with drag forces reduction, measured using a force balance. This flow control result is obtained using a genetic algorithm technique with a reactive loop. Integral scales of the pressure spectra and characteristics of the vortex structures enable then to propose a flow control model applied to set the amplitude and the frequency of the pulsed jets. The understanding of the pressure increase on the tailgate involves cross correlations with velocity fields on specific cut planes in the wake. Amplitudes of dynamic modes linked to the instantaneous pressure and velocity fields enable to check the most efficient blowing frequencies related to the jet location. The Dynamic Modal Decomposition (DMD) technique is used to get these modes and could be introduced in the optimisation loop in order to improve the energy efficiency of this active flow control system.

The statistical operators typically applied in postprocessing numerical databases for statistically steady turbulence are a mixture of physical averages in homogeneous spatial directions and in time. Alternative averaging operators may involve phase or ensemble averages over different simulations of the same flow. In this paper, we propose straightforward metrics to assess the relative importance of these averages, employing a mixed averaging analysis of the variance. We apply our novel indicators to two statistically steady turbulent flows that are homogeneous in the spanwise direction. In addition, this study highlights the local effectiveness of the averaging operator, which can vary significantly depending on the mean flow velocity and turbulent length scales. The work can be utilized to identify the most effective averaging procedure in flow configurations featuring at least two homogeneous directions. Thus, this will contribute to achieving better statistics for turbulent flow predictions or reducing computing time.

Purpose

A key component of aircraft acoustic installation effects relevant for under-wing turbofan-powered airliners, is studied: jet-flap interaction noise.

Observations

First, noise measurements performed on laboratory jets and on realistic engine exhaust geometries are analyzed to gain understanding both on surface pressure in the jet near-field and on far-field acoustics. The analysis of experimental datasets at various scales underlines intense, advecting, coherent and exponentially-growing pressure signatures in the jet near field and on the wing under-side. The outcome confirms our hypothesis for the main mechanism driving jet-flap interaction noise: coherent organized turbulent structures.

Methods

Relevant physical models are selected and chained together. RANS CFD and stability analysis model the characteristics of jet wavepackets as noise sources, analytical tailored Green’s functions and Boundary Element Method (BEM) predict the diffraction of the wavepackets by the airframe.

Results

For academic configurations where a flat plate models the wing and flap, the wavepacket model is found able to capture noise directivity and trends. The significant impact of a swept trailing edge and the contributions of other plate edges lead us to design, test and simulate a plate with realistic wing plan form. The wavepacket-BEM simulation reproduces jet-surface interaction for the wing plan-form plate, as well as jet-flap interaction on realistic models tested at ONERA CEPRA19 facility during large-scale wind-tunnel tests. Wing-mounted unsteady pressure sensors are utilized as first control points. Then, polar and azimuthal acoustic directivity is examined. Discrepancies between experiments and simulations are identified. Finally an installation geometrical effect is computed: the vertical separation H between nozzle and wing is varied to replicate the tests.

Conclusion

The diffraction of coherent organized turbulent structures generates jet-flap interaction noise in the academic jet laboratory, in large-scale wind-tunnel test and on the full-scale aircraft. We conclude on the potential and the limits of the proposed wavepacket-BEM model to predict the sound field, and we outline the perspectives for future modelling and testing.

This study investigates the influence of forebody configuration on aerodynamic noise generation and radiation in standard squareback vehicles, employing a hybrid computational aeroacoustics approach. Initially, a widely used standard squareback body is employed to establish grid-independent solutions and validate the applied methodology against previously published experimental data. Six distinct configurations are examined, consisting of three bodies with A-pillars and three without A-pillars. Throughout these configurations, the reference area, length, and height remain consistent, while systematic alterations to the forebody are implemented. The findings reveal that changes in the forebody design exert a substantial influence on both the overall aerodynamics and aeroacoustics performance of the vehicle. Notably, bodies without A-pillars exhibit a significant reduction in downforce compared to their A-pillar counterparts. For all configurations, the flow characteristics around the side-view mirror and the side window exhibit an asymmetrical horseshoe vortex with high-intensity pressure fluctuations, primarily within the confines of this vortex and the mirror wake. Side windows on bodies with A-pillars experience more pronounced pressure fluctuations, rendering these configurations distinctly impactful in terms of radiated noise. However, despite forebody-induced variations in pressure fluctuations impacting the side window and side-view mirror, the fundamental structure of the radiated noise remains relatively consistent. The noise pattern transitions from a cardioid-like shape to a monopole-like pattern as the probing distance from the vehicle increases.

In this study, we have investigated the effects of inflow spurious periodicity on a turbulent boundary layer subjected to a weak pressure gradient due to a gently backward ramp utilizing wall-resolved large-eddy simulation. The spurious periodicity is generated through repeating segments of inflow data produced via a long boundary layer precursor simulation. Using a short segment of inflow data and recycling it while introduces a spurious periodicity, can help to reduce the computational cost of the precursor simulation as well as the storage needed to record the massive data. However, there is no quantitative analysis as to how far the results are affected by the spurious periodicity in case of a separated turbulent boundary layer. This study compares several cases with various inflow segment lengths with respect to a reference case with no periodicity. The inflow segments are created by truncating the reference case and thus an intrinsic disturbance is associated with them. An additional case without the disturbance is also considered to examine the latter’s influence on the results. First- and second-order flow statistics are assessed, and spectral analysis is conducted to scrutinize the impact of spurious periodicity. An additional second-order parameter is introduced as an indicator of the spurious periodicity influence. Finally, an especial configuration with active flow control using a synthetic jet actuator is investigated with and without the spurious periodicity. This will help to assess if the dominant frequency of the active flow control is affected by recycling inflow data.

The effects of different vortex breakdown states on the evaporation process characterizing air-acetone vapor swirling jets laden with liquid acetone droplets in the dilute regime are discussed based on results provided by direct numerical simulations. Adopting the point-droplet approximation, the carrier phase is solved using an Eulerian framework, whereas a Lagrangian tracking of the dispersed phase is used. Three test cases are investigated: one with fully-turbulent pipe inflow conditions and two with a laminar Maxworthy velocity profile at different swirl rates. Consequently, turbulent, bubble-type, and regular conical vortex breakdown states are established. Following phenomenological and statistical analyses of both phases, a significant enhancement of the overall droplet evaporation process due to the onset of the conical vortex breakdown is observed due to the strongest centrifugal forces driving the entire liquid drops towards the low-saturation mixing layer of the jet. The effects of droplet inertia on evaporation are isolated through an additional set of simulations where liquid droplets are treated as Lagrangian tracers. While it is found that inertial effects contribute to enhanced vaporization near the mixing layer under bubble vortex breakdown conditions, droplet inertia plays a secondary role under both turbulent and conical vortex breakdown due to intense turbulent mixing and high centrifugal forces, respectively.