Flow, Turbulence and Combustion最新文献

The experimental investigation of the effects of scarfed nozzles on jet-installation noise was conducted using unheated subsonic jets in an anechoic jet noise facility. Four different types of scarfed nozzles with increasing nozzle lip angles were examined to study the installation effects at various plate distances away from the jet. Mach numbers ranging from 0.3 to 0.8 were investigated in the experiments. The use of scarfed nozzles is known to result in the deflection of flow away from the centre axis, inducing asymmetry in the jet shears, leading to azimuthal variation in the spectra, and ultimately, noise reduction. This study aims to explore the possibility of reducing jet-installation noise using scarfed nozzles at subsonic flow conditions. The characteristics of jet hydrodynamic pressure fluctuations were investigated in the axial direction using far-field measurements. The near-field flow features were studied using surface pressure transducers installed on the flat plate for the installed configurations. Detailed spectral, coherence, and correlation analyses were carried out to determine the noise reduction mechanisms associated with scarfed nozzles in the proximity of a flat plate. The results of the study showed that the use of scarfed nozzles significantly reduced the jet-installation noise. The reduction was attributed to the generation of an asymmetric flow field induced by the nozzle geometry. The reduction in noise levels was also observed to increase with increasing nozzle lip angle. The detailed analyses revealed that the noise reduction mechanism was associated with a decrease in the acoustic power generated by the jet. Overall, the results suggest that scarfed nozzles can be an effective means of reducing jet-installation noise in subsonic flow conditions.

With the growth of modern turbofan engines, their integration under the wing becomes challenging and induces aerodynamic and acoustic interactions between the jet exhaust and the airframe. Jet noise reduction techniques have been widely studied over the past decades but their efficiency has still to be demonstrated once installed. The present lab-scale jet experiments at Mach 0.6 compare the noise radiated by beveled and rectangular installed nozzles to circular ones on a quarter-sphere radiation map using a microphone antenna. For all radiation angles, modified nozzles show an amplitude decrease of the jet-plate interaction tones of the noise spectra attributed to a strong coupling between the jet shear layers and the sound scattering at the plate trailing edge. Beveled nozzles achieve a noise reduction for all radiation angles with a maximum decrease up to 2 dB at receiver locations perpendicular to the plate. While rectangular nozzles show a similar behavior, a sound increase is observed for listeners parallel to the plate when the height-to-width ratio is small.

The specific heat capacity of supercritical fluids (SCFs) exhibits a sharp variation near the pseudo-critical temperature, resulting in the emergence of a localized region characterized by significantly large specific heat capacity within SCF flows. To comprehensively examine the influence of this prominent local specific heat capacity on turbulence and heat transfer in SCF flows, a series of direct numerical simulations are executed under supercritical pressure conditions, with an inlet bulk Reynolds number of ({Re}_{in}= 2700). Four cases sharing identical geometry yet differing in thermophysical properties are simulated and systematically compared after isolating the specific heat capacity from the other thermophysical factors. The findings reveal that the large local specific heat capacity results in heightened enthalpy fluctuations and fosters the enhancement of turbulent heat transfer. Furthermore, an observed quenching effect attributed to the substantial local specific heat capacity becomes evident within the near-wall region, stemming from fluctuations in thermal diffusivity. Notably, the decomposition of wall heat flux underscores the significant influence of the large local specific heat capacity on the primary turbulent heat flux governing SCF heat convection. The impact exhibits a nuanced complexity, simultaneously manifesting in a simultaneous increase in mean enthalpy gradient and reduction in turbulence.

Among the different passive and active techniques for skin friction drag reduction for turbulent boundary layers, near wall forcing through moving walls is one of the most promising techniques at low Re(_tau). Fewer studies have looked at the mechanism at high Re(_tau), closer to flight conditions, largely because, in this regime, numerical simulations become harder and experiments more challenging. To that end, there is the need of a systematic study for different surface waves and flow conditions. This work introduces a new model using a kagome lattice and an experimental setup which combines simultaneous measurements of surface displacement and velocity in the boundary layer. Here the results from a shortened version of the model at Re(_tau approx) 1000 are presented to demonstrate the capability of the experimental setup which is developed in view of further investigation at higher Reynolds number.

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.

Abstract

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.