Flow, Turbulence and Combustion最新文献

To investigate the turbulence characteristics over a streamwise-preferential porous substrate, we design a layered porous medium that satisfies the turbulent drag-reducing conditions suggested by the direct numerical simulation (DNS) of Gómez-de Segura and García-Mayoral (J Fluid Mech 875:124–172, 2019). Planar particle image velocimetry (PIV) measurements are carried out for fully developed turbulent flows over substrates made of the layered porous medium. Two (square duct and two-dimensional channel) flow cases are considered. Streamwise-wall-normal plane measurements are performed at the bulk Reynolds numbers (Re_b=)5000–15000 for the square duct flows. The measurement data indicate that with the drag-reducing conditions, which are suggested by the DNS, turbulence over the porous substrate is suppressed to a similar level to that near a solid smooth wall. For further discussion, we then carry out another PIV campaign for channel flows. With streamwise-wall-normal and streamwise-spanwise plane measurements of channel flows at (Re_b=)3000–15000, it is observed that the turbulence level near the porous medium is more significant than that near the solid wall. To investigate why turbulence over the layered porous medium behaves unlike in the DNS, we analyse the data comparing with our previously studied porous medium turbulence. With the spanwise streak distributions and quadrant analyses, their similarity and dissimilarity of turbulence structures are discussed.

For the development of upper stage rocket engines with laser ignition, the transition of oxidizer and fuel from the pure cryogenic liquid streams to an ignitable mixture needs to be better understood. Due to the near vacuum conditions that are present at high altitudes and in space, the injected fuel rapidly atomizes in a so-called flash boiling process. To investigate the behavior of flashing cryogenic jets under the relevant conditions, experiments of liquid nitrogen have been performed at the DLR Lampoldshausen. The experiments are accompanied by a series of computer simulations and here we use a highly resolved LES to identify 3D effects and to better interpret results from the experiments and existing 2D RANS. It is observed that the vapor generation inside the injector and the evolution of the spray in the combustion chamber differ significantly between the two simulation types due to missing 3D effects and the difference in resolution of turbulent structures. Still, the observed 3D spray dynamics suggest a suitable location for laser ignition that could be found in regions of relative low velocity and therefore expected low strain rates. Further, measured droplet velocities are compared to the velocities of notional Lagrangian particles with similar inertia as the measured droplets. Good agreement between experiments and simulations exists and strong correlation between droplet size and velocity can be demonstrated.

In this work, we revisit the application of the compressible linear eddy model for large eddy simulation (CLEM-LES) of calorically perfect gas detonations in an attempt to clarify if the Kolmogorov number can be treated as a constant instead of a tuning parameter when no-slip boundary conditions are included in three-dimensional simulations. In its early development, the CLEM-LES with a one-step combustion chemistry model was used to simulate two-dimensional methane-oxygen detonations to gain insight on the roles and impact of turbulent mixing rates on the presence of unburned pockets of reactive gas and cellular structure. In these past simulations, special treatment of the boundary conditions was not considered, and therefore wave speeds always recovered the Chapman-Jouguet (CJ)-velocity. Moreover, tuning of the Kolmogorov number was required in order to qualitatively capture the experimentally observed flow fields. In this work we carefully perform three-dimensional simulations of detonation propagation using the CLEM-LES, and include no-slip walls as boundary conditions. Also, instead of tuning the Kolmogorov number to obtain the correct cell size, as was done in the past, we instead use a standard value of 1.5. We found that by carefully specifying the boundary conditions, and treating the Kolmogorov as a constant (thus no model calibration), both the expected propagation velocity deficit and cellular structure are recovered. Finally, upon constructing the resulting energy spectrum, we found that the kinetic energy cascade follows the well-known −5/3 power law description of incompressible turbulence in the inertial subrange, but was not symmetric nor isotropic.

Most studies of secondary currents (SCs) over streamwise aligned ridges have been performed for rectangular ridge cross-sections. In this study, secondary currents above triangular ridges are systematically studied using direct numerical simulations of turbulent channel flow. The influence of ridge spacing on flow topology, mean flow, and turbulence statistics is investigated at two friction Reynolds numbers, 550 and 1000. In addition, the effects of ridge width on SCs, which have not previously been considered for this ridge shape, are explored. The influence of SCs on shear stress statistics increases with increased ridge spacing until SCs fill the entire channel. One of the primary findings is that, for ridge configurations with pronounced secondary currents, shear stress statistics exhibit clear Reynolds number sensitivity with a significant growth of dispersive shear stress levels with Reynolds number. In contrast to rectangular ridges, no above-ridge tertiary flows are observed for the tested range of ridge widths. Flow visualisations of SCs reveal the existence of corner vortices that form at the intersection of the lateral ridge sides and the smooth-wall sections. These are found to gradually disappear as ridges increase in width. Premultiplied spectra of streamwise velocity fluctuations show strong dependency on the spanwise sampling location. Whereas spanwise averaged spectra show no strong modifications by SCs, a significant increase of energy levels emerges at higher wavelengths for spectra sampled at the spanwise locations that correspond to the centres of the secondary currents.

The Head-on Quenching (HoQ) of laminar premixed ammonia–hydrogen-air flames under lean to stoichiometric condition is numerical investigated. Detailed chemistry including 34 reactive species and detailed multi-component transport model including thermal diffusion (Soret effect) are applied. The quenching distance is considered as a representative quantity for the HoQ process, and the influence of different system parameters on it has been investigated. These parameters involve fuel/air equivalence ratios, hydrogen content in gas mixture and pressure. It was found that an increase of quenching distance can be caused by a lower hydrogen addition and a leaner mixture condition. Furthermore, it was found that, regardless of the gas mixture, the quenching distance decreases monotonically with increasing pressure, obeying a power function with the exponent (-) 0.7. Moreover, numerical results show a relation between the quenching Peclet number and the dimensionless wall heat flux normalized by the flame power. Additionally, sensitivities of quenching distances with respect to the transport model, considering the heat loss in the wall and the chemical kinetics are studied.

Air intakes are an integral part of contemporary passenger and military aircraft engines. Their impact on aerodynamic performance across the entire flight envelope is critical to aircraft flight safety, efficiency, and manoeuvrability, especially at high Mach numbers due to shock waves. The high demand for reductions in aircraft weight and size and enhancements in durability, comfort, and thermal and radar signatures compel researchers and engineers to explore new designs and develop efficient air intakes for high-performance aircraft engines. Although a number of studies on air intake have been conducted and reported in the open literature, there is little information available in the public domain on bifurcated twin air intakes using synthetic jet. As a result, the primary goal of this research is to use computational fluid dynamics modelling to investigate the effects of synthetic jets on swirl inflow variable geometry twin air intake aerodynamic performance over a range of Reynolds numbers. Some important parameters (distortion coefficient, non-uniformity index, swirl coefficient, and static and total pressure coefficients) were investigated. Both static and total pressure recovery have been increased at all swirl numbers. A significant decrease in distortion coefficient and swirl coefficient has also been achieved, reaching a 53% reduction in the distortion coefficient and a 62% reduction in the swirl coefficient. The reduction in the non-uniformity index is achieved by 62% for the controlled flow case. The findings show that synthetic jets are effective in controlling the flow separation in the twin air intakes and enhancing aerodynamic performance.

This paper presents experimental investigations of the polyurethane foam influence on the combustion dynamics of hydrogen-air flames propagating in a channel with a sudden change in cross-section (i.e. expansion). The channel is open at both ends. Porous media of various lengths and pore size are considered. The porous inserts are placed downstream of the sudden expansion, inside the diagnostic section of dimensions 20 × 40 mm. A Schlieren visualization technique is used to monitor flame shape and propagation dynamics. Various equivalence ratios ranging from 0.3 to 1.0 are tested. The results show that depending on the equivalence ratio, porous length and pore size, the mixture can either propagate throughout the foam or be quenched. In propagating regime, it is found that the output velocity just behind the foam increases linearly with porous matrix length, indicating that the tortuous flow within the foam plays a significant role in the propagation of the flame. These results could be used both to increase the efficiency of gaseous combustion and to ensure the explosion safety of the gas equipment.

The unravelling of multilength-scale insert-generated turbulence, particularly, the induced-forcing plays critical role in the fundamental comprehension of energy formation and decay as a function of grid conformation. This study experimentally investigates the flow mechanical characteristics at Re_Dh = 4.1 × 10⁴ for a regular-grid (RG), single-square-grid (SSG) and six 2D planar space-filling square-fractal-grids (SFG) of different fractal iterations (N), thickness ratios (t_r) and blockage ratios (σ) via piezoelectric thin-film flapping velocimetry (PTFV). Thin-film’s tip-deflection (δ_rms) and voltage response (V_rms) analysis along the grids’ centreline reveals increasing flow fluctuation strength with increasing σ, t_r and decreasing N, owing to higher shedding intensity of lower frequency, larger scale energy-containing vortices from thicker first iteration bar. However, higher: energy dissipation rate, centreline mean velocity decrement rate and local deceleration experienced in the turbulence decay region of larger t_r grid, along with additional fractal scales lead to less potent flow-structure-interplay on thin-film undulation. More importantly, SSG-generated turbulence enables the generation of average (V_rms, δ_rms) and millinewton turbulence forcing F_rms that are respectively, 9× and 5× larger than RG of similar σ, and 2× larger than the best performing N = 3 SFG. Our findings disclose the importance of grid geometrical management for effective utilisation of turbulence-generating grids in engineering applications.

A direct numerical simulation of a three-dimensional diffuser at Reynolds number Re = 10,000 (based on inlet bulk velocity) has been performed using a low-dissipation finite element code. The geometry chosen for this work is the Stanford diffuser, introduced by Cherry et al. (Int. J. Heat Fluid Flow 29:803–811, 2008). Results have been exhaustively compared with the published data with a quite good agreement. Additionally, further turbulent statistics have been provided such as the Reynolds stresses or the turbulent kinetic energy. A proper orthogonal decomposition and a dynamic mode decomposition analyses of the main flow variables have been performed to identify the main characteristics of the large-scale motions. A combined, self-induced movement of the large-scales has been found to originate in the top-right expansion corner with two clear features. A low-frequency diagonal cross-stream travelling wave first reported by Malm et al. (J. Fluid Mech. 699:320–351, 2012), has been clearly identified in the spatial modes of the stream-wise velocity components and the pressure, associated with the narrow band frequency of (St in [0.083,0.01]). This movement is caused by the geometrical expansion of the diffuser in the cross-stream direction. A second low-frequency trait has been identified associated with the persisting secondary flows and acting as a back and forth global accelerating-decelerating motion located on the straight area of the diffuser, with associated frequencies of (St < 0.005). The smallest frequency observed in this work has been (St = 0.0013). This low-frequency observed in the Stanford diffuser points out the need for longer simulations in order to obtain further turbulent statistics.

This paper provides a preliminary study of two different methods to handle the limitations of the Lagrangian point-force approach in the context of unstructured LES solvers. Large deviations in mass, momentum, and energy exchanges between the gas and liquid phases may occur if the assumptions of the point-force approach are not verified. In particular, the point-force approach considers the particles to be subgrid-scale phenomena but the use of more and more refined grids for the carrier flow allowed by today’s computer power leads to cell sizes of the order or smaller than the particle diameters. Several methods are found in the literature to tackle this problem. However, they are usually suited for structured solvers. In the case of unstructured solvers handling several hundred thousand particles in unsteady flows, such methods are far too expensive. In this work, two original methods adapted for spray calculation in unstructured solvers are implemented and compared: the particle-bursting method, and the multigrid method. In this preliminary study, only the correction on the evaporation model is studied, with drag being neglected. Both methods greatly improve the accuracy of the evaporation model but only the multigrid method is independent of the Eulerian mesh refinement. The results presented show that the two methods are relevant to correct the evaporation source terms when the limits of the point-force model are reached. However, the study should be extended to consider the impact of the methods on the drag model calculation and the effects on the interaction with a reactive gaseous phase.