Flow, Turbulence and Combustion最新文献

This study presents novel drag reduction active-flow-control (AFC) strategies for a three-dimensional cylinder immersed in a flow at a Reynolds number based on freestream velocity and cylinder diameter of (Re_D=3900). The cylinder in this subcritical flow regime has been extensively studied in the literature and is considered a classic case of turbulent flow arising from a bluff body. The strategies presented are explored through the use of deep reinforcement learning. The cylinder is equipped with 10 independent zero-net-mass-flux jet pairs, distributed on the top and bottom surfaces, which define the AFC setup. The method is based on the coupling between a computational-fluid-dynamics solver and a multi-agent reinforcement-learning (MARL) framework using the proximal-policy-optimization algorithm. This work introduces a multi-stage training approach to expand the exploration space and enhance drag reduction stabilization. By accelerating training through the exploitation of local invariants with MARL, a drag reduction of approximately (9%) is achieved. The cooperative closed-loop strategy developed by the agents is sophisticated, as it utilizes a wide bandwidth of mass-flow-rate frequencies, which classical control methods are unable to match. Notably, the mass cost efficiency is demonstrated to be two orders of magnitude lower than that of classical control methods reported in the literature. These developments represent a significant advancement in active flow control in turbulent regimes, critical for industrial applications.

A joint study utilising experimental and numerical methods was carried out to investigate the aerodynamic effect of a rudder-mounted slat on a vertical stabiliser. The wind tunnel test results showed that the side force coefficient was increased more than 3% with a negligible increase in drag when the rudder deflection angle was set to δ = 30°. Large eddy simulation (LES) results suggested that the rudder-mounted slat can increase the circulation around the vertical stabiliser, showing that the flow from the upstream recirculating regions was drawn towards the rudder surface. Associated changes in the turbulent flow field, including the mean and turbulent flow field and the vortical structure are also presented to understand the flow control mechanism by the rudder-mounted slat.

Interactions between attractive spheroidal particles are studied in boxes of chaotic flow under the action of a homogeneous and isotropic forcing technique. The fully resolved fluid field and structure-resolved particle–fluid coupling regime are obtained through direct numerical simulation and an immersed boundary method. Agglomeration outcomes are accommodated through attractive van der Waals forces, suitably adapted to consider the orientational dependencies associated with the non-spherical shape. Binary particle interactions are first studied in quiescent conditions, as well as in a periodic box of chaotic fluid flow. The latter is forced using a stochastic method, where the magnitude of the velocity fluctuations and Taylor–Reynolds number are chosen based on those typically seen in nuclear waste processing scenarios. Differences in particle interaction behaviours are presented for the cases of disks and needles, with the role of orientation and kinetic energy in determining interaction outcomes analysed and contrasted with spheres. Results indicate that needles have the highest agglomeration propensity in the chaotic fluid, followed by spheres, and then disks. Lastly, the inclusion of attractive orientationally-dependent interaction forces promotes alignment between the symmetry axes of spheroidal particle pairs, whilst the increased action of the fluid was also seen to promote alignment between the interacting particles when compared to the quiescent case.

Ammonia combustion is gaining interest as a feasible alternative to traditional fossil fuels because of to the low environmental impact and as hydrogen and energy carrier. This study used Computational Fluid Dynamics (CFD) simulations to compare various turbulence models for premixed ammonia/hydrogen combustion in a swirl-stabilized burner. The primary aim was to identify the best turbulence model for accurately predicting the flow dynamics, combustion behaviour, and emissions profiles of ammonia/hydrogen fuel blends. The turbulence models evaluated were Large Eddy Simulation (LES), Realizable k-(epsilon), Renormalization Group (RNG) k-(epsilon), k-(omega) SST, and Reynolds Stress Model (RSM). On the LES side, a further comparison of two subgrid models (Smagorinsky-Lilly and WALE) was investigated. The Flamelet Generated Manifold (FGM) method was utilized with a detailed chemistry scheme taking into consideration all (hbox {NO}_x) reactions. To improve the prediction of (hbox {NO}_x) emissions, additional scalar transport equations for NO and (hbox {NO}_2) were included. This methodology aimed to be a balance between computational efficiency and the accuracy expected of detailed chemistry models. Validation was done with a swirl burner from Cardiff University’s Gas Turbine Research Centre. Results showed that all turbulence models accurately captured flame characteristics in terms of exhaust temperature and axial velocity with minor differences in the recirculation zones, where only the RSM model can predict the velocity trend as the LES simulation while other RANS models differ by at least 7 m/s. The temperature reached by the LES resulted 100 K higher than the other models in the flame zone. LES simulation can predict the emission value with an error of less than 10(%). Moreover, the error related to emissions derived from the RANS simulations was not negligible, underestimating (hbox {NO}_x) emissions by about 35(%). However, RSM model produced results that were closer to those derived from the high-fidelity LES when compared to the others RANS models, particularly in terms of flame thickness and emissions. It was concluded that it is mandatory to perform an unsteady analysis to reach reasonable results.

Quantifying the similarity of velocity vector fields is a critical task across numerous applications within fluid mechanics research, such as computational fluid dynamics validation and quantifying the levels of variability in a flow field. However, this task remains challenging for widely used vector comparison metrics at present. Traditional metrics include the Relevance Index (RI) and Magnitude Similarity Index (MSI) as well as their local versions, Local Structural Index (LSI) and Local Magnitude Index (LMI). These metrics, however, are often sensitive to low-velocity magnitude areas, which can distort the results. To address this, improved metrics like the Weighted Relevance Index (WRI), the Weighted Magnitude Index (WMI), and their amalgamated Combined Magnitude And Relevance Index (CMRI), have been introduced in the literature. Despite having reduced sensitivity to low-velocity areas, CMRI in its original form does not equally consider the significance of WRI and WMI, and introduces a degree of subjectivity. In the present work, we propose two enhanced metrics to address this problem: the modified CMRI for one-by-one flow field comparison, and the ensemble CMRI for comparing collections of vector fields. We compare their properties to the previously developed CMRI and spatially averaged CMRI, and investigate their usage in an applied example for quantifying cyclic variations in a flow from a combustion engine cylinder. The newly proposed metrics were found to more robustly isolate the effects of discrepant vector magnitudes and directions, leading to improved diagnostics of in-cylinder flow fields. In particular, the modified CMRI, which ensures equal treatment of WMI and WRI, can serve as a baseline for flow field comparison, providing a more objective target for quantifying flow similarity.

The work constructs the Gas kinetic solver (GKUA) to solve the Boltzmann model equation. Then the solver is respectively confirmed by NS, DSMC and experiments in typical conditions during reentry. Furthermore, the Maxwellian gas-surface interaction model is utilized to study the effects of reflected gas molecules state ((alpha_{e})) on flow field and aerodynamic properties at various extent of gas rarefaction. Results reveal the temperature is more susceptible to the state of reflected gas molecules compared with pressure. And the larger gas rarefaction tends to weaken the effects. As for surface heat flux, it just increases with (alpha_{e}) in lower gas rarefaction, while it behaves as the opposite trend with larger gas rarefaction. Freestream condition (H = 50km,Ma = 8.0,AOA = 60^{o}) is set for booster model in practical application. We experience the shrinks of aerodynamic pitch moment coefficient with more (alpha_{e}). These results are valuable for the construction of expired spacecraft forecasting platform which integrates exterior ballistics with aerothermodynamic computations to obtain tracks of spacecraft fragments in advance.

Experimental studies in industrial-relevant geometries are of great value for validating computational fluid dynamics (CFD). This study provides such data using Magnetic Resonance Velocimetry (MRV) in a replica of the single-phase and isothermal OECD/NEA-KAERI rod bundle benchmark exercise based on the MATiS-H test facility at the Korea Atomic Energy Research Institute (KAERI). The geometry is a 5 × 5 nuclear fuel assembly model of a pressurized water reactor with a split-type mixing grid inducing a swirling flow in each sub-channel. The Reynolds number based on the hydraulic diameter is 50,250. Recent studies demonstrated that MRV enables a comprehensive validation of CFD results in industrial-relevant test cases by providing time-averaged, three-dimensional measurement data from complex opaque structures. Nevertheless, there was still some potential left to improve the accuracy of the measurement. This study uses a newly developed MRV method to accurately measure the mean velocity vectors and the Reynolds stress tensor in three dimensions. The measurement volume reaches from shortly upstream to 10 times the hydraulic diameter downstream of the mixing grid. The estimated mean measurement uncertainty of the velocity data is 1.9% based on the bulk velocity of 1.72 m/s. In the case of the Reynolds stress data, the estimated mean uncertainty for each component is between 0.7 and 1.8% based on the square of the bulk velocity. The comparison to previously published Laser Doppler velocimetry measurements confirms the high accuracy of the reported 3D MRV data. The study includes a detailed description of the technique and boundary conditions. The measurement data is available to interested parties upon request.

Direct numerical simulations (DNS) have been utilised to investigate the impact of different thermal wall boundary conditions on premixed V-flames interacting with walls in a turbulent channel flow configuration. Two boundary conditions are considered: isothermal walls, where the wall temperature is set either equal to the unburned mixture temperature or an elevated temperature, and adiabatic walls. An increase in wall temperature has been found to decrease the minimum flame quenching distance and increase the maximum wall heat flux magnitude. The analysis reveals notable differences in mean behaviours of the progress variable and non-dimensional temperature in response to thermal boundary conditions. At the upstream of the flame–wall interaction location, higher mean friction velocity values are observed for the case with elevated wall temperature compared to the other cases. However, during flame–wall interaction, friction velocity values decrease for isothermal walls but initially rise before decreasing for adiabatic walls, persisting at levels surpassing isothermal conditions. For all thermal wall boundary conditions, the mean scalar dissipation rates of the progress variable and non-dimensional temperature exhibit a decreasing trend towards the wall. Notably, in the case of isothermal wall boundary condition, a higher scalar dissipation rate for the non-dimensional temperature is observed in comparison to the scalar dissipation rate for the progress variable. Thermal boundary condition also has a significant impact on Reynolds stress components, turbulent kinetic energy, and dissipation rates, showing the highest magnitudes with isothermal case with elevated wall temperature and the lowest magnitude for the isothermal wall with unburned gas temperature. The findings of the current analysis suggest that thermal boundary conditions can potentially significantly affect trubulence closures in the context of Reynolds averaged Navier–Stokes simulations of premixed flame–wall interaction.

The statistical behaviours of wall heat flux and wall shear stress and their interdependence during unsteady head-on quenching of statistically planar turbulent premixed flames within turbulent boundary layers due to heat loss through the cold wall have been analysed using three-dimensional Direct Numerical Simulation data with friction Reynolds numbers of (Re_tau =110) and 180. In both cases, the mean wall shear stress decreases during flame-wall interaction, whereas the mean wall heat flux magnitude increases with time as the flame approaches the wall and eventually assumes a maximum value before decreasing with the progress of flame quenching. The integral length scales of wall heat flux in both streamwise and spanwise directions have been found to grow with time after the maximum mean heat flux magnitude is obtained for the two (Re_tau) cases considered. However, the integral length scale of wall shear stress in the streamwise direction grows but the integral length scale of wall shear stress in the spanwise direction decreases with time after the maximum mean heat flux magnitude is reached. Moreover, the correlation coefficient between the wall heat flux magnitude and wall shear stress becomes increasingly negative while the mean wall heat flux increases with time, but this negative correlation weakens with the progress of flame quenching. The first few (i.e., most energetic) Proper Orthogonal Decomposition (POD) modes of wall shear stress and the wall heat flux magnitude have been found to capture the qualitative nature of the correlation between these quantities and their spatial variations. It is found that tens of most energetic POD modes are needed to capture the mean and variances of wall heat flux and wall shear stress. The number of most energetic modes, which contribute significantly to the statistics of both wall heat flux and wall shear stress, decreases with decreasing (Re_tau) and also with the progress of flame quenching due to the weakening of turbulence effects.