Aerodynamic and Acoustic Optimization for Multi-element Airfoils

With the development of computational power, large eddy simulation (LES) method is increasingly used in simulating complex flow. However, there still exist many factors affecting the LES quality and appropriate mesh resolution is among one of them. This work aims to develop an automatic procedure to refine the LES mesh by combining adaptive mesh refinement (AMR) and LES quality criteria. An LES refinement criterion is developed by estimating the proper grid length scale which meets the accuracy requirement of LES method. With this criterion, the baseline mesh is automatically refined with the AMR method. In this work, an efficient one-shot refinement strategy is also proposed to reduce the overall simulation time. Current AMR-based LES method is verified with the typical LES test case about the flow past circular cylinder at ReD = 3900. Results show that the automatically refined mesh provides systematically better agreement with experimental results and with current method the balance between accuracy and computational expense for LES can be obtained.

The present paper describes a study that is aimed at establishing and quantifying the benefits of the Large Eddy Simulation (LES) method for predicting scalar turbulent transport in a combustor relevant jet-mixing problem. A non-reacting co-annular jet mixing configuration is considered for which comprehensive experimental data for both velocity and scalar fields have recently been obtained. Detailed comparisons are presented for the development of the axial velocity field in terms of both mean and turbulence intensity. Similarly, the mixing between the jets is examined by comparison with measurements for the mean concentration and the variance of concentration fluctuations. Agreement with these statistically averaged fields is demonstrated to be very good, and a considerable improvement over the standard eddy viscosity RANS approach. Illustrations are presented of the time-resolved information that LES provides such as time histories, and also conserved scalar pdf predictions. The LES results are shown, even using a simple Smagorinsky sub-grid-scale model, to predict correctly lower values of the turbulent Prandtl number (∼ 0.6) in the free shear regions of the flow, as well as higher values (∼ 1.0) in the wall-affected regions. The ability to predict turbulent Prandtl number variations (rather than input these as commonly done in most combustor RANS CFD models) is an important and promising feature of the LES approach for combustor simulation since it is known to be important in determining combustor exit temperature traverse.

Due to the limited length of cylinders, their use in practical engineering inevitably involves end effects, which results in three-dimensional flows at the ends of cylinders. These flow fields are the main factor influencing the aerodynamic and flow field characteristics of cylinders. Regarding a finite-length cylinder rotor, a specific pattern of tip vortices will form under the action of rotation, resulting in notable differences in the aerodynamic characteristics between an ideal two-dimensional cylinder and a finite-length cylindrical rotor. In the present study, the large eddy simulation method is used to systematically investigate cylinder rotors with various aspect ratios. By analyzing the sectional aerodynamic and flow field characteristics, the variation laws of the aerodynamic force, wind pressure, and flow field characteristics of cylinder rotors under the influence of end effects were summarized. The results show that the influence range and intensity of the tip vortices on the rotor flow field and sectional aerodynamic characteristics are dominated by the dimensionless rotating speed, which in turn affects the range of the end effects. The development trend of the tip vortices is analyzed and discussed from multiple aspects, including sectional aerodynamics, the pressure coefficient, and the flow correlation, and an attempt is made to explain changes in the phenomenon.

Natural convection in a volumetrically heated fluid is of significant importance related to the In-Vessel Retention (IVR) issue. The COPRA facility is designed to simulate the lower plenum of reactor vessel at 1:1 scale for the Chinese advanced PWR, which is a two-dimensional 1/4 circular slice structure with an inner radius of 2.2 m. Water was used as simulant in COPRA-L1 tests. Due to the full scale geometry, The Rayleigh numbers within the corium pools could reach to 1016, matching those in the prototypical situation during severe accident. Due to the high Rayleigh numbers and high turbulence in the melt pool, we choose the Large-Eddy Simulation (LES) method to do the numerical calculations. The LES method can better capture the heat transfer characteristic in a high-Rayleigh number volumetrically heated liquid pool. The numerical results of pool temperature and heat flux distribution were compared with those from COPRA-L1 tests.

The explicit filtering method for large eddy simulations of a turbulent premixed flame

Since 1960s, from experimental observation, there are several stages for liquid droplets in multi-phase combustion: pure heating, pure evaporation without individual droplet combustion, and individual droplet combustion (burning) with individual flame around which enhances evaporation. As for individual burning droplets, they have envelope flame or wake flame regimes. From experimental and theoretical research, in liquid fuel combustion chambers, according to the space between the droplets, there are four typical droplet group combustion modes. Generally, the external group combustion mode and the internal group combustion mode are the two main modes, and there are individual burning droplets in multiphase combustion. Up to now, based on the literature information, no matter in Reynolds Averaged Navier Stokes (RANS) method, Large Eddy Simulation (LES) method or Point Direct Numeral Simulation (PDNS) method, droplets in multi-phase combustion are treated as pure evaporation points, corresponding to the bright blue flame condition, i.e., small droplets in hot gas mixture with the pure evaporation stage, so in species equation and energy equation, the droplet points are mass source and heat sink. As for the individual droplets burning mode, in point source simulation method, corresponding to the big droplet in cold gas phase mixture with yellow trail flame condition, the source terms in the species equation and energy equation should be the mass sink and heat source. The individual burning droplets affects the NO formation because the flame around the droplet has higher temperature than the local gas phase mixture mostly for the quasi-stoichiometric ratio reaction in individual droplet flame. In this paper, a new spray combustion model with the consideration of individual droplets combustion mode is brought forward. The existing modeling approach is extended with an ignition criterion and a droplet combustion correlation. The ignition criterion, which judges the droplet stage between pure evaporation and burning, and the droplet combustion correlation, which integrates the micro-scales droplet burning effects into point source term frame, are both selected from pioneer’s experimental work and added to the present existing widely used multi-phase combustion model. Then a jet spray flame is tested by the new model with RANS and LES method. The prediction results are compared with the literature experimental results. New model gives better simulation results in downstream regions. According to the results, this model added the individual droplet combustion mode in multi-phase combustion model, while its combustion correlations are quite rough at present. More verify studies should be carried in the future.

Large eddy simulations of mixing effects on the course of precipitation process

The speed increase in high-speed trains is a critical procedure in the promotion of high-speed railway technology. As an indispensable and complex structure of high-speed trains, the pantograph’s aerodynamic drag and noise is a significant limitation in the speed increase process of high-speed trains. In the present study, the hybrid method of large eddy simulation (LES) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is applied to analyze the aerodynamic and aeroacoustic performances of pantograph installed in different ways, i.e., sinking platform and fairing. The results of simulation show that the application of pantograph fairing can reduce the aerodynamic drag greatly. In addition, compared with the pantographs installed alone on the train roof, the installation of the sinking platform brings about 2 dBA reduction in sound pressure level (SPL). Meanwhile, the utilization of the pantograph fairing mainly decreases the noise in the frequency band above 1000 Hz and the largest SPL reduction is up to 3 dBA among the monitoring points. Further analysis shows that the influence of different diversion strategies on the spectral characteristics actually attenuates the dominant frequency of the panhead. In the horizontal plane, the noise directivity of the pantograph installed with a fairing is similar to the pantograph installed alone on the train roof.

Computational analysis of locally forced flow over a wall-mounted hump at high- Re number

Convergence of cognitive and adaptive decline in chronic schizophrenia

The critical review discusses the most accurate methods for description of turbulent flows: the computationally very expensive direct numerical simulation (DNS) and slightly less accurate and slightly less expensive large eddy simulation (LES) methods. Both methods have found their way into nuclear thermal hydraulics as tools for studies of the fundamental mechanisms of turbulence and turbulent heat transfer. In the first section of this critical review, both methods are briefly introduced in parallel with the basic properties of the turbulent flows. The focus is on the DNS method, the so-called quasi-DNS approach, and the coarsest turbulence modeling approach discussed in this work, which is still on the very small-scale, wall-resolved LES. Other, coarser turbulence modeling approaches (such as wall-modeled LES, Reynolds Averaged Navier-Stokes (RANS)/LES hybrids, or RANS) are beyond the scope of the present work. Section II answers the question: “How do the DNS and LES methods work?” A short discussion of the computational requirements, numerical approaches, and computational tools is included. Section III is about the interpretation of the DNS and LES results and statistical uncertainties. Sections IV and V give some examples of the DNS and wall-resolved LES results relevant for nuclear thermal hydraulics. The last section lists the conclusions and some of the challenges that might be tackled with the most accurate techniques like DNS and LES.

Regularization method for large eddy simulations of shock-turbulence interactions

The so-called T-shaped reducing tees are typically used to divide, change and control (to a certain extent) the flow direction in pipe networks. In this study, the Ffowcs Williams–Hawkings (FW-H) equation and the Large Eddy Simulation (LES) methods are used to simulate the flow-induced noise related to T-shaped reducing tees under different inlet flow velocities and for different pipe diameter ratios. The results show that the maximum flow velocity, average flow velocity, and vorticity in the branch pipe increase gradually as the related diameter decreases. Strong vorticity and secondary flows are also observed in the branch pipe, and the associated violent pressure fluctuations are found to be the main sources of flow-induced noise. In particular, as the pipe diameter ratio decreases from 1 to 0.45, the Total Sound Pressure Level (TSPL) increases by 6.8, 6.26, and 7.43 dB for values of the inlet flow velocity of 1, 2, and 3 m/s, respectively. The distribution characteristics of the flow-induced noise in the frequency domain follow similar trends for different pipe diameter ratios.

Multi-element airfoils are high-lift devices and provide improved aerodynamic characteristics, which are beneficial for several applications. The performance of high lift devices was investigated numerically by adapting the 2D viscous, steady, pressure model equations together with the SST k-ω turbulence model. Simulations were performed using ANSYS Fluent 24 for the NHLP2D airfoil with a leading edge slat and trailing edge flap. The flow is considered at a Reynolds number of 1.6x105 and varying angles of attack from 0 to 28 deg. The parameters that describe the positioning of the slat and flap were tested variables. These variables include deflection angles, gaps, and overlaps. The lift and drag coefficients are evaluated for various configurations. The results reveal that slats increase the maximum lift on the wing and delay stall, increasing the maximum angle of attack. However, they reduce the lift. In contrast, the flaps increase the overall high-lift configuration airfoil camber, thereby increasing lift. It was found that for the tested slats and flaps locations, slats with a 2.6% gap, -1.5% overlap and 30° deflection, and flap with a 1.3% gap, 5.3% overlap and 30° deflection was the one with the most favorable performance, generating a Clmax of 3.81 and Cd of 0.158.

The noise-generating mechanism of rod-airfoil configuration and the effect of spanwise length on noise prediction