Drag reduction using riblets downstream of a high Reynolds number inclined forward step flow

Micro-riblet is an efficient passive method for controlling turbulent boundary layers, with the potential to reduce frictional drag. In various applications within the transportation industry, flow separation is a prevalent flow phenomenon. However, the precise drag reduction performance of riblets in the presence of flow separation remains unclear. To address this, an inclined forward step model is proposed to investigate the interaction between riblet and upstream flow separation. The large eddy simulation (LES) method is applied to simulate the flow over geometries with different step angles and riblet positions. The results show riblets still reduce wall frictional resistance when subjected to the upstream flow separation. Remarkably, as the angle of the step increases from 0 degrees to 30 degrees, the drag reduction experiences an increment from 9.5% to 12.6%. From a turbulence statistics standpoint, riblets act to suppress the Reynold stress in the near-wall region and dampen ejection motions, thus weakening momentum exchange. Quadrant analysis reveals that with the augmentation of flow separation, the Q2 motion within the flow field intensifies, subsequently enhancing the riblet-induced drag reduction. Moreover, the position of the rib lets has a significant impact on the pressure drag. Riblets close to the point of separation enhance flow separation, altering the surface pressure distribution and thus increasing the resistance. The results reveal that when the riblets are positioned approximately 160 riblet heights away from the step, their effect on the upstream flow separation becomes negligible. The precise performance of riblets under complex flow conditions is important for their practical engineering application

高速列车内飞沫传播特性及抑制方法研究

采用基于Euler-Lagrange的数值模拟方法，对高速列车车厢内气流组织及飞沫扩散传播特性进行了研究，提出并验证了合理的抑制飞沫传播扩散方法。研究结果表明：高速列车上送下回的通风方式会在车厢内形成大范围的旋涡结构，对飞沫的传播扩散影响较大；通过在车厢顶部加装整流网的方式，可以使飞沫的传播距离缩短34.5%,减少乘客间的交叉感染。控制方案可为高速列车的车厢通风系统设计及保障旅客健康安全方面提供参考

Reverse jet parameters study on aerodynamic thermal uncertainty of a blunt body

Adding head opposing jets is a common flow control approach to reduce the aerodynamic heating of hypersonic vehicles with blunt-shape head. However, interests of present studies are mostly focused on surface heating reduction originate from opposing jets only on single-and-determinate free incoming conditions, rather than that under existence of freestream perturbations, to the best of knowledge. In this work, two-layers research structure is formed to investigate the jet design parameters on surface thermal uncertainty with occurrence of free incoming perturbations. In the inner layer, the Polynomial Chaos method is applied for quantifying the surface thermal uncertainty, including the mean value and the standard deviation, under free incoming perturbations. In the outer layer, the Variance Analysis method is employed to describe the jet geometry and flow parameters on surface thermal uncertainty. As a consequence, the parameters impact on the mean surface heating under perturbations are found similar to that in baseline freestream condition. Moreover, only jet entry diameter and total-pressure ratio of jetto-freestream are important when it comes to the standard deviation of surface heating. This research is anticipated to be available of robust thermal-reduction optimization on hypersonic vehicles with blunt shape head in a wide range of free incoming conditions for reference. (c) 2020 Elsevier Masson SAS. All rights reserved

Influence of rectangular strips size on aerodynamic performance of a high-speed train subjected to crosswind

Conventional studies usually assume that the train surface is smooth, so as to simplify the numerical calculation. In fact, the surface of the train is irregular, which will change the flow characteristics in the boundary layer and further affect the aerodynamic performance of a train. In this work, roughness is applied to the roof of a 1:25 scaled train model in the form of longitudinal strips. Firstly, the improved delayed detached eddy simulation (IDDES) method is adopted to simulate the aerodynamic performance of the train model with both smooth and rough surface, which are subjected to crosswind. Results show that the side force coefficient and the roll moment coefficient subjected to rough model decreased by 3.71% and 10.56% compared with the smooth model. Then, the width, height and length of the strips are selected as variables to design different numerical simulation schemes based on the orthogonal experimental design method. Through variance analysis, it can be found that four design parameters have no significant effect on the side force coefficient. Meanwhile, for the roll moment coefficient, the length of the strips in the straight region of the train has a significant effect and the width of the strips has a highly significant effect on it. These conclusions can provide a theoretical basis to improve the aerodynamic performance of the high-speed train subjected to crosswind. © 2021 American Society of Mechanical Engineers (ASME). All rights reserved

Multi-objective design optimization of the combinational configuration of the upstream energy deposition and opposing jet for drag reduction in supersonic flows

Optimization design has been widely used in the supersonic vehicle design process and the drag reduction characteristic is an important objective of the optimization. The drag reduction mechanism applied to the blunt body with the combinational configuration of the upstream energy deposition and opposing jet for drag reduction has been conducted numerically. In the current study, the three-dimensional coupled implicit compressible Reynolds Averaged Navier-Stokes equations and Menter's shear stress transport turbulence model are employed to simulate the flow fields around the blunt body with the combined method. The results show that in the jet-to-freestream total-pressure ratio of 0.2 and 0.4, the drag is reduced by 47.44% and 45.96%, respectively. Further, the Latin hypercube method is used for the generation of initial samples for optimization and the multi-objective design optimization algorithm coupled with the Kriging model surrogate model is applied to determine optimal flow control parameters. The drag reduction factor R-d and drag reduction effectiveness E-eff are selected as optimization objectives. The Pareto-optimal front for the multi-objective design optimization results is acquired and there exists a challenging tradeoff between the two optimization objectives. The drag reduction factor R-d and drag reduction effectiveness E-eff further increase as much as 28.16% and 116.47%, respectively. The jet has a stronger penetration in the optimum design condition, and the findings suggest that the strategy of adding energy spot to the upstream flow field of the opposing jet can be an effective way for drag reduction. (C) 2020 Elsevier Masson SAS. All rights reserved

沟槽微结构尺寸对高速列车横风特性影响研究

随着列车运行速度的不断提升,气动效应对列车运行安全性产生的影响越来越突出。目前针对高速列车横风效应的研究通常假定列车表面光滑,实际上列车表面是非光滑的,边界层内的流动特性有所不同。利用微结构进行非光滑表面设计的新型技术手段可能改善高速列车在横风条件下的气动性能。以在车顶加设矩形条带组的方式,对1∶25比例的列车模型进行局部非光滑设计;采用改进的延迟分离涡模拟(IDDES)方法对横风作用下光滑表面和粗糙表面的列车模型进行气动性能模拟。结果表明,与光滑模型相比,粗糙模型下的侧向力系数和倾覆力矩系数分别降低了3.71%和10.56%。选取条带的宽度、高度和长度为设计变量,基于正交试验设计方法设计不同的数值模拟方案,利用方差分析和极差的方法探索矩形条带几何参数与列车侧向力和倾覆力矩间的关系,给出条带外形设计的优选方案。本研究可为横风作用下如何提升高速列车的气动性能提供理论依据

Robust optimisation of the streamlined shape of a high-speed train in crosswind conditions

Traditional deterministic aerodynamic optimisation cannot consider environmental uncertainty, which may lead to sensitivity issues. The present study proposes a robust design framework for the aerodynamic optimisation of high-speed trains, which accounts for the uncertain wind and its impact on crosswind stability. In this framework, a variance analysis method based on the Non-Intrusive Polynomial Chaos is proposed to determine the deformation area, and a parametric model is subsequently established. The Non-dominated Sorting Genetic Algorithm-II (NSGA-II) is used as the optimiser to minimise the mean and variance of the aerodynamic response. The mean and variance can be quickly predicted by an uncertainty analysis approach combining Monte Carlo simulation and Kriging model. The framework is then applied to the optimisation of a high-speed train under crosswind. The results of the robust optimisation are compared with those of the baseline geometry and deterministic optimisation. The mean and variance of the rolling moment under crosswind are reduced by 2.26% and 3.37% respectively after optimisation, indicating that the performance and robustness are both improved. The proposed framework is effective for the engineering design of high-speed trains and can also provide a reference for the robust design of other aerodynamic shapes

Aerodynamic-Aeroacoustic Optimization of a Baseline Wing and Flap Configuration

Optimization design was widely used in the high-lift device design process, and the aeroacoustic reduction characteristic is an important objective of the optimization. The aerodynamic and aeroacoustic study on the baseline wing and flap configuration was performed numerically. In the current study, the three-dimensional Large Eddy Simulation (LES) equations coupled with dynamic Smagorinsky subgrid model and Ffowcs-William and Hawkings (FW-H) equation are employed to simulate the flow fields and carry out acoustic analogy. The numerical results show reasonable agreement with the experimental data. Further, the particle swarm optimization algorithm coupled with the Kriging surrogate model was employed to determine optimum location of the flap deposition. The Latin hypercube method is used for the generation of initial samples for optimization. In addition, the relationship between the design variables and the objective functions are obtained using the optimization sample points. The optimized maximum overall sound pressure level (OASPL) of far-field noise decreases by 3.99 dB with a loss of lift-drag ratio (L/D) of less than 1%. Meanwhile, the optimized performances are in good and reasonable agreement with the numerical predictions. The findings provide suggestions for the low-noise and high-lift configuration design and application in high-lift devices

A practical approach to flow field reconstruction with sparse or incomplete data through physics informed neural network

High resolution flow field reconstruction is prevalently recognized as a difficult task in the field of experimental fluid mechanics, since the measured data are usually sparse and incomplete in time and space. Specifically, due to the limitations of experimental equipment or measurement techniques, the expected data cannot be measured in some key areas. In this paper, a practical approach is proposed to reconstruct flow field with imperfect data based on the physics informed neural network (PINN), which integrates those known data with the physical principles. The wake flow past a circular cylinder is taken as the test case. Two kinds of the training set are investigated, one is the velocity data with different sparsity, and the other is the velocity data missing in different regions. To accelerate training convergence, the learning rate schedule is discussed, and the cosine annealing algorithm shows excellent performance. Results reveal that the proposed approach not only can reconstruct the true velocity field with high accuracy, but also can predict the pressure field precisely, even when the data sparsity reaches 1% or the core flow area data are truncated away. This study provides encouraging insights that the PINN can serve as a promising data assimilation method for experimental fluid mechanics

Parametric study on wing-lambda-shock formation

It is well-known that a wing is one of the most important parts of an aircraft as it is used to generate lift force. According to a wing moving at sufficiently high subsonic speeds, the flow speed on the wing's upper surface can be supersonic due to acceleration through the curvature-created suction, thereby forming a shock wave in a lambda shape. Additionally, the lambda shock can interact with the boundary layer flow. These phenomena relate to disturbances in the flow field, including flow separation, thus causing undesirable effects on lift production. Hence, a better understanding of the phenomenon of wing-lambda-shock formation and its nature is essential. This study presents a numerical investigation of the lambda-shock formation on an ONERA M6 wing, which is known as a swept, semi-span wing with no twist, under parametric effects of angleof-attack, and free-stream Mach number, which is increased up to the supersonic regime. The pressure coefficients obtained by simulations are validated by open data. Then, numerical results in terms of the local pressure coefficient, local Mach number, averaged lift and drag coefficients, and?-shape characteristics based on Mach number and pressure coefficients are discussed under an investigated range of the parameters. Results show that the angle-of-attack and free-stream Mach number can affect the lambda shock formation on the wing upper surface physically. Specifically, an iso-sonic surface with lambda shock waves is disturbed when the angle-of-attack and free-stream Mach number vary in an investigated range. This also affects lift and drag coefficients of the wing. © 2021 by ASME