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Perturbations of flow control parameters may yield a significant alteration in the boundary layer stability. Based on the previously established parameter-associated sensitivity, the present work derives the optimal minor parameter perturbation analytically under the constraint of base flow energy variation. Specifically, the steady blowing-suction factor and the generalized Hartree parameter are examined at Mach number 4.5 to stabilize the mode S. Good agreement between the linear stability theory calculation, sensitivity theory, and Lagrangian approach is achieved for the optimal parametric state. The optimal state occurs if the contribution of the base velocity distortion has the greatest advantage over the temperature counterpart. Contributions of various physical sources to the growth rate behave similarly and collapse onto one correlation if normalized by the maximum, particularly for the major four: advection, mean shear, base temperature gradient, and pressure gradient. When the parameter perturbation further becomes finite, the optimal state is found on the constraint border of control parameters. Although the favorable pressure gradient and wall suction stabilize the broadband mode S, an unusual opposite tendency may occur for a single-frequency disturbance. In this unusual parametric range, positive contributions of both the major and minor physical sources to the growth rate are promoted. The contributive increase in major and minor sources are attributed to the enhancement of mean shear and viscous effect, respectively. Whether the parametric influence is stabilization or destabilization is intrinsically determined by the sensitivities, and the intermediate process is analyzed. Finally, given the modification to the critical Reynolds number, the input control parameter perturbation is inversely obtained and verified. [ABSTRACT FROM AUTHOR]

This paper investigates a Mach 6 oblique breakdown via direct numerical simulation in conjunction with stability and quadrant analyses. Particular emphasis is placed on, first, the heat transfer and mean flow distortion in the near-wall and outer transitional boundary layer, and, second, the flow events that are responsible for the production of the Reynolds stress. The energy budget reveals that enhancement of viscous dissipation due to mean flow distortion dominates the heat transfer overshoot, while the dissipation due to fluctuations is lesser but not negligible. Downstream of the location of the peak mean heat flux, the wall temperature gradient (non-dimensionalized by the freestream temperature and local boundary layer thickness) varies little, owing to the occurrence of breakdown and the establishment of self-similarity. Renormalized by the boundary layer thickness, a new correlation of the Stanton number shows no overshoot or difference between the original overshoot region and the turbulent region, which indicates the possibility of similarity once breakdown has occurred. In the outer region, enhanced advective heat exchange strongly reshapes the mean temperature profile. Because of successive modal growth and nonlinear saturation, the contributions of the primary oblique mode, streak mode, and a superharmonic to the outer advective heat transfer are found to compete near the location of the peak heat flux. From the perspective of fluid motions, quadrant analysis highlights the evenly and broadly distributed joint probability density function (PDF) of the fluctuating velocities during transition, which results in overproduction of the Reynolds stress, while the PDF is concentrated around zero in the turbulent region. The flow event Q2 (ejection) overtakes Q4 (sweep) in the outer boundary layer of the transitional region, mainly owing to the primary mode, while the two events become attenuated and nearly achieve balance when transition is complete. [ABSTRACT FROM AUTHOR]

Direct numerical simulation of shock-turbulent mixing layer interaction (STMLI) is conducted in this paper to study the influence of shock-turbulent interaction (STI) on the turbulence evolution and shock-associated noise. The results show that turbulent kinetic energy and pressure fluctuation around the interaction point of STI are both first increased and then reduced to a smaller value than that in the fully developed region of the turbulent mixing layer, while the Reynolds-stress anisotropy at the upper edge of STMLI is changed under the compression–expansion effect induced by the distorted shock tip and the reflected expansion wave. Additionally, it is found that shock-associated noise would increase the overall sound pressure level (OASPL) and amplify the high-frequency noise at the upstream observers. By applying the shock-leakage theory, the turbulence scale analysis, and the spectrum analysis, two generation mechanisms of shock-associated noise are identified: first, the influence of turbulence on the shock wave results in the shock unsteady movement, which generates a sound wave with cylindrical wave front; second, STI decreases the turbulence scale and increases the pressure fluctuation in the high-frequency band so as to strengthen the small-scale turbulence to radiate out more high-frequency noise. Finally, the shock strength effect on shock-associated noise is explored, and the shock-associated noise reduction is observed when decreasing the shock strength. By converting the OASPL difference to the equivalent acoustic pressure difference, a linear correlation between the shock-associated noise source strength and the shock strength is found. [ABSTRACT FROM AUTHOR]

Cartesian cut-cell meshes are favored for their excellent complex geometric adaptability, orthogonality, and mesh generation convenience. However, the difficulty in constructing shape function for hanging-node and irregular cut-cell elements limits their use in a standard finite element method (FEM). Inspired by the point interpolation method shape function used in a smoothed finite element method (S-FEM) which adapts to the arbitrary shape of an element, this work proposes a cell-based S-FEM using Cartesian cut-cell meshes for incompressible flows. Four different types of cell-based smoothing domains (CSDs) are constructed and compared in the Cartesian cut-cell mesh, involving node-based CSD (NCSD), face-based CSD (FCSD), mixed CSD (MIXCSD), and tetrahedral CSD (T4CSD). The smoothed Galerkin weak form and semi-implicit characteristic-based split (CBS) scheme are employed for spatial discretization and stabilization of Naiver–Stokes (N–S) equations, respectively. Several numerical examples are utilized to compare the convergences, computational accuracy, and computational efficiency of proposed CSDs. The numerical results demonstrate that FCSD and T4CSD exhibit instability. Conversely, NCSD and MIXCSD exhibit good stability, and NCSD shows slightly higher computational accuracy than MIXCSD, but at a lower computational efficiency. Additionally, the results show that Cartesian cut-cell meshes offer superior computational accuracy compared to tetrahedral meshes. Therefore, the present method provides an attractive numerical technique for solving flow problems with complex geometries. [ABSTRACT FROM AUTHOR]

This research aims to explore the influence of transient pressure fluctuation inside high-speed trains passing throught tunnels on the fetal growth of Sprague – Dawley (SD) rats. A pressure variation simulation system was designed and exposure experiments were performed on SD rats. Forty-eight SD rats are divided into two control groups and two experimental groups, and are then exposed to transient pressure alternation (−1200 Pa ~1200 Pa) from gestation day 0 to gestation day 5 (GD 0–5). Fetal growth and development indicators on GD12 and GD18 between experimental and control groups were compared. Statistical results showed that, compared to the control group, the key indicators in the experimental group, including placental weight, placental diameter, fetal weight, and crown-to-rump length have decreased by 4.77%, 3.38%, 6.20%, and 3.75% respectively on GD18. The findings imply that the pressure fluctuation environment of high-speed trains has potential effects on the fetal growth of SD rats. [ABSTRACT FROM AUTHOR]

• Turbulence noise is amplified by the shock/turbulence interaction. • The normal stresses in transverse directions contribute greatly to the noise. • Three mechanisms of noise variation under shock/turbulence interaction are found. • Acoustic intensity parts for different turbulence scales are increased under STI. • Acoustic intensity increase for interaction of smaller and larger scales is larger. Direct numerical simulation of shock/isotropic turbulence interaction combining Lighthill's acoustic analogy is conducted to study the influence of shock/turbulence interaction (STI) on the turbulence noise in the farfield. The acoustic statistics are adopted to quantify the influence of STI on the turbulence noise. The results show that STI increases the power spectral density in almost the whole frequency band and obviously amplifies the acoustic intensity. Based on the numerical results and the theoretical analysis, the primary noise sources are identified. The normal Reynolds stress components in the transverse direction contribute greatly to the noise at the observers. By applying the Rankine-Hugoniot relation and the spatial and temporal Fourier analysis on the simplified noise source, the mechanisms of turbulence noise variation under STI are also investigated, which can be divided into three parts. First, the compression effect of the shock wave increases the downstream density. Second, STI increases the value of turbulent kinetic energy spectrum over the entire wavenumber range. Third, the noise sources in the high frequency band are amplified under STI. Additionally, the transverse velocities are filtered in wavenumber space and the influence of STI on turbulence noise is studied from the perspective of the turbulence scales. The contribution of acoustic intensity radiated from "smaller" or "larger" turbulence scales is decreased, and the contribution of acoustic intensity radiated from the interaction of "smaller" and "larger" turbulence scales is increased under STI. [ABSTRACT FROM AUTHOR]

In this study, the sliding grid method is used to simulate a 350 km/h high-speed train passing through a 350 m standard single-line tunnel. The cross-sectional pressure distribution ahead of the train is investigated, the three-dimensional characteristics and propagation mechanism of pressure waves are described based on iso-pressure surfaces, and a newly accurate pressure sensor layout is proposed. The results show that the farther the cross-section is from the train, the smaller the cross-sectional pressure difference is. Influenced by the shape of the train nose and tunnel cross-section, the pressure waves are not simple planar waves, but spherical waves that vary in complexity over the propagation process. As the pressure difference of the 16.6 m cross-section before the train is within 1 Pa, the pressure waves behind this cross-section are defined as planar waves. Besides, the pressure peak-to-peak value difference between the wall and construction clearance reaches 13% at 70–90 m cross-sections, so the pressure sensors should be placed on the surface of the appurtenances rather than on the tunnel wall. In summary, this study is significant for understanding the formation and transmission mechanisms of pressure waves and guiding the scientific layout of pressure sensors in tunnels. [ABSTRACT FROM AUTHOR]

• A novel model of using nSFEM and DPM to solve particle-laden flows. • The super robustness in handling extremely distorted polygonal meshes of the present method. • The unified implementation of mean value coordinates interpolation as shape function and fluid drag interpolation with high-fidelity. • The polygon mesh in this paper shows a great adaptation to complex geometry. In this work, an n-sided polygonal smoothed finite element method (nSFEM) using mean value shape function for concave polygonal element coupled with the discrete phase model (DPM) is developed to solve particle-laden flows. The fluid phase is solved using nSFEM stabilized by the well-developed characteristic-based split (CBS) algorithm. The concave polygonal element is successfully constructed via the intrinsic characteristic of mean value shape function and a new smoothing domain (SD) construction with more physical meaning; thus, addressing the severely distorted local mesh, as well as retaining good geometric adaptability. In the meantime, the modeling of the coupling between the fluid and the particle is achieved by reusing the mean value interpolation to obtain the fluid drag force acting on the particle. The accurate capture of fluid velocities at particle positions within arbitrary polygonal elements via mean value interpolation ensures fluid drag with high-fidelity. Several classical numerical examples, including particle-laden flow around a circular cylinder, are presented to demonstrate the high accuracy and good robustness, and the capability of the proposed method for predicting particle motion in the analysis of complex particle-laden flows. [ABSTRACT FROM AUTHOR]

• A coupled S-FEM and Lagrangian particle tracking model is presented for the solution of 3D dilute particle-laden flows. • The unified solution of particle-fluid systems on unstructured meshes with multi-type elements based on S-FEM. • A completely new application of S-FEM on multiphase flows. In this study, a coupled solution algorithm for three-dimensional dilute particle-laden flows is proposed by integrating the Lagrangian particle tracking model into the smoothed finite element method (S-FEM). Initially, an unstructured mesh fluid solver with multi-type elements is developed using the cell-based S-FEM (CS-FEM) in the Eulerian framework. Subsequently, a fluid force-driven strategy is employed to trace the particle trajectories based on the Lagrangian approach. Moreover, a one-way coupling strategy is designed to perform the solution of the particle-fluid system. To ensure accurate computation of fluid information at the particle positions, we introduce the spherical mean value interpolation algorithm that is compatible with polyhedral elements, enabling uniform interpolation across different types of elements. The correctness of both the Eulerian and Lagrangian solvers is validated independently using benchmarks. Numerical results, validated by references and the finite volume method (FVM) software Fluent, demonstrate the effective prediction of particle trajectories and distributions by the proposed algorithm. Overall, this algorithm expands the application of CS-FEM to multiphase flows and exhibits its capability to handle practical particle-laden flow problems. [ABSTRACT FROM AUTHOR]

• Field measurements of the through-draught in subway carriages are carried out. • The generation of the through-draught and its matching relation with the train's running state are analyzed. • Velocity distribution of the through-draught along the train length is described. • The impact of train acceleration and station spacing on airflow variation is discussed. The through-draught effect inside a subway significantly impacts the flow field profile, thus causing a potential threat to pollutant dispersion and human health. This study conducted multiple field measurements to systematically analyze the through-draught characteristics during actual train operation. The results show that the total variation of airflow velocity has a high consistency with train operation parameters, which reaches its peak value at the end of the acceleration period (EAP). And the airflow direction reverses as the running speed falls to nearly 0.45 times the maximum of that. For the whole subway train, the air velocity grows continuously in the first four carriages and then drops from the fifth carriage, with a maximum speed of 2.37 ± 0.12 m/s. Moreover, the airflow velocity distribution in a carriage exhibits an approximate "W-shape", and the profile of the maximum airflow velocity is equivalent to a "parabolic surface", with a velocity difference of less than 0.45 m/s. Finally, the through-draught effect is comprehensively affected by train acceleration and station spacing which further impact the air inertia forces and internal pressure difference, respectively; the wind speed peak positively correlates with train acceleration and subjects to an inverse exponential function with the station spacing. The research outcome can provide a valid basis for airflow control and air quality improvement in subway carriages operating in the underground tunnel. [ABSTRACT FROM AUTHOR]

This paper aims to study a new connected structure composed of parallel headings and cross passages, mitigating micro-pressure waves (MPWs) at the 400 km/h railway tunnel exit. The numerical method based on the sliding mesh method with the RNG k-ε turbulence model is utilized to simulate the relative motion of the train and tunnel, of which the accuracy of the method is verified by moving model tests. Then, the paper analyzes the effects of the distance of cross passage to tunnel entrance (l 1), length of cross passage (l 2), number of cross passages (n), spacing of cross passages (l 3), and layout location on MPWs. For one cross passage, as the increase of l 1 , MPWs gradually decrease, reaching an optimal value at l 1 = 25 m, but as the increase of l 2 , MPWs gradually increase. When l 1 = 25 m, l 2 = 10 m, MPWs are reduced by about 34.4%. For multiple cross passages, when n = 2, the reduction rate of MPWs reaches a stable level of about 40%. l 3 does not significantly affect MPWs. The connected structure placed at the tunnel entrance is more effective in mitigating MPWs. In addition, we find that the combination of the connected structure and a hat oblique tunnel portal can reduce MPWs by 52%. This study makes reasonable use of the parallel heading left by the tunnel construction to significantly alleviate MPWs without increasing the construction cost. • A new connected structure applied to mitigate micro-pressure waves at 400 km/h railroad tunnels is studied. • The optimal parameters of the connected structure are obtained. • The mechanism of the connected structure to alleviate micro-pressure waves is revealed. • Combining the connected structure with the existing tunnel portal can further decrease micro-pressure waves with a reduction rate of 52%. [ABSTRACT FROM AUTHOR]

• Obtained the mechanism of pressure wave variation under different geothermal distributions. • Studied and summarized the influence of different geothermal distributions on the pressure wave peak. • Obtained the variation law of the dynamic temperature field in the tunnel. In this paper, experiments and numerical simulations are used to study the pressure wave transient laws, propagation characteristics and dynamic temperature field changes in tunnels located near geothermal hot springs. To simulate different temperature distributions, areas of 0–80 m, 80–160 m, 160–240 m and 270–350 m are heated, and the tunnel pressure wave peak value and train surface pressure wave peak value are studied accordingly. The research shows that when the heating zone is 0–80 m, the influence on the initial pressure wave is the greatest, the change rate is 8–10 %, and the initial expansion wave change rate is 5–10 %. When the heating zone is 80–160 m, there is little change in the pressure wave peak value in the first and second stages, and the expansion wave peak value is larger than the normal temperature in the third stage. When the heating zone is 160–240 m, the change rate of the pressure wave peak is small. When the heating zone is 270–350 m, there is a negligible change in the initial compression wave and expansion wave peaks in the first and second stages, and the change rate in the third stage is 2–4 %. Moreover, the heating zone located in front of the tunnel has a greater impact on the pressure at the tip of the train's nose, and the heating zones located in the middle and end of the tunnel have little effect on the initial positive peak value of the train pressure wave but a greater effect on the negative peak value. This study explores the influence mechanism of different geothermal distributions on pressure waves, which can provide more accurate boundary data for improving passenger comfort and the train thermal structural performance in high-temperature tunnels. [ABSTRACT FROM AUTHOR]

When a high-speed train (HST) passes the snowy ballast bed in winter, the slipstream can induce snow drifting and generate snow accumulation problem in the bogie region. The conventional numerical method has utilized the way that snow particles drift with the incoming flow around a stationary train with moving ground (STMG). In this paper, the sliding mesh technique is applied to reproduce the actual scene of a moving train with stationary ground (MTSG). Coupled with the discrete phase model, a novel simulation method for snow accumulation is presented. The comparisons between the STMG and MTSG are conducted, including the flow field, snow distribution and accumulation features. The results show that similar flow field around the HST is captured by the two methods. The MTSG predicts more dispersed snow distribution in the Bogie 2 region than the STMG. The total mass of accumulated snow on bogies and cavities obtained by MTSG is 18.31% more than that derived from STMG. The presented method achieves the relative motion among the HST-snow-ground and the sweeping process of the slipstream on the snow surface is fully considered. In engineering applications, it provides a feasible method to explore the snow accumulation phenomenon of HSTs in more scenes, such as the intersecting of two HSTs. • A novel snow accumulation simulation method for high-speed trains is presented. • The presented method achieved the relative motion among the train-snow-ground. • The slipstream-induced sweeping process on snow surface is considered. • The comparisons between the conventional and presented methods were conducted. • The mass was predicted 18.31% more by the presented method than the conventional. [ABSTRACT FROM AUTHOR]

• Study the mitigation effect of pressure fluctuations caused by train intersection in a tunnel with the enlarged end. • Consider the effects of the total length of tunnels and unit number of trains on the optimal length of the enlarged section. • Analyse the mitigation principle of the pressure fluctuations impacted upon trains. • Obtain an optimal length coefficient to attain the maximum mitigation of the pressure fluctuations. Intense pressure fluctuations generated by the intersection of high-speed trains (HSTs) in a tunnel are unfavourable for the comfort of passengers and the safety of HSTs and tunnel facilities. This study numerically investigated the mitigation principle of the pressure fluctuation impacted upon HSTs using the enlarged tunnel end with taking into account the total tunnel length and the unit number of HSTs. Moving model tests were also performed to validate the numerical method based on the unsteady RANS equations and RNG k - ε turbulence model. The numerical results show that the enlarged end of a tunnel can manifestly alleviate the negative pressure induced by the train intersection and the length ratio of the optimal length of the enlarged section to the total length is in the range of 0.26–0.28. The optimal length of the enlarged section can be implemented by synchronising the contact of the derived waves with the train intersection. Moreover, the increase in the marshalling number of HST augments the mitigation effect of the enlarged section. This study gives insight into the mitigation of the pressure fluctuation in tunnels and can provide valuable data and a theoretical basis for the tunnel design when considering the aerodynamic effect of HST intersection. [ABSTRACT FROM AUTHOR]

In this study, tests and simulations are used to investigate the changes in micro-pressure waves (MPWs) generated by high-speed trains passing through high geo-temperature tunnels. First, moving model tests of high-speed trains passing through local high-temperature tunnels are conducted, and then the simulation results are compared with the experimental results to verify the accuracy of the numerical algorithm. The results show that high temperature reduce the pressure gradient and alleviate MPWs. When the length of the high-temperature zone in the tunnel is within the streamlined length of the train (12.5 m), MPWs decrease as this length increases. When the length exceeds the streamlined length, MPWs basically no longer change. When the length equals the streamlined length and the air temperature in the tunnel reaches approximately 323 K, MPWs are reduced by 9.1%. This paper also presents the influence of the circumferential angle of the tunnel high-temperature zone on MPWs. MPWs gradually decrease with increasing heating angle when the angle is less than 225° and remain basically unchanged when the angle is greater than 225°. The results provide a method for mitigating MPWs and guidance for scholars who study MPWs mechanism in high geo-temperature tunnels. • Study the micro-pressure waves performance of trains passing through the high geo-temperature tunnels. • The characteristics of micro-pressure waves at different lengths L H in the high-temperature zone of the tunnel are analyzed. • The change law of micro-pressure waves under different circumferential angles i in the high-temperature zone are revealed. • Provide a method for mitigating noise pollution caused by micro-pressure waves of railway tunnels. [ABSTRACT FROM AUTHOR]

In this study, through experiments and numerical simulations, micro-pressure waves (MPWs) resulting from the movement of high-speed trains through high-temperature geothermal tunnels were studied, and a self-satisfying MPW mitigation method was proposed. First, a model experiment of a moving high-speed train under local high-temperature conditions was carried out. Then, a relationship between the MPW and length of the heating zone (L H) and the length of the train streamline (12.5 m) was established via numerical simulation. For L H < 12.5 m, the MPW amplitude gradually decreases with increasing L H. For L H > 12.5 m, the MPW amplitude remains basically unchanged. The influence of the circumferential distribution of the heating zone along the tunnel wall on the MPWs was further studied. The research results inspired an MPW mitigation method for tunnels, considering a heating length of 12.5 m and an angle of 225°. Moreover, in a tunnel, electric energy could be generated under slipstream action through a wind turbine installed to provide heating equipment with power, thereby achieving self-satisfaction. This research proposed a new MPW mitigation method for research on MPW mitigation. Moreover, new mitigation methods could facilitate the construction of an environmentally friendly society and provide a new approach to the sustainable development of cities. [Display omitted] • Obtained the influence of the length of the heating zone on MPW amplitude. • The relationship between the MPW and L H and the length of the train streamline was established. • Obtained the influence of the circumferential distribution of the heating zone on MPW. • A new self-satisfying MPW wave mitigation system is proposed. • A new MPW mitigation method and outlined new directions for research on MPW mitigation. [ABSTRACT FROM AUTHOR]

• Obtained the influence of the length of the heating zone on the peak value of the initial compression wave. • Obtained the influence of the length of the heating zone on the peak value of the initial expansion wave. • Study the effect of different heating zone lengths on the peak value of the train's initial compression wave. • The result of the initial temperature field of the tunnel is obtained. • Study the dynamic change process of temperature during train operation. In this paper, through experimental and numerical simulation methods, the influence of the tunnel heating zone length on the pressure wave peak and temperature field changes is investigated in tunnels with different heating zone lengths. The accuracy of the applied simulation algorithm is verified against experimental data from moving model tests. Via numerical simulations, the changes in the pressure wave peak value and temperature field are explored with five heating zone lengths. For the compression wave, this research revealed that when L H (length of the heating zone) < L T (length of the train) and that with an increase in L H , the peak value of the initial compression wave notably decreases. When L H > L T , the compression wave peak value remains basically stable. In regard to the expansion wave at tunnel measurement points, for L (from tunnel entrance) ⩽ 110 m, the initial expansion wave peak exhibits small fluctuations. At L > 110 m, with an increase in the heating zone length, the peak value of the expansion wave peak gradually decreases. In addition, the dynamic temperature field changes. The obtained research results provide a new idea to reduce the peak pressure value in a tunnel. In addition, this study provides a method for pressure wave mitigation during tunnel design and guidance for passenger comfort improvement in future city railway tunnels. [ABSTRACT FROM AUTHOR]