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Computing the Casimir force and energy between objects is a classical problem of quantum theory going back to the 1940s. Several different approaches have been developed in the literature often based on different physical principles. Most notably a representation of the Casimir energy in terms of determinants of boundary layer operators makes it accessible to a numerical approach. In this paper, we first give an overview of the various methods and discuss the connection to the Krein-spectral shift function and computational aspects. We propose variants of Krylov subspace methods for the computation of the Casimir energy for large-scale problems and demonstrate Casimir computations for several complex configurations. This allows for Casimir energy calculation for large-scale practical problems and significantly speeds up the computations in that case. [ABSTRACT FROM AUTHOR]

In this paper, different compressible correction strategies for k-ω turbulence model are proposed. The dilatation dissipation term, which is significant in compressible flow, is taken into account and modeled in the Favre-averaged k-equation and ω-equation. We propose correction model for the dilation dissipation term based on the gradient Mach number in this paper, the coefficients of which are calibrated from experimental data on compressible turbulent mixed layers, and compare it to the Wilcox, Sarkar, and Heinz correction models. In slope-cavity flow and oblique shock wave/boundary layer interactions, the performance of these corrections is evaluated and compared to experimental data. The new correction has a similar or even better effect in separation prediction, heat flux prediction, and other aspects, and the uncertainty of model form and coefficients can be investigated further. • A general compressible correction model is conducted with the consideration of dilatation and structural compressibility. • The experimental data of compressible turbulent mixed layers is employed to calibrate the model coefficients. • Verification and validation are carried out in slope-cavity flow and incident oblique shock wave/boundary layer interactions. • The performance of the model and its comparison with other correction models are discussed. [ABSTRACT FROM AUTHOR]

This paper addresses the fault-tolerant attitude tracking control problem for spacecraft with limited communication capability and input saturation. The design of a hysteresis quantizer aims to alleviate communication burden between the controller module and the actuator module. Then, the attitude tracking problem for spacecraft with input quantization, input saturation, actuator faults and external disturbances is transformed into an attitude control problem with uncertain input coefficients and bounded disturbances. Thereafter a dynamic loop gain function-based approach and a hyperbolic tangent function term with a time-varying boundary layer are introduced to address the uncertainties of input coefficients and the bounded disturbances, respectively. To facilitate the boundness analysis of the signals in the closed-loop system, a pertinent lemma about the dynamic loop gain function is proved. Finally, numerical simulations are employed to validate the effectiveness of the proposed scheme. • The spacecraft attitude tracking problem with disturbances and uncertainties. • The quantized control law alleviates communication burdens of spacecraft. • A dynamic loop gain function method is used to address the impact of uncertain. • A hyperbolic tangent term is devised to counteract bounded disturbances. [ABSTRACT FROM AUTHOR]

This research paper presents a thorough examination of shock wave train phenomena in various duct structures, utilizing a machine learning-optimized k-ω model. The focus of the study is to apply machine learning techniques to adapt the constant coefficients of the k-ω turbulence model, improving its accuracy and computational efficiency in capturing the dynamics of shock waves. An important aspect of this investigation is the impact of diverging sections within the duct, specifically how changes in the divergence angle, while maintaining a constant ratio of the exit area to the throat area, affect compressible flow parameters, shock wave positions, and other related characteristics of shock trains. The study systematically explores these effects and provides fresh insights into the behavior of shock wave trains under different divergence conditions. In a departure from the usual practices in supersonic research, this study compares the phenomena of shock wave trains in rectangular and circular duct geometries. The findings contribute valuable data and analyses to a field that has traditionally focused on rectangular configurations. The results indicate that the shape of the duct has a significant influence on the shock wave train, emphasizing the importance of considering geometric diversity in the study of supersonic flows and shock wave phenomena. This research sets the stage for further investigations into the interaction between duct geometry, shock wave propagation, and the dynamics of compressible flows. • Improved k-ω SST model accuracy to 95.5 % with machine learning. • Longer duct diverging sections intensify the first lambda shock. • Duct shape critically affects shock wave and boundary layer interactions. [ABSTRACT FROM AUTHOR]

This paper proposes a decentralized practical prescribed-time (PT) tracking design using quantized output feedback (QOF) for uncertain interconnected lower-triangular systems with unknown time-delay interconnections. The local output signals are assumed to be only measured and quantized for the PT tracker design under a band-limited network. By employing a PT-dependent scaling function, a decentralized memoryless PT observer based on quantized local outputs is developed to estimate local unmeasurable state variables. Owing to output quantization, the available output feedback signals become discontinuous. As a result, the tracking error between the actual (i.e., unquantized) local output and the local desired signal cannot be utilized in the local virtual controller. To address this issue, a novel adaptive compensation mechanism is derived to design the local PT neural network tracking laws using only quantized local outputs and estimated states. The proposed PT tracking controller does not require information on the interconnected nonlinear functions and interaction delays. During the Lyapunov stability analysis, the boundary layer error decomposition approach is employed to address the issue of non-differentiability in the local virtual control laws. The proposed QOF control system achieves practical PT stability. It is shown that the settling time of local tracking errors can be predetermined, regardless of the design parameters and initial conditions. Finally, the proposed QOF decentralization strategy is supported with illustrative examples and a comparison to demonstrate its benefits. • A decentralized prescribed-time (PT) tracking problem is addressed using quantized output feedback (QOF). • A decentralized memoryless PT state observer based on quantized local outputs is developed. • The practical PT stability of the proposed QOF control system is analyzed mathematically. [ABSTRACT FROM AUTHOR]

The design and analysis of a finite difference method for solving a class of time-fractional singularly perturbed convection-diffusion problems is the subject of this paper. The time-fractional derivative is taken in the Caputo sense with the order α ∈ (0 , 1). The solution to this class of problems possesses weak singularity in the vicinity of the initial time t = 0 and an exponential boundary layer at the right lateral surface as the singular perturbation parameter ε → 0. To tackle the initial layer at time t = 0 , a classical L 1 finite difference scheme is employed on a graded mesh to discretize the time-fractional derivative. Further, a standard upwinding procedure in the spatial direction is used on a piecewise uniform Shishkin mesh. It is shown that the resultant fully discrete scheme is parameter uniformly convergent with order of convergence O (M − p + N − 1 log ⁡ N) , where p = min ⁡ { 2 − α , r α } , r is the graded mesh exponent and N , M are the number of mesh subintervals in space and time direction respectively. Lastly, numerical results illustrate that the proposed scheme is simple, efficient, robust and accurate as well as validate the error estimates presented in the paper. [ABSTRACT FROM AUTHOR]

The literature is rich in fluid–structure interaction studies of a single rod in oscillating fluid (or the reciprocate), however, there is a dearth of experimental data for rod bundle oscillation, particularly if the bundle is in a confined environment. The presence of the fluid surrounding the moving solid is modeled with force coefficients such as inertial and drag. In case of small amplitudes oscillating flow, these force coefficients depend non-linearly on the Keulegan–Carpenter number K C. The dependence present different regimes, separated by threshold K C numbers. At different regimes correspond different fluid dynamics, hence different force coefficients. The aim of this paper is to experimentally investigate the fluid dynamic for a rod bundle oscillating in a stagnant fluid. The velocity fields have been measured with Particle Image Velocimetry (PIV). Both the rod bundle and the fluid have the same refractive index, which allow to measure the velocity fields within the rod bundle in an non intrusive manner. The bundle is hosted in a double tank, immersed in a refractive index matched solution and placed on an earthquake shake table. The shake table sets the bundle into motion along a single direction. Data enable the identification of a threshold effect in the fluid response to the oscillating assembly. This threshold effect is due to the formation of vortexes in the fluid. Data show a clear trend on how the development of turbulence occurs firstly inside the rod assembly and then in the bypass between the assembly and the tank wall. The experimental results indicate the presence of a viscous boundary layer developed along the rod, above the threshold K C. The results presented in this paper represent a step forward the comprehension of the damping effect induced by the fluid at different oscillating amplitudes and frequencies, due to different K C regimes. • PIV measurements with refraction index match conditions for an oscillating rod bundle in stagnant oil. • Determination of Keulegan Carpenter instability threshold. • Comparison fluid dynamics in different points of the rod bundle inner part. [ABSTRACT FROM AUTHOR]

The paper investigates the problem of fire safety in habitable space structures. The flame propagation over thermally destructing material is studied theoretically within the framework of the assumptions of the boundary layer theory and the presence of a diffusion flame. The solution is developed based on the concept of thermally thin fuel layer. Flame propagation velocity over structured material, on which different widths ribs are attached, in an opposed flow is determined analytically. PMMA sample of 10 mm thickness, on which ribs of widths 1–10 mm are attached in the direction of flow, is considered as an example. The analytical solution is compared with experimental data. • Flame propagation velocity over structured material in an opposed flow is determined analytically. • The flame propagation velocity decreases with an increase in the width of the rib. • Slowing down the blower speed can help reduce the speed of flame propagation and the intensity of combustion. • The ribs create more favorable conditions for flame propagation. [ABSTRACT FROM AUTHOR]

Supersonic and hypersonic vehicles have complex shock wave/boundary layer interaction problems during flight, and it adversely affect the aerodynamic performance of the vehicle. In this paper, the bump passive control approach is introduced to control the shock wave/boundary layer interaction, and its configuration is optimized with the minimum separation area and the maximum total pressure recovery coefficient. The Kriging model is established by means of the Latin hypercube experimental design approach to obtain the sample points. The Non-dominated Sorting Genetic Algorithm-II (NSGA-II) algorithm is utilized to optimize the parameters of the bump configuration, and the flow field structures of the uncontrolled condition, the special basic condition and the optimized condition are compared and analyzed. The obtained results show that the surrogate-based optimized model shows better performances than the basic condition. In the parameter setting range considered in this study, the optimized Case 3c with the middle position in the optimization front owns a larger total pressure recovery coefficient and a smaller separation area, with its separation area and total pressure recovery coefficient being 0.80 mm2 and 67.39% respectively. • Bump was used to control the shock wave/boundary layer interaction. • Bump configuration was optimized firstly by surrogate-based optimization algorithm. • An optimized case was obtained with smaller separation area and larger total pressure recovery coefficient. [ABSTRACT FROM AUTHOR]

Investigating the effects of periodic magnetic field and triple diffusion on bioconvection flow through a rotating cone populated by oxytactic bacteria and an Eyring-Powell fluid is innovative and significant. The magneto-bioconvection flow of an Eyring-Powell multi-diffusive fluid across a spinning cone is investigated in this paper, considering the effect of oxytactic microorganisms. The non-similar technique is used in the mathematical analysis of the flow over a spinning cone. The flow under consideration contains two diffusive species: liquid hydrogen and liquid oxygen. In light of the periodic magnetic field, the surface gradients, notably skin friction, exhibit wavy effects in the boundary layer domain. The governing equations for the fluid flow in the current flow problem, accompanied by heat diffusion, species diffusion, rotation, bioconvection, and periodic magnetic, are highly coupled nonlinear PDEs dependent on the proper initial and boundary conditions. Mangler's transformations convert them into non-dimensional forms, and numerical non-similar solutions are produced using implicit finite difference approximation and quasi-linearisation. The enriching values of bioconvective Rayleigh number Rb decline the velocity F, as a result, lessen the friction between the surrounding fluid and the cone's surface. The improving Peclet number Pe and microbial density difference σ reduced the microorganism density profile and heightened the microorganism density number R e - 1 / 2 N n. [ABSTRACT FROM AUTHOR]

The horizontal drainage layer is widely used in blow-filling and road foundation-filling projects. Due to the change in the groundwater table and horizontal drainage layer, the double-layer ground sometimes shows two states of unsaturated and saturated. Meanwhile, the two-layered unsaturated–saturated soil system ground is very widely distributed in nature. In this paper, based on the soil consolidation theory, semi-analytical solutions of excess pore pressures and the settlement of a two-layered unsaturated–saturated soil system containing a horizontal drainage layer under different drainage layer boundary are obtained by using the Laplace transform technique and decoupling method. And the model in this paper can be well degraded to two classic soil consolidation ones and presents an excellent agreement with references, showing excellent accuracy and reliability of the solutions obtained in this paper. Further, by selecting parameters to analyze their effects on the consolidation of the modelling in this paper. The results show that the horizontal drainage layer can provide additional drainage channels to accelerate the consolidation of two-layered ground in an unsaturated–saturated soil system, while the location of the horizontal drainage layer at 0.3 H -0.5 H can most efficiently accelerate the ground consolidation process. When drainage conditions are poor, the settlement completion time is prolonged. [ABSTRACT FROM AUTHOR]

In this paper, second-order rotational velocity correction (RVC) finite element method is designed for solving the two-dimensional/three-dimensional (2D/3D) incompressible thermally coupled magnetohydrodynamics (MHD) system. This format not only reduces the complexity of solving the model, i.e. the strongly coupled nonlinear saddle-point problem is transformed into a series of small elliptic equations, but also the limitation of artificial Neumann numerical boundary layer of pressure is overcome and the precision of pressure is improved. Then, we use the Gauge-Uzawa formula to convert the rotation term into a telescopic symmetrical form to establish and prove the stability of the second-order RVC algorithm in a linear case. Finally, the numerical results demonstrate the effectiveness and stability of second order rotational velocity corrected projection finite elements in solving the problem. [ABSTRACT FROM AUTHOR]

Heat transport (HT) efficiency in boundary layer flow (BLF) across wedges is critical for many industrial processes, as well as aerospace and defence research. The HT analysis in a power-law fluid along a moving permeable wedge is described in this paper. A changeable parameter is the injection velocity at the wedge surface. The governing equations are dimensionless, and the shooting technique is used to achieve a non-similar solution. Graphs depict the impacts of various factors on the velocity and temperature contours, local Nusselt number, and skin friction coefficient. The results show that the dilatant fluid has a velocity decline near the wedge surface and achieves its maximum velocity away from the wedge surface. The findings of this study may aid in the development of better HT systems for a variety of applications. [ABSTRACT FROM AUTHOR]

The time-dependent nonlinear convective Williamson nanofluid flow over a rough conical surface in the presence of numerous diffusions and Arrhenius activation energy effects are the main focus of the investigation in this paper. The unsteady flow is essentially due to the impulsive mainstream flow. Two diffusive species, namely, liquid Hydrogen and liquid Nitrogen, occur in the flow under investigation. A sinusoidal waveform mathematically models the rough conical surface with small amplitude and high frequency. Thus, the surface gradients, namely skin friction, exhibit wavy effects in the boundary layer regime. In the current flow problem, the governing equations for the fluid flow are highly coupled nonlinear PDEs that depend on the proper initial and boundary conditions. Mangler's transformations are used to convert them into non-dimensional forms, and implicit finite difference approximation and quasilinearization are employed to produce numerical semi-similar solutions. The numerical results representing the boundary layer effects are displayed graphically and analyzed for parametric values. The friction experienced by the rough surface is strongly fluctuating for higher values of Williamson parameter W. The heat transfer rate Re x - 1 / 2 N u increases approximately by 61% and 63% at ξ = 0.8 when Nt decreases from Nt = 0.5 to Nt = 0.1 , for the cases of W = 0 (Newtonian fluid) and W = 1(Williamson fluid), respectively. Additionally, there is a strong correlation between the current findings and the related results previously published in the literature. [ABSTRACT FROM AUTHOR]

• This paper does not linearize or average orbit parametersfor J 2 perturbation. • Gauss's variation equations and the coordinate transformation method are combined. • The open-loop control of spacecraft hovering with J 2 perturbation is derived. • A closed-loop controller that keeps the spacecraft hovering state stable is obtained. For spacecraft hovering in low orbit, a high precision spacecraft relative dynamics model without any simplification and considering J 2 perturbation is established in this paper. Using the derived model, open-loop control and closed-loop control are proposed respectively. Gauss's variation equations and the coordinate transformation method are combined to deal with the relative J 2 perturbation between the two spacecraft. The sliding mode controller is adopted as the closed-loop controller for spacecraft hovering. To improve the control accuracy, the relative J 2 perturbation is regarded as a known parameter term in the closed-loop controller. The external uncertainty perturbations except J 2 perturbation are estimated by numerical difference method, and the boundary layer method is used to weaken the impact of chattering on the sliding mode controller. The open-loop control of spacecraft hovering with the relative J 2 perturbation and without the relative J 2 perturbation are simulated and compared, and the results prove that the accuracy of open-loop control with relative J 2 perturbation has been significantly improved. Similarly, the simulation of the closed-loop control are presented to validate the effectiveness of the designed sliding mode controller, and the results demonstrate that the designed sliding mode controller including the derived relative J 2 perturbation can guarantee the high accuracy and robustness of spacecraft hovering in long-term mission. [ABSTRACT FROM AUTHOR]

• Using ammonia combustion to reduce drag for the first time. • Ammonia demonstrates superior local drag reduction. • The self-ignition point decides overall anti-drag effects. • The inlet decomposition ratio is not the higher the better. Boundary layer combustion is an effective method to reduce friction in scramjet combustors. In existing research on drag-reducing fuels, hydrocarbons pose challenges related to carbon deposition and emissions. Meanwhile, hydrogen presents significant difficulties in storage and transportation. For the first time, this paper explores ammonia as a drag-reducing alternative. We employed the Reynolds-averaged Navier-Stokes (RANS) method to investigate the characteristics of drag reduction by ammonia combustion in supersonic flow. Numerical results indicate that the local drag reduction by ammonia combustion is better than hydrocarbon and worse than hydrogen compared to existing research. Ammonia exhibits superior potential for drag reduction. However, the overall drag reduction at room ammonia injection temperature is limited due to the significant distance between the ammonia self-ignition point and the injection port. To address this challenge, we proposed two optimization strategies: increasing the ammonia injection temperature and increasing the inlet ammonia decomposition ratio. As the ammonia injection temperature increases, the self-ignition position moves towards the inlet, and the combustion drag reduction region expands upstream. Interestingly, a higher ammonia inlet decomposition ratio does not necessarily lead to a better drag reduction effect. In fact, an excessively high decomposition ratio can inhibit the combustion drag reduction effect. The most effective drag reduction occurs when the ammonia inlet mass fraction is 50%, resulting in a 42.8% reduction in integrated drag compared to the case without NH 3 injection. Overall, this paper offers critical insights into the utilization of ammonia combustion in scramjets to reduce drag. [Display omitted] [ABSTRACT FROM AUTHOR]

For hypersonic boundary layer, the porous coating comprising small blind holes is an effective passive method that can delay the laminar-turbulent transition by damping the second mode wave. In this paper, surface with two-dimensional rectangular cavities with larger size comparable to the thickness of boundary layer is numerically investigated. The micro-cavity surface affects not only the second mode wave, but also the base flow due to the roughness effects. The numerical results showed that the total drag of base flow can be reduced by the micro-cavity surface compared to the smooth surface, more significant with smaller cavity width and higher porosity, while the cavity depth has little influence on the drag performance. The total drag can be further reduced when the micro-cavity surface suffers moderate static deformation under the pressure difference in the base flow. The micro-cavity surface is able to damp the second mode wave like the porous coating, and thus also has the potential to delay the transition. The effect on the second mode wave is stronger with higher porosity and smaller cavity width, while not monotonic with the cavity depth. The static deformation of structure causes local change of cavity characteristics, resulting in the second mode more damped over the deformed micro-cavity surface. When the structure dynamically responses to the fluctuating pressure of the second mode wave, the intensity of wave is slightly further decreased. • A surface with micro cavities is able to suppress the growth of the second mode wave. • A micro-cavity surface with small cavity width can reduce the flow drag, and the opposite is true when the cavity width is large. • The pressure drag increases while friction drag decreases when the micro-cavity surface suffers deformation, but the total drag is almost unchanged. • The deformed micro-cavity surface leads to the second mode wave more damped, but the drag fluctuation is significantly enhanced. [ABSTRACT FROM AUTHOR]

In order to efficiently perform the direct numerical simulation for shock wave boundary layer interference, a large grid computing scale is needed, and a single graphics processing unit cannot meet the urgent needs of computing. This paper proposes a parallel algorithm based on the architecture of multiple graphics processing units to effectively deal with the direct numerical simulation for shock wave boundary layer interference. The algorithm achieves a high overlap between graphics processing unit computation and boundary data exchange to improve the parallel efficiency when the grid computing scale is large. The working principle of the multi-graphics-processing-unit parallel algorithm is introduced, and the accuracy and feasibility of the algorithm are verified. Finally, through the application of the algorithm in the research of the compression-expansion slope boundary layer, fine flow structures such as large-scale vortex structure, shocklet, and attached shock wave can be captured. As a conclusion, the multi-graphics-processing-unit parallel algorithm proposed in this paper can accurately display the flow field of shock wave boundary layer interference. • A parallel algorithm based on multiple GPUs is proposed. • The accuracy, feasibility, and efficiency of the parallel algorithm are verified. • The DNS of the SWBLI based on the algorithm is tested and analyzed. [ABSTRACT FROM AUTHOR]

A Rotated Lattice Boltzmann Flux Solver (RLBFS) with improved stability is proposed in this paper for the effective simulation of compressible flows with intense shock waves at high Mach numbers. Unlike the conventional LBFS or conventional Riemann solvers, which evaluate its mass, momentum and energy fluxes in the normal direction of each cell interface, the present RLBFS decomposes the outer normal direction of the cell interface into two perpendicular directions of the velocity difference and their counterparts. The fluxes in these two directions are respectively evaluated by the LBFS and the final fluxes at each interface are obtained by weighted summations of the two obtained fluxes. The matrix-based stability theory is applied to analyze the proposed solver by simulating the strong normal shock problem from Mach number 1.5 to 10. It reveals that the RLBFS has a smaller negative eigenvalue than the original LBFS, indicating that the RLBFS is more stable. The numerical accuracy of the RLBFS is evaluated by simulating the advection of density perturbation. The performance of the RLBFS is further demonstrated by simulating seven more compressible flows, including the transonic flow over RAE2822 airfoil, odd-even grid perturbation problem, a Mach 3 wind tunnel with a step, supersonic corner flow, double Mach reflection of a strong shock, shock-boundary layer interaction and hypersonic viscous flow around a blunt body. The obtained results agree well with the published data, showing that the RLBFS can effectively avoid the phenomenon of shock instability and thus has a potential for simulating practical compressible flows with intense shock waves at high Mach numbers. • A rotated lattice Boltzmann flux solver with improved stability is proposed. • A matrix-based stability theory is used to analyze the RLBFS and the LBFS. • Theoretical analysis show that the RLBFS is more stable than LBFS. • Numerical examples show the reliability and capability of the RLBFS. [ABSTRACT FROM AUTHOR]

This article is related to analysing the consequences of joule heating and thermal (hot) radiation on the limit layer stream of a non-Newtonian liquid (Powell-Eyring fluid) while electro transversal magnetic field is present and Cattaneo Christov double diffusion via a convectively heated Riga Plate. The effects of the coefficient of thermophoresis and Brownian motion on joule heating are included in this mathematical model. In this paper, we introduced a new condition namely zero mass flux. A rectangular coordinates system is being employed for the flow equations to get the momentum, energy equations and concentration equations mathematically. By employing a similarity transformation, established (partial differential equations) PDE is reduced into an ordinary differential equations ODE. This method is integrated with the Runge-Kutta RK technique to solve this nonlinear ODE numerologically. With Graph, we show different parameters of velocity and temperature etc. Skin friction, with the help of a graph we can also inspect the Nusselt number and Sherwood number in detail. While studying we noticed that the increase of Hartmann number and fluid parameter is caused to increases in fluid velocity and thickness of the boundary layer. Prandtl number values are increases while decreasing in temperature of flowing fluid and concentration distribution of flowing fluid. This exploration motivates the previous research, and for further study, it gives a platform to analyse on flow past of nanofluid over a Riga plate. [ABSTRACT FROM AUTHOR]

The presented article is based on the detection and resolution algorithms that are assigned to the adaptive finite element methods for solid mechanics problems, in which the boundary layer phenomenon cannot be neglected. In the considered problems, the quantity of interest is the global strain energy. The algorithms were applied within the exemplary hpq -adaptive finite element method composed of 3D-based hierarchical modelling and approximation, residual equilibration method of error estimation and a three step adaptive procedure. This procedure is extended through the inclusion of the additional step of adaptation where the boundary layer is a posteriori detected and resolved. The detection algorithm is based on a simple sensitivity analysis performed locally for the finite elements adjacent to the boundary. The detection algorithm is also extended onto: assessment of the boundary layer significance, determination of the optimal ratio of the exponential refinement, and iterative repetition of the refinement. The resolution of the boundary layer was based on the exponential (or graded) refinement of the mesh in the direction normal to the boundary. In the paper, our theoretical considerations, algorithms, and numerical examples are consistently limited to elliptic shells and plates, however the proposed approach can be extended to parabolic and hyperbolic shells. [ABSTRACT FROM AUTHOR]

The paper analyzes the possibility of improving the efficiency of the propeller as one of the main directions of improving the main propulsion plant. The requirements of the MARPOL-73 convention, as amended, and the need to adapt the main propulsion plant to changing operating conditions require improving the design of the fixed pitch propeller. The paper presents the results of CFD modeling and studies of the flow around the NACA-2412 blade with the supply of additional medium along the chord, regarding the suction surface. This work is an adaptation of the method used in aviation for increasing lift force by blowing the boundary layer. The magnitude of the lift force has a significant impact on the dynamic characteristics of the propeller. CFD modeling of the propeller blade profile in compressible and incompressible media, and also the calculation of the parameters of the NACA-2412 profile with the supply of additional medium were performed. The modeling results are confirmed by comparison with known modeling results for compressible media and experimental data. The possibility of increasing the efficiency of the propeller due to the jet supply of additional water to the suction surface of the blade along the chord has been proved. The results of the work can be used to control the operating mode of the main engine by changing the thrust and the hydrodynamic torque of resistance to rotation of the propeller. [ABSTRACT FROM AUTHOR]

Though the boundary layer formation in the chemotactic process has been observed in experiment (cf. [63]), the mathematical study on the boundary layer solutions of chemotaxis models is just in its infant stage. Apart from the sophisticated theoretical tools involved in the analysis, how to impose/derive physical boundary conditions is a state-of-the-art in studying the boundary layer problem of chemotaxis models. This paper will proceed with a previous work [24] in one dimension to establish the convergence of boundary layer solutions of the Keller–Segel model with singular sensitivity in a two-dimensional space (half-plane) with respect to the chemical diffusion rate denoted by ε ≥ 0. Compared to the one-dimensional boundary layer problem, there are many new issues arising from multi-dimensions such as possible Prandtl type degeneracy, curl-free preservation and well-posedness of large-data solutions. In this paper, we shall derive appropriate physical boundary conditions and gradually overcome these barriers and hence establish the convergence of boundary layer solutions of the singular Keller–Segel system in the half-plane as the chemical diffusion rate vanishes. Specially speaking, we justify that the boundary layer converges to the outer layer (solution with ε = 0) plus the inner layer as ε → 0 , where both outer and inner layer profiles are precisely derived and well understood. By doing this, the structure of boundary layer solutions is clearly characterized. We hope that our results and methods can shed lights on the understanding of underlying mechanisms of the boundary layer patterns observed in the experiment for chemotaxis such as the work by Tuval et al. [63] , and open a new window in the future theoretical study of chemotaxis models. [ABSTRACT FROM AUTHOR]

• The effective permeability model of rough and dense porous media is studied. • The effect of relative roughness on the effective permeability is analyzed. • The relationship between effective permeability and pore structure is discussed. The research in this paper explores the roughness of pores in rough and dense porous media using the model of rough capillary bundles. By taking into account the influence of boundary layer effects and microscale phenomena on fluid flow, a fractal model is derived to determine the efficient permeability of fluid flow in rough dense porous media. The connection between effective permeability and relative roughness and pore microstructure structure is discussed. It is revealed that the effective permeability exhibits an inverse proportionality to the relative roughness and viscosity, while concurrently displaying a direct proportionality to the pore fractal dimension. Furthermore, through the validation of experimental data and a comparative analysis with existing research models, it is evident that the model studied in this paper aligns more closely with real-world conditions. [ABSTRACT FROM AUTHOR]

In this paper, we explore a deep learning feedforward artificial neural network (ANN) framework as a numerical tool for approximating the solutions to singularly perturbed delay differential equations (SPDDE). Our approach trains the network with fewer uniform data points along with linear interpolation as a dependent variable. More importantly, the exact solution is not used for training the network. A mean square or Euclidean norm type total loss function is utilized to assess the deep learning network's performance. In the training and testing phase, our network adjusts itself to fit the tangent and the curvature by minimizing the total loss function and fine-tuning the network's hyperparameters during the backpropagation stage. Our focus in this contribution is to investigate an answer to the question- "whether neural networks can learn to solve the differential equations with both delay and boundary layer behavior"? Traditional numerical methods are known to perform poorly in approximating the solutions to SPDDE due to boundary layer behavior, which is the sharp change in the gradient of the solution inside a region of very small width. Our proposed network architecture is used to investigate the above question for three varieties of SPDDE. The numerical results demonstrate that the fine-tuned adaptive deep learning architecture can effectively approximate the solution to SPDDE for varieties of delay and perturbation parameters. It is expected that the current method can be extendable to approximate 2D convection-diffusion partial differential equations with a boundary layer. • The paper presents a deep learning feedforward neural network method to approximate the solution to singularly perturbed delay differential equations. • The network is trained without the exact solution and by using only few data points. The algorithm is built to adjust itself for the curvature in the problem. • A fine tuning of the network hyper parameters is done using backpropagation stage. • The efficacy of the method is shown by solving different second order singularly perturbed differential equations. [ABSTRACT FROM AUTHOR]

This paper presents computational fluid dynamics (CFD) results of a transonic transport aircraft wing, featuring a hybrid laminar flow control (HLFC) and variable camber (VC) technology coupling. The goal of the paper is a quantitative assessment of synergistic effects for aerodynamic drag reduction, when combining the possibility of actively shaping the surface pressure distribution of the wing through VC with the passive natural laminar flow aspect of HLFC. Modeling approaches for both VC integration, achieved through deflections of an Adaptive Dropped Hinge Flap (ADHF), and boundary layer suction into a CFD workflow are presented. Transition prediction is accomplished with the γ − R e θ + Cross-flow (C F) transition turbulence model, and linear stability theory (LST) coupled to a two- N -factor transition prediction method. Both methods reflect synergy-driven effects. While the extent of laminar flow predicted by the γ − R e θ + C F model is limited, both favorable ADHF settings and boundary layer suction lead to an overall drag reduction. This is correspondingly reflected through the analyses via LST. Furthermore, the latter predicts a pronounced downstream shift in transition position with increasing suction strength, where the inherent differentiation of transition mechanisms in the two- N -factor method between Tollmien-Schlichting (TSI) and cross-flow instabilities (CFI) reflects a marked reduction in TSI N -factors with increasing ADHF deflection angles. Even though N -factors associated to CFI partially increase through ADHF deflections, the intended synergistic effects are represented throughout the investigated parameter range, leading to an increase in the extent of laminar flow for adequate ADHF deflection angles. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of film cooling characteristics of pressure-side film hole on trailing-edge cutback in the high-pressure vane of gas turbines. This paper employs the Pressure Sensitive Paint (PSP) technique and Particle Image Velocimetry (PIV) technique to obtain the time-averaged adiabatic film cooling effectiveness and details of the mixing region, respectively. Numerical simulations with the DDES turbulence model are also used to predict the flowfields. Two configurations are examined: one with a cutback without pressure-side film holes (Config A) and another with a cutback that includes pressure-side film holes (Config B). This paper investigates eight blowing ratios (M slot = 0.3–1.2) of slot coolant in cutback for Config A and a suitable blowing ratio (M hole = 0.4) of film holes for Config B, using both experimental and numerical methods. The results of both experiments and simulations for Config A and B show good agreement and reveal a non-monotonic trend of film cooling effectiveness with increasing blowing ratio. The non-monotonic trend observed in Config A is attributed to an increase in the velocity gradient in the boundary layer on the coolant side with increasing blowing ratio, which leads to a different shedding vortex structure. The results for Config B show that the coolant from the pressure-side film holes changes the mixing structure downstream of the lip by rebuilding the mainstream's shear layer on the mainstream side. Adiabatic film cooling effectiveness increases significantly under low and moderate slot blowing ratios, but non-monotonicity is maintained. The Fast Fourier Transform method is used to investigate the impact of the jet stream, which affects only the area downstream of the film holes, while other regions along the pitch-wise direction are less affected. This paper also attempts to combine the effectiveness empirical correlations of 2D slot and film holes using the superposition model to quantitatively analyze the deterioration of cooling effectiveness resulting from the mixing of coolant and mainstream. [ABSTRACT FROM AUTHOR]

In this paper, a new linear active disturbance rejection control (LADRC) is proposed for servo systems based on active damping (AD), internal model control (IMC) and sliding mode control (SMC). The IMC rules are applied to tune the controller gains for a prescribed tracking performance. The AD method injects a virtual damping force into the motion system to actively attenuate disturbances. By deriving the sliding dynamics of IMC, a boundary layer solution of SMC is employed to enhance the robustness of control system. As the improved robust IMC scheme is applied as the controller of LADRC, the uncompensated disturbance of linear extended state observer can be better suppressed. With this method, a higher tracking control accuracy is expected to be obtained in the presence of uncertain dynamics. Theoretical stability was analyzed based on the Lyapunov method. Comparative experiments were conducted on a permanent magnet synchronous motor to validate the superiority of the proposed approach. • Servo systems may encounter large amount of uncertainties and load disturbances. • Active damping (AD) method is applied for a better damping condition. • Robust internal model control (IMC) is designed via sliding mode approach and AD method. • A linear active disturbance rejection control (LADRC) is proposed via robust IMC scheme. • The proposed LADRC has superior performance over some of state-of-the-art methods. [ABSTRACT FROM AUTHOR]

• A steam condensation model and an entropy generation approach are developed. • The non-equilibrium condensation characteristics and irreversible loss under different swirl strengths are investigated. • To maintain optimal performance of the nozzle, it is advisable to limit the swirl intensity to a value below 0.30. The energy loss within a supersonic separator is highly dependent on the cyclonic intensity. In this paper, an entropy production analysis model and a nonequilibrium condensation model are developed to study the liquefaction performance of the supersonic nozzle and the energy loss under different swirl intensities. With an increase in swirl intensity from 0 to 0.94, the nucleation starting point and Wilson point at the centerline move forward by 45.05 mm and 69.17 mm, respectively. The mass-average liquid fraction at the nozzle outlet increases by 28.81 %, but the flow rate decreases by 43.02 %, and the total entropy production experiences a 19.66 % increase. Indirect (or turbulent) dissipation dominates the total entropy production of wet steam flow resulting from the large gradient of velocity at the boundary layer, whereas phase change entropy production ranks second and mainly occurs in the nucleation region. Overall, the swirl intensity should be kept below 0.30 to achieve a balance between gas handling capacity, liquefaction performance, and energy loss for the supersonic nozzle. [Display omitted] [ABSTRACT FROM AUTHOR]

An exact similarity solution of Carreau fluid flow has been attempted in this paper. Whereas, the flow is maintained inside a channel of both parallel and inclined walls. The previous issues of pseudo-similarity solutions for such problems have been resolved here. Moreover, a single model is devised for the flow of Carreau fluid in both Poiseuille and Jeffery-Hammel type structures. Furthermore, the problem has exactly described the two previous (classical) simulations for flow of Carreau fluid. The present investigations have been carried out without incorporating the boundary layer approximation and stream function assumptions and we utilized the full form of constitutive equations. The system of PDEs is exactly converted into and exact system of ODEs and later the final equation are solved by a powerful and well established numerical tool. The analysis has been independently carried out for straight and inclined walls channels, whereas, the classical results of both types have been retrieved in special circumstances. It is observed that parabolic profiles for velocity spread over the width of the channel large value of Weissenberg number σ 1 < 0 and an increase in the velocity is noted for large σ 1 < 0. It demonstrates the nozzle flow of Carreau fluid inside a converging channel of inclined walls. [ABSTRACT FROM AUTHOR]

CO 2 -based binary mixtures are promising in energy engineering to improve efficiencies of thermodynamic cycles where supercritical CO 2 prevails. Indeed, under supercritical pressure and transcritical temperature conditions, special thermophysical properties of binary mixtures make the heat transfer process accompanied by enhanced cross-diffusion effects, i.e., the Soret effect and the Dufour effect. Their influences on heat transfer have not been well understood yet. This paper numerically investigates turbulent Rayleigh–Bénard convection in CO 2 - C 2 H 6 binary mixture. Calculations are performed based on a spectral element method solver considering various temperature differences and pressures. Results indicate that the flow field presents a double-vortex structure. The thickness of the boundary layer is negatively correlated with the flow intensity. Under a positive separation ratio, the concentration gradient induced by cross-diffusion effects enhances buoyancy, which in turn reinforces the natural convection and heat transfer. The enhancement becomes stronger as the critical pressure is approached. Turbulent fluctuations are also strengthened by cross-diffusion effects, except for temperature fluctuations. The unusual reduction in temperature fluctuations is attributed to the increased size of the thermal plume caused by the Dufour effect. • Transcritical temperature conditions and cross-diffusion effects are studied in Rayleigh–Bénard convection. • Time-averaged quantities and fluctuations for various fields are analyzed. • Present cases indicate heat transfer can be enhanced by cross-diffusion effects by more than 20%. • Unique behavior of temperature fluctuation is identified and explained. [ABSTRACT FROM AUTHOR]

• Effects of persistent trapped vortex ring on the impingement heat transfer was revealed. • The tunable Generalized k - ω model was used to calibrate the jet dynamics. • Heat transfer characteristics of the steady jet and sweeping jet impingement were compared. Jet impingement-based heat transfer enhancement is widely employed due to the significant mass, momentum, and energy it accompanied. While the steady jet with high energy flux is commonly favored, it still exhibit few defects, including heat transfer deterioration away from the stagnation point and mutual flow interference in the jet arrays, which are particularly critical in demanding applications. Recently, the sweeping jet has been proposed to resolve the above challenges owing to its self-excited and self-sustaining unsteady oscillation. However, current research predominately focuses on impinging on the flat surface, neglecting impinging on the concave surface, such as internal cooling at the gas turbine blade leading-edge as well as the hot gas de-icing at the aircraft wing leading-edge. Understanding the heat transfer characteristics in these scenarios is crucial as they notably differ from those on flat surfaces. Therefore, this paper investigates the heat transfer characteristics and the related essential flow dynamics of a sweeping jet with a Reynolds number of 2577 impinging on a concave surface, and those of a steady jet under the same conditions are compared. The jet-to-wall distance and the curvature of the impingement surface are 4 and 10 times the jet's hydraulic diameter, respectively. To correctly capture the three-dimensional characteristics of the sweeping jet, this research employs the Reynolds stress model for turbulence modeling and the Generalized k - ω (GEKO) model for better calibration of the jet's spreading and dissipation rates, as well as the characteristics of trapped vortex generated due to the confined concave flow channel. The study initially focuses on the average Nusselt number on the impingement surface, revealing a 15.2 % increase in heat transfer intensity of the sweeping jet compared to the steady jet, and the sweeping jet covers a larger area with enhanced uniformity. Subsequent analysis on streamlines in two perpendicular planes and Q criterion iso-surfaces in the spatial domain shows the formation of a trapped vortex ring with a constant location around the jet, restricting the effective cooling range to its inner side and thus resulting in a steep temperature rise on the two lateral sides of the vortex ring. The stability of the trapped vortex is enhanced by the curved impingement wall in the oscillation plane, which on the one hand forces the high-temperature fluid that has exchanged heat with the wall to repeatedly wash the same location, and on the other hand significantly reduces the jet oscillation angle compared to a free sweeping jet. Consequently, the heat transfer intensity of the sweeping jet is lower in the main oscillation plane than in the non-oscillation plane. Analysis of the near-wall flow parameters indicates that the sweeping jet generates higher velocity and turbulence kinetic energy in the wall jet zone far away from the stagnation point compared to the steady jet, which results in a higher local heat transfer intensity. The unsteady transverse oscillation of the sweeping jet disturbs the wall jet zone and disrupts the formation of the wall boundary layer, causing a higher velocity gradient at the wall and hence the lower thicknesses of the velocity boundary layer and the thermal boundary layer than those of the steady jet. In conclusion, this study reveals the flow and heat transfer features of the sweeping jet impingement on confined concave surface, with the influence of the trapped vortex ring mattering significantly. The findings provide a reference for designing a more effective jet arrangement. Future work will focus on investigating the effects of jet Reynolds number and impingement distance variations on the trapped vortex ring, and how to mitigate the adverse effects of the trapped vortex ring on impingement heat transfer. [ABSTRACT FROM AUTHOR]

• Effects of wind on the pool fire flame morphology and flow field were revealed. • Flame half-width of pool fires under wind were quantified using scaling analysis. • The crosswise profiles of heat flux and temperature show a similar trend. • The downstream crosswise heat flux was described by the Boltzmann function. This paper reveals the crosswise profiles of heat flux and temperature over the downstream surface (ground) of a pool fire in relative strong wind, which was not well addressed in the past. Two square gaseous burners (25 cm, 30 cm) are used with propane as the fuel producing different heat release rates to simulate pool fires. The wind speed ranges from 1.5 m/s to 4 m/s (with 0.5 m/s intervals for various cases), concerning relatively strong wind conditions (Fr > 0.87). The crosswise heat flux profile first shows to be nearly stable (fluctuation less than 20 %) in the continuous flame region, then a sharp decrease in the intermittent flame region and finally goes to near zero. The temperature profile shows a similar trend as heat flux. For the near stable region, the flame boundary layer goes into the counter-rotating streamwise vortex pairs to form the streak like flames. And the counter-rotating pair (CVP) on the flame makes the streaks on the sides of the burner to be even larger, as verified by the CFD simulation. The heat flux is different under the streaks and troughs of the flame (higher under trough than in streaks), which interprets the fluctuation in the nearly stable region. Then the sharp decrease happens in the intermittent flame region. The flame half-width is obtained, which sits at about the middle of the sharp decrease region. And finally, for the position further away in the crosswise direction, the profile turns to be nearly zero. The crosswise profiles can be well represented by a Boltzmann function. The obtained experimental results and the proposed functions on the crosswise profiles of heat flux and temperature provide an essential base to estimate the heat feedback to the downstream surface of a wind-driven fire. [ABSTRACT FROM AUTHOR]

• Effect of acoustic excitation on separated boundary layer is experimentally studied. • Influence of Reynolds number and sound pressure level is examined. • The Kelvin-Helmholtz instability and the Klebanoff streaks have been identified. • Acoustic excitation accelerates the l-t transition onset. • Acoustic excitation leads to shrinking or suppression of separation bubble. The paper discusses experimental results on the effect of broadband acoustic excitation on laminar-to-turbulent transition in a separated shear layer developing on a flat plate subjected to an adverse pressure gradient (APG) and freestream turbulence level equal to Tu ≅ 1 %. The study encompasses the influence of Reynolds number (Re x = 185 000 and 370 000) and sound pressure level (SPL). The inherent complexity of the problem is simplified by providing an acoustic excitation from a controlled source (loudspeaker), acting on the boundary layer developing on the flat plate with a given streamwise pressure gradient. Two types of instabilities were identified in the pre-transitional boundary layer in unexcited flows. One was related to the inviscid Kelvin-Helmholtz (K-H) instability, while the second one was associated with formation of streamwise-oriented Klebanoff streaks (so-called Klebanoff mode). In the low Reynolds number case (Re x = 185 000), the K-H was responsible for transition onset, while in the high Reynolds number flow (Re x = 370 000), the Klebanoff distortions dominated the turbulent breakdown with the minor effect of the K-H instability. In addition to the naturally developing boundary layer, the flow was exposed to a pink noise characterized by SPL = 125 dB and 135 dB. In the low Reynolds number case, the acoustic excitation enhanced the K-H instability. It resulted in an earlier laminar-to-turbulent transition in case with higher sound pressure level (135 dB). In the high Reynolds number flow, the acoustic excitation enhanced the mixed-type transition mechanism with dominant role of the Klebanoff streaks. Shrinking or complete suppression of the separation bubbles was observed, depending on the applied sound pressure level (125 and 135 dB). [ABSTRACT FROM AUTHOR]

In this paper we study the boundary layer flow of a particular class of non-Newtonian fluids, namely piezo-viscous fluids, past a stretching wedge (Falkner–Skan flow). These fluids are essentially Navier–Stokes fluids with pressure-dependent viscosity. We consider the two-dimensional steady flow and introduce some similarity variables, so that the problem is reduced to a third-order nonlinear BVP. We solve the problem numerically by means of a shooting procedure combined with Newton's method, and we find local similarity solutions. We investigate the behaviour of the velocity field, of the drag and of the thickness of the boundary layer for different values of the involved parameters, making a comparison between the piezo-viscous and the Newtonian case. We find that, depending on some material parameters, the effect of pressure is that of reducing or increasing the boundary layer with respect to the Newtonian case. • We investigate the Falkner–Skan boundary layer flow of a piezo-viscous fluid. • We consider a deformable wedge with suction/injection. • We look for local self-similar solution. • We solve the problem numerically for the main variables of the problem. • We discuss the effects of the pressure on the solution, making a comparison with the Newtonian case. [ABSTRACT FROM AUTHOR]

The moving discontinuous Galerkin method with interface condition enforcement (MDG-ICE) is a high-order, r -adaptive method that treats the grid as a variable and weakly enforces the conservation law, constitutive law, and corresponding interface conditions in order to implicitly fit high-gradient flow features. In this paper, we develop an optimization solver based on the Levenberg–Marquardt algorithm that features an anisotropic, locally adaptive penalty method to enhance robustness and prevent element degeneration in the computation of hypersonic, viscous flows. Specifically, we incorporate an anisotropic grid regularization based on the mesh-implied metric that reduces grid motion in directions with small element length scales, an element shape regularization that mitigates excessive element curvature, and a penalty regularization that penalizes degenerate elements. Additionally, we introduce a procedure for locally scaling the regularization operators in an adaptive, elementwise manner in order to maintain grid validity. We apply the proposed MDG-ICE formulation to two- and three-dimensional test cases involving viscous shocks and/or boundary layers, including Mach 17.6 hypersonic viscous flow over a circular cylinder and Mach 5 hypersonic viscous flow over a sphere, which are very challenging test cases for conventional numerical schemes on simplicial grids. Even without artificial dissipation, the computed solutions are free from spurious oscillations and yield highly symmetric surface heat-flux profiles. [ABSTRACT FROM AUTHOR]

• The paper reports results of first experiment on three tandem cylinders with helical strakes. • Three vibration modes are proposed through analysis of amplitude and frequency. • Helical strakes can reduce multi frequency instability phenomenon. • The best WIG suppression occurs in "S" helical strakes, and the vibration suppression efficiency of up to 665 %. • Further explanation of WIG suppression mechanism through vortex dynamics. Helical strakes as a classic passive control method for vortex-induced vibration (VIV), have been extensively studied, but wake-induced galloping (WIG) control of three tandem cylinders has received no previous attention. The current wind tunnel experimental study tries to fill this gap in the context of mechanical science. WIG of three tandem cylinders considering different structures of helical strakes is modeled and investigated. Helical strakes with varying heights G , pitches P , and cross-sectional shapes (square "S", involute "I", and D-shaped "D") are first designed. Wind tunnel tests are conducted to experimentally study the dynamics responses of tandem cylinders (m*ξ = 0.09) at different spacings, and three vibration modes are obtained. This study delves into the characterization and quantification of the impact of various parameters of helical strakes on WIG of three tandem cylinders. These aspects, scarcely addressed in existing research, are thoroughly explored through statistical analysis, focusing on vibration amplitude, vibration frequency, and wavelet analysis. It is found that helical strakes can effectively suppress the multi-frequency vibration instability of bare cylinders. For wind speeds below 6 m/s, the "S" helical strakes achieves a vibration suppression efficiency of up to 665 %; Three-dimensional computational fluid dynamics (3D-CFD) demonstrate the coupling effect in the flow field, revealing that the "S" helical strakes has a greater impact on boundary layer formation than the "D" helical strakes; Reducing the pitch of the helical strakes can enhance the disturbance ability of the spanwise position, thereby increasing its suppression effect. This work improves vibration reduction and sheds light on how the helical strakes' cross-section controls vibrations in the tandem cylinders. [Display omitted] [ABSTRACT FROM AUTHOR]

Inverters are essential in converting solar panel-generated direct current (DC) into alternating current (AC) for seamless integration with electrical grids. This paper presents the modeling and analysis of a single-phase grid-connected H-bridge neutral point clamped inverter using the Sliding Mode Control (SMC) method. Addressing challenges in meeting control objectives – like fast dynamic response, precise reference tracking, robustness against parameter variations, cost-effectiveness, and compliance with grid standards such as low harmonic distortion and minimal DC transfer to the grid – the study employs the boundary layer method to minimize the SMC method's chattering phenomenon. The use of proportional-resonant (PR) control maintains the robustness of the SMC method against parameter changes, while the boundary layer effectively reduces chattering. A transfer function between the measured grid current and the reference grid current was also derived to investigate the effect of the filter parameters on the system. Furthermore, the proposed control method naturally mitigates resonance in the LCL filter and minimizes leakage current via a symmetrical LCL filter and specialized modulation scheme. Simulations conducted using MATLAB/Simulink® validate achievement of control objectives, steady-state performance, dynamic response, robustness to parameter variations, and adherence to grid standards. [ABSTRACT FROM AUTHOR]

This paper considers a two-dimensional singularly perturbed reaction-diffusion equation with a discontinuous source term. Due to this discontinuity, interior, corner, and boundary layers appear in the solution for adequately small values of the perturbation parameter ϵ. To achieve a decent estimate of the solution, we construct a numerical approach adopting an efficient hybrid finite difference method that includes a proper layer adapted piece-wise uniform Shishkin mesh. Further, we prove that the hybrid finite difference method is almost second-order uniformly convergent with respect to the perturbation parameter. We have implemented our method to test examples. Numerical results are verifying the theoretical results. [ABSTRACT FROM AUTHOR]

Highlights • Gas injection increases a maximal velocity within the boundary layer. • Gas suction decreases an overshoot phenomenon till his absolute vanishing. • Separation of thermal boundary layer at gas injection. • Conventional point of thermal boundary layer separation is proposed. Abstract The paper reports on numerical simulation of an accelerated laminar boundary layer over the heated permeable wall. Xenon was used as a working gas. The finite difference method was applied to solve the system of laminar boundary layer equations with variable gas properties. It has been shown that the gas injection in the mentioned conditions increases maximal velocity within the boundary layer and decreases skin-friction coefficient and thermal Stanton number. On the contrary, gas suction decreases an overshoot phenomenon down to its vanishing and increases skin-friction coefficient and thermal Stanton number up to the value at asymptotic suction. The paper considers a separation of thermal boundary layer at gas injection. The proposed conventional point of this separation is situated farther from the leading edge than the separation point of the dynamic boundary layer. [ABSTRACT FROM AUTHOR]

Abstract A numerical study aiming at the enhancement of the operating range of supersonic cascade via boundary layer suction is presented. The purpose of this paper is to improve the maximum backpressure ratio of the flowfield by suction control and to learn the flowfield behavior and characteristics with different suction control parameters. An S-shaped supersonic cascade developed and tested at DLR (DLR-PAV-1.5) that operates with a relative entrance Mach number of 1.5 is selected in this paper. A two-dimensional multiblock N S solver has been successfully applied to the cascade flow which is validated using the data of the experiment of Schreiber. An in-deep numerical study has been carried out to investigate the effects of suction locations, widths and angles by comparing the operating range and aerodynamic performance parameters of the supersonic cascade. The results indicate that the maximum backpressure ratio can be enhanced by 17.4% with a loss of 3% mainstream mass flow by suction slot in 60% or 70% length of the axial chord. As the width of the suction slot increases to 7 mm, the maximum backpressure ratio is enhanced by 24%, and the total pressure recovery coefficient is improved to 95.3%. The range of suction slot angle that can effectively improve the maximum backpressure ratio is 60–90° and −90 to −60°. This paper provides guidance for the supersonic cascade to improve its performance parameters through suction control. Highlights • Shock-wave evolution with the increase of static pressure ratio was discussed. • Influence of suction static pressure ratios on suction efficiency was investigated. • Flowfield characteristics with boundary layer suction were reported and studied. • Effect of suction parameters on supersonic cascade performance was analyzed. [ABSTRACT FROM AUTHOR]

• Pool boiling at atmospheric and sub-atmospheric pressure using water. • Comparative assessment of contributing mechanisms to bubble growth. • Comparison of models of bubble growth with new experimental results. • Recommendation for predicting bubble departure time and bubble radius. • Recommendation for bubble growth rates. Prediction of bubble growth rate is very important for the development of accurate models for bubble departure diameter and thus the heat transfer rates in nucleate boiling. This paper presents an evaluation study to the existing homogeneous and heterogeneous bubble growth models using our experimental data for bubble growth in saturated pool boiling of deionized water on a plain copper surface. The experiments were conducted at pressures 1, 0.5 and 0.15 bar and superheat in the range 5.1 – 19.5 K. To start with, the paper presents a critical review on bubble growth models in homogeneous and heterogeneous boiling. It was found that homogeneous growth models achieved some partial agreement with the experimental data at some conditions and thus they should be used carefully in heterogeneous boiling. There was a good agreement between some of the models that were suggested based on the assumption that bubble growth occurs due to evaporation from the superheated boundary layer around the bubble. The models based on microlayer evaporation only could not explain the experimental data, i.e. partial agreement at some conditions. The model that predicted the data very well at all conditions was the "relaxation boundary layer" model by Van Stralen [25]. This model was generalized in the current study by suggesting two new empirical models for the departure diameter and departure time. [ABSTRACT FROM AUTHOR]

The accumulation of aerodynamic heat at the rudder shaft of a hypersonic vehicle will reduce the vehicle's aerodynamic performance. This paper proposes a trailing-edge curved-surface jet with longitudinal characteristics to solve this problem. We used high-speed photography and force measurement techniques in the experiment. The results show that the interaction between the expansion wave formed by the curved surface jet and the hypersonic flow will generate an oblique shock wave and finally form a unique flow structure. The oblique shock wave in the new flow structure is the key to realizing the rudderless flight, and it can linearly control the pitching characteristics of the hypersonic airfoil. That is, it can linearly control the attitude of the model. The diffusion angle of the jet has a significant influence on this oblique shock wave. Experiments show that the dimensionless parameter pressure ratio PR and the jet diffusion angle roughly present a linear relationship. Therefore, there is approximately a linear relationship between PR and the flow control effect of the curved-surface jet. When the PR is within 90∼250, the curved-surface jet can increase the pitching moment coefficient by 0.033∼0.132. The dynamic mode decomposition results show that expansion wave, jet instability, oblique shock wave, and boundary layer are the dominant factors affecting the new flow structure. This paper proposes a new theoretical model to reveal the flow control effect and the physical mechanism of the curved-surface jet. The results provide an experimental and theoretical basis for realizing rudderless hypersonic vehicles. [ABSTRACT FROM AUTHOR]

Boundary layer transition is an important factor in the design process of hypersonic vehicles, and crossflow instability is a critical mode. In this paper, the traveling crossflow instability in a laminar boundary layer at Mach 6 is experimentally investigated. Experiments were performed over a 7 ∘ half-angle cone at 6 ∘ angle of attack, with the unit Reynolds number of R e ∞ = 5. 75 × 1 0 6 m − 1. Flush-mounted fast-response pressure transducers and time-averaging temperature-sensitive paint were used to capture the traveling crossflow instability and the stationary crossflow instability, respectively. The traveling crossflow waves with characteristic frequencies between 10 and 25 kHz are detected in the planes with the azimuthal angle between 23 ∘ and 143 ∘ away from the leeward ray. The three-dimensional amplitude growth was obtained. In general, the amplitude growth of the traveling crossflow instability is roughly leeside-forward and windside-aft. Although the traveling crossflow instability grows earlier on the leeward side, it has a larger amplitude on the windward side. Compared with the stationary mode, the traveling mode grows faster and reaches the highest amplitude earlier. In addition, a high-frequency instability with characteristic frequencies between 100 and 200 kHz is observed. It may be the secondary instability of the traveling crossflow waves, but whether it is true or not needs further experimental and numerical studies. • In a hypersonic flow, the three-dimensional growth of the traveling crossflow instability on a yawed cone was measured. • Compared with the stationary crossflow instability, the traveling crossflow instability grows faster and reaches the highest amplitude earlier. • A high-frequency instability with f = 100–200 kHz was found, and it may be the secondary instability of the traveling crossflow waves. [ABSTRACT FROM AUTHOR]

The complexity of blasting designs and the extensive computational cost pose significant challenges in accurate prediction using three-dimensional numerical methods. This paper proposes a feasible approach for engineering-scale blasting numerical modelling with the integration of charge structures and layout design in the four-dimensional lattice spring model (4D-LSM). The approach enables the 4D-LSM to simulate intricate blasthole designs and varied charge structures through theoretical derivation and equivalence analysis of blasthole boundary conditions and initiation point, and significantly improves the computational efficiency by combining the submodel method. The accuracy and correctness of the 4D-LSM blasting model were verified through qualitative and quantitative comparison with available single-hole blasting experiments and other numerical predictions. On this basis, the blasting design of the vertical shaft was optimized by combining the 4D-LSM blasting model validated by field monitoring data with the response surface method (RSM). In addition, the applicability of the 4D-LSM blasting model was discussed by examining the effects of particle size, number of boundary layers, and dynamic constitutive model parameters on the calculation results. This study not only further improves the application of the 4D-LSM in the field of large-scale blasting engineering, but also provides an efficient numerical calculation tool for complex blasthole design. [ABSTRACT FROM AUTHOR]

This paper presents a set of high-performing heat sinks that exhibit twice the thermohydraulic performance in terms of conductance compared to conventional rectangular fin heat sinks. The heat sinks presented here are designed through three-dimensional topology optimization (TO), manufactured using additive manufacturing (AM), and their performance is validated through experimental testing. The heat sink design is governed by steady-state Navier-Stokes equations and the energy equation. The objective is to minimize the average temperature of the heat source surface with a constant heat flux. Our design process incorporates two constraints: the pressure drop constraint and the project undercut perimeter (PUP) based overhang angle constraint. The incorporation of the overhang angle constraint ensures that the optimized heat sink design is self-supported and amenable to additive manufacturing without the need for additional support structures. Post-optimization CFD investigations revealed that the optimized heat sink offers improved thermal performance, attributed to 2 kinds of three-dimensional convection effects, thermal boundary layer re-initialization, and efficient mixing. The optimized heat sink designs are manufactured using laser-powder bed fusion process, an additive manufacturing technique, and their superior performance relative to a conventional rectangular heat sink is validated through experimental measurements. The experimental tests are in good agreement with CFD simulations, confirming a remarkable 100% increase in conductance for the TO designs compared to a conventional heat sink. • Geometrically intricate heat sink design: achieved through topology optimization and additive manufacturing. • Improved thermohydraulic performance: twice the heat conductance over rectangular fin design. • Cooling mechanism: optimized designs disrupt boundary layer, enables full-field mixing of cold and hot flow. • Smaller surface area: optimized designs possess smaller surface area than rectangular fins. [ABSTRACT FROM AUTHOR]