Flow Boundary Research Articles

BFANet: Effective segmentation network for low altitude high-resolution urban scene image

The semantic segmentation of low altitude high-resolution urban scene images taken by UAV plays an important role in city management. However, such images have the characteristics of inter-class homogeneity and intra-class heterogeneity. How to segment these images quickly and accurately is still challenging. In this paper, we propose a novel double-branch network. For the challenge of inter-class homogeneity, a boundary flow module is designed to enhance the flow of latent semantic information between two branches by imposing boundary constraints between classes. To alleviate intra-class heterogeneity, a context extraction module based on adaptive dynamic fusion is designed, which effectively captures the long-term relationship of features with very low parameters. Experiments on two typical datasets show that our approach achieves the best balance between accuracy and speed. Specifically, we achieve 65.8% mIoU and 74.1% mIoU on UAVid test set and UDD validation set respectively, and 60FPS on an NVIDIA TITAN Xp.

Canards in a bottleneck

In this paper, we investigate the stationary profiles of a nonlinear Fokker–Planck equation with small diffusion and nonlinear inflow and outflow boundary conditions. We consider corridors with a bottleneck whose width has a unique global nondegenerate minimum in the interior. In the small diffusion limit, the profiles are obtained constructively by using methods from geometric singular perturbation theory (GSPT). We identify three main types of profiles corresponding to: (i) high density in the domain and a boundary layer at the entrance, (ii) low density in the domain and a boundary layer at the exit, and (iii) transitions from high density to low density inside the bottleneck with boundary layers at the entrance and exit. Interestingly, solutions of the last type involve canard solutions generated at the narrowest point of the bottleneck. We obtain a detailed bifurcation diagram of these solutions in terms of the inflow and outflow rates. The analytic results based on GSPT are further corroborated by computational experiments investigating corridors with bottlenecks of variable width.

Numerical simulation on flow and heat transfer mechanism of n-decane in tube with round protrusions under supercritical pressure

The numerical simulation about flow and heat transfer characteristics of n-decane under supercritical pressure in the tube with round protrusions is researched in this study. The heat transfer enhancement mechanism of protuberant tube is conducted by realizable k-ε model. The average outer wall temperature of protuberant tube is reduced by approximately 60–80 K and the Nu is 45–55 greater rather than smooth structure within the operating cases. When comparing with smooth tube, the thermal performance of protuberant tube increased by 62.5%–81.4%, and the whole heat transfer performance increased by 15.6%–32.4%. The obvious heat transfer enhancement of protuberant tube is attributed to the effective destruction of thermal and flow boundary layer. Forced mixing effects of protrusions enhance flow and heat transfer further. Besides, the effects of protrusion height, pitch and circumferential number on flow and heat transfer performance are also studied. The results reveal that the increase of protrusion height and circumferential number can simultaneously rise the Nuc/Nus and fc/fs, while the increase of protrusion pitch is contrary. Among all working conditions, the protuberant tube of H = 0.5 mm, P = 4 mm and Nc = 6 obtains the largest PEC = 1.35.

The onset of turbulence in decelerating diverging channel flows

This work investigates the stability and transition to turbulence in a diverging channel subjected to a time-varying trapezoidal-shaped inflow boundary condition. Numerical simulations are performed for different deceleration rates and Reynolds numbers while maintaining a constant acceleration rate. The flow transition begins with two-dimensional primary instability with the formation of inflectional velocity profiles, followed by local separation and the emergence of an array of shear layer vortices. We divide simulation cases systematically into three categories based on the onset of secondary instability and the generation of streamwise vorticity. At low and medium Reynolds numbers (type I), the spanwise vortex rolls formed by inflectional instability remain two-dimensional and diffuse at the channel centre without exhibiting further instabilities. At high Reynolds numbers and deceleration rates (type II), the rolled shear layer exhibits secondary instability during the zero mean inflow phase, followed by local incipient turbulent structure formation. The streamwise vorticity that develops over the shear layer structures causes oscillations with a spanwise wavelength similar to those associated with the elliptic instability in a counter-rotating vortex pair. Using the Lamb–Oseen approximation of vortices in conjunction with the dynamic mode decomposition algorithm of the three-dimensional flow field, we captured successfully the characteristics of the secondary instability. The third category (type III) is characterized by periodic unsteady separation, secondary instability, and merging of shear layer vortices, which occurs when Reynolds numbers are high and deceleration rates are low.

Optimal control flow encoding for time-efficient magnetic resonance velocimetry

Phase contrast velocimetry relies on bipolar gradients to establish a direct and linear relationship between the phase of the magnetic resonance signal, and the corresponding fluid motion. Despite its utility, several limitations and drawbacks have been reported, the most important being the extended echo time due to the encoding after the excitation. In this study, we elucidate a new approach based on optimal control theory that circumvents some of these disadvantages. An excitation pulse, termed FAUCET (flow analysis under controlled encoding transients), is designed to encode velocity into phase already during the radiofrequency excitation. As a result of concurrent excitation and flow encoding, and hence elimination of post-excitation flow encoding, FAUCET achieves a shorter echo time than the conventional method. This achievement is a matter of significance not only because it decreases the loss of signal due to spin–spin relaxation and B0 inhomogeneity, but also because a shorter echo time is always preferred in order to reduce the dimensionless dephasing parameter and the required residence time of the flowing sample in the detection coil. The method is able to establish a non-linear bijective relationship between phase and velocity, which can be employed to enhance the resolution over a specific range of velocities, for example along flow boundaries. A computational comparison between the phase contrast and optimal control methods reveals that the latter’s encoding is more robust against remnant higher-order-moment terms of the Taylor expansion for faster voxels, such as acceleration, jerk, and snap.

Steady solver for incompressible Navier-Stokes equation with automated adaptive mesh refinement using FEniCS

This study demonstrates steady incompressible flow over a cylinder section using the finite element method. An open-source framework was used to increase the accuracy of the results of the analysis. The Python-based open-source computing platform FEniCS was used for the problem formulation. The mesh generator used is Gmsh. FEniCS offers automated adaptive mesh refinement and higher-order elements of order three and higher, which produce more accurate results. These features are not available in most FEM software. The problem DFG benchmark 2D-1 in Featflow was analyzed using the open-source framework. The drag and lift coefficients and the pressure difference between the front and back of the cylinder were found and compared with the benchmark values. The results obtained prove that the proposed open-source framework is incredibly effective in solving fluid flow problems. The FEniCS code to perform this DFG 2D-1 benchmark analysis is given in this work. It shows the problem formulation, setting of boundary conditions, and the adaptive solver. Thus, the same open-source framework can be used to analyze any laminar incompressible flow by simply creating a different geometry and varying the flow characteristics and boundary conditions.

Research on the Flow Field of Electrolytic Processing for Shaped Holes with Large Depth-to-diameter Ratio

For parts with large deep-diameter ratio shaped hole features, the complex structure of the profile and large deep-diameter ratio led to poor profile machining accuracy. The shaped hole processing requires high efficiency, high precision and low cost. Therefore, it is difficult to use the traditional processing method, and the electrolysis method based on the anodic dissolution mechanism is adopted, which has no shear stress and accurate profile forming. The 3D model is established based on the part, and the cathode and tooling structure is designed. Based on COMSOL software, we designed the flow channel uniformity of electrolytic processing of shaped holes and analyzed the arrangement of cathode outlet holes in electrolytic processing, which affects the uniformity of the processed surface and flow field. The arrangement of cathode exits holes to meet the uniform distribution of electrolyte at the surface boundary and high-speed flow at the surface was obtained by simulation analysis. By analyzing the flow field of different inlets, the flow rate magnitude of the shaped boundary and surface of the surface shaped hole is positively correlated with the inlet pressure of the electrode liquid.

The linearized 3d Euler equations with inflow, outflow

In 1983, Antontsev, Kazhikhov, and Monakhov published a proof of the existence and uniqueness of solutions to the 3D Euler equations in which on certain inflow boundary components fluid is forced into the domain while on other outflow components fluid is drawn out of the domain. A key tool they used was the linearized Euler equations in vorticity form. We extend their result on the linearized problem to multiply connected domains and establish compatibility conditions on the initial data that allow higher regularity solutions.

The circulation growth of non-impulsive starting jet

An analytical model that can be used to predict the circulation growth process for non-impulsive starting jets has been developed by extending the over-pressure correction model for impulsive starting jets. Only the jet velocity function U0(t) is needed to obtain the circulation growth process of non-impulsive starting jet. The non-impulsive starting jets generated by nozzle and orifice configurations are performed numerically with two acceleration schemes (represented by two kinds of velocity functions) and different acceleration stage stroke ratios LaD (modifying the acceleration time ta in each velocity function to obtain LaD=0.05:0.2:1.25) to verify the accuracy of the proposed analytical model. The influence of radial velocity distribution in the jet inflow boundary is ignored in the process of constructing this model, but this has been proven to be reasonable. The time-averaged relative error with the growth process of the total circulation predicted by this model is about 11% to 26%, which is smaller than the 44% to 88% obtained by the classical slug model. For the range of LaD from 0.05 to 1.25, the relative error with the prediction of the total circulation at the end of the acceleration stage is from 4% to 28%, as compared to the 39% to 96% for the slug model.

Global Stability Analysis of the Interaction Between a Longitudinal Vortex and an Oblique Shock Wave

AbstractIn this study, the interaction of a longitudinal vortex with an oblique shock was studied numerically. A global stability approach was used to investigate the onset of unsteadiness due to vortex breakdown. For this purpose, steady and unsteady laminar flow simulations were performed. A longitudinal vortex with an exponentially decaying tangential velocity profile, a so‐called Erlebacher vortex, was introduced into the flow field by a modification of the inflow boundary, while the oblique shock was created by a ramp in supersonic flow. Parametric studies concerning the influence of the axial velocity deficit and the tangential velocity component on the stability of the vortex were performed. Several amplified global eigenmodes could be identified and it could be shown that the frequencies predicted by the global stability analysis were in reasonable agreement with dominant frequencies observed in the unsteady simulations.

Pore-scale study on the stress jump coefficient in porous composite system

The stress jump coefficient at the fluid/porous interface is a fundamental parameter to study the velocity distribution in a porous composite system. In this paper, a substantial work is carried out to investigate the characteristics of the stress jump coefficient. To this end, the real pore structure of metal foam is constructed using the Weaire–Phelan model, and the macro model and the pore-scale model are presented to simulate a complex three-dimensional porous composite system. Furthermore, a novel method to determine the stress jump coefficient is proposed. The influences of the inlet velocity, the rotation number, the porosity, the free fluid layer thickness, and the flow pattern (the Poiseuille flow, the free boundary flow, and the rotating channel flow) on the stress jump coefficient are studied. The results show that the stress jump coefficient varies with the porosity, which shows that it is dependent on the porous structure. It also found that the stress jump coefficient is independent of the inlet velocity, the rotation number, and the flow pattern. When the thickness of the free fluid layer is large, the stress jump coefficient is also independent of the thickness of the fluid layer.

Analytical investigation of a grain boundary model that accounts for slip system coupling in gradient crystal plasticity frameworks

In this work, a physically based dislocation theory of plasticity is derived within an extended continuum mechanical context. An orientation-dependent grain boundary flow rule is introduced for the modelling of dislocation pile-up at grain boundaries and dislocation transmission through grain boundaries. With the conventional grain boundary modelling approach according to Gurtin (Gurtin. 2008 J. Mech. Phys. Solids 56 , 640–662. ( doi:10.1016/j.jmps.2007.05. 002 )) the single-crystal consistency check for the limit case of adjacent grains that hold no misorientation is not satisfied. In order to overcome this modelling shortcoming, a slip system coupling based on a geometric measure of slip system compatibility is introduced. In order to investigate the grain boundary modelling approaches, the analytical solution of a three-phase periodic laminate is used to study the interactions of dislocations and grain boundaries within the gradient crystal plasticity framework. With the developed grain boundary model two grain boundary states, i.e. microhard and microcontrolled, are observed for misaligned grains. This allows the modelling of slip activation at grain boundaries based on the dislocation pile-up stress.

A singularly perturbed convection-diffusion parabolic problem with incompatible boundary/initial data

A singularly perturbed parabolic problem of convection-diffusion type with incompatible inflow boundary and initial conditions is examined. In the case of constant coefficients, a set of singular functions are identified which match certain incompatibilities in the data and also satisfy the associated homogeneous differential equation. When the convective coefficient only depends on the time variable and the initial/boundary data is discontinuous, then a mixed analytical/numerical approach is taken. In the case of variable coefficients and the zero level of compatibility being satisfied (i.e. continuous boundary/initial data), a numerical method is constructed whose order of convergence is shown to depend on the next level of compatibility being satisfied by the data. Numerical results are presented to support the theoretical error bounds established for both of the approaches examined in the paper.

Numerical and Experimental Analysis of the Cold Flow Physics of a Nonpremixed Industrial Gas Burner

Abstract The flow field of a nonpremixed industrial gas burner is analyzed with Reynolds-averaged Navier–Stokes computational fluid dynamics validated against velocity and pressure measurements. Combustion is not modeled because the aim is optimizing the predictive capabilities of the cold flow before including chemistry. The system's complex flow physics, affected by a 90 deg turn, backward and forward facing steps, and transversal jets in the mainstream is investigated at full and partial load. The sensitivity of the computed flow field to inflow boundary condition setup, approach for resolving/modeling wall-bounded flows, and turbulence closure is assessed. In the first sensitivity analysis, the inflow boundary condition is prescribed using measured total pressure or measured velocity field. In the second, boundary layers are resolved down to the wall or modeled with wall functions. In the third sensitivity analysis, the turbulence closure uses the k−ω shear stress transport eddy viscosity model or two variants of the Reynolds stress model. The agreement between the predictions of most simulation setups among themselves and with the measurements is good. For a given type of inflow condition and wall flow treatment, the ω-based Reynolds stress model gives the best agreement with measurements among the considered turbulence models at full load. At partial load, the comparison with measured data highlights some scatter in the predictions of different patterns of the flow measurements. Overall, the findings of this study provide insight into the fluid dynamics of industrial gas burners and guidelines for their simulation-based analysis.

Rapid and integrated petroleum systems analysis with dynamic gas fraction and flexural reconstructions: Application to hydrocarbon charge and commodity risk

The most time consuming and computationally intensive steps in petroleum systems analysis involve building heat flow boundary conditions that are consistent with regional geologic observations, solving the transient heat equation, and translating a range of hydrocarbon charge predictions from source intervals into prospect specific predictions that can be used for decision making in oil and gas exploration. Furthermore, three-dimensional regional flexure, which can have a significant impact on paleo-migration pathways, often is not included in the critical step of translating charge calculations into trap specific predictions due to the time-consuming nature of map preparation. In this paper, an efficient and integrated petroleum systems analysis (IPA) workflow is presented with a new fast, semi-analytical, decoupled approach for solving the transient heat equation that is fully integrated with a rapid inversion methodology for transient heat flow boundary conditions and a 3D regional flexural reconstruction that uses standard basin modeling inputs and requires little to no additional map preparation. A charge-to-trap model is also described that translates hydrocarbon expulsion calculations with dynamic parameterized gas fractions that vary with transformation ratio into trap specific predictions using flexurally restored migration polygons. This fully integrated and efficient approach enables rapid interactive investigation of uncertainty space and fast probabilistic modeling that is ideal for evaluating hydrocarbon charge and commodity risk in data-sparse frontier basins. The applicability of this approach is demonstrated for assessing charge and commodity risk with a case study from the Colorado Basin, Argentina, whereby two alternative hydrocarbon accumulation hypotheses that postulate that traps are filled to structural spill with undersaturated oil are tested for reservoirs located in the upper Colorado Formation that are charged by postulated Aptian and Neocomian source rocks. This application indicates that it is unlikely that the tested traps are filled to structural spill with undersaturated oil, and how incorrect conclusions on hydrocarbon commodity may be reached if 3D regional flexure is not considered during petroleum systems analysis. Finally, it is also shown how this integrated and efficient approach can be implemented with low-cost computational platforms such as Python Anaconda or in spreadsheet programs that empower modelers to rapidly translate their ideas into benchmarked prototypes that can be immediately leveraged by the geoscience community (www.github.com/eakneller/ipatoolkit3D).

Boundaries in free higher derivative conformal field theories

We consider free higher derivative theories of scalars and Dirac fermions in the presence of a boundary in general dimension. We establish a method for finding consistent conformal boundary conditions in these theories by removing certain boundary primaries from the spectrum. A rich set of renormalization group flows between various conformal boundary conditions is revealed, triggered by deformations quadratic in the boundary primaries. We compute the free energy of these theories on a hemisphere, and show that the boundary a-theorem is generally violated along boundary flows as a consequence of bulk non-unitarity. We further characterize the boundary theory by computing the two-point function of the displacement operator.

Diffuse-Interface Blended Method for Imposing Physical Boundaries in Two-Fluid Flows.

Multiphase flows are commonly found in chemical engineering processes such as distillation columns, bubble columns, fluidized beds and heat exchangers. The physical boundaries of domains in numerical simulations of multiphase flows are generally defined by a conformal unstructured mesh which, depending on the complexity of the physical system, results in time-consuming mesh generation which frequently requires user-intervention. Furthermore, the resulting conformal unstructured mesh could potentially contain a large number of skewed elements, which is undesirable for numerical stability and accuracy. The diffuse-interface approach allows for the use of a simple structured meshes to be used while still capturing the desired physical (e.g., solid-fluid) boundaries. In this work, a novel diffuse-interface method for the imposition of physical boundaries is developed for the incompressible two-fluid multiphase flow model. This model is appropriate for dispersed multiphase flows which are pervasive in chemical engineering processes, in that this flow regime results in high levels of mass and energy transfer between phases. A diffuse interface is used to define the physical boundaries and boundary conditions are imposed by blending the conservation equations from the two-fluid model with that of the nondeformable solid. The results from the diffuse-interface method are compared with results from a conformal unstructured mesh for different interface functions and widths. For small interface widths, the accuracy of the flow profile is unaffected by the choice of interface function and the phase fraction distribution and flow behavior are within 3% compared to those from a conformal mesh. As the interface width increases, the diffuse-interface solution deviates from the conformal mesh solution in both the localized gas fraction and the overall gas hold-up, resulting in a difference up to 30%. In the case of flow past a cylinder, where the solid interacts with the flow, the presence of the diffuse interface extends the thickness of the solid boundary and results in a deviation from the conformal mesh solution as time increases.

Performance enhancement for membrane distillation by introducing insoluble particle fillers in the feed

Temperature and concentration polarizations are typical near-surface phenomena that limit the performance of membrane distillation. An effective method for enhancing the performance of membrane distillation is to mechanically disturb or break the polarization effect near the membrane surface. In this study, we focused on disturbing the flow boundary layer formed near the membrane surface to eliminate the influence of temperature and concentration polarizations on the membrane distillation performance by introducing insoluble particles as fillers into the feed. It is believed that the sliding/scrolling of insoluble particle fillers on the membrane surface is a practical approach for breaking the boundary layer on the membrane surface and eliminating near-surface polarization. The effects of breaking the near-surface polarization on the membrane distillation performance were systematically investigated via experiments under various conditions and verified with related computational fluid dynamics simulations. The test results demonstrated that the permeate flux, temperature polarization coefficient, and system efficiency of membrane distillation can all be improved with particle fillers. The permeate flux does not increase indefinitely with the rise of particle filler mass flow rate. Moreover, the incorporation of particle fillers is especially effective in enhancing the permeate flux at low feed concentration.

Magneto radiative and heat convective flow boundary layer in Maxwell fluid across a porous inclined vertical plate

Heat transport in a 2D steady radiative boundary layer with Maxwell fluid flow and the influence of heat generation and MHD has been studied across a porous inclined vertical plate. Through similarity transformation, the mathematical modelling is converted to ODEs, and the built-in solver Bvp4c via MATLAB is used to solve. The linear movement of an inclined porous plate introduced the flow. The MHD (M), Prandtl number (Pr), radiation (Rd), Rayleigh number (Ra), local Nusselt number (Nux), angle of inclination (γ), and material relaxation time (β) have a considerable impact on the flow field as a result. The local Nusselt numbers and the skin friction coefficient are also given as numbers. The validation with the numerical solution is presented. The results are shown, and a thorough physical analysis has been done. The temperature of the fluid rises due to the greater electric field, increasing the heat transfer on the inclined plate. However, skin friction increases dramatically as the heat radiation parameter rises. The critical findings of this study are that the temperature profile increases and the velocity profile lower as the inclination angle increases. The Maxwell fluid parameter raises the velocity profile as well.

Managing erosion and deposition to stabilize a silt-laden river

Human regulations are involved in the hydrogeomorphic processes of silt-laden rivers with unprecedented intensity, and further, affect the structures and functions of the riverine social-ecosystem. The braided reach (BR) of the lower Yellow River is one of the world's most sediment-rich and dynamic rivers. In the recent twenty years, the Xiaolangdi Reservoir constructed upstream and the growing river training works have deeply changed the conditions of the BR, however, the behaviors of the fluvial system under multiple human influences and their mechanisms remain unexplored. Here we systematically analyze the changes in the BR in the past four decades from the view of a coupled human and natural system. We find that compared with the pre-dam period, the channel of the BR in the post-dam period is 60 % narrower and 122 % deeper. Meanwhile, the lateral erosion rate and lateral accretion rate have decreased by 164 m yr−1 and 236 m yr−1, and the flood transport capacity has increased by nearly 79 %. These changes were mainly caused by anthropic flow regime changes and boundary modifications, whose relative contributions were 71 ± 10 % and 29 ± 10 %, respectively. The interactions among channel morphology change, regional flood risk and human activities underpinned the evolution of the fluvial system by shifting the human-river relationship. Reach-scale stabilization of a silt-laden river needs the effective management of erosion and deposition processes, which calls for integrated management of soil conservation, dam regulation, and floodplain governance at a basin scale. Lessons from the lower Yellow River have important implications for other rivers faced with siltation problems, especially in the Global South.