Flow Boundary Research Articles

Characteristics of limited flow and soil water infiltration boundary of a subsurface drip irrigation emitter in silty loam soil

The accurate estimation of the infiltration boundary of a buried point source is required in the hydraulic design of a subsurface drip irrigation (SDI) system, as well as in the numerical simulation of soil water movement. In this study, an experiment was conducted to measure the variation in emitter flow rate and soil back pressure in silty loam soil. The infiltration cavity was observed, and the radius was estimated by matching the measured emitter flow rate and soil back pressure using HYDRUS-2D simulations. The relationship between the cavity radius, emitter flow rate, and soil back pressure, as affected by the soil bulk density, working pressure, and nominal emitter flow rate, are discussed. The results indicate that the infiltration cavity formed rapidly during irrigation with an irregular shape. The surface area increased as nominal emitter flow rate increased and soil bulk density increased. The variation of flow rate as a function of time had no necessary relationship with that of soil back pressure, due to the formation of the cavity. When the infiltration process became stable, higher soil bulk density resulted in increase in soil back pressure and decrease in emitter flow rate. However, the effects of working pressure and nominal emitter flow rate on soil back pressure were unclear. The emitter flow rate and back pressure, solved by an existing analytical model using estimated radius, demonstrated satisfactory agreement with the measured data, with the minimum root-mean-square error (RMSE), compared with those using a constant radius. When the infiltration boundary was taken as a constant head, the soil back pressure and cavity both significantly affected the simulated soil wetting patterns. The effect of cavity radius on the wetting front could be neglected for the constant flux boundary.

Numerical Study of Unsteady Boundary Layer Flow through A Vertical Surface with Convective Boundary Conditions

In the present-day paper the effort is to see the properties of heat allocation over a vertical flat surface for an unsteady mixed, boundary layer convective flow. The non-linear partial differential equations of the present model are transformed in to ordinary differential equations by means of suitable similar transformations. The consequent ordinary differential equations are resolved mathematically by means of MATLAB program bvp4c and discussed the effects of skin friction, suction and injection for temperature and velocity.

Simulation of crucible-free growth of monocrystalline silicon fibres for mirror suspension in gravitational-wave detectors

Monocrystalline silicon fibres are a promising candidate to support the test mass needed in gravitational-wave detectors. Stringent requirements on material purity and fibre diameter underscore the imperative for crucible-free crystal growth methods, such as Pedestal and Floating Zone techniques. We developed a multiphysics numerical model that simulates the coupling between electromagnetism, heat transfer, fluid flow and phase boundaries for these techniques within the same mathematical framework, which facilitates direct comparisons of both growth techniques. Model validation was addressed using experimental measurements and other simulation tools, and satisfactory agreement was obtained. Our findings indicate that electromagnetic forces are the primary mechanism responsible for stirring the molten silicon at small crystal diameters. The resulting fluid velocity develops slightly transient behaviour, although this has little impact on the phase boundary shapes. The understanding of the growth process enabled by the model will facilitate future optimization of crystal quality and diameter stability.

Verification of the γ-Reθt Transition Model in OVERFLOW and FUN3D

The findings of the recent AIAA and NATO workshops on transition modeling have underscored the importance of verifying the transition models that are based on Reynolds-averaged Navier–Stokes. To that end, the present work aims to verify the Langtry–Menter (LM2009) transition model coupled with Menter’s shear-stress transport (SST) turbulence model. Specifically, the SST-2003-LM2009 model, as implemented in the OVERFLOW structured overset solver and the FUN3D unstructured solver, is applied to both natural and bypass transition scenarios on a flat plate and to natural and separation-bubble-induced transition on the NLF-0416 airfoil. The Richardson extrapolation method was used to analyze and report the grid convergence of integrated and local load coefficients upstream, within, and downstream of the transition region, based on solutions obtained using the two solvers on identical grids. Reasonable agreement is obtained between the two codes for both global and local surface load coefficients when further refining the workshop mesh series, although with a grid-convergence index for the local friction coefficient that may exceed 1% in certain cases. The study also highlights the solution sensitivity to the choice of turbulence inflow boundary conditions and model options, along with the necessity to unambiguously specify this information for benchmark exercises.

Particle Image Velocimetry Measurements in a High-Speed Low-Reynolds Low-Pressure Turbine Cascade

Abstract Particle image velocimetry (PIV) measurements in the blade-to-blade (B2B) plane and cascade outlet plane (COP) of a high-speed low-pressure turbine (LPT) cascade were performed at engine-representative outlet Mach number (0.70–0.95), and Reynolds number (70–120 k) under steady flow conditions. The freestream turbulence characteristics were imposed by means of a passive turbulence grid. The PIV results on the B2B plane were compared against five-hole probe (5HP) and Reynolds-averaged Navier–Stokes (RANS) computations to assess the validity of measurement and simulation techniques in the engine-relevant LPT cascade flows. The PIV captured the wake depth and width measured by the 5HP whereas the RANS displayed an overprediction of the wake Mach number deficit. The 5HP was found to impose sinewave fluctuations of the measured flow angle downstream, around three times higher than PIV. Additionally, PIV estimated turbulence intensity (TI) in the cascade, showing TI decay along a streamline. At the highest Mach number, a peak TI occurred past a shock wave. Measurements of the outlet flow field highlighted a high TI in the secondary flow region whereas high degree of anisotropy (DA) was registered in the boundary of the secondary flow and freestream regions. The contribution of the streamwise fluctuation component was found to be less than the crosswise and radial components in the freestream region. Increasing the cascade outlet Mach number, the contribution of streamwise fluctuation to the DA was observed to decrease.

Thermal analysis of thermoelectric active cooling including external thermal resistances

Thermoelectric active cooling uses nontraditional thermoelectric materials with high thermal conductivity, high thermoelectric power factor, and relatively low figure of merit (ZT) to transfer large heat flows from a hot object to a cold heat sink. However, prior studies have not considered the influence of external thermal resistances associated with the heat sinks or contacts, making it difficult to design active cooling thermal systems or compare the use of low-ZT and high-ZT materials. Here, we perform a non-dimensionalized analysis of thermoelectric active cooling under forced heat flow boundary conditions, including arbitrary external thermal resistances. We identify the optimal electrical currents to minimize the heat source temperature and find the crossover heat flows at which low-ZT active cooling leads to lower source temperatures than high-ZT and even ZT→+∞ thermoelectric refrigeration. These optimal parameters are insensitive to the thermal resistance between the heat source and thermoelectric materials, but depend strongly on the heat sink thermal resistance. Finally, we map the boundaries where active cooling yields lower source temperatures than thermoelectric refrigeration. For currently considered active cooling materials, active cooling with ZT < 0.1 is advantageous compared to ZT→+∞ refrigeration for dimensionless heat sink thermal conductances larger than 15 and dimensionless source powers between 1 and 100. Thus, our results motivate further investigation of system-level thermoelectric active cooling for applications in electronics thermal management.

Quantum simulation for partial differential equations with physical boundary or interface conditions

This paper explores the feasibility of quantum simulation for partial differential equations (PDEs) with physical boundary or interface conditions. Semi-discretization of such problems does not necessarily yield Hamiltonian dynamics and even alters the Hamiltonian structure of the dynamics when boundary and interface conditions are included. This seemingly intractable issue can be resolved by using a recently introduced Schrödingerisation method [1,2] – it converts any linear PDEs and ODEs with non-Hermitian dynamics to a system of Schrödinger equations, via the so-called warped phase transformation that maps the equation into one higher dimension. We implement this method for several typical problems, including the linear convection equation with inflow boundary conditions and the heat equation with Dirichlet and Neumann boundary conditions. For interface problems we study the (parabolic) Stefan problem, linear convection, and linear Liouville equations with discontinuous and even measure-valued coefficients. We perform numerical experiments to demonstrate the validity of this approach, which helps to bridge the gap between available quantum algorithms and computational models for classical and quantum dynamics with boundary and interface conditions.

Aerodynamic designs in FORMULA ONE cars

To many of us, FORMULA ONE (F1) racing is an expensive sport that requires high-level entry requirements. However, compared to its value for entertainment purposes, the significance of its technological contribution to transportation in all different areas is enormous. This study focuses on understanding the physics and implementation behind aerodynamic designs in modern F1 racing cars. For this purpose, this research is conducted by analyzing the structure of the individual parts and their integrated effects on F1 aerodynamic performance. The result revealed that generating appropriate vortices around the boundary flows and guiding the airflow in an optimized direction can boost the performance of F1 cars. This study emphasizes the importance of utilizing airflow control for maximum maneuverability and might benefit many transportation-related industries.

Influence factors and prediction model of surface roughness in single-point diamond turning of polycrystalline soft metal

Ultrasmooth surface of soft metals is required in the field of advanced technology. However, the strong plasticity of soft metals makes it hard to achieve high surface finishing in ultraprecision machining. In this paper, the ultraprecision machining experiments of polycrystalline tin are carried out, and the factors including tool nose radius, cutting parameters, and material properties affecting the surface roughness in single-point diamond turning are investigated. Side flow, anisotropy, and grain boundary step are identified as the main causes of increasing of surface roughness, with grain boundary step demonstrating the largest impact. A surface roughness prediction model of tin in single-point diamond turning is established, which takes the minimum undeformed chip thickness, side flow, anisotropy and grain boundary step into consideration. The experiments show that the maximum prediction error of this prediction model is 6.51%.

A comparative study of enhanced thermal performance in Tesla-type microchannels

Enhancing convective heat transfer in microchannels is significant for cooling of micro-electro-mechanical systems (MEMS). The dominant laminar flow in confined domain at microscale normally results in thick thermal boundary layers, seriously deteriorate the heat transfer process because of limited disturbance of flow. In this study, a novel Tesla-type microchannel configuration with significant flow diodicity was proposed to significantly interrupt boundary layers and promote fluid mixing. Hydrodynamic and thermal performances of this as-designed configuration were comprehensively investigated through experimental and numerical studies. Thermal characteristics such as Nusselt number (Nu), thermal resistance (Rth), and performance evaluation criterion (PEC) were computed. Diodic performance was also evaluated by measuring pressure drop diodicity and thermal diodicity in different flow directions. The results demonstrate that convection heat transfer is significantly enhanced because of strong mixing and periodic interruption of liquid flow and thermal boundary layers. Compared to conventional plain-wall microchannel, the average wall temperature is dramatically reduced by about 10 K at Re = 250 because of enhanced mixing effect. Accordingly, Rth is significantly decreased by ∼ 45 %. A PEC of 1.3 is achieved as well. Overall, the Nu is ranging from 10 to 25.2 for Re ranging 100 to 450, about 1.3 ∼ 2.1 times higher than that of plain-wall microchannel. Diodic performances in terms of Dip and Dit are about 1.9 and 1.3 for Dh = 0.4 mm. The new understandings of this study might provide insights for the design of new heat sinks for potential applications in electronics cooling.

Impact of thermodynamical rotational flow of cerebrospinal fluid in the presence of elasticity

ObjectiveTo explore the experimental justification of cerebrospinal fluid (CSF) amplitude and elastic fluctuations of ventricles, we extend our previous computational study to models with rotational flow and suitable boundary conditions. In the present study, we include an elastic effect due to the interaction with the thermal solutal model which accounts for CSF motion which flows rotationally due to hydrocephalus flows within the spinal canal.MethodsUsing an analytical pertubation method, we have attempted a new model to justify CSF flow movement using the influences of wall temperature difference.ResultsThis paper presents results from a computational study of the biomechanics of hydrocephalus, with special emphasis on a reassessment of the parenchymal elastic module. CSF amplitude in hydrocephalus patients is 2.7 times greater than that of normal subjects.ConclusionsThis finding suggests a non-linear mechanical system to present the hydrocephalic condition using a numerical model. The results can be useful to relieve the complexities in the mechanism of hydrocephalus and can shed light to support clinically for a convincing simulation.

Comparison of the LBM snowdrift model output with the observation results

In this work, snowdrift experiments which are equivalent to one drifting snow event are performed by the snowdrift model. The model consisted of the computational fluid dynamics part of the large-eddy simulation with the lattice Boltzmann method and the drifting snow part of the conventional advection algorithm for representative Lagrangian particles. The observed vertical wind profile of a 4 h drifting snow event in Teshikaga Town was used as the inflow boundary conditions in the model to compare the results of the snowdrift estimated by the model and the observed snowdrift distribution. Parallelization enabled us to simulate the snowdrift distribution in a realistic domain and on the time scale of a single drifting snow event. We demonstrated that the upgraded model could quantitatively reproduce the height and position of the observed snowdrift along the center of a three-dimensional fence.

The interplay between bulk flow and boundary conditions on the distribution of microswimmers in channel flow

While previous experimental and numerical studies of dilute microswimmer suspensions have focused on the behaviours of swimmers in the bulk flow and near boundaries, models typically do not account for the interplay between bulk flow and the choice of boundary conditions imposed in continuum models. In our work, we highlight the effect of boundary conditions on the bulk flow distributions, such as through the development of boundary layers or secondary peaks of cell accumulation in bulk-flow swimmer dynamics. For the case of a dilute swimmer suspension in Poiseuille flow, we compare the distribution (in physical and orientation space) obtained from individual-based stochastic models with those from continuum models, and identify under what conditions it is mathematically sensible to use specific continuum boundary conditions to capture different physical scenarios (i.e. specular reflection, uniform random reflection and absorbing boundaries). We identify that the spread of preferred cell orientations is dependent on the interplay between rotation driven by the shear flow (Jeffery orbits) and rotational diffusion. We find that in the absence of hydrodynamic wall interactions, swimmers preferentially approach the walls perpendicular to the surface in the presence of high rotational diffusion, and that the preferential approach of swimmers to the walls is shape-dependent at low rotational diffusion (when suspensions tend towards a fully deterministic case). In the latter case, the preferred orientations are nearly parallel to the surface for elongated swimmers and nearly perpendicular to the surface for near-spherical swimmers. Furthermore, we highlight the effects of swimmer geometries and shear throughout the bulk-flow on swimmer trajectories and show how the full history of bulk-flow dynamics affects the orientation distributions of microswimmer wall incidence.

Heat transport and flow morphology of geostrophic rotating Rayleigh–Bénard convection in the presence of boundary flow

Using direct numerical simulations, we investigate the heat transport in bulk and boundary flows separately in rotating Rayleigh–Bénard convection in cylindrical cells. In the bulk we observe a steep scaling relationship between the Nusselt number ( $Nu$ ) and the Rayleigh number ( $Ra$ ), which is consistent with the results from simulations using periodic boundary conditions. For the boundary flow, we observe a power law $Nu_{BF}\sim (Ra/Ra_w)^1$ at the leading order, where $Nu_{BF}$ is the local Nusselt number of the boundary flow and $Ra_w$ is the onset Rayleigh number of the wall mode. We develop a model using the boundary layer marginal stability theory to explain this power law, and further show that a more precise description of the data can be obtained if a higher-order correction is introduced. A striking finding of our study is the observation of a sharp transition in flow state, manifested by a sudden drop in $Nu_{BF}$ with a corresponding collapse of the boundary flow coherency. After the transition, the boundary flow breaks into vortices, leading to a reduction in flow coherency and heat transport efficiency. As the physical properties of the vortices should not depend on the aspect ratio, $Nu_{BF}$ for all aspect ratios collapse together after the transition. Moreover, the centrifugal force helps trigger the breakdown of the coherent boundary flow state. For this reason, $Nu_{BF}$ for the cases with non-zero centrifugal force collapse together. We further develop a method that enables us to separate the contributions from the bulk and boundary flows in the global Nusselt number using only the global $Nu$ and it does not require the centrifugal force to be absent.

Developing a Computational Fluid Dynamics-Finite Element Method Model to Analyze Thermal-Mechanical Stresses in a Heavy-Duty Medium-Speed Diesel Engine Piston During Warm-Up

Abstract Pistons play a vital role in internal combustion engines, affecting both performance and reliability, and are subjected to intense thermal-mechanical loads that have become more challenging due to improved engine efficiency and power. This study examines the thermal and mechanical stress experienced by a piston in a heavy-duty medium-speed diesel engine during warm-up. The heavy-duty diesel engines are typically used in heavy trucks, locomotives, and ships. A combination of computational fluid dynamics, finite element method, and matlab was used to consider factors such as oil temperature and flowrate, coolant temperature, component temperature, and boundary conditions during engine transient conditions. The results highlight the significant variations in the thermal and mechanical stress on the piston, particularly in the piston head under different warming-up conditions. It is noted that the variation in oil temperature is a crucial factor affecting the thermal stress on the piston. Low oil temperature can result in reduced heat exchange coefficient and inadequate cooling of the piston due to low flowrate of the cooling oil. During engine warm-up, both thermal and combined stresses reach maximum values and then decrease when the engine reaches stable operating conditions. By selecting appropriate warming-up modes, the quality of the warm-up process and the strength and longevity of the engine could be improved. This study also provides useful insights for technicians to prevent critical conditions that may damage the piston and reduce its strength and lifespan.

Three-dimensional global instability analysis for high-speed boundary layer flow

Herein, a numerical approach for the three-dimensional global instability analysis of high-speed boundary layer is developed. The approach is based on the Krylov–Schur method for large-scale eigenproblems. A shift-and-invert spectral transformation is employed to accelerate the convergence for eigenvalues of interest. Iterative linear solver is adopted in the transformation to reduce memory usage by avoiding the fill-in problem of direct solver and the explicit construction of matrices. A Robin condition based on BiGlobal instability analysis is imposed at the inflow boundary, and a non-reflecting condition derived from inviscid flow is used for the far-field boundary. The approach is validated in a Blasius boundary layer. According to a comparison between the performances of several algorithms in this case, the conjugate gradient squared method with no preconditioning is chosen as the linear solver. The results provided by three-dimensional global instability analysis consist with those from linear stability theory and BiGlobal instability analysis. A further demonstration of our approach is provided for a three-dimensional roughness-disturbed boundary layer. The fluctuations extracted from direct numerical simulation results illustrate that the symmetric component is the dominant unstable mode in roughness wake, as it grows twice as fast as the antisymmetric component. Two eigenvalues are obtained from the three-dimensional global instability analysis, and these represent the symmetric and antisymmetric unstable modes. The wavelengths and growth rates of both modes delivered by instability analysis agree well with the direct numerical simulation results, and the profiles of disturbance amplitude match the features of the varicose and sinuous modes.

Uncertainty analysis on flood routing of embankment dam breach due to overtopping failure

The breach risk of reservoir dam exists objectively while exerting benefits. There is great uncertainty in the overtopping breach process of embankment dam and the dam break flood routing which affects the accuracy of dam risk analysis and may cause unnecessary waste in emergency scheduling. However, the uncertainty analysis of the breach process and its consequence is currently inadequate. Therefore, a stratified sampling Monte Carlo method is proposed to simulate uncertain overtopping breach flood of embankment dams. The main sources of uncertainty are analyzed and determined as uncertain dam breach and flood routing processes. The uncertain breach process of dam is studied by presenting a sensitive study between 3 mechanism breach models and 5 parametric breach models. The random dam breach process is restored using HEC-RAS semi-empirical breach model by estimating breach characteristics through multiple common breach models. The random flood routing is carried out through 1D–2D coupled unsteady flow analysis in which the random breach process is adopted as an upper inflow boundary condition. According to the case study results, though parameters have been controlled in a limited range, the flood routing results in the early stage of dam overtopping failure present greater uncertainty. As the flood progresses further downstream, the uncertainty will gradually decrease. This study could serve as a reference for dam breach risk map making.

Realistic turbulent inflow conditions for estimating the performance of a floating wind turbine

Abstract. A novel method for generating turbulent inflow boundary conditions for aeroelastic computations is proposed, based on interfacing hybrid hot-wire and particle image velocimetry measurements performed in a wind tunnel to a full-scale load simulation conducted with FAST. This approach is based on the use of proper orthogonal decomposition (POD) to interpolate and extrapolate the experimental data onto the numerical grid. The temporal dynamics of the temporal POD coefficients is driven by the high-frequency hot-wire measurements used as input for a lower-order model built using a multi-time-delay linear stochastic estimation (LSE) approach. Being directly extracted from the data, the generated three-component velocity fields later used as inlet conditions present correct one- and two-point spatial statistics and realistic temporal dynamics. Wind tunnel measurements are performed at a scale of 1:750, using a properly scaled porous disk as a floating wind turbine model. The motions of the platform are imposed by a linear actuator. Between all 6 degrees of freedom (DOFs) possible, the present study focus on the streamwise direction motion of the model (surge motion). The POD analysis of the flow, with or without considering the presence of the surge motion of the model, shows that a few modes are able to capture the characteristics of the most energetic flow structures and the main features of the wind turbine wake, such as its meandering and the influence of the surge motion. The interfacing method is first tested to estimate the performance of a wind turbine in an offshore boundary layer and then those of a wind turbine immersed in the wake of an upstream wind turbine subjected to a sinusoidal surge motion. Results are also compared to those obtained using the standard inflow generation method provided by TurbSim available in FAST.

Experimental Study on Flow Structure Characteristics of Gap Flow Boundary Layer Based on PIV

The boundary layer is the main source of frictional resistance in gap flow, and the study of the flow structure characteristics of the gap flow boundary layer is of great significance for the study of gap flow theory. In this study, the PIV technique was utilized to experimentally investigate the gap flow boundary layers with Reynolds numbers of 16,587–56,870 and gap ratios of 0.6–0.8. The characteristics of the wall friction velocity, the boundary layer thickness, and the wall function of the gap flow boundary layer were analyzed, and the influences of the mean velocity of the gap flow and the gap ratio on the flow structure characteristics of the boundary layer were explored. The results show that using PIV to measure the velocity profile in the viscous sub-layer to solve for the wall friction velocity had good precision. The boundary layer thickness was inversely proportional to the mean velocity of the gap flow and the gap ratio. The wall functions of the boundary layer were as follows: in the viscous sub-layer (y+< 5.5), u+=y+; in the transition layer (5.5 <y+< 26), u+=10.071tanh0.071y+; and in the logarithmic layer (y+> 26), u+=2.78lny++3.8. The thickness of the logarithmic layer was proportional to the mean velocity of the gap flow and inversely proportional to the gap ratio. The inner region of the boundary layer extended to y< 0.18δ or y< 0.13(h/2).

Analysis of the effect of turbulent inflow on the flow-field and noise of supersonic jet flow using large eddy simulation

The impact of inflow flow state on the flow-field and far-field noise of supersonic jet flow has been studied. A new turbulent inflow boundary condition based on the synthetic eddy method is implemented in an in-house LES code. The implementation was validated using a turbulent channel flow. Simulation results of an under-expanded supersonic jet with a converging nozzle have shown that turbulent inflow leads to thicker and turbulent initial shear layer with rich vortical structures. The thick initial shear layer hinders the growth of flow instability and break the acoustic feedback loop, which is vital to the generation of screech tone. The alternation of the flow-field by the turbulent inflow also leads to changes in the far-field noise radiation. The screech tone is eliminated, the noise in the downstream direction is considerably suppressed, and the noise radiates in the upstream direction is slightly amplified.