Results Of 3D Numerical Simulations Research Articles

Визуализация тепловых потоков в высокоскоростном течении за ударной волной

The paper presents the thermographic studies of unsteady heat fluxes behind a plane shock wave in the rectangular 24x48 mm shock tube test section. Consecutive panoramic visualization of the heat fluxes plots on streamlined walls after the plane shock wave interaction with a rectangular obstacle fixed on the channel wall are obtained. The duration of the recorded thermal processes is up to 40 milliseconds after the shock wave passage. The heating and cooling of the test chamber walls streamlined by supersonic flow are visualized using the Telops FAST M200 high-speed infrared camera (operating range 1.5 – 5.1 microns) through the quartz windows transparent to infrared radiation. Visualization of the thermal fields were compared with the shadow images and results of 2D numerical simulation of a nonstationary gas dynamic process after the diffraction of a shock wave with Mach numbers M=2.0-4.5.

Evaluation of Hydraulics and Downstream Fish Migration at Run-of-River Hydropower Plants with Horizontal Bar Rack Bypass Systems by Using CFD

Anthropogenic structures often block or delay the downstream migration of fish in rivers, thereby affecting their populations. A potential solution at run-of-river hydropower plants (HPPs) is the construction of a fish guidance structure in combination with a bypass system located at its downstream end. Crucial to fish guidance efficiency and thus to fish behavior are the hydraulic flow conditions in front of the fish guidance structure and upstream of the bypass entrance, which have not thus far been investigated in depth. The present study aims to extend the knowledge about the flow conditions at these structures. Based on the results of 3D numerical simulations of two idealized block-type HPPs with horizontal bar rack bypass systems, the flow conditions were examined, and the fish guidance efficiency was predicted. Herein, a new method was used to represent the fish guidance structure in the numerical model. The results show that the approach flow to fish guidance structures at block-type HPPs varies significantly along their length, and areas with unfavorable flow conditions for downstream fish migration frequently occur according to common guidelines. Subsequently, eight variations were performed to investigate the effect of key components on the flow field, e.g., the bypass discharge. Finally, the results were compared with literature data and discussed.

Midlatitude Sporadic E‐Layer Horizontal Structuring Modulated by Neutral Instability and Mixing in the Lower Thermosphere

AbstractObservations of 30‐MHz coherent backscatter from sporadic‐E ionization layers were obtained with a VHF imaging radar located in Ithaca, New York. The volume probed by the radar lies at relatively high magnetic latitudes, on the northern edge of the mid‐latitude region and underneath the ionospheric trough. Banded, quasi‐periodic (QP) echoes observed from Ithaca are similar to those found in lower midlatitude regions. The Doppler shifts observed are smaller and, so far, do not appear to reach the threshold for Farley‐Buneman instability. However, many of the echoes exhibit fine‐scale structure, with secondary bands or braids oriented obliquely to the primary bands. Secondary bands have been seen only rarely at lower middle latitudes. In previous observations, the QP scattering has been linked to unstable neutral wind shears. Neutral wind shear commonly found in the lower thermosphere could play a key role in the formation of these irregularities and explain some morphological features of the resulting plasma density irregularities and the radar echoes. We consider whether neutral instability and turbulence in the lower thermosphere is the likely cause for some of the structuring in the sporadic‐ E layers. Results of 3D numerical simulations of atmospheric dynamics in the mesosphere to lower thermosphere support the proposition. In particular, we focus on Ekman‐type instabilities that, like the more common Kelvin‐Helmholtz instabilities, are inflection point instabilities, although specifically associated with turning shears, and result in convective rolls aligned close to the mean wind direction, with smaller‐scale secondary waves aligned normal to the primary structures.

A polyhedral discrete de Rham numerical scheme for the Yang–Mills equations

We present a discretisation of the 3+1 formulation of the Yang–Mills equations in the temporal gauge, using a Lie algebra-valued extension of the discrete de Rham (DDR) sequence, that preserves the non-linear constraint exactly. In contrast to Maxwell's equations, where the preservation of the analogous constraint only depends on reproducing some complex properties of the continuous de Rham sequence, the preservation of the non-linear constraint relies for the Yang–Mills equations on a constrained formulation, previously proposed in [10]. The fully discrete nature of the DDR method requires to devise appropriate constructions of the non-linear terms, adapted to the discrete spaces and to the need for replicating the crucial Ad-invariance property of the L2-product. We then prove some energy estimates, and provide results of 3D numerical simulations based on this scheme.

Divertor plasma detachment: roles of plasma momentum, energy, and particle balances

Contrary to statements from some recent papers, the bifurcation of the scrape-off layer and divertor plasma parameters, as described within a 1D model, does exist. The analysis of both numerical simulations and experimental data shows that the real physical reasons that can cause a strong reduction of the plasma particle flux to the target are impurity radiation loss and volumetric plasma recombination. Fitting the parameter describing the ‘plasma momentum removal’, based on the results of 2D numerical simulations, then using this fit in subsequent analytic expressions for , and finally interpreting the evolution of as the result of elastic plasma-neutral interactions break the causality principle and lead to a wrong physical picture of divertor plasma detachment.

Numerical Simulation of Friction and Heat Exchange in Viscous-Gravitational Fluid Flow in a Loop Thermosiphon

Numerical simulation of velocity and temperature fields with laminar liquid flow in a laboratory experimental loop thermosiphon (natural circulation loop) has been made. A vertical electrically heated pipe served as a lift section. Calculations were carried out for the case of heating the entire length of the pipe under conditions of constant heat flux density on the wall. The construction of the loop is considered, in which local pressure losses and friction losses of pressure in the descending pipe are negligibly small in comparison with friction losses of pressure in the ascending section. The object of the analysis is the laminar mode of flow. In this regime, in the flows originating exclusively under the action of thermogravitational forces, the skin-friction coefficient varies in the most sophisticated way along the flow. Using the results of calculation of velocity and temperature fields, the longitudinal changes of the skin-friction coefficients and of heat transfer coefficients are determined. As a whole, according to the results of 2D numerical simulation, the skin-friction coefficients depend in the main on the change of the velocity gradient on the wall along the flow as a result of practically continuous rearrangement of velocity profile along the entire heating zone. The form of the velocity profiles and the degree of their deformation depend in turn on the heat flow density on the wall and on the hydraulic diameter. It is shown that in a single-phase loop of natural circulation (i.e., under conditions where the liquid moves exclusively under the action of thermogravitational forces) the shear stresses on the wall change along the heating zone in a complex way, and the skin-friction coefficient cannot be described by the simple dependence of the form ξ = a/Reb for being used in one-dimensional calculations. In all of the calculated regimes, including the one with the least (of those considered) heat flux density on the wall, the Nusselt numbers exceeded the stabilized values under the conditions of forced flow with constant thermophysical properties. With increase in distance from the entrance into the zone of heating, the Nusselt numbers first decrease monotonically, attain minimum values, and then begin to increase.

Measurement and modeling of thermal evolution during induction heating and thixoforming of low carbon steel

A comparative study of thixoforming and forging of double-cup parts in low carbon steel was performed using a combination of process in-situ experimental analysis and modeling. First, the thermal profile resulting from induction heating of the slug prior thixoforming was experimentally determined using thermocouples and compared to the results of 3D numerical simulation of induction heating. The thermal profile was found to be heterogeneous with temperature ranging from 1150 °C to 1425 °C. Numerical modeling of the thermocoupled slug demonstrated that the geometry alteration induced by thermocouples disturbs the magnetic field and thermal diffusion during induction heating resulting in a significant modification of the temperature distribution within the slug. Thermal evolution during transfer from the heating cell to the press was also monitored and the data were used to determine the heat transfer coefficient which was found to be approximately 150 W.m−2.K−1 on a temperature range of 1300 °C–1400 °C. Numerical simulation of thixoforming and forging processes was performed using a micro-macro model and the calculated forming load evolution was compared with the in-situ experimental data collected in-situ using a force sensor located on the upper punch. The comparative study demonstrates the essential influence of the input thermal profile to carry out reliable numerical simulation. The load applied to thixoform the double-cup part was found to be 40 % lower than in forging condition whereas the extruded length resulting from thixofming is approximately 12 % higher than for the forged specimen. Post-process analysis shows that thixofomring does not affect the microstructure and tnsile propeties of the specimen in comparison with forging, except for a sligh grain coarsening of the non-extruded part of the specimen.

Flow Structure in the Eclipsing Polar V808 Aur. Results of 3D Numerical Simulations

The structure of the flows in the eclipsing polar V808 Aur is studied. Comparison of computations with observed light curves is used to identify and clarify a number of features in the system, such as the drift of the hot spot—the place where the stream from the inner Lagrange point approaches the surface of the white dwarf, changes in the brightness in the secondary minimum, a dip in the light curve in the high state before entering the eclipse, the asymmetrical eclipse profile in high state, and others. A three-dimensional numerical MHD model based on the approximation of modified magnetic hydrodynamics is used in these studies. Numerical computations were carried out for several mass-exchange rates corresponding to different states of activity of the V808 Aur system. The computations show that, as the mass-exchange rate increases, the length of the ballistic part of the accretion stream increases, leading to changes in the spatial configuration of the flow and an appreciable drift of the region of energy release on the surface of the white dwarf (by up to 30° in longitude). These changes in the flow structure lead to effects in the light curve that are in good agreement with the available observations.

Dynamic behavior of pile foundations under vertical and lateral vibrations

ABSTRACTPile foundations supporting machines, buildings under seismic effect, and wind turbines are subjected to dynamic loads. Under such conditions, there is a necessity to evaluate the dynamic behavior of piles. Therefore, a 3D numerical modeling technique is needed to consider the complicated dynamic interaction between the piles and soil (pile-soil interaction) and between adjacent piles in the same group (pile-soil-pile interaction). To validate the results of 3D numerical simulation of piles, the results obtained from the numerical model has been compared to the measurements of a selected case study. The 3D finite element model has also been used to evaluate the impedance parameters and induced peak displacements. The study is performed for single piles and pile groups. The different techniques to model the piles are evaluated. In addition, the effect of related parameters, such as excitation force frequency (f), soil modulus of elasticity (Es), pile slenderness Ratio (L/D), dimensionless spacing ratio (S/D) and pile group size (ng) have been studied.

Johnson–Holmquist-II(JH-2) Constitutive Model for Rock Materials: Parameter Determination and Application in Tunnel Smooth Blasting

The Johnson–Holmquist-II(JH-2) model is introduced as the constitutive model for rock materials in tunnel smooth blasting. However, complicated and/or high-cost experiments need to be carried out to obtain the parameters of the JH-2 constitutive model. This study chooses Barre granite as an example to propose a quick and convenient determination method for the parameters of the JH-2 model using a series of computational and extrapolated methods. The validity of the parameters is verified via comparing the results of 3D numerical simulations with laboratory blast-loading experiments. Subsequently, the verified parameter determination method, together with the JH-2 damage constitutive model, is applied in the numerical simulation of smooth blasting in Zigaojian tunnel, Hangzhou–Huangshan high-speed railway. The overbreak/underbreak induced by rock blasting and joints/discontinuities is well estimated through comparing the damage contours resulting from the numerical study with the tunnel profiles measured from the tunnel site. The peak particle velocities (PPVs) of the near field are extracted to estimate the damage scope and damage degree for the surrounding rock mass of the tunnel on the basis of PPV damage criteria. This method can be used in the excavation of rock tunnels subjected to large strains, high strain rates, and high pressures, thereby reducing safety risk and economic losses.

Trimming and sheared edge stretchability of automotive 6xxx aluminum alloys

Edge quality produced by shearing processes often leads to reduced material formability which was observed in multiple studies and summarized in the reference literature. The intention to make the sheared edge performance more predictable has motivated the development of several experimental techniques such as the hole expansion test and the tensile test with one side of the sample sheared in various cutting conditions. The paper presents a review of published results for both of these techniques and illustrates very limited research dedicated to sheared edge performance of aluminum alloys. The experimental study was performed on three broadly used in automotive industry 6xxx serious aluminum alloys which have similar mechanical properties and chemical composition with some variation in processing regimes and individual elements quantities. The effects of cutting clearance on longitudinal, transverse and diagonal orientations of the trim line relative to the rolling direction were discussed for all three alloys. The results of standard tensile test of all three materials were very similar with rather small difference in work hardening and almost no difference in total elongations measured by an extensometer. Measurements of elongations as a result of stretching of samples along orthogonally trimmed sheared surfaces indicated that the increase of the cutting clearance above 5% for Alloy 1 and above 15% for Alloy 2 usually leads to formation of burrs and reduced elongations of trimmed samples before fracture occurs. The effect of cutting clearance on sheared edge stretching performance for Alloy 3 is significantly less pronounced than for Alloys 1 and 2, which makes Alloy 3 very attractive for exterior panel automotive applications in which significant stretching of sheared edge can be anticipated. Detailed discussion of mechanism of blank separation during trimming process was based upon the results of 2D numerical simulation and interrupted trimming process with partially propagated cracks.

Performance of an existing raft foundation rested on granular soils due to TBM tunneling process

In urban areas, there is a necessity to evaluate the performance of existing structures during and after the construction of nearby tunnels. Therefore, a 3D numerical modeling technique is needed to consider the geometrical interactions between the tunnel and the existing structures. To validate the results of 3D numerical simulation for the tunneling process, the surface settlement trough resulted from the numerical model has been compared with field measurements of a selected case study (Second Heinenoord Tunnel in Netherlands). The 3D finite element model has also been used to evaluate the raft settlements and bending moments. The study is performed during and after the tunnel advancement process. In addition, the effect of related parameters, such as soil relative density (Dr), tunnel diameter (D), tunnel cover (Z), horizontal clearance between the raft centerline and the tunnel centerline (CL), raft inclination angle (i), and raft thickness (d), have been studied.

Model for pressure drop and flow deflection in the numerical simulation of stents in aneurysms.

The numerical simulation of flow diverters like stents contributes to the development and improvement of endovascular stenting procedures, leading ultimately to an improved treatment of intracranial aneurysms. Due to the scale difference between the struts of flow diverters and the full artery, it is common to avoid fully resolved simulations at the level of the stent porosity. Instead, the effect of stents on the flow is represented by a heuristic continuum model. However, the commonly used porous media models describe the properties of flow diverters only partially, because they do not explicitly account for the deflection of the flow direction by the stent. We show that this deficiency can be circumvented by adopting the theoretical framework of screen models. The article first reviews existing screen models. It then proposes an explicit formula for the drag and the deflection coefficient, as predicted by each model, for both perpendicular and inclined angles. The results of 2D numerical simulations are used to formulate a generalization of these formulas, to achieve best results in the case of stent modeling. The obtained model is then validated, again through 2D numerical simulation.

Numerical simulation of heat and mass transfer in a 3D model of a loop heat pipe evaporator

The article presents results of 3D numerical simulation of flow and conjugate heat and mass transfer in a model of the TacSat-4 satellite loop heat pipe evaporator. The mathematical model includes the Reynolds-averaged Navier–Stokes equations describing the flow in the liquid and vapor regions, Darcy's law for filtration modeling in the wicks and the energy equation with accurate coupling of connected sub-domains including effects of evaporation on interfaces between the porous and vapor regions. According to the simulation results, the evaporation localizes mainly at the vapor groove corners near the evaporator body. The vapor grooves operate under essentially different conditions, as a result, the flow rates differ by several times. Significant thickening of the evaporator body yields only weak reduction in a level of the grooves’ flow rate non-uniformity.

Transient characteristics of induced polarization in inhomogeneous media (from results of 2D numerical simulation)

This paper presents the results of 2D mathematical modeling of induced polarization (IP) in the cases of profiling and electrical tomography for piecewise homogeneous media containing ore bodies with different structures, sizes, depths, relaxation times, chargeability, and electrical resistivity. The standard and spectral approaches to data analysis are compared. The cases where analysis of transient IP characteristics yields new information of practical importance are considered. The main features of transient IP characteristics in 2D inhomogeneous media are determined from the modeling results. Analysis of transient IP characteristics is found to yield the best results when the depth of occurrence of polarizable bodies is minimal and their electrical conductivity is an order of magnitude lower than that of the host tock.

The Shock Dynamics of Heterogeneous YSO Jets:3D Simulations Meet Multi-epoch Observations

Abstract High-resolution observations of young stellar object (YSO) jets show them to be composed of many small-scale knots or clumps. In this paper, we report results of 3D numerical simulations designed to study how such clumps interact and create morphologies and kinematic patterns seen in emission line observations. Our simulations focus on clump scale dynamics by imposing velocity differences between spherical, over-dense regions, which then lead to the formation of bow shocks as faster clumps overtake slower material. We show that much of the spatial structure apparent in emission line images of jets arises from the dynamics and interactions of these bow shocks. Our simulations show a variety of time-dependent features, including bright knots associated with Mach stems where the shocks intersect, a “frothy” emission structure that arises from the presence of the Nonlinear Thin Shell Instability along the surfaces of the bow shocks, and the merging and fragmentation of clumps. Our simulations use a new non-equilibrium cooling method to produce synthetic emission maps in Hα and [S ii]. These are directly compared to multi-epoch Hubble Space Telescope observations of Herbig–Haro jets. We find excellent agreement between features seen in the simulations and the observations in terms of both proper motion and morphologies. Thus we conclude that YSO jets may be dominated by heterogeneous structures and that interactions between these structures and the shocks they produce can account for many details of YSO jet evolution.

Phase distortion suppression in a nonlinear optical system with integral feedback

We consider a mathematical model of an optical system where the phase modulation introduced by a nonlinear optical element involves a phase shift in an additional time-integral feedback loop, which is determined by the gain coefficient . In the case of , this model describes the dynamics of a nonlinear interferometer, where the effect of phase distortion compensation has been numerically and experimentally investigated before. In this paper, the results of 2D numerical simulations are presented for the case . It is shown that different types of small-amplitude stationary phase inhomogeneities can be fully compensated with time. The effect of the partial suppression of dynamic distortions given in the form of traveling waves and Kolmogorov-spectrum atmospheric irregularities is demonstrated; the quality of the distortion compensation is analyzed, depending on the spatiotemporal characteristics of the waves. The obtained results confirm the potential of using the optical feedback configuration with an additional integral circuit for suppression of both static and dynamic phase distortions.

Multilevel approach to lifetime assessment of steam turbines

• Deterministic and probabilistic method of lifetime assessment is presented. • Effect of creep and fatigue damage content on failure probability is analyzed. • Practical application shown by example of lifetime calculation of turbine rotor. This paper presents a multilevel methodology for a steam turbine lifetime assessment based on the damage calculation, probabilistic analysis and fracture mechanics considerations. Creep-fatigue damage calculations serve as a basis for evaluating the current lifetime expenditure and for defining additional steps of analysis. The need for the use of probabilistic analysis results from the inherent uncertainty in estimating the lifetime expenditure primarily caused by scatter in material properties. Fracture mechanics considerations are helpful in determining additional safety margins for components containing cracks. This methodology has been illustrated using an example of the lifetime calculations of a high-temperature steam turbine rotor. The calculations were based on the results of 2D numerical simulations performed for steady state and transient operating conditions.

Size determination of subsurface defect by active thermography – Simulation research

This paper presents the results of 3D numerical simulations of thermal phenomena in uniform material with subsurface defect located at some depth. The material is aimed to be tested by means of IR active thermography. The temperature field of the front surface of material tested is observed and analysed. The results of modelling are suitable for step heating as an external thermal stimulation – heat flux. The heating duration needed for accurately defect sizing is discussed. Also, an effect of two defects with different thermal properties and simultaneously occurring in the material tested is analysed.

The transitional backward-facing step flow in a water channel with variable expansion geometry

The backward-facing step flow is investigated experimentally and numerically at moderate Reynolds numbers. Different channel expansion ratios (ER=1.43, 2, 2.5, and 4) and inlet flow conditions (steady and pulsatile) are applied with the aim to analyze the structure and stability of flow behind the step. Electrodiffusion technique is used to measure the wall shear rate along the experimental water channel. Direction sensitive sensors detect the near-wall extent of different flow-recirculation regions (primary recirculation and secondary corner, roof, and bottom eddies). The results of 2D numerical simulations performed in commercial CFD software FLUENT provide additional information on global flow rearrangement caused by the change of operation parameters. As the channel expansion ratio is increased, the steady recirculation pattern observed in the laminar flow regime becomes more complex. The obtained experimental and numerical data suggest possible scaling for the reattachment length and roof eddy size. In the transitional regime the near-wall flow exhibits an unsteady character with a high sensitivity to external low-frequency perturbations. The inlet pulsatile forcing is found to affect strongly the overall flow structure behind the step. A significant reduction of the reattachment length and an intensification of pulsatile back flow can be achieved by applying an appropriate forcing at frequencies close to that of the global flow instability.