Assumption Of Laminar Flow Research Articles

Thorough Observation of Real Contact Area of Copper Gaskets Using a Laser Microscope With a Wide Field of View

To quantitatively predict the leakage rates of static metal seals, it is important to observe the real contact area at seal surfaces because the leakage path consists of the noncontact portions between the flange and gasket surfaces. In a previous study, we observed the real contact situation using a thin polymer film 1 μm in thickness. In the present study, we observed the real contact area on gasket surfaces using a laser microscope with a wide field of view. With this method, observation time over the whole gasket surface could be greatly reduced compared with conventional methods. The observations indicated that the leakage paths on the gasket surfaces were in the radial direction perpendicular to a lathe-turned groove and the circumferential direction along the groove. As the closing loads increased, the leakage paths in the radial direction disappeared and only the leakage path in the circumferential direction remained. When the closing loads increased further, the widths of the leakage paths at both the inside and outside on the gasket surface became narrower. The critical contact pressure where the leakage paths in the radial direction disappear was determined from the observation of the contact surface of the gasket. The leakage rates obtained from the experiments showed good agreement with the calculated values under the assumption of laminar flow along the turned groove over the critical contact pressure.

Study of natural circulation in a VHTR after a LOFA using different turbulence models

Natural convection currents in the core are anticipated in the event of the failure of the gas circulator in a prismatic gas-cooled very high temperature reactor (VHTR). The paths that the helium coolant takes in forming natural circulation loops and the effective heat transport are of interest. The heated flow in the reactor core is turbulent during normal operating conditions and at the beginning of the LOFA with forced convection, but the flow may significantly be slowed down after the event and laminarized with mixed convection. In the present study, the potential occurrence and effective heat transport of natural circulation are demonstrated using computational fluid dynamic (CFD) calculations with different turbulence models as well as laminar flow. Validations and recommendation on turbulence model selection are conducted. The study concludes that large loop natural convection is formed due to the enhanced turbulence levels by the buoyancy effect and the turbulent regime near the interface of upper plenum and flow channels increases the flow resistance for channel flows entering upper plenum and thus less heat can be removed from the core than the prediction by laminar flow assumption.

Comparison of turbulence models for a free-burning high-intensity argon arc

Free-burning arcs where the work piece acts as an anode are frequently used for a number of industrial applications. Our investigation was concerned with developing a capability to model free-burning high-intensity argon arcs and enhancing the accuracy of numerical results according to a comparative study of turbulence models. For the arc modeling involved complicated interactions between the flow and electromagnetic fields, we modified the Navier-Stokes equations to take into account the radiation transport, the electrical power input and the electromagnetic driving forces with the relevant Maxwell equations. For the turbulence modeling, which is critical to the arc edges due to a steep temperature gradient between the arc column and the surrounding gas, zero- and two-equation eddy-viscosity models were used. The major arc parameters of temperature, axial velocity, electric potential difference and pressure-rise from ambient atmospheric pressure were summarized under laminar and turbulent conditions. It was found that the standard k-ɛ model modified to take into account the effect of a steep temperature gradient at the edge of free-burning arcs could predict reasonable temperature profiles better than the cases with the assumption of laminar flow and Prandtl’s mixing length model used for switching arcs.

Numerical Methods and Applications in Biomechanical Modeling

Numerical Methods and Applications in Biomechanical Modeling

State of the Art: Permeability of Asphalt Concrete

The findings of an extensive literature review on the permeability of hot-mix asphalt concrete are detailed in a state-of-the-art report on the measurement and interpretation of asphalt concrete permeability data. The permeability of asphalt concrete is affected by a range of factors with various levels of importance, which are reviewed along with their impact on the coefficient of permeability. Many theoretical, empirical, semiempirical, and numerical models have been developed to predict permeability, using a range of indicators. Some of these models are reviewed and their advantages and shortcomings discussed. Recent advances in X-ray tomography studies are also summarized. The review reveals that field permeability measurements are not reported to match numerically well with laboratory measurements, though there is some correlation. The reviewed test methods for permeability all rely on an assumption of laminar flow that is unlikely in more porous mixtures. Attempts to measure connected air voids improve the chances of obtaining more meaningful correlations between permeability and air voids, regardless of the mathematical model used to link the two quantities. The lift-thickness to nominal maximum aggregate size (NMAS) ratio and/or changes in the binder content has a less significant effect on permeability than changes in the porosity and/or mix gradation. Air void gradients and distributions in compacted asphalt concrete mixtures can now be assessed with X-ray techniques: the distribution of air voids appears to be nonuniform in laboratory-prepared specimens.

Two-dimensional numerical simulation on galloping detonation in a narrow channel

The numerical simulations of the two-dimensional galloping detonation performed by using two-dimensional full Navier–Stokes simulations with a detailed chemistry model are presented. The detonation in a narrow channel with d=5mm, which is approximately twice the half-reaction length of hydrogen, shows a feature of galloping detonation with two initiations during its propagation under the laminar flow assumption. The distance between these two initiations is approximately 1300mm, which causes the induction time behind the leading shock wave. As the channel width increases, the galloping feature diminishes. The detonation propagates approximately 4% lower than DCJ for d=10 and 15mm. By increasing the channel width, the strength of the detonation increases, as shown in the maximum pressure histories. The effects of turbulence behind the detonation show that the galloping feature disappears, although its propagation velocity becomes 0.9DCJ. The strength of the detonation becomes significantly weak compared with the detonation propagating in the wide channel widths, and this feature is similar to the laminar assumption. The trend of the velocity deficits in the NS simulations agrees fairly well with the trend of the modified ZND calculations with η=0.25.

The importance of turbulence during condensation in a horizontal circular minichannel

Three-dimensional simulations of condensation of refrigerant R134a in a horizontal minichannel are presented. Mass fluxes ranging from 50kgm−2s−1 up to 1000kgm−2s−1 are considered in a circular minichannel of 1mm diameter, and uniform wall and vapour–liquid interface temperatures are imposed as boundary conditions. The Volume of Fluid (VOF) method is used to track the vapour–liquid interface; the effects of interfacial shear stress, gravity and surface tension are taken into account. The influence of turbulence in the condensate film is analysed and compared against the assumption of laminar condensate flow by employing different computational approaches and validating the results against experimental data. Under the assumption of laminar condensate flow, experimental heat transfer coefficient values at low mass fluxes can be predicted, but the computed heat transfer coefficient is found to be almost independent of mass flux and vapour quality. Only when turbulence in the condensate film is taken into account does the numerical model capture the influence of mass flux that is observed in the experimental measurements.

Modeling of Analysis ICP Torch at Atmospheric Pressure With Applied Voltage

In this paper, a new 2-D nonlinear direct-coupled model for the simulation of inductively coupled plasma torches working at atmospheric pressure is presented. Steady fluid flow and temperature equations are simultaneously solved (direct method) using a finite-element formulation for optically thin argon plasmas under the assumptions of local thermodynamic equilibrium and laminar flow. The electromagnetic field equations are formulated in terms of potential vector with applied voltage source.

On the stability of pseudo-shocks in transonic flows from an asymptotic viewpoint

The global stability properties of weak pseudo-shocks in transonic diffusers are discussed based on a mathematically consistent treatment of strong shock–boundary layer interaction. The equations governing the viscous–inviscid interactions taking place are obtained by matched asymptotic expansions starting from first principles of fluid dynamics. The global stability analysis reveals that the interaction of the pseudo-shock with a shock-induced separation bubble is at the root of self-sustained shock oscillations widely known from technical transonic diffuser flows. It is thus demonstrated that the resulting problem conserves essential properties of transonic flows through diffusers even under the restrictive set of assumptions needed in course of the problem formulation like assumption of laminar boundary flow. Therefore, the presented problem could serve as a suitable starting point to obtain a deeper theoretical insight into the problem of shock oscillations by a suitable generalization of the global linear stability analysis to the weakly non-linear case in future efforts.

Magma ascent and effusion from a tensile fracture propagating to the Earth's surface

SUMMARY An effusive volcanic eruption results from a sequence of different processes, such as the pressurization of a magma chamber, the propagation of a dyke and the flow of lava at the Earth' surface. The aim of this paper is to establish relationships between the different quantities describing such processes. We consider a spherical magma chamber filled with a low-viscosity magma and included in a homogeneous and isotropic elastic half-space. We assume that, as a result of the inflow of fresh magma or a phase transition, the pressure in the chamber increases slowly during a finite time interval. Assuming that the pressure increase is linear in time, we calculate the stress field generated in the surrounding medium considering the chamber as a centre of dilation. We assume that a vertical tensile fracture originates at the top of the magma chamber after the rock strength is exceeded. The fracture is assumed to propagate quasi-statically along a vertical plane, driven by the stress distribution: both the cases of positive and negative buoyancy force are considered. The problem is solved in two dimensions by considering the fracture as a tensile Somigliana dislocation and expanding the associated stress release into Chebyshev polynomials. The fracture may reach the Earth's surface or not, depending on the depth and radius of the magma chamber, the rate and duration of pressure increase, the rock and magma densities and the rock strength. When the fracture reaches the Earth's surface, we assume that it becomes a vertical conduit. Magma pours out from the vent, driven by the pressure gradient in the conduit. Under the assumption of laminar flow of a Newtonian fluid, we evaluate the initial effusion rate as a function of the relevant model parameters. The flow rate is found to be a non-linear function of the density contrast. We also establish a relationship between the flow rate in the conduit and the initial thickness of the ensuing lava flow, in the case of effusion on a steep slope.

Investigation into Fluid Velocity Field of Wedge-Shaped Gap in Grinding

In the grinding process, grinding fluid is delivered for the purposes of chip flushing, cooling, lubrication and chemical protection of work surface. Hence, the conventional method of flood delivering coolant fluid by a nozzle in order to achieve high process performance purposivelly. However, hydrodynamic fluid pressure can be generated ahead of the grinding zone due to the wedge effect between wheel peripheral surface and part surface. In this paper, a theoretical fluid velocity field modeling is presented for flow of coolant fluid of wedge-shaped gap in flood delivery surface grinding, which is based on navier-stokes equation and continuous formulae. The numerical simulation results showed that the velocity in the x direction was dominant and the side-leakage in the y direction existed. The velocity in the z direction was smaller than the others because of the assumption of laminar flow. The smaller the gap is, the larger the velocity in the x direction. The magnitude of the velocity is also proportional to the surface velocity of the wheel.

Numerical simulation of gas flow through porous sandstone and its experimental validation

Due to post-lithogenetic fracturing or weathering, sandstone can contain complex dual porosity structure, which makes it difficult to identify the fluid flow behaviour through it. The main objective of this study is to develop a three dimensional numerical model to simulate the flow of gas through porous sandstone under triaxial test condition. For the purpose of the modelling study, laboratory data which have been obtained from triaxial experiments conducted on porous sandstone samples [1] were used. In this study, a commercial reservoir simulator, COMET 3 was used to model the gas flow in the test samples. The COMET 3 model closely predicts the gas flow through the porous sandstone sample for the range of confining and gas injection pressures studied in the laboratory and for low gas flow rates (laminar flow). However the model fails at high flow rates (turbulent flows), due to the fact that the COMET 3 simulator uses Darcy’s law for flow simulations. Therefore, caution is required when interpreting the model results if the assumption of laminar flow does not apply to the flow conditions. An empirical relationship has been developed in this study which can be used to obtain an estimation of the flowrate when the flow has become non-Darcian. It is however important to be able to identify the transition point from Darcy to non-Darcy flow when the assumption of Darcian flow in COMET 3 no longer applies. Experiments conducted by Yi et al. [1] show very clearly the transition points, which are dependent on the injection and confining pressures. The model developed using COMET 3 appeared to give better prediction of the gas flow rate in sandstone sample, under triaxial test condition, than the COMSOL Multiphysics model given in the study in Yi et al. [1].

Experimental Study of the Performance of Static Seals Based on Measurements of Real Contact Area Using Thin Polycarbonate Films

To clarify the sealing characteristics of metal gasket seals, leakage rates of gas and the real contact area of the seal surfaces were measured under several closing loads. The static seal consisted of a ring-shaped copper gasket and the two steel flanges that held the gasket in place. Gasket widths in the radial direction were 2 mm, 3 mm, and 5 mm. The contact surfaces of the flanges were finished by lathe turning. To determine the leakage flow paths between the gasket and the flanges, the real contact situation between them was observed using a thin polymer film 1 μm in thickness. The results indicated the leakage flow paths on the gasket surface were the radial direction perpendicular to the lathe-turned groove and the circumferential direction along the groove. As the closing loads increased, the leakage flow in the radial direction ceased and only that in the circumferential direction remained. Therefore, the cross-section of the aperture for the leakage flow in the circumferential direction was evaluated from the measured real contact area, and the leakage rates were estimated by assumption of laminar flow. The results agreed well with the measured leakage rates.

The effect of various coil parameters on ICP torch simulation

In this paper numerical simulation is performed to investigate the effects of coil parameters on the characteristics of argon discharges in 2D axi-symmetric inductively coupled plasma (ICP) torch working at atmospheric pressure. Simulations were carried out using an indigenously developed CFD code in FORTRAN for ICP torch. The code uses the standard RF-ICP torch geometry to solve continuity, momentum, energy and vector potential equations under the assumption of LTE, steady and laminar flow. Using ICP torch argon plasma is simulated at oscillator frequency of 3.0 MHz with coil current 152 Amp. The coil parameters such as number of turns of the coil (Nc), spacing between the turns of the coil (Lc), radius of coil turn (Rc), axial variation of first coil position (Pc) and non uniform spacing of the coil turn, are varied to study their effect on temperature and flow fields in ICP torch. The results indicate that increase in number of turns of the coil increase total power dissipated into discharge almost linearly. The spacing between turns of the coil drastically affects the flow pattern and the total power dissipated into discharge. Increase in radius of the coil turn decreases the maximum energy dissipation rate and hence affects the temperature field. The axial variation of coil position affects the flow field .Wall temperature profile plays an important role in deciding the practical suitability of a particular coil configuration. It has been shown that temperature and flow fields can be controlled by changing various coil parameters and one can design an ICP torch system suitable for a particular process application.

The effect of swirl velocity on ICP torch simulation

In this paper numerical simulation is performed to investigate the effects of swirl velocity on the characteristics of argon discharges in 2D axi-symmetric inductively coupled plasma (ICP) torch working at atmospheric pressure. Simulations were carried out using an indigenously developed CFD code for the standard RF-ICP torch geometry under the assumption of local thermodynamic assumption (LTE), steady and laminar flow. The argon plasma is simulated at oscillator frequency of 3.0 MHz with coil current 150 Amp. Study has been done for different swirl velocities of plasma and sheath gas. In this paper, we consider the swirl of sheath and plasma gas and reverse swirl. In reverse swirl, at the inlet, swirl is given to plasma gas in one direction and sheath gas in the opposite directions. The swirl effect has been adjudged from thermal energy transport point of view. It has been found that swirl of sheath gas increases the wall heat loss and decreases the axial temperature towards the outlet of the torch. The increased heat transfer rate along the periphery of plasma core, due to swirl velocity, increases the stability of plasma core.

Numerical and Analytical Study of Reversed Flow and Heat Transfer in a Heated Vertical Duct

Both analytical and numerical calculations are performed to study the buoyancy effect on the reversed flow structure and heat transfer processes in a finite vertical duct with a height to spacing ratio of 12. One of the walls is heated uniformly and the opposite wall is adiabatic. Uniform air flow is assumed to enter the duct. In the ranges of the buoyancy parameter of interest here for both assisted and opposed convection, a reversed flow, which can be observed to initiate in the downstream close to the exit, propagates upstream as Gr/Re2 increases. The increase in the Reynolds number has the effect of pushing the reversed flow downstream. Simple analytical models are developed to predict the penetration depth of the reversed flow for both assisted and opposed convection. The models can accurately predict the penetration depth when the transport process inside the channel is dominated by natural convection. Local and average Nusselt numbers at different buoyancy parameters are presented. Comparison with the experimental data published previously was also made and discussed. Good agreement confirms many of the numerical predictions despite simplifications of the numerical process made, such as two-dimensional and laminar flow assumptions.

Stability of Jeffcott Rotor Operating in Tilting 3-Pad Journal Bearings with Turbulent Oil Film

This paper introduces the results of theoretical investigation on the dynamic characteristics of tilting 3-pad journal bearing that operates with turbulent oil film. The Reynolds, energy, viscosity and geometry equations determine the oil film pressure, temperature distributions, and oil film resultant force that are the grounds for the dynamic characteristics of bearing. These equations were solved simultaneously on the assumption of adiabatic laminar or adiabatic turbulent oil flow in the bearing gap. The stability and system damping of Jeffcott rotor operating in tilting 3-pad journal bearing was determined.

Thickness Effect on the Thrust Generation of Heaving Elliptic Airfoils

The influence of the Reynolds number on the propulsive characteristics of insect wings is investigated by focusing on the aerodynamic transition from drag to thrust. The effect of both the thickness ratio and the Reynolds number on the thrust generation of elliptic airfoils is investigated using a lattice Boltzmann method. Three Reynolds numbers (Re = 50,100, and 185) and two Strouhal numbers (Sr = 0.2 and 0.4) are treated, while the thickness ratio is varied from 0.05 to 1.0. For all investigated Reynolds numbers, it is found that the airfoils oscillating at Sr = 0.2 do not produce thrust. For Sr = 0.4, it is found that thrust is produced at Re = 185. It is found that an airfoil of approximately 10% thickness produces maximum thrust. Thus, it can be said that, for thrust generation, there exists critical Reynolds and Strouhal numbers, and the thickness ratio is also a crucial parameter. By investigating the vortex pattern, velocity profiles, and vorticity intensities of the vortex cores behind the oscillating airfoils, it is found that 1) mushroom vortex patterns indicative of thrust do not completely guarantee the thrust generation, and 2) a phase lag between the thrust-force indicating vortex pattern and the resulting thrust production is observed. The present investigation shows that thrust-producing airfoils should produce strong vortices, enough to overcome the momentum deficit due to the boundary layer. Present results are obtained within the limitation of the laminar flow assumption.

Physical mechanism and theoretical model of thermoacoustic heat exchangers

Unlike a regenerator, there are both oscillating heat exchange flux (first order) and non‐zero time‐averaged heat exchange flux (second order) in the hot and cold end heat exchangers of thermoacoustic systems. The non‐zero time‐averaged heat flux just reflects the net heat exchange between the working substance and external heat sinks, which is more important and interesting for design. In this paper, the physical mechanism of oscillating flow heat exchanger is first analyzed. Based on understanding of the heat transfer mechanism, the theoretical model of heat transfer is then developed; in this part, the key of the problem is to obtain transversal distribution of the second order time‐averaged temperature, T20(y). Eventually, an analytical expression of Nusselt number for the thermoacoustic heat exchangers is obtained under the assumption of laminar flow.

Vortex Induced Vibrations of Finned Cylinders

This article presents the results of a numerical simulation on the vortex induced vibration of various finned cylinders at low Reynolds number. The non-dimensional, incompressible Navier-Stokes equations and continuity equation were adopted to simulate the fluid around the cylinder. The cylinder (with or without fins) in fluid flow was approximated as a mass-spring system. The fluid-body interaction of the cylinder with fins and uniform flow was numerically simulated by applying the displacement and stress iterative computation on the fluid-body interfaces. Both vortex structures and response amplitudes of cylinders with various arrangements of fins were analyzed and discussed. The remarkable decrease of response amplitude for the additions of Triangle60 fins and Quadrangle45 fins was found to be comparable with that of bare cylinder. However, the additions of Triangle00 fins and Quadrangle00 fins enhance the response amplitude greatly. Despite the assumption of two-dimensional laminar flow, the present study can give a good insight into the phenomena of cylinders with various arrangements of fins.