The following remarks outline the structure of the Newtonian Mach’s principle and its implications for fluid motion and turbulence. This principle can only be understood as part of classical thermomechanics on a global scale and is directly related to both the first axiom of Newtonian mechanics and to global total energy covariance.

The magnetoplasmadynamic (MPD) arcjet is a promising thruster which is developed for exploration missions to the moon and Mars, and for raising orbits of large space structures. The MPD arcjet utilizes mainly electromagnetic force, i.e Lorentz force J × B, which is generated in this work by interaction between the current density and a coaxial magnetic field azimuthally induced by the total discharge current. In the present notes, we describe the implementation of a density–pressure-based method for the simulation of the magnetohydrodynamic (MHD) equations under a finite volume formulation. This new algorithm was developed for both ideal and resistive MHD equations and make use of the central-upwind schemes of Kurganov and Tadmor for flux calculation. As we assume that the plasma flow is a continuum fluid, electrical conductivity is predicted according to the Spitzer-Harm formulation. With the developed model, a limited set of computer runs was performed to assess the effect of geometric scale changes on an Argon self-field MPD thrusters performance. The results are reported and discussed.

A high speed camera investigation is presented into the behavior of CO₂ dry-ice particles in an application of dry-ice blasting to the defouling of commercial aircraft engine compressors. An image acquisition system is deployed in the compressor section of an aircraft engine and is used to determine the evolution of dry-ice particle size and velocity from the nozzle exit to the entrance to the engine’s high pressure compressor as the engine is cranked. A comparison study between CO₂ dry-ice particle laden flows and airflows with single Polyoxymethylene (POM) particles of various diameters is also presented. Measurements are made using a range of blasting system pressures and using sonic and supersonic blasting nozzles. The behavior of large CO₂ dry-ice particles (dP ≥ 1 mm) in this discontinuously and inhomogenously laden flow is compared to that of single POM particles under similar flow conditions and is found to behave similarly. The experiments presented turn out to be useful for supporting development of special purpose dry-ice blasting systems.

Among basic problems associated with additive manufacturing of metal objects by accretion of moltenmetal droplets is a number of questions of fluid-mechanical character. Authors concentrate attention on two alternative designs of droplets generation: one using centrifugal force and the other with liquid surface deformed electrostatically into a cone producing the droplets at its tip.

This paper presents a newly developed overlapping domain decomposition (ODD) method, which forms the basis of a near-far field coupling solver for a wide range of wave-structure interaction problem. In this method, the computational domain is decomposed into near- and far-fields which are then modeled separately by solving the viscous Navier-Stokes equation (NSE) and the Potential Laplacian equation (PLE), respectively. The free-surface problem is solved in both domains but using totally different strategies: a moving mesh-free surface tracking method is adopted in the potential domain; and the volume of fluid (VOF) method is used to track the free surface in the viscous domain. The novelty of the reported method lies in two-folds. First, similar to the relaxation zone technique, the introduction of the overlapped buffer zone eliminates the need of performing time time-consuming iterative schemes in the non-ODD methods to ensure the matching of free-surface elevation at the domain boundaries. Second, an in-house developed OVERSET method is adopted for the viscous domain solver to handle large object displacement in the case of extreme event. Finite volume method (FVM) is adopted to discretize both the NS and PL equations. The proposed method has been implemented in the OpenFOAM platform. To validate the method, is firstly employed to simulate a solitary wave and the resulting wave parameters are compared to the analytical solution, which suggests the accuracy and efficiency of the proposed method.

The use of compressed natural gas (CNG) as a fuel in internal combustion engines brings significant advantages in terms of reduction of CO₂-emissions and fuel consumption. Compared to standard gasoline combustion, the CO₂-production can be clearly reduced by using a methane-based fuel as it has a beneficial H/C ratio. The high knock resistance of methane allows higher compression ratios so that the thermodynamic efficiency is enhanced. Furthermore, the realization of a stratified mixture formation concept shows great potential to further increase fuel savings due to the reduced throttling losses.

In the present work, an URANS-based simulation strategy using the commercial code AVL Fire for the direct injection (DI) of CNG and the mixture formation, including a discretisation of the full nozzle and cylinder geometry is presented. High pressure ratios between the injector and the cylinder lead to a choked flow inside the nozzle. A supersonic region, including shock-occurrence follows as the jet is expanded further downstream the orifice. To successfully resolve the multi-scale flow phenomena the mesh generation process involves the design of a fine hexahedral mesh for the injector, which is merged to the moving cylinder mesh by an arbitrary interface. Turbulence is modelled using the k-ζ-f model. To estimate the grid-induced error of the simulation, a set of calculations was performed on meshes of decreasing cell dimension. Different nozzle geometries are investigated and evaluated with regard to their mixture formation suitability as well as the effect of increasing rail pressure. Variations include an inward opening multi-hole injector and an outward opening annular ring injector. The results show a strong tendency of the gas jets to interact with each other and with the surrounding walls.

The implementation of subsea separation and liquid boosting is becoming a common development scheme for the exploration of deep water fields. Subsea separation is an attractive and economic solution to develop deep offshore fields producing fluid without hydrate or wax. Recently the subsea separation system is designed for a water depth of 3,000 m and internal design pressure up to 690 bar. Development and application of subsea separation system are relatively common in the developed countries and many studies have been conducted previously, but it is still a new field in Korea and this is the significance of this study. In this study, gas-liquid mixture flow characteristics inside in-line type subsea separation system are investigated by numerical and experimental studies for the development of subsea separator. For the subsea separator designed in this study, it is predicted to have a separating efficiency of 70%.

To model submarine flows of granular materials we propose an extension of the drift-flux approach. The extended model is able to represent dilute suspensions as well as dense granular flows. The dense granwular flow is modelled as a Herschel–Bulkley fluid, with a yield stress that depends on the dispersed phase pressure. Qualitative numerical experiments show that the model is able to correctly reproduce the stability of submerged sand heaps with different internal angles of friction and initial slopes. When initially starting with heaps with an angle smaller than the internal angle of friction, the heaps are stable. When starting with heaps with angles larger than the internal angle of friction, a flow of solid material is initiated. The flow later stops when the bed is at an angle smaller than the internal angle of friction.

Suspensions of nanoscale particles and fluids have been recently subject of intense research, since it was proved that they considerably improve heat transfer capabilities of the fluid which can be crucial in several technological processes. Several applications can be found in the field of porous media flow, such as oil recovery systems, thermal and geothermal energy, nuclear reactors cooling. Since nanofluids are a mixture of a solid and fluid phase, in general, the two phase mathematical model would be the most appropriate to use. However, due to very small size of nanoparticles (1–100 nm) can be assumed, that they behave as a water molecule and a single phase model along with empirical correlations for nanofluid properties can be used. In the present study a convective flow through porous cavity fully saturated with nanofluid is analyzed in detail using the single phase mathematical model based on the Navier-Stokes equations taking into account the non-Darcy parameters. The mathematical model is written at a macroscopic level enabling the simulation of the porous media flow. The solutions are obtained with the in house numerical code based on the Boundary Element Method, which was already proved to have some unique advantages when considering fluid flow problems in different configurations. The effects of the presence of different types of nanoparticles as well as the porous matrix were investigated in detail for different values of governing parameters in order to examine the improved heat transfer characteristics of added nanoparticles.

A numerical study on the laminar vortex shedding frequency on a porous and partially porous circular tube has been made. Porous tubes characterised by an inner and outer diameter of the porous zone are subjected to a uniform flow. The porous region is divided in eight segments of 45° with a different porous structure. The porous behaviour has been modelled by a Darcy-equation. To ensure a laminar vortex shedding, the Reynolds number has been chosen between 40 and 200. The influence of the Reynolds number and the properties of the porous zone (thickness, pressure drop), characterised by a dimensionless thickness L and a Darcy number Da on vortex shedding frequency (dimensionless Strouhal number Sr) and volume flow through the porous zone (dimensionless volume flow P) has been analysed.

Effects of circulation on the evolution of vortex tubes and the associated response of near-wall flows in the shear of laminar boundary-layer flows are investigated using a model proposed by Hon and Walker (Hon, T.L. & Walker, J.D.A, Computers & Fluids, 20(3), pp. 343–358, 1991). Direct numerical simula- tions with freestream Mach number of 0.5 are conducted. Firstly, the dynamics of single hairpin vortex is investigated. Numerous secondary hairpin vortices, much more than previously reported, which are regularly aligned in the streamwise direction are allowed to be newly generated according to the shear-layer instability of the legs of an initial hairpin vortex. Small-scale turbulence is then produced when the circulation is sufficiently large. Secondly, a straight vortex tube model is investigated. Sinu- ous deformation of a shear layer, which leads to the generation of discrete hairpin vortices, becomes obvious especially near the upper region of the vortex tube. In order to quantify the initial instability triggering the generation of the secondary hairpin vortices, quasi-linear stability analysis is conducted. While only one unstable mode appears when the circulation is small, two modes, that is, off-wall mode and near-wall mode, appear when the circulation is large. The cases of circulation where the two modes appear correspond to those of circulation where the production of small-scale turbulence is observed in the simulations of the single hairpin vortex.

Cavitation is the formation of vapour cavities in a liquid due to a local low pressure. The traditional cavitation number is used to predict the occurrence of cavitation in liquid flows through devices such as pumps, propellers or dam spillways. However, this number can only be applied when cavitation is produced by changes in the dynamic and static pressure in a liquid flow. There are other means to pro- duce cavitation where the traditional cavitation number cannot be applied. The purpose of this research is to formulate a new dimensionless number valid to predict cavitation in some scenarios where the traditional cavitation number fails. The ‘tube-arrest’ method produces cavitation by subjecting a col- umn of liquid to a high acceleration without the need of any velocity between the liquid and the tube. In this scenario, the traditional number is not useful due to the low values of relative velocity between liquid and walls. However, the dimensionless number reported here predicts accurately the occurrence of cavitation in the ‘tube-arrest’ method, as it is shown by Finite Element Method analysis. There is another scenario where the dimensionless number is tested successfully; that is, in the bulk of a liquid downstream of a closing valve. A systematic comparison between the values of the dimensionless number and the occurrence of cavitation predicted by the FEM analysis is given. On the other hand, the values of the traditional cavitation number are calculated and it is shown that these values are meaning- less in these scenarios. In contrast, the agreement between the prediction of the dimensionless number and the simulations is excellent. It is concluded that the new dimensionless number predicts cavitation in scenarios where the traditional number is meaningless.

We present an experimental investigation of water flow around a hydrofoil with a superhydrophobic patterned surface. The experimental setup uses a water tunnel to measure the drag over the hydrofoil and acquire velocity field measurements using particle image velocimetry (PIV). Drag reduction on the order of 10% or higher was observed on hydrofoils with irregular surface textures combined with super-hydrophobic coating, leading to effective Navier slip on the surface. However, we report that other macroscopic flow characteristics, including the stall angle, are also changed by application of the coating.

This study presents an experimental investigation on a diffuser augmented wind turbine (DAWT). A screen mesh is used to simulate the energy extraction mechanisms of a wind turbine in experiment. Different screen porosities corresponding to different turbine loading coefficients are tested. Measure- ments of the axial force and of the velocity distribution in radial direction are reported. The general purpose is to highlight the dependency between the diffuser and the screen, and to compare the radial velocity distributions in the diffuser between unloaded and loaded conditions. It is shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition. More- over, the thrust on the diffuser largely depends on the screen loading. For the present configuration, the thrust on the screen with high loading coefficient contributes for more than 70% of the total thrust on the DAWT. Smoke visualizations and radial velocity profiles reveal that the high loading screen induces flow separation on the outer surface of the diffuser, justifying the results of the thrust measurements. It is also inferred that the flow separation leads to loss of thrust and has a great effect on the total pressure drag. It should be emphasized that the experimental results indicate that the flow field around the diffuser is strongly affected by the choice of screen porosity, that is, turbine loading. And that, the thrust coef- ficient of the diffuser does not show a linear dependence on the thrust coefficient of the screen. The axial momentum theory, therefore, is not a solid predictor for DAWT performance with high loaded screens.

Diffuser Augmented Wind Turbines (DAWTs) have been a highlight of research in small size wind turbines due to their potential capability of exceeding Betz Limit. Bringing a wall in close proximity of blade tip may have strong influences on physics of tip leakage flow. In current study, the effect of the diffuser wall on tip leakage flow is examined. Different tip gap to diffuser radius ratios are numerically studied in a two dimensional domain. As tip leakage flow in DAWTs has not been studied before, the methodology for the current work is adopted from similar research in gas turbines. Obtained results indicate that as the tip gap to diffuser radius ratio is reduced from 0.067 to 0.02, tip leakage mass flow rate reduces to 6% and the total pressure loss increases by 56%. This behavior of tip leakage flow is further elaborated by discussing the flow physics in the gap region.

In this paper, a computational approach, based on the solution of Reynolds-averaged-Navier–Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported. In order to reduce the computational cost, the turbine is modeled as an actuator disc (AD) that imposes a resistance to the passage of the flow. The effect of the AD is modeled applying two body forces, upstream and downstream of the AD, such that they impose a desired pressure jump. Com- parison with experiments carried out in similar conditions shows a good agreement suggesting that the adopted methodology is able to carefully reproduce real flow features.

In this paper, we study the problem of scattering of surface water waves by a thin circular arc shaped porous plate submerged in the deep ocean. The problem is formulated in terms of a hypersingular integral equation of the second kind in terms of an unknown function representing the difference of potential function across the curved barrier. The hypersingular integral equation is then solved by a collocation method after expanding the unknown function in terms of Chebyshev polynomials of the second kind. Using the solution of the hyper-singular integral equation, the reflection coefficient, trans- mission coefficient and energy dissipation coefficient are computed and depicted graphically against the wave number. Known results for the rigid curved barrier are recovered. It is observed that the poros- ity of the barrier reduces the reflection and transmission of the waves and enhances the dissipation of wave energy. The reflection coefficient and dissipation of wave energy decreases as the length of the porous curved barrier increases. Also the reflection coefficient is almost independent of the inertial force coefficient of the material of the porous barrier. However, the inertial force coefficient of the material of the porous barrier enhances transmission and reduces dissipation of wave energy.

A numerical formulation of flow-induced instability modelling of laminated anisotropic pipelines is derived. The analysis is based on fluid-structure interaction equations and FEA. Taking into account the flow parameters and the material properties, critical flow velocities causing instability are calculated for fibre-reinforced polymeric (FRP) pipelines resting on elastic supports. A parametric study of the effect of fibre orientation, stiffness of elastic supports and span length between supports is carried out. The results are commented and discussed.

The compatibility of the semiempirical turbulence theory of L. Prandtl with the actual flow pattern in a turbulent boundary layer is considered in this article, and, based on this theory, the final calculation results of the boundary layer are analyzed. These show that the accepted additional conditions and relationships, which integrate the differential equation of L. Prandtl, associating the turbulent stresses in the boundary layer with the transverse velocity gradient, are fulfilled only in the near-wall region, where the mentioned equation loses meaning, and are inconsistent with the physical meaning in the main part of integration.

It is noted that an introduced concept regarding the presence of a laminar sublayer between the wall and the turbulent boundary layer is the way to give a physical meaning to the logarithmic velocity profile.

It shows that coincidence of the experimental data with the actual logarithmic profile is obtained as a result of the use not of a particular physical value, as an argument, but of a function of this value. In this way, the coincident experimental points in general are concerned with different sections of the boundary layer. Accordingly, the informational value of the comparison of the calculations and the experiment given in the literature is actually eliminated.

Particle interactions in highly-viscous nonlinear and linear shear flows play an important role in a variety of applications including composite materials processing, microfluidics, chromatography, and particle resuspension, to name a few. Binary interactions among particles can provide information used in rheological models for suspension flows such as migration rates and self-diffusivity. In past numeri- cal studies, particle roughness has been treated, for the most part, as a constant, static quantity. In the current study, roughness is treated as a stochastic parameter. Hence, quantities such as dispersion, net particle migration, and self-diffusivity also become stochastic parameters. Numerical simulations are performed using a semi-analytic solution for the motion of two particles in an arbitrary unbounded flow field to determine the effects of random particle roughness.

Surface cleaning prior to coating and fabrication processes is an important process to ensure the quality and proper functioning of the products. The current work involves the modeling of a cleaning process where viscous media is removed from surfaces using mechanical force from impinging water jets. The jets are mounted on a rotating nozzle carrier, which combine the normal force with increased tangential force resulting in the removal of oil present in the grooves of metal surfaces.

The modeling of such a process is performed with the volume of fluid (VOF) method with an open source Computational Fluid Dynamics (CFD) code Open-FOAM®. Rectangular grooves, which represent an idealized form of roughness are considered for numerical analysis. The oil present in the roughness grooves is resolved by the computational mesh.

The inlet is modified to model the phenomenon of rotating jets. In order to model the effect of rotating jets, a reference vector is transformed by a time-dependent rotational tensor. All the faces which make a certain angle (opening angle) with the reference vector are activated. This results in the formation of jets with a thickness which can be controlled by the opening angle. The numerical model is used to study the influence of the frequency of rotation, nozzle exit velocity, viscosity of oil and the aspect ratio of the grooves on surface cleaning. An impinging turbulent jet is modeled using the k-epsilon turbulence model.

Finally, the CFD simulations are qualitatively compared with a previously developed semi-empirical model and experiments conducted in an industrial setup. The tendencies of oil removal due to the effect of the process parameters observed in the simulation are in close agreement with the semi-empirical model and experimental results. Thus, the cleaning model can be used to conduct sensitivity analysis to achieve an optimal performance of the cleaning process.

The characteristics of the measurement capacity of air-coupled ultrasonic sensor influenced by the incident angle were investigated analytically and experimentally. The optimized incident angle between ultrasound and test pipe was determined. The air-coupled ultrasonic sensor with this determined incident angle was applied to the flowrate measurement in the aluminium pipe. The measurement results were compared to those obtained by using the electromagnetic flowmeter. Since the measurement results show good linearity, the capacity of the air-coupled ultrasonic flowmeter is revealed.

Nowadays open-source CFD codes provide suitable environments for implementation and testing low-dissipative algorithms typically used for turbulence simulation. Moreover these codes produce a reliable tool to test high-fidelity numerics on unstructured grids, which are particularly appealing for industrial applications. Therefore in this work we have developed several solvers for incompressible Navier-Stokes equations (NSE) based on high-order explicit and implicit Runge-Kutta (RK) schemes for time-integration. Note that for NSE space discretization the numerical technology available within OpenFOAM (Open-source Field Operation And Manipulation) library was used.

Specifically in this work we have considered explicit RK projected type schemes for index 2 DAE system and L-stable Singly Diagonally Implicit Runge-Kutta (SDIRK) techniques. In the latter case an iterated PISO-like procedure based on Rhie-Chow correction was used for handling pressure-velocity coupling within each RK stage. The accuracy of the considered algorithms was evaluated studying the Taylor-Green vortex. Moreover several benchmark solutions have been computed in order to assess the reliability, the accuracy and the robustness of the presented solvers.

The Czochralski crystal growth manufacturing process results in small periodic and undesirable fluctuations in the crystal diameter under certain conditions. These fluctuations have strong, non-linear characteristics and are likely to appear at combinations of critical values of certain parameters, such as the rotational velocity, the ratio of crystal radius to crucible radius, and the temperature gradient.

This paper uses perturbation theory to try to identify the critical combinations of parameters that lead to these fluctuations. Firstly, the zero and first-order equations are obtained. Secondly, numer-ically-based steady-state solutions of these equations are calculated, and finally, the stability of the steady-state solutions is examined. It is observed that the steady-state solutions do not exhibit any unusual patterns for any values of the configuration parameters. Furthermore, all the steady-state solutions are found to be stable for all initial conditions; therefore, the steady-state solutions and the analysis of their stability did not indicate the source of the observed fluctuations. This analysis suggests that a better approximation of the equations such as second order perturbation analysis may be needed to identify the conditions that lead to the observed fluctuations.

The process of cooling caused by a water droplet contacting a surface has been extensively reported in the literature; however, the effect of surface wettability on the outcome of the cooling rate has yet to be analyzed. Due to optical limitations inside a liquid droplet, a three-dimensional (3D) computational fluid dynamics (CFD) model, including coupling between multiphase flow and the conjugated heat transfer module was developed to simulate the impact, spreading and transient heat transfer between a cold-water droplet and a heated surface. The total heat transfer results were calculated for both superhydrophobic and hydrophilic surfaces. The Navier-Stokes equation expressing the flow distribution of the liquid and the gas, coupled with the volume of fluid (VOF) method for tracking the liquid interface, was solved numerically using the finite volume methodology. The grid dependency test was examined for the 3D model, even though the convergence of the results was not exact. The 2 mm diameter water droplet with the Weber numbers 7, 25 and 62, which correspond to non-splashing regimes, were impinged onto two different surfaces. We showed that spray cooling on a superhydrophobic substrate was capable of improving the efficiency of the cooling process up to 40% compared to that of a hydrophilic surface. Additionally, the critical Weber regime was obtained for the optimal heat transfer between the droplet and the two substrates.