DNS and RANS simulations were carried out for core-annular flow in a horizontal pipe and results were compared with experiments carried out with water and oil in our lab. In contrast to most existing studies for core-annular flow available in the literature, the flow annulus is not laminar but turbulent. This makes the simulations more challenging. As DNS does not contain any closure correlations, this approach should give the best representation of the flow (provided a sufficiently accurate numerical mesh and numerical method is used). Various flow configurations were considered, such as without gravity (to enforce an on-average concentric oil core) and with gravity (to allow for eccentricity in the oil core location). Both single-phase and two-phase conditions were considered; single-phase flow refers to the water annulus with imposed wavy wall, whereas two-phase flow includes the determination of the wavy interface. Mesh refinement was carried out to assess the numerical accuracy of the simulation results.

@en

The Reynolds-Averaged Navier Stokes (RANS) with the Launder & Sharma low-Reynolds number k−ε model was used to simulate core-annular flow in the same configuration with vertical upflow as considered by Kim & Choi (2018), who carried out Direct Numerical Simulations (DNS), and by Vanegas Prada (1999), who performed experiments. The DNS are numerically very accurate and can thus be used for benchmarking of the RANS turbulence model. There is a large ratio between the oil and water viscosities, and the density difference between the water and oil is only small. The frictional pressure drop was fixed and the water holdup fraction was varied. Differences between the RANS and DNS predictions, e.g. in the wave structure and in the Reynolds stresses, are discussed. Despite the shortcomings of the considered Launder & Sharma low-Reynolds number k−ε model in RANS, in comparison to DNS, the RANS approach properly describes the main flow structures for upward moving core-annular flow in a vertical pipe, like the travelling interfacial waves in combination with a turbulent water annulus. The Fanning friction factor with RANS is 18% lower than with DNS, and the holdup ratio with RANS is only slightly higher than with DNS (i.e. it has a slightly larger tendency to accumulate water in RANS than in DNS).

@en

Interfacial waves in core-annular pipe flow are studied through two-phase numerical simulations. Here the water annulus is turbulent, whereas the oil core stays laminar. The low-Reynolds number Launder & Sharma k−ε model is applied. By extracting the moving wave shape from the two-phase results and imposing this as a solid boundary in a single-phase simulation for the water annulus gives single-phase results (for the pressure drop and holdup ratio) that are in close agreement with values obtained from the two-phase approach. The influence of wave amplitude and wave length on the pressure drop and hold up ratio is then studied by using the single-phase flow model. This gives insight in the appearance of core-annular flow, where the water-based Fanning wall-friction factor and the hold-up ratio are selected as the most important quantities. The effect of watercut and eccentricity on these quantities is also investigated.

@en

A three-dimensional (3D) numerical simulation was performed using a combined stroke swimmer (deformable sphere) in an incompressible fluid of an infinite domain. The time-dependent deformation of the swimmer surface was assumed independent of the circumferential cross section in the flow direction of the swimmer. The 3D numerical simulation is an extension of our previous study that considered an axisymmetric numerical simulation. In particular, different fluid viscosities were considered for the same stroke of the swimmer. The effect of the swimmer inertia was studied by gradually decreasing the fluid viscosity. When the fluid viscosity decreased, the mean velocity of the swimmer changed its direction between Re = 0.00189 and Re = 0.0103. There is a transition between Re = 0.0103 and Re = 9.90 from the axisymmetric to three-dimensional flow that exhibits planar symmetry.

@en

1D, 2D and 3D numerical simulations were carried out with the Reynolds-Averaged Navier-Stokes equations (RANS) for horizontal oil-water core-annular flow in which the oil core stays laminar while the water layer is turbulent. The turbulence is described with the Launder-Sharma low-Reynolds number k−ϵ model. The simulation results are compared with experiments carried out in our lab in a 21 mm diameter pipe using oil and water with a viscosity ratio of 1150 and a density ratio of 0.91. The 1D results represent perfect turbulent CAF (i.e. no gravity, no interfacial waves), the 2D results represent axi-symmetric CAF (i.e. no gravity, with interfacial waves), and the 3D results represent eccentric CAF (i.e. with gravity, with interfacial waves). The simulation results typically show a turbulent water annulus in which the structure of the (high-Reynolds number) inertial sublayer can be recognized. The pressure drop reduction factor (which is the ratio between the pressure drop for CAF and the pressure drop for single phase viscous oil flow) for the 2D and 3D results is about the same, but its value is about 35% higher than in the experiment. The hold-up ratio in the 3D model is close to the experimental value, but the 2D prediction is slightly lower. The eccentricity predicted by the 3D simulations is much higher than in the experiment. Most likely, the observed differences between the simulations and the experiments are due to limitations of using a low-Reynolds number k−ϵ model. In particular the water layer at the top in the 3D results shows a relaminarization, which might be absent in the experiment.

@en

Core-annular flow is an efficient way of transporting viscous oil through a pipeline. A sharp increase in the pressure drop will occur when the oil waves at the water-oil interface touch the pipe wall. Depending on the oil and pipe material physical properties, the oil may adhere to the wall leading to fouling. Therefore, a necessary requirement for the onset of oil fouling of the pipe wall is that the flow hydrodynamics allow the oil to reach and touch the wall. With respect to the problem statement, this study deals with finding the hydrodynamic conditions under which core-annular flow becomes unstable and the oil waves touch the pipe wall. The method that is followed is to resolve the first-principle set of equations that describe the hydrodynamics: the Reynolds-Averaged Navier-Stokes (RANS) equations are solved using Computational Fluid Dynamics (CFD) in the opensource package OpenFOAM. Simulations were carried out for the horizontal pipe with two diameters (10.5 and 21 mm), at a range of imposed pressure drops and water holdup fractions (giving the mixture velocity and watercut as output). Most simulations were carried out for an oil to water viscosity ratio of 1040 (but also a variation of this was considered). For each value of the pressure drop (or mixture velocity) there is a critical value of the watercut below which the oil reaches the pipe wall. This critical value of the watercut is lower for the larger pipe diameter of 21 mm, namely about 9.6%, than for the smaller pipe diameter of 10.5 mm, namely about 14% (for a viscosity ratio m = 1040). Wall touching occurs when the mixture velocity is too low, but this lower limit is significantly higher for the larger pipe diameter of 21 mm, namely about 1.1 m/s, than for the smaller pipe diameter, namely about 0.3 m/s (for a viscosity ratio m = 1040). The main conclusion is that a state-of-art CFD approach is capable of simulating the growth of waves at the oil-water interface until they touch the pipe wall, which is a necessary condition for the onset of fouling.

@en

A numerical simulation has been made of the combined stroke swimmer (a deformable sphere) and compared with the results of the second-order perturbation theory of Felderhof and Jones (2017). At a small ratio of the amplitude of the deformation of the sphere and the radius of the sphere the numerical and theoretical results agree well. However for a larger value of this ratio the results deviate due to inertia. The streamlines, as calculated numerically, change significantly with increasing inertia.

@en

Nuclear reactor designs such as PWR and BWR are susceptible to vibrations induced on the nuclear fuel rods due to fast flowing coolants around the rods. The non-linear behaviour of flexible components have always been a challenge to compute especially when dealing with strongly coupled fluid–structure interaction cases as found in the reactors. Simulating such a behaviour involves a two-way coupling of a well resolved turbulent flow Computational Fluid Dynamics (CFD) solver to a Computational Solid Mechanics (CSM) solver. The use of a high fidelity CFD solver to resolve turbulent flows in an FSI (Fluid-Structure Interaction) simulation is computationally expensive ergo is not practical in for industrial purposes. To address this issue, a different approach is discussed in this article to simulate turbulence induced vibrations through the use of U-RANS models. The method is based on computing the modeled turbulent pressure and velocity fluctuations from values obtained by solving U-RANS (Unsteady-Reynolds Averaged Navier Stokes) equations. The calculated turbulent fluctuating field is combined with mean values to compute an instantaneous turbulent pressure field to apply an external pressure and shear force on the structure and vice-verse until convergence is achieved. This method can be used to accurately estimate the behaviour of a flexible structure in a turbulent flow. The article provides a detailed explanation of the model followed by validation with three numerical test cases. The first case involves a CFD simulation where results from the pressure fluctuation model (PFM) is compared to a benchmark DNS (Direct Numerical Simulation) of a turbulent channel flow with friction Reynolds number, Re_τ=640. Later the PFM is applied to a 2-dimensional strongly-coupled FSI simulation with a flexible steel flap in turbulent water flow to study the feasibility and stability of PFM applied to an FSI problem. Finally, the PFM is used to simulate an experimental case of a brass rod excited by turbulent water performed by Chen and Wambsganns (1972). The results show that the PFM is capable of simulating turbulence induced vibration (TIV) with low-fidelity U-RANS models.

@en

The vestibular system in the inner ear senses angular head manoeuvres by endolymph fluid which deforms a gelatinous sensory structure (the cupula). We constructed computer models that include both the endolymph flow (using CFD modelling), the cupula deformation (using FEM modelling), and the interaction between both (using fluid–structure interaction modelling). In the wide utricle, we observe an endolymph vortex. In the initial time steps, both the displacement of the cupula and its restorative forces are still small. As a result, the endolymph vortex causes the cupula to deform asymmetrically in an S-shape. The asymmetric deflection increases the cupula strain near the crista and, as a result, enhances the sensitivity of the vestibular system. Throughout the head manoeuvre, the maximal cupula strain is located at the centre of the crista. The hair cells at the crista centre supply irregularly spiking afferents, which are more sensitive than the afferents from the periphery. Hence, the location of the maximal strain at the crista may also increase the sensitivity of the semicircular canal, but this remains to be tested. The cupula overshoots its relaxed position in a simulation of the Dix-Hallpike head manoeuvre (3 s in total). A much faster head manoeuvre of 0.222 s showed to be too short to cause substantial cupula overshoot, because the cupula time scale of both models (estimated to be 3.3 s) is an order of magnitude larger than the duration of this manoeuvre.

@en

Computational fluid dynamics (CFD) is a powerful method to investigate aneurysms. The primary focus of most investigations has been to compute various hemodynamic parameters to assess the risk posed by an aneurysm. Despite the occurrence of transitional flow in aneurysms, turbulence has not received much attention. In this article, we investigate turbulence in the context of abdominal aortic aneurysms (AAA). Since the clinical practice is to diagnose an AAA on the basis of its size, hypothetical axisymmetric geometries of various sizes are constructed. In general, just after the peak systole, a vortex ring is shed from the expansion region of an AAA. As the ring advects downstream, an azimuthal instability sets in and grows in amplitude thereby destabilizing the ring. The eventual breakdown of the vortex ring into smaller vortices leads to turbulent fluctuations. A residence time study is also done to identify blood recirculation zones, as a recirculation region can lead to degradation of the arterial wall. In some of the geometries simulated, the enhanced local mixing due to turbulence does not allow a recirculation zone to form, whereas in other geometries, turbulence had no effect on them. The location and consequence of a recirculation zone suggest that it could develop into an intraluminal thrombus (ILT). Finally, the possible impact of turbulence on the oscillatory shear index (OSI), a hemodynamic parameter, is explored. To conclude, this study highlights how a small change in the geometric aspects of an AAA can lead to a vastly different flow field.

@en

Simultaneous particle-image velocimetry and laser-induced fluorescence combined with large-eddy simulations are used to investigate the flow and pollutant dispersion behaviour in a rural-to-urban roughness transition. The urban roughness is characterized by an array of cubical obstacles in an aligned arrangement. A plane fence is added one obstacle height h upstream of the urban roughness elements, with three different fence heights considered. A smooth-wall turbulent boundary layer with a depth of 10h is used as the approaching flow, and a passive tracer is released from a uniform line source 1h upstream of the fence. A shear layer is formed at the top of the fence, which increases in strength for the higher fence cases, resulting in a deeper internal boundary layer (IBL). It is found that the mean flow for the rural-to-urban transition can be described by means of a mixing-length model provided that the transitional effects are accounted for. The mixing-length formulation for sparse urban canopies, as found in the literature, is extended to take into account the blockage effect in dense canopies. Additionally, the average mean concentration field is found to scale with the IBL depth and the bulk velocity in the IBL.

@en

Both large-eddy simulations (LES) and water-tunnel experiments, using simultaneous stereoscopic particle image velocimetry and laser-induced fluorescence, have been used to investigate pollutant dispersion mechanisms in regions where the surface changes from rural to urban roughness. The urban roughness was characterized by an array of rectangular obstacles in an in-line arrangement. The streamwise length scale of the roughness was kept constant, while the spanwise length scale was varied by varying the obstacle aspect ratio l / h between 1 and 8, where l is the spanwise dimension of the obstacles and h is the height of the obstacles. Additionally, the case of two-dimensional roughness (riblets) was considered in LES. A smooth-wall turbulent boundary layer of depth 10h was used as the approaching flow, and a line source of passive tracer was placed 2h upstream of the urban canopy. The experimental and numerical results show good agreement, while minor discrepancies are readily explained. It is found that for (Formula presented.) the drag induced by the urban canopy is largest of all considered cases, and is caused by a large-scale secondary flow. In addition, due to the roughness transition the vertical advective pollutant flux is the main ventilation mechanism in the first three streets. Furthermore, by means of linear stochastic estimation the mean flow structure is identified that is responsible for street-canyon ventilation for the sixth street and onwards. Moreover, it is shown that the vertical length scale of this structure increases with increasing aspect ratio of the obstacles in the canopy, while the streamwise length scale does not show a similar trend.

@en

An experimental and numerical study has been made of oil-water core-annular flow in a horizontal pipe with special attention for the turbulence in the water. An experimental set-up was built to be able to compare numerical predictions with detailed experimental results. The oil density was considerably smaller than the water density. Full 3D and time-dependent numerical simulations of some of the experiments were done. Only when a turbulence model is applied the agreement with the experiments is reasonably good.@en

A Random Positioning Machine (RPM) is a device used to study the role of gravity on biological systems. This is accomplished through continuous reorientation of the sample such that the net influence of gravity is randomized over time. The aim of this study is to predict fluid flow behavior during such RPM simulated microgravity studies, which may explain differences found between RPM and space flight experiments. An analytical solution is given for a cylinder as a model for an experimental container. Then, a dual-axis rotating frame is used to mimic the motion characteristics of an RPM with sinusoidal rotation frequencies of 0.20.2 Hz and 0.10.1 Hz while Particle Image Velocimetry is used to measure the velocity field inside a flask. To reproduce the same experiment numerically, a Direct Numerical Simulation model is used. The analytical model predicts that an increase in the Womersley number leads to higher shear stresses at the cylinder wall and decrease in fluid angular velocity inside the cylinder. The experimental results show that periodic single-axis rotation induces a fluid motion parallel to the wall and that a complex flow is observed for two-axis rotation with a maximum wall shear stress of 8.08.0 mPa (80 mdyne/cm2mdyne/cm2). The experimental and numerical results show that oscillatory motion inside an RPM induces flow motion that can, depending on the experimental samples, reduce the quality of the simulated microgravity. Thus, it is crucial to determine the appropriate oscillatory frequency of the axes to design biological experiments.0@en

Turbulence Induced Vibrations (TIV) or Turbulence Buffeting in a nuclear reactor is an undesirable side effect of achieving high coolant flow rates. The turbulent pressure field of these flows force the structure to vibrate within the setup. High amplitude vibrations of the fuel rods can lead to structural damage within the reactor and create a safety hazard. TIV is a difficult phenomenon to predict and limited experimental data is available. Therefore, the most pragmatic method of estimating behavior of structures is through computational approach. Fluid-structure interaction problems like these are tackled by partitioned coupling of CFD (Computational Fluid Dynamics) and CSM (Computational Solid Mechanics) solvers. To simulate TIV, it is necessary to capture the turbulent eddies by resolving the fluid flow field up to the smallest scales. Use of high-fidelity CFD solvers such as LES is an ideal approach. However, this modeling approach is very expensive when an FSI simulation is considered. Low-fidelity models such as URANS are able to simulate only the large fluid structures and are inadequate to model the fluctuation at the smaller scales, resulting in an inaccurate approach for TIV. This article proposes the use of Presure Fluctuation Model for FSI simulations to estimate the effects of pressure fluctuations (p⁰) which trigger TIV. In the proposed model, standard URANS (Unsteady Reynolds Averaged Navier-Stokes) models are complemented with this model to calculate p⁰. The calculated p⁰ is summed to the mean pressure (p) to acts as a source of external excitation for the structure in an FSI simulation. The proposed model has an advantage over high fidelity models in terms of reduced computational costs and accurate estimation of TIV. The method is validated against a DNS of a channel flow and the results have shown to be in good agreement. Afterwards, the model is applied to two coupled FSI test cases. The results conclude that the pressure fluctuation model is capable of simulating TIV without requiring an external perturbation to trigger these vibrations.

@en

BACKGROUNDANDPURPOSE: Hemodynamics are thought to play an important role in the rupture of intracranial aneurysms.Wetested whether hemodynamics, determined from computational fluid dynamics models, have additional value in discriminating ruptured and unruptured aneurysms. Such discriminative power could provide better prediction models for rupture. MATERIALS AND METHODS: A cross-sectional study was performed on patients eligible for endovascular treatment, including 55 ruptured and 62 unruptured aneurysms. Association with rupture status was tested for location, aneurysm type, and 4 geometric and 10 hemodynamic parameters. Patient-specific spatiotemporal velocities measured with phase-contrast MR imaging were used as inflow conditions for computational fluid dynamics. To assess the additional value of hemodynamic parameters, we performed 1 univariate and 2 multivariate analyses: 1 traditional model including only location and geometry and 1 advanced model that included patient-specific hemodynamic parameters. RESULTS: In the univariate analysis, high-risk locations (anterior cerebral arteries, posterior communicating artery, and posterior circulation), daughter sacs, unstable inflow jets, impingements at the aneurysm body, and unstable complex flow patterns were significantly present more often in ruptured aneurysms. In both multivariate analyses, only the high-risk location (OR, 3.92; 95% CI, 1.77-8.68) and the presence of daughter sacs (OR, 2.79; 95% CI, 1.25-6.25) remained as significant independent determinants. CONCLUSIONS: In this study population of patients eligible for endovascular treatment, we found no independent additional value of aneurysmal hemodynamics in discriminating rupture status, despite high univariate associations. Only traditional parameters (high-risk location and the presence of daughter sacs) were independently associated with ruptured aneurysms.

@en