This study presents a detailed quantification of how flow orientation affects mass transfer and frictional resistance in periodically confined channels, offering novel insights into the physical similarity relations governing these phenomena. We constitute that the Sherwood number and friction factor adhere to universal scaling laws of the form Sh = A1+B sin(2α) Re1 2 and f = A1+B sin(2α) Re−1 2 , where α depicts the orientation of the periodically confined channel. It is found that the flow orientation and the cross flow velocity independently affect both, the Sherwood number and the friction factor. A key contribution of this work is the explicit characterization of the flow orientation: a 45° rotation of the flow relative to the spacer structure increases the Sherwood number by nearly 25%, while the friction factor rises by approximately 20%. These findings highlight a fundamental trade-off between mass transfer enhancement and flow resistance, suggesting that any process optimization must carefully balance the gains in mixing efficiency against the increased energy dissipation. This study provides a robust framework for further investigations into how periodic geometrical constraints influence transport processes in complex flow systems.
Helically wound structures are widely used in practical engineering due to its excellent mechanical property, such as the steel rope, the reinforced armor layer of the marine flexible pipe/cables. The shear stiffness plays an important role in exactly predicting the mechanical response of the helically wound structure, especially for the short structure. There has been no general methodology to directly calculate the shear effect of this type of structure because of the geometrical complexity. This paper introduces and modifies a novel implementation of asymptotic homogenization method so-called NIAH to effectively calculate the shear property of the helically wound structure. This modification of NIAH is derived based on the strain energy equivalence of the macroscopic structure and the microscopic structure so-called the microscopic unit cell, which can be used to quickly calculate the effective stiffness and effective stress. In this paper, the effective stiffness is only discussed. The mechanical mechanism of the shear effect of helically wound structures is firstly explained, and then taking into account the shear effect, a quickly effective analysis method of the mechanical response for the helically wound structure is proposed. The efficient and accurate finite-element model of the unit cell which is used in the numeric implementation of the effective analysis method, is determined through the sensitivity analysis of meshing methods and periodic boundary conditions. Considering the practical application, the implementation of this method is validated for helically wound structures with equal-scale subcomponents and non-equal-scale subcomponents. The influence of the slenderness ratio on the shear effect is also explored in this work. This study provides a meaningful reference for the loading analysis and structural design of helically wound structures.
For the assessment of experimental measurements of focused wave groups impacting a surface-piecing fixed structure, we present a new Fully Nonlinear Potential Flow (FNPF) model for simulation of unsteady water waves. The FNPF model is discretized in three spatial dimensions (3D) using high-order prismatic - possibly curvilinear - elements using a spectral element method (SEM) that has support for adaptive unstructured meshes. This SEM-FNPF model is based on an Eulerian formulation and deviates from past works in that a direct discretization of the Laplace problem is used making it straightforward to handle accurately floating structural bodies of arbitrary shape. Our objectives are; i) present detail of a new SEM modelling developments and ii) to consider its application to address a wave-body interaction problem for nonlinear design waves and their interaction with a model-scale fixed Floating Production, Storage and Offloading vessel (FPSO). We first reproduce experimental measurements for focused design waves that represent a probably extreme wave event for a sea state represented by a wave spectrum and seek to reproduce these measurements in a numerical wave tank. The validated input signal based on measurements is then generated in a NWT setup that includes the FPSO and differences in the signal caused by nonlinear diffraction is reported.
Corrosive wear of cylinder liners in large two-stroke marine diesel engines that burn heavy fuel oil containing sulfur is coupled to the formation of gaseous sulfur trioxide (SO3) and subsequent combined condensation of sulfuric acid (H2SO4) and water (H2O) vapor. The present work seeks to address how fuel sulfur content, charge air humidity and liner temperature variations affects the deposition of water and sulfuric acid at low load operation. A phenomenological engine model is applied to simulate the formation of cylinder/bulk gas combustion products and dew points comply with H2O–H2SO4 vapor liquid equilibrium. By assuming homogenous cylinder gas mixtures condensation is modeled using a convective heat and mass transfer analogy combined with realistic liner temperature profiles. Condensation of water is significantly altered by the liner temperature and charge air humidity while sulfuric acid condensation (the order is a few mg per cylinder every cycle) is proportional to the fuel sulfur content. Condensation takes place primarily in the upper part of the cylinder liner where a reduction of the surface temperature or saturated charge air provides that the deposited acid can be highly diluted with water.
In this work, three-dimensional computational fluid dynamics (CFD) studies of sulphur oxides (SOx) and sulphuric acid (H2SO4) formation processes in a large, low speed two-stroke marine diesel engine are carried out. The current numerical study aims to investigate the conversion of sulphuric dioxide (SO2) to sulphuric trioxide (SO3) and the possibility of H2SO4 condensation which are the prerequisites to better understand the corrosion-induced wear phenomenon. This is achieved with the aid of the implementation of a multicomponent surrogate model, which comprises a skeletal n-heptane mechanism and a reduced sulphur subset mechanism. In the present work, performance of the coupled CFD-chemical kinetic model is evaluated using both qualitative and quantitative methods. The modelling results show that the temporal and spatial evolutions of SOx predicted by the skeletal model are similar to those by the base mechanism. Predictions of the variations of SOx and the associated SO2 to SO3 conversion in response to the change of fuel sulphur content, swirl velocity, start of injection, scavenge pressure and humidity qualitatively agree with numerical and experimental results from the literature. The model is further evaluated using the measured SO2 to SO3 conversion levels in a low load, low scavenge pressure case and a low load, high scavenge pressure case. The absolute values of simulated and measured conversion levels are close, although the former appear to be higher. The current results show that the flame impinges at the cylinder liner near top dead centre. The gas is cooled rapidly by the wall temperature and H2SO4 is produced in the region where the local temperature is less than 600 K. Based on the flue gas correlation, the acid dew point temperature is higher than the wall temperature, suggesting that acid condensation may begin early at the top part of the cylinder liner. The predicted distribution corresponds well with the distribution of corroded parts observed in service engines. The model is expected to serve as an important tool to simulate the rates of SO2 absorption into lubricating oil film and H2SO4 condensation in this combustion system.
An adaptive machine learning framework is established for an implicit determination of the performance degradation of a ship due to marine growth, i.e., biofouling. The framework is applied in a case study considering telemetry data of a cruise ship operating predominantly in the Caribbean Sea. The dataset encompasses seven years including three dry-docking intervals and several in-water cleaning events. The COVID-19 period receives special focus due to the drastic change in the operational profile. A main outcome of the study is a comparison of the derived performance estimate to the corresponding results of the industry standard ISO 19030. Additional aspects of the present study include the use of special regularization techniques for incremental machine learning and the increase of transparency through the implementation of prediction intervals indicating model uncertainty. Overall, it is found that the developed machine learning framework shows good agreement with the industry standard underlining its plausibility.
In a number of experiments and field tests of point absorbers, snap loads have been identified to cause damage on the mooring cables. Snap loads are basically propagating shock waves, which require special care in the numerical modeling of the mooring cable dynamics. In this paper we present a mooring cable model based on a conservative formulation, discretized using the Runge-Kutta discontinuous Galerkin method. The numerical model is thus well suited for correctly capturing snap loads. The numerical model is verified and validated using analytic and experimental data and the computed results are satisfactory.
Mobilization of residual oil droplets is the key process for enhanced oil recovery. Visualization of the droplet movement at a pore level provides insights on the underlying physical mechanisms. We couple a microfluidic droplet generator and a thin glass capillary to study the movement of oil droplets under salinity gradients with visualization of individual droplet movements. The driving forces that affect the movement of the droplets are discussed. We demonstrate experimentally that oil droplets in micro-confined channels can be mobilized and move against pressure under the concentration gradients of dissolved salts. The gradient-driven movement can be strong enough to drive a droplet through a narrow constriction in the middle of the capillary channel. The droplet movement can be understood by combining a Marangoni stress due to surfactant redistribution, electrostatic interaction and diffusiophoresis. This suggests that the abrupt change of salinity may be one of the physical mechanisms of smart waterflooding.
We present recent progress on the development of a new fully nonlinear potential flow (FNPF) model for estimation of nonlinear wave-body interactions based on a stabilized unstructured spectral element method (SEM). We introduce new proof-of-concepts for forced nonlinear wave-body interaction in two spatial dimensions to establish the methodology in the SEM setting utilizing dynamically adapted unstructured meshes. The numerical method behind the proposed methodology is described in some detail and numerical experiments on the forced motion of (i) surface piercing and (ii) submerged bodies are presented.
The future fuel for marine engines is not yet decided. However, it is well-known that utilizing green alternative e- fuels is a big step in the way of decarbonization. Dual-fuel (DF) engines offer great fuel flexibility with possibility of using different green gaseous e-fuels like methane and ammonia. The ignition of the lean premixed gaseous fuel in a DF engine depends on the auto-ignition of the injected pilot diesel fuel. Therefore, a proper auto- ignition of the pilot diesel is important in these engines. In the present study, large eddy simulation is carried out to study the auto-ignition process of pilot diesel in premixed methane-diesel DF combustion in a constant volume combustion chamber. The entire DF combustion processes involving methane injection, methane/air mixing, pilot diesel injection, and ignition are simulated. The numerical model is validated against experimental data. The base case has a pilot diesel injector with 8 nozzle holes. The auto-ignition process in two other cases with 4 nozzle holes are investigated and compared with the base case. Considering same amount of pilot fuel, the injection rate is assumed to be double in one the cases, while in the other case, the injection duration is doubled. The results show that the ignition process in the case with 4 nozzle holes and double injection rate is incomplete due to flame impingement on the walls. However, a reduction of the nozzle hole numbers can improve diesel pilot ignition in the case with 4 nozzle holes and double injection duration. The higher fuel mass per orifice leads to an increased fuel concentration within the diesel pilot sprays and higher combustion rate than the base case. Furthermore, more confined spray envelope in the case with double injection duration leads to an increased reactivity and more efficient auto-ignition process than the case with double injection rate.