Twin Cylinders Research Articles

Two-cylinder Synchronous Electro-hydraulic Servo System and its Control Technology Development

Background: Electro-hydraulic servo control system synchronous control in industrial application is very extensive because the application of electro-hydraulic servo control system is mostly in a poor working environment, there are a variety of complex external factors. Objective: Improve the comprehensive performance indicators such as accuracy, speed, and stability of synchronous control of hydraulic cylinders Methods: In the process of synchronous movement, the typical hardware structure and control methods and algorithm strategies of the double hydraulic cylinder are analyzed in detail, and the basic principle of common hydraulic double cylinder synchronous control is summarized. Results: Through the comprehensive analysis of papers and patent documents related to the synchronous control of hydraulic twin cylinders, these papers and patented technologies study the control algorithm, and apply the new technology to the synchronous control of electro-hydraulic servo hydraulic cylinders to improve its comprehensive performance indicators such as work accuracy, speed and stability. Conclusion: Referring to the development trajectory of control algorithms in this regard, combined with new control theory and new technology, many achievements can be applied and researched in hydraulic cylinder synchronous control, and better develop the synchronous control system when double hydraulic cylinders and more hydraulic cylinders work together. By collating and analyzing the relevant research results, the two-cylinder synchronous control technology is comprehensively evaluated and prospected, which provides a valuable reference for the research and application in this field.

Data-driven identification and pressure fields prediction for parallel twin cylinders based on POD and DMD method

The mode analysis of parallel twin cylinders is conducted in this paper using two data-driven methods: proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD). First, a high-fidelity computational fluid dynamics (CFD) model of parallel twin cylinders is established, and numerical simulations of the model are carried out. Subsequently, the fundamental principles of the POD and DMD algorithms are systematically introduced. Utilizing snapshots obtained from the high-fidelity CFD model, the POD and DMD methods are employed to extract the dominant flow structures. Furthermore, a comparison between the two data-driven methods is conducted by analyzing modal frequencies, pressure distribution, and the reconstruction errors of pressure fields. Finally, the pressure fields of non-sample points are predicted based on the POD–backpropagation neural network (BPNN) surrogate model and the DMD method, and the predicted results are compared with the CFD simulation results. It found that (i) the DMD method is capable of extracting the main coherent structures of the pressure fields, directly obtaining flow modes and their corresponding frequencies, and assessing the stability of flow modes; (ii) the DMD method can capture the main flow features of the pressure fields in both spatial and temporal dimensions, while the POD method is primarily efficient at capturing the spatial features of the pressure fields; (iii) in contrast to the frequency-ranked DMD method, the energy-ranked POD method can reconstruct the pressure fields using a smaller number of modes, indicating that the POD method has an advantage in terms of mode reduction; (iv) in contrast to the energy-ranked POD method, the frequency-ranked DMD method has a wider applicability to the range of flow types and has more advantages in stability analysis of complex dynamic systems; (v) the predicted pressure fields around the cylinder using the first five-order POD modes or DMD modes closely align with CFD calculation results. Additionally, the evolution of pressure fields predicted by the POD–BPNN surrogate model with the first five-order POD modes or the DMD method with the first 200-order DMD modes significantly agrees with CFD simulation results; (vi) the combined use of the POD–BPNN surrogate model and DMD methods allows efficient interpolation and extrapolation of samples, delivering exceptional predictive performance. This study offers insight into the coherent structures in parallel twin cylinders.

Flow fields prediction for data-driven model of parallel twin cylinders based on POD-RBFNN and POD-BPNN surrogate models

Flow around cylinders is an important phenomenon in many different engineering fields. In this paper, the fast prediction of the pressure fields of parallel twin cylinders is implemented based on a data-driven algorithm. Firstly, the pressure fields of parallel twin cylinders with a low Reynolds number are obtained through the Computational Fluid Dynamics (CFD) method. The pressure fields at different time steps are collected to form a snapshot matrix. The Proper Orthogonal Decomposition (POD) algorithm is then applied to obtain the POD basis vectors of the snapshot matrix, enabling the reconstruing the pressure fields. Subsequently, two reduced-order models (ROM) called the POD-RBFNN and POD-BPNN surrogate models are proposed in this paper. The POD-RBFNN surrogate model uses the Radial Basis Function Neural Network (RBFNN) to train the POD mode coefficients obtained from the POD algorithm, while the POD-BPNN surrogate model uses the Backpropagation Neural Network (BPNN) for the same purpose. Linearly combining the POD mode coefficients predicted by the POD-RBFNN or POD-BPNN surrogate models with the POD basis vectors obtained from the POD algorithm enables fast and efficient prediction of pressure fields for non-sample points. Finally, comparisons are made between the predicted pressure fields obtained from these two surrogate models and the actual values obtained through CFD simulations. It is found that both the POD-RBFNN and POD-BPNN surrogate models proposed in this paper not only significantly improve efficiency but also maintain a high level of accuracy. However, the training time of the POD-RBFNN surrogate model is significantly shorter than that of the POD-BPNN surrogate model. Additionally, the POD-RBFNN surrogate model exhibits smaller Root Mean Square Errors (RMSE) and Mean Absolute Error (MAE). For the data-driven model of parallel twin cylinders described in this paper, the POD-RBFNN surrogate model is more suitable to predict the pressure fields. The research results in this paper are believed to hold significant value for CFD calculations of parallel twin cylinder models, offering essential guidance for a deeper understanding of cylinder flow problems and optimizing engineering design.

Generalized hydrodynamic coefficients of twin connected circular cylinders in finite water depth

In this work the generalized hydrodynamic coefficients of connected twin circular cylinders are studied. Firstly, a matched boundary element method with eigen-function expansion is applied to study the generalized hydrodynamic coefficients of floating and submerged tunnels with twin horizontal cylinders in finite water depth. By numerical examination it is found that gap resonance problem can be neglected for the submerged floating tunnels with a certain submergence depth, and the axial wave number play an important role for the added mass. Then, analytic solutions are derived for the high frequency approximations of the generalized added masses of twin connected cylinders by the eigenfunction expansion in the polar coordinate system with aid of their images about the x-axis in the infinite fluid domain. The high frequency approximations of generalized added masses decrease with increasing vibration wave number. When T/a and B/a are larger than 3 (where T is the cylinder submergence, B is the spacing distance between the twin cylinders and a is the radius of the cross-section of the cylinder), the hydrodynamic interference between the two cylinders can be neglected and the generalized added mass coefficients of connected twin cylinders can be derived from these of single isolated cylinders.

Flow regime identification and flow instability analysis of oscillatory flows over twin circular cylinders

Oscillatory flows past two identical circular cylinders are investigated by two-dimensional direct numerical simulations in the parameter space of gap ratio (0.5 ≤ G ≤ 4.0), angle of flow incidence (0° ≤ α ≤ 90°) and Keulegan–Carpenter number (4 ≤ KC ≤ 12) with a constant Reynolds number Re = 150, where G = L/D, KC = UmT/D and Re = UmD/υ with D being the dimeter of the identical cylinders, L the shortest surface-to-surface distance between the two cylinders, Um and T being the velocity amplitude and period of the sinusoidal oscillatory flow, respectively, and α is defined as the angle between the flow direction to the line connecting the centers of the two cylinders. Comparing with the tandem or side-by-side arrangements of twin circular cylinders in oscillatory flows, the staggered twin cylinders (0° < α < 90°) involve more diverse flow regimes, including the periodic, quasi-periodic and chaotic flow states, due to the inherent asymmetric flow interference around the cylinder pair. In addition to introducing four flow regimes for the tandem and side-by-side arrangements, this study newly identifies 11 flow regimes for the staggered twin cylinders. The newly reported flow regimes in this work are collaboratively identified through the flow visualizations, steady streaming, frequency spectra of fluid forces and Lissajous phase diagrams, as well as the temporal-spatial symmetry features of the wake flows. Connecting with the previous work by Zhao and Cheng [“Two-dimensional numerical study of vortex shedding regimes of oscillatory flow past two circular cylinders in side-by-side and tandem arrangements at low Reynolds numbers,” J. Fluid Mech. 751, 1–37 (2014)], this study presents overall regime maps in the KC-α plane for varied gap ratios. It is found that the flow regimes previously and presently identified for the tandem and side-by-side arrangements may also appear for the staggered twin cylinders. The present numerical results suggest the sensitive dependence of the flow regimes on the parameters of KC, α, and G. It is also found that a specific flow regime with narrow parameter bands may appear within another flow regime, forming the abnormal regime hole in the regime map. To understand the profound influence of the control parameters on the flow regime transition, and the relevant temporal-spatial symmetry breaking, the linear Floquet stability analysis is conducted in this work. It was confirmed that the variation of the KC number may result in the Ky symmetry breaking over several periodic flow regimes, while the change of the angle of flow incidence may account for the H2 symmetry covering various periodic and quasi-periodic flow regimes. The stability analysis also explains the temporal flow transition and the abnormal occurrence of the regime holes within either quasi-periodic or chaotic flow regimes.

Wake Control of Flow Past Twin Cylinders via Small Cylinders

The drag and lift force of a twin-cylinder structure are often greater than those of a single cylinder, causing serious structural safety problems. However, there are few studies on the passive control of twin cylinders. The study aimed to investigate the performance of passive drag reduction measures using small cylinders on twin cylinders at a Reynolds number of 100. The effects of small cylinder height (HD/D = 0~1.0, D is the side length of the twin cylinder) and cross-sectional shape on fluid force and flow structures were studied by direct numerical simulations. The control mechanism was analyzed using high-order dynamic mode decomposition (HODMD). The results showed that significant drag reduction occurred in the co-shedding state, particularly when the gap length of the twin cylinders L/D = 6.0. The small control cylinders with HD = 0.6, by contrast, showed the best performance in reducing the mean drag and fluctuating lift of the twin cylinders. It reduced the mean drag of the upstream cylinder (UC) by 2.58% and the downstream cylinder (DC) by more than 62.97%. The fluctuating lift coefficient for UC (DC) was also decreased by more than 70.41% (59.74%). The flow structures showed that when the flow hit UC under the action of small control cylinders, a virtual missile-like aerodynamic shape was formed at the leading edge of UC. In this way, the gap vortex consisted of two asymmetric steady vortices and the vortex length significantly increased. This was also confirmed by HODMD. The coherence modes in the gap were suppressed and thus the interaction between gap flow and wake flow was mitigated, which resulted in the fluid force reduction.

Mode interpretation and force prediction surrogate model of flow past twin cylinders via machine learning integrated with high-order dynamic mode decomposition

Understanding and modeling the flow field and force development over time for flow past twin tandem cylinders can promote insight into underlying physical laws and efficient engineering design. In this study, a new surrogate model, based on a convolutional neural network and higher-order dynamic mode decomposition (CNN-HODMD), is proposed to predict the unsteady fluid force time history specifically for twin tandem cylinders. Sampling data are selected from a two-dimensional direct numerical simulation flow solution over twin tandem cylinders at different aspect ratios (AR = 0.3–4), gap spacing (L* = 1–8), and Re = 150. To promote insight into underlying physical mechanisms and better understand the surrogate model, the HODMD analysis is further employed to decompose the flow field at selected typical flow regimes. Results indicate that CNN-HODMD performs well in discovering a suitable low-dimensional linear representation for nonlinear dynamic systems via dimensionality augment and reduction technique. Therefore, the CNN-HODMD surrogate model can efficiently predict the time history of lift force at various AR and L* within 5% error. Moreover, fluid forces, vorticity field, and power spectrum density of twin cylinders are investigated to explore the physical properties. It was found three flow regimes (i.e., overshoot, reattachment, and coshedding) and two wake vortex patterns (i.e., 2S and P). It was found the lift force of the upstream cylinder for AR < 1 is more sensitive to the gap increment, while the result is reversed for the downstream cylinder. It was found that the fluctuating component of the wake of cylinders decreases with increasing AR at L* = 1. Moreover, flow transition was observed at L* = 4.

Sensitivity-aided active control of flow past twin cylinders

The flow-induced vibration (FIV) of twin structures always experiences significantly larger vibration amplitude than that of a single one, which causes severe structural safety problems. However, FIV control methods specifically for twin structures are still lacking. This study investigates the FIV and fluid forces of twin square cylinders under a jet flow control for a laminar flow. The control effects are studied for stationary and free-oscillating cylinders with a gap distance of L = 6.0D in co-shedding regimes (D is the side length of the cylinder). Global linear instability, adjoint method, and sensitive analysis are conducted to find a position where the jet flow should be placed. It was found that the most sensitive region is located in the near-wake region, and we thus set at jet flow at the rear of the cylinders. The control effects show that for stationary cylinders, UC achieves its maximum mean drag (CD) reduction of 11% when Uj > 0.3, meanwhile the CD of the downstream cylinder (DC) and fluctuating lift (CL') ≈ 0. UC plays a primary role in determining the control effects on fluid forces compared to DC, which coincides with the sensitivity analysis. For the free-oscillating case, much larger control energy is required to completely suppress vortex-induced vibration (VIV) with Uj > 2.4. In addition to the splitting effects of the jet flow, the neutralization effect of jet-induced vorticity was found plays a significant role in suppressing VIV, which can neutralize the vorticity generated from the side edge of the cylinders. High-order dynamic mode decomposition (HODMD) is also applied to uncover the underlying control mechanism. It demonstrates that the global energy of the dynamic mode energy is significantly decreased, meanwhile, the primary dynamic mode (i.e., the mode shape introducing VIV) is significantly weakened by the jet flow due to splitting and neutralizing effects.

Receptivity-orientated drag reduction of twin cylinders by steady leading-edge suction control based on adjoint method

This study investigates the drag reduction of two tandem square cylinders under steady suction control at Reynolds numbers 50–200. The position where the suction flow should be placed is determined by using a receptivity analysis based on the adjoint method, and we investigate how control affects the fluid force and flow structures. High-order dynamic mode decomposition (HODMD) is applied to analyze the dynamic coherence modes and uncover the underlying control mechanism. The adjoint modes show that the regions of maximum receptivity to momentum forcing are localized on each side of the up-cylinder (UC) near the leading edge (LE). Thus, the suction flow is placed on the LE. The drag can be significantly reduced at wide gap distances, especially for the co-shedding regime. Under suction flow control, the separation is suppressed near the LE, and the gap vortices are no longer fed by the vorticity generated by the separated shear layer; they only result from the trailing-edge separation, which weakens and shrinks. Subsequently, the interaction between the gap flow and the down-cylinder (DC) is weakened, which reduces the drag and lift forces. The decrease in drag exceeds 66.4% for the UC and reaches 81.6% for the DC. The fluctuating reduction in the lift for the UC (DC) exceeds 59.0% (75.7%). HODMD results show that, as the suction flow velocity increases, the LE suction flow modifies the local time-averaged modes rather than the global mode energy. Conversely, the dynamic mode energy decreases significantly, whereas the mode shape remains unchanged except for a phase shift.

VIV of twin square cylinders in various configurations at a low Reynolds number

Vortex-induced vibration (VIV) of twin square cylinders is numerically investigated at Re = 150 with tandem, staggered and side-by-side configurations. Three VIV categories are identified due to dynamic responses, flow-induced forces and flow fields. For Case-Ⅰ (α = 0°), two cylinders only display the typical synchronization region (the reduced velocity of the synchronization region, Vr ≈ 5–6). The maximum transverse amplitudes of twin cylinders are 2 times and 2.4 times that of the single one (Ay,max = 0.3B, B is cylinders' characteristic length), respectively. There is also sizable streamwise oscillation for Cylinder-2 (Ax,max = 0.48B). Compared to Case-Ⅰ, streamwise and transverse amplitudes of twin cylinders systematically rise for Case-Ⅱ (α = 15°, 30°), where Ay,max and Ax,max of two cylinders is approximately 1.9 times and 2.9 times those in Case-Ⅰ, respectively. Two cylinders exhibit not only a typical synchronization region (Vr ≈ 5–6) but also a non-typical synchronization region at high Vr ≈ 10–12. Unlike the typical one, Cylinder-2 within the non-typical region shows stronger vortexes, more pronounced vibrations and lower fluctuating lift. For Case-Ⅲ (α = 45°–90°), the flow interference of twin square cylinders mainly occurs in the wake area and marginally affects the dynamic responses. The VIV characteristics of twin cylinders are similar to those of an isolated cylinder.

Fully nonlinear dynamics of floating solar platform with twin hull by tubular floaters in ocean waves

Sustainability in harvesting solar power offshore depends on the survival of their platforms under environmental loads such as ocean waves. Therefore, a framework is established in this study to estimate the dynamics of floating solar platforms under ocean waves experimentally and numerically. Hence the platform is taken twin hull with double circular cylinders which exposed to a broad range of waves in the calibrated physical wave tank. On the other hand, the high order boundary element method and mixed Eulerian-Lagrangian approach are used in the numerical model to solve the fully nonlinear fluid field and the acceleration potential method is employed to approximate the response of the platform implicitly. To validate the framework, numerical and experimental results are compared for a single cylinder under the incident waves. Then, the comparison is extended for a twin hull platform. The effect of nonlinear free surface flow amongst the twin cylinders on the dynamics of the platform is studied parametrically. Afterwards, the performance of the present framework is assessed for a near breaking incident wave and an irregular wave. The stream function theory and flux of superposed wave components are used to emulate the high steep wave and the irregular wave respectively.

Nonlinear wave resonance due to oscillations of twin cylinders in a uniform current

A higher order finite element method with 8-node element is adopted to analyse the nonlinear wave resonance generated by oscillations of twin cylinders in a uniform current. The velocity potential in the fluid domain at each time step is obtained based on the finite element method through an iteration procedure. Numerical results are provided for twin rectangular cylinders undergoing specified oscillations in a uniform current at resonant frequencies. The effects of current on the wave and hydrodynamic force at the resonant frequencies are studied in detail and it is found that the resonance happens at all first-order resonant frequencies for both symmetric and antisymmetric motions of the cylinders. In particular, in addition to the first-order resonant frequency, the maximum wave elevation and horizontal force at resonance always regularly increase or decrease as the increase of the absolute value of Froude number or the spacing between two cylinders within the range of larger spacings. A similar trend can be also observed in the oscillational frequency of cylinders at resonance. Some results are also compared with those by linear solution and its superposition with the second-order, their difference at different Froude numbers are also discussed.

Vortex structure behind highly heated tandem twin cylinders

Vortex structure behind highly heated tandem twin cylinders

Numerical simulation of the VIV of twin tandem diamond cylinders at low Reynolds numbers

To further reveal vibration interference of a multicylinder system, the vortex-induced vibration of two-degree-of-freedom upstream and downstream diamond cylinders with a center-to-center distance L = 5B (B is characteristic length of cylinders) and a mass ratio of the oscillator fixed at m* = 2 was numerically investigated. The influences of the Reynolds number (Re = 40–200) and the reduced velocity (Ur = 3–12) on the dynamic characteristics were considered. The results show that Re has a remarkable effect on the VIV characteristics of twin cylinders, especially on the downstream cylinder. With increasing Re numbers, the lock-in region for the upstream cylinder remains almost constant, while that of the downstream cylinder increases. Unlike the low Re numbers (Re = 40–80), the trajectory of the upstream cylinder at high Re numbers is a closed ring shape rather than a figure “8” within the lock-in region. The downstream cylinder is dominated by transverse vibration in most cases of Re = 40, while it exhibits a significant vibration in both the streamwise and transverse directions at Re = 80–200. Additionally, the wake of twin cylinders at Re = 40 and Ur = 3–4 displays a steady state. With increasing Re numbers, the steady state disappears, and the wake modes with various reduced velocities at high Re (Re = 120–200) are more complicated and disordered.

Aerodynamic Characteristics of Coupled Twin Circular Bridge Hangers with Near Wake Interference

Much work has been devoted to the investigation and understanding of the flow-induced vibrations of twin cylinders vibrating individually (e.g., vortex-induced vibration and wake-induced galloping), but little has been devoted to coupled twin cylinders with synchronous galloping. The primary objective of this work is to investigate the aerodynamic forcing characteristics of coupled twin cylinders in cross flow and explore their effects on synchronous galloping. Pressure measurements were performed on a stationary section model of twin cylinders with various cylinder center-to-center distances from 2.5 to 11 diameters. Pressure distributions, reduced frequencies and total aerodynamic forces of the cylinders are analyzed. The results show that the flow around twin cylinders shows two typical patterns with different spacing, and the critical spacing for the two patterns at wind incidence angles of 0° and 9° is in the range of 3.8D~4.3D and 3.5D~3.8D, respectively. For cylinder spacings below the critical value, vortex shedding of the upstream cylinder is suppressed by the downstream cylinder. In particular, at wind incidence angles of 9°, the wake flow of the upstream cylinder flows rapidly near the top edge and impacts on the inlet edge of the downstream cylinder, which causes a negative and positive pressure region, respectively. As a result, the total lift force of twin cylinders comes to a peak while the total drag force jumps to a higher value. Moreover, there is a sharp drop of total lift coefficient for α = 9–12°, indicating the potential galloping instability. Finally, numerical simulations were performed for the visualization of the two flow patterns.

Experimental study of wake-induced instability of coupled parallel hanger ropes for suspension bridges

Hangers in suspension bridges are usually composed of closely-spaced multiple cables, and they may suffer from server wind-induced vibrations due to the wake interference effect. The primary objective of this research was to examine the wake-induced instability of the coupled twin or quadruple cylinders that are tied together to have the same motion, instead of separate cylinders vibrating independently. The elastically mounted twin cylinders free to oscillate in streamwise and transverse directions were tested at various cylinder spacings. The Scruton (Sc) number, Sc = 2mδ/ρD2 (D is cylinder diameter, δ is logarithmic decrement, m is cylinder mass per unit length, and ρ is fluid density), was varied from about 65–420 by changing the damping, and the critical damping necessary to suppress the wake-induced instability was established. The wake-induced instability occurred with an elliptical orbit at cylinder spacing of 3.2D and wind incidence angle of 10°, while no instability was found at other spacings. The results of force measurements were presented and used to discuss the excitation mechanism based on the 1-DoF and 2-DoFs quasi-steady model. Despite some differences between theoretical analysis and measurements, the 2-DoFs model outperforms greatly the 1-DoF model in predicting the critical wind speed. For the coupled quadruple cylinders arranged in rectangular configuration, no large amplitude vibrations were found at all wind incidence angles due to complicated flow interference. Finally, the wake-induced instability of hanger subspans divided by spacers was investigated with a new aeroelastic model, and it was found that four spacers placed at equal intervals along hanger length are sufficient to suppress the wind-induced instability within the subspan for hangers in the Xihoumen Bridge.

Variation of flow field around twin cylinders with and without the outer perforated cylinder — Numerical study

Presence of the outer perforated cylinder reduces the direct wave impact on the inner cylinder, which has been testified by many researchers. However, the force reduction mechanism, which is complicated due to the wave-porous structure interaction, needs to be addressed in detail. The present study explains the mechanism with the aid of the computational fluid dynamics (CFD) tool STAR CCM+. This package is chosen for its capabilities to simulate viscous and turbulence effects caused by passage of waves. For the present study, flow fields around the twin cylinders with different orientations are examined with and without the outer perforated cover. Mechanism contributing to the reduction of force on the existing structure is explained in physical terms, and force reduction is quantified. The present study has direct application in the retrofitting application of offshore members.

Wake of tandem cylinders near a wall

This article reports an experimental investigation on the near wake of two identical circular cylinders in crossflow arranged in tandem configuration in the streamwise direction and with the additional interference of the ground. The Reynolds number based on the cylinders diameter is 4.9×103. The present study analyses the effect of longitudinal pitch-to-diameter ratios spanning the well known flow regimes of tandem cylinders in absence of ground effect, i.e. single bluff body, shear layer reattachment and vortex co-shedding of twin cylinders. Particle Image Velocimetry measurements are performed to extract first and second order statistics of the wake. The wake features have been analyzed and compared with the bluff-body wake models present in literature. Additionally, the flow fields are decomposed in Proper Orthogonal modes to characterize the main coherent structures. The time coefficients of the modes are analyzed to extract phase relations between vortical features.Far enough from the wall the cylinders wake is symmetric, with Von Kármán vortices shed symmetrically in the wake with respect to cylinders centerline. The measured average wake characteristics and vortical structures are consistent with the data reported in the literature for tandem cylinders without ground effect. For a wall-to-cylinder gap equal to 1 diameter, the ground strongly influences the flow field, introducing asymmetry in the typical Von Kármán wake. The ground boundary layer is thickened past the downstream cylinder and a wall-jet appears, enclosed between the wall boundary layer and the cylinders wake. Eventually these flow structures are not distinguishable when the cylinders wake and the wall boundary layer are grown enough to merge. From POD analysis, vorticity blobs appear on the wall, paired with vortex shedding.If the wall gap is decreased to 0.3 diameters, an extended low speed region appears close downstream of the cylinders. A POD mode similar to a vortex shedding is still present; however it cannot be associated to an alternate Von Kármán street. The shed structures show a shorter wavelength than the Von Kármán shedding in far-from-the ground cases and are accompanied by a flapping jet-like structure ejected from the wall gap between the cylinder and the wall. The jet strongly changes the features of the wake on the side opposite to that of the wall, thus suggesting a possible coexistence between jet oscillations and shedding.

Friction behavior of a multi-interface system and improved performance by AlMgB14–TiB2–C and diamond-like-carbon coatings

This study investigated friction behavior of a bearing system with two interfaces involved: a roller component experiencing rolling–sliding interaction against twin cylinders under point contacts while simultaneously undergoing pure sliding interaction against a socket under a conformal contact. Lubrication modeling predicted a strong correlation between the roller׳s rolling condition and the system׳s friction behavior. Experimental observations first validated the analytical predictions using steel and iron components. Diamond-like-carbon (DLC) coating and AlMgB14–TiB2 coating with a carbon topcoat (BAMC) were then applied to the roller and twin cylinders, respectively. Testing and analysis results suggest that the coatings effectively decreased the slip ratio for the roller–cylinder contact and the sliding friction at both bearing interfaces and, as a result, significantly reduced the system frictional torque.

Fully nonlinear simulations of interactions between solitary waves and structures based on the finite element method

Fully nonlinear interactions between solitary waves and structures are studied based on a finite element method (FEM) in two-dimensions. A mesh with higher order 8-node quadrilateral isoperimetric element is used in the simulation. The velocity potential is obtained by solving a linear matrix system using the conjugate gradient (CG) method with a symmetric successive overelaxlation (SSOR) preconditioner, and the velocity in the fluid domain is calculated through differentiating shape functions directly. The wave elevation and potential on the free surface are updated by the fourth order Runge–Kutta method. Waves and hydrodynamic forces are obtained for single solitary waves acting on a wall and propagating on a tank with a step, collision between two solitary waves, interactions between a solitary wave and a single and twin rectangular cylinders on free surface. Some results are compared with previous studies of analytical solutions and those by experiment. The simulations have provided many results to show the nonlinearities of waves and hydrodynamic forces with different incoming wave amplitudes, and the effect of dimensions of the single cylinder and the spacing between twin cylinders on the wave and forces are also discussed.