Side Of Cylinder Research Articles

Optimizing reduced frequency using genetic algorithms for plasma flow control to achieve drag reduction on a circular cylinder

A closed-loop parameter optimization system around a cylinder is built by integrating the plasma actuation and genetic algorithms in this research, employing numerical simulations and experimental methods. The study aims to minimize the total drag on the cylinder by optimizing the reduced frequency. A pair of surface dielectric barrier discharge plasma actuators, powered by alternating-current high-voltage sources, is symmetrically positioned at ±90° azimuth angles on the two sides of a circular cylinder, and the Reynolds (Re) number is 1.5×104 based on the cylinder diameter. Numerical simulations were first used to determine the optimization space for the reduced frequency, followed by wind tunnel experiments to further search for the optimal research within this space. Particle image velocimetry and hot-wire anemometry were used to investigate the flow field's instantaneous and time-averaged characteristics. Ultimately, the optimal reduced frequency was identified based on duty-cycle frequency, free-stream velocity, and cylinder diameter. The results show that the optimal duty-cycle frequency obtained through genetic algorithm optimization in numerical simulations and wind tunnel experiments is the same, at 140 Hz, corresponding to a reduced frequency of approximately 1.372. The drag reduction rates are also similar, at 73.9% and 73.6%, respectively. During plasma flow control with the optimal reduced frequency, the dominant frequency of the overall motion of the separated vortex field is no longer the natural shedding frequency of the baseline flow. Still, it is instead controlled by the plasma duty-cycle frequency. Compared to the baseline flow, the plasma flow control at the optimal reduced frequency transforms the large-scale alternating vortices into small-scale shedding vortices, resulting in a time-averaged narrow and stable velocity deficit region, leading to reduced energy loss and significantly lower time-averaged drag coefficient. Meanwhile, the interaction between plasma-induced vortices and the Kármán vortex street in the cylinder wake enhances mixing, significantly suppressing turbulence intensity. The results demonstrate the effectiveness of genetic algorithms in identifying the global optimal reduced frequency of plasma actuation, achieving maximum drag reduction.

Numerical Analysis of Hydrogen Behavior Inside Hydrogen Storage Cylinders under Rapid Refueling Conditions Based on Different Shapes of Hydrogen Inlet Ports

In order to investigate the effects of different shapes of hydrogen inlet ports on the behavioral characteristics of hydrogen in Type IV hydrogen storage cylinders under rapid refueling conditions, a mathematical model of hydrogen temperature rise and a three-dimensional numerical analysis model were developed. The rectangular, hexagonal, triangular, Reuleaux triangular, circular, elliptical and conical inlet ports were researched by using computational fluid dynamics methods. The results showed that, for the same refueling flow rate and cross-sectional area, the hydrogen temperature inside a cylinder with a rectangular inlet port is higher and the jet tilt angle is larger than for a hexagonal port, while the thermal stratification phenomenon is not obvious. The hydrogen temperature inside a cylinder with a triangular inlet port is lower than that with a Reuleaux triangle port and the jet tilt angle is larger, and neither has significant thermal stratification. The hydrogen temperature inside a cylinder with a circular inlet port is higher than that with an ellipse port, the jets are not tilted on either one, and the phenomenon of thermal stratification is prominent. Further analysis indicated that enlarging the cross-sectional area and increasing the refueling flow rate results in a higher hydrogen temperature and intensified thermal stratification and an upward-angled jet can effectively reduce or eliminate thermal stratification.

Vortex induced vibration control of a cactus-shaped cylinder with porous spines

A numerical study is conducted to examine the vortex induced vibration (VIV) control of a cactus-shaped cylinder with 8 porous spines, and the Reynolds number is 150. The effects of full porous spines and partial porous spines on the inhibition of VIV are investigated in this study. The results indicate that while full porous spines offer a specific level of control effectiveness, an optimal distribution of these porous spines can substantially improve their inhibitory impact. Combining porous spines and solid spines alters the separation point of the shear layer and reduces the interactions among shear layers. However, misuse of porous spines can lead to significant vibrations. The most effective control is achieved by replacing 5 spines on the windward side of the cactus-shaped cylinder with porous spines at the Darcy number of 0.01. The corresponding reductions of vibration amplitude, drag force, and lift force are 31.3%, 26.8%, and 65.3%, respectively.

Integrated neural network based simulation of thermo solutal radiative double-diffusive convection of ternary hybrid nanofluid flow in an inclined annulus with thermophoretic particle deposition

Numerical simulation of magnetohydrodynamic radiative double-diffusive convective heat and mass transfer of a ternary hybrid nanofluid with thermophoretic particle deposition in an inclined annulus is studied. Both sides of cylinders are preserved at uniform temperatures, while the remaining sides are thermally insulated. The coupled nonlinear partial differential equations are solved using a finite difference approach. The detailed computational results of heat and mass transfer rate, temperature, concentration and flow fields are presented and discussed for a distinct range of significant physical parameters. The results demonstrated that increasing the angle of the inclination factor increases fluid flow. In light of buoyancy, when the annular cavity is set downward, the enhanced shear velocity is transported upwards, where it reaches its optimal temperature. The higher temperature difference leads to augmented dynamic fluid motion, particularly in the radial direction, as the system seeks to balance the thermal energy through convective transfer. The Brownian motion parameter has abrupt mobility of nanoparticles in liquid, which promotes particle collisions with liquid molecules, yielding kinetic energy. Increased values of thermophoretic parameters augment the thermophoresis force. Thermophoretic particle deposition increases the concentration of nanoparticles in an annulus due to the migration of particles from hot to cold regions under the influence of temperature gradient. The heat and mass transfer characteristics of ternary hybrid nanofluid are forecasted through an artificial neural network-based Levenberg–Marquardt backpropagated algorithm. The built model shows the heat and mass transfer rate root mean squared values through the Levenberg–Marquardt algorithm as one and mean squared error values as 1e-07 and 1e-08 respectively.

Prediction of Microstructure Evolution in Ball Mill Liner Forging Process

The liner is affixed to the inner side of the ball mill cylinder to protect the cylinder. Through isothermal compression experiments, Arrhenius constitutive models, peak strain models, critical strain models, dynamic recrystallization dynamic models, and grain size models suitable for the forging process of Mn–Cr–Ni–Mo steel used in ball mill liners were established. By utilizing Deform software, a 3D thermo‐force‐structure coupling model for the hot forging process of ball mill liners was constructed, and the volume fraction of dynamic recrystallization and average grain size during forging was predicted. The response surface model was employed to investigate how process parameters interacted with each other and affected microstructure uniformity in ball mill liners. After optimization, the optimal parameters were determined: initial forging temperature at 1200 °C, forging speed at 30 mm s−1, and friction coefficient at 0.3. Subsequently, a hot forging experiment on ball mill liners was conducted using these optimized parameters; samples were analyzed through backscattered electron diffraction device experiments and microscopic tissue observations. Results demonstrated that microstructural changes observed during actual forging processes aligned with numerical simulation results—thus verifying both the accuracy of the Mn–Cr–Ni–Mo steel material model and numerical simulation method.

Direct numerical simulation of 45° oblique flow past surface-mounted square cylinder

Comprehensive coherent structures around a surface-mounted low aspect ratio square cylinder in uniform flow with an oblique angle of $45^{\circ }$ were investigated for cylinder-width-based Reynolds numbers of 3000 and 10 000 by direct numerical simulation based on a topology-confined mesh refinement framework. High-resolution simulations and the critical-point concept were scrutinized to reveal for the first time the reasonable and compatible topologies of flow separation and complete near-wall structures, due to their extensive impact on various engineering applications. Large-scale horseshoe vortices are observed at two notable foci in the viscous sublayer. Within this layer, a wall-parallel jet is formed by downflow intruding into the bottom surface at the half-saddle point, then deflecting in the upstream direction and finally penetrating the bottom surface until the half-saddle point. A pair of conical vortices on the cylinder's top surface switch themselves on two sides of the square cylinder, where the switching frequency is identical with that of the sway of the side shear layer. The undulation of the Kelvin–Helmholtz instability is identified in the instantaneous development of a conical vortex and side shear layer, where the scaling of the ratio of the Kelvin–Helmholtz and von Kármán frequencies follows the power-law relation obtained by Lander et al. (J. Fluid Mech., vol. 849, 2018, pp. 1096–1119). Large-scale arch-shaped vortex is often detected in the intermediate wake region of a square cylinder, involving two interconnected portions, such as the leg portion separated from leeward surfaces and head portion rolled up from the top surface. The leg portion of the arch-shaped vortex was rooted by two foci near the bottom-surface plane.

Using a fiber Bragg grating technique to investigate temperature cycling strain of pressed TATB‐based polymer‐bonded explosives

AbstractPressed 1,3,5‐triamino‐2,4,6‐trinitrobenzene (TATB)‐based polymer‐bonded explosive (PBX) exhibits significant anisotropic and irreversible expansion characteristics during temperature cycling, resulting in both safety and service issues. The anisotropic and irreversible expansion properties of PBX have shown to be closely related to the microstructures, including explosive crystals, binders, micro voids, el. Fiber Bragg grating (FBG) technology has advantages of high reliability, accuracy, and sensitivity, which is more conducive to real‐time capture of the changes in strain throughout the temperature cycling. In this study, temperature cycling strain monitoring experiments of PBX pressed under different thermal–mechanical coupling loading conditions were studied using FBG technology. The effect of the pressing parameters on the anisotropic and irreversible expansion characteristics of the PBX's during temperature cycling was measured and analyzed, taking into account potential microstructural factors such as crystal orientation, binders, and porosity. The results indicated that the strain on the side and top of PBX cylinders were distributed with increasing strain in the direction of higher density. The strain stratification was significant during the high‐temperature stage of temperature cycling and was not obvious during the low‐temperature stage. The degree of strain stratification decreased with an increase in temperature cycling cycles and increased with higher pressing temperatures. The anisotropic expansion of PBX cylinders increased with an increase in temperature during cycling and decreased with a reduction in temperature. The axial expansion degree of PBX cylinders pressed under different compression conditions in the later cycles of temperature cycling is consistent with the different crystal orientations intensity inside them.

Experimental study on diesel spray flame and wall heat transfer of two-dimensional combustion chamber: effect of common rail pressure

Reducing the heat transfer occurring during the spray combustion of diesel engines remains a challenging task. This study investigated the spray flame behaviour, wall heat flux, and heat transfer phenomena of diesel engines at varying common rail pressures. A rapid compression and expansion machine (RCEM) was employed to simulate a single cycle of diesel spray combustion using split injection sprays. The two-dimensional (2D) stepped piston cavity is a uniquely shaped model used to describe the spray flame behaviour in the combustion chamber. To investigate the heat transfer phenomena, we installed wall heat flux sensors in the cylinder and cavity side. The results indicated that higher common rail pressures increased the peak value of the wall heat flux and reduced the combustion duration and the luminous flame. The higher common rail pressure led to an increase in spray velocity, thereby enhancing the turbulence level. This increase in turbulence to higher peak value of the wall heat flux. The highest peak value of the wall heat flux was reached by the cylinder head owing to the intense combustion region, followed by the cavity bottom owing to the highest flame temperature occurring around this location. Additionally, the heat transfer phenomena in diesel engines were successfully represented by local Nu-Re correlations. Our results contribute to the discussion on heat transfer phenomena that influence the thermal efficiency of diesel combustion engines.

Parametric analysis of the flow past a square cylinder in the interface of two different-velocity streams

We address the linear stability of the two-dimensional incompressible flow past a square cylinder immersed in the wake of an upstream splitter plate, which separates two streams of different velocities, UT (top) and UB (bottom), to three-dimensional perturbations. The analysis is done in the so-called wake transition regime, across which two-dimensional vortex shedding incorporates spanwise modulation. In addition to the top and bottom stream Reynolds numbers (ReT,B=DUT,B/ν, with D the square cylinder side and ν the kinematic viscosity of the fluid), a parametric analysis is conducted to gauge the effects of splitter plate length (S) and the gap between the trailing edge of the splitter plate and the front face of the cylinder (G) on the leading three-dimensionalizing instabilities. Modes akin to the A- and B-type instabilities that characterize the wake transition regime past circular and square cylinders in homogeneous incoming flow are observed only at very small values of the top-to-bottom Reynolds number ratio R=ReT/ReB, while a third mode, mode C, is ubiquitous beyond more than very mild values of R. Increasing S at constant small G has a stabilizing effect on mode C, whose onset is pushed to higher values of R. Only for long S is mode A observed. Fixing a short S and increasing G results instead in a destabilization of mode C, and mode B is favored over mode A.

Measurements of flow-induced vibration of a flexible splitter plate mounted on a cylinder in free stream flow

We experimentally study flow-induced vibration (FIV) of a thin, cantilevered, flexible plate attached on the lee side of a circular cylinder in a free stream airflow. The Reynolds number (ReD) based on the diameter of the cylinder was varied in the range of (4 × 103, 5×104). The plates of different lengths and the same span were tested with varying airflow velocities (U) in a wind tunnel. The reduced velocity (UR) based on the length and the second natural frequency of the plate was in the range of (1, 11). We describe the plate dynamics showing displacement, frequency, phase plane, amplitude spectral density (ASD), oscillation envelope, modal energy contribution, and flow structures. Based on the motion of the plate, three FIV regimes are observed, namely, initial excitation, transition, and lock-in. During lock-in, the plate attains self-sustained limit cycle oscillation. We observe the presence of small-amplitude higher harmonics in the ASD plot. The second mode is dominant around the onset of lock-in, while the first mode is dominant later in the lock-in regime. The dimensionless displacement increases with an increase in mass ratio (Ms) in the lock-in regime. With a decrease in Ms, the slope of the dimensionless frequency with UR increases. To explain measurements, we develop a wake oscillator model (WOM) in which the plate and fluctuating lift force are modeled as an Euler–Bernoulli cantilever beam and van der Pol oscillator, respectively. The coupled WOM qualitatively and quantitatively predict the measured amplitude and frequency response, respectively, in the lock-in regime.

Effect of spacing ratios on coupling mechanism of three tandem cylinders in planar shear flow

Using four-step semi-implicit characteristic-based split method, flow-induced vibrations of three tandem cylinders in planar shear flow are numerically simulated at Re = 100. The vibration responses are more complicated than that of a single cylinder due to the gap development and mutual interference with each other. The spacing ratio (L/D) between three tandem cylinders of equal size ranges from 2.5 to 5.5, with a mass ratio (mr) of 2.0 attributed to each one. The reduced velocity (Ur) spans from 3 to 30, and the shear ratio (k) varies as 0, 0.05 and 0.1 respectively. The numerical results show that spacing ratio and shear ratio have an influence on the dynamic responses, performances of frequency, phase and energy for three tandem cylinders. The vibration responses of each cylinder can be clearly divided in three major branches, which first increase then decrease with increasing Ur. The gap flow inside cylinders at small L/D heightens the probability of vortex merging, intensifying mutual interference and consequently amplifying the amplitude. It is worth noting that as k increases to 0.1, downstream cylinders exhibit different vibration responses at L/D ≥ 4.0. Specifically, a secondary increase occurs after Ur > 18 at L/D = 4.0, and the amplitude is completely suppressed after Ur > 25 at L/D = 5.5. With the help of other performances, the differences in vibration responses of three tandem cylinders at different k and L/D are revealed.

Scattering of water waves by multiple segmented concentric semi-closed breakwaters and an interior cylinder: An analytical approach

This study aims to provide a theoretical analytical model for the design of breakwaters like Palm Jumeirah breakwater in Dubai by investigating the hydrodynamic characteristics of segmental semi-enclosed breakwaters simplified from it. An analytical solution to the interaction of water waves and a cylinder and multiple concentric segmented arc-shaped breakwaters has been developed based on linear potential theory. The eigenfunction matching method and separation of variables have been applied to expand the unknown function in terms of Chebyshev polynomials. By considering the boundary conditions, the integral equation has been simplified into a set of algebraic equations, which are then solved to determine the unknown function. The accuracy of the model has been confirmed by comparing the results with those in previous articles. The numerical analysis has revealed the relationship between variations in the hydrodynamic loads, wave elevation of the cylinder, and factors including the gap width between the arcs, opening angle, wave incidence angle, and porous-effect coefficient. The results indicate that segmented breakwaters offer better protection to the internal structure than unsegmented breakwaters. Moreover, segmented breakwaters with smaller gaps are less sensitive to diagonal waves and the gap can serve as a navigational channel, which is an extra benefit.

Short crested wave–current interaction with a cylinder and two concentric asymmetric arc-shaped walls

The diffraction problem of the interaction of short-crested incident waves and currents with a combined structure is studied. The combined structure consists of two concentric asymmetric exterior porous arc-shaped cylinders and an impermeable interior cylinder. A variable separation and an eigenfunction expansion method are used to derive the velocity potential for the entire fluid domain. The accuracy of the current model is verified by comparing the results of this study with published calculations. Numerical results reveal the variation of wave force and run-up on the combined structure with current speed, opening angle, and the locations of the two arc exterior cylinders. Moreover, the specific parameter conditions under which the resonance of the wave and structure may occur are analyzed, and the resonance phenomenon may be enhanced owing to the presence of a uniform current. Furthermore, the hydrodynamic performance of segmented arcs in the limiting case is examined. The results show that the segmented arc is not effective in protecting the interior structure when the wavelength is short. The findings of this study are anticipated to provide theoretical direction for the design of nearshore architecture.

The threshold of instability of Taylor–Couette flow of a Newtonian fluid in quasi-periodic modulation with two immense frequencies using spectral Chebychev collocation methods

This study investigates the dynamic flow of a Newtonian fluid through two coaxial cylinders, each rotating at speeds Ω1(t)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Omega }_{1}(t)$$\\end{document} (inner cylinder) and Ω2(t)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\Omega }_{2}(t)$$\\end{document} (outer cylinder). We derive equations of motion for disturbances in balance, yielding a controlled system characterized by parameters such as Taylor number, wave number, frequency ratio, and interior cylinder frequency. We introduce numerical techniques for solving this system, employing spectral Chebychev collocation for spatial resolution and a combined approach of Floquet theory and Runge–Kutta method for temporal resolution. Our refined approach enables comprehensive analysis of fluid dynamics within the rotating coaxial cylinders, showcasing the interplay of various control parameters.

Flow control over a circular cylinder using a slot and axially arranged holes

Circular cylinder flow is controlled using a slot and axially arranged holes (AAH) at a Reynolds number of 32000. When the slot and AAH are placed at small incidence angles from the free-stream direction, the passive jet generated on the leeward cylinder side pushes the wake vortex downstream, reducing the drag compared to the baseline cylinder. If the incidence angle is larger than the critical angle, the alternate blowing and suction induced by the slot and AAH promote a boundary-layer transition on one side and a shear-layer transition on the other. The earlier transition to turbulent flow substantially shortens the separated shear layers on both sides, shifting the wake formation region toward the base and leading to an increase in drag. The AAH has a higher critical incidence angle than the same-size slot, as the passive jet developed from the AAH undergoes a transition to alternate blowing and suction at a higher incidence angle than the slot. Just before the incidence angle of the AAH reaches the critical angle, the weak jet flow induced on the windward cylinder side delays the boundary-layer separation through early separation and laminar reattachment without shortening the separated shear layer, thereby allowing additional drag reduction.

A petal–cylindrical seismic metamaterial occupying low-frequency wide bandgaps in horizontally stratified soils

Earthquake prevention is of fundamental interest in seismic research area. Seismic metamaterials proved very useful for this purpose, but because they are mainly based on resonant structures, the working frequency bandwidth of these metamaterials is narrow. This work aims to increase the bandwidth by improving cylindrical resonant structure. Two types of cylindrical structures with four petals attached to the four sides of hollow and solid cylinders are proposed. Their bandgap properties have been investigated by using the finite element method. Particularly, considering more realistic site conditions for seismic surface wave propagation, we explored the properties of the proposed structures for horizontally stratified soils, besides homogeneous soil. The results show that a low-frequency wide bandgap is provided in case of homogeneous soil. Excitedly, in case of a stratified substrate, an additional bandgap is obtained in lower frequency region. Moreover, to confirm the feasibility of the proposed structures, the transmission coefficients for the finite unit cells are calculated for both homogeneous and horizontally stratified soils along with the different directions. The finding substantiates that the finite system for stratified soils can effectively attenuate seismic surface waves within the two low-frequency bandgaps occupying the relative bandwidths of 154% and 81%.

Wake control of a square cylinder: Impinging the vortex cores with dual flexible splitter plates

Numerous works have been conducted to study the wake control of a bluff body and more efficient strategies are still what we seek for. This paper studies the complex fluid–structure interaction (FSI) problem of a square cylinder mounted with dual flexible splitter plates from a new perspective: impinging the vortex core with flapping plate tip. The Reynolds number (Re) based on the side length of the square cylinder is fixed at 332.6. The splitter plates are attached at both sides of the square cylinder. Structural parameters, such as plate length and bending stiffness, are considered to achieve an optimal arrangement of this configuration. Compared to the traditional single splitter plate arrangement, only a short splitter length is needed for this dual splitter plates configuration. Flow visualization indicates that the drag reduction is associated with the increase of base pressure and the lift suppression is related to the inhabitation of the interaction of the shear layer and split effect of the tip vortices. The dimensionless bending stiffness plays a vital role in determining the behavior of the splitter plate. Two vibration modes, i.e., off-center vibration and asymmetry vibration modes are captured and the up and down splitter plates tend to vibration in-phase. The Lissajous trajectories of the free tip mainly exhibits arc-shape and sickle-shape. Given the performance of the square cylinder-dual splitter plates configuration, splitter length of l/L=1.5 is an optimal configuration, which is less affected by flexural rigidity. The maximum drag reduction could reach 23.1% and the lift force exerting on this configuration does not increase significantly, which is beneficial for the structural stability.

Experimental study on wall heat transfer from diesel spray flame in two-dimensional combustion chamber operated with rapid compression and expansion machine (RCEM)

Heat transfer phenomenon in internal combustion diesel engine is one of problems to be solved. This study was conducted to investigate the relationship between diesel spray impingement, combustion processes, and heat loss using a rapid compression and expansion machine (RCEM). Furthermore, spray combustion and heat transfer processes were investigated using multiple injection strategies with a 6-holes injector. The amounts of diesel fuel used in the pilot, pre-injection, and main injection sprays were 3.964, 0.747, and 11.205 mg, respectively. Variations in common rail pressure conditions (30, 120, and 180 MPa) were applied to properly investigate spray evaporation, air–fuel mixture formation, and heat transfer in the combustion chamber. In addition, a local wall heat flux sensor was installed in the cylinder (head and liner) and piston cavity (upper lip, lip, and bottom) to measure the heat flux distribution, and a diffused backlight illumination method was used to visualize the spray and flame behavior in the combustion chamber and 2D piston cavity. The results showed that the impingement flame surrounding the cylinder head contributed to the highest magnitude and longest duration of the heat flux on the cylinder side owing to the spray flame velocity, impingement, and residence time. Meanwhile, the highest different temperature at cavity bottom induced more heat flux on the cavity side than that on the squish side. In addition, the higher injection pressure increased the spray momentum and spray tip penetration which led to a better air–fuel mixture, better evaporation rate, and less flame residence that contributed to reduce the accumulated heat loss in combustion chamber. Furthermore, a low common rail pressure was found to induce a greater difference in the accumulated heat flux ratio than a high common rail pressure which is owing to longer flame residence time in the combustion chamber. They were approximately 14% and 6% greater than that compared to the baseline common rail pressure which mean that a high common rail pressure contributes for the improving thermal efficiency in spray combustion.

Flow-induced vibration of a cylinder-plate assembly in laminar flow: Branching behavior

The transverse flow-induced vibration of an elastically supported cylinder-plate assembly (viz., a rigid splitter-plate attached to the downstream side of a circular cylinder) with a low mass ratio of 10 and a zero structural damping coefficient at a Reynolds number of 100 is investigated in the present work. A careful identification of all the branches in the amplitude response of an assembly with various plate lengths is undertaken, in conjunction with the associated flow dynamics responsible for these branches involving various aspects of the flow, such as the vortex-shedding in the far wake and the evolution of the shear layers generated on the upper and lower surfaces of the cylinder in the near wake. This knowledge offers crucial new perspectives on the nature and physical mechanisms behind the complex dynamics of a cylinder-plate system. These investigations involve a wide range of plate lengths LSP/D=0–4 (where D is the diameter of the circular cylinder) over an extensive span of reduced velocities Ur = 2–30. For LSP/D≤0.5, a self-limiting oscillation is induced in the structure—this can be either a vortex-induced vibration (VIV) or an integrated VIV-galloping response. For LSP/D≥0.75, the amplitude response is non-limited in the sense that the amplitude increases linearly with increasing Ur. More precisely, the amplitude response consists of either a strongly correlated VIV-galloping regime (at LSP/D=0.75) or two clearly separated regimes of VIV and galloping (for LSP/D>0.75). In the galloping regime, both odd- and even-multiple synchronizations between the system oscillation and the vortex shedding are supported. “Kinks” in the amplitude response signal the onset of synchronization branches in the galloping regime. Two new branches have been identified for a cylinder-plate assembly with longer plate lengths, namely, an initial galloping branch and a still (quiescent) branch. The initial galloping branch is associated with wake meandering. For the still branch, the assembly is stationary (no vibratory motion), and flow over the assembly is steady (no vortex shedding or shear-layer meandering).

Deep reinforcement learning-based active flow control of vortex-induced vibration of a square cylinder

We mitigate vortex-induced vibrations of a square cylinder at a Reynolds number of 100 using deep reinforcement learning (DRL)-based active flow control (AFC). The proposed method exploits the powerful nonlinear and high-dimensional problem-solving capabilities of DRL, overcoming limitations of linear and model-based control approaches. Three positions of jet actuators including the front, the middle, and the back of the cylinder sides were tested. The DRL agent as a controller is able to optimize the velocity of the jets to minimize drag and lift coefficients and refine the control strategy. The results show that a significant reduction in vibration amplitude of 86%, 79%, and 96% is achieved for the three different positions of the jet actuators, respectively. The DRL-based AFC method is robust under various reduced velocities. This study successfully demonstrates the potential of DRL-based AFC method in mitigating flow-induced instabilities.