Cylinder Spacing Research Articles

Numerical Simulation and Investigation of Flow Patterns Around Three Side-by-Side Cylinders Arranged in Line with an Oscillating Central Cylinder

A flow past three cylinders arranged in a vertical row in a side-by-side configuration in which both side cylinders were kept fixed and an oscillatory movement provided to the middle cylinder was simulated. This particular configured setup holds significant importance in engineering and presents a compelling resemblance to numerous real-world scenarios especially in electricity production using oscillatory motion. In the present study, free-stream flow across three side-by-side cylinders with a transversely oscillating middle cylinder, was simulated using the commercial CFD software tool ANSYS 2021 R2 at a low Reynolds number (Re) = 100, with non-dimensional cylinder spacing (s/d) = 2;. For the middle cylinder, non-dimensional amplitude (amplitude ratio), A/d = 0.5 and a frequency ratio of fe/fo=0.8, 1 and 1.2 were considered (fe/fo, where fe is the cylinder oscillation frequency and fo is the corresponding vortex shedding frequency for stationary cylinders). In this study, re-meshing was done using the dynamic mesh technique, and pre-defined oscillatory motion was provided with the help of an additionally hooked User Defined Function (UDF) in the commercial CFD software tool. The objective of the present study was to investigate the effect of governing parameters on flow physics and wake nature, as well as the effect of the flow transition on engineering parameters. The interesting finding to report here that for oscillation frequency fe/fo= 1 there exist synchronous flow confirmed from wake diagram having comparatively more drag with that of other two higher fe/fo= 1.2 and lower oscillation frequency ratio fe/fo= 0.8 for cylinder spacing (s/d) = 2.

Wave propagation through a dense cluster of cylinders: a radiative transfer model

Clusters of cylinders are a useful way to model the wave propagation through several natural and complex media: crowds of people, forests, arrays of scatterers, or various materials. Depending on the absorption properties of the cylinders, as well as their size- and distance-to-wavelength ratio, different physical mechanisms are at play: scattering, viscous and thermal dissipation, absorption within the material of the cylinders themselves. We consider the low frequency case when the wavelength is larger than both the cylinder diameter and spacing. Most multiple scattering theories require larger than wavelength spacing and are not applicable in the case of dense clusters and at low frequencies. As a first approximation, a porous medium model is used to calculate the speed of sound in the cluster. However, this approach does not readily give access to the coherency of the signal. Recently, a radiative transfer approach was used to model forest acoustics to examine the transformation of the coherent sound field into an incoherent sound field. We use this approach to examine the coherent to incoherent field transition depending on the wavelength to diameter and spacing.

The evolution of the bubble collapse morphology between two cylinders within a confined space

This work investigates the dynamic bubble behaviors between two cylinders within a confined space using high-speed photographic experiments and Kelvin impulse theory. First, the evolution of the collapse morphologies of bubbles located at the origin and along the y axis between two cylinders is qualitatively investigated. The effects of the cylinder spacing and bubble ordinate on the characteristics of the bubble deformation and the liquid velocity are then explored. The variations of the bubble interface velocities, the roundness of the bubble cross section, and the bubble radius are quantitatively analyzed. The conclusions can be summarized as follows: (1) The experimental bubble collapse phenomena at the origin can be divided into three cases: hourglass-shaped collapse, “8”-shaped collapse, and capsule-shaped collapse. Bubble collapse at the y axis can also be divided into three scenarios: awl-shaped collapse, spindle-shaped collapse, and inverted triangle-shaped collapse. (2) The cylinder spacing and the bubble ordinate significantly affect the experimental bubble collapse behaviors and the theoretical liquid flow field. (3) High-velocity liquid regions are generated around the bubble when it oscillates freely, and the nearby cylinders always lead to low-velocity regions between them and the bubble. The closer the bubble is to the cylinder, the smaller the low-velocity regions and the larger the high-velocity regions.

Deep reinforcement learning based proactive dynamic obstacle avoidance for safe human-robot collaboration

Ensuring the health and safety of human operators is paramount in manufacturing, particularly in human-robot collaborative environments. In this paper, we present a deep reinforcement learning-based trajectory planning method for a robotic manipulator designed to avoid collisions with human body parts in real-time while achieving its goal. We modelled the human arm as a freely moving cylinder in 3D space and formulated the dynamic obstacle avoidance problem as a Markov decision process. The algorithm was tested in a simulated environment that closely mimics our laboratory environment, with the goal of training a deep reinforcement learning model for autonomous task completion. A composite reward function was developed to balance the effects of different environmental variables, and the soft-actor critic algorithm was employed. The trained model demonstrated a 93% success rate in avoiding dynamic obstacles while achieving its goals when tested on a generated data set.

Fluid flow control around unequal cylinder spacing behind three side-by-side cylinders

Fluid flow control around unequal cylinder spacing behind three side-by-side cylinders

Experimental investigation of the Eckert-Weise effect (aerodynamic cooling) of pair side-by-side circular cylinders in a compressible cross-flow

The rear surface of a thermally insulated circular cylinder placed in a subsonic high-velocity cross-flow is significantly cooled, this phenomenon is known as aerodynamic cooling or the Eckert–Weise effect. This effect is associated with the redistribution of the stagnation temperature (energy separation) in the unsteady vortex wake. Experiments show that the effect is maximal at Mach numbers of the free-stream flow of 0.5–0.7, resulting in negative values of the temperature recovery factor (the temperature of the cylinder rear surface becomes lower than the static temperature of the free-stream flow). In this case, the cooling of the rear surface can reach tens of degrees. This effect can be utilized as the basis for a novel energy separation device, as it enables heat transfer between flows with the same stagnation temperatures. As shown in a numerical study (Aleksyuk, 2021), the effect is highly sensitive to the flow interference behind side-by-side cylinders, this is especially important for its practical application. In this study, the aerodynamic cooling of a pair identical circular cylinders in side-by-side arrangement, is experimentally investigated within the Mach and Reynolds number ranges of M = 0.34–0.58 and ReD = 1.6∙105–3.1∙105. The cylinder spacing considered in this study (P/D = 1.1; 1.5; 2.0; 3.0; and 4.0, where P is the center-to-center cylinder spacing and D is the diameter) covered the key regimes of vortex interference, that is, single, bistable, and coupled vortex streets. Also, investigations were conducted for a single circular cylinder with the same diameter within the Mach and Reynolds number ranges of M = 0.34–0.68 and ReD = 1.6∙105–3.4∙105. The channel blockage ratio was found to be 8% for a single cylinder and 16% for a pair of cylinders. Consequently, this study focuses on confined flow or flow past a confined cylinder with a uniform profile of oncoming flow velocity. It is shown that the Eckert-Weise effect for the pair of cylinders can either increase or decrease depending on the interference regimes and is sensitive to the blockage ratio value.

Spacing effects on flows around two square cylinders in staggered arrangement via LBM

This study presents a computational analysis of fluid flow characteristics around two staggered arranged square cylinders using the Lattice Boltzmann Method (LBM). With Reynolds number (Re) fixed at 200, numerical simulations explore the influence of varying gap ratios (G) ranging from 0 to 10 times the cylinder size. Emphasis is placed on understanding the impact of cylinders spacing on flow structure mechanisms and induced forces. Investigation of fluid flow parameters includes vorticity behavior, pressure streamlines, and variations in drag and lift coefficients alongside the Strouhal number under different values of G. From the results, four distinct flow patterns emerge: single bluff body flow, flip flopping flow, modulated synchronized flow, and synchronized flow, each exhibiting unique characteristics. This study reveals the strong dependence of fluid forces on G, with low spacing values leading to complex vortex structures and fluctuating forces influenced by jet flow effects. At higher spacing values, proximity effects between cylinders diminish, resulting in a smoother periodic flow. The Strouhal number, average drag force and the rms values of drag and lift force coefficients vary abruptly at narrow gaps and become smooth at higher gap ratios. Unlike the tandem and side-by-side arrangements the staggered cylinders arrangement is found to have significant impact on the pressure variations around both cylinders. Overall, this research could contribute to a comprehensive understanding of staggered cylinder arrangements and their implications for engineering applications.

Effect of Flow Interference between Cylinders Subjected to a Cross Flow over a Cluster of Three Equally Spaced Cylinders

In this paper, flow over a cluster of three equally spaced circular cylinders was studied by numerical simulation based on the turbulence model k-kl-ω for two incidence angles β = 0° and 60°, at different Reynolds numbers, and flow interference pattern characteristics between cylinders, characteristics of force parameters, and Strouhal number of each cylinder with different spacing ratios ranging from 1.5 to 4 at Re 8 × 104, 2 × 105 and 2 × 106 were obtained. Analyzing the flow field around three cylinders, the following results have been obtained: (1) at incidence angle β = 0° and 60°, the wake was nearly symmetrical if S/D ≥ 2.0; (2) at β = 60°, S/D = 1.5, and Re = 2 × 105, an asymmetric periodic flow pattern occurred in the wake region which produced a significant effect on the surface mechanical parameters and Strouhal number, and this was observed for the first time. The periodic flow regime of the wake region also occurred at S/D = 1.35 and 1.5, without the same phenomenon at S/D = 1.7 and 2.0; this phenomenon is described for the first time in this paper; (3) the phenomenon of periodic flow regime in the wake region was intrinsic and related to Reynolds number and space ratio. In addition, the characteristics of force parameters of three cylinders were mainly affected by the interference between cylinders, at 1.5 < S/D < 4, which indicated that the drag coefficient of three cylinders reduces with different Reynolds numbers and increases with enlargement of the spacing ratio for upstream cylinders at incidence angle 0°. At incidence angle 60° and S/D = 1.5~4, the Strouhal number decreases with the enlargement of spacing ratio for the upstream cylinders, but the Strouhal number increases for the downstream cylinder, which is another prominent flow interference influence. The results indicated the effect of flow interference between cylinders subjected to a cross flow over a cluster of three equally spaced cylinders, considering the flow pattern, surface mechanical parameters and Strouhal number, which should be considered in the establishment of standard design codes in fields such as offshore wind turbine engineering for flow interference around groups of cylinders.

Adaptive unified gas-kinetic scheme for diatomic gases with rotational and vibrational nonequilibrium

Multiscale nonequilibrium physics at large variations of local Knudsen number are encountered in applications of aerospace engineering and micro-electro-mechanical systems, such as high-speed flying vehicles and low pressure of the encapsulation. An accurate description of flow physics in all flow regimes within a single computation requires a genuinely multiscale method. The adaptive unified gas-kinetic scheme (AUGKS) is developed for such multiscale flow simulation. The AUGKS applies discretized velocity space to accurately capture the non-equilibrium physics in the multiscale UGKS, and adaptively employs continuous distribution functions following Chapman–Enskog expansion to efficiently recover near-equilibrium flow region in GKS. The UGKS and GKS are dynamically connected at the cell interface through the fluxes from the discretized and continuous gas distribution functions, which avoids any buffer zone between them. In this study, the AUGKS with rotation and vibration non-equilibrium is developed based on a multiple temperature relaxation model. The real gas effect in different flow regimes has been properly captured. To capture aerodynamic heating accurately, the heat flux modifications from the rotation and vibration modes are also included in the current scheme. Unstructured discrete particle velocity space is adopted to further improve the computational performance of the AUGKS. Numerical tests, including Sod tube, normal shock structure, high-speed flow around the two-dimensional cylinder and three-dimensional sphere and space vehicles, and an unsteady nozzle plume flow from the continuum flow to the background vacuum, have been conducted to validate the current scheme. In comparison with the original UGKS, the current scheme speeds up the computation, reduces the memory requirement, and maintains the equivalent accuracy for multiscale flow simulation, which provides an effective tool for nonequilibrium flow simulations, especially for the flows at low and medium speed.

On the Stability of Killing Cylinders in Hyperbolic Space

On the Stability of Killing Cylinders in Hyperbolic Space

Numerical investigation of entropy generation and Magnetohydronamic natural convection in a porous square cavity with four embedded cylinders

In this study, we investigated buoyancy-induced convection in a permeable square hollow containing four embedded cylinders and subjected to a magnetic field using numerical methods. The finite element approach was used to solve the governing equations of the system as well as the initial and boundary conditions. We analyzed the effects of the emerging non-dimensional quantities on the flow pattern and thermal field, as well as entropy production, in relation to the thermophysical properties of the obstacles. In the limiting case, we compared our results with already published work and found outstanding concurrence. Our simulations revealed that increasing cylinder spacing leads to higher thermal entropy generation, while fluid friction irreversibility has the opposite effect. Additionally, the imposed magnetic field significantly suppressed temperature distribution and flow field, resulting in low thermal transmission within the cavity.

Wake-induced torsional oscillation of two tethered cylinders for energy harvesting

When two cylinders submerged in a uniform flow are arranged in tandem, the downstream cylinder can oscillate in response to the wake from the upstream cylinder. In this investigation, the downstream cylinder is allowed to oscillate freely around the center of a fixed upstream cylinder, mimicking a pendulum-like motion. Our findings suggest that the wake stiffness concept, initially verified for relatively large cylinder spacing (ℓ≥4) and linear transverse cylinder motion, is also relevant for characterizing the wake-induced vibration (WIV) response observed for two tandem tethered cylinders situated in close proximity (2≤ℓ≤4) for Reynolds numbers in the range 1.0×104≲Re≲1.2×105, with tests conducted up to Re≈1.4×105. For small cylinder spacing (ℓ≤2.5), the downstream cylinder attains a maximum oscillation angle amplitude and exhibits consistent vibration, providing reliable potential for energy harvesting. We also explore hysteresis in the WIV response, which is observed to depend on the history of Reynolds number variation. Our findings reveal hysteresis at both the onset and termination of oscillation.

Patient's Healthy-Limb Motion Characteristic-Based Assist-As-Needed Control Strategy for Upper-Limb Rehabilitation Robots.

The implementation of a progressive rehabilitation training model to promote patients' motivation efforts can greatly restore damaged central nervous system function in patients. Patients' active engagement can be effectively stimulated by assist-as-needed (AAN) robot rehabilitation training. However, its application in robotic therapy has been hindered by a simple determination method of robot-assisted torque which focuses on the evaluation of only the affected limb's movement ability. Moreover, the expected effect of assistance depends on the designer and deviates from the patient's expectations, and its applicability to different patients is deficient. In this study, we propose a control method with personalized treatment features based on the idea of estimating and mapping the stiffness of the patient's healthy limb. This control method comprises an interactive control module in the task-oriented space based on the quantitative evaluation of motion needs and an inner-loop position control module for the pneumatic swing cylinder in the joint space. An upper-limb endpoint stiffness estimation model was constructed, and a parameter identification algorithm was designed. The upper limb endpoint stiffness which characterizes the patient's ability to complete training movements was obtained by collecting surface electromyographic (sEMG) signals and human-robot interaction forces during patient movement. Then, the motor needs of the affected limb when completing the same movement were quantified based on the performance of the healthy limb. A stiffness-mapping algorithm was designed to dynamically adjust the rehabilitation training trajectory and auxiliary force of the robot based on the actual movement ability of the affected limb, achieving AAN control. Experimental studies were conducted on a self-developed pneumatic upper limb rehabilitation robot, and the results showed that the proposed AAN control method could effectively estimate the patient's movement needs and achieve progressive rehabilitation training. This rehabilitation training robot that simulates the movement characteristics of the patient's healthy limb drives the affected limb, making the intensity of the rehabilitation training task more in line with the patient's pre-morbid limb-use habits and also beneficial for the consistency of bilateral limb movements.

Rayleigh–Faber–Krahn and Hong–Krahn–Szegö-type inequalities for the absolute value of the heat operator

The main aim of this paper is to show that the first singular number of the generalized Cauchy–Dirichlet heat operator is minimized by a circular cylinder among all domains of the same measures with the circular cylinder in Euclidean space. This result gives us an analogue of the celebrated Rayleigh–Faber–Krahn inequality for the absolute value of the heat operator. Further, we obtain an analogue of the Hong–Krahn–Szegö inequality for the same operator.

Experimental Study on Wave Transmission Coefficient of Double Cylindrical Floating Breakwater in a Wave Basin

Floating breakwater is a better alternative than the conventional method. It is an effective coastal engineering unit used to protect coastal regions such as harbours and marinas from erosive waves. It has many other applications such as offshore renewable energy generation, infrastructure preservation, temporary structure, and provides a controlled environment for aquaculture activities. The floating breakwater such as the double cylindrical floating breakwater (DCFB) is claimed to have effective wave attenuation characteristics. A lab scale investigation was done by placing the DCFB in a controlled wave basin in with wave generator to generate wave and wave gauge to quantify the wave transmission coefficient. The wave transmission coefficient represents the effectiveness of the breakwater system in attenuating incoming waves, where an effective breakwater system will result in a low transmission coefficient value. In this study, the DCFB’s effectiveness in terms of wave transmission coefficient was investigated with two different configurations where the spacing between the cylinders of the DCFB units are 0.2 m and 0.6 m respectively. Results show that the configuration of DCFB units with 0.5 m cylinder spacing are more effective in absorbing incoming wave height resulting in lower transmission coefficient. Furthermore, some prospective areas had been identified for future enhancement.

Experimental Investigations on the Wake‐Induced Vibration of an Electromagnetic Energy‐Harvesting System

The wake energy of an electromagnetic harvesting device is derived from the phenomena of wake‐induced vibration (WIV). Experiments were conducted to determine the effects of the Reynolds number, the resistance of the circuit, and the distance between two concentric circular cylinders on the wake energy of the flow. The mechanism of the diaphragm energy harvester (DEH) consists of a single and tandem circular cylinders, which were examined separately. An elastic diaphragm with a magnet was also mounted in the wake of the cylinders. The Reynolds number varies between 2000 and 5000. The converter transforms the wake energy of the flow into electricity due to the vibration of the cylinder in a magnetic field. This research fills a critical gap in the literature by investigating the energy performance of a DEH under the wake‐induced vibration of two fixed circular bluff bodies, demonstrating for the first time the feasibility of capturing vortex energy through the wake in the presence of a diaphragm. This unique approach distinguishes our work in the field of electromagnetic energy‐harvesting devices, showcasing the potential for practical applications in renewable energy generation. The test results indicate that increasing the Reynolds number enhances the converter’s power output. Additionally, it has been observed that two critical distances are significant for the output voltage: the gap between the tandem cylinders (L∗) and the distance between the edge of the diaphragm and the center of the cylinder (X∗). The voltage output of tandem cylinders suggests that an optimal cylinder spacing falls within the range of 4 ≤ L∗ < 5.

Turbulence intensity effect on axial-flow-induced vibration of a two-cylinder cluster

This numerical study aims to investigate the effect of inlet turbulence intensity Tu (0.05 – 5.0%) on flow-induced vibration of an elastic cylinder with fixed ends placed next to a rigid cylinder of the same diameter and length in axial flow. Turbulent flow structure and fluid-structure interaction are captured using large eddy simulation and two-way coupling calculation, respectively. It is found that Tu produces a pronounced effect on vibration characteristics of the elastic cylinder. Given a dimensionless velocity U̅ (i.e., 3.49, less than critical U̅ for onset of buckling), root-mean-square vibration amplitude Arms⁎ increases rapidly with increasing Tu from 0.7% to 5.0% for any given center-to-center cylinder spacing P⁎ (1.20 – ∞). This change in Arms* is attributed to the interactions between turbulent structures of different scales, where interactions between large-scale eddies result in the formation of small-scale eddies in the interstitial flow between the cylinders, thus enhancing greatly the flow instability and hence Arms*. Another interesting phenomenon takes place at U̅ = 7.62 (beyond the critical U̅ for onset of buckling) and P⁎ = 1.80, where the elastic cylinder buckles into a new dynamic equilibrium state. As Tu increases from 0.3% to 1.5%, the flutter instability occurs merely along the y direction that is normal to the plane of two cylinders axes and also normal to main stream direction. It has been found that the high-Tu flow triggers two vortices in this direction, giving rise to two regions of different pressures around the cylinder. The two regions reposition quasi-periodically themselves, thus producing lateral forces that account for this flutter instability in the y direction. Scaling analysis of the dependence of Arms⁎ on Tu and U̅ reveals that the relationship Arms⁎ = g1(Tu, U̅) may be reduced to Arms⁎ = g2(U̅eff); g1 and g2 are different functions and the scaling factor U̅eff = U̅ TF, where TF is the turbulence factor that accounts for the effect of Tu on Arms⁎. The scaling law is discussed in detail, along with the physical meanings of U̅eff.

Large Eddy Simulation of Flow Characteristics around Cylinders with Crosswise and Streamwise Arrangements in Ocean Energy

The flow around cylinders is one of the most fundamental phenomena in extracting wave energy from ocean waves. Compared with flows around a single cylinder, the investigation of flows around multiple cylinders is still limited and requires further studies to reveal flow characteristics. To this end, large eddy simulations are conducted to investigate the flow around double cylinders with crosswise and streamwise arrangements. Systematic studies on the influence of the number of mesh cells, the first near-wall mesh size, and the transient time step are carried out to achieve accurate and efficient simulations. The drag coefficient, flow separation, and flow pattern for different arrangements under various cylinder spacings are analyzed according to simulation results. For the crosswise arrangement, the flow pattern switches from the single-body regime to the synchronized vortex-shedding regime as the spacing increases. For the streamwise arrangement, the flow pattern develops from the reattachment regime to the vortex-shedding regime as the spacing increases.

Mixed Convective Flow Past Reverse Doublet like Rotating Side by Side Cylinders

The flow unsteadiness and the consequent evolution of the vortex shedding for flows around bluff objects are significantly important in fluid dynamics. Rotating the bluff objects may help in controlling such flow instabilities. Depending on the rotational speed, the unsteady flow may be transfigured into a steady pattern. On the other hand, multiple objects placed transverse to the incoming flow have destabilizing wake interactions. The instability even gets amplified with the introduction of thermal buoyancy. Under such conditions, the rotation plays an important role in stabilizing the wake dynamics. In view of the above, the present study aims to numerically investigate the wake and the thermal characteristics around a pair of circular cylinders arranged in side-by-side fashion in an unconfined medium. The top and the bottom cylinders are rotated in the counterclockwise and the clockwise directions, respectively, replicating a reverse doublet like configuration. The Reynolds and the Prandtl numbers are fixed at 100 and 0.71, respectively. The effect of the rotation on the flow instability is studied for Richardson number 0 to 1. The computations are performed using a finite volume method for the dimensionless cylinder spacings 0.7, 1.5, 3.0, and 5.0. The unsteady and the steady attributes are examined using the vorticity, isotherm-streamline contours, and lift signals. Finally, a regime diagram is constructed to exhibit the critical rotational speeds at which the flow becomes steady for various cylinder spacings.

Investigation of the Influence of Wake Field Characteristic Structures on Downstream Targets Using the POD Method

This research investigated the impact of complex low-speed wake flow structures on the aerodynamic characteristics of objects downstream. It employed the proper orthogonal decomposition (POD) method and the domain precursor simulation method to compare traditional methods and validate this approach. The study generated several flow structures of parallel dual-cylinder wakes with different scales and spacing. The variations in the aerodynamic coefficient of three downstream objects at various times passing through wakes of varying scales were appropriately compared and analyzed. The study established that the wake with a cylinder spacing of G = 1.5 has a more compact and concentrated modal structure than that with a cylinder spacing of G = 0.35. Smaller objects were more responsive to the wake flow structure with a spacing of G = 1.5, whereas larger objects responded more to the flow structure with a spacing of G = 0.35. The achieved results also revealed that the aerodynamic force coefficients of objects passing through the wakefield at different times were closely related to the temporal characteristics of the wake flow structure with different scales.