Free Cylinder Research Articles

Simulation of fluid‐structure interaction using the boundary data immersion method with adaptive mesh refinement

SummaryThe fluid‐structure interaction is simulated using the boundary data immersion method. As the fluid‐structure interface is smeared in the smoothing region, deviations are incurred in fluid simulations. For compressible flow, high order difference schemes with more mesh cells for the stencils are usually employed to achieve high overall accuracy, but near interfaces it requires wider smoothing region of several mesh cells for computational stability and hence lowers its accuracy significantly. To address this issue, the proposed algorithm switches to lower order difference schemes near the interfaces and applies adaptive mesh refining there to compensate the accuracy loss. Implemented with Structured Adaptive Mesh Refinement Application Infrastructure (SAMRAI), the algorithm shows notable improvement in the overall accuracy and efficiency in cases such as channel flow and flow past a cylinder. The algorithm is used to simulate the shock wave past a fixed or free cylinder with Ma and Re , which reveals the relaxation process and the temporal evolution of the drag coefficient, it goes through a valley and maintains at relatively high value for the fixed cylinder, while that of the free cylinder tends to decrease in fluctuation which is found to be caused by the interaction between the forward moving cylinder and vortexes in the unsteady wake.

Using high-frequency household surveys to describe energy use in rural North India during the COVID-19 pandemic

COVID-19 continues to exact a substantial toll on health. While mortality and morbidity associated with the pandemic are the most obvious impacts, social and economic disruptions are becoming apparent. There is reason to believe that the COVID-19 pandemic has slowed or reversed gains in clean household energy use in rural India. Here we describe phone surveys deployed repeatedly in Jharkhand and Bihar to describe pandemic-related changes in household socio-economic conditions and energy-use patterns. Over three-quarters of households reported hardships during the pandemic, including loss of employment and an inability to search for jobs. In turn, some of these households relied more on polluting fuels. Despite nearly all households preferring gas and electricity, we observed varied behaviours related to the cost of and access to these modern energy sources. We highlight the success of India’s three-free-cylinders scheme, with 90% of households aware of the programme and utilizing at least one free cylinder. These findings illustrate the utility of high-frequency energy-related questionnaires and suggest that interventions to improve clean fuel accessibility and affordability can increase the resilience of transitions to clean household energy.

Development of Initial Disturbances in the Circulating Flow with Variable Vorticity around the Free Cylinder

Development of Initial Disturbances in the Circulating Flow with Variable Vorticity around the Free Cylinder

Flow-Induced Locomotion of a Flexible Filament in the Wake of a Cylinder in Non-Newtonian Flows

The problem of a flapping filament mounted to a free cylinder provides insight into computational fluid dynamics. We study the extent to which the length of the flexible filament and the non-Newtonian power-law fluids should be implemented to stabilize the flapping of an immersed structure in the flow with the Reynolds number of 100. We propose a hybrid model that includes an explicit Lattice Spring Model and Immersed Boundary non-Newtonian Lattice Boltzmann Method to simulate the behavior of the filament in the vicinity of a fluid flow. We validate this hybrid model using bench-marked problems, where a non-Newtonian fluid flow passes over a fixed cylinder in an unconfined channel and in a Newtonian flow over a cylinder containing a flexible fin. In the result section, we study the effects of fluid and structural characteristics on the motion of the filament and cylinder. When shear-thinning, Newtonian, and shear-thickening fluids are used, the results show that the cylinder keeps its minimum distance to the channel centerline at a specific length. This is due to the flapping of the filament generating the low-pressure region downstream from the cylinder and filament structure. We also analyze the effects of the bending stiffness. We find that from the lowest bending stiffness of 0.003 to the highest of 0.007, flexibility has no impact on the cylinder's center of mass, when the filament length is set as large as the critical length. The vibration amplitude and frequency of the filaments are also measured in different fluids and in varied filament stiffnesses. When n = 0.7, the bending stiffness of 0.007 causes a high-frequency vibration with the highest amplitude, whereas a bending stiffness of 0.005 achieves the minimum amplitude.

Acoustic radiation force on a rigid cylinder between two impedance boundaries in a viscous fluid

Acoustofluidic technology combines acoustic and microfluidic technologies to realize particle manipulation in microchannels driven by acoustic waves, and the acoustic radiation force (ARF) with boundaries is important for particle manipulation in an acoustofluidic device. In the work reported here, the ARF on a free cylinder immersed in a viscous fluid with an incident plane wave between two impedance boundaries is derived analytically and calculated numerically. The influence of multiple scattering between the particle and the impedance boundaries is described by means of image theory, the finite-series method, and the translational addition theorem, and multiple scattering is included partly in image theory. The ARF on a free rigid cylinder in a viscous fluid is analyzed by numerical calculation, with consideration given to the effects of the distances from cylinder edge to boundaries, fluid viscosity, cylinder size, and boundary reflectivity. The results show that the interaction between the two boundaries and the cylinder makes the ARF change more violently with different frequencies, while increasing the viscosity can reduce the amplitude of the ARF in boundary space. This study provides a theoretical basis for particle manipulation by the ARF in acoustofluidics.

Comparative investigation: Closed versus semi-closed vertical cylinder-shaped fish cage in waves

A comparative analysis between a closed cage (CC) and a semi-closed/open-bottom cage (OC) in regular, white-noise and irregular waves is presented. Both cages consist of a vertical circular free surface-piercing cylinder with an external toroidal floater. A main target is to examine experimentally and theoretically the similarities and differences of the hydrodynamic behavior between the two cages. Transfer functions obtained from regular and white-noise waves test show that the two cages have similar performance in surge motion, ovalizing deformations, interior wave elevation and mean drift loads in shorter waves (wavelength to cage diameter ratio λ/D<1). This is not the case for heave, with OC experiencing much larger heave. A clear sinkage (minus average heave) is also observed for OC, which is almost proportional to the square of the incident wave amplitude. The linear potential flow solver WAMIT can provide a reasonable prediction of surge and pitch motions, and mean drift loads for OC in shorter waves (λ/D<1.5), but is unsatisfactory in longer waves and, in general, for heave motion.From the analysis of irregular wave tests, there is a small difference in the standard deviations of the surge motion (wave-frequency component) and interior waves for the semi-closed and closed cages. The slow-drift component is dominant in surge for both cages, with OC having a smaller value due to larger viscous damping associated with flow separation at the bottom. Theoretical evaluations of standard deviations based on transfer functions from a white-noise test and from WAMIT are performed, assuming that the linear superposition principle applies. Reasonable agreement against results from experiments in irregular waves is achieved except for response variables with strong nonlinear effects.Survival conditions for both cages are determined through systematic evaluations. The platform freeboard is identified as the most critical parameter. The minimum freeboard for the closed and semi-closed cage should be at least 1.05 and 1.3 m, respectively, to operate at moderate exposure sea states. The standard deviation of vertical acceleration due to rigid body motions and interior waves could reach 0.72 m/s2 for both cages at high exposure sea states, which might be of concern for fish inside.

Investigating the influence of compressibility on the second mode flutter instability of a clamped–free cylinder in axial flow using fluid–structure interaction simulations with the Chimera technique

In this research a numerical framework is presented and validated to investigate the influence of compressibility on the flutter instability of a clamped–free cylinder in axial flow by means of fluid–structure interaction (FSI) simulations. The FSI simulations consist of a computational fluid dynamics (CFD) model coupled with a finite-element model. In the CFD model the large deformation of the cylinder is handled by employing a Chimera technique in which the background mesh remained fixed.The framework was first validated by performing simulations in water flow and comparing these to experimental data from literature. Afterwards, the framework was applied to air flow simulations. The inlet velocity and structural parameters were adapted to investigate the effect of compressibility at different Mach numbers. The dimensionless velocity is kept constant in order to remain in the same working point with second mode flutter.The simulations showed that for a low Mach number there is negligible influence of compressibility on the established flutter regime. At higher Mach numbers the compressibility triggers a more violent flutter regime in which the contact between the cylinder and the channel wall extends over a larger length. In the simulations the flutter motion starts out planar and, given sufficient time, usually develops into a rotational motion. It was also observed that supersonic zones and, consequently, shocks are created in the flow while the inlet is subsonic.

Effect of Radiation on Natural Convection Heat Transfer from Heated Horizontal Cylinder in Vented Enclosure

The natural convection heat transfer from a heated circular cylinder placed in a square vented enclosure was investigated considering the effect of the thermal radiation from cylinder using finite volume method with simple algorithm that used pressure-velocity coupling to solve Navier-Stockes and energy equations using Computational Fluid Dynamics CFD (Fluent software). The flow domain is filled with air. The enclosure width is 12.5 cm, two symmetrical openings at its lower and upper walls with size O = 2.5 cm and the cylinder diameter is 5 cm. Different rang of cylinder’s surface emissivity with Rayleigh number range between 103 to 106 was applied in this study as operating condition. The effect of the Ra, cylinder radiation for vented square enclosure on the convection, radiation and total Nusselt numbers Nu and flow and thermal patterns were investigated. The numerical results for pure natural convection were compared with available experimental results, good agreement was obtained. The results revealed that the natural convection heat transfer from the cylinder in the enclosure is increased as compared with that of the unbounded cylinder without radiation effect. The results also revealed that there is a linear relationship between the heat transfer enhancement and the emissivity, there is a significant effect of the cylinder surface radiation which gives high augmentation compared with that of the free cylinder and natural convection heat transfer without radiation effect. A maximum total Nusselt number is obtained at high emissivity compared with that of natural convection heat transfer, the maximum Nu enhancement ranged between 70 to 350 % for low Ra and 25 to 125% for high Ra.

Acoustic radiation force and motion of a free cylinder in a viscous fluid with a boundary defined by a plane wave incident at an arbitrary angle

The acoustic radiation force (ARF) is derived for a free cylinder immersed in a viscous fluid with a boundary defined by a plane wave incident at an arbitrary angle. Trajectories of a free rigid cylinder under the action of ARF are also investigated. Various aspects affecting the ARFs and trajectories of a free rigid cylinder, such as fluid viscosity, the incident angle of the plane wave, the density ratio of the fluid to particle, the particle radius, and boundary, are addressed in numerical simulations. Results show that ARFs are positive or negative depending on the various factors considered in this work. Moreover, the amplitude of the total ARF on a free cylinder in a bounded viscous fluid defined by a plane wave incident at a certain angle may decrease with increasing viscosity, which is significantly different from the case of a fixed cylinder immersed in a boundless viscous fluid. Furthermore, the trajectory of the particle changing with different conditions is investigated. We can predict and regulate the particle trajectory by selecting relevant parameters. The finite element method is implemented to validate the theoretical results. The finite element results and theoretical results are in good agreement. This work helps better understand the underlying mechanism of the particle manipulation using ARF.

Adaptive mesh refinement for simulating fluid‐structure interaction using a sharp interface immersed boundary method

SummaryIn this article, adaptive mesh refinement (AMR) is performed to simulate flow around both stationary and moving boundaries. The finite‐difference approach is applied along with a sharp interface immersed boundary (IB) method. The Lagrangian polynomial is employed to facilitate the interpolation from a coarse to a fine grid level, while a weighted‐average formula is used to transfer variables inversely. To save memory, the finest grid is only generated in the local areas close to the wall boundary, and the mesh is dynamically reconstructed based on the location of the wall boundary. The Navier‐Stokes equations are numerically solved through the second‐order central difference scheme in space and the third‐order Runge‐Kutta time integration. Flow around a circular cylinder rotating in a square domain is firstly simulated to examine the accuracy and convergence rate. Then three cases are investigated to test the validity of the present method: flow past a stationary circular cylinder at low Reynolds numbers, flow past a forced oscillating circular cylinder in the transverse direction at various frequencies, and a free circular cylinder subjected to vortex‐induced vibration in two degrees of freedom. Computational results agree well with these in the literature and the flow fields are smooth around the interface of different refinement levels. The effect of refinement level has also been evaluated. In addition, a study for the computational efficiency shows that the AMR approach is helpful to reduce the total node number and speed up the time integration, which could prompt the application of the IB method when a great near‐wall spatial resolution is required.

Galilean invariance study on different lattice Boltzmann fluid–solid interface approaches for vortex-induced vibrations

The present work compares fluid–solid interface approaches for the lattice Boltzmann method (LBM) to study vortex-induced vibrations (VIV). Two classes of fluid–solid interface approaches, namely the partially saturated methods (PSM) (Noble and Torczynski (1998), Holdych (2003), Krause et al. (2017), Trunk et al. (2018)) and moving boundary methods (MBM) (Bouzidi et al. (2001), Lallemand and Luo (2003), Yu et al. (2003), Ginzburg et al. (2008) and Filippova and Hänel (1998), Mei et al. (1999, 2000)), are investigated. First, the Galilean invariance of aerodynamic coefficients obtained by each scheme is examined. The Bhatnagar–Gross–Krook (BGK) LBM is utilized to simulate an eccentrically positioned cylinder in a transient Couette flow. In addition, various refill methods for MBM and volume approximation techniques for PSM are tested. Besides different error norms and a grid independence study of each method, the Galilean invariance violating frequencies are studied as well. These error calculations are used to choose a representative for each class to simulate vortex shedding. The VIV test case describes a transverse oscillation of a cylinder in a freestream at a Reynolds number of 100. Both free and forced cylinder oscillations are examined to study known phenomena as lock-in and lock-out zones. The results of MBM and PSM are in good agreement to literature values and prove the suitability for VIV simulations.

Dynamics of Separation Points Upon Impact of a Floating Circular Cylinder

The two-dimensional problem of vertical separation impact of a circular cylinder under the free surface of a heavy liquid is considered. The problem is studied in a linearized formulation corresponding to low velocities of the body and liquid and taking into account the dynamics of the points of separation of the cavitation zone. A coupled nonlinear problem is formulated, which includes a mixed boundary-value problem of potential theory with one-sided constraints on the surface of the body and an equation defining the law of motion of the cylinder. Examples demonstrating the dynamics of separation points during forced or free cylinder motions are considered. Numerical results obtained using the proposed mathematical model are compared with the results of asymptotic analysis of the initial nonlinear problem for small times.

An internal combustion alternator with both free piston and cylinder

The free-piston generator has more merits than the traditional reciprocating engines, and has been under extensive investigation. This study focuses on the development two novel perspective ideas unlike already advanced the linear free piston generator. These are: (1) to make both piston and cylinder free that provides ideally balancing of the system; (2) to turn from the linear scheme of a generator to rotary design because of simplicity and compactness. The mathematical simulation of operation of the linear two-stroke cycle alternator with both free piston and cylinder was conducted with using the model of zero-dimensional dynamics. Combustion of methane-air mixture with excess air ratio of 1.2 was considered. Both heat and friction losses were taken into account. Operation particularities of such system were analyzed. It was found that oscillation amplitude of moving parts did not exceed of 70 mm at frequency of ∼50 Hz for alternator with following basic parameters: cylinder bore of 80 mm, length of 400 mm, piston length of 120 mm. Mass of both piston and cylinder was chosen identical and equal of 3 kg. It was shown that the efficiency of the chemical energy conversion reached of 46%, and specific power was high as 40 kW/l at compression ratio of 40. The lab scale pneumatic alternator in rotor variant with both free rotor and cylindrical body was assembled for experimental demonstration of rotary design capability. The alternator was fed with compressed air of low pressure below of 6 atm. The electrical power in the load reached of 10 W at input power with intake compressed air about 100 W. The results indicate that alternators with both free piston and cylinder in linear and rotary designs have the potential to achieve high efficiency and provide the vibration – free operation.

Characteristics of the wake formation and force distribution of the bistable flow on two cylinders side-by-side

This numerical study with experimental support examines the bistable phenomenon along two parallel smooth cylinders submitted to a uniform turbulent cross-flow. The spacing ratio P/D of the cylinders is 1.26, being D the cylinder diameter and P the distance between centers. The computational domain and the experimental tests section are equivalents. The Reynolds number based on the free mean velocity and cylinder diameter was Re = 2.2 × 104. For the numerical analysis, the Navier–Stokes equations were solved using finite volumes method for a URANS simulation (unsteady Reynolds averaged Navier–Stokes) with the kω-SST-SAS (shear stress tensor-scale adaptive simulation) turbulence model. Measurements were performed by means of hot wire anemometry technique, firstly to determine actual boundary conditions of flow velocity and turbulence intensity at the test section entrance, secondly to obtain velocity and velocity fluctuation at the wake of the cylinders in different positions along the cylinder height to corroborate the numerical results. By means of the numerical results of velocity, the process of the wake formation is described, showing that the bistability does not occur simultaneously along the cylinder, corroborated by a delay on the switching process of the simultaneously measured velocities and the cross-correlation function. This effect appears also in the results of cylinder wall pressure distribution as well as in lift and drag coefficients.

Geometry of motion and nutation stability of free axisymmetric variable mass systems

In classical mechanics, the ‘geometry of motion’ refers to a development to visualize the motion of freely spinning bodies. In this paper, such an approach of studying the rotational motion of axisymmetric variable mass systems is developed. An analytic solution to the second Euler angle characterizing nutation naturally falls out of this method, without explicitly solving the nonlinear differential equations of motion. This is used to examine the coning motion of a free axisymmetric cylinder subject to three idealized models of mass loss and new insight into their rotational stability is presented. It is seen that the angular speeds for some configurations of these cylinders grow without bounds. In spite of this phenomenon, all configurations explored here are seen to exhibit nutational stability, a desirable property in solid rocket motors.

Tuning Elastic Waves in Soft Phononic Crystal Cylinders Via Large Deformation and Electromechanical Coupling

Soft electroactive materials can undergo large deformation subjected to either mechanical or electrical stimulus, and hence, they can be excellent candidates for designing extremely flexible and adaptive structures and devices. This paper proposes a simple one-dimensional soft phononic crystal (PC) cylinder made of dielectric elastomer (DE) to show how large deformation and electric field can be used jointly to tune the longitudinal waves propagating in the PC. A series of soft electrodes, which are mechanically negligible, are placed periodically along the DE cylinder, and hence, the material can be regarded as uniform in the undeformed state. This is also the case for the uniformly prestretched state induced by a static axial force only. The effective periodicity of the structure is then achieved through two loading paths, i.e., by maintaining the longitudinal stretch and applying an electric voltage over any two neighboring electrodes or by holding the axial force and applying the voltage. All physical field variables for both configurations can be determined exactly based on the nonlinear theory of electroelasticity. An infinitesimal wave motion is further superimposed on the predeformed configurations, and the corresponding dispersion equations are derived analytically by invoking the linearized theory for incremental motions. Numerical examples are finally considered to show the tunability of wave propagation behavior in the soft PC cylinder. The outstanding performance regarding the band gap (BG) property of the proposed soft dielectric PC is clearly demonstrated by comparing with the conventional design adopting the hard piezoelectric material. One particular point that should be emphasized is that soft dielectric PCs are susceptible to various kinds of failure (buckling, electromechanical instability (EMI), electric breakdown (EB), etc.), imposing corresponding limits on the external stimuli. This has been carefully examined for the present soft PC cylinder such that the applied electric voltage is always assumed to be less than the critical voltage except for one case, in which we illustrate that the snap-through instability of the axially free PC cylinder made of a generalized Gent material may be used to efficiently trigger a sharp transition in the BGs.

Vibration analysis of a cylinder with slight diameter and thickness variations

The cross section of circular cylinder in their dynamic model is always considered as a perfect circle, which means radii at every point on the circle are the same. In real engineering structure, there are slight fluctuations in shape of the circular cylinder which is different from those in ideal model. Meanwhile, effects of structural fluctuations on its dynamic characteristic are rarely analyzed before. To study problem mentioned above, the geometric shape of a typical, apparently symmetrical cylinder is examined experimentally to demonstrate that a small variation in diameter and thickness indeed exists in practice firstly. Because fluctuations in diameter and thickness of the cylinder are related to each other, we need to separate effects of a slight variation in its diameter and thickness on structural dynamic characteristics to search the key factor of influence. Then, two simplified modes, which are modeled by finite element method, are used to study the effects of diameter or thickness variation alone on the natural frequencies and modal shapes of the free cylinder. It is revealed that the diameter variation described by the simplified model captures the key influencing elements which affect the modal characteristics of the free cylinder. Finally, a free cylinder with both variations is analyzed numerically and the results are verified experimentally. This work illustrates that significant discrepancies inevitably exist between the measured results of an actual free cylinder and an assumed symmetrical model even if there is only a very slight variation in its geometric shape.

A REVIEW ON THE TIME-DEPENDENT VORTEX SHEDDING BY MEANS OF STREAMFUNCTIONS' STRICTLY ROTATIONAL COMPONENT

A computational analysis is performed on the two-dimensional time-dependent Navier–Stokes equations in their streamfunction–vorticity transport form, for a numerical investigation on vortex shedding past a circular cylinder. The numerical technique is a mixed spectral-finite analytic scheme in which the flow fields are expanded in the Fourier series along the azimuthal direction and the convolutions arising from the convective terms of the vorticity transport equation are calculated by means of Fast Fourier Transform algorithms; the time advancement is performed by using a fourth-order Runge–Kutta scheme (first four iterations) and a Predictor–Corrector algorithm (subsequent iterations). The flow of a viscous incompressible fluid around an impulsively started circular cylinder at Reynolds number Re = 1000 (based on free stream velocity and cylinder diameter) is examined with particular concern to the mechanism of vortex shedding and more particularly to the shedding of the viscous part of the flow field, when separated from the inviscid. The nonsymmetric configuration of the wake is promoted by imposing at the nondimensional time t = 0 an initial perturbation to the initially irrotational flow. The perturbation consists in a slightly rotational field implemented into the computational code by means of an appropriate perturbation function; six different perturbations are tested. The results are presented for several time steps of integration, in terms of streamlines generated by the strictly rotational component of the streamfunction. The mutual interaction of the primary vortices in the near wake as a fundamental mechanism of the "viscous" vortex shedding, is demonstrated; moreover, different perturbations induce vortex shedding phenomena with different characteristics.

Two-degree-of-freedom vortex-induced vibrations of a circular cylinder at Re=3900

The vortex-induced vibrations of an elastically mounted circular cylinder are investigated on the basis of direct numerical simulations. The body is free to move in the in-line and cross-flow directions. The natural frequencies of the oscillator are the same in both directions. The Reynolds number, based on the free stream velocity and cylinder diameter, is set to 3900 and kept constant in all simulations. The behavior of the coupled flow-structure system is analyzed over a wide range of the reduced velocity (inverse of the natural frequency) encompassing the lock-in range, i.e. where body motion and flow unsteadiness are synchronized. The statistics of the structural responses and forces are in agreement with prior experimental results. Large-amplitude vibrations develop in both directions. The in-line and cross-flow oscillations are close to harmonic; they exhibit a frequency ratio of 2 and a variable phase difference across the lock-in range. Distinct trends are noted in the force-displacement phasing mechanisms in the two directions: a phase difference jump associated with a sign change of the effective added mass and a vibration frequency crossing the natural frequency is observed in the cross-flow direction, while no phase difference jump occurs in the in-line direction. Higher harmonic components arise in the force spectra; their contributions become predominant when the cylinder oscillates close to the natural frequency. The force higher harmonics are found to impact the transfer of energy between the flow and the moving body, in particular, by causing the emergence of new harmonics in the energy transfer spectrum.

Electron beam induced THz emissions from nanotube array

A new scheme of terahertz radiation generation by passing an ultrashort electron bunch over a planar array of nanotube/nanorod across their lengths is proposed and analyzed. The beam pulse exerts a repulsive impulse on the free electron cylinder of each nanorod and displaces them with respect to ion cylinder. After the passage of the pulse, the electron cylinders oscillate at their natural frequency ωp/2 (where ωp is the plasma frequency of electrons in each carbon nanotube) acting as phased array dipole antennae, emitting THz radiation.