Wavy Cylinder Research Articles

Wake of wavy elliptic cylinder at a low Reynolds number: wavelength effect

Effects of the spanwise wavelength (λ) of a sinusoidal wavy cylinder with elliptic cross-section on wake structures and fluid forces are numerically investigated at a Reynolds number Re = 100. A wide range of the wavelength, $0.43 \le \lambda /{D_m} \le 8.59$ , is considered with a wave amplitude of a/Dm = 0.048, where Dm is the hydraulic diameter of the wavy cylinder. Based on vortical structures, Strouhal number (St) and wake closure length (Lc), fluid forces, streamline topologies and spatio-temporal evolutions of the near wake, five distinct flow patterns (I–V) are identified depending on λ/Dm. The drag force reaches its minimum in pattern III, the fluctuating lift force is zero in flow patterns III and IV. Distinct from the classical flow where alternate vortex shedding occurs synchronously over the entire cylinder span, flow pattern IV has alternate vortex shedding over a one half-wavelength of the wavy elliptic cylinder, antiphased with that over the other half-wavelength, thus leading to zero fluctuating lift over one complete wavelength. A thorough comparison of the wakes is made between the wavy elliptic cylinder and wavy circular or square cylinder, distinguishing the underlying flow physics behind the salient behaviours observed.

Energy harvesting based on flow-induced vibration of a wavy cylinder coupled with tuned mass damper

We investigate the flow-energy harvesting performance of a system consisting of a wavy cylinder and a tuned mass damper (TMD), through experiments, theoretical modeling and CFD simulations. The wavy cylinder is elastically supported, being able to oscillate transversally in cross flows and further enforcing a piezoelectric sheet to generate electricity. Results indicate that the output power strongly depends on wavelength λz, amplitude a, and the tip mass of TMD. The wavy cylinder outperforms the smooth cylinder when λz≥ 3.6Dm and peaks at λz = 6.0Dm and a = 0.25Dm with an improvement of 70%. This superiority also exists in a wider range of reduced velocities, leading to a power increase of 121%. The behavior of the oscillation echoes the observations made on the energy. The flow field results reveal that at λz = 6.0Dm and a = 0.25Dm, hairpin vortexes with legs attaching on the two sides of the node are observed, producing a wide wake near the node while a narrow wake near the saddle. However, an opposite scenario is observed for the case with the worst performance, i.e., λz = 1.8Dm and a = 0.25Dm. The former wake distribution seems to create a larger pressure difference over the wavy cylinder, induce a larger oscillation and hence improve the power output.

Effect of inclined magnetic field on natural convection and entropy generation of non-Newtonian ferrofluid in a square cavity having a heated wavy cylinder

Effect of inclined magnetic field on natural convection and entropy generation of non-Newtonian ferrofluid in a square cavity having a heated wavy cylinder

Magneto-bioconvection flow in an annulus between circular cylinders containing oxytactic microorganisms

The present work focuses on the numerical investigations of the existence of oxytactic microorganisms on the bioconvection flow of nano-encapsulated phase change material (NEPCM) inside an annulus. The novel geometry of an annulus contains an inner wavy cylinder and an outer circular cylinder considering a rotational circular velocity of an inner wavy cylinder. The incompressible smoothed particle hydrodynamics (ISPH) method solved the dimensionless form of controlling equations. The novel annulus between the embedded circular cylinder and the outer circular cylinder can be applied in the tube bundles in heat exchangers. Further, rotation of an inner wavy cylinder in a circular cavity can be used in thermal fin's vibration and cooling of the electric devices. The effects of pertinent parameters such as a rotational frequency number ω, bioconvection Rayleigh number Rab, thermal radiation Rd, Lewis number Le, and Hartmann number Ha on the contours of oxygen concentration ϕ, temperature θ, heat capacity ratio Cr, oxytactic microorganisms N, and velocity field V as well as the average Nusselt & Sherwood numbers Nu¯ and Sh¯ are conducted. Increasing ω enhances the maximum of V by 56.6% and lowers Nu¯. The variations of Rab are enhanced the contours of N and maximized the velocity field from 0.86 at Rab=1 to 8.22 at Rab=100 due to the power in bioconvection flow. Increasing Le diminishes the strength of oxygen concentration ϕ and microorganisms N. Increasing Ha from 0 to 50 slows down the velocity field's maximum by 47.2% and enhances the value of Nu¯.

On the limits of wavy cylinder wavelength and amplitude for effective wake and vortex-shedding control

On the limits of wavy cylinder wavelength and amplitude for effective wake and vortex-shedding control

Investigation of improved heat transport featuring in dissipative ternary nanofluid over a stretched wavy cylinder under thermal slip

Purposeand Methodology: The heat transport investigation in non-Newtonian fluids suspended by distinct sort of nanoparticles is a rich motive in the present time. Therefore, inspired by the physical characteristics of base fluid and ternary nanoparticles (Al2O3-CuO-Cu), a comprehensive analysis conducted over a cylinder with wavy radius with special emphasis in Saddle and Nodal points. The key effects of dissipation, the first order thermal slip, surface convection and stretching/shrinking added in the model. The acquired model is then examined via RK-scheme and portrayed the results against the physical ranges of the parameters. Key findingsA comprehensive discussion of the results provided that the nanofluid velocity increased for higher Casson number (α=0.5,1.0,1.5,2.0) and is examined optimum for stretched cylinder surface. The higher viscous dissipation which results the effects of Eckert number (Ec=0.01,0.02,0.03,0.04), surface convection due to Biot number (B1=0.1,0.2,0.3,0.4) and the first order thermal slip (α1=0.1,0.3,0.5,0.7) are the key physical factors to acquire the favorable heat transfer amount for the practical interest. Further, the particles concentration in the range of 1%–6% is observed good to increase the heat transmission in ternary nanoliquid and the values of effective characteristics (dynamics viscosity and thermal conductivity) upsurges against the concentration factor up to 6%.

Performance enhancement of the wavy cylinder-based piezoelectric wind energy harvester with different wave-shaped attachments

The vortex-induced vibration (VIV)-based piezoelectric wind energy harvester (PWEH) has a small working bandwidth and poor wind energy harvesting performance. In order to improve the output power of the PWEH and achieve sustainable power supply for microelectronic products, this study proposes a novel wavy cylinder-based PWEH with two wavy attachment rods attached to the surface of the wavy cylinder, the mathematical model of PWEH is developed. The flow-induced vibration (FIV) response mechanism and the energy harvesting characteristics of the wavy cylinder-based PWEH are numerically explored. The wavy cylinder with wavy-shaped attachments produces a three-dimensional shear layer that can increase the wavy cylinder’s aeroelastic instability range and realize the synergistic effects of VIV and galloping, which increases the wave cylinder’s vibrational amplitude and lowers the galloping critical wind speed. Compared with the typical VIV-PWEH, the novel wind energy harvester can work outside the VIV lock-in range, and its output power and voltage rise as wind speed does. The wavy attachment rods with triangular cross section have greater advantages than other rods, its output power is 6.13 mW at a wind speed of 5.7 m/s and load resistance of 100 KΩ, compared with the circular cylinder PWEH, the average power increases by approximately 166%. The study’s findings can theoretically promote the enhancement of PWEH’s energy harvesting efficiency.

Experiment on Noise Reduction of a Wavy Cylinder with a Large Spanwise Wavelength and Large Aspect Ratio in Aeroacoustic Wind Tunnels

Current research shows that the wavy shape can play an important role in drag reduction. Meanwhile, it also has the potential of noise reduction. In the present study, a kind of wavy shape of periodic cosine profile with a large spanwise wavelength and large aspect ratio was applied to the circular cylinder model. The experiments on the influence of various aspect ratios (ratio of wave wavelength to amplitude) on the far-field noise of the wavy cylinder were carried out in a 0.55 m × 0.4 m aeroacoustic wind tunnel. It is shown that the maximum decrease of the far-field SPL (Sound Pressure Level) between the wavy cylinder and baseline cylinder exceeded 37 dB within the frequency between 200 Hz and 1000 Hz. The noise reduction effect of the wavy cylinder will become better along with the increasing aspect ratio. However, there exists a critical aspect ratio near λ/a = 30. If the aspect ratio continues increasing, the noise reduction effect of the wavy cylinder will decrease instead of increasing. Finally, the computational fluid dynamics method is applied to reveal the noise reduction mechanism of this kind of wavy cylinder with a large spanwise wavelength and large aspect ratio. It is found that the periodic shedding vortex is disturbed and tends to be banded instead of showing alternate oscillation. The turbulence intensity and velocity fluctuation around the wavy cylinder will be also reduced. According to the vortex and sound theory, these changes are beneficial to the noise reduction. The large spanwise wavelength and large aspect ratio play a significant role in controlling the shedding vortex variation and adjusting the local flow field around the crest and trough of the wavy cylinder, which is the key factor to change the flow field and reduce the flow-noise of the wavy cylinder.

Experimental investigation of aerodynamic forces and vortex-induced vibrations of wavy cylinders at subcritical Reynolds numbers

Wavy cylinders have been reported to exhibit satisfactory aerodynamic performance at low Reynolds numbers. However, aerodynamic characteristics are still ambiguous at higher Reynolds numbers. A series of wind tunnel tests were conducted for different wavy cylinders with Reynolds numbers (Re) from 0.75 × 104 to 2.00 × 105, including measurements of the overall aerodynamic forces, local surface static pressure, and vortex-induced vibrations (VIVs). The experiments yielded the following results. First, the mean drag and fluctuating lift coefficients of wavy cylinders were generally lower than those of a circular cylinder in the tested Re range. The maximum drag reduction was up to 18%. Second, a very slight waviness of a/D = 0.03 could improve aerodynamic performance, which could be taken as a reference for practical application. Third, the aerodynamic characteristics of wavy cylinders exhibited obvious three-dimensional (3D) features. This is reflected by various parameters varying along the spanwise direction. Fourth, the mechanisms of stronger 3D properties and less forces might be different. The former is dependent on the degree of obliqueness, while the latter is more related to the spanwise wavelength. Fifth, compared to a circular cylinder, the VIV amplitude was reduced and the lock-in range was shifted to higher wind velocities for wavy cylinders.

Double-diffusive magneto-natural convection of nanofluid in an enclosure equipped with a wavy porous cylinder in the local thermal non-equilibrium situation

The emphasis of this study is the numerical analysis via finite volume method of magneto-natural convection induced by double-diffusion of a nanofluid within an enclosure equipped with a wavy porous cylinder. The porous cylinder which is under the local thermal non-equilibrium (LTNE) situation is modeled using Darcy-Brinkman-Forchheimer design as momentum equation. Results are interpreted and evaluated in terms of the dimensionless controlling variables such as Rayleigh number (Ra), Lewis number (Le), Hartman number (Ha), buoyancy ratio parameter (N), Darcy number, (Da), and medium's porosity (ε) at a given heat transfer coefficient for the solid/fluid inter-phase (H) and modified conductivity ratio (γ) of the porous wavy cylinder region. Corcione's correlations with 4% of Al2O3 nanoparticles is used to estimate nanofluid's thermal conductivity and viscosity. Findings show that the heat and mass transfers are improved by Ra, Da, and ε augmentation and Ha and Le diminution. In general, as Le increases, the mass transfer rate rises while the corresponding heat transfer rate drops. Also, we identified a value of N where Le-dependent heat and mass transfer rates are minimal. Moreover, the heat transfer rate is found to be more sensitive to the changes in the porous medium's properties of the wavy porous cylinder than the mass transfer rate. Furthermore, the LTNE situation everywhere exists in the porous wavy cylinder except for the center where the LTE is realized.

Numerical investigations of flow over wavy cylinders at sub-critical Reynolds number

Numerical investigations of flow over wavy cylinders at sub-critical Reynolds number

Reynolds number effects on the bistable flows over a wavy circular cylinder

The wake of wavy cylinder has been shown to exhibit bistability. Depending on the initial condition, the final state of the wake can either develop into steady flow (state I), or periodic shedding (state II). In this paper, we perform numerical simulations to reveal the Reynolds number effects (Re=30 – 300) on these two wake states. With increasing Reynolds number, the steady vortical structures in state I wake sways back and forth in the spanwise direction, resulting in low-frequency fluctuations in drag forces, but not in lift. For state II, the increase in Reynolds number is associated with the emergence of another spectral peak in the lift coefficient. The secondary frequency is associated with highly three-dimensional vortical structures in the wake. For both states, the wakes transition to turbulent flows at higher Reynolds numbers, with the development of small-scale vortices. We further study the streamwise gust flows over the wavy cylinder. The time-varying inflow velocity results in a wide range of instantaneous Reynolds number spanning from the absolutely unstable flow regime to the bistable regime. Depending on the period of the inflow velocity variation, the wake perturbations grown at the absolutely unstable flow regime can be damped out in state I wake, or grow large enough to trigger the transition to state II, resulting in loss of flow control efficacy. The above analyses reveal novel flow physics of the bistable states at unexplored Reynolds numbers, and showcase the complex transition behavior between the two states in unsteady flows. The insights gained from this study improve the understanding of the wake dynamics of the wavy cylinder.

Non-homogeneous two-component buongiorno model for nanofluid flow toward Howarth's wavy cylinder with activation energy

We use computational methods to examine the behavior of a nanofluid in three dimensions flows through a circular cylinder whose radius varies sinusoidally. The fundamental equations are derived and simplified to highly nonlinear ODEs by using similarity transformations. The bvp4c performs a numerical solution to the resultant nonlinear system. Sherwood number, Nusselt number and Skin-friction are also assessed because of their importance in the engineering field. Viscous dissipation's influence is crucial to the heating and cooling processes. The thermal boundary layer improves and the concentration boundary layer weakens as the Eckert number rises. In proportion to the magnitude of activation energy is augmented, while flow of wall mass is diminished. Besides, the results shows that the Nusselt number rises with progresses in the instability parameter and the Lewis number. An important point to note is that the Nusselt number decrements with increasing Eckert number and enhance with rising Brownian parameter. Moreover, when the nodal/saddle point parameter is increased, the longitudinal skin-friction coefficient decreases while the skin-friction coefficient in the vertical direction rises. The novelty of this study is related to: i) the considering the energy activation in the problem, ii) the analyzing in the form of unsteady, iii) the comprehensive solutions for magnetic effect. The results of this work can be applicable in industries dramatically where local cooling and heating through impingement jets are required in electronic devices, heat sinks, drying technologies, etc.

Near wake flow and forced convection heat transfer of sinusoidal wavy cylinder based on flow decomposition

Based on flow decomposition we investigate the near wake flow and forced convection heat transfer around sinusoidal wavy cylinders, with an attempt to deepen our understanding of inherent correlations between near wake flow and heat transfer/transport. Large eddy simulations (LES) are conducted at a Prandtl number Pr = 0.7 and a subcritical Reynolds number Re = 3.0 × 103 for the unsteady flow of the sinusoidal wavy cylinder of different wavelengths λ (= 1.89, 3.79 and 6.06Dm) and a fixed wave amplitude a (= 0.152Dm), where Dm is the mean diameter of the cylinder. The near wake flow of the wavy cylinder, together with heat transport, is decomposed via the traditional triple decomposition and a newly-proposed quadruple decomposition, taking into account dominant periodic vortex shedding and/or spanwise heterogeneity of the flow. While heat transfer from the surface of the wavy cylinder to the surrounding flow is distinctly different from that of the smooth cylinder, results from the flow decomposition reveal intrinsic characteristics of the near wake flow and heat transport for the wavy cylinder with different wavelengths. It is observed that heat transport in the near wake of the wavy cylinder is predominantly associated with the coherent spanwise vortices of dominant shedding frequency as well as the spanwise time-invariant components of the mean flow. It is also found that the major contributor to Reynolds stresses (heat flux) is the temporal random stresses (temporal random heat flux) in the near wake of the wavy cylinder. Furthermore, results from the quadruple decomposition show that the spanwise-averaged heat flux in the near wake of the smooth cylinder and the wavy cylinder with λ = 1.89Dm encompasses both spatiotemporal random and temporal dispersive heat fluxes outside the recirculation bubble and only the former heat flux inside the recirculation bubble. However, the spanwise-averaged heat flux in the near wake of the wavy cylinder with λ = 3.79 or 6.06Dm is profoundly and exclusively contributed by the spatial dispersive heat flux outside the recirculation bubble.

Aerodynamic performance of a sinusoidal wavy cylinder at subcritical and critical Reynolds numbers

Previous studies have shown that in the subcritical regime, wavy circular cylinders exhibit less wind-induced drag than standard circular cylinders. However, the aerodynamic performance of wavy cylinders at higher Reynolds numbers is still unknown. Hence, a series of wind tunnel tests including measurement of the surface pressure, vortex-induced vibration (VIV) and dry galloping were conducted for a wavy cylinder and a corresponding standard cylinder at Re = 1.00 × 105-3.86 × 105. The results indicate that the aerodynamic characteristics, including the mean and fluctuating pressure distributions, mean and fluctuating aerodynamic force coefficients, Strouhal number, flow regime classification, and non-Gaussian characteristics, are significantly modified when shifting from the standard cylinder to the wavy cylinder. Furthermore, these modifications highly depend on the Reynolds number. Regarding the wavy cylinder, related parameters obviously vary along the spanwise direction, indicating three-dimensional aerodynamic characteristics. Moreover, the maximum VIV amplitude of the wavy cylinder is 40% lower than that of the standard cylinder. The velocity corresponding to the maximum amplitude is increased by 11% for the wavy cylinder. Additionally, the maximum dry galloping amplitude is reduced by 11%.

On the Contact Problem for a Wavy Cylinder and an Elastic Half-Plane

On the Contact Problem for a Wavy Cylinder and an Elastic Half-Plane

Numerical Investigation of Flow Past Bio-Inspired Wavy Leading-Edge Cylinders

A numerical investigation is proposed to explore the flow past a novel wavy circular cylinder as a passive flow control, whose shape is determined by a sinusoidal function applied to its leading edge line, similar to studies with wavy leading-edge airfoils. The latter are motivated by the wavy-shaped tubercles found in the flippers of humpback whales, which are believed to improve their maneuverability. Our attempt is, therefore, to assess the effects of leading-edge waviness now on a simpler and canonical geometry: circular cylinders. The present work relies on iLES simulations conducted with Nektar++ at a Reynolds number of 3900. Besides the straight cylinder, two wavy geometries are assessed, which are determined by a single wavelength of 37.5% for two amplitudes, 3% and 11%, based on the mean diameter of the wavy cylinder. Our results showed that, contrary to what is usually the case with traditional wavy cylinders at similar Reynolds numbers, waviness caused a reduction in the near-wake recirculation length and an increase in the mean near-wake turbulent kinetic energy compared to the straight cylinder. This was followed by a reduction in base pressure (up to about 36%) leading to a rise in lift oscillations and also to a significant increase in the mean drag coefficient of up to about 28%. An attempt to detail the flow phenomena is provided, evidencing the emergence of counter-rotating pairs of streamwise vortices between peaks. It is argued that the differences observed in recirculation length, turbulent kinetic energy, and force coefficients start even prior to the formation of these coherent structures and end up with interactions with the near wake.

Numerical investigation of thermal energy storage system loaded with nano-enhanced phase change material with Koch snowflake fractal cross-section

The melting of nano-enhanced phase change material (NEPCM) in a latent heat thermal storage system is investigated for PCM loaded within an annulus between an outer wavy cylinder and an inner Koch snowflake cylinder. The inner Koch snowflake cylinder is used in this study to increase the heating surface area and enhance the NEPCM melting process. The inner cylinder is hot, while the outer one is insulated. The system governing equations and the associated boundary conditions were numerically solved by employing the Galerkin weighted residual finite element method. The impacts of nanoparticle concentration, undulation number of the outer cylinder, annulus eccentricity, and inner cylinder geometry on the melting performance of the storage unit were examined and represented by the liquid fraction, isotherms, Bejan number, velocity, and Nusselt number. The usage of more blades and snowflake surfaces near the hot source's periphery portions disrupts the PCM surface's cohesiveness and increases heat penetration between the PCM layers. The nanoparticle concentration of 6 vol% enhances the melting time by about 32 %, and the undulation number of 8 participates in 25 % of performance enhancement. Putting the hot cylinder closer to the bottom of the storage unit expands the buoyancy effect and enhances the melting process by >3.2 times. The utilization of a Koch snowflake cylinder instead of a triangular one also shows an enhancement of >30 %.

Experimental Investigation of Turbulent Cross-flow in a Staggered Tube Bundle with Wavy Cylinders

This paper presents the results of an investigation on the effects of wavy cylindrical tubes on the turbulent cross-flow in a staggered tube bundle arrangement with transverse and longitudinal pitch-to-diameter ratios of 3.8 and 2.1, respectively. Two models of staggered tube bundles equipped with wavy cylinders are tested; the Model-A having a wavelength ratio (λ/DSUBm/SUB= 2) corresponding to a wave steepness (a/λ = 0.1) and the Model-B having (λ/DSUBm/SUB= 1) corresponding to (a/λ= 0.2).Experimental measurements were performed using a subsonic wind tunnel to study and compare the flow characteristics of the new configuration of cylindrical tubes with that of a similar arrangement with smooth cylinders for Reynolds numbers ranging from 0.5×10⁴ to 2×10⁴ based on the mean tube diameter and the free stream velocity. Experimental results show that the circumferential minimum pressure coefficients for wavy tube bundles are greater than at the bundle with smooth cylinders. As a result, the lift forces of wavy tube bundles are suppressed and a significant drag reduction is observed up to 20% for both wavy models. This continuous reduction of the drag force becomes larger with the increase of Reynolds number.

Three-dimensional effects for flow past a wavy cylinder at low Reynolds numbers

The three-dimensional (3D) flow characteristics affected by the structural heterogeneity of a wavy cylinder varied sinusoidally along the spanwise direction at a low Reynolds number (50∼350), are studied using numerical simulations. Compared with a uniform cylinder, the transition routes and vortex shedding modes of a wavy cylinder can be revealed based on the Strouhal number–Reynolds number relationship, with new vortex structures features discovered in each mode. Unsynchronized transitions between several 3D shedding modes and the resulting hysteresis-like phenomenon attributed to different initial conditions. The extrinsic 3D effects caused by the structural wavy perturbations afford the inhomogeneous spanwise variations in flow physical quantities, such as the pressure coefficient, time-averaged separation angles, and vortex formation lengths. The effects of the Reynolds number on the wake quantities of the wavy cylinder are compared with those of a uniform cylinder, which is described by a single function form in the laminar regime and irregular features in the 3D wake-transition regime. In addition, the deviations of the time-averaged separation angles between the two-dimensional (2D) and 3D results are adopted to reveal the strengthened three-dimensionality of the flow for a wavy cylinder and its variations with shedding mode transitions.