Wavy Circular Cylinder Research Articles

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.

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.

RETRACTED: Aqueous aluminium–copper hybrid nanofluid flow past a sinusoidal cylinder considering three-dimensional magnetic field and slip boundary condition

We analyzed the problem of the steady general three-dimensional magnetohydrodynamics stagnation-point boundary layer flow past an impermeable wavy circular cylinder considering aluminium–copper/water hybrid nanofluid as the working fluid and velocity slip as well as temperature jump boundary conditions. The induced magnetic field effect was also taken into account. The analytical procedure is based on the model that implements the nanoparticles and base fluid masses to formulate the equivalent volume fraction, equivalent density, and equivalent specific heat at constant pressure which is then substituted in the chosen single-phase thermophysical properties. Then, the foregoing relations were used in basic governing PDEs (partial differential equations), according to Tiwari–Das nanofluid scheme. It is worth mentioning that the bvp4c code from MATLAB software that is a famous finite-difference method has been exploited for solving the final similarity ODEs (ordinary differential equations). Results demonstrate that the developed mass-based model can be successfully employed with great confidence to study the flow and heat transfer of hybrid nanofluid in other similar problems. Moreover, it is proved that the nodal stagnation points possess higher values of skin friction coefficients and local Nusselt numbers relative to those for the saddle stagnation points. Besides, enhancing the second nanoparticle's mass leads to increase in all parameters of engineering interest including skin friction coefficients along x and y directions as well as local Nusselt number.

Optimization of wavy cylinder for aerodynamic drag and aeroacoustic sound reduction using computational fluid dynamics analysis

Motivated by the requirement to lessen the aerodynamic drag and aeroacoustic sound of the bluff bodies, the present paper is devoted to a numerical analysis of the aerodynamics and aeroacoustics related with the flow past wavy circular cylinders. Based on the efficient flow control method, as has been presented by preceding researchers, the existing work embarks upon an investigation on the wavy cylinder at a various wavelength and amplitude conditions. Computations are performed for a circular cylinder of the length-to-diameter ratio ( L/ D) of 25 at a Reynolds number ( Re) of 97300 using large eddy simulation and Ffowcs Williams- Hawking’s acoustic analogy. Firstly, the cylinder without waviness is subjected to a uniform incoming flow is considered for validation against measurements. Secondly, various collection of wave shape parameters, specifically dimensionless wavelength λ/ D (=1 to 2.5), and wave amplitude a/ D (=0.05 to 0.2) have been taken into consideration. It is disclosed that the proper selection of shape parameters could significantly reduce the drag and sound emission levels, compared to the normal cylinder. Finally, a multi-objective particle swarm optimization was performed using the radial basis neural network to simultaneously reduce the aerodynamic drag and sound emission, with λ/ D and a/ D as design variables. We recognized a critical λ/ D and a/ D for the wavy circular cylinder at the considered subcritical Re.

Influence of metallic nanoparticles in water driven along a wavy circular cylinder

In the present study, simultaneous effects of metallic nanoparticles and magnetohydrodynamic due to stagnation point flow of nanofluid along a wave circular cylinder is presented. The effect of induced magnetic field is incorporated to deal the boundary and thermal boundary layer domain. Mathematical modelling for momentum and energy equation is constructed that is based upon three different kinds of nanoparticles namely: copper (Cu), Titanium di oxide (TiO2), and alumina (Al2O3) within the working fluid water. Each mixture is analysed at the individual level and made comparison amongst all the mixture to examine the resistance and thermal conductivity of nanofluid within the boundary layer region. The solutions are exposed via boundary value problem using shooting method along with the Runge-Kutta-Fehlberg method. The characteristics of emerging parameters for the fluid flow and heat transfer are discussed through graphs and tables. The effects of ϕ (nanoparticle volume fraction) on heat transfer and shear stress at the wall are analysed in detail. It is finally concluded that by increasing the ratio of nanoparticles there is a significant increase in the temperature but slight decrease in the velocity profile.

Numerical simulation for thermal radiation and porous medium characteristics in flow of CuO-H2O nanofluid

Researchers have a wide-ranging tradition in endeavoring to rise the thermophysical aspects of convection heat transferors for illustration, transformer oil and water. Technological advancements in recent years permit the dispersal of elements having ranges between 10 and 100 nm in such liquids. Recent researches regarding nanoliquids have been elaborated to exhibit anomalously higher convection heat transportation. Keeping such implications in mind, we formulated CuO-H2O nanoliquid flow with wavy circular cylinder as heater subjected to magnetohydrodynamics. The well-known Darcy model featuring porous medium along with KKL (Koo–Kleinstreuer–Li) model is considered simultaneously for analysis. Heat transportation is reported considering radiation effect. The impact of shape factor of nanoparticles is also considered. Simulations are presented employing the novel control volume finite element method. The influences of notable parameters like Darcy number, Rayleigh and Hartmann numbers, radiation parameter, amplitude of undulations, nanofluid volume fraction and shape factor of nanoparticles have been investigated on flow and heat transfer features. Moreover, a novel correlation regarding average Nusselt number has been developed subject to analysis’s active parameters. Our outcomes report that lower values of amplitude of undulations provide the uppermost estimations of average Nusselt number.

Effect of the spanwise computational domain size on the flow over a two-dimensional bluff body with spanwise periodic perturbations at low Reynolds number

To effectively control the flow over a two-dimensional bluff body, three-dimensional perturbations including spanwise-varying shape modifications and surface actuations have been suggested by previous studies. One of the computational issues is how big the spanwise computational domain size (Lz) should be as compared to the wavelength of these spanwise-varying perturbations (λ), to accurately estimate the effect of these perturbations. To investigate this issue, we consider three different two-dimensional bluff bodies with spanwise periodic perturbations: wavy square cylinder, wavy circular cylinder, and spanwise-varying blowing and suction on a circular cylinder at Re = 100. The spanwise length scales of largest vortical structures existing in the wake significantly depend on Lz, and they are larger than λ, reaching 2λ, 3λ, and even bigger than or equal to 12λ for the cases considered. These largest structures are composed of multiple of elementary vortical structures whose length scales are smaller than or equal to Lz. The amounts of drag reduction by periodic perturbations at Lz > λ are smaller than those at Lz = λ, indicating that the computational domain size equal λ overestimates their performances.

Heat transfer by natural convection of Fe3O4-water nanofluid in an annulus between a wavy circular cylinder and a rhombus

Researchers in modern science always attempt to find new techniques to optimize the performance of energy devices through heat transfer enhancement. It comes to the knowledge that such enhancement can be done with the help of nanofluids. Therefore, the present study is aimed to investigate the role of natural convection and thermal radiation on thermo-hydrodynamics of nanofluid heat transfer in an annulus between a wavy circular cylinder and a rhombus enclosure subject to a uniform magnetic field. A new model for viscosity called magnetic field dependent (MFD) viscosity is used. Mathematical formulation of the current physical model is based on the continuity, momentum and energy equations. Numerical simulation of the reduced problem is done with the help of Control Volume based Finite Element Method (CVFEM). The impacts of controlling physical parameters, for instance, Rayleigh number, radiation parameter, Hartmann number, aspect ratio, shape factor of nanoparticles and solid volume fraction of nanoparticles on thermo-hydrodynamics of flow are studied. The quantities of physical interest are graphically presented and discussed in detail. From the present study it becomes apparent that the local heat transfer rate decreases with an increase in aspect ratio in the absence of Hartmann number. In addition, for decreasing values of aspect ratio, the space for fluid flow inside enclosure becomes wider.

Finite element based large eddy simulation of flow over bluff bodies

Large eddy simulation (LES) of bluff body flows is performed using a finite element based formulation which employs a combined subgrid-scale model. The combined model uses the residual based variational multiscale (RBVMS) approach along with the dynamic Smagorinsky eddy viscosity model. Specifically, the RBVMS model is used to represent the cross-stress terms while the dynamic eddy viscosity model is used for the Reynolds stresses. The dynamic procedure is based on a localized version of the variational Germano identity. Two sets of cases are considered: flow over straight and wavy circular cylinders at ReDm=3000, and flow over a straight square cylinder at ReD=22,000. For the straight circular cylinder case, we compare LES predictions based on the RBVMS model and the combined subgrid-scale model against the experimental data. The combined model provides better predictions (for both force data and centerline profiles in the wake) and we select it in the rest of the cases. For the wavy circular cylinder case, LES predictions (based on the combined model) shows a good agreement with the experimental data including mean velocity profiles in the wake. It accurately captures the elongation of the recirculation region. In the square cylinder case too, LES predictions (based on the combined model) show a very good agreement with the direct numerical simulation and multiple experimental datasets.

파형 실린더 주위의 유동 및 열 전달 특성

Abstract Three-dimensional characteristics of fluid flow and heat transfer around a wavy circular cylinder having sinusoidal variation in cross sectional area along the spanwise direction are numerically investigated using the immersed boundary method. Th e three different wavelengths of π4 , 3 and π2 at the fixed wavy amplitude of 0.1 have been considered to investigate the effects of waviness especially on the forced convection heat transfer around a wavy cylinder when the Reynolds and Prandtl n umbers are 300 and 0.71, respectively. The present computational results for a wavy cylinder are compared with those for a smooth cylinder. The time- and total surface-averaged Nusselt number for a wavy cylinder with λ=π/2 is larger than that for a smooth cylinder, whereas that with λ=π/4 and π/3 is smaller than that for a smooth cylinder. However, because t he surface area exposed to heat transfer for a wavy cylinder is la rger than that for a smooth cylinder, the total heat transfer rate for a wavy cylinder with different wavelengths of

Waviness effect of a wavy circular cylinder on the heat transfer at a Reynolds number of 300

The present study investigates three-dimensional characteristics of fluid flow and heat transfer around a wavy cylinder which has the sinusoidal variation in the cross sectional area along the spanwise direction. The three different wavelengths of π/4, π/3 and π/2 at the fixed wavy amplitude of 0.1 have been considered to investigate the effect of waviness on especially the forced convection heat transfer around a wavy cylinder when the Reynolds and Prandtl numbers are 300 and 0.71, respectively. The numerical solution for unsteady forced convective heat transfer is obtained using the finite volume method. The immersed boundary method is used to handle the wavy cylinder in a rectangular grid system. The present computational results for a wavy cylinder are compared with those for a smooth cylinder. The fluid flow and heat transfer around the wavy cylinder depends on both the location along the spanwise direction and the wavelength. The time- and total surface-averaged Nusselt number for a wavy cylinder with λ = π/2 is larger than that for a smooth cylinder, whereas that with λ = π/4 and π/3 is smaller than that for a smooth cylinder. However, because the surface area exposed to heat transfer for a wavy cylinder is larger than that for a smooth cylinder, the total heat transfer rate for a wavy cylinder with different wavelengths of λ = π/4, π/3 and π/2 is larger than that for a smooth cylinder.