Near Wake Of Circular Cylinder Research Articles

The Interaction of Porous Material Coating With the Near Wake of Bluff Body

The flow around a circular cylinder with porous material coating (PMC) is numerically investigated based on unsteady Reynolds-averaged Navier–Stokes (URANS) method at subcritical Reynolds number. The results are compared with some available results in the open literature. The interaction of PMC with the near wake of a circular cylinder such as streamline, vorticity field, and shear stress are studied in detail. Subsequently, the fluctuation forces and velocity distribution in the boundary layer are analyzed and the effect of various thicknesses of PMC is investigated. The numerical results reveal that PMC has prominently modified the flow characteristic of the near wake of circular cylinder and significantly mitigated the fluctuations of aerodynamic forces from two aspects of frequency and amplitude. It means that the vortex shedding from the bluff body is suppressed. It also is found that the thickness of the PMC is a sensitive parameter to the aerodynamic forces and velocity distribution in the boundary layer. Furthermore, the mean drag can be reduced to a certain extent when the thickness is appropriate. It is expected that the modification of flow characteristic and aerodynamic forces is closely related to the flow-induced noise reduction. Those results will be helpful to understand the mechanism of flow control on bluff body flow by using porous material coating and accumulate meaningful information for further industrial application.

Near Wake of a Horizontal Circular Cylinder in Stably Stratified Flows

The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder; the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.

Numerical investigations of lift suppression by feedback rotary oscillation of circular cylinder at low Reynolds number

This article describes a strategy of active flow control for lift force reduction of circular cylinder subjected to uniform flow at low Reynolds numbers. The flow control is realized by rotationally oscillating the circular cylinder about its axis with ω(t)=−λCL(t), where ω(t) is the dimensionless angular speed of rotation cylinder, λ is the control parameter and CL(t) is the feedback signal of lift coefficient. The study focuses on seeking optimum λ for the low Reynolds numbers of 60, 80, 100, 150, and 200. The effectiveness of the proposed flow control in suppressing lift force is examined comprehensively by a numerical model based on the finite element solution of two-dimensional Navier–Stokes equations. The dependence of lift reduction on the control parameter λ is investigated. The threshold of λ, denoted by λc, is identified for the Reynolds numbers considered in this work. The numerical results show that the present active rotary oscillation of circular cylinder is able to reduce the amplitude of lift force significantly as long as λ≤λc, at least 50% for the laminar flow regime. Meanwhile, the present active flow control does not result in the undesirable increase in the drag force. The Strouhal number is observed to decrease slightly with the increase of λ. As for a specific Reynolds number, the larger λ gives rise to the larger amount of lift reduction. The lift reduction reaches the maximum at λ=λc. The mechanism behind the present lift reduction method is revealed by comparing the flow patterns and pressure distributions near the active rotationally oscillating circular cylinder and the stationary circular cylinder. It is found that the critical value λc generally increases with Reynolds number. Two types of lift shift are observed in the numerical results for the cases with λ>λc. The first is characterized by the regular fluctuation of lift coefficient but with nonzero mean value, while the second is associated with the sustaining increase of lift coefficient. The phenomenon of lift shift is found to be related closely to the evolution of vortex pattern in the near wake of circular cylinder.

Global frequency selection in the observed time-mean wakes of circular cylinders

Observations have been made of the time-mean velocity profile at midspan in the near-wake of circular cylinders at moderate Reynolds numbers between 600 and 4600, well beyond the Reynolds number of approximately 200 at which the wake becomes three-dimensional. The measured profiles are found to be represented quite accurately by a family of function profiles with known linear instability characteristics. The complex instability frequency is then determined as a function of wake position, using the function profiles. In general, the near wake undergoes a transition from convective to absolute instability; the distance downstream to the point of transition is found to increase over the Reynolds number range investigated. The emergence of a significant region of convective instability is consistent with the known appearance of Bloor–Gerrard vortices. The selected frequency of the wake instability is determined by the saddle-point criterion; the Strouhal numbers for Bénard–von Kármán vortex shedding are found to compare well with the values in the literature.

Experimental and numerical modelling of the three-dimensional incompressible flow behaviour in the near wake of circular cylinders

An experimental investigation was carried out to study the structure of the flow field around three-dimensional circular cylinders. The study of the flow field around an obstacle was performed in a wind tunnel using a particle image velocimetry (PIV) system. The flow of a fluid around an obstacle with a different velocity to the oncoming flow was examined. The results showed the dependence of the flow structure around the obstacle on its Reynolds number, and the spacing between a pair of obstacles. Detailed quantitative information of turbulence parameters in the vicinity of the obstacle was attained. Extensive wind tunnel experimental results are presented and compared with numerical simulation. A three-dimensional numerical model with Reynolds stress model (RSM) turbulence and a non-uniform grid system were used to examine the effects of a single cylinder and two cylinders in tandem on the flow. The principal objective was to analyse three-dimensional flow past a single cylinder and two circular cylinders placed in tandem by combining the application of a PIV experimental technique and an RSM turbulence model. For the case of two cylinders in tandem, the flow patterns are characterized in the gap region as a function of the distance between the cylinders. A good level of agreement was found between the experimental results of flow and numerical simulation.

Bistable flow in lower transition regime of circular cylinder

Based on flow visualization, LDA and DPIV, the present study establishes the switching phenomenon of near wake of circular cylinder of low aspect ratios (AR ≤ 4) within the lower transition from the laminar shedding regime to the wake transition regime. The phenomenon occurs at and below the critical Reynolds number, which involves switching between the wide wake mode without the large-scale primary vortex and the regular shedding mode with primary vortices. The switching is suggested to play an important role in the transition.In the switching regime the small-scale secondary vortices and their pairings are identified. Their appearance, pairings and phase difference on the two sides of the wake play important roles in the switching and the enhancement or suppression of the primary vortices. The present study further establishes the fact that the two-dimensionality of the secondary vortices is important in the shift of the transition to high Reynolds number at low aspect ratios.

Near wake of a circular cylinder submitted to blowing – II: Impact on the dynamics

The near wake of a porous circular cylinder in cross-flow submitted to blowing through its whole surface is experimentally studied. The blowing impact on the Strouhal number exhibits a linear decrease of the vortex shedding frequency with the blowing ratio. A simple model representing this decrease as a function of the injection rate is developed, based on the wake static pressure profile evolution. The main flow temperature influence is also investigated in case of non-isothermal blowing and is shown to have no effect on the Strouhal number evolution. Finally, the interaction between the blowing and the shear layer is investigated through a spectral analysis of the velocity signal. A modification of the shear layer power spectrum is observed when injection occurs. The dynamics are slowed down and characteristic patterns, denoted sub-peaks, appear while the relationship between the von Kármán and the shear layer frequencies without blowing remains valid.

An investigation of the near wake of a circular cylinder using a video-based digital cross-correlation particle image velocimetry technique

A cross-correlation particle image velocimetry (PIV) technique has been developed to measure the spatiotemporal in-plane velocity vector field evolution of time-dependent flows. A novel iterative two-stage cross-correlation scheme of two sequential images of flow tracers has been incorporated in the image analysis. The implementation in hardware and software of this complete recording and analysis system are described. The expected accuracy of the velocity measurements was investigated and is discussed. The technique has been applied to study the near wake behind a circular cylinder at low Reynolds numbers (Re d). The measurements presented pertain to cylinders with d = 12.5 and 25 mm ( 1 d = 19.5 and 9.8 , respectively). The respective Reynolds numbers Red are 875 and 769. Two planes of this flow are considered in this study: (1) plane normal to the cylinder axis ( xy plane) and (2) a plane containing the cylinder axis and the stream direction ( xz plane). Instantaneous in-plane velocity vector fields and out-of-plane vorticity fields are presented for both planes. The effect of spatial resolution on peak vorticity is discussed using velocity vector field measurements in the near wake of the cylinder that were conducted using different spatial resolutions. The three-dimensional nature of the near wake of circular cylinders at low Re d is demonstrated using quantitative in-plane velocity vector field and out-of-plane vorticity measurements. An upstream influx of relatively high velocity fluid into the stagnant near-wake region in the xy plane and the subsequent deflection of the fluid normal to this plane as it approaches the stagnation region at the cylinder are shown to be responsible for the generation of three-dimensional flow in the near wake of a circular cylinder.

Secondary vortices in the wake of circular cylinders

Using both the hydrogen-bubble flow-visualization technique and hot-wire-anemometry measurements, secondary vortices have been detected in the near-wake of circular cylinders and their characteristics studied over a Reynolds-number range of 1200–11000. The vortex-shedding characteristics of these secondary vortices clearly indicate that ‘transition waves’, detected in the cylinder near wake by Bloor (1964), and secondary vortices are identical phenomena. It is established that the non-dimensional shedding frequency of the secondary vortices demonstrates a 0.87 power-law relationship relative to Reynolds number, contrary to the 0.5 power-law reported by Bloor. The results suggest that the vortices result from a near-wake, free-shear instability which causes the separated cylinder boundary layer to roll up into the secondary vortices. Visual observations indicate that immediately following their formation the vortices undergo a strong three-dimensional distortion, which may provide the mechanism for the transition from laminar to turbulent Strouhal vortices.

Flame structures in the near wake of circular cylinders

Flame structures in the near wake of circular cylinders

Secondary vortex generation in the near wake of circular cylinders (Secondary vortex generation in near wake of circular cylinders under forced oscillation, analyzing motion dependent transition regimes using hydrogen bubbles flow visualization)

Secondary vortex generation in the near wake of circular cylinders (Secondary vortex generation in near wake of circular cylinders under forced oscillation, analyzing motion dependent transition regimes using hydrogen bubbles flow visualization)