Numerical Investigations of Flow Past a Rotating Stepped Cylinder

2018 ◽  
Author(s):  
Jamie Shin ◽  
Hamid Rahai ◽  
Shahab Taherian ◽  
Ryan J. Ferris
Keyword(s):  
Vortex Shedding ◽  
Vortical Structure ◽  
The Other ◽  
Stream Velocity ◽  
Time Step ◽  
Numerical Investigations ◽  
Flow Past ◽  
L Cell ◽  
Large Cylinder ◽  
Small Cylinder

Unsteady numerical investigations of flow past a partially rotating stepped cylinder have been performed. The objective of the study was to investigate whether the wake characteristics could be controlled with rotation of one cylinder while the other remains stationary and how partial rotation impacts the aerodynamic forces. The stepped cylinder was 2 m in length where the first meter was a round cylinder 5 cm in diameter followed by a 2:1 step down cylinder. Two round end plates, 0.1 cm thick and 40 cm in diameter, were placed at each end. The end plates were positioned at 5 degrees with respectto the incoming flow to remove the end effect on vortex shedding. All simulations were performed using the Siemens PLM STAR-CCM+ CFD software with K-ω turbulence model. The time step was 0.00083 second to resolve the flow for each 10 degrees rotation. 1200 time steps were used. The investigations were performed with one cylinder rotating while the other remains stationary. Four cases were investigated. When either cylinder was rotating, the RPM was maintained at either 2000 or 4000 while the free stream velocity was maintained at 10 m/sec. The Reynolds number for the large and small cylinders were approximately 32,258 and 16,129, respectively. The corresponding velocity ratios λ for the large cylinder rotating were 0.5 and 1.0, and 0.25 and 1.0 for the small cylinder. Previous investigations have classified vortical structure in the wake of a step cylinder in terms of L-cell (for large cylinder), S-cell (for small cylinder) and N-cell (the region in between). When the large cylinder is rotating, at λ = 1.0, the velocity and vorticity in the wake of the large cylinder is increased. The N-cell initially has a larger velocity than the L-cell and is at a slanted angle. A suction effect was observed in the near wake region, causing the flow in the L-cell to coalesce near its midsection. The vortices originated at the step were connected to the S-cell at a lower speed. The overall lift to drag ratio (L/D) for this case was 1.14. When λ = 0.5, vortex structures were maintained through the three different cells with increased variations in cell frequency across the large cylinder, the L/D was reduced to 0.36. When the small cylinder was rotating, at λ = 0.5, vortex shedding was suppressed within the S-cell and considerable distortion was observed in the vortical structure in the wake of the large cylinder. However, the N-cell had similar structure as when large cylinder was rotating, but connecting to the L-cell at a larger slanted angle. When λ was reduced to 0.25, shedding was observed across the length of the cylinder with increased variations. The corresponding L/D ratios for these cases were both at 0.2.

An Experimental Study of Flow Past a Dual Step Cylinder

2010 ◽  
Author(s):  
Chris R. Morton ◽  
Serhiy Yarusevych
Keyword(s):  
Flow Visualization ◽  
Vortex Shedding ◽  
Large Diameter ◽  
Three Dimensional ◽  
Small Diameter ◽  
Single Vortex ◽  
Aspect Ratios ◽  
Flow Past ◽  
Large Cylinder ◽  
Small Cylinder

Flow past a dual step cylinder has been investigated using experimental flow visualization methods. The dual step cylinder model is comprised of a small diameter cylinder (d) and a large diameter cylinder (D) mounted at the mid-span of the small cylinder. The experiments have been performed for ReD = 1050, D/d = 2, and a range of large cylinder aspect ratios (L/D). The focus of the study is on vortex shedding and vortex interactions occurring in the large and small cylinder wakes. A flow visualization study completed using hydrogen bubble technique and planar laser induced fluorescence has shown that the flow development is highly dependent on the aspect ratio of the large cylinder, L/D. The results identify four distinct flow regimes: (i) for L/D ≥ 17, three vortex shedding cells form in the wake of the large cylinder, one central cell and two cells of lower frequency extending over about 4.5D from the large cylinder ends, (ii) for 7 < L/D ≤ 14, a single vortex shedding cell forms in the wake of the large cylinder, whose shedding frequency decreases with decreasing L/D, (iii) for 2 ≤ L/D ≤ 7, vortex shedding in the wake of the large cylinder is highly three-dimensional, such that each vortex deforms while it is shed into the wake, (iv) for 0.2 ≤ L/D ≤ 1, only small cylinder vortices are shed in the wake and often form vortex connections across the wake of the large cylinder.

Vortex Shedding From a Bluff Body Adjacent to a Plane Sliding Wall

1991 ◽  
Vol 113 (3) ◽  
pp. 384-398 ◽  
Author(s):  
M. P. Arnal ◽  
D. J. Goering ◽  
J. A. C. Humphrey
Keyword(s):  
Reynolds Number ◽  
Vortex Shedding ◽  
Bluff Body ◽  
Solid Wall ◽  
Reynolds Numbers ◽  
Careful Examination ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Flow Past ◽  
Lower Reynolds Number

The characteristics of the flow around a bluff body of square cross-section in contact with a solid-wall boundary are investigated numerically using a finite difference procedure. Previous studies (Taneda, 1965; Kamemoto et al., 1984) have shown qualitatively the strong influence of solid-wall boundaries on the vortex-shedding process and the formation of the vortex street downstream. In the present study three cases are investigated which correspond to flow past a square rib in a freestream, flow past a rib on a fixed wall and flow past a rib on a sliding wall. Values of the Reynolds number studied ranged from 100 to 2000, where the Reynolds number is based on the rib height, H, and bulk stream velocity, Ub. Comparisons between the sliding-wall and fixed-wall cases show that the sliding wall has a significant destabilizing effect on the recirculation region behind the rib. Results show the onset of unsteadiness at a lower Reynolds number for the sliding-wall case (50 ≤ Recrit ≤100) than for the fixed-wall case (Recrit≥100). A careful examination of the vortex-shedding process reveals similarities between the sliding-wall case and both the freestream and fixed-wall cases. At moderate Reynolds numbers (Re≥250) the sliding-wall results show that the rib periodically sheds vortices of alternating circulation in much the same manner as the rib in a freestream; as in, for example, Davis and Moore [1982]. The vortices are distributed asymmetrically downstream of the rib and are not of equal strength as in the freestream case. However, the sliding-wall case shows no tendency to develop cycle-to-cycle variations at higher Reynolds numbers, as observed in the freestream and fixed-wall cases. Thus, while the moving wall causes the flow past the rib to become unsteady at a lower Reynolds number than in the fixed-wall case, it also acts to stabilize or “lock-in” the vortex-shedding frequency. This is attributed to the additional source of positive vorticity immediately downstream of the rib on the sliding wall.

Vortex Shedding From Low Aspect Ratio Dual Step Cylinders

2012 ◽  
Author(s):  
Chris Morton ◽  
Serhiy Yarusevych
Keyword(s):  
Aspect Ratio ◽  
Vortex Shedding ◽  
Small Diameter ◽  
Dominant Frequency ◽  
Cylinder Wake ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Half Loop ◽  
Large Cylinder ◽  
Small Cylinder

A low aspect ratio dual-step cylinder is comprised of two cylinders of different diameters (D and d). The large diameter cylinder (D) with low aspect ratio (L/D) is attached to the mid-span of a small diameter cylinder (d). This geometry is relevant to many engineering applications, e.g., finned-tube heat exchangers, underwater cables, and cylindrical support structures. The present study investigates the effect of Reynolds number (ReD) and L/D on dual step cylinder wake development for 1050 ≤ ReD ≤ 2100, D/d = 2, and 0.2 < L/D ≤ 3. Experiments have been performed in a water flume facility utilizing flow visualization, Laser Doppler Velocimetry (LDV), and Particle Image Velocimetry (PIV). The results show that vortex shedding occurs from the large and small diameter cylinders at distinct frequencies for L/D ≥ 1 & ReD = 2100 and L/D ≥ 2& ReD = 1050. At these higher aspect ratios investigated, large cylinder vortices predominantly form closed vortex loops in the wake and small cylinder vortices form half-loop vortex connections. In contrast, at lower aspect ratios, vortex shedding from the large cylinder ceases, with the dominant frequency centred-activity in the large cylinder wake attributed to the passage of vortex filaments connecting small cylinder vortices. The presence of the large cylinder distorts the vortex filaments causing cyclic vortex dislocations accompanied by the formation of half-loop vortex connections. Increasing L/D decreases the frequency of occurrence of vortex dislocations and increases the dominant frequency in the large cylinder wake. The results also show that the Reynolds number has a substantial effect on wake vortex shedding frequency, which is more profound than that expected for a uniform cylinder.

Vortex shedding from square prisms in smooth and turbulent flows

1986 ◽  
Vol 164 ◽  
pp. 77-89 ◽  
Author(s):  
Yasuharu Nakamura ◽  
Yuji Ohya
Keyword(s):  
Flow Visualization ◽  
Turbulent Flows ◽  
Cross Section ◽  
Vortex Shedding ◽  
Large Scale ◽  
Resonant Interaction ◽  
The Other ◽  
Flow Past ◽  
Scale Turbulence ◽  
Main Effects

Visualization and measurements of velocity and pressure were made for the flow past prisms of variable length with square cross-section, placed normal to smooth and turbulent approaching flows. Square prisms shed vortices in one of the two fixed wake planes. The plane of shedding is switched irregularly from one to the other. Flow visualization confirms the two main effects of small– and large-scale turbulence on the flow past square prisms that had previously been suggested. In particular, large-scale turbulence intensifies vortex shedding from square prisms through resonant interaction, thereby reducing the base pressure considerably.

COMPARISON OF LES WITH DSM, k-ε AND EXPERIMENTS FORTURBULENT VORTEX SHEDDING FLOW PAST 2D SQUARE CYLINDER

1994 ◽  
Vol 59 (459) ◽  
pp. 49-56 ◽  
Author(s):  
Shigehiro SAKAMOTO ◽  
Akashi MOCHIDA ◽  
Shuzo MURAKAMI ◽  
Wolfgang RODI
Keyword(s):  
Vortex Shedding ◽  
Square Cylinder ◽  
Flow Past

scholarly journals On the effect of postponing pregnancy in a Zika transmission model

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Edy Soewono ◽  
Glenn Lahodny
Keyword(s):  
Stochastic Analysis ◽  
Infection Intensity ◽  
Disease Outbreak ◽  
Recruitment Rate ◽  
The Other ◽  
Transmission Model ◽  
Basic Reproductive Ratio ◽  
Numerical Investigations ◽  
The Individual ◽  
Endemic State

AbstractWe construct a Zika transmission model to investigate the effect of postponing pregnancy on the infection intensity. We perform analytical and numerical investigations for deterministic and stochastic analysis to obtain the basic reproductive ratio, endemic state, probability of disease extinction, and the probability of outbreak. The results indicate that by reducing the pregnancy rate the mosquito-to-human ratio increases, and, consequently, the basic reproductive ratio increases. Simultaneously, the probability of disease extinction decreases, and the probability of disease outbreak increases. On the other hand, the endemic state of infected infants initially increases with the decrease of the pregnancy recruitment rate, up to a certain level, and decreases as the recruitment rate of pregnancy tends to zero. This work highlights that postponing pregnancy that gives the individual temporary protection for unexpected infected newborns may increase the population infectivity.

The flow past a rapidly rotating circular cylinder

1957 ◽  
Vol 242 (1228) ◽  
pp. 108-115 ◽  
Keyword(s):  
Boundary Layer ◽  
Circular Cylinder ◽  
Present Theory ◽  
Stream Velocity ◽  
Two Dimensional ◽  
Rotating Circular Cylinder ◽  
Flow Past ◽  
Separating Flow ◽  
Two Dimensional Flow ◽  
Uniform Stream

This paper considers the two-dimensional flow past a circular cylinder immersed in a uniform stream, when the cylinder rotates about its axis so fast that separation in suppressed. The solution of the flow in the boundary layer on the cylinder is obtained in the form of a power series in the ratio of the stream velocity to the cylinder's peripheral velocity, and expressions are deduced for the value of the circulation and the torque on the cylinder. The terms calculated explicitly are sufficient to give reliable numerical values over the whole range of rotational speeds for which the postulate of non-separating flow is justifiable. The previously accepted theory, due to Prandtl, predicted that the circulation should not exceed a certain limit, while the present theory indicates that the circulation increases indefinitely with increase of rotaional speed. Strong arguments against the older theory are put forward, but the experimental evidence available is inconclusive.

Accelerated flow past a symmetric aerofoil: experiments and computations

2007 ◽  
Vol 591 ◽  
pp. 255-288 ◽  
Author(s):  
T. K. SENGUPTA ◽  
T. T. LIM ◽  
SHARANAPPA V. SAJJAN ◽  
S. GANESH ◽  
J. SORIA
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Angle Of Attack ◽  
Stream Velocity ◽  
Published Data ◽  
Navier Stokes Equation ◽  
Moment Coefficient ◽  
Flow Past ◽  
Naca 0015 ◽  
Accelerated Flow

Accelerated flow past a NACA 0015 aerofoil is investigated experimentally and computationally for Reynolds number Re = 7968 at an angle of attack α = 30°. Experiments are conducted in a specially designed piston-driven water tunnel capable of producing free-stream velocity with different ramp-type accelerations, and the DPIV technique is used to measure the resulting flow field past the aerofoil. Computations are also performed for other published data on flow past an NACA 0015 aerofoil in the range 5200 ≤ Re ≤ 35000, at different angles of attack. One of the motivations is to see if the salient features of the flow captured experimentally can be reproduced numerically. These computations to solve the incompressible Navier–Stokes equation are performed using high-accuracy compact schemes. Load and moment coefficient variations with time are obtained by solving the Poisson equation for the total pressure in the flow field. Results have also been analysed using the proper orthogonal decomposition technique to understand better the evolving vorticity field and its dependence on Reynolds number and angle of attack. An energy-based stability analysis is performed to understand unsteady flow separation.

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