Turbulent wake of an inclined cylinder with water running

2007 ◽  
Vol 589 ◽  
pp. 261-303 ◽  
Author(s):  
MD. MAHBUB ALAM ◽  
Y. ZHOU
Keyword(s):  
Fluid Dynamics ◽  
Stagnation Point ◽  
Vortex Shedding ◽  
Flow Separation ◽  
Vortex Formation ◽  
Cylinder Surface ◽  
Stagnation Line ◽  
Separation Line ◽  
Fluid Forces ◽  
Aerodynamic Pressure

This paper presents the results of an experimental study of fluid dynamics around an inclined circular cylinder with and without water running over its surface, covering water rivulet formation, fluid forces on the cylinder, near wake and their interrelationships. The cylinder inclination angle (α) with respect to incident flow was between 55° and 80°. It has been found that water running over the cylinder surface may behave quite differently, depending on the Reynolds number (Re), and subsequently impact greatly upon the fluid dynamics around the cylinder. As such, five flow categories are classified. Category A: one water rivulet was observed, irrespective of α, at the leading stagnation point at a small Re. Category B: the rivulet splits into two, symmetrically arranged about the leading stagnation line, once Re exceeds a critical value that depends on α. The two rivulets may further switch back to one, and vice versa. Category C: two symmetrical straight rivulets occur. Category D: the two rivulets shift towards the flow separation line with increasing Re and oscillate circumferentially. The oscillation reaches significant amplitude when the rivulets occur at about 70° from the leading stagnation point. This increased amplitude is coupled with a rapid climb in the mean and fluctuating drag and lift, by a factor of near 5 for the fluctuating lift at α = 80°. Meanwhile, the flow structure exhibits a marked variation. For example, Strouhal number and vortex formation length decrease, along with an increase in spanwise vorticity concentration, velocity deficit, and coherence between vortex shedding and fluctuating lift. All these observations point to the occurrence of a ‘lock-in’ phenomenon, i.e. the rivulet oscillation synchronizing with flow separation. A rivulet–vortex-induced instability is proposed to be responsible for the well reported rain–wind-induced vibration associated with the stay cables of cable-stayed bridges. Category E: the two rivulets shift further downstream just beyond the separation line; the shear layers behind the rivulets become highly turbulent, resulting in weakened vortex shedding, flow fluctuating velocities and fluctuating fluid forces. Based on the equilibrium of water rivulet weight, aerodynamic pressure and friction force between fluid and surface, an analysis is developed to predict the rivulet position on the cylinder, which agrees well with measurements.

Fluid Dynamics Around an Inclined Cylinder With Running Water Rivulets

2006 ◽  
Author(s):  
Md. Mahbub Alam ◽  
Y. Zhou
Keyword(s):  
Fluid Dynamics ◽  
Stagnation Point ◽  
Vortex Shedding ◽  
Flow Separation ◽  
Incident Flow ◽  
Stagnation Line ◽  
Running Water ◽  
Separation Line ◽  
Fluid Forces ◽  
Aerodynamic Pressure

This paper presents the experimental study of fluid dynamics around an inclined circular cylinder with and without water running over its surface, covering the water rivulet formation, the fluid forces on the cylinder, the near-wake structure and their interrelationships. Two cylinder inclination angles (α) were investigated, i.e., 80° and 55°, respectively, with respect to incident flow. It has been found that water running over the cylinder surface may behave quite differently, depending on the incident flow velocity (U∞), which has subsequently a great impact upon fluid dynamics around the cylinder. As such, five flow categories are classified. Category A: one water rivulet was observed, irrespective of α, at the leading stagnation point at a small U∞. Category B: the rivulet splits into two, symmetrically arranged about the leading stagnation line, once U∞ exceeds a α-dependent critical value. The two rivulets may further switch back to one, and vice versa. Category C: two symmetrical straight rivulets occur constantly. Category D: the two rivulets shift towards the flow separation line with increasing U∞ and oscillate circumferentially. The oscillation reaches significant amplitude when the rivulets occur at about 70° from the leading stagnation point. This increased amplitude is coupled with a rapid climb in the mean and fluctuating drag and lift, fluctuating lift rising by a factor of near 5 at α = 80°. Meanwhile, the flow structure exhibits a marked variation, including a declining Strouhal number, reduced vortex formation length, velocity fluctuation and velocity deficit, improved two-dimensionality of the flow, increased coherence between vortex shedding and fluctuating lift, and dipped fluid damping at the vortex shedding frequency. All these observations point to the occurrence of a ‘lock-in’ phenomenon, i.e. the rivulet oscillation synchronizing with flow separation. Category E: the two rivulets shift further downstream just beyond the separation line; the shear layers behind the rivulets become highly turbulent, resulting in weakened vortex shedding, fluctuating fluid forces and fluctuating wake velocity. Based on the equilibrium of water rivulet weight, aerodynamic pressure and friction force between fluid and surface, analysis is developed to predict the rivulet position on the cylinder, which agrees well with measurements.

Near Wake Properties of a Strumming Marine Cable: An Experimental Study

1985 ◽  
Vol 107 (1) ◽  
pp. 86-91 ◽  
Author(s):  
R. D. Peltzer ◽  
D. M. Rooney
Keyword(s):  
Cross Section ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Wind Tunnel Experiments ◽  
Near Wake ◽  
Cable System ◽  
Fluid Forces ◽  
Flow Speeds ◽  
Shedding Frequency ◽  
Marine Cable

Resonant flow-induced oscillations of a flexible cable can occur when the damping of the cable system is sufficiently small. The changes in the flow field that occur in the near wake of the cable during these resonant oscillations are closely related to the changes in the fluid forces that accompany these oscillations. The present wind tunnel experiments were undertaken to examine the effects that forced synchronized vibration and the helically-wound cross section of the cable have on near wake vortex shedding-related parameters; specifically the shedding frequency, vortex formation length Lf, reduced velocity Ur, vortex strength and the wake width Lw. The range of flow speeds over which the vortex shedding was locked on to the vibration frequency varied directly with the vibration amplitude. The helical cross section and the synchronized vibration caused significant changes in the near wake development that could be directly related to the increase in hydrodynamic forces associated with unforced synchronized vibration.

An Experimental Study on the Vertical Flow Past a Finite-Length Horizontal Cylinder at Low Reynolds Numbers

2014 ◽  
Vol 493 ◽  
pp. 68-73 ◽  
Author(s):  
Willy Stevanus ◽  
Yi Jiun Peter Lin
Keyword(s):  
Finite Length ◽  
Vortex Shedding ◽  
Vortex Formation ◽  
Reynolds Numbers ◽  
Horizontal Cylinder ◽  
Vortex Street ◽  
Vertical Flow ◽  
Low Reynolds Numbers ◽  
Vortex Shedding Frequency ◽  
Shedding Frequency

The research studies the characteristics of the vertical flow past a finite-length horizontal cylinder at low Reynolds numbers (ReD) from 250 to 1080. The experiments were performed in a vertical closed-loop water tunnel. Flow fields were observed by the particle tracer approach for flow visualization and measured by the Particle Image Velocimetry (P.I.V.) approach for velocity fields. The characteristics of vortex formation in the wake of the finite-length cylinder change at different regions from the tip to the base of it. Near the tip, a pair of vortices in the wake was observed and the size of the vortex increased as the observed section was away from the tip. Around a distance of 3 diameters of the cylinder from its tip, the vortex street in the wake was observed. The characteristics of vortex formation also change with increasing Reynolds numbers. At X/D = -3, a pair of vortices was observed in the wake for ReD = 250, but as the ReD increases the vortex street was observed at the same section. The vortex shedding frequency is analyzed by Fast Fourier Transform (FFT). Experimental results show that the downwash flow affects the vortex shedding frequency even to 5 diameters of the cylinder from its tip. The interaction between the downwash flow and the Von Kármán vortex street in the wake of the cylinder is presented in this paper.

Experimental study of a pitching and plunging wing

2018 ◽  
Vol 90 (7) ◽  
pp. 1136-1144 ◽  
Author(s):  
Dimitris Gkiolas ◽  
Demetri Yiasemides ◽  
Demetri Mathioulakis
Keyword(s):  
Experimental Data ◽  
Flow Visualization ◽  
Wind Turbine ◽  
Flow Separation ◽  
Flow Behavior ◽  
Lift Coefficient ◽  
Turbine Blades ◽  
Content Type ◽  
Separation Line ◽  
Wing Chord

Purpose The complex flow behavior over an oscillating aerodynamic body, e.g. a helicopter rotor blade, a rotating wind turbine blade or the wing of a maneuvering airplane involves combinations of pitching and plunging motions. As the parameters of the problem (Re, St and phase difference between these two motions) vary, a quasi-steady analysis fails to provide realistic results for the aerodynamic response of the moving body, whereas this study aims to provide reliable experimental data. Design/methodology/approach In the present study, a pitching and plunging mechanism was designed and built in a subsonic closed-circuit wind tunnel as well as a rectangular aluminum wing of a 2:1 aspect-ratio with a NACA64-418 airfoil, used in wind turbine blades. To measure the pressure distribution along the wing chord, a number of fast responding transducers were embedded into the mid span wing surface. Simultaneous pressure measurements were conducted along the wing chord for the Reynolds number of 0.85 × 106 for both steady and unsteady cases (pitching and plunging). A flow visualization technique was used to detect the flow separation line under steady conditions. Findings Elevated pressure fluctuations coincide with the flow separation line having been detected through surface flow visualization and flattened pressure distributions appear downstream of the flow separation line. Closed hysteresis loops of the lift coefficient versus angle of attack were measured for combined pitching and plunging motions. Practical implications The experimental data can be used for improvement of unsteady fluid mechanics problem solvers. Originality/value In the present study, a new installation was built allowing the aerodynamic study of oscillating wings performing pitching and plunging motions with prescribed frequencies and phase lags between the two motions. The experimental data can be used for improvement of computational fluid dynamics codes in case that the examined aerodynamic body is oscillating.

Analytical Modeling of Turbine Cascade Leading Edge Heat Transfer Using Skin Friction and Pressure Measurements

2007 ◽  
Author(s):  
Brian M. Holley ◽  
Lee S. Langston
Keyword(s):  
Heat Transfer ◽  
Skin Friction ◽  
Stagnation Point ◽  
Analytical Modeling ◽  
Exact Analytical Solution ◽  
Leading Edge ◽  
Cfd Modeling ◽  
Turbine Cascade ◽  
Stagnation Line ◽  
Hiemenz Flow

The flow near the leading edge stagnation-line of a plane turbine cascade airfoil is analyzed using measurements, analytical modeling, and computational fluid dynamics (CFD) modeling. New measurements of skin friction and pressure indicate that the aerodynamics of the leading edge are well described by an exact analytical solution for stagnation-point or Hiemenz flow. The skin friction measurements indicate the extent over which the analytical model applies. Based on measurements from an earlier study, the highest heat transfer levels occur along the leading edge stagnation-line. The same parameters that characterize Hiemenz flow also characterize a stagnation-point potential flow, which is used to accurately predict the heat transfer levels along the stagnation-line. CFD analysis indicates that pressure predictions are better than skin friction predictions for characterizing the analytical modeling that is used for more accurate heat transfer evaluation. This provides an approach for predicting the peak heat transfer coefficient in a cascade based only upon surface static pressure calculations.

scholarly journals Comparison of Blade Element Method and CFD Simulations of a 10 MW Wind Turbine

Fluids ◽  
2018 ◽  
Vol 3 (4) ◽  
pp. 73 ◽  
Author(s):  
Galih Bangga
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Flow Separation ◽  
Spatial Interpolation ◽  
Flow Conditions ◽  
Cfd Simulations ◽  
Computational Fluid Dynamics Cfd ◽  
Correction Terms ◽  
Selection Of ◽  
Blade Element

The present studies deliver the computational investigations of a 10 MW turbine with a diameter of 205.8 m developed within the framework of the AVATAR (Advanced Aerodynamic Tools for Large Rotors) project. The simulations were carried out using two methods with different fidelity levels, namely the computational fluid dynamics (CFD) and blade element and momentum (BEM) approaches. For this purpose, a new BEM code namely B-GO was developed employing several correction terms and three different polar and spatial interpolation options. Several flow conditions were considered in the simulations, ranging from the design condition to the off-design condition where massive flow separation takes place, challenging the validity of the BEM approach. An excellent agreement is obtained between the BEM computations and the 3D CFD results for all blade regions, even when massive flow separation occurs on the blade inboard area. The results demonstrate that the selection of the polar data can influence the accuracy of the BEM results significantly, where the 3D polar datasets extracted from the CFD simulations are considered the best. The BEM prediction depends on the interpolation order and the blade segment discretization.

Control of Vortex Shedding Using a Screen Attached on the Separation Point of a Circular Cylinder and Its Effect on Drag

2017 ◽  
Vol 139 (7) ◽  
Author(s):  
Gokturk Memduh Ozkan ◽  
Erhan Firat ◽  
Huseyin Akilli
Keyword(s):  
Circular Cylinder ◽  
Surface Area ◽  
Vortex Shedding ◽  
Control Method ◽  
Separation Point ◽  
Whole Body ◽  
Cylinder Surface ◽  
Control Effect ◽  
Permeable Plate ◽  
Turbulent Statistics

The control of flow in the wake of a circular cylinder by an attached permeable plate having various porosity ratios was analyzed experimentally using both particle image velocimetry (PIV) and dye visualization techniques. The force measurements were also done in order to interpret the effect of control method on drag coefficient. The diameter of the cylinder and length to diameter ratio of the plate were kept constant as D = 50 mm and L/D = 1.0, respectively. The porosity ratio, β, which can be defined as the ratio of open surface area to the whole body surface area, was taken as β = 0.4, 0.5, 0.6, 0.7, and 0.8 (permeable plates). The study was performed considering deep water flow conditions with a constant Reynolds number of ReD = 5000 based on the cylinder diameter. Each permeable plate was attached on the separation point and the results were compared with the results of cylinder without permeable plate (plain cylinder) in order to understand the control effect. Both qualitative and quantitative results revealed that the permeable plates of 0.4 ≤ β ≤ 0.6 are effective on controlling the unsteady flow structure downstream of the cylinder, i.e., the vortex formation length was increased, turbulent statistics was reduced and vortex shedding frequency was diminished when the permeable plate attached normal to the cylinder surface from the lower separation point. However, the drag force acting on the cylinder was found to be increased due to the increased cross-sectional area.

Computational Fluid Dynamics Modelling of a Load Sensing Proportional Valve

2019 ◽  
Author(s):  
Alessandro Corvaglia ◽  
Giorgio Altare ◽  
Roberto Finesso ◽  
Massimo Rundo
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Steady State ◽  
Complex Geometry ◽  
Ansys Fluent ◽  
Proportional Valve ◽  
Fluid Forces ◽  
Load Sensing ◽  
Cfd Models ◽  
Dynamics Modelling

Abstract In this paper, two 3D CFD models of a load sensing proportional valve are contrasted. The models were developed with two different software, Simerics PumpLinx® and ANSYS Fluent®. In both cases the mesh was dynamically modified based on the fluid forces acting on the local compensator. In the former, a specific template for valves was used, in the latter a user-defined function was implemented. The models were validated in terms of flow rate and pressure drop for different positions of the main spool by means of specific tests. Two configurations were tested: with the local compensator blocked and free to regulate. The study has brought to evidence the reliability of the CFD models in evaluating the steady-state characteristics of valves with complex geometry.

A Fluid Dynamics Study of a Modified Low-Reynolds-Number Flapping Motion

2010 ◽  
Author(s):  
M. R. Amiralaei ◽  
H. Alighanbari ◽  
S. M. Hashemi
Keyword(s):  
Fluid Dynamics ◽  
Reynolds Number ◽  
Phase Angle ◽  
Fluid Dynamic ◽  
Low Reynolds Number ◽  
Dynamic Performance ◽  
Navier Stokes ◽  
Fluid Forces ◽  
Low Reynolds ◽  
Volume Method

The objective of the present study is to investigate the low Reynolds number (LRN) fluid dynamics of an elliptic airfoil performing a novel figure-eight-like motion. To this mean, the influence of phase angle between the pitching and translational (heaving and lagging) motions and the amplitude of translational motions on the fluid flow is simulated. Navier-Stokes (NS) equations with Finite Volume Method (FVM) are used and the instantaneous force coefficients and the fluid dynamics performance, as well as the corresponding vortical structures are analyzed. Both the phase angle and the amplitudes of horizontal and vertical motions are of great importance to the fluid dynamic characteristics of the model as they are shown to change the peaks of the fluid forces, fluid dynamic performance, and the vortical patterns around the model.

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