Impulsively Started Steady Flow About Rectangular Prisms: Experiments and Discrete Vortex Analysis

1986 ◽  
Vol 108 (1) ◽  
pp. 47-54 ◽  
Author(s):  
T. Sarpkaya ◽  
C. J. Ihrig
Keyword(s):  
Reynolds Number ◽  
Steady Flow ◽  
Strouhal Number ◽  
Relative Displacement ◽  
Shear Layers ◽  
Discrete Vortex ◽  
Vortex Model ◽  
Vorticity Distribution ◽  
Initial Rise ◽  
Lift And Drag

Impulsively started steady flow about sharp-edged rectangular prisms has been investigated experimentally and numerically. The forces acting on the bodies have been determined at a Reynolds number of about 20,000 for various angles of incidence as a function of the relative displacement of the fluid. The results have shown that the shedding of the first few vortices has profound effects on both the lift and drag coefficients, often resulting in a large initial rise in drag. The surface-vorticity-distribution version of the discrete vortex model has shown that the strength of the vortex clusters varies from 80 to 90 percent of the vorticity generated in the shear layers. The Strouhal number is correctly predicted but the calculated forces are somewhat larger than those measured experimentally.

A numerical study of vortex interaction

1984 ◽  
Vol 146 ◽  
pp. 331-345 ◽  
Author(s):  
I. G. Bromilow ◽  
R. R. Clements
Keyword(s):  
Flow Visualization ◽  
Numerical Study ◽  
Experimental Studies ◽  
Shear Layers ◽  
Controlled Study ◽  
Vortex Interaction ◽  
Flow Conditions ◽  
Discrete Vortex ◽  
Vortex Model ◽  
Discrete Vortex Model

Flow visualization has shown that the interaction of line vortices is a combination of tearing, elongation and rotation, the extent of each depending upon the flow conditions. A discrete-vortex model is used to study the interaction of two and three growing line vortices of different strengths and to assess the suitability of the method for such simulation.Many of the features observed in experimental studies of shear layers are reproduced. The controlled study shows the importance and rapidity of the tearing process under certain conditions.

An Experimental Investigation Assessing the Validity of Quasi-Static Calculations for an Oscillating Hydrofoil

2011 ◽  
Author(s):  
Rajan Fernandez ◽  
Keith Alexander
Keyword(s):  
Steady Flow ◽  
Strouhal Number ◽  
Strong Dependence ◽  
Lift Coefficient ◽  
Experimental Program ◽  
Design Data ◽  
Drag And Lift ◽  
Drag And Lift Coefficient ◽  
Lift And Drag ◽  
Human Powered

Inspired by animals, flapping wing propulsion has been of interest since the early 1900s. Flapping hydrofoil propulsion has been attempted by designers of human powered watercraft because of the novelty and the apparent high theoretical efficiency, but with limited success. The earliest human powered hydrofoil, the Wasserlaufer, was invented by Julius Schuck in 1953. The first really successful human powered hydrofoil, the Trampofoil, was invented by Alexander Sahlin in 1998. While these craft function adequately the design data for flapping hydrofoils is inadequate or not available. This paper describes an experimental program and initial results for the required data. To design a vehicle with a lifting and thrusting oscillating hydrofoil the force that the hydrofoil will exert on the vehicle through its entire oscillating cycle must ideally be known. The force profiles could be estimated via quasi-static calculations based on steady flow lift and drag coefficients, but these often do not cover the full 360 degree range that can be required and there is doubt that the steady flow coefficients properly represent the dynamic situation of an oscillating hydrofoil. Hence a valuable process would be one that could determine dynamic drag and lift coefficient loops as function of the Strouhal number, heaving and pitching profiles. To work toward the collection of this information, experimental data is being recorded in a towing tank with an oscillating NACA4415 hydrofoil over a range of Strouhal numbers and types of oscillating profiles. While there are still some limitations to the experimental equipment preliminary experimental results show the limitations of using quasi-static calculations and go some way to providing the design data for the hydrofoil section tested. We conclude that quasi-static calculations based on the gliding coefficient curve for for an oscillating hydrofoil are only valid for very small Strouhal numbers (St≪0.05). We have shown that as the Strouhal number increases, the error in such calculations increases very rapidly. We also note that the lift coefficient of the hydrofoil has a strong dependence on the angle of attack and is not affected by the gliding stall.

An inviscid numerical simulation of vortex shedding from an inclined flat plate in shear flow

1977 ◽  
Vol 82 (2) ◽  
pp. 241-253 ◽  
Author(s):  
Masaru Kiya ◽  
Mikio Arie
Keyword(s):  
Shear Flow ◽  
Flat Plate ◽  
Strouhal Number ◽  
Vortex Shedding ◽  
Initial Conditions ◽  
Shear Layers ◽  
Discrete Vortex ◽  
Uniform Shear ◽  
Uniform Shear Flow ◽  
Shear Parameter

Two-dimensional vortex shedding behind an inclined flat plate in uniform shear flow is numerically investigated by means of an inviscid discrete-vortex approximation. The points of appearance of the vortices are fixed in the plane of the plate at a short distance downstream of the edges of the plate. The strengths of the vortices are determined from the Kutta condition. On the assumption that the steadily periodic flow patterns are independent of initial conditions, the numerical calculations are performed for an inclined flat plate immersed in an incompressible fluid which is set in motion impulsively from rest with the velocity profile of uniform shear flow. The results of analysis show that the Strouhal number of vortex shedding and the time-averaged values of other hydrodynamic characteristics of the flow such as the outer-edge velocity of the separated shear layers, the convective velocity of the shear layers and the drag force exerted on the plate vary closely linearly with the shear parameter of the approaching shear flow. A linear relation between the Strouhal number and the shear parameter is confirmed by an air-tunnel experiment. The effects of the shear parameter on the calculated vortex patterns in the wake are also presented.

Flow Characteristics of Oscillating Flow in a Channel Obstructed by an Array of Circular Cylinders

2000 ◽  
Author(s):  
Ken-ichi Sawada ◽  
Hiroyuki Murata ◽  
Michiyuki Kobayashi
Keyword(s):  
Reynolds Number ◽  
Steady Flow ◽  
Strouhal Number ◽  
Oscillation Amplitude ◽  
Flow Characteristics ◽  
Circular Cylinders ◽  
Oscillating Flow ◽  
Mean Velocity ◽  
Cross Sectional ◽  
Flow Experiments

Abstract Flow visualization experiments on oscillating flow in a channel obstructed by an array of circular cylinders were performed. First of all, steady flow experiments without oscillation were carried out. The Karman vortices began to shed in a range of the Reynolds number: 53<Re<68. The Strouhal number showed a tendency to decrease with an increase of the Reynolds number. Oscillating flow experiments were carried out secondly. When the oscillation amplitude was small, Karman vortices shed periodically and its Strouhal number agreed with that in the steady flow. But, when the oscillation amplitude was large, shedding of Karman vortex was well controlled by the oscillating flow. The vortex shedding frequency became large with an increase of the cross-sectional mean velocity. On the other hand, the amplitude of velocity fluctuation in the wake was small in the accelerating phase and large in the decelerating phase.

Identification of flow states around three staggered square cylinders at two symmetrical arrangements by a numerical investigation

2020 ◽  
Vol 31 (11) ◽  
pp. 2050151
Author(s):  
Salwa Fezai ◽  
Fakher Oueslati ◽  
Brahim Ben-Beya
Keyword(s):  
Reynolds Number ◽  
Fluid Flow ◽  
Strouhal Number ◽  
Physical Parameters ◽  
Steady Regime ◽  
Drag And Lift Coefficients ◽  
Square Cylinders ◽  
Drag And Lift ◽  
Lift And Drag ◽  
Obstacle Geometry

The fluid flow over three staggered square cylinders at two symmetrical arrangements has been numerically investigated in this study. The numerical calculations are carried out for several values of the Reynolds number (Re) ranging from 1 to 180. The results are presented in the form of vorticity contours and temporal histories of drag and lift coefficients. Furthermore, the physical parameters, namely, the average drag and lift coefficients and Strouhal number are presented as a function of Re. Two different states of flow are found in this work by systematically varying Re: steady and unsteady states. The transition to unsteady state regime is exhibited via Hopf bifurcation first in the second configuration followed consequently by the first one with critical Reynolds number of Re[Formula: see text] and Re[Formula: see text], respectively. It is observed that the bifurcation point of the steady regime to the unsteady one is very much influenced by the change in the geometry of the obstacle. The unsteady periodic wake is characterized by the Strouhal number, which varies with the Reynolds number and the obstacle geometry. Hence, the values of vortex shedding frequencies are estimated for both the considered configurations. Computations obtained also reveal that the spacing in the wake leads to reducing the pressure and enhancing the fluid flow velocity for both arrangements by monotonically strengthening the Reynolds number value. Furthermore, the drag and lift coefficients are determined, which allowed determining; the best configuration in terms of both lift and drag. It is observed that the drag force is dependent on the obstacle geometry and strengthens while lowering the Reynolds number. On the other hand, an opposite trend of the lift drag evolutions is observed for both configurations and considerably affected by the arrangements shape.

Numerical Simulation of Flow Past an Elliptical Cylinder Undergoing Rotationally Oscillating Motion

2015 ◽  
Vol 137 (3) ◽  
Author(s):  
Esam M. Alawadhi
Keyword(s):  
Reynolds Number ◽  
Drag Coefficient ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Angle Of Attack ◽  
Local Maximum ◽  
Elliptical Cylinder ◽  
Equivalent Diameter ◽  
Lift And Drag ◽  
Oscillating Motion

The finite element method is used to simulate the near-wake of an elliptical cylinder undergoing rotationally oscillating motion at low Reynolds number, 50 ≤ Re ≤ 150. Reynolds number is based on equivalent diameter of the ellipse. The rotationally oscillating motion was carried out by varying the angle of attack between 10 deg and 60 deg, while the considered oscillation frequencies are between St/4 and 4 × St, where St is the Strouhal number of a stationary elliptical cylinder with zero angle of attack. Fluid flow results are presented in terms of lift and drag coefficients for rotationally oscillating case. The details of streamlines and vorticity contours are also presented for a few representative cases. The result indicates that at when the frequency is equal to the Strouhal number, the root-mean-square (RMS) of lift coefficient reaches its local minimum, while the average of drag coefficient reaches its local maximum. Increasing the Reynolds number increases the RMS of lift coefficient and decreases average of drag coefficient.

Investigation of Turbulent Mixing in a Macro-Scale Multi-Inlet Vortex Nanoprecipitation Reactor by Stereoscopic-PIV

2014 ◽  
Author(s):  
Zhenping Liu ◽  
James C. Hill ◽  
Rodney O. Fox ◽  
Michael G. Olsen
Keyword(s):  
Reynolds Number ◽  
Turbulent Mixing ◽  
Three Dimensional ◽  
Stereoscopic Particle Image Velocimetry ◽  
Vortex Center ◽  
Mean Velocity ◽  
Vortex Model ◽  
Functional Nanoparticles ◽  
Turbulent Characteristics ◽  
Macro Scale

Flash Nanoprecipitation (FNP) is a technique to produce monodisperse functional nanoparticles through rapidly mixing a saturated solution and a non-solvent. Multi-inlet vortex reactors (MIVR) have been effectively applied to FNP due to their ability to provide both rapid mixing and the flexibility of inlet flow conditions. Until recently, only micro-scale MIVRs have been demonstrated to be effective in FNP. A scaled-up MIVR could potentially generate large quantities of functional nanoparticles, giving FNP wider applicability in the industry. In the present research, turbulent mixing inside a scaled-up, macro-scale MIVR was measured by stereoscopic particle image velocimetry (SPIV). Reynolds number of this reactor is defined based on the bulk inlet velocity, ranging from 3290 to 8225. It is the first time that the three-dimensional velocity field of a MIVR was experimentally measured. The influence of Reynolds number on mean velocity becomes more linear as Reynolds number increases. An analytical vortex model was proposed to well describe the mean velocity profile. The turbulent characteristics such as turbulent kinematic energy and Reynolds stress are also presented. The wandering motion of vortex center was found to have a significant contribution to the turbulent kinetic energy of flow near the center area.

Strouhal Number for VIV Excitation of Long Slender Structures

2018 ◽  
Author(s):  
Andrew E. Potts ◽  
Douglas A. Potts ◽  
Hayden Marcollo ◽  
Kanishka Jayasinghe
Keyword(s):  
Reynolds Number ◽  
Strouhal Number ◽  
Degrees Of Freedom ◽  
Measurement Techniques ◽  
Cross Flow ◽  
Degree Of Freedom ◽  
Circular Cylinders ◽  
Two Degrees Of Freedom ◽  
Shedding Frequency ◽  
Eddy Shedding

The prediction of Vortex-Induced Vibration (VIV) of cylinders under fluid flow conditions depends upon the eddy shedding frequency, conventionally described by the Strouhal Number. The most commonly cited relationship between Strouhal Number and Reynolds Number for circular cylinders was developed by Lienhard [1], whereby the Strouhal Number exhibits a consistent narrow band of about 0.2 (conventional across the sub-critical Re range), with a pronounced hump peaking at about 0.5 within the critical flow regime. The source data underlying this relationship is re-examined, wherein it was found to be predominantly associated with eddy shedding frequency about fixed or stationary cylinders. The pronounced hump appears to be an artefact of the measurement techniques employed by various investigators to detect eddy-shedding frequency in the wake of the cylinder. A variety of contemporary test data for elastically mounted cylinders, with freedom to oscillate under one degree of freedom (i.e. cross flow) and two degrees of freedom (i.e. cross flow and in-line) were evaluated and compared against the conventional Strouhal Number relationship. It is well established for VIV that the eddy shedding frequency will synchronise with the near resonant motions of a dynamically oscillating cylinder, such that the resultant bandwidth of lock-in exhibits a wider range of effective Strouhal Numbers than that reflected in the narrow-banded relationship about a mean of 0.2. However, whilst cylinders oscillating under one degree of freedom exhibit a mean Strouhal Number of 0.2 consistent with fixed/stationary cylinders, cylinders with two degrees of freedom exhibit a much lower mean Strouhal Number of around 0.14–0.15. Data supports the relationship that Strouhal Number does slightly diminish with increasing Reynolds Number. For oscillating cylinders, the bandwidth about the mean Strouhal Number value appears to remain largely consistent. For many practical structures in the marine environment subject to VIV excitation, such as long span, slender risers, mooring lines, pipeline spans, towed array sonar strings, and alike, the long flexible cylinders will respond in two degrees of freedom, where the identified difference in Strouhal Number is a significant aspect to be accounted for in the modelling of its dynamic behaviour.

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