P. Anagnostopoulos

Response characteristics of a vortex-excited cylinder at low reynolds numbers

An experimental investigation has been conducted into vortex-induced, cross-flow oscillations of a circular cylinder mounted elastically in a water channel. The Reynolds number ranged between 90 and 150, a regime where the vortex street is fully laminar. The frequency and amplitude of cylinder oscillation and the vortex-shedding frequency were measured in and around the lock-in region. Fluid velocities in the cylinder wake and cylinder response were recorded simultaneously at Re = 115 in the lock-in region, and traces of cylinder motion were taken in and around resonant conditions. The maximum oscillation amplitude occurs near the lower limit of the lock-in region, while no substantial differences in final response are detectable between the cylinder amplitude developing from rest and the free stream velocity being varied with the cylinder already oscillating. The cylinder oscillation frequency was found to be slightly higher than its natural frequency in air, in the greatest part of the lock-in region.

NUMERICAL STUDY OF THE BLOCKAGE EFFECTS ON VISCOUS FLOW PAST A CIRCULAR CYLINDER

In various numerical solutions of flow around bluff bodies the unbounded physical domain is replaced by a restricted computational one whose extent depends on the size of the computational grid network. The truncation of the solution domain in the cross-flow direction reduces the computer time required for the solution, but introduces numerical blockage effects which influence considerably the values of the various flow parameters. In the present paper the finite element solution of steady and unsteady flow around a circular cylinder at Re = 106 is presented for blockage ratios of 0.05, 0.15 and 0.25. A boundary condition was tested for which the streamfunction values at the outer boundaries were those of the irrotational solution around a circular cylinder. The size of the standing vortices decreases with the blockage ratio when the flow is steady, while the spacing of the vortices decreases in both directions with increasing blockage ratio when the wake becomes unsteady. The hydrodynamic forces on the cylinder and the Strouhal number are magnified as the blockage ratio increases. The application of the streamfunction values derived from the irrotational solution at the outer boundaries reduced blockage effects only at high blockage ratio.

Viscous oscillatory flow around a circular cylinder at low Keulegan–Carpenter numbers and frequency parameters

SUMMARY The results of a numerical study of the viscous oscillating flow around a circular cylinder at low Keulegan‐ Carpenter numbers (KC) and frequency parameters (b) are presented in this paper. The finite element method was used for the solution of the Navier‐Stokes equations in the formulation where the streamfunction and vorticity are the field variables. The computation was conducted at Keulegan‐Carpenter numbers extending up to KCa 15 and frequency parameters ranging between ba 6 and 100. At low values of the Keulegan‐Carpenter number the flow remains symmetrical. As the Keulegan‐Carpenter number is increased over a certain value which depends also on the frequency parameter, asymmetries appear in the flow which are eventually amplified and lead finally to complex vortex-shedding patterns, some of which are markedly different from those observed at higher frequency parameters. The solution revealed that although for certain values of KC and b the shedding of vortices is periodic, there also exists a complicated flow regime in which the flow is not periodic but switches between different modes in consecutive cycles of flow oscillation. For the various flow cases examined, the traces of the hydrodynamic forces are presented and the hydrodynamic coefficients and RMS values of the inline force are compared with experimental evidence. # 1998 John Wiley & Sons, Ltd.

Modelling of water pollution in the Thermaikos Gulf with fuzzy parameters

Abstract The fuzzy set theory is applied for the definition of the imprecise parameters, which are used in a water pollution model. The physico-chemical coefficients and the loads of pollution sources are expressed in the form of triangular fuzzy numbers. A two-dimensional finite element algorithm, combined with fuzzy logic analysis, is used for the solution of the advection–dispersion equation for ten different water quality variables. The characteristic Galerkin technique was employed for the temporal discretisation, whereas interval operations were conducted for the solution of the algebraic system of equations with fuzzy coefficients. The model was applied for the study of pollution in the Gulf of Thermaikos, located in Northern Greece, for water velocities obtained from a wind-induced circulation model. The concentration distributions of the water quality variables considered are derived in the form of fuzzy numbers, and are presented in relevant diagrams.

Numerical solution for laminar two-dimensional flow about a fixed and transversely oscillating cylinder in a uniform stream

Abstract A finite element solution is presented for laminar 2-dimensional flow past a fixed and transversely oscillating cylinder. The solution of vortex street development behind the cylinder is obtained when the cylinder remains fixed. Then the cylinder which is assumed elastically mounted is allowed to oscillate in the cross-flow direction under the action of the fluctuating lift force. The grid system is translated with the cylinder at each time step and the field variables are interpolated to new grid locations. The computer results predict the lock-in phenomenon which occurs when oscillation frequency is near the natural vortex-shedding frequency. Particular attention was given to flow configuration, by means of streamlines, filament lines, and equi-vorticity lines. The effect of cylinder oscillation on lift and drag forces and separation angles is extracted from the numerical results. Computer results at Reynolds numbers 106 and 115 are in good agreement with experimental measurements conducted by the author.

Numerical visualization of oscillatory flow around a circular cylinder at Re=200 and KC=20—an aperiodic flow case

The results of a numerical study of the viscous oscillating flow around a circular cylinder at Reynolds number=200 and Keulegan–Carpenter number=20 are presented. The finite element method was used for the solution of the Navier–Stokes equations, in the formulation where the streamfunction and the vorticity are the field variables. The computation revealed a complicated flow pattern at which the flow is not periodic but switches between different modes at various oscillation cycles. The vortex shedding patterns occurring at different cycles were identified by means of numerical flow visualization. The traces of the hydrodynamic forces were derived from the solution, and the hydrodynamic coefficients of the in-line force were evaluated.

Numerical Study of Oscillatory Flow Past a Pair of Cylinders in a Side-by-Side Arrangement

The results of a numerical study of the viscous oscillating flow around a pair of circular cylinders are presented herein, for a constant frequency parameter, β, equal to 50, and Keulegan-Carpenter numbers, KC, ranging between 0.2 and 10. The cylinders were placed side-by-side to the oncoming flow, for a pitch to diameter ratio, P/D, equal to 2. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables. The vorticity contours generated from the solution were used mainly for the flow visualization, whereas the stream-lines and isobars are shown in some cases. At low values of the Keulegan-Carpenter number the flow remains symmetrical with respect to the horizontal axis of symmetry of the solution domain. As the Keulegan-Carpenter number is increased asymmetries appear in the flow, which are eventually amplified and lead finally to more complicated vortex-shedding patterns. These asymmetries generate an aperiodic flow configuration at consecutive cycles, which becomes almost chaotic as KC grows larger. For the various Keulegan-Carpenter numbers examined the time-histories of the hydrodynamic forces are presented, and the r.m.s. values of the hydrodynamic forces and the coefficients of the in-line force were evaluated.Copyright

A Numerical Hydrodynamic-ecological ModelFor The Prediction Of Pollution In TheThermaikos Gulf- Effect Of The Wind Direction

A hydrodynamic-ecological model is used for the numerical study of coastal pollution in the Thermaikos Gulf. It is composed by a wind-induced circulation component, and by an advection-dispersion module of ten different quality parameters. These parameters are: chlorophyll-a, organic nitrogen, ammonia nitrogen, nitrite nitrogen, nitrate nitrogen, organic and inorganic phosphorus, BOD, dissolved oxygen, and coliforms. The study of the results obtained for different conditions revealed the influence of the wind direction on the distribution of pollution loads in the Thermaikos Gulf.

Numerical Study of Oscillatory Flow Past a Pair of Cylinders at Low Reynolds and Keulegan-Carpenter Numbers

The results of a numerical study of the viscous oscillating flow around a pair of circular cylinders are presented herein, for a constant frequency parameter, β, equal to 50, and Keulegan-Carpenter numbers, KC, ranging between 0.2 and 10. The cylinders were placed side-by-side to the oncoming flow, for a pitch to diameter ratio, P/D, equal to 1.2. The finite-element method was employed for the solution of the Navier-Stokes equations, in the formulation where the stream function and the vorticity are the field variables. The streamlines and vorticity contours generated from the solution were used for the flow visualization. At low values of the Keulegan-Carpenter number the flow remains symmetrical with respect to the horizontal axis of symmetry of the domain. As the Keulegan-Carpenter number is increased asymmetries appear in the flow, which are eventually amplified and lead finally to more complicated vortex-shedding patterns. These asymmetries generate an aperiodic flow configuration at consecutive cycles, which becomes almost chaotic as KC grows larger. For the various Keulegan-Carpenter numbers examined the traces of the hydrodynamic forces are presented, and the r.m.s. values of the hydrodynamic forces and the coefficients of the in-line force were evaluated.© 2002 ASME

Mass transfer effects on free-convection flow past an impulsively started porous infinite vertical limiting surface

The effects of heat and mass transfer on the flow field of a laminar boundary layer is considered. The flow is that of an incompressible viscous fluid past an impulsively started permeable vertical limiting surface with constant heat flux. The solution of the problem was obtained numerically, using an implicit finite difference scheme. The solution is given in a number of diagrams, which depict the influence ofG on velocity,P on temperature and Sc on concentration. The influence ofG on skin friction is also given.