The Proper Orthogonal Decomposition in the Analysis of the wake behind a Foamed and a Finned Circular Cylinder (original) (raw)
Related papers
The Analysis of the Wake behind a Foamed and a Finned Cylinder
Power and Energy Systems, 2013
Particle Image Velocimetry (PIV) has been carried out to investigate the wake region behind a foamed and a finned cylinder. The purpose of this analysis is to develop one-and two-point correlations and to investigate the flow characteristics for these two cases. The experiments are conducted for two Reynolds numbers (based on the mean air velocity and the cylinder diameter) 2000 and 8000. Two dimensional results of planar PIV reveal the important aspects of the local flow features of the circular finned and foamed cylinders. These include turbulent boundary layer development over the surface and a delayed separation of the flow resulting in a smaller wake size in each case. The application of Proper Orthogonal Decomposition (POD) to the PIV velocity fields of the two cylinder types is also discussed. The POD computed for the measured velocity fields for both cases shows that the first two spatial modes contain most of the kinetic energy of the flow irrespective to the cylinder type. These two modes are also responsible for the large-scale coherence of the fluctuations. For finned and foamed cylinder types, the first four eigenmodes of the ve locity fie ld were measured and their organizations were investigated. These eigenmodes disclose the overall mean flow structure, and the large-scale structure being essentially connected to the most robust flow motion.
A comparison between the wake behind finned and foamed circular cylinders in cross-flow
Experimental Thermal and Fluid Science, 2014
The flow pattern behind a circular cylinder is associated with various instabilities. These instabilities are characterized by the Reynolds number and include the wake, separated shear layer and boundary layer. Depending on the physical application of the cylinder, increasing the level of turbulence on the surface of the cylinder would be a target for drag reduction or heat transfer enhancement. Particle Image Velocimetry (PIV) has been carried out to investigate the wake region behind a foamed and a finned cylinder. The purpose of this analysis is to investigate the flow characteristics for these two cases. The experiments are conducted for a wide range of Reynolds numbers (based on the mean air velocity and the cylinder diameter) from 1000 to 10000. Two dimensional results of planar PIV reveal the important aspects of the local flow features of the circular finned and foamed cylinders. These include turbulent boundary layer development over the surface and a delayed separation of the flow resulting in a smaller wake size at each speed. The application of Proper Orthogonal Decomposition (POD) to the PIV velocity fields of the two cylinder types is also discussed. The POD computed for the measured velocity fields for all cases shows that the first two spatial modes contain most of the kinetic energy of the flow, irrespective to the cylinder type. These two modes are also responsible for the large-scale coherence of the fluctuations. For three different cylinder types, the first four eigenmodes of the flow field were calculated and their structures were analysed.
Experimental Thermal and Fluid Science, 1996
HA 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 crosscorrelation 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 (Red). The measurements presented pertain to cylinders with d = 12.5 and 25 mm (l/d = 19.5 and 9.8, respectively). The respective Reynolds numbers Re d 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.
Proper Orthogonal Decomposition and Spectral Analysis of a Wall-Mounted Square Cylinder Wake
Journal of Aerospace Technology and Management
The fl ow patterns over a fi nite square cylinder of aspect ratio of 3 were analyzed experimentally in a subsonic wind tunnel using the time-resolved particle image velocimetry (TR-PIV) techniques. The near wake fl ow structures and vortex shedding characteristics were investigated using mean fl ow analysis, spectral analysis and proper orthogonal decomposition (POD). The cylinders were fi xed on a elliptical leading edge fl at plate, creating a boundary layer which interacted with the cylinder wake. The 2D PIV measurements were conducted at a low horizontal plane, z/h = 0.3, to investigate possible boundary layer interactions. Due to the complexity of the phenomena, the fl ow was characterized both in terms of average behavior and time-resolved velocity fi elds. Both symmetrical and anti-symmetrical vortices structures occur in the cylinder wake, which can be identifi ed based on the coeffi cients of the fi rst four POD modes. The results indicated that the alternating Karman vortex structures are dominant, described by the fi rst two POD modes.
Experiments in Fluids, 2005
This study focuses on the flow modifications generated by a flat plate stabilizer placed downstream of a half-cylinder compared to the wake formation without the plate. For moderate Reynolds number, a snapshot proper-orthogonal-decomposition method based on particle-image-velocimetry (PIV) is used to analyze the complex dynamics of the flows and to highlight typical swirl formation mechanisms. Using this tool, the nearwake structures, the kinetic aspects, the energy contributions, the degree of organization of the structures and their temporal behaviour are identified. A strong link between the Strouhal number, the mean vorticity and the Total Energy is observed. The interference of the shear layer with the stabilizer has an atypical influence on the wake characteristics.
On Vorticity Dynamics in Turbulent Wake behind Circular Cylinder
MATEC Web of Conferences
The circular cylinder turbulent wake is studied experimentally using Particle Image Velocimetry (PIV) method. The vorticity component parallel to the cylinder axis is evaluated from measured velocity field. Dynamics of the vorticity field is analysed using the Oscillation Pattern Decomposition (OPD) method. The 3 dominant modes corresponding to the 1st, 2nd and 3rd harmonics of Strouhal frequency are detected and presented. The corresponding topologies are characterized by waves of vortical systems. The results are similar to the dynamics of velocity field, however the procedure is much easier to implement, as only half number of degrees of freedom are active.
Flow structure around geometries similar to circular cylindrical geometries in various industrial and daily life applications must be known in respect of energy efficiency and stability for different flow conditions. In this sense, flow structures in the wake of a circular cylinder located in a boundary layer flow and a flow region near to free flow surface were examined by means of PIV system in an open water channel. Circular cylinder was parallel to boundary layer plate and perpendicular to flow direction. Experiments were conducted primarily in a region near to the free surface of the flow and then in the boundary layer flow at various locations of the cylinder diameter (D) ratios of contacting case, 0.00D, 0.25D, 0.50D and 1.0D. Cylinder diameter was 50 mm and Reynolds number calculated according to this diameter was selected 5000. For the experiments, Stereo Particle Image Velocimetry Setup in Selcuk University Mechanical Engineering Department Hydraulic Laboratory was utilized. Results obtained as the consequence of investigation of flow around circular cylinder at different flow conditions are presented in the form of time averaged streamwise velocity component contours and timeaveraged vorticity contours. It is shown that wake structures of circular cylinder immersed in a boundary layer flow exhibited considerably different characteristics compared to the flow structures obtained at flow region near to free surface of the flow.
Longitudinal vortex structures in a cylinder wake
Physics of Fluids, 1994
This paper presents the velocity field of the longitudinal vortices found in the wake of a circular cylinder, as measured using digital particle image velocimetry (PIV). Vorticity and circulation of the longitudinal vortices are presented, based on instantaneous velocity distributions in a transverse plane behind the cylinder. Recently, flow visualizations by Wei and Smith,' Williamson,' Welsh et a1.,3 and Bays-Muchmore and Ahmed4 have shown that three-dimensional (3-D) vertical structures develop in the wake of a bluff body. The 3-D vertical structures were found to include pairs of counterrotating longitudinal vortices superimposed on the nominally two-dimensional (2-D) Kirmin vortex street as speculated
Experiments in Fluids, 2005
The vortex formation and shedding process in the near wake region of a 2D square-section cylinder at incidence has been investigated by means of particle image velocimetry (PIV). Proper orthogonal decomposition (POD) is used to characterize the coherent large-scale flow unsteadiness that is associated with the wake vortex shedding process. A particular application of the POD analysis is to extract the vortex-shedding phase of individual velocity fields, which were acquired at asynchronous low rate with respect to the vortex shedding cycle. The phase of an individual flow field is determined from its projection on the first pair of POD modes, allowing phase averaging of the measurement data to be performed. In addition, a low-order representation of the flow, constructed from the mean and the first pair of POD modes, is found to be practically equivalent to the phase-averaged results. It is shown that this low-order representation corresponds to the basic Fourier component of the flow field ensemble with respect to the reconstructed phase. The phase-averaged flow representations reveal the dominant flow features of the vortex-shedding process and the effect of the angle of incidence upon it.