Analyzing three-dimensional wake vortex dynamics...
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17 th International Symposium on Application of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 07 – 10, 2014 2.10.6 Analyzing three-dimensional wake vortex dynamics using time-resolved planar PIV C. Morton 1,* , S. Yarusevych 1 1: Dept. of Mechanical and Mechatronics Engineering, University of Waterloo, Canada * Correspondent author: [email protected] Keywords: PIV processing, POD analysis, multi-plane PIV, vortex shedding, phase-averaging Traditional phase-averaging of PIV measurements in bluff body wakes involves obtaining a reference signal for resolving the phase variation of dominant coherent structures. This typically requires a time-resolved quantitative measurement of the flow velocity or surface pressure. Once phase variation is obtained in the measurement signal, the PIV data can be grouped according to specific phase angles and then averaged. This type of procedure provides a statistical description of the dominant structures, and has been successfully applied in several investigations (e.g., Konstantinidis et al., 2005). The present study considers flow development past a dual step cylinder, for which the dominant coherent structures in the wake are comprised of multiple dominant frequencies. One approach to investigating such flows is to obtain multiple reference signals simultaneously, with each signal containing a distinct phase reference for one of the dominant frequencies. If the phase of each dominant frequency is known for all vector fields, a conditional phase-averaging approach can be employed. Although an array of sensors may lead to reasonable results, the performance is expected to depend dramatically on the placement of the probes so as to ensure that the wake structures are being properly detected. In the present study, it is shown that no external reference signal is required for phase-averaging time- resolved planar PIV data, as the temporal coefficients obtained from Proper Orthogonal Decomposition (POD) of the PIV data can be used instead (van Oudheusden et al., 2005). Uniform flow past a dual step cylinder for D/d = 2, L/D = 1, and ReD = 2100 was investigated in a water flume facility at the University of Waterloo. Two-dimensional velocity measurements were completed using a LaVision PIV system comprised of a Photonics Nd-YLF pulsed laser and two Photron SA4 high speed cameras. The PIV system was arranged in order to acquire measurements in vertical (x-z) and horizontal (x-y) planes simultaneously (Fig. 1). Flow visualization results described in the full paper show that vortex shedding occurs from the small diameter and large diameter cylinders at different frequencies. Hence, the vortices continuously move in and out of phase, resulting in a complex arrangement of vortex interactions that is presumably dependent on their relative phase alignment. Using PIV data, it is possible to perform conditional phase averaging to reconstruct wake topologies observed through various relative phase alignments of vortices in the large and small cylinder wakes. Figure 2 shows two three-dimensional reconstructions of the vorticity field in the wake of a dual step cylinder. The results in Fig. 2 illustrate the dominant periodic vortex interactions occurring between the large and small cylinder vortices in the near wake. On the average, small cylinder vortices form vortex connections with large cylinder vortices in the near wake, though the arrangement of these connections is dependent upon their relative phase alignment (Fig. 2). Farther downstream, vortex connections between large and small cylinder vortices are maintained when these vortices are aligned in phase (e.g., Fig. 2c). However, when the phase separation becomes significant, consecutive small cylinder vortices cannot maintain direct connections with large cylinder vortices and form half-loop connections instead. The POD-based methodology for reconstructing three- dimensional wake topology from planar velocity measurements is a technique applicable primarily to strongly periodic flows which can consist of more than one dominant frequency. It relies on time-resolved planar velocity measurements in multiple two-dimensional planes and velocity measurements in an orthogonal plane, which are not required to be simultaneous. This technique is a significant improvement over traditional phase averaging methods and can be used as a statistical tool for the analysis of vortex dynamics in turbulent flows. References Konstantinidis E, Balabani S, Yianneskis M (2005) “Conditional averaging of PIV plane wake data using a cross-correlation approach,” Experiments in Fluids 39: 38-47. van Oudheusden BW, Scarano F, van Hinsberg NP, Watt DW (2005) “Phase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence” Experiments in Fluids 39:86-98. Fig. 1 PIV measurement setup. Measurements are conducted simultaneously in the x-z plane at y/D = -0.5 and x-y plane, with 18 x-y planes investigated. Velocity fields are shown in place of the raw particle image pairs. (a) (b) Fig. 2 Reconstructed three-dimensional wake topology of a dual step cylinder for a large cylinder phase of zero degrees and a small cylinder phase of: (a) 180°, (b) 290°.
Transcript of Analyzing three-dimensional wake vortex dynamics...
17th International Symposium on Application of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 07 – 10, 2014
2.10.6
Analyzing three-dimensional wake vortex dynamics using time-resolved planar PIV
C. Morton1,*, S. Yarusevych1
1: Dept. of Mechanical and Mechatronics Engineering, University of Waterloo, Canada * Correspondent author: [email protected]
Keywords: PIV processing, POD analysis, multi-plane PIV, vortex shedding, phase-averaging
Traditional phase-averaging of PIV measurements in bluff
body wakes involves obtaining a reference signal for resolving the phase variation of dominant coherent structures. This typically requires a time-resolved quantitative measurement of the flow velocity or surface pressure. Once phase variation is obtained in the measurement signal, the PIV data can be grouped according to specific phase angles and then averaged. This type of procedure provides a statistical description of the dominant structures, and has been successfully applied in several investigations (e.g., Konstantinidis et al., 2005).
The present study considers flow development past a dual step cylinder, for which the dominant coherent structures in the wake are comprised of multiple dominant frequencies. One approach to investigating such flows is to obtain multiple reference signals simultaneously, with each signal containing a distinct phase reference for one of the dominant frequencies. If the phase of each dominant frequency is known for all vector fields, a conditional phase-averaging approach can be employed. Although an array of sensors may lead to reasonable results, the performance is expected to depend dramatically on the placement of the probes so as to ensure that the wake structures are being properly detected. In the present study, it is shown that no external reference signal is required for phase-averaging time-resolved planar PIV data, as the temporal coefficients obtained from Proper Orthogonal Decomposition (POD) of the PIV data can be used instead (van Oudheusden et al., 2005).
Uniform flow past a dual step cylinder for D/d = 2, L/D = 1, and ReD = 2100 was investigated in a water flume facility at the University of Waterloo. Two-dimensional velocity measurements were completed using a LaVision PIV system comprised of a Photonics Nd-YLF pulsed laser and two Photron SA4 high speed cameras. The PIV system was arranged in order to acquire measurements in vertical (x-z) and horizontal (x-y) planes simultaneously (Fig. 1).
Flow visualization results described in the full paper show that vortex shedding occurs from the small diameter and large diameter cylinders at different frequencies. Hence, the vortices continuously move in and out of phase, resulting in a complex arrangement of vortex interactions that is presumably dependent on their relative phase alignment. Using PIV data, it is possible to perform conditional phase averaging to reconstruct wake topologies observed through various relative phase alignments of vortices in the large and small cylinder wakes.
Figure 2 shows two three-dimensional reconstructions of the vorticity field in the wake of a dual step cylinder. The results in Fig. 2 illustrate the dominant periodic vortex interactions occurring between the large and small cylinder vortices in the near wake. On the average, small cylinder vortices form vortex connections with large cylinder vortices in the near wake, though the arrangement of these connections is dependent upon their relative phase alignment (Fig. 2). Farther downstream, vortex connections between large and small cylinder vortices are maintained when these vortices are aligned in phase (e.g., Fig. 2c). However, when the phase separation becomes significant, consecutive small cylinder vortices cannot maintain direct connections with large cylinder vortices and form half-loop connections instead.
The POD-based methodology for reconstructing three-
dimensional wake topology from planar velocity measurements is a technique applicable primarily to strongly periodic flows which can consist of more than one dominant frequency. It relies on time-resolved planar velocity measurements in multiple two-dimensional planes and velocity measurements in an orthogonal plane, which are not required to be simultaneous. This technique is a significant improvement over traditional phase averaging methods and can be used as a statistical tool for the analysis of vortex dynamics in turbulent flows.
References
Konstantinidis E, Balabani S, Yianneskis M (2005) “Conditional averaging of PIV plane wake data using a cross-correlation approach,” Experiments in Fluids 39: 38-47. van Oudheusden BW, Scarano F, van Hinsberg NP, Watt DW (2005) “Phase-resolved characterization of vortex shedding in the near wake of a square-section cylinder at incidence” Experiments in Fluids 39:86-98.
Fig. 1 PIV measurement setup. Measurements are conducted simultaneously in the x-z plane at y/D = -0.5 and x-y plane, with 18 x-y planes investigated. Velocity fields are shown in place of the raw particle image pairs.
(a) (b) Fig. 2 Reconstructed three-dimensional wake topology of a dual step cylinder for a large cylinder phase of zero degrees and a small cylinder phase of: (a) 180°, (b) 290°.
