Averaged Bin 4 Vorticity Averaged Bin 5 Vorticity 6...
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Experimental Analysis of Coherent Structures in the Unsteady Wake of a Circular Cylinder Jacob J. Morrida & Melissa A. Green, Syracuse University, Syracuse NY,13244, USA Introduction: Controlling or preventing the vortex shedding phenomenon that occurs in the wake of a cylinder placed in a cross flow is of common interest in several areas of fluid dynamics. Experiments were done to measure the vortex shedding behind a circular cylinder without flow control using particle image velocimetry (PIV) equipment. The purpose of this experiment was to obtain a basis with which to compare with flow control experiments and to develop analysis methods for temporally resolved and under-resolved data. Experiment/Method Overview: •Velocity field data in the wake of a circular cylinder at Reynolds number of 550 was collected using PIV. •150 snapshots were taken at a rate of 4 Hz •The data was phase-averaged by sorting the images into bins based on the location of maximum vorticity. This was done to give a clear picture of the procession of the vorticies after being shed. Strouhal number Our shedding frequency, determined from phase-averaging analysis, was 0.571 Hz, yielding St=0.212. Published St for a cylinder at Re=550 is approximately 0.21 (Vortex Dynamics in the Cylinder Wake. C H K Williamson, 1996) Particle Imaging Velocimetry (PIV): The Dantec Dynamics PIV system used for this experiment is capable of 3D stereo experiments, implementing 2 cameras. However, the system was only used to gather 2D velocity data with the use of one camera. Motivation: Vortex shedding is a phenomenon that leads to several aerodynamic issues, including unwanted vibrations that lead to noise and structural fatigue, as well as increased drag. If shedding can be reduced or eliminated, it will lead to less maintenance, reduced noise, and increased efficiency. Results: •Phase-averaged plots depict the vorticity moving downstream by the length of one bin between each image. However, better time resolution would yield more accurate results. •Best results were achieved by using 7 bins at the optimal bin distribution. Using more bins resulted in double-binning and other errors. Future work will approach this problem by both varying experiment parameters and refining the phase-averaging software. Karman Vortex Sheet, “An Album of Fluid Motion”, Van Dyke, 1982 Laser Power Supply Laser Camera Test Section Future Work: •Repeat experiments at lower Reynolds number for better time resolution. •Refine phase-averaging analysis to increase time resolution of post-processing technique •Compare experimental results with numerical simulations. •Use Eulerian and Lagrangian post-processing analysis methods for investigation of wake structures Binning from 20≤x≤90 mm Optimal Binning from 45≤x≤99 mm Examples of Instantaneous Vorticity Fields and Binning Windows •In the image on the left, there are 2 positive vortices within the bin range. The spread of the bins is too large, and the first bin is too close to the cylinder so that it includes structures that have not shed yet. •The image on the right depicts the optimal bin locations, which do not pick up unshed vortices, and have a small enough spread so that there is only one positive vortex within the bin range at a time. Choosing Bin Locations: First and last bins too far apart: more than one vortex of the same sign will be binned in the same image. First bin at x location too far upstream: the first bin will pick up vorticity from structures still attached, and will bin those rather than the vortex of the same sign that has already shed. Last bin at x location too far downstream: vorticies disperse and weaken before they reach the last bin. Vorticity in immersed boundary simulation, Re=100 (From Munday & Taira, FSU) Q criterion and LCS of immersed boundary simulations, Re=100 -20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 x [mm] y [mm] -8 -6 -4 -2 0 2 4 6 8 x 10 -3 x [mm] y [mm] Averaged Bin 6 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 5 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 4 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 3 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 2 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 1 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 x [mm] y [mm] Averaged Bin 7 Vorticity -20 0 20 40 60 80 100 120 20 40 60 80 100 120 -4 -3 -2 -1 0 1 2 3 4 x 10 -3 -20 0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 x [mm] y [mm] -8 -6 -4 -2 0 2 4 6 8 x 10 -3
Transcript of Averaged Bin 4 Vorticity Averaged Bin 5 Vorticity 6...
Experimental Analysis of Coherent Structures in the Unsteady Wake of a Circular Cylinder Jacob J. Morrida & Melissa A. Green, Syracuse University, Syracuse NY,13244, USA
Introduction: Controlling or preventing the vortex shedding phenomenon that occurs in the wake of a cylinder placed in a cross flow is
of common interest in several areas of fluid dynamics. Experiments were done to measure the vortex shedding behind a circular cylinder without flow control using particle image velocimetry (PIV) equipment. The purpose of this experiment was to obtain a basis with which
to compare with flow control experiments and to develop analysis methods for temporally resolved and under-resolved data.
Experiment/Method Overview: •Velocity field data in the wake of a circular cylinder at Reynolds number of 550 was collected using PIV. •150 snapshots were taken at a rate of 4 Hz •The data was phase-averaged by sorting the images into bins based on the location of maximum vorticity. This was done to give a clear picture of the procession of the vorticies after being shed.
Strouhal number Our shedding frequency, determined from phase-averaging analysis, was 0.571 Hz, yielding St=0.212. Published St for a cylinder at Re=550 is approximately 0.21 (Vortex Dynamics in the Cylinder Wake. C H K Williamson, 1996)
Particle Imaging Velocimetry (PIV): The Dantec Dynamics PIV system used for this experiment is capable of 3D stereo experiments, implementing 2 cameras. However, the system was only used to gather 2D velocity data with the use of one camera.
Motivation: Vortex shedding is a phenomenon that leads to several aerodynamic issues, including unwanted vibrations that lead to noise and structural fatigue, as well as increased drag. If shedding can be reduced or eliminated, it will lead to less maintenance, reduced noise, and increased efficiency.
Results: •Phase-averaged plots depict the vorticity moving downstream by the length of one bin between each image. However, better time resolution would yield more accurate results. •Best results were achieved by using 7 bins at the optimal bin distribution. Using more bins resulted in double-binning and other errors. Future work will approach this problem by both varying experiment parameters and refining the phase-averaging software.
Karman Vortex Sheet, “An Album of Fluid Motion”, Van Dyke,
1982
Laser Power Supply Laser Camera Test Section
Future Work: •Repeat experiments at lower Reynolds number for better time resolution. •Refine phase-averaging analysis to increase time resolution of post-processing technique •Compare experimental results with numerical simulations. •Use Eulerian and Lagrangian post-processing analysis methods for investigation of wake structures
Binning from 20≤x≤90 mm Optimal Binning from 45≤x≤99 mm
Examples of Instantaneous Vorticity Fields and Binning Windows
•In the image on the left, there are 2 positive vortices within the bin range. The spread of the bins is too large, and the first bin is too close to the cylinder so that it includes structures that have not shed yet. •The image on the right depicts the optimal bin locations, which do not pick up unshed vortices, and have a small enough spread so that there is only one positive vortex within the bin range at a time.
Choosing Bin Locations: First and last bins too far apart: more than one vortex of the same sign will be binned in the same image. First bin at x location too far upstream: the first bin will pick up vorticity from structures still attached, and will bin those rather than the vortex of the same sign that has already shed. Last bin at x location too far downstream: vorticies disperse and weaken before they reach the last bin.
Vorticity in immersed boundary simulation, Re=100 (From Munday & Taira, FSU)
Q criterion and LCS of immersed boundary simulations, Re=100
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