Time Resolved Particle Image Velocimetry Techniques with Continuous Wave Laser and their Application to Transient Flows
Item
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Title
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Time Resolved Particle Image Velocimetry Techniques with Continuous Wave Laser and their Application to Transient Flows
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Identifier
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d_2009_2013:a8ad745067a4:11160
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identifier
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11595
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Creator
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Elzawawy, Amir,
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Contributor
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Yiannis Andreopoulos
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Date
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2012
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Language
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English
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Publisher
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City University of New York.
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Subject
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Mechanical engineering | Aerospace engineering | Aerodynamics | Experimental Techniques | Flapping | Fluid Mechanics | PIV CW laser | Vortex Dynamics
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Abstract
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The demand to increase the temporal resolution of Stereo-Particle Image Velocimetry systems used in the measurement of highly unsteady flow fields is limited by the low repetition rate of the pulse lasers and cameras. The availability of high-frame-rate digital cameras and CW lasers opens new possibilities in the development of continuous PIV systems with increased temporal resolution. Time-Resolved Particle Image Velocimetry (TR-PIV) with continuous wave (CW) laser sheet technique and a high frame-rate camera is introduced here to be used in gas flows at low to moderate Reynolds numbers. This experimental technique can measure velocity of the flow in a planar field with good spatial and temporal resolution. Additional modifications led to the development of a Split view TR-PIV system capable of resolving three-component velocity fields. The optical setup consists of a single high-frame-rate camera which can accommodate two simultaneous stereo view images of the deforming fluid on its CMOS chip obtained by using four different planar mirrors, appropriately positioned. This approach offers several advantages over traditional systems with two different cameras. First, it provides identical system parameters for the two views which minimize their differences and thus facilitating robust stereo matching. Second, it reduces calibration time since only one camera is used and third its cost is substantially lower than the cost of a system with two cameras.;The TR-PIV with the CW laser technique has been evaluated in canonical turbulent boundary layer flows and the results were compared to data from the vast literature. Particular attention has been given to the performance of the system components, such as the high speed cameras, and the CW lasers. The techniques were also investigated in terms of the duration of exposure of PIV images. The effect of the duration of exposure was proven to be particularly important, and it has a negative effect for the case with higher freestream velocity of 11m/sec.;In the second part of the present work, an application of the technique on transient flow with moving boundaries was attempted. This involved the investigation of the unusually high unsteady aerodynamic forces generated on a flat panel (actuator) during its impulsive deployment against the incoming turbulent boundary layer flow. The actuator is embedded in the wall and it's fully deployed when it reaches 90 degree angle. Also, experiments at different deployment speeds (1 to 20 rad/sec) and different Reynolds numbers (23,000 and 68,200) were carried out to investigate their effects on the force generation. In particular three different sets of experiments were carried out. The first one aimed at directly measuring the aerodynamic forces with the means of a force balance. In these experiments, significantly higher aerodynamic forces are seen compared to the steady state case for the actuator at full deployment. These higher aerodynamic forces increase with the increasing values of the Strouhal number. The other two sets were carried out to understand the flow dynamics during the deployment. In these sets, flow visualization and TR-PIV experiments were carried out to investigate the flow structure around the actuator. It is has been observed that at the early stages of the deployment a tip vortex is developed. This vortex carries significant strength as seen by the vorticity magnitude during the deployment. This vortex is separated from the actuator during the early stages of the deployment. However, it stays in the vicinity of the wake of the actuator during most of the deployment period, while a large increase in its strength is observed. It is believed that this strong vorticity field of the tip vortex structure is initially generated on the surface of the actuator during the early part of deployment, while the actuator is moving with high acceleration. This vortical structure, which is not observed in the steady state case, is seen to be convected downstream by the time the actuator is fully deployed. Other vortices of weaker strength continue to shed off the tip after the full deployment of the actuator as in the case for the steady state. A strong standing horseshoe vortex is seen to be established in front of the actuator at a late stage of the deployment.;The velocity data obtained by the TR-PIV techniques is also used to analyze the aerodynamic forces based on the flow field velocity data. This analysis shows a significant contribution of force terms involved unsteady vorticity, lamb vector, and inertial motion of the actuator. Viscous terms had almost no contribution to the total aerodynamic forces in both the drag and the lift directions.
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Type
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dissertation
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Source
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2009_2013.csv
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degree
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Ph.D.
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Program
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Engineering
