Pulsed Jet Micropropulsion
Experimental Fluid Dynamics Laboratory
Department of Mechanical Engineering
Southern
Small flight-capable or submersible vehicles are of great technological interest because their diminutive size permits increased portability and access to otherwise inaccessible locations. Applications for such vehicles range from battlefield/urban surveillance to undersea exploration of small caverns to precise drug delivery within patients and microsurgery. The present investigation is focused on extending pulsed-jet propulsion schemes to the micropropulsion level (i.e., involving vehicles less than 1 mm in size). Pulsed jets – consisting of a series of jet pulses – are appealing for micropropulsion because the highly unsteady nature of the flow leads to relatively large thrust and potentially high efficiencies (higher than those achieved with traditional propulsion schemes at small scales). Additionally, they are relatively simple to implement mechanically and can be readily manufactured at the micro scale.
To investigate pulsed jets as candidates for micropropulsion applications, two complimentary avenues are being pursued:
1. Vortex ring formation at low Reynolds number: Each pulse of a pulsed jet engenders the formation of a vortex ring (or “smoke ring”), so studying vortex rings provides useful information about pulsed jets. The goal in studying vortex ring formation at low Reynolds number is to understand how the physical mechanisms (such as fluid entrainment and vorticity roll-up) scale as Reynolds number becomes smaller (approaching the micropropulsion regime). This information is necessary for scaling, modeling, and optimizing pulsed jets for micropropulsion applications.
2. Performance and Optimization of a Pulsed Jet Vehicle: While pulsed jets themselves are interesting, the ultimate goal is to use them to propel a vehicle. To that end, the goal of this study is to develop a pulsed jet vehicle, evaluate its performance in the micropropulsion (low Reynolds number) environment, and investigate strategies for optimizing its performance. Two approaches to performance optimization are through the jetting parameters (e.g., pulse duration and frequency) and vehicle configuration (e.g., flow intake geometry and placement). The results of the study on vortex ring formation at low Reynolds number will provide guidance for the former.
Summaries of current progress in the project areas are given below. See also the article from the SMU Research magazine.
Vortex Ring Formation at Low Reynolds Number
The vortex ring formation process is being studied using a piston-cylinder mechanism (see picture on the left below). A vortex ring is generated by translating a piston of diameter D through a length L in the cylinder, thereby ejecting a slug of fluid. Ring formation as visualized through planar laser induced fluorescence (PLIF) is shown below (see picture on the right below). PLIF measurements allow a direct investigation of the entrainment process since the dye marks fluid ejected from the cylinder while black regions indicate ambient fluid (i.e., fluid initially outside the vortex ring). Preliminary measurements indicate that the majority of fluid entrained into the ring is entrained after the piston motion has stopped, showing that the bulk of the entrainment process occurs during the impulse-preserving phase of the vortex motion. Future measurements will include digital particle image velocimetry (DPIV) measurements of the velocity and vorticity field during vortex ring formation in the Reynolds number range 1 to 1000 and 0.2 < L/D < 4.
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Piston-Cylinder Vortex Ring Generator |
Vortex Ring Formation at Re = 1000 and L/D = 2.0 (imaged using PLIF) |
Graduate student: Ali Olcay
Faculty Advisor: Paul S. Krueger
Publications:
Ali B. Olcay and Paul S. Krueger, “Ambient Fluid Entrainment by Vortex Ring Formation,” Bull. Am. Phys. Soc., vol. 49, no. 9, 57th Annual Meeting of the Division of Fluid Dynamics, November, 2004, Seattle, WA.
Performance
and Optimization of a Pulsed Jet Vehicle
[Coming soon… Watch this
space. Current efforts are focused on
the design and development of a pulsed-jet vehicle based on a
volume-displacement mechanism (similar to that used by squid) to generate the
pulsed jet. (Any pulsed jet device will
use a similar mechanism.) To simulate
micropropulsion settings, the vehicle performance will be evaluated and
optimized at Reynolds numbers between 1 and 1000.]
Acknowledgement: This material is based upon work supported by the National Science Foundation under Grant No. 0347958. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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