Laboratory for Underwater Hydrodynamics

J. Grun,¹ T. Jones,¹ C. Manka,² and L.D. Bibee³
¹Plasma Physics Division
¹Research Support Instruments
¹Marine Geosciences Division

The LUH: The Laboratory for Underwater Hydrodynamics (LUH) is an indoor laboratory facility designed to provide a well-controlled and a well-diagnosed environment for the performance of precise, small-scale hydrodynamics experiments for the Navy (Fig. 9). In the LUH, underwater shocks and bubbles are generated by the rapid heating of a small volume of water or solid material inside a pressurized water tank by a short and powerful (5-ns duration, 500-J energy) laser pulse focused to a small spot inside the tank. Upon being heated, the small volume of water or other material turns to gas and expands, thereby forming a bubble and generating the pressure for launching a shock. Cavitation bubbles that are generated by this mechanism have kilobar pressures at centimeter distances; unlike bubbles formed by explosives, they are visually transparent, allowing diagnosis of the inside of the bubble. The laser focusing optics can be set up to create a point-shaped focus, a disk-shaped focus, or a line-shaped focus useful for simulating the behavior of different shapes of charges, including a line charge.

FIGURE 9
The NRL Laboratory for Underwater Hydrodynamics.

Besides containing water, the tank holds objects scaled to model underwater structures or surfaces of interest to the Navy. One of these objects is a large tray with a porous bottom designed to hold up to 20- cm deep sand. This water flow and sand tray are designed to model littoral water and beach conditions. De-aired, gas-saturated, or super-saturated water can be pumped through the porous bottom of the sand tray, thereby changing the air content of the sand over the full range observed in natural environments. The chamber can also be pressurized to 2 atmospheres or partially depressurized to 0.1 atmosphere to simulate conditions at various depths.

The LUH facility is equipped with a large number of state-of-the-art diagnostics, such as high-speed imaging (500 frames at 200,000 frames/second), Schlieren shadowgraphy, time-resolved interferometry, spectroscopy, and miniature fiber-optic or electric pressure gauges. Range gating is used to image through murky water.¹ Sand conditions are diagnosed with sand core samplers and geo-acoustic probe arrays capable of producing tomographic, three-dimensional images of sand/air content (Fig. 10).

In constructing the LUH facility we took advantage of previous experiments that used nonexplosive methods to generate underwater shocks or cavitation. Among them are interesting experiments that used lasers 5,000 times less energetic than ours to study shock hydrodynamics associated with ocular laser surgery.² We also considered using spark-generated shocks and bubbles,³ but chose to use a laser instead because it does not require the use of electrodes, which may interfere with bubble dynamics.

FIGURE 10
Tomographic reconstruction provides a precise threedimensional image of the air content in the sand tray. The air content in this sample varies from 1% at the bottom to 0% at the top.

Sample Results: Experiments performed thus far on the LUH include propagation of ultra-short (picosecond) laser light pulses through water, freefield shock and bubble dynamics, bubble-jet formation near a rigid boundary, shock transmission through a gas channel in water, and interaction of line-charge-like cylindrical shocks and point-chargelike spherical shocks with sand containing different amounts of air. Results of these experiments were used to help validate DYSMAS and GEMINI, the Navy's main predictive hydrodynamics and materials codes, which are run by the Naval Surface Warfare Center at Indian Head, Maryland.

Figure 11 shows an example of the experimental results that LUH produces, cavitation bubbles with no boundary present and a comparison to DYSMAS simulations. Here, a laser-generated bubble is photographed using high-speed imaging. The images (every eighth one is presented) show a thin piece of plastic being held on a stalk in water. The laser pulse heats this plastic, vaporizing it so that it expands, forming the bubble that is just beginning to appear in the first frame. The bubble in the image sequence is seen to expand, collapse, and then rebound again before the experiment is concluded. A DYSMAS simulation of the bubble formation shows an almost perfect match between the measured and calculated bubble trajectory.

FIGURE 11
Every eighth image of the evolution of a bubble formed by a laser and a comparison of experimental and computational shock pressure time history and bubble dynamics.

Acknowledgments: The experiments performed on the LUH are designed and interpreted jointly with the Naval Surface Warfare Center in Indian Head, Maryland, in particular with Drs. Alexandra Landsberg, Daniel Tam, and Gregory Harris. The authors also thank Dr. Judah Goldwasser of ONR for his encouragement and support.

[Sponsored by ONR]

References
¹ E.A. Mclean, H.R. Burris, and M.P. Strand, "Short-pulse Rangegated Optical Imaging in Turbid Water," Applied Optics 34, 4343-4351 (1995).
² W. Lauterborn and H. Bolle, "Experimental Investigations of Cavitation-bubble Collapse in the Neighborhood of a Solid Boundary," J. Fluid Mech. 72(2), 391-401 (1975).
³ G.L. Chahine, G.S. Frederick, C.J. Lambrecht, G.S. Harris, and H.U. Mair, "Spark-generated Bubbles as Laboratory-scale Models of Underwater Explosions and Their Use for Validation of Simulation Tools," Proceedings of 66th Shock and Vibration Symposium, Biloxi, MS, Vol. 2, pp. 265-276 (1995) (available from the Shock and Vibraton Analysis Center ).