High speed PIV measurements in water hammer.

An experimental study addressing the challenge to measure the relaxation coefficient between vapor and liquid phase in very fast phenomena such as water hammers is presented. A titanium projectile encapsulating a sapphire tube containing water is accelerated and impacts a metal wall creating a water hammer. A custom-built compressed air system is used to accelerate the projectile to speeds up to 30 m/s and create the impact leading to the water hammer. The system guarantees repeatability of the impact speed within 0.1% to allow to perform statistics on the collected data. The sapphire tube, being transparent to green and UV light allows the deployment of non-intrusive laser-based diagnostic techniques such as Particle Image Velocimetry, Shadowgraphy, and Fiber Optic Hydrophone pressure measurements. These imaging techniques are deployed at a very high repetition rate (> 300 kHZ) to visualize the water hammer shock wave traveling through the projectile, while the fiber optic hydrophone measures the pointwise pressure with a bandwidth of 10 MHz. Simultaneous Shadowgraphy, PIV, and pressure measurements during the water hammer are presented and discussed.