Photon Doppler velocimetry for resolving vertical penetration into sand targets

This paper presents details for using a photon Doppler velocimeter (PDV) to produce high fidelity velocity penetration data from vertical penetration into granular soil targets. The PDV theory used for collecting data is presented, along with details relevant to soil penetration testing. Experimental aspects related to specific equipment used, measurement configurations implemented, power losses, and probe alignment are presented in order to facilitate replication. The PDV equipment were mounted on a vertical gas gun and used to measure conical and blunt nose rod projectile velocities in penetration into loosely packed and densely packed soil targets. An in-depth discussion of data smoothing and filtering, data sampling and repeatability is presented. Penetration tests in several soil targets are used to reveal important phenomenology of penetration in granular media. The roles of soil target bulk density and pore saturation are investigated in terms of Poncelet drag and bearing resistance parameters. It is shown that bulk density plays a major role in penetration resistance while saturation has a secondary effect. The Poncelet drag coefficient was found to be projectile nose shape dependent. The drag coefficient was as high as 2.0 in penetration of blunt nose projectiles in densely packed sand, while a conical nose reduced the drag coefficient to 1.4. Saturation reduced the drag coefficient by 32 % and 28 % in densely packed and loosely packed soil targets, respectively.