A non-intrusive technique capable of measuring the instantaneous spatial pressure, velocity and material acceleration distributions simultaneouly over a sample area in a turbulent flow field has been developed at the Laboratory for Experimental Fluid Dynamics at the Johns Hopkins University. A four-exposure PIV system is used for measuring the distribution of material acceleration by comparing the velocity of the same group of particles at different time, and then integrating it to obtain the pressure distribution. Two 2K´2K CCD cameras and perpendicularly polarized Nd:Yag lasers are used for recording four exposures, with images 1 and 3 recorded by camera 1 giving the first velocity vector map and images 2 and 4 recorded by camera 2 giving the subsequent second velocity vector map. Exposing both cameras to the same particle field at the same time and cross-correlating the images enables precision matching of the two fields of view. Application of local image deformation correction to velocity vectors measured by the two cameras reduces the error due to relative misalignment and image distortion to about 0.01 pixels in synthetic images. An omni-directional virtual boundary integration scheme is introduced to integrate the acceleration while minimizing the effect of the local random errors in acceleration. Further improvements are achieved by iterations to correct the pressure along the boundary. Typically 3 to 5 iterations are sufficient for reducing the incremental mean pressure change in each iteration to less than 0.1% of the dynamic pressure. Validation tests of the principles of the technique using synthetic images of rotating and stagnation point flows show that the standard deviation of the measured pressure from the exact value is about 1.0%. This system is used to measure the instantaneous pressure, velocity and acceleration distributions of a 2D open cavity flow field in a water tunnel. Since cavitation is an effective method for detecting pressure peaks, we use cavitation as a method of "validating" the pressure measurements, especially where there is no other available technique to do so, e.g. in turbulent shear flows and near corners. Principles and procedures of the pressure measurement technique, the experimental setup and the pressure-related turbulence statistical data, togehter with the data on occurrence of caitation in the 2D open caity flow, can be found on this website.

(This research project is funded by ONR)

Journal Papers:

Liu, X. and Katz, J., Cavitation phenomena occurring due to interaction of shear layer vortices with the trailing corner of a 2D open cavity, Physics of Fluids, 20, 041702, DOI:10.1063/1.2897320. (2008). (download a pdf version)

Liu, X. and Katz, J., Instantaneous pressure and material acceleration measurements using a flour-exposure PIV system, Experiments in Fluids, 41, 227-240. (2006). (download a pdf version)

Conference Papers and Presentations:

Liu, X. and Katz, J., Measurement of Pressure-Rate-of-Strain, Pressure Diffusion and Velocity-Pressure-Gradient Tensors in a Cavity Shear Layer by Integrating the Material Acceleration, Bulletin of the American Physical Society, Vol. , No. . p . (2007).

Liu, X. and Katz, J., A Comparison of cavitation inception index measurements to the spatial pressure distribution within a 2D cavity shear flow, FEDSM2007-37090, 5th Joint ASME/JSME Fluids Engineering Conference, San Diego, California, July 30-August 2. (2007). (download a pdf version)

Liu, X. and Katz, J., Measurement of pressure fluctuation and pressure-velocity correlations in a cavity shear layer by integrating the material acceleration, Bulletin of the American Physical Society, Vol. 51, No. 9. p95. (2006).

Liu, X. and Katz, J., Development of a PIV based technique for measurements of instantaneous pressure distributions, Bulletin of the American Physical Society, Vol. 50, No. 9, p219. (2005).

Liu, X. and Katz, J., Instantaneous pressure and material acceleration measurements using a four exposure PIV system, 6th international symposium on particle image velocimetry, Pasadena, California, Sept. 21-23. (2005). (download a pdf version)

Liu, X., Instantaneous pressure distribution measurements by integrating material accelerations, American Institute of Aeronautics and Astronautics Region I Technical Mini-Conference, Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, October 30. (2004).

Liu, X. and Katz, J., Measurements of the pressure distribution in a cavity flow by integrating the material acceleration, HT-FED2004-56373, 2004 ASME Heat Transfer/Fluids Engineering Summer Conference, Charlotte, North Carolina, USA, July 11-15.(2004). (download a pdf version)

Liu, X., Instantaneous pressure distribution measurements by integrating material accelerations, 2004 Research Symposium on Environmental and Applied Fluid Mechanics, Center for Environmental and Applied Fluid Mechanics, Johns Hopkins University, May 18. (2004).

Liu, X. and Katz, J., Measurements of the instantaneous pressure distribution by integrating the material acceleration, Bulletin of the American Physical Society, Vol. 48, No. 10, p96. (2003).

Liu, X. and Katz, J., Measurements of pressure distribution by integrating the material acceleration, Cav03-GS-14-001, Proc. of the 5th International Symposium on Cavitation (Cav2003), Osaka, Japan, Nov. 1-4. (2003).

Liu, X., Gopalan, S. and Katz, J., Development and application of experimental tools for measuring the flow and unsteady pressure fields in cavitating flows", Workshop on Current Issues in Cavitation Research, the Center for Environmental and Applied Fluid Mechanics, Johns Hopkins University, January 20-21. (2003).

Liu, X., Katz, J., and Gopalan, S., Measurements of pressure distribution by integrating the material acceleration, Bulletin of the American Physical Society, Vol. 47, No. 10, 2002, p197 (the 55th Annual Meeting of the Division of Fluid Dynamics of the American Physical Society, November 24-26, 2002, Dallas, Texas). (2002).

Last updated on 01/20/2008