SAFL High Speed Water Tunnel
Description of Water Tunnel and Experimental Equipment
Figure 1: SAFL High Speed Water Tunnel
The high-speed water tunnel at St. Anthony Falls Laboratory (SAFL) is a recirculating, closed-jet facility with absolute pressure regulation. It is capable of velocities in excess of 20 m/s. The test section measures 0.19 m (W) x 0.19 m (H) x 1.0 m (L). It is instrumented with pressure taps and hydrophones and provides optical access from 3 sides. Test section windows can be fitted with interchangeable mounting plugs to install instrumented hydrofoils, lift and drag force balances and scale models. The water tunnel has recently undergone renovations that included replacement of the old 150HP Direct Current Motor with a modern 75HP Alternating Current Motor. Available experimental tools include digital strobe photography, digital video, high resolution Stereo Time-Resolved Particle Image Velocimetry (TR-PIV), high resolution Time-Resolved Particle Shadow Velocimetry (TR-PSV), 2-component Laser Doppler Velocimetry (LDV) with automated traversing system and Phase Doppler Anemometry (PDA) for particle/bubble sizing.
The water tunnel is specially designed to remove large quantities of air that may be injected in the test section during experiments. Of particular interest are the modifications made for hydroacoustic studies (Killen, 1981). In its current configuration the tunnel features a closed test section with a thin Mylar roof between the test section and a tank containing hydrophones. The measured uniformity of the flow is better than 1% and the turbulence level is found to be approximately 0.3%.
A variety of cavitation research is currently being performed in this facility including ONR-supported work on the stability and control of high speed cavity running bodies, a new low drag partially cavitating hydrofoil supported by DARPA and an integrated numerical/experimental study of cavitation induced flow instabilities funded by NSF. In the latter study, unsteady lift and drag is measured using a specially designed force balance. Hydrofoils can be highly instrumented with an array of static pressure ports and an array of piezoelectric film transducers (Arndt et al, 1995, 1997), quartz crystal transducers, and miniature accelerometers. The pressure transducers were manufactured from circular sections of polyvinylidine fluoride film (PVDF). Simultaneous sampling of the instantaneous distribution of pressure along the chord length of the foil is made possible with the array of pressure sensors. Photographic observations can be made utilizing a 35 mm camera and strobe lighting triggered by specially conditioned pressure signals measured on the foil. By capturing a sequence of photos at different time delays, equivalent framing rates as high as 100,000 per second can be achieved. In this manner the instantaneous pressure distribution could be correlated with the extent of sheet cloud cavitation occurring at any given moment. The SAFL facility is also equipped for acoustic studies, using an array of Bruel and Kjaer Type 8103 miniature hydrophones mounted in the dead water space above the test section.
Figure 2: Typical setup for hydrofoil studies
Also available to the project is a TSI Stereoscopic PIV System, which is capable of measuring all three components of the velocity vector in a plane in a single-phase flow (the vorticity component normal to the plane can also be measured). Alternatively, the system can be set up as a 2-camera, filtered PIV system to obtain the velocity fields of both the gas and liquid phases in a two-phase flow. For this purpose the liquid phase is seeded using fluorescent particles, so that the light scattered by the gas phase (same wavelength as that of the incident light of the pulsed YAG laser) and liquid phase (Laser Induced fluorescence) will be at different wavelengths. PIV imaging of the flow field is then accomplished with optical filters. The camera with a filter corresponding to the wavelength of the laser will be used to capture images of the bubbles, while the camera with the filter corresponding to the wavelength of the fluoresced light will be used to capture images of the liquid phase.
The PIV system comprises two high resolution (4 megapixel CCD), 12-bit dynamic range TSI PowerView 4M cameras with an assortment of fixed focal length (28mm, 60mm, 105mm) NIKKOR lenses, Mini-Dual YAG lasers (120mJ/pulse), Synchronizer, Light Arm and INSIGHT 6.1 Data Acquisition, Analysis and Display Software. The SAFL PIV system is capable of what TSI refers to as “UltraPIV”, where the spatial resolution can be increased by an order of magnitude or higher while maintaining measurement accuracy. This improvement is achieved exclusively on the software side by three distinct steps: conditioning of the raw data (image fields) to be optimal for the used algorithm, a Hart cross-correlation algorithm, and progressive optimization with double correlation validation. Progressive optimization enables the software to extract accurate velocity measurements from small interrogation regions. UltraPIV represents the most detailed, most accurate velocity measurements possible today. In addition, SAFL has all the necessary components to employ the SAFL PIV system as a MicroPIV system. The key feature of this micro PIV system is an incident-light optical set-up with a pair of Mitutoyo Infinity-Corrected Long Working Distance Objectives (20x magnification, Plano Apochromatic objective, 95mm parfocalization length, tube lens focal length of 200mm). With this optical setup, digital image spatial resolutions on the order of 1 µm/pixel can be achieved. For MicroPIV red fluorescing polymer microspheres with 1.0 µm diameter are used as tracer particles (Duke Scientific Corp., R0100, particles are tagged with a fluorescent dye).
The PIV, LDV and PDA systems can be used with the water and wind tunnels available at SAFL, or with specially designed desktop experiments. Versatile structural aluminum framing is available to quickly assemble custom experimental setups supporting experiments and equipment. Workstation computers equipped with National Instruments E-Series Multifunctional DAQ-card and LabView data acquisition software are used for data recording and processing in both facilities, which allows for compatibility of the data sets between SAFL and other laboratories. An added benefit is the ability to carry out on-line comparisons while experiments are progressing in different laboratories simultaneously.