In our laboratories, a variety of flow facilities and instrumentation are employed to study complex flows and induced forces, with application to blading and turbomachinery, tubes in fossil and nuclear energy conversion systems, cavities in propulsion systems, cables and risers in offshore ocean environments. The common theme, from a practical standpoint, is to understand, predict, and control flow-induced vibration, noise generation, mixing and heat transfer processes. In the following, we describe details of our systems and techniques.
Flow Facilities. Two large-scale water channels, specially designed for three-dimensional illumination and image acquisition, are employed for experiments involving steady inflow. A variety of test sections can be placed within the channels, in order to simulate interaction of turbulent and vortical flow fields with bodies, including cylinders, spheres, delta wings, streamlined leading-edges. In addition, fully-developed turbulent boundary layers on flat plates and turbulent flows within channels are obtainable through inserts into the main test section of the water channels.
A wave tank system, again designed specifically for three-dimensional laser illumination and image acquisition, generates waves of desired amplitude and frequency through use of a unique force feedback-controlled wave paddle (Edinburg Designs Ltd). This system generates high quality progressive waves while minimizing contamination due to reflection effects, attainable with the feedback system. Experiments in air are carried out in an open wind tunnel system, housed in the same laboratories as our water-based facilities, in order to allow direct comparison of unsteady and turbulent flow events in water and air.
Instrumentation. An extensive range of laser systems allows illumination of complex flows for the technique for high-image-density particle image velocimetry. Two Yag lasers (135 mJ), two high-powered Argon-ion lasers (25 watts), five medium-powered Argon- ion lasers (3 to 5 watts) and two low-powered helium-neon lasers are employed in our experimental systems. This laser-based instrumentation is coupled with one of six different laser scanning units, employing multi-faceted rotating mirrors or oscillation galvanometer-driven mirrors, in order to provide a laser sheet for two- and three- dimensional scanning. These systems include custom-designed optical trains, in addition to the rotating and the oscillating mirror components.
Surface pressure fluctuations are measured using one of a large number of piezo-based, high- sensitivity pressure transducer systems, with attendant amplifiers and filters. Mobile data acquisition work stations are employed at four different locations in our laboratories in order to acquire pressure, force and velocity data. [Force measurements are undertaken using one of four different types of force stings, which are designed to provide high signal to noise ratio output for low-level forces. Continuous records of unsteady velocity fluctuations are acquired using hot film/wire and LDA systems.]
Control and Forcing Systems. A total of eight computer-controlled high resolution stepping motor systems are employed with appropriate indexers, in conjunction with a central laboratory microcomputer, in order to impart controlled motion to cylinders, spheres, wings, and other configurations. These control systems allow synchronization of the body motion with acquisition of flow images, unsteady forces and pressures.
Image Acquisition Units. For the technique of high-image-density particle image velocimetry, as well as for qualitative flow visualization methods, including the hydrogen bubble technique, and dye and smoke injection, a variety of camera and lens systems are available. A total of eight motor-driven, 35 mm Nikon and Canon cameras, a Hulcher cinema framing camera, a Kodak ES-1 high resolution video camera, a Kodak Megaplus camera and two black and white video cameras are employed in our investigations. Lens systems provide the possibility of relatively high magnification. A total of four image shifting mirror systems are employed to preclude directional ambiguity during acquisition of particle image patterns.
Image Processing Systems. Two Nikon digitizing systems are employed for digitizing patterns of particle images from 35 mm film. In addition, a Leaf system allows high resolution digitizing of medium and large format film. These digitizing systems are linked to two special-purpose work stations. In addition, a total of nine workstations are employed for extensive types of image post-processing in our laboratories. Such post- processing includes not only the evaluation of patterns of vorticity and streamline topology, but also image enhancement and transformation, pattern recognition, and other state-of-the-art image processing techniques. In addition, these systems, which are housed in our laboratories, are linked via a local area network to a Titan mini- supercomputer, and a total of fifteen Silicon Graphics and IBM Workstations at other locations in the Department of Mechanical Engineering and Mechanics.