Hot-wire anemometry
Hot wire anemometry is a well established technique in which the speed of an air flow is measured by sensing the rate of cooling of fine, electrically heated wires. Most of the velocity and turbulence measurements we make are made using sub-miniature 4-sensor hot-wire probes. These consist of four 5-micron diameter tungsten wires held on eight independent prongs within a 0.5mm3 measurement volume.
The sensors are arranged as in a Kovaznay vorticity probe - but here are used to sense only the 3 instantaneous components of velocity. Ironically we use the approximate vorticity sensing capability of the probe to eliminate vorticity related gradient errors from our velocity measurements, which can be substantial with more conventional multiple hot-wire probes.
The hot-wires supported on a fully computer-controlled traverse gear. Measurements are made by reading the 4 simultaneous unlinearized voltages output by the sensors using 4 buck and gain amplifiers and a 400kHz simultaneous sample and hold A/D converter. Measurements are linearized and processed on line using array processors to produce near-instantaneous results. Every A/D conversion is recorded on optical discs so that the experiment may be re-run electronically and a wide variety of analysis performed. The hot-wires are directly calibrated for angle response and we have the capability of directly calibrating frequency response too, using a pulsed laser to provide impulsive heat inputs to the sensors. We repair and are starting to manufacture our own hot-wire probes.
Because we have computer controlled traversing and measurements detailed measurements can be made in relatively short times. The vortex icon appearing along with the title at the head of this page was measured by Joe Miranda in our 3'x2'x20' subsonic tunnel using the hot-wire system. The icon only shows the turbulence kinetic energy field behind a lifting rectangular wing but all mean velocities, turbulence stresses and triple products were measured, and some 3-component spectra. The total measurement time was about 3 hours.
Diode array velocimetry
Diode array velocimetry (or DAV) is a new laser-based velocity measurement technique developed in previous years by our research group. This technique, currently in the patenting process, works by timing the passage of seed particles as the pass through a small portion of a laser beam. This is done by tracking images of the particles as the pass across a segmented photodiode array. The array, and the receiving angle and form of the laser beam are designed to make the determination of velocity components simple and accurate. This technique has advantages over laser Doppler anemometry since it requires only a single beam, regardless of the number of components measured and produces signals (electrical pulses indicating transit time) that are much easier and cheaper to process the frequency-modulated bursts. We have demonstrated this new technology by building a DAV for single-component velocity measurements and using it for accurate velocity and turbulence measurements in attached and separated flows. This technique and are results are detailed in - Devenport W J and Smith E J, 1994, "The Diode-Array Velocimeter", Journal of Fluid Mechanics, vol. 259, pp 167-189.
Particle Image Velocimetry
We are currently acquiring the hardware needed for a very high speed quantitative flow visualization system. The heart of the system is a 45W copper-vapor laser and a 1000 frame per second digital video camera, interfaced to a high-speed computer. Initially this system will be set up in the towing tank facility, the first objective being to obtain instantaneous flowfield measurements of a tip vortex in a convected frame of reference. Initially we intend to develop this hardware into a particle image velocimeter, in which the paths of seed particles introduced into the water flow are individually tracked to reveal the evolution of the instantaneous flow field. However, the hardware could be reconfigured as a scalar image velocimeter. This device measures the complete instantaneous flow field over a continuous volume by using a scanning laser and camera to measure the concentration distribution of dye introduced to the flow.
