Development of a "Digital Bridge" Thermal Anemometer for Turbulence Measurements

Thermal anemometry (a.k.a. hot-wire anemometry) has been a key experimental technique in fluid mechanics for many decades. Due to the small physical size and high frequency response of the sensors (resulting in excellent spatial and temporal resolution), the technique has been widely used for studies of turbulent flows. Even with the advent of nonintrusive techniques such as Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV), hot wire anemometry is uniquely capable of extremely high frequency response and fine spatial resolution measurements. ViGYAN proposes a fundamental change to the anemometer configuration, with two related aspects. First, the circuitry to power the sensor and establish its operating point is packaged immediately adjacent to the sensor, i.e. in the typical probe holder, removing the effect of the cable connecting the sensor to an external anemometer. Second, modern analog-digital conversion hardware will be employed to the maximum extent possible, potentially including directly driving the sensor. Data transmission will then be fully digital, immune to environmental variations or electrical noise. Further, direct excitation would permit the choice of operating modes (not just variation of operating point) by changes in software. The ultimate objective of the research is therefore referred to as a "Digital Bridge".

A primary objective of the Digital Bridge design is, of course, to improve the performance of thermal anemometry in demanding aerospace applications, particularly large transonic wind tunnels. Assuming success, the miniaturized, localized and substantially digitized electronics package could be used for acquisition and processing of signals from hot film arrays, often used for boundary layer studies in both wind tunnel and flight environments. Other potential research applications could include planetary atmosphere measurements. One of the key issues here has been the large number of sensors, but the digital bridge approach lends itself to effective multiplexing of a large number of sensors across a smaller number of anemometers. Such an approach would allow for the use of hot wire sensors analogous to the shift to electronically scanned pressure (ESP) transducers widely used in wind tunnels.

Looking to broader markets, the majority of air mass flow sensors for automotive applications rely on thermal anemometry in one way or another. The monitoring and control of heating, ventilation, and air conditioning (HVAC) systems is also done with thermal anemometry systems. A digital bridge approach will offer improved environmental tolerance and greater reliability for both applications, with its digital outputs being easily integrated into the overall automotive or industrial control systems used.

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