An adaptive response compensation technique has been proposed to compensate for the response lag of the con stant-current hot-wire anemometer (CCA) by taking advantage of digital signal processing technology. First, we have developed a simple response compensation scheme based on a precise theoretical expression for the frequency response of the CCA (Kaifuku et al. 2010, 2011), and verified its effectiveness experimentally for hot-wires of 5 μ m, 10 μ m and 20 μ m in diameter. Then, another novel technique based on a two-sensor probe technique—originally developed for the response compensation of fine-wire thermocouples (Tagawa and Ohta 1997; Tagawa et al. 1998)—has been proposed for estimating thermal time-constants of hot-wires to realize the in-situ response compensation of the CCA. To demon strate the usefulness of the CCA, we have applied the response compensation schemes to multipoint velocity measure - ment of a turbulent wake flow formed behind a circular cylinder by using a CCA probe consisting of 16 hot-wires, which were driven simultaneously by a very simple constant-current circuit. As a result, the proposed response com pensation techniques for the CCA work quite successfully and are capable of improving the response speed of the CCA to obtain reliable measurements comparable to those by the commercially-available constant-temperature hot-wire anemometer (CTA).
In recent years, particle image velocimetry (PIV) has become one of the most popular techniques for measuring velocity fields. The hot-wire anemometry [1-5], on the other hand, has long been used mainly for measuring turbulent gaseous flows because of its simple and highlyreliable measurement systems and wide range of applicability. Thus, the hot-wire anemometry is still frequently utilized as a reliable research tool for statistical and frequency analyses of turbulent flows.
The hot-wire anemometry is generally driven at three operation modes, i.e., the constant temperature, constantcurrent and constant voltage modes. For the application of the hot-wire anemometry, the constant-temperature anemometer (CTA) is commercially available and almost always used as a standard system for driving the hot-wire, while the other two modes are rarely used, primarily due to their response lag during velocity fluctuation measurement. However, the electric circuit of the CTA is not simple, and the measurement system is fairly expensive. On the other hand, the constant-current hot-wire anemometer (CCA) can be set up with a very simple and low-cost electric circuit for heating the hot-wire. Thus, if we improve the response characteristics of the CCA with the aid of digital signal processing, the CCA will have a great advantage over the CTA and will be a promising tool for multipoint turbulence measurement.
When a hot-wire is driven at a constant electric current, the wire temperature, which corresponds to the hot-wire output, does not respond correctly to high-frequency velocity fluctuations because of the thermal inertia of the wire. Thus, the CCA output needs to be compensated adequately for the response lag to reproduce high frequency components of the measurement data. In order to investigate the response characteristics of the hot-wire, Hinze [
If the time-constant value is known in advance, we may apply the existing response compensation techniques for the first-order system [4,5] to recover the response delay in the CCA measurement. In reality, however, since the time-constant value of the CCA changes largely depending on flow velocity [
In the present study, first, we thoroughly tested the theoretical expression for the frequency response of the CCA by applying it extensively to the digital response compensation of three different hot-wire probes consisting of tungsten wires 5 μm, 10 μm and 20 μm in diameter. Secondly, we have proposed a novel approach to the response compensation of the CCA output that will work without our knowing in advance the geometrical parameters of the hot-wire probe. This approach is based on a two-sensor probe technique for compensating the response delay of fine-wire thermocouples [9,10], and enables in-situ estimation of the time-constant values of the hot-wire probe, and will realize adaptive response compensation of the CCA outputs. Specifically, in the twosensor probe technique, two hot-wires of unequal diameters—having different response speeds—are used simultaneously, and the time-constant values of the two hotwires can be obtained from the measurement data itself without carrying out any dynamic calibration of the hotwire probe.
Finally, in order to demonstrate the usefulness of the CCA, we have applied the above response compensation schemes to multipoint velocity measurement of turbulent flows e.g., [11-13]. In the present multipoint measurement, we have simultaneously used 16 hot-wires driven by the CCA to measure a turbulent wake flow formed behind a cylinder. In these verification experiments, we have measured turbulence intensities (r.m.s. values), power spectra and instantaneous signal traces of the velocity fluctuations, and have compared them with those obtained by a hot-wire probe driven by a commercially available constant-temperature hot-wire anemometer.