Since this will strongly depend on the thermodynamic conditions at the wire location, the adjustment is more difficult to perform in supersonic flows. (2005) have shown that in supersonic boundary layers, both CTA and CVA can have bandwidths well suited for moderate supersonic regimes, with typical values going up to 300–500 kHz.įor each operating mode, it is necessary to measure and adjust the time constant of the anemometer. Kegerise and Spina (2000) and Weiss et al. It should be emphasized, however, that the signal-to-noise ratio is an important issue that has to be addressed whatever the electronic circuit (operating mode). This is a very promising method with many advantages related to bandwidth and ease of use.Īlthough the CCA and CTA methods are complementary, it appears that the CTA is probably the simpler to use ( Bestion, Gaviglio, and Bonnet, 1983).
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The electronics are optimally suited for obtaining the necessary measurement bandwidths. This method has been only recently introduced by Sarma and Comte-Bellot ( Sarma, 1998 Comte-Bellot and Sarma, 2001 Sarma, 1998 Comte-Bellot and Sarma, 2001). The voltage of the wire is kept constant and the anemometer operates as a constant voltage anemometer (CVA). The bandwidth of this apparatus is better adapted to measurements when the overheat ratio is high. Appropriate closed loop electronics with wide bandwidths and signal-to-noise ratio have to be used. The temperature of the wire is maintained constant by a closed loop control and the anemometer operates as a constant temperature anemometer (CTA). This is performed by appropriate electronic and/or numerical amplification of a high-pass filtered signal. Since the natural frequency of the wire is quite low, the output signal needs to be amplified and carefully post-processed to enhance the low bandwidth. The current level sets the temperature of the wire in the flow, and this temperature difference is related to the overheat ratio, which is a key parameter for the resolution of the operating mode. The current passing through the wire is maintained constant for the constant current anemometry (CCA). Three operating modes are possible that can affect the characteristics of the sensor: The time constant of the heat transfer of HWA depends on several parameters: the wire (sensor) temperature, the aerothermodynamic conditions, and the operating electronics. For supersonic flows, the prongs have to be designed for minimizing parasitic effects of shock waves and, as described in Section 4.1.2, the bandwidth of the system has to be particularly high when compared to low speed flows. As is well known, a sensor composed of a thin wire (typically 2.5–5 μm in diameter and 0.5–1 mm long) is soldered or welded between prongs and placed in the flow.
The principle behind the hot-wire anemometer can be simply described.
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