Response of hot-wire anemometry to transient flow induced by weak pressure waves

When a pressure wave propagates in a stationary fluid, the fluid flow is accelerated from zero to a certain value. Although hot-wire anemometry is among the most reliable measurement techniques for small-amplitude and high-frequency fluctuations of velocity or mass flow rate (mass flux), few reports on their response to step accelerations from zero velocity exist. In this study, the response of hot-wire anemometry operated in constant temperature mode (HW-CTA) to an accelerated flow with an initial flow velocity, \(U\) $U$ , to \(U+\Delta u\) $U+\mathrm{\Delta }u$ , where \(\Delta u\) $\mathrm{\Delta }u$ is the increase in flow velocity due to a pressure wave, was investigated. In the experiments, the opening valves generated a pressure wave in a pipe with a radius of 50 mm. The velocity signals measured by HW-CTA and flush-mounted pressure transducer were compared for several values of \(U\) $U$ and \(\Delta u\) $\mathrm{\Delta }u$ . A non-negligible dead time and increased time constant were found in the signal measured by HW-CTA when compared with those measured by the pressure transducer for \(U=0\) $U=0$ , whereas a negligible delay was observed for \({{\text{Re}}}_{U}>0.2\) ${\text{Re}}_U>0.2$ \(\left(U>0.5 {\text{m}}/{\text{s}}\right)\) $\left(U,>,0.5,\text{m},/,\text{s}\right)$ . \({{\text{Re}}}_{U}\) ${\text{Re}}_U$ is the wire Reynolds number based on \(U\) $U$ and the hot-wire diameter. Finally, it was revealed that the sum of the dead time and time constant corresponded to the flow transitions from natural to forced convection.