Effects of hot-wire length on the measurement of turbulent spectra in anisotropic flows

Fluid velocity measurements obtained using hot-wire anemometry are subject to spatial filtering due to the finite length of the heated sensing element. This effect can lead to a negative bias error in the magnitude of the velocity fluctuations in turbulent flows when the smallest length scales of the flow are comparable to the wire length. Here an analytical model is used to provide a quantitative description of these errors in anisotropic flows. The model is then applied to a high Reynolds number pipe flow in which two hot-wire probes of different lengths were used to measure velocity spectra at three locations in the boundary layer. These data were used to estimate the anisotropy of the turbulence as a function of frequency (or streamwise wavenumber) and wall-normal distance.