Long hot wires and the measurement of large scale turbulent fluctuations

An analysis is presented which describes the use of long sensing length hot wire anemometers for the reduction of spatial aliasing. The reduction of spatial aliasing benefits those studying large scale structures in turbulent velocity fields because the aliasing places unwanted spectral energy into the low wave numbers (which are generally characteristic of large scale motion) and thus contaminates the structures. The long wires reduce spatial aliasing by attenuating the energy in the high wave numbers thereby limiting the amount of energy which can be aliased. A numerical study of the long hot wire shows that the finite length of the hot wire reduces the amount of energy in the high wave number region of the three dimensional isotropic spectrum. The reduction of energy in the spectrum at the high wave numbers is dependent on probe length and orientation, with only the direction in which the probe is oriented being affected. As a result of the three dimensional spectrum being attenuated, the one dimensional spectrum is also attenuated, but in this case the attenuation is over all wave numbers. The largest reduction of spectral energy for the one dimensional spectrum occurs in the high wave numbers. To supplement the numerical study, an experimental investigation was also performed. The results of the experimental study are consistent with the numerical findings for the one dimensional spectrum. Energy is reduced in all wave numbers (frequency), and the largest reduction in energy occurs in the high wave numbers (frequency).