

Article
Correcting hotwire spatial resolution effects in third and fourthorder velocity moments in wallbounded turbulence
AbstractSpatial averaging, resulting from the finite size of a hotwire probe, significantly affects the accuracy of velocity measurements in turbulent flows close to walls. Here, we extend the theoretical model, introduced in Segalini et al. (Meas Sci Technol 22:104508, 2011) quantifying the effect of a linear spatial filter of hotwire probes on the mean and the variance of the streamwise velocity in turbulent wallbounded flows, to describe the effect of the spatial filtering on the third and fourthorder moments of the same velocity component. The model, based on the three(four) point velocitycorrelation function for the third(fourth) order moment, shows that the filtering can be related to a characteristic length scale which is an equivalent of the Taylor transverse microscale for the secondorder moment. The capacity of the model to accurately describe the attenuation is validated against direct numerical simulation (DNS) data of a zero pressuregradient turbulent boundary layer. The DNS data allow the filtering effect to be appraised for different wire lengths and for the different moments. The model shows good accuracy except for the thirdorder moment in the region where a zerocrossing of the thirdorder function is observed and where the equations become illconditioned. An “a posteriori” correction procedure, based on the developed model, to correct the measured third and fourthorder velocity moments is also presented. This procedure, based on combining the measured data by two single hotwire sensors with different wire lengths, is a natural extension of the one introduced by Segalini et al. (Exp Fluids 51:693–700, 2011) to evaluate both the turbulence intensity and the transverse Taylor microscale in turbulent flows. The technique is validated against spatially averaged simulation data showing a good capacity to correct the actual profiles over the entire height of the boundary layer except, as expected, for the thirdorder moment in the region where the latter exhibits a zerocrossing. Moreover, the proposed method has been tested on experimental data from turbulent pipe flow experiments.
http://dx.doi.org/10.1007/s0034801314964

