Statistics of filtered velocity in grid and wake turbulence

Data on spatially filtered turbulence are commonly needed for a priori sub- grid model studies and for a posteriori testing of large eddy simulation (L ES) codes. In this paper, hot-wire anemometry is used to record very long r ecords of such data, required for good convergence of high-order statistics . An array consisting of four X-wire probes placed transversal to the flow direction is built. Unlike previous single-probe hot-wire measurements, whi ch only allowed stream-wise filtering using Taylor's hypothesis, the array permits cross-stream filtering as well. Measurements which are spatially fi ltered at a length-scale Delta pertaining to the inertial-range of turbulen ce are performed in grid and wake turbulence. The data can be used directly to compare with results from LES. From the data, fundamental differences b etween filtered and unfiltered velocity fields are examined through probabi lity density functions and the scaling behavior of high-order structure fun ctions. A comparative study of probability density functions of filtered an d unfiltered velocity increments shows that the tails of the distributions are affected by the filtering even at scales much larger than the filter sc ale. Significant differences are also observed in regard to the scaling of structure functions. It is shown that extended self-similarity, a recent te chnique for measuring inertial range scaling exponents, yields questionable results when applied to structure functions of filtered velocity. (C) 2000 American Institute of Physics. [S1070-6631(00)02605-2].