A WIND-TUNNEL STUDY OF AIR-FLOW IN WAVING WHEAT - 2-POINT VELOCITY STATISTICS

Two-point, space-time correlations of streamwise and vertical velocity were obtained from a wind tunnel simulation of an atmospheric surface layer with an underlying model wheat canopy constructed of flexible n ylon stalks. Velocity data extend from 1/6 canopy height to several ca nopy heights, with in excess of 2000 three-dimensional vector separati ons of the two x-wire probes. Isocorrelation contours over an x, z sli ce show the streamwise velocity autocorrelation to be tilted approxima tely 18 degrees in a downwind direction, and the vertical velocity aut ocorrelation to be roughly circular, such that vertical velocities at the same horizontal position but different heights are closely in phas e. Cross-correlations between the two velocity components reflect this difference to some extent. Lateral displacements of the probes reveal ed side lobes with correlations of reversed sign but we cannot positiv ely link this pattern to particular vorticular structures. Integral le ngth scales obtained directly from the spatial correlations match simi lar scales deduced from single-point rime series with Taylor's hypothe sis at 2 to 3 times the canopy height but greatly exceed such scales a t lower levels, particularly within the wheat. We conclude that the re versed sign lateral lobes are important components of the correlation field and that an integral length scale for the lateral direction must be defined such that they are included. Convective velocities obtaine d from the time lag to optimally restore correlation lost by physical separation of the probes change only slowly with height and greatly ex ceed the mean wind velocity within and immediately above the canopy. T hus, mean wind velocity is not a suitable proxy for convective velocit y in the application of Taylor's hypothesis in this situation. The rat io of vertical to longitudinal convective velocity for the streamwise velocity signal yields a downwind tilt angle of about 39 degrees which is probably abetter estimate of the slope of the dominant fluid motio ns than the tilt of the major axis of the isocorrelation contours ment ioned previously.