Wake Structures of Rectangular Bodies with Radiused Edges Near a Plane Surface 1999-01-0648

Almost all published results of wake measurements for ground vehicles or similar shapes have included very limited information on streamwise development of wake structures. This is typically a result of the fact that the wake measurements have been conducted as parts of particular vehicle development efforts. So the focus has been on the incremental changes in the wakes associated with alternative geometries or buildup of various parts. The objectives are typically reached by limiting the surveys to a single streamwise plane. The present study, by contrast, is a study of wake development for a series of relatively simple rectangular shapes with radiused edges with a systematic variation in the ratio of height to width or “Aspect Ratio”. Measurements include surveys in several streamwise planes and surveys in several longitudinal planes selected to produce volumetric data sets that give good description of the development of the wakes to about 2 hydraulic diameters downstream of the base. The baseline shapes are smooth on all surfaces. In addition to the effect of aspect ratio, a systematic variation of roughness on the lower surface is included. The present study was motivated by recent tests for a commercial van with various roof heights in which force measurements suggested that drag coefficient for taller (higher aspect ratio) vehicles is lower than drag coefficient for shorter (lower aspect ratio) vehicles. This is the opposite of the trend expected from data on basic shapes in the literature. The primary force data and comparisons to other vehicles are given in another paper. This paper provides an extensive data set on wakes for a basic series of shapes and for selected variations in lower surface roughness which is intended to encompass typical values representative of current vehicles.