2. 3 Cylinder Array

Cylinders are set in the test section with their axes normal to the flow direction. We use two types of cylinder array mounts, that is, the regular arrangement and the staggered arrangement. Figure 6 shows the positions of the cylinders for both arrangements. By selecting the type of array mount and the connectors to set cylinders, various patterns of the cylinder arrangement are available. For connectors not in use, small plugs are inserted to make the wall of the test section flat and smooth. The nominal cylinder diameter is 4 mm. The large cylinders with 8 mm diameter are also tested to investigate the effects of the cylinder diameter. Note that the diameter of the pressure sensor element is 5.5 mm and it is too large to resolve the spatial variation of the pressure in the scale smaller than the cylinder diameter.

The cylinder arrangements tested in the present experiment are divided into seven categories as listed in Table 1 . In the table, the illustration of the cylinder arrangement is also given. For convenience of discussion, each cylinder arrangement is identified by the code number consisting of four digit number and alphabets. The first digit represents the number of cylinders in the most upstream row. The second, third and fourth digit numbers are defined in the same way. Note that we call the line of cylinders in the incident flow direction and that in the transverse direction as "column" and "row" respectively The alphabet "R","S" and "L" denote the regular arrangement staggered arrangement (see Fig. 6) and the large cylinders, respectively.