Compressible flows in the wakes of a square cylinder and thick symmetrical airfoil arranged in tandem

Abstract

Compressible flows in the wakes of a two-dimensional square cylinder (side length D = 20 mm) and thick symmetrical airfoil (NACA 0018, chord length 20 mm) arranged in tandem have been examined experi­mentally, at free-stream Mach numbers between 0.15 and 0.91, at free-stream Reynolds numbers (based on the side length) between 7.0 x 10 4 and 4.2 x 10 5 , and with spacing (or central distance) L between the cylinder and airfoil ranging from 22.5 to 110 mm. When the Mach number is smaller than about 0.63, the flow can be divided into three patterns depending upon the spacing. In the first flow pattern, with small spacing, the airfoil is enclosed completely within the vortex formation region of the square cylinder. In the second flow pattern, the separating shear layers from the square cylinder reattach to the airfoil. In the third flow pattern, with large spacing, the separating shear layers roll up upstream of the airfoil. The Strouhal number becomes a minimum at the critical spacing of about 3.3 D and then experiences a sudden jump, practically at the value found for the single square cylinder, which corresponds to the transition from the second flow pattern to the third flow pattern. Once the Mach number becomes larger than about 0.63, the critical spacing disappears. However, although no local flow regions are supersonic, acoustic waves propagating upstream have been observed most clearly when the vortex shed from the square cylinder is incident on the leading edge of the airfoil. Whereas once the local flow regions become supersonic, i. e. the Mach number is larger than about 0.7, the downstream airfoil provides a streamlining effect on the flow behind the square cylinder, and thus lets the alternating vortices form downstream of the trailing edge of the airfoil. The alternating vortices are shed through the gap between the two shock waves formed on the upper and lower separating shear layers. The pressure amplitude in the test section decreases suddenly and various frequency components other than the vortex shedding frequency appear.