Drag Measurement in Unsteady Compressible Flow

• Main Author: Efune, Marc
• Format: Others
• Language: en
• Published: 2006
• Subjects: Drag; Cones; Compressible Flow
• Online Access: http://hdl.handle.net/10539/1844

Faculty of Engineering and Biult Enviroment School of Mechanical,Industrial And Aeronautical Engineering 9807537d [e-mail] === Drag over a wide range of shapes is well established for steady flow conditions. Drag in unsteady flow, however, is for the most part not well understood. The research presented herein examines the drag over cones in unsteady compressible flow. This was achieved by constraining cones, with half-vertex angles ranging from 15° to 30°, in a shock tube and passing shock waves over them. The resulting drag was measured directly using a stress wave drag balance (SWDB). Tests were run at shock Mach numbers between 1.12 and 1.31 with corresponding post-shock Reynolds numbers between 2 × 105 and 6 × 105. The drag on the four cone geometries as well as one sphere geometry was modelled numerically. Density contours of the flow fields, obtained from the numerical simulations were used to visualise the shock/model interactions and deduce the causes of any variations in drag. It was thus proved that post-shock fluctuations are due to shock wave reflections off the shock tube walls and the model support. The maximum unsteady drag values measured experimentally ranged from 53.5 N for the 15° cone at a Mach number of 1.14 to 148.6 N for the 30° cone at a Mach number of 1.29. The drag obtained numerically agreed well with experimental results, showing a maximum deviation in peak drag of 9.6%. The drag forces on the conical models peaked as the shock wave reached the base of the cone whereas the drag on the sphere peaked just before the shock reached the equator of the sphere. The negative drag and large post-shock drag fluctuations on a sphere measured by Bredin (2002) were present in the numerical results and thus confirm that these features were not due to balance error. The large post-shock drag fluctuations were also present on the cones. The unsteady drag was shown to increase as both the shock wave Mach number and the cone angle were increased. The ratio of the maximum unsteady drag to the compressible steady state drag varied from v 4.4:1 to 9.8:1, while the ratio of the maximum unsteady drag to the incompressible steady state drag varied from 8.3:1 to 22.2:1. The steady state drag values were shown to be of the same order of magnitude as the post shock unsteady drag. Further numerical work is recommended to confirm that drag fluctuations are in fact due to shock reflections and to better establish the relationship between the unsteady drag and the cone angle.