| Summary: | Faculty of Engineering and Biult Enviroment
School of Mechanical,Industrial And Aeronautical Engineering
9807537d
efunemarc@hotmail.com === 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.
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