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.

Cricket balls behave differently at various stages of the game depending upon how much wear and tear has taken place due to use. The playing performance of cricket balls depends largely on the surface texture. The ball is swung using the primary seam during the early stages of the game but later the surface roughness starts to affect the lateral movement. This work attempts to find a quantitative measure of the surface roughness of cricket balls and then uses it to discriminate between new and old balls. Area-scale fractal analysis is used to find the surface roughness in order discriminate between the balls. FTEST (a statistical tool) is also used to establish a discriminatory criterion between the old and new balls. Wind tunnel test results are presented to show the relationship between the surface roughness and drag. Finally a correlation between the roughness and drag of the cricket is shown.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords:Spacecraft; electric propulsion; LEO; GRACE; Drag-free. Includes bibliographical references (leaves 111-112).

Thesis (D.Tech.-Mechanical engineering)--Cape Technikon, 1997. / Includes bibliography. Also available online.

The fluid forces on a single cylinder in cross flow are measured experimentally by strain gauges. The drag and lift coefficients of the cylinder are measured with momentum injection of various direction and magnitude. The results show that the drag coefficient of the cylinder is reduced with momentum injection. For higher Reynolds numbers, the magnitude of momentum injection must be increased to maintain the effectiveness of momentum injection. The influence of the drag coefficient is reduced when the angle of momentum injection is increased, and then the lift coefficient is varied obviously.

An investigation of turbulent structure modification of a boundary layer for a fully developed channel flow by microbubble injection close to the upper wall was carried out using Particle Image Velocimetry (PIV). Two-dimensional velocity components in an x-y plane at Reynolds number of 5128 based on the half height of the channel and bulk velocity were measured. Microbubbles, with an average diameter of 30 ??m were produced by electrolysis and injected in the buffer layer. Different values of the void fraction were attained and used to evaluate the effects of the presence of microbubbles and their concentration within the boundary layer. A reduction in drag was observed due to the injection of microbubbles. Drag reduction augments as the value of the void fraction increases. Furthermore, increases in both the non-dimensional values of streamwise and normal turbulent intensities, normalized by the friction velocity were observed with the void fraction growth. A gradual decrease in the Reynolds shear stresses was achieved as the void fraction increases. This effect is due to a ??decorrelation?? or ??decoupling?? between the streamwise and normal fluctuating velocities. Modifications in the length and time scales due to the presence of microbubbles were detected by calculating two-point correlation coefficients in one and two dimensions and the autocorrelation coefficient at various locations within the measurement zone. Streamline length and time scales were increased. On the contrary, the normal length and time scales were decreased. The vorticity and strain rate values decreased with the injection of microbubbles. Turbulent energy production was also decreased within the boundary layer. Quadrant analysis was used to find out the contribution of the u?? and v?? fluctuating velocity components to the Reynolds stress. The presence of microbubbles reduces the contribution to the Reynolds stresses by Q4 events (sweeps), which are responsible for the production of skin friction. Vortical structure detection in the measurement area was pursued. The structure with and without the microbubble injection is compared. In this study the presence of microbubbles within the boundary layer has produced several modifications in the flow structure as well as reduction in the drag.

Thesis (M.S. in Engineering Science (Mechanical Engineering))--Naval Postgraduate School, December 2003. / Thesis advisor(s): Garth V. Hobson, Rayomnd P. Shreeve. Includes bibliographical references (p. 67). Also available online.

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