Although complex, inconsistent and fickle, the time-averaged flow over a stationary slender forebody is generally well-understood. However, the nature of unsteady, time-varying flows over slender forebodies - whether due to the natural unsteadiness or forced maneuvering - is not well-understood. This body of work documents three experimental investigations into the unsteadiness of the flow over a 3.5 caliber tangent-ogive cylinder at high angles of incidence. The goal of the investigations is to characterize the natural and forced flow unsteadiness, using a variety of experimental tools. In the first investigation, flow data are collected over a stationary model in a water tunnel. Particle-Image Velocimetry (PIV) is employed to acquire time-dependent planes of velocity data with the model at several angles of attack. It is discovered that the asymmetric flow associated with the tangent-ogive forebody exhibits a large degree of unsteadiness, especially for data planes located far from the forebody tip. Vortex shedding of the type exhibited by a circular cylinder in crossflow is observed, but this shedding is skewed by the presence of the tip, the shedding process does not require equal periods of time from each side of the body, and this results in a time-averaged flowfield that is asymmetric, as expected. The rms values of the time-averaged velocity, as well as the turbulent kinetic energy and axial vorticity are calculated. In the second investigation, surface pressure data are acquired from several circumferential rings of pressure ports located on two models undergoing ramp coning motions in two different wind tunnel facilities. The surface pressure data are integrated to determine the sectional yaw forces. Coning motions were performed at several different reduced frequencies, and pneumatic control actuation from the nose was employed. The chosen control actuation method used a small mass flow rate ejected very close to the forebody tip, so as to leverage the inherent convective instability. The data resulting from these tests were analyzed in order to determine how the coning motions affect the distribution of surface pressure and yaw forces, how quickly the flow reacts to the motion, and the extent of control authority of the pneumatic actuation. It was discovered that the yaw forces increase in the direction of the motion for small reduced frequencies, but in the direction opposite to the motion for large reduced frequencies. The effects of the motion tend to dominate the control method, at least for the reduced frequencies and setup tested in the low-speed wind tunnel. The results from the high-speed testing with transitional separation give a preliminary indication that the control method could have sufficient control authority when the reduced frequencies are low. The third investigation involves tangent-ogive cylinder undergoing a pitching maneuver in a water tunnel. Laser-Doppler Velocimetry (LDV) is used in order to map out several planes of velocity data as the model is pitched. The LDV data is used to calculate vorticity and turbulent kinetic energy. Variables that are proportional to the flow asymmetry and proximity to the steady-state flow are defined. All of these variables are displayed as a function of time and space (where appropriate). The delay in the development of the asymmetry and the flow progression to the steady state are determined to be a function of pitch-axis location. The propagation velocity of the convective asymmetry is faster than expected, most likely because of the increased axial velocity in the vortex cores. Vortex breakdown of one of the vortices is observed, with loss of axial velocity and dilution of the vorticity over a large area. The cause of this phenomenon is not yet understood, but it is reminiscent of vortex breakdown over delta wings. / Ph. D.

The natural unsteadiness in the post-breakdown flowfield of a 75° sweep delta wing at 40° angle of attack was studied with dual and single point hot-wire anemometry in the Engineering Science and Mechanics (ESM) Wind Tunnel at a Reynolds number Re = 210,000. Data were taken in five crossflow planes surrounding the wing's trailing edge. Results showed a dominant narrowband Strouhal frequency of St = 1.5 covering approximately 80% of the area with lower-intensity broadband secondary frequencies over 15% of that region. Cross-correlations between a fixed and traversing wire were calculated and phase and coherences mapped to determine the convection speed and trajectory of the helical mode instability. High-speed Particle Image Velocimetry (PIV) was conducted over a 75° sweep delta wing at 40° angle of attack in the ESM Water Tunnel II at Re = 45,000. Data were taken along the axis of the vortex in the breakdown flowfield at a speed of 0.1% of the convective time scale of the flow. Animations of instantaneous streamlines and velocity vectors revealed the impression of a helically spiralling vortex core on the measurement plane. Spectral analysis of the PIV data showed reduced frequencies which confirmed those found with the single-point measurements made in the ESM Wind Tunnel. The effect of four novel control surfaces on the breakdown flowfield of the delta wing was studied with surface pressure measurements along the axis of the vortex in the ESM Wind Tunnel. The apex flap was found effective and delayed vortex breakdown by 8° for a fixed wing. The flowfield was characterized over the delta wing executing a pitch-up maneuver at a reduced frequency of 0.06. Surface pressure measurements were taken in the ESM Wind Tunnel and Laser Doppler Velocimetry (LDV) was employed in the ESM Water Tunnel I as both the unmodified wing and then the wing with an apex flap deployed at an optimal angle <font face="Symbol">b</font> = 15° executed the pitch-up. Both sets of data confirmed the hysteresis of the flowfield. The LDV data, taken in two crossflow planes throughout the maneuver, showed an asymmetric breakdown development. As a practical extension of the study of the breakdown wake flowfield, hot-wire measurements were made over an F/A-18 model to determine the spectral characteristics of the flowfield. Three-dimensional vortex interactions were investigated with helium bubble flow visualization in the VPI Stability Tunnel. / Ph. D.

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