Unsteady flow over a 6:1 prolate spheroid

Abstract

The flow over a 6:1 prolate spheroid undergoing transient maneuvers is studied. The Dynamic Plunge-Pitch-Roll (DyPPiR) model mount provides the unique capability to simulate truly unsteady maneuvers in a wind tunnel. The DyPPiR is presented along with several transducer systems used to study specific aspects of the spheroid flow field. The types of unsteady measurements performed include hot-film surface skin friction measurements, surface pressure measurements, and force and moment measurements. Unsteady measurements are much more difficult to perform than steady ones and require many special considerations. In light of this fact, each transducer system is examined in detail for its appropriateness in detecting unsteady phenomena and the associated uncertainties. Recommendations are made for each of these systems for the improvement of their accuracy and relevance in studying unsteady phenomena. In particular, time dependent separation locations are measured successfully for the first time, as are time dependent force and moment measurements. Steady and unsteady data are presented for each of these systems for two maneuvers: a 0° to 30°, 0.33 second pure ramp pitchup about the model center, and thus referred to as the Pitchup Maneuver, and a 0° to 13.5° pure pitchup about the model center that simulates the time dependent sideslip angle of a submarine entering a turning maneuver, thus referred to as the Submarine Maneuver. These data are coupled with steady oil flow visualizations and data sets from other researchers to describe the spheroid flow field in detail in both steady and unsteady cases. This flow field is characterized by complex, three-dimensional cross-flow separations that are highly non-linear and are expected to have very complex time dependencies in unsteady flows. It is shown that, especially at higher angles of attack, significant lags occur in the flow field during the maneuvers compared to the steady cases. In particular, separation is delayed at all locations of the model by up to 10° higher angle of attack in the unsteady maneuvers compared to the steady data. Equivalently, the separation structure during the unsteady maneuvers lag the steady data by from 1.5 to 4.5 non-dimensional time units (t’). The range of these time constants and the fact they are constant for neither the entire model or a given sensors shows for the first time the complex nature of the time dependency of three-dimensional crossflow separation. In addition, normal force and pitch moment lags on the order of 1 time unit are demonstrated.