. "Unsteady Three-Dimensional Cross-Flow Separation Measurements on a Prolate Spheroid Undergoing Time-Dependent Maneuvers." Twenty-First Symposium on Naval Hydrodynamics. Washington, DC: The National Academies Press, 1997.
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Twenty-First Symposium on NAVAL HYDRODYNAMICS
techniques fail to capture the non-linearities in such flows and CFD techniques cannot accurately compute the highly complex, separated flow fields of full vehicle geometries in steady conditions, let alone unsteady ones. Such methods are deficient because they lack physical models on which to base their computations that accurately describe the complexities of a time-dependent, turbulent, separated flow field. These models can only be developed with suitable experimental flow field data from sufficiently realistic flows. A new apparatus at Virginia Tech, the Dynamic Plunge-Pitch-Roll (DyPPiR) model mount, provides for the first time the capability to model truly time-dependent, high-excursion, high Reynolds number flows in a laboratory setting (1,2).
“Dynamic” testing has been an important part of design and validation of various types of craft for decades. Typically these techniques are only “quasi-steady”, relying on very small amplitude sinusoidal oscillations that can describe small-excursion maneuvers reasonably well (10). Even in these tests however, the measurements are usually limited to forces and moments. Seldom are flow field data taken in dynamic tests.
The DyPPiR is installed in Virginia Tech's 1.8m square Stability Wind Tunnel. The DyPPiR combines three 20.6 MPa hydraulic actuators to plunge a model through a 1.5m range vertically, pitch the model through a +/–45° range, and roll the model through a +/–140º range. Typical models are on the order of up to 2m long, and typical maneuvers last several tenths of a second. Even at Reynolds numbers of over 4 million the maneuvers are fast enough to exhibit significant unsteadiness. All degrees of freedom are digitally controlled by a personal computer, which allows the researcher to program not only sinusoidal trajectories, but more importantly, user-defined trajectories. Thus, the DyPPiR successfully fills the need of forcing a model to perform general, rapid, truly unsteady, high excursion, high Reynolds number maneuvers.
The DyPPiR has been used primarily to study submarine-like configurations. While some force and moment measurement capability has been developed for use with these configurations on the DyPPiR, the primary measurements made during DyPPiR tests are time-dependent surface pressure measurements and time-dependent, constant temperature surface hot-film measurements. The pressure measurements are useful in determining how unsteadiness affects the forces and moments and the structure of the vortical separations that form on the leeward side. The hot-film sensors are used to measure wall shear. Minima in the wall shear are interpreted as near separation locations. Thus, hot-film measurements provide the capability to study separation formation and migration during transient maneuvers.
To relate unsteady wind tunnel tests to real-time maneuvers, the time is non-dimensionalized by the time for flow to pass over a model, L/U∞ (8):
Most parameters studied are related to the instantaneous angle of attack. The DyPPiR specifically sets a pitch actuator position during a maneuver. Since for all of the maneuvers studied the model is rotated about the model center, it can be stated that the DyPPiR pitch angle is equal to the instantaneous angle of attack of the model, referenced at the model center. Due to the rotational motion of the model, the instantaneous local angle of attack varies linearly from the nose to the tail of the model, with the nose being at a lower angle of attack than the model center and the tail being at a higher angle of attack than the model center. The magnitude of the local induced increment in angle of attack is a function of distance from the model center and rotational speed. In all cases studied here, these angle of attack increments, relative to the model center angle of attack, are less than 1.4° at the extreme nose and tail.
6:1 Prolate Spheroid
The prolate spheroid is an interesting geometry because although the body shape is very simple, the flow field is very complicated. In addition, the prolate spheroid flowfield carries over qualitatively to submarines, missiles, torpedoes, and to a certain extent aircraft fuselages. The prolate spheroid is a fairly well-documented flow. Other simple bodies that have been studied that are of interest are ellipsoid-cylinders and ogive cylinders. Until the present, all data taken on prolate spheroids has been steady. Wetzel (14) gives a brief overview of the steady prolate spheroid literature along with a detailed description of the steady flowfield. The prolate spheroid flowfield at angle of attack is characterized by massive crossflow separation that forms on the tail at low angles of attack and migrates windward and noseward at increasing angles of attack (2). The circumferential location of separation and even the separation topology are highly dependent on the state of the local boundary layer, that is, whether it is laminar, transitional, or turbulent. In all tests in this report, trip strips at x/L=0.20 guaranteed a relatively Reynolds number-insensitive separation over the rear 80% of the model. The separation on the nose, however, was governed by the laminar flow upstream of the trips. Here, at high angles of attack, the laminar flow will separate, undergo transition, reattach, and re-separate as a second, turbulent separation. These two separation lines merge downstream of the trip strips (14).
Overview of Present Research Program
The present work represents the first unsteady data taken on the DyPPiR at Virginia Tech. The focus up