S. G. Liou

Velocity measurements of airframe effects on a rotor in a low-speed forward flight

Aerodynamic interactions between the rotor and the airframe of a rotorcraft can have severe effects and are difficult to predict analytically. To attack this problem, the velocity field of a two-bladed rotor has been measured in a wind tunnel with and without an airframe model in proximity. The periodic and time-averged velocity fields were measured using a laser velocimeter in planes both parallel to and above and below the rotor tip path plane at an advance ratio of 0.1 and a rotor tip Mach number of 0.29. The data were shown to be free of tunnel wall effects. The effect of including the cylindrical airframe model were measured. For the geometry studied, airframe influence on the rotor flowfield was mostly confined to the front half of the rotor disc. Hub effects were noticeable, even with the minimal-sized hub used. Strong vortex interaction effects were observed using strobed laser sheet flow visualization and measured using laser velocimetry.

Flow over a twin-tailed aircraft at angle of attack. II - Temporal characteristics

Revised May 5, 1991 Flow Over a Twin-Tailed Aircraft at Angle of Attack. Part I: Spatial Characteristics N.M. Komerath1, S.G.Liou2, R.J. Schwartz3, J.M. Kim3 School of Aerospace Engineering Georgia Institute of Technology Atlanta, Georgia 30332 ABSTRACT A quantitative study is reported on the low-speed flow environment of scale models of a twin-tailed fighter aircraft at high angles of attack. Laser sheet flow visualization is used to observe the various sources of vortex generation, and the evolution of these vortex flows. Surface tufts are used to observe the nature of flow separation on the vertical tails as angle of attack is varied. Laser Doppler velocimetry is used to quantify the time-averaged three-dimensional velocity field, and histograms of velocity, in selected planes proceeding from the inlets to the vertical tails. No concentrated vortex is observed near the vertical tails, however, the tails are seen to be immersed in a vortex flow of large radius. Flow separation propagates up the outside surfaces of the vertical tails, with increasing angle of attack; however, the flow on the inside surfaces of the tails remains largely attached. The flow angularity at the tails varies widely along the tail span, is sensitive to angle of attack, and fluctuates over a wide range at each location. These results are found to be relatively insensitive to the precise modeling of inlet through-flow and inlet attitude. Contours of the root-mean square velocity fluctuations indicate that the largest fluctuations occur in the separated flow immediately above the wing surfaces. ______________ __________________________________________________________ 1: Associate Professor. Member, AIAA 2. Post-Doctoral Fellow. Member, AIAA 3. Graduate Research Assistant. Student Member, AIAA

Measurements of the unsteady vortex flow over a wing-body at angle of attack

Measurements of the unsteady vortex flow over a wing-body at high angles of attack were carried out on a generic test model of a pointed body of revolution with double-delta wings. Vortex patterns and trajectories were quantified from digitized laser sheet video images. The velocity-field measurements showed the jetlike flow in the unburst vortex, unsteady secondary structures below the primary core, and then the reversed flow in the burst vortex. Results of hot-film anemometry revealed the presence of peak frequencies in the velocity spectra over the wing and near the trailing edge, which varied linearly with freestream speed and increased as the measurement point moved upstream. Good Strouhal correlation was found with previous results obtained for a smaller generic wing-body model.

MEASUREMENT AROUND A ROTOR BLADE EXCITED IN PITCH, PART 2: UNSTEADY SURFACE PRESSURE

This paper describes measurements of chordwise distributions of unsteady pressure at three radial locations on a stiff two-bladed teetering model helicopter rotor in hover, with the blades executing simple harmonic pitch oscillations about the quarter-chord pitch axis. The objective of the present work is to provide a data base to correlate the measured unsteady blade pressure distributions with dynamic inflow to establish the validity or deficiencies of available analytical methods for predictions of unsteady aerodynamic phenomena. The effect of dynamic inflow on rotor unsteady surface pressure has been

Rotor wake interaction with separated flow

Aerodynamic interactions represent one of the toughest challenges to computational methods for the prediction of rotorcraft performance, loads, and noise. Efforts over the past ten years using a simple hemisphere/cylinder airframe under a teetering two-bladed rotor have shown that most of the dominant effects can be predicted using potential-flow concepts, in the absence of massive flow separation and strong vortex-surface collision. Here, the prospects for modeling the separated flow interaction are studied. The rotor wake interacts with the separated flow over a pressure-instrumented boom, downstream of a backstep cut into the original cylinder. The undisturbed backstep flow is characterized using spectral analysis of the velocity and surface pressure fields. The interaction between the tip vortex and the backstep free shear layer is visualized using laser sheet videography, and measured in detail using laser velocimetry and pressure sensing. The tip vortex dominates the interaction, causing periodic destruction and reconstruction of the shear layer, and large-amplitude longitudinal motion of the reattachment zone. However, the characteristics of the undisturbed shear layer are still detected in the surface pressure spectra. The tip vortex and its collision with the boom surface appear to be unaffected by the shear layer interaction. It is argued that this experiment is a conservative representation of the interaction, so that occurrences on real rotorcraft should be less complex. Thus, while the separated flow interaction appears to be extremely complex at first sight, the dominant effects are surprisingly simple, and may be easily modeled.