Robert C. Nelson

The unsteady aerodynamics of slender wings and aircraft undergoing large amplitude maneuvers

Abstract Aircraft that maneuver through large angles of attack will experience large regions of flow separation over the wing and fuselage. The separated flow field is characterized by unsteadiness and strong vortical flow structures that can interact with various components of the aircraft. These complicated flow interactions are the primary cause of most flight dynamic instabilities, airload nonlinearities and flow field time lags. The aerodynamic and the vortical flow structure over simple delta wings undergoing either a pitching or rolling motion is presented. This article reviews experimental information on the flow structure over delta wings and complete aircraft configurations. First, the flow structure of leading-edge vortices and their influence on delta wing aerodynamics for stationary models is presented. This is followed by a discussion of the effect of large amplitude motion on the vortex structure and aerodynamic characteristic of pitching and rolling delta wings. The relationship between the flow structure and the unsteady airloads is reviewed. The unsteady motion of the delta wing results in a modification of the flow field. Delays in flow separation, vortex formation, vortex position and the onset of vortex breakdown are all affected by the model motion. These flow changes cause a corresponding modification in the aerodynamic loads. Data is presented which shows the importance of flow field hysteresis in either vortex position or breakdown and the influence on the aerodynamic characteristics of a maneuvering delta wing. The free-to-roll motion of a double-delta wing is also presented. The complicated flow structure over a double-delta wing gives rise to damped, chaotic and wing rock motions as the angle of attack is increased. The concept of a critical state is discussed and it is shown that crossing a critical state produces large transients in the dynamic airloads. Next, several aircraft configurations are examined to show the importance of unsteady aerodynamics on the flight dynamics of aircraft maneuvering at large angles of attack. The rolling characteristics of the F-18 and X-31 configurations are examined. The influence of the vortical flow structure on the rolling motion is established. Finally, a brief discussion of nonlinear aerodynamic modeling is presented. The importance of critical states and the transient aerodynamics associated with crossing a critical state are examined.

Separated Flowfield on a Slender Wing Undergoing Transient Pitching Motions

An experimental investigation on the flowfield surrounding a delta wing undergoing a transient pitching motion was conducted. The leading-edge vortices were marked with smoke, and the model and vortex motions were recorded using high-speed motion picture photography. These film records were then analyzed to yield information on the vortex location and trajectory as functions of both angle of attack and time. Hysteresis effects on the vortex dynamics were apparent for both the pitch-up and pitch-down motions

Unsteady surface pressure measurements on a slender delta wing undergoing limit cycle wing rock

An experimental investigation of slender wing limit cycle motion known as wing rock was investigated using two unique experimental systems. Dynamic roll moment measurements and visualization data on the leading edge vortices were obtained using a free to roll apparatus that incorporates an airbearing spindle. In addition, both static and unsteady surface pressure data was measured on the top and bottom surfaces of the model. To obtain the unsteady surface pressure data a new computer controller drive system was developed to accurately reproduce the free to roll time history motions. The data from these experiments include, roll angle time histories, vortex trajectory data on the position of the vortices relative to the models surface, and surface pressure measurements as a function of roll angle when the model is stationary or undergoing a wing rock motion. The roll time history data was numerically differentiated to determine the dynamic roll moment coefficient. An analysis of these data revealed that the primary mechanism for the limit cycle behavior was a time lag in the position of the vortices normal to the wing surface.

An experimental investigation of the planar turbulent wake in constant pressure gradient

This paper describes an experimental investigation into the development of a planar turbulent wake under constant adverse and favorable pressure gradient conditions. The focus of the study is on the near-wake due to its relevance to high-lift systems for commercial transport aircraft. The wake is generated by a flat splitter plate with tapered trailing edge. The pressure gradients are imposed as the wake passes through a wind tunnel diffuser test section with fully adjustable top and bottom wall contours. The streamwise pressure gradients imposed on the wake flow field are held constant in each case. The wake initial conditions are maintained identical upstream of the location where the pressure gradient is first imposed. The use of constant pressure gradients, coupled with identical initial conditions, facilitates isolation of the effect of streamwise pressure gradients on the near-field evolution of the wake and provides a clean test case for computational models. In this paper we focus on characterizing the mean flow widening, streamwise velocity defect variation, and the streamwise evolution of turbulence statistics for both favorable and adverse streamwise pressure gradients. The imposed pressure gradients are shown to have a very significant effect on both the mean and turbulent flow quantities.

Measurement of unsteady surface pressure on a slender wing undergoing a self-induced oscillation

SummaryThe methodology outlined here was shown to be an effective means by which detailed unsteady surface pressure measurements may be obtained on a wing undergoing a self-induced oscillation in a wind tunnel study. Previous methods have relied on forcing the wing through a sinusoidal motion at a fixed amplitude and frequency. The advantage of the technique outlined here is that pressure measurements are obtained for the exact motion time history, including all transient motions. In fact, almost any self-induced or forced motion may be implemented by the system. The method also has the advantage of a significant reduction in the cost of pressure transducers due to the repeatability of the unsteady flowfield.

Flow about yawed, stranded cables

The development of a steady lift force on a stranded cable, which is yawed with respect to a flow, is a unique characteristic of a cable when compared to a circular cylinder. Comparisons of lift and normal drag coefficients and wake characteristics were made between stranded cable models and the cylinder. These were based upon surface pressure and hot-wire measurements and flow visualization studies conducted in a low speed wind tunnel on rigid cables and cylinders. The models were yawed to four different yaw angles and tested within the Reynolds number range of 5,000 and 50,000. Pressure profiles for the yawed cables indicated that the lift force is directed towards the side where the primary strands are more nearly aligned with the flow. The pressure profiles also indicated that the lift force is generated by asymmetric separation. The small scale irregularities associated with wires within individual strands also appeared to have an effect on the cables lift and drag characteristics. Results show that cables have significantly different shedding characteristics and near-wake shear layer structure when compared to the circular cylinder. For the flow regime tested, the Strouhal number showed no dependence on Reynolds number nor spanwise position along the cable.

An experimental investigation of delta wing vortex flow with and without external jet blowing

A visual and quantitative study of the vortex flow field over a 70-deg delta wing with an external jet blowing parallel to and at the leading edge was conducted. In the experiment, the vortex core was visually marked with TiCl4, and LDA was used to measure the velocity parallel and normal to the wing surface. It is found that jet blowing moved vortex breakdown farther downstream from its natural position and influenced the breakdown characteristics.

A discrete vortex model for predicting wing rock of slender wings

The fluid mechanism responsible for generating wing rock of slender sharp-edged delta wings was investigated using an unsteady discrete vortex model developed for that purpose, which is based on results of experimental investigations. Combined experimental and computational results indicate that wing rock is sustained by a lag in the position of the leading edge vortices normal to the surface. Results of comutations also indicate that certain complex aerodynamic problems may be governed primarily by unsteady inviscid phenomena.

An Experimental Study of the Stability of a Four-Vortex System

An experiment is currently being conducted at the University of Notre Dame to investigate the interactions, and ultimately the instabilities, that persist in a four-vortex wake. A wake consisting of four trailing vortices was created in Notre Dame’s atmospheric boundary layer wind tunnel. The advantage of this tunnel is the long test section, which permits the observation of wake interactions at distances up to thirty wake-spans downstream of the point of generation. The wake was created by four high aspect ratio wing models such that the circulation strength and the spacing between each vortex in the wake may be controlled. The principle aim of this study is to experimentally determine the optimal configurations, based on circulation strength and spacing, that are consistent with the rapid amplification of certain wake vortex instability modes. The primary method for evaluating the experimental facility has been flow visualization. Helium bubbles were introduced into the core region of individual vortices to visualize these wake structures. This paper presents some early results from the visualization of the four-vortex wake. These results suggest that the use of helium bubbles is a suitable method for visualizing wake vortices for the full length of the test section.

EXPERIMENTAL INVESTIGATION OF TURBULENT BOUNDARY LAYER RELAMINARIZATION WITH APPLICATION TO HIGH-LIFT SYSTEMS: PRELIMANARY RESULTS

An experimental investigation of turbulent boundary layer relaminarization is underway at the Hessert Center for Aerospace Research at the University of Notre Dame. A turbulent boundary layer was exposed to a large favorable pressure gradient for several values of Launders relaminarization parameter. It was observed that many of the turbulent boundary layer parameters behaved as if the boundary layer relaminarized, despite values of the relaminarization parameter below which relaminarization would be expected. Preliminary results lead one to question the validity of the relaminarization parameter as the proper criterion for the onset of relaminarization. Further, evidence suggests the turbulent burst-sweep event may be a direct indicator of relaminarization and preliminary experimental data supports this claim.