R. Mark Rennie

Forcing of a Two-Dimensional, Weakly-Compressible Subsonic Free Shear Layer

Forcing of a two-dimensional, weakly-compressible subsonic shear layer has been experimentally demonstrated. The measurements were made in a new compressible shearlayer facility that mixes highand low-speed flows with speeds up to M = 1. The forcing was performed using voice-coil actuators mounted at the trailing edge of the splitter plate separating the highand low-speed flows at the inlet to the mixing section. The ability of the forcing actuator to organize the shear layer into large-amplitude vortical structures was verified from Malley-probe measurements of shear-layer turbulent spectra, flow visualization, and optical wavefront measurements. The ramifications of the results on adaptive-optic control of the optical aberrations imposed by the shear layer are discussed.

Aero-Optical Measurements of High-Mach Supersonic Boundary Layers

Aero-optical measurements of a boundary layer at high supersonic speeds of M = 3.0 and 4.3 were performed in the Trisonic Wind Tunnel at the US Air Force Academy. Overall levels of aero-optical distortions, convective speeds and the aperture functions were calculated from the data. It was found that the statistics of the aero-optical distortions were similar to subsonic boundary layers. Several modified models were shown to properly predict aerooptical levels of boundary layers up to M = 5. As a separate experiment, a wedge model was placed inside the tunnel to study the aero-optical effects of the attached oblique shock. Aerooptical structures with an abnormally-low convective speed were observed in the laser beam traversing the oblique shock around the wedge. These structures were speculated to be related to convecting unsteady distortions imposed on the oblique shock by the naturallyvibrating wedge.

Management of Wind Tunnel Performance Data Using Neural Networks

This paper describes the use of neural networks to organize and manipulate wind-tunnel performance data into formats that are useful to tunnel operators. The neural networks were trained and tested using a mathematical model that was developed from basic fluidmechanic equations, and calibrated to accurately simulate the behavior of a subsonic, closedcircuit wind tunnel. The ability of the trained neural networks to extract secondary results such as the model drag area from a database composed of typically-measured wind-tunnel control and status parameters, is illustrated. Methods to identify the parameters required in the neural networks are also discussed.

Aerodynamic Design of an Aircraft-Mounted Pod for Improved Aero-Optic Performance

A new approach for the shaping of pods and fairings for optical systems is presented in which the objective of the approach is to prevent or mitigate aero-optic effects. Basic streamlined shapes are developed which prevent supersonic flow and shock formation around the optical aperture. Furthermore, an innovative design for the shaping of cutouts that extend the maximum aft lookback angle of the optical system is described. The design approach is supported by computational fluid dynamic analyses, and the design of planned wind-tunnel tests is presented.

An Experimental Investigation of Lift Enhancement and Roll Control Using Plasma Actuators

This paper describes an experimental investigation into the use of near-trailing-edge mounted plasma actuators for lift enhancement and roll-control. Data are presented showing the effect that the actuators had on lift, pitch, and roll moment coefficients as a function of angle of attack, wind speed and actuator excitation power. The results indicate that the performance of the actuators is influenced by Reynolds number effects. In particular, the lift-enhancement effect of the actuators is shown to have a linear dependence on wind speed/Reynolds number at low wind speeds. The implications of these findings on the utility of plasma actuators as lift and roll-control devices are discussed.

Mathematical Model for the Thermal Behavior of a Wind Tunnel

The development from basic fluid-mechanic principles of a mathematical model for the thermal behavior of a closed-circuit wind tunnel is presented. As part of the development of the model, parameters such as total-pressure loss factors and convective heat-transfer coefficients are identified that must be determined from experimental measurement. Techniques for the measurement of these parameters, as well as results for the Mach 0.6 capable 3 Foot Wind Tunnel at the University of Notre Dame, are presented. After incorporating these experimentally-measured parameters into the mathematical model, the model is shown to predict the behavior of the 3 Foot Wind Tunnel with reasonable accuracy.

Mathematical Modeling of Wind-Speed Transients in Wind Tunnels

A mathematical model for the time-accurate variation of flow conditions within a wind tunnel undergoing an acceleration of the main fan is presented. The mathematical model is developed from basic fluid-mechanic principles following the “lumped parameter” approach, in which the tunnel circuit is discretized and the assumptions of one-dimensional, constant-property flow are applied to each section. The predictions of the mathematical model are shown to compare closely to dynamic measurements of the flow within a lowspeed, closed-circuit wind tunnel with different breather conditions.

Measurement of Beacon Anisoplanatism Through a Two- Dimensional Weakly-Compressible Shear Layer

An experimental investigation was conducted into the effectiveness with which aero-optic aberrations imposed on a collimated reference beam could be evaluated using a point-source beacon. The experiments were conducted in the University of Notre Dame’s Compressible Shear-Layer Wind Tunnel which was used to create an optically-active shear-layer flow with high-speed Mach number of 0.8. Anisoplanatic effects included a difference in wavefront shape between the (spherical wavefront) beacon and the (planar wavefront) reference beam, and a difference in the regions of the flow sampled by the beacon and reference beams. A modal compensation approach was used to minimize the anisoplanatism between the beacon and reference wavefronts, which showed that the best compensation results were obtained when the shear layer was regularized using mechanical forcing.

Characterization of a Forced Two-Dimensional, Weakly- Compressible Shear Layer

Experimental data are presented consisting of fluid mechanic and optical measurements made in a forced, two-dimensional, weakly-compressible subsonic shear layer. The measurements were made in the University of Notre Dame’s compressible shear-layer facility that mixes highand low-speed flows with speeds up to M = 1.0. The forcing was performed using voice-coil actuators mounted at the trailing edge of the splitter plate separating the highand low-speed flows at the inlet to the mixing section. Numerical results were also computed using a discrete-vortex, weakly-compressible computer code, and are used to interpret the experimental data. The experimental and numerical results are used to develop a simple yet effective feedforward adaptive-optic correction strategy for the optical aberrations imposed by the shear layer on a traversing laser beam.

The Aero-Optical Environment of a Helicopter in Hover

Aero-optic aberrations for an optical system carried by a helicopter are expected to originate primarily from rotor-blade tip vortices that propagate down into the line of sight of the optical system. Landgrebe’s prescribed wake model in conjunction with a revised Biot-Savart law is used to calculate the velocity field beneath a medium-sized helicopter in hover. A thermodynamic numerical routine calculates the corresponding density and indexof-refraction fields associated with the velocity field. The approach is used to determine spatiallyand temporally-resolved predictions of the aero-optic aberrations on an optical system mounted on a hovering helicopter. Metrics for the farfield optical performance and potential adaptive-optic correction approaches are discussed.