Brian K. Crawford

Effects of Step Excrescences on Swept-Wing Transition

An in-flight test article mounted on a Cessna O-2A Skymaster studying the effect of forwardand aft-facing steps on swept-wing boundary-layer transition is described. An accurate instrumentation suite, consisting of a five-hole probe, static pressure ports, and an infrared camera is presented. Pressure coefficient profiles compare favorably with a computational validation. Infrared thermography is used as a global transition detection technique. Step height and transition location based Reynolds numbers are computed and compared with unswept transition studies. Based on the current data, the addition of the crossflow instability is believed to reduce the critical step height Reynolds number for both forwardand aft-facing steps.

Effects of Step Excrescences on a Swept Wing in a Low-Disturbance Wind Tunnel

A 30° swept-wing model with a movable, leading-edge extending to 15% chord is used in low-disturbance wind-tunnel tests to study the effect of two-dimensional, step excrescences in a three-dimensional boundary layer. Forwardand aft-facing steps are modulated during the tests. Pressure measurements are compared with computational results, infrared thermography is used to globally detect boundary-layer transition, and hotwire measurements provide details of the boundary-layer profiles in the vicinity of the steps. An analysis of the results is provided including data from tests in a flight environment and from experimental studies of an unswept model of equivalent 2-D pressure gradient.

Effects of Step-Excrescence Location on Swept-Wing Transition

Two-dimensional step excrescences on a swept-wing model are studied in two lowdisturbance environments, wind-tunnel and flight. The present study focuses at 1% chord. An extensive comparison is made with previous flight and wind-tunnel results at 15% chord using the same test article and same initial and boundary conditions. The combined sensitivity of curvature effects and streamwise location of the step excrescence is examined. Infrared thermography and hotwire anemometry results are presented and will be used to validate the companion computational effort by Tufts et al.

Quantitative Boundary-Layer Transition Measurements Using IR Thermography

Infrared (IR) thermography has been frequently utilized for detection of laminar-turbulent transition. This technique exploits the large differential in shear stress, and thereby convection rate between laminar and turbulent flow. Typically, the model is either heated or cooled during the test. The turbulent region is then driven closer to the freestream temperature by the locally high convection rate, while the laminar region is comparatively insulated from the freestream. This differential can then be detected using an IR camera. Details of this technique can be found in Ref. 1. This technique provides a global measurement of the transition front with high fidelity. However, the resulting images are often noisy due to reflections on the surface, warm spots caused by heat from internal electronics, nonuniform temperatures generated by the model’s internal structure, sunlight and shadows on the model, etc. While these effects can be mitigated by careful test and model design, they cannot, in general, be entirely avoided. Because of this, image filtering is highly desirable to make the images easier to interpret. Traditionally, these images are then analyzed by a human in order to visually extract a transition location. This procedure is typically limited to an accuracy of approximately ±5% chord, and is subject to significant human bias error. As such, a more quantitative method is required. Additionally, manually adjusting the colorization and filtering of IR images using the stock FLIR ExaminIR software, and then visually extracting a representative transition location is a time consuming process. This rapidly becomes time prohibitive when hundreds or thousands of images require processing and analysis.

Flight experiments on the effects of step excrescences on swept-wing transition

A 30° swept-wing model with a movable, leading-edge extending to 15% chord is used in flight tests to study the effect of two-dimensional, step excrescences on swept-wing transition, where stationary-crossflow waves are typically the dominant instability. Transport unit Reynolds numbers are achieved using a Cessna O-2A Skymaster. Forward- and aft-facing steps are modulated in-flight. Pressure measurements are compared with CFD. Infrared thermography is used to globally detect boundary-layer transition. When the 2-D pressure gradient matches the unswept case, the swept-wing case has a lower Rekk,crit. However, there is still potential to relax conventional, laminar-flow tolerances for steps.

Computational design of a test article to investigate 2-D surface excrescences on a swept laminar-flow wing

The construction of a spanwise-invariant swept-wing test article designed to facilitate the inclusion of a range of two-dimensional (2-D) step and gap excrescences in flight via an internal articulation mechanism has been completed. Using a finite-element Navier-Stokes solution and a spectrally accurate boundary-layer solver, coupled with stability analyses, the model has been designed to be subcritical to all instabilities except the crossflow instability. The model can be safely flown in a flight experiment, and be mounted in the Klebanoff-Saric Wind Tunnel at Texas A&M University. The model can be tested at multiple angles of attack (pressure gradients), as well as multiple Reynolds numbers, including unit Reynolds numbers typical of transports. Stability behaviour of the test article was designed to be conducive to a thorough examination of the interaction between an inherent crossflow instability and the shear layer created by the step and gap excrescences. The computations and experiments will together provide data and correlations complementing the previous studies of 2-D excrescences on an unswept flat plate and wedges subject to favourable pressure gradients by The Northrop Grumman Corporation. The current paper focuses on the design and stability analyses of the smooth-wing test article, that is, without excrescences present, as a baseline configuration.

Computational Investigation of the Effects of Surface Imperfections and Excrescences on the Crossflow Instability

This paper describes a computational investigation and companion to the current work at the Texas A&M Engineering Experiment Station (TEES) Flight Research Lab (FRL). The FRL is both operating an O-2A aircraft and conducting wind tunnel tests in the Klebanoff Saric Wind Tunnel (KSWT) in the investigation of surface excrescences on the transition of laminar to turbulent flow. Using a finite-element Navier-Stokes solution and a spectrally accurate boundary-layer solver, coupled with linear and nonlinear stability analyses, it is proposed to quantify the effect of surface imperfections and outer mold line (OML) non-uniformities on crossflow instabilities. The quantitative goal of the proposed research is to computationally evaluate a wide parameter space of step heights and gaps, and develop correlation models between geometry and estimated effect on transition for use by designers. The proposed computations will be validated against experiments on the physical model both in-flight and in the KSWT, and vice versa, in a tightly integrated program. This paper is a companion paper to “Effects of Step Excrescences on Swept-Wing Transition” by Duncan et al. also submitted to the 31st Applied Aerodynamics Conference.

Flexible Flight Research Platform at Texas A&M University Flight Research Laboratory

The Flight Research Laboratory (FRL) at Texas A&M University has completed several collaborative projects to gather flight data for several laboratory experiments. The experiments were hosted on a restricted-category O-2A aircraft for inexpensive, reliable, and flexible flight research. For example, a LADAR system was installed for airborne detection of simulated wildlife targets against various backgrounds. Also, a shear-sensitive paint was applied to a pylon-mounted surface and imaged at a range of airspeeds. Another project required aerial photography on a pre-programmed ground track that was coupled to the autopilot. Finally, an atmospheric sampling mission over industrial areas was planned and executed.