Even as computer technology evolves and databases of CFD results grow, only flow visualization offers an accurate and quick portrayal of the global flow field. In an arena of empirical correction factors and simplified systems of nonlinear partial differential equations, it is a welcome sanity check.
The ongoing quest for improved performance and drag reduction motivates visualization of boundary layer state. Infrared imaging has remained a relatively unexploited technique in this application. Previous investigations have been mostly limited to controlled wind tunnel environments. Exceptions have mostly required specialized expensive installations.
Infrared, however, can be used in situ quickly and inexpensively by deploying imaging systems remotely, as outlined in the following diagram.
With remote infrared, a surface can be analyzed with minimal modification to the subject surface. Unlike other visualization methods, no environmentally controvertible substances are employed and studies are not restricted to single test points for a given flight. Readily available commercial infrared imaging systems can be used.
Several flight experiments, performed in conjuction with Dan Banks of NASA Dryden Flight Research Center, have been performed to investigate the feasibility and extensibility of flow visualization via remote infrared.
The first flight experiment employed a T-34C as the subject aircraft and NASA Dryden's F/A-18 #846 with a preexisting AN/AAS-38 NITE Hawk targetting FLIR as the imaging aircraft. The T-34C is a good candidate for subsonic study, with sufficient overlap in performance envelope with the F/A-18 for the two aircraft to be flown together.
The infrared footage returned surface patterns that correlated with expected transition location. In the figure below, the outboard region of the right wing is shown. The leading edge is to the left. The whiter regions are warmer and correspond to laminar flow. Darker regions are cooler and are turbulent. Note the turbulent wedge coming off the trip near the leading edge.
Separation visualization studies with the T-34C were deferred because performance differences with the F/A-18 were too great. It was proposed that separation could be induced over a limited portion of the wing by installing a small "tab" normal to the wing surface. By inducing separation in this manner, the T-34C would be able to maintain sufficient speed to fly in formation with the F/A-18. However, CFD studies showed that such a tab would generate a stable vortex on the wing surface aft of the tab instead of separated flow characterized by a wake.
The second set of flight experiments was conducted at transonic speeds with a Learjet Model 24 as the subject aircraft.
The Learjet 24 employed had the Softflite kit installed for improved stall handling characteristics. The Softflite kit includes boundary layer energizers" (BLEs) on the outboard portions of the wing and raised triangles on the leading edge.
At 40,000 feet, Mach 0.77, the following infrared image of the outboard portion of the right wing was captured.
The leading edge is right of the center. The reflective chrome surface of the leading edge has a low emissivity and appears black. The tip tank is at the bottom of the image. The subject area, with black, insulative film applied, appears as a lighter region with distinct edges. A circular trip made with reflective aluminum tape is visible on the subject area near the leading edge. Boundary layer energizers, the boundary layer fence, and a reflection of the right engine nacelle are also visible. Neither intensity changes indicating transition nor a turbulent wedge off the trip are visible.
The lack of a turbulent wedge originating at the trip suggests that flow over the entire subject area is turbulent. The leading edge triangles of the Softflite kit likely contaminate the attachment line boundary layer, causing turbulent flow over the entire surface aft of them.
The inboard section of the right wing is shown below. The Softflite kit's leading edge triangles are not installed this far inboard. This thermogram was taken at 40,000 feet at approximately Mach 0.79.
Dan Banks, NASA Dryden Flight Research Center
