Andy P. Broeren

Spanwise Variation in the Unsteady Stalling Flowfields of Two-Dimensional Airfoil Models

Recent investigations of two-dimensional airfoil stalling characteristics have revealed low-frequency and highly unsteady flow in some cases and large-scale three-dimensional structures in other cases. The latter were referred to as stall cells and can form on two-dimensional configurations where the ends of the airfoil model are flush with tunnel side walls or end plates. We present results of detailed investigations of the stalling characteristics of several airfoils that exhibited both low-frequency unsteadiness and large-scale three-dimensional structures. The airfoils were wind-tunnel tested in a two-dimensional configuration. The primary measurements were spanwise wake velocity and mini-tuft flow visualization. The results showed that airfoils with trailing-edge separations at and above maximum lift (static stall) exhibited stall-cell patterns. Conversely, airfoils that had leading-edge separation bubbles that grew in size as the angle of attack was increased into stall developed the low-frequency, highly unsteady flow

Fundamental Ice Crystal Accretion Physics Studies

Ice accretion within an engine due to ice crystal ingestion is being investigated because of numerous engine power-loss events associated with high-altitude convective weather. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. Two weeks of testing occurred at the NRC Research Altitude Facility in November 2010. The tests utilized a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15 °C. Ice and liquid water contents ranged up to 20 and 3 g/m3, respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leading-edge ice growth to a thickness greater than 15 mm in 3 min. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared correlate with the wet-bulb temperature, which was estimated to be above 0 °C in tests at 93 kPa and below 0 °C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component such as a stator blade to degrade significantly, and could damage downstream components if shed.

Effect of High-Fidelity Ice-Accretion Simulations on Full-Scale Airfoil Performance

The simulation of ice accretion on a wing or other surface is often required for aerodynamic evaluation, particularly at small scale or low Reynolds number. Although there are commonly accepted practices for ice simulation, there are no established and validated guidelines. The purpose of this paper is to report the results of an experimental study establishing a high-fidelity, full-scale, iced-airfoil aerodynamic performance database. This research was conducted as a part of a larger program with the goal of developing subscale aerodynamic simulation methods for iced airfoils. Airfoil performance testing was carried out at the ONERA F1 pressurized wind tunnel using a 72 in. (1828.8 mm) chord NACA 23012 airfoil over a Reynolds number range of 4.5 x 10 6 to 16.0 × 10 6 and a Mach number range of 0.10 to 0.28. The high-fidelity ice-casting simulations had a significant impact on the aerodynamic performance. A spanwise-ridge ice shape resulted in a maximum lift coefficient of 0.56 compared with the clean value of 1.85 at Re = 15.9 x 10 6 and M = 0.20. Two roughness and streamwise shapes yielded maximum lift values in the range of 1.09 to 1.28, which was a relatively small variation compared with the differences in the ice geometry. The stalling characteristics of the two roughness ice simulations and one streamwise ice simulation maintained the abrupt leading-edge stall type of the clean NACA 23012 airfoil, despite the significant decrease in maximum lift. Changes in Reynolds and Mach numbers over the large range tested had little effect on the iced-airfoil performance.

A Wind Tunnel Study of Icing Effects on a Business Jet Airfoil

Aerodynamic wind tunnel tests were conducted to study the effects of various ice accretions on the aerodynamic performance of a 36-inch chord, two-dimensional business jet airfoil. Eight different ice shape configurations were tested. Four were castings made from molds of ice shapes accreted in an icing wind tunnel. Two were made using computationally smoothed tracings of two of the ice shapes accreted in the icing tunnel. These smoothed profiles were then extended in the spanwise direction to form a twodimensional ice shape. The final two configurations were formed by applying grit to the smoothed ice shapes. The ice shapes resulted in as much as 48% reduction in maximum lift coefficient from that of the clean airfoil. Large increases in drag and changes in pitching moment were also observed. The castings and their corresponding smoothed counterparts yielded similar results. Little change in performance was observed with the addition of grit to the smoothed ice shapes. Changes in the Reynolds number (from 3×10 to 10.5×10) and Mach number (from 0.12 to 0.28) did not significantly affect the iced-airfoil performance coefficients.

Effect of Intercycle Ice Accretions on Airfoil Performance

Results are presented of an experimental study designed to characterize and evaluate the aerodynamic performance penalties of residual and intercycle ice accretions that result from the cyclic operation of a typical aircraft deicing system. Icing wind-tunnel tests were carried out on a 36-in. chord NACA 23012 airfoil section equipped with a pneumatic deicer for several different Federal Air Regulation 25 Appendix C cloud conditions. Results from the icing tests showed that the intercycle ice accretions were much more severe in terms of size and shape than the residual ice accretions. Molds of selected intercycle ice shapes were made and converted to castings that were attached to the leading edge of a 36-in. chord NACA 23012 airfoil model for aerodynamic testing. The aerodynamic testing revealed that the intercycle ice shapes caused a significant performance degradation. Maximum lift coefficients were typically reduced about 60% from 1.8 (clean) to 0.7 (iced) and stall angles were reduced from 17 deg (clean) to 9 deg (iced). Changes in the Reynolds number (from 2.0 × × 10 6 to 10.5 × 10 6 ) and Mach number (from 0.10 to 0.28) did not significantly affect the iced-airfoil performance.

Aerodynamic Simulation of a Horn-Ice Accretion on a Subscale Model

The objective of this experimental investigation was to determine the geometric simulation fidelity required to accurately model the aerodynamics of a horn-ice accretion in a wind tunnel. A casting and a 2-D smooth simulation with variable horn geometry were constructed to model a horn-ice accretion on a NACA 0012 airfoil. Several simulations of differing fidelity, including a casting, were constructed to model a horn-ice accretion on a NACA 23012 airfoil. Aerodynamic testing was performed in the University of Illinois 3 x 4 ft wind tunnel at a Reynolds number of 1.8 x 10 6 and a Mach number of 0.18. Minor changes to the upper-horn geometry of the NACA 0012 2-D smooth simulation were found to have notable impacts on drag and maximum lift. Therefore, spanwise variations in the ice accretion geometry must be carefully examined so that an appropriate cross section can be chosen from which to generate a tracing for a 2-D simulation. Such a 2-D smooth simulation, as was constructed for the NACA 23012 airfoil, can model maximum lift to within 1 % of that of the casting. This type of simulation can also provide an estimate of drag that is within the uncertainty of the casting due to spanwise variation, although it does not reproduce three dimensionality in the iced-airfoil flowfield.

Boundary layer trips on airfoils at low reynolds numbers

Three categories of boundary layer trips (single 2-D plain, multiple 2-D plain, and 3-D trips) were tested on the M06-13-128, E374, and SD7037 airfoils over the Reynolds numbers of 100,000 to 300,000. Flow visualization and drag data were acquired for a number of trip heights and locations. To facilitate comparisons between airfoils, trips were located relative to untripped laminar separation locations. Drag data showed dramatic drag reductions for relatively thin trips, with thicker trips having slightly better performance. The trip location proved to be of little significance for trips located upstream of laminar separation. Little advantage was seen in utilizing multiple 2-D trips or complex 3-D trips over single 2-D trips. Finally, through the application of trips, it was not possible to improve the performance of an airfoil exhibiting large laminar separation bubbles over that of an untripped airfoil with small bubbles.

Airfoil Ice-Accretion Aerodynamics Simulation

NASA Glenn Research Center, ONERA, and the University of Illinois are conducting a major research program whose goal is to improve our understanding of the aerodynamic scaling of ice accretions on airfoils. The program when it is completed will result in validated scaled simulation methods that produce the essential aerodynamic features of the full-scale iced-airfoil. This research will provide some of the first, high-fidelity, full-scale, iced-airfoil aerodynamic data. An initial study classified ice accretions based on their aerodynamics into four types: roughness, streamwise ice, horn ice, and spanwise-ridge ice. Subscale testing using a NACA 23012 airfoil was performed in the NASA IRT and University of Illinois wind tunnel to better understand the aerodynamics of these ice types and to test various levels of ice simulation fidelity. These studies are briefly reviewed here and have been presented in more detail in other papers. Based on these results, full-scale testing at the ONERA F1 tunnel using cast ice shapes obtained from molds taken in the IRT will provide full-scale iced airfoil data from full-scale ice accretions. Using these data as a baseline, the final step is to validate the simulation methods in scale in the Illinois wind tunnel. Computational ice accretion methods including LEWICE and ONICE have been used to guide the experiments and are briefly described and results shown. When full-scale and simulation aerodynamic results are available, these data will be used to further develop computational tools. Thus the purpose of the paper is to present an overview of the program and key results to date.

Implementation and Validation of 3-D Ice Accretion Measurement Methodology

A research program has been implemented to develop and validate the use of a commercial 3-D laser scanning system to record ice accretion geometry in the NASA Icing Research Tunnel. A main component of the program was the geometric assessment of the 3- D laser scanning system on a 2-D (straight wing) and a 3-D (swept wing) airfoil geometries. This exercise consisted of comparison of scanned ice accretion to castings of the same ice accretion. The scan data were also used to create rapid prototype artificial ice shapes that were scanned and compared to the original ice accretion. The results from geometric comparisons on the straight wing showed that the ice shape models generated through the scan/rapid prototype process compared reasonably well with the cast shapes. Similar results were obtained with the geometric comparisons on the swept wing. It was difficult to precisely compare the scans of the cast shapes to the original ice accretion scans because the cast shapes appear to have shrunk during the mold/casting process by as much as 0.10-inch. However the comparison of the local ice-shape features were possible and produced better results. The rapid prototype manufacturing process was shown to reproduce the original ice accretion scan normally within 0.01-inch.

Development of 3D Ice Accretion Measurement Method

A research plan is currently being implemented by NASA to develop and validate the use of a commercial laser scanner to record and archive fully three-dimensional ice shapes from an icing wind tunnel. The plan focused specifically upon measuring ice accreted in the NASA Icing Research Tunnel (IRT). The plan was divided into two phases. The first phase was the identification and selection of the laser scanning system and the post-processing software to purchase and develop further. The second phase was the implementation and validation of the selected system through a series of icing and aerodynamic tests. Phase I of the research plan has been completed. It consisted of evaluating several scanning hardware and software systems against an established selection criteria through demonstrations in the IRT. The results of Phase I showed that all of the scanning systems that were evaluated were equally capable of scanning ice shapes. The factors that differentiated the scanners were ease of use and the ability to operate in a wide range of IRT environmental conditions.