Edward B. White

A Runback Criterion for Water Drops in a Turbulent Accelerated Boundary Layer

Predicting the runback threshold for liquid drops in aerodynamic boundary layers is a challenging problem with numerous applications including aircraft icing simulations. The critical parameters that govern drop runback are investigated in this experiment by using a wind tunnel that provides a turbulent accelerated flow similar to flows near an unswept wings leading edge. The experiments feature water drops on aluminum with a contact angle of 70 ±5 deg. Results show that significant water/air interface unsteadiness precedes drop runback. This is likely due to air-flow separation in the drop wakes. For displacement-thickness-scaled Reynolds numbers ranging from 348 to 429, a constant-Weber-number runback threshold We =3.45 ± 0.09 is found to adequately correlate the runback results.

Realistic Leading-Edge Roughness Effects on Airfoil Performance.

Wind farm operators observe power production decay over time, with the exact cause unknown and difficult to quantify. A likely explanation is blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Difficulty arises in understanding and quantifying performance degradation. Historically, wind turbine airfoil families were designed for the lift to be insensitive to roughness by simulating roughness with 2D trip strips. Despite this, roughness is still shown to negatively affect airfoil lift performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. Therefore, to better understand how real roughness effects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. This data will serve to validate and calibrate a one-equation, computational roughness amplification model that interacts with the Langtry-Menter transition model. The observed roughness contains 2D steps, heavy 2D erosion, pitting, insects, and repairs. Of these observations, 2D steps from paint chips were characterized and recreated for this particular wind tunnel entry. The model was tested at chord Reynolds numbers up to 3.6 × 10. Measurements of lift, drag, and pitching moment were made with and without roughness contamination. Transition location was acquired with infrared thermography and a hotfilm array. The paint roughness yields a consistent increase in drag compared to the clean configuration. Numerical simulations are only compared to the clean configuration and match well to lift, drag, and transition for Rec = 1.6 × 10. However, drag is overpredicted at Rec = 3.2 × 10.

Influence of 2D Steps and Distributed Roughness on Transition on a NACA 63(3)-418

Wind farm operators observe power production decrease over time. Quantifying performance degradation on individual components is difficult, exacerbating the problem. One potential explanation is accumulation of blade surface roughness, as wind turbines are continuously subjected to environmental hazards. Historically, wind turbine airfoils were designed for lift to be insensitive to roughness by simulating roughness with 2D trip strips. However, roughness was still shown to negatively affect airfoil performance. Experiments have also illustrated that random-distributed roughness is not properly simulated by trip strips. To better understand how real-world roughness affects performance, field measurements of turbine-blade roughness were made and simulated on an airfoil section in a wind tunnel. Of the observed roughness types, insect roughness and paint chips were characterized and recreated as distributed roughness and a forward-facing step. Distributed roughness was tested in three heights and two density configurations. The model chord Reynolds number was varied between 0.8 to 4.4 × 10. Measurements of lift, drag, and pitching moment were completed. Transition location was determined using both infrared thermography and hotfilm anemometry. Results indicate minimal performance loss due to paint-chip roughness. At Rec = 2.4 × 10, L/Dmax decreased 40% for the dense 140 μm, sparse-extended 140 μm, and sparse 200 μm roughness. This indicates that both density and height are critical performance parameters. Lastly, all but one configuration had Rek,crit within predictions from literature.

Experimental Measurement and CFD Model Development of Thick Wind Turbine Airfoils with Leading Edge Erosion

Leading edge erosion and roughness accumulation is an issue observed with great variability by wind plant operators, but with little understanding of the effect on wind turbine performance. In wind tunnels, airfoil models are typically tested with standard grit roughness and trip tape to simulate the effects of roughness and erosion observed in field operation, but there is a lack of established relation between field measurements and wind tunnel test conditions. A research collaboration between lab, academic, and industry partners has sought to establish a method to estimate the effect of erosion in wind turbine blades that correlates to roughness and erosion measured in the field. Measurements of roughness and erosion were taken off of operational utility wind turbine blades using a profilometer. The field measurements were statistically reproduced in the wind tunnel on representative tip and midspan airfoils. Simultaneously, a computational model was developed and calibrated to capture the effect of roughness and erosion on airfoil transition and performance characteristics. The results indicate that the effects of field roughness fall between clean airfoil performance and the effects of transition tape. Severe leading edge erosion can cause detrimental performance effects beyond standard roughness. The results also indicate that a heavily eroded wind turbine blade can reduce annual energy production by over 5% for a utility scale wind turbine.

Measurements of Water Droplet Movement in a Stagnation Point Boundary Layer

Improving numerical simulations of aircraft ice accretion will require empirical data on the behavior of liquid water on rough wing surfaces under glaze icing conditions. Toward this end, the present work presents a technique that provides detailed, full-field droplet interface height measurements using laser speckle shift correlations. The technique consists of illuminating a rough surface with coherent laser light and measuring the deformation of the resulting speckle pattern when a water drop is present on the rough surface. Computer algorithms have been developed that recover and integrate the speckle shift field and provide the reconstructed the droplet profile. Preliminary results are presented and possibilities for future improvements are discussed.

Analysis of the Impact of Leading Edge Surface Degradation on Wind Turbine Performance

Over time it has been reported wind turbine power output can diminish below manufacturers promised levels. This is clearly undesirable from an operator standpoint, and can also put pressure on turbine companies to make up the difference. A likely explanation for the discrepancy in power output is the contamination of the leading edge due to environmental conditions creating surfaces much coarser than intended. To examine the effects of airfoil leading edge roughness, a comprehensive study has been performed both experimentally and computationally on a NACA 633 − 418 airfoil. A description of the experimental setup and test matrix are provided, along with an outline of the computational roughness amplification model used to simulate rough configurations. The experimental investigation serves to provide insight into the changes in measurable airfoil properties such as lift, drag, and boundary layer transition location. The computational effort is aimed at using the experimental results to calibrate a roughness model that has been implemented in an unsteady RANS solver. Furthermore, a blade element momentum code was used to assess the impact on the performance of a turbine as whole due to discrepancies in clean vs. soiled airfoil characteristics. The results have implications in predicting the power loss due to leading edge surface roughness, and can help to establish an upper bound on admissible surface contamination levels.

Real-Time Measurement of Skin Friction Using Calibrated Multi-Element Hotfilm Arrays

This paper describes a general procedure for real-time calibrated measurement of skin friction using hotfilm sensors controlled by a constant voltage anemometer (CVA). Hotfilms are sensitive to air and substrate temperature and compensating for temperature dependence requires in situ estimates of heated and unheated hotfilm resistance. This can be done by rapidly alternating between overheat ratios. To verify the usefulness of this approach, a CVA was constructed to make in situ estimates of conductive dissipation into the substrate. Following previous work, the analysis approach assumes the air and substrate are in thermal equilibrium. Wind tunnel tests were conducted within a 3×4 foot wind tunnel at Texas A&M University. Hotfilms were installed along the chord of a NACA 0018 airfoil and calibrated to measure shear stress. The validity of the calibration approach using multiple overheats was tested for changing air and substrate temperature over 12C ranges. The calibration remains valid over temperature changes of this magnitude when the air and model remain in thermal equilibrium. However, when the air and substrate were not in thermal equilibrium over this temperature range the calibration failed. Future work must consider a more general formulation that accommodates unequal air and substrate temperatures and avoids electronic noise.