Airfoil transition and separation studies using an infrared imaging system

Gartenberg, Ehud; Roberts, A. S.

doi:10.2514/3.46016

Cited by 26 publications

(12 citation statements)

References 4 publications

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“…They are equal to zero up to x/c ≈ 0.3, indicating that the boundary layer is laminar up to this point, and their subsequent progressive increase is due to the appearance and development of the vortical structures visible in Figure (5). Transition from a laminar state thus appears to begin considerably too close to the leading-edge, as experimental data 18,27,28 indicate the transition zone to start between x/c = 0.6 and x/c = 0.7. Figure (5) presents a snapshot of the instantaneous vorticity field around the NACA0012 airfoil.…”

Section: Two-dimensional Airfoilmentioning

confidence: 98%

“…the beginning of the fully turbulent zone-at x/c = 0.78. Gartenberg and Roberts, 27 working at a lower Reynolds number of 3.75 × 10 5 , found the boundary layer to remain laminar up to x/c = 0.8, while Lee and Kang 18 found the transition zone for an airfoil at a Reynolds number of 6 × 10 5 to be located between x/c = 0.62 and x/c = 0.78. It should be noted that the location and length of the transition zone is very dependent on experimental conditions, and in particular on the background turbulence level of the upstream flow.…”

Section: Three-dimensional Airfoilmentioning

confidence: 99%

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Noise Radiated by a High-Reynolds-number 3-D Airfoil

Marsden

Bogey

Bailly

2005

11th AIAA/CEAS Aeroacoustics Conference

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A large-eddy simulation (LES) of the flow around a NACA0012 airfoil at a chord-based Reynolds number of 5.0 × 10 6 and a Mach number of 0.22 is performed, and its results are compared with experimental data. The airfoil is placed without incidence to the turbulence-free incoming flow. The boundary layer in this configuration is expected to be initially laminar, and to transition to a turbulent state along the second half of the airfoil chord, and as such constitutes a challenging test-case for LES computations to reproduce. The LES calculation is performed with a parallel code resolving the full compressible Navier-Stokes equations on structured curvilinear grids with optimized explicit high-order finite-difference schemes and filters. A preliminary two-dimensional simulation and a full three-dimensional simulation are performed for the same airfoil configuration. The twodimensional simulation shows substantial discrepancies with available experimental data. On the other hand, flow results from the three-dimensional simulation compare favorably, and the LES is shown to capture boundary layer dynamics reasonably well. These results are a first step towards the development of the direct computation of noise emitted by a high-Reynolds-number airfoil.

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Section: Two-dimensional Airfoilmentioning

confidence: 98%

Section: Three-dimensional Airfoilmentioning

confidence: 99%

Noise Radiated by a High-Reynolds-number 3-D Airfoil

Marsden

Bogey

Bailly

2005

11th AIAA/CEAS Aeroacoustics Conference

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“…The local surface temperature of the rotor blade depends on the local flow-related heat transfer coefficient. [20,21]. This results in a thermal fingerprint that correlates with the friction coefficient of the different flow areas of the boundary layer.…”

Section: Thermographymentioning

confidence: 99%

Investigation of Laminar–Turbulent Transition on a Rotating Wind-Turbine Blade of Multimegawatt Class with Thermography and Microphone Array

et al. 2019

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Knowledge about laminar–turbulent transition on operating multi megawatt wind turbine (WT) blades needs sophisticated equipment like hot films or microphone arrays. Contrarily, thermographic pictures can easily be taken from the ground, and temperature differences indicate different states of the boundary layer. Accuracy, however, is still an open question, so that an aerodynamic glove, known from experimental research on airplanes, was used to classify the boundary-layer state of a 2 megawatt WT blade operating in the northern part of Schleswig-Holstein, Germany. State-of-the-art equipment for measuring static surface pressure was used for monitoring lift distribution. To distinguish the laminar and turbulent parts of the boundary layer (suction side only), 48 microphones were applied together with ground-based thermographic cameras from two teams. Additionally, an optical camera mounted on the hub was used to survey vibrations. During start-up (SU) (from 0 to 9 rpm), extended but irregularly shaped regions of a laminar-boundary layer were observed that had the same extension measured both with microphones and thermography. When an approximately constant rotor rotation (9 rpm corresponding to approximately 6 m/s wind speed) was achieved, flow transition was visible at the expected position of 40% chord length on the rotor blade, which was fouled with dense turbulent wedges, and an almost complete turbulent state on the glove was detected. In all observations, quantitative determination of flow-transition positions from thermography and microphones agreed well within their accuracy of less than 1%.

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“…The extent of the laminar and turbulent flow regimes is marked in each image. The evaluation of the mean value shows the existing temperature gradients within the laminar and turbulent flow regimes due to the chord-position dependency of the friction coefficient and, thus, heat flux [1,5,10]. Additionally, the area in the middle of the image at the end of the laminar flow regime is superimposed with a reflection from the camera lens.…”

Section: Flow Visualizationmentioning

confidence: 99%

“…One possibility of visualizing the boundary layer flow on an airfoil is given by the thermographic flow visualization that makes use of the relation between the heat transfer coefficient and the local skin friction between the fluid and the surface [2]. The technique is an already long established method in wind tunnel experiments to visualize the boundary layer flow [3][4][5] and enables the analysis of the laminar-turbulent flow transition [6,7], the laminar separation bubble [8,9] and turbulent separation [10]. For wind turbines in operation, the thermographic flow visualization is particularly suitable because it is a noninvasive, contactless approach without the need for surface preparation [11].…”

Section: Introductionmentioning

confidence: 99%

Laminar-Turbulent Transition Localization in Thermographic Flow Visualization by Means of Principal Component Analysis

et al. 2021

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Thermographic flow visualization is a contactless, non-invasive technique to visualize the boundary layer flow on wind turbine rotor blades, to assess the aerodynamic condition and consequently the efficiency of the entire wind turbine. In applications on wind turbines in operation, the distinguishability between the laminar and turbulent flow regime cannot be easily increased artificially and solely depends on the energy input from the sun. State-of-the-art image processing methods are able to increase the contrast slightly but are not able to reduce systematic gradients in the image or need excessive a priori knowledge. In order to cope with a low-contrast measurement condition and to increase the distinguishability between the flow regimes, an enhanced image processing by means of the feature extraction method, principal component analysis, is introduced. The image processing is applied to an image series of thermographic flow visualizations of a steady flow situation in a wind tunnel experiment on a cylinder and DU96W180 airfoil measurement object without artificially increasing the thermal contrast between the flow regimes. The resulting feature images, based on the temporal temperature fluctuations in the images, are evaluated with regard to the global distinguishability between the laminar and turbulent flow regime as well as the achievable measurement error of an automatic localization of the local flow transition between the flow regimes. By applying the principal component analysis, systematic temperature gradients within the flow regimes as well as image artefacts such as reflections are reduced, leading to an increased contrast-to-noise ratio by a factor of 7.5. Additionally, the gradient between the laminar and turbulent flow regime is increased, leading to a minimal measurement error of the laminar-turbulent transition localization. The systematic error was reduced by 4% and the random error by 5.3% of the chord length. As a result, the principal component analysis is proven to be a valuable complementary tool to the classical image processing method in flow visualizations. After noise-reducing methods such as the temporal averaging and subsequent assessment of the spatial expansion of the boundary layer flow surface, the PCA is able to increase the laminar-turbulent flow regime distinguishability and reduce the systematic and random error of the flow transition localization in applications where no artificial increase in the contrast is possible. The enhancement of contrast increases the independence from the amount of solar energy input required for a flow evaluation, and the reduced errors of the flow transition localization enables a more precise assessment of the aerodynamic condition of the rotor blade.

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Airfoil transition and separation studies using an infrared imaging system

Cited by 26 publications

References 4 publications

Noise Radiated by a High-Reynolds-number 3-D Airfoil

Noise Radiated by a High-Reynolds-number 3-D Airfoil

Investigation of Laminar–Turbulent Transition on a Rotating Wind-Turbine Blade of Multimegawatt Class with Thermography and Microphone Array

Laminar-Turbulent Transition Localization in Thermographic Flow Visualization by Means of Principal Component Analysis

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