Extraction of skin-friction fields from surface flow visualizations as an inverse problem

Liu, Tianshu

doi:10.1088/0957-0233/24/12/124004

Cited by 39 publications

(36 citation statements)

References 41 publications

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“…Using the OFV, it was possible to qualitatively resolve streamlines associated with an equivalent skin-friction vector field near the surface of the suction side of the airfoil [15][16][17] , as seen in Fig. 5 for Re c = 3.64•10 5 .…”

Section: Resultsmentioning

confidence: 99%

“…Using the flow visualization, the skin-friction field near the surface was obtained by solving the Euler-Lagrange equations [15][16][17] :…”

Section: Methodsmentioning

confidence: 99%

“…where g is the normalized luminescent intensity of the oil,  is equivalent skin-friction vector, and  is a Lagrange multiplier chosen to allow the solution to converge; the value of f comprises an additional set of parameters, as defined by Liu 17 . By performing this analysis, along with a multi-scale decomposition 15 , it is assumed that the thickness of the oil at a given location is proportional to the skin-friction.…”

Section: Methodsmentioning

confidence: 99%

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Measurements of 3-D Stall Cells on 2-D Airfoil

DeMauro

Dell'Orso

Sivaneri

et al. 2015

45th AIAA Fluid Dynamics Conference

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The present work aims to identify and quantify underlying physics behind the formation of three-dimensional stall cell using oil flow visualization and stereoscopic particle image velocimetry (SPIV) on a two-dimensional NACA 0015 airfoil. The three dimensional structures were explored at various angles of attack ( = 14 o to 18 o ) and Reynolds numbers (Re c = 1.62•10 5 to 4.2•10 5 ). Surface oil flow visualization was used to qualitatively identify the stall cells and resolve associated equivalent near surface skin friction. In addition, SPIV measurements were taken in order to visualize the stall cells above the surface of the airfoil. Results showed that the stall cells are highly sensitive to Reynolds number and angle of attack, with evidence of an apparent bi-stable state existing at a Reynolds number of 3.83•10 5 . Using SPIV the flow fields and the associated vorticity fields for the various cases were measured and correlated to the surface oil flow visualization.luminescent intensity of oil n = Number of stall cells Re c = Reynolds number based on airfoil's chord (Re c = U ∞ c  ) AIAA Aviation t = Time (s) U = Time-averaged streamwise velocity (m/s) U ∞ = Freestream velocity (m/s) V = Time-averaged cross-stream velocity (m/s) V tot = Time-averaged total velocity (m/s) W Time-averaged spanwise velocity (m/s) x = Streamwise direction (m) y = Cross-stream direction (m) z = Spanwise direction (m)  = Angle of attack (°)  = Lagrange multiplier  = Kinematic viscosity of air (m 2 /s)  = Equivalent skin-friction vector  tot = Time-averaged total vorticity (1/s) AR = Airfoil's aspect ratio (AR = b c ) OFV = Oil Flow Visualization SPIV = Stereoscopic Particle Image Velocimetry IntroductionThe study of flow about nominally two-dimensional airfoils or bluff bodies has been used by researchers for years as a way of defining aerodynamic characteristics, sometimes disregarding end conditions. Through the use of end plates or by extending the edges of a model to the walls of a wind tunnel, experiments attempt to suppress strong spanwise velocity components, nominally resulting in two-dimensional separation lines and shear layers. However, work by researchers such as Gregory et al. 1 identified that three-dimensionalities can develop downstream of the separation point and are independent of the type of end condition employed. Crow 2 indicated that structures developed from two nearly parallel vortex lines whose induced velocity fields amplified the slight random displacements inherently present in their behavior. Moreover, Humphreys 3 and Schewe 4 reported that these three-dimensionalities resulted in cellular patterns downstream of boundary layer separation on circular cylinders.For airfoils at post-stall angles of attack, these three-dimensional structures are termed "stall cells" and may be a source of buffeting and unsteady forces 5 . As outlined by Boiko et al. 6 , knowledge of the existence of these structures and attempts to control them has been reported in the literature for years. In each case, it was noted...

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Section: Resultsmentioning

confidence: 99%

“…Using the flow visualization, the skin-friction field near the surface was obtained by solving the Euler-Lagrange equations [15][16][17] :…”

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Measurements of 3-D Stall Cells on 2-D Airfoil

DeMauro

Dell'Orso

Sivaneri

et al. 2015

45th AIAA Fluid Dynamics Conference

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show abstract

“…Validation of this scenario can be provided by high-resolution skin friction maps on the cylinder surface. To this aim, following the physics-based optical flow methodology developed for global skin friction evaluation from TSP images (Liu and Woodiga 2011;Liu 2013) mean skin friction lines are calculated. From the mean surface temperature field obtained with an appropriate TSP calibration curve (not shown here), the variational method has been used to find the inverse solution to a Horn-Schunck optical flow equation where the optical flow is replaced by the skin friction vector (Liu and Woodiga 2011).…”

Section: Secondary Separation Linementioning

confidence: 99%

“…In this respect, Fey et al (2013) make use of a newly developed TSP to investigate a water flow around a circular cylinder up to Reynolds number 14,500. The feasibility of TSP as a tool to obtain an estimate of global skin friction was investigated by Liu and Woodiga (2011) and discussed from the unified perspective of an inverse problem in Liu (2013).…”

mentioning

confidence: 99%