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Book ChapterDOI

Investigation of Flow Separation Lines Over a Finite Wing

01 Jan 2021-pp 247-255
TL;DR: In this article, the location of flow separation lines over finite wings is carried out using numerical simulations in ANSYS (FLUENT) using numerical simulation in finite wing is not straight forward as flow has an additional degree of freedom to move when it detaches from surface of wing.
Abstract: Flow over airfoil is extensively studied to identify point of flow separation, transition, re-attachment (formation of laminar separation bubble), and to understand relation between aerodynamic forces generated by airfoil with change in angles of attack Extension of flow separation concept to finite wing is not straight forward as flow has an additional degree of freedom to move when it detaches from surface of wing A preliminary investigation of location of flow separation lines over finite wings is carried out using numerical simulations in ANSYS (FLUENT)
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Journal ArticleDOI
TL;DR: The proposed networks provide a general framework for instance detection, keypoint detection and classification that can be fine-tuned to the specific experimental application and imaging parameters using synthetic data.
Abstract: 3D particle streak velocimetry (3D-PSV) and surface flow visualization using tufts both require the detection of curve segments, particle streaks or tufts, in images. We propose the use of deep learning based instance segmentation neural networks Mask region-based convolutional neural network (R-CNN) and Cascade Mask R-CNN, trained on fully synthetic data, to accurately identify, segment, and classify streaks and tufts. For 3D-PSV, we use the segmented masks and detected streak endpoints to volumetrically reconstruct flows even when the imaged streaks partly overlap or intersect. In addition, we use Mask R-CNN to segment images of tufts and classify the detected tufts according to their range of motion, thus automating the detection of regions of separated flow while at the same time providing accurate segmentation masks. Finally, we show a successful synthetic-to-real transfer by training only on synthetic data and successfully evaluating real data. The synthetic data generation is particularly suitable for the two presented applications, as the experimental images consist of simple geometric curves or a superposition of curves. Therefore, the proposed networks provide a general framework for instance detection, keypoint detection and classification that can be fine-tuned to the specific experimental application and imaging parameters using synthetic data.

3 citations

References
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Journal ArticleDOI
TL;DR: In this paper, the authors derived an exact theory of three-dimensional steady separation and reattachment using nonlinear dynamical systems methods and obtained criteria for separation points and separation lines on fixed no-slip boundaries in compressible flows.
Abstract: We derive an exact theory of three-dimensional steady separation and reattachment using nonlinear dynamical systems methods. Specifically, we obtain criteria for separation points and separation lines on fixed no-slip boundaries in compressible flows. These criteria imply that there are only four basic separation patterns with well-defined separation surfaces. We also derive a first-order prediction for the separation surface using wall-based quantities; we verify this prediction using flow models obtained from local expansions of the Navier–Stokes equations.

135 citations

Journal ArticleDOI
TL;DR: In this paper, a comprehensive approach applicable to incompressible and compressible steady-state flows as well as incompressibly unsteady flow is presented, which is similar to earlier topological approaches to separation but is more complete and adds more emphasis to certain points than in the past.
Abstract: In recent years there has been extensive research on three-dimensional flow separation. There are two different approaches: the phenomenological approach and a mathematical approach using topology. These two approaches are reviewed briefly and the shortcomings of some of the past works are discussed. A comprehensive approach applicable to incompressible and compressible steady-state flows as well as incompressible unsteady flow is then presented. The approach is similar to earlier topological approaches to separation but is more complete and in some cases adds more emphasis to certain points than in the past. To assist in the classification of various types of flow, nomenclature is introduced to describe the skin-friction portraits on the surface. This method of classification is then demonstrated on several categories of flow to illustrate particular points as well as the diversity of flow separation. The categories include attached, two-dimensional separation and three different types of simple, three-dimensional primary separation, secondary separation, and compound separation. Hypothetical experiments are utilized to illustrate the topological terminology and its role in characterizing these flows. These hypothetical experiments use colored oil injected onto the surface at singular points in the skin-friction portrait. Actual flow-visualization information, if available, is used to corroborate the hypothetical examples.

70 citations

Proceedings ArticleDOI
24 Jun 2013
TL;DR: In this paper, the results from an ongoing effort to develop an aerodynamic database from Reynolds-Averaged Navier-Stokes (RANS) computational analysis of airfoils and wings at stall and post-stall angles of attack are presented.
Abstract: This paper presents selected results from an ongoing effort to develop an aerodynamic database from Reynolds-Averaged Navier-Stokes (RANS) computational analysis of airfoils and wings at stall and post-stall angles of attack. The data obtained from this effort will be used for validation and refinement of a low-order post-stall prediction method developed at NCSU, and to fill existing gaps in high angle of attack data in the literature. Such data could have potential applications in post-stall flight dynamics, helicopter aerodynamics and wind turbine aerodynamics. An overview of the NASA TetrUSS CFD package used for the RANS computational approach is presented. Detailed results for three airfoils are presented to compare their stall and post-stall behavior. The results for finite wings at stall and post-stall conditions focus on the effects of taper-ratio and sweep angle, with particular attention to whether the sectional flows can be approximated using two-dimensional flow over a stalled airfoil. While this approximation seems reasonable for unswept wings even at post-stall conditions, significant spanwise flow on stalled swept wings preclude the use of two-dimensional data to model sectional flows on swept wings. Thus, further effort is needed in low-order aerodynamic modeling of swept wings at stalled conditions.

33 citations

Proceedings ArticleDOI
01 Jun 1998
TL;DR: The correlation between flight and wind tunnel forebody pressure distributions for the F-18 HARV were improved by using twin longitudinal grit strips on the forebody of the wind-tunnel model and the detrimental effect of a very sharp nose apex was demonstrated on the X-31 aircraft.
Abstract: Lessons learned from comparisons between ground-based tests and flight measurements for the high-angle-of-attack programs on the F-18 High Alpha Research Vehicle (HARV), the X-29 forward-swept wing aircraft, and the X-31 enhanced fighter maneuverability aircraft are presented. On all three vehicles, Reynolds number effects were evident on the forebodies at high angles of attack. The correlation between flight and wind tunnel forebody pressure distributions for the F-18 HARV were improved by using twin longitudinal grit strips on the forebody of the wind-tunnel model. Pressure distributions obtained on the X-29 wind-tunnel model at flight Reynolds numbers showed excellent correlation with the flight data up to alpha = 50 deg. Above (alpha = 50 deg. the pressure distributions for both flight and wind tunnel became asymmetric and showed poorer agreement, possibly because of the different surface finish of the model and aircraft. The detrimental effect of a very sharp nose apex was demonstrated on the X-31 aircraft. Grit strips on the forebody of the X-31 reduced the randomness but increased the magnitude of the asymmetry. Nose strakes were required to reduce the forebody yawing moment asymmetries and the grit strips on the flight test noseboom improved the aircraft handling qualities.

11 citations

Journal ArticleDOI
V C Patel1
TL;DR: The grammar that is used to describe various facets of separation of three-dimensional flow is reviewed, and some recent numerical and experimental results are used to illustrate the outstanding difficulties of the subject.
Abstract: Separation of three-dimensional flow, although much more common than its two-dimensional counterpart, has defied precise description and definition in spite of numerous attempts. Here, we briefly review the grammar that is used to describe various facets of the phenomenon, and use some recent numerical and experimental results to illustrate the outstanding difficulties of the subject.

8 citations