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Kamran Mohseni - One of the best experts on this subject based on the ideXlab platform.
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the vortex entrainment sheet in an inviscid fluid theory and separation at a sharp edge
2019Co-Authors: Adam Devoria, Kamran MohseniAbstract:In this paper a model for viscous boundary and shear layers in three dimensions is introduced and termed a vortex-entrainment sheet. The vorticity in the layer is accounted for by a conventional vortex sheet. The mass and momentum in the layer are represented by a two-dimensional surface having its own internal tangential flow. Namely, the sheet has a mass density per-unit-area making it dynamically distinct from the surrounding outer fluid and allowing the sheet to support a pressure jump. The mechanism of entrainment is represented by a discontinuity in the normal component of the velocity across the sheet. The velocity field induced by the vortex-entrainment sheet is given by a generalized Birkhoff–Rott equation with a complex sheet strength. The model was applied to the case of separation at a sharp edge. No supplementary Kutta Condition in the form of a singularity removal is required as the flow remains bounded through an appropriate balance of normal momentum with the pressure jump across the sheet. A pressure jump at the edge results in the generation of new vorticity. The shedding angle is dictated by the normal impulse of the intrinsic flow inside the bound sheets as they merge to form the free sheet. When there is zero entrainment everywhere the model reduces to the conventional vortex sheet with no mass. Consequently, the pressure jump must be zero and the shedding angle must be tangential so that the sheet simply convects off the wedge face. Lastly, the vortex-entrainment sheet model is demonstrated on several example problems.
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the vortex entrainment sheet in an inviscid fluid theory and separation at a sharp edge
2018Co-Authors: Adam Devoria, Kamran MohseniAbstract:In this paper a model for viscous boundary and shear layers in three-dimensions is introduced and termed a vortex-entrainment sheet. The vorticity in the layer is accounted for by a conventional vortex sheet. The mass and momentum in the layer are represented by a two dimensional surface having its own internal tangential flow. Namely, the sheet has a mass density per-unit-area making it dynamically distinct from the surrounding outer fluid. The mechanism of entrainment is represented by a discontinuity in the normal component of velocity across the sheet. The sheet mass is able to support a pressure jump, which in turn may cause additional entrainment. This feature was confirmed when the model was used to represent the Falkner-Skan boundary layers. The velocity field induced by the vortex-entrianment sheet is given by a generalized Birkhoff-Rott equation with a complex sheet strength. The model was also applied to the case of separation at a sharp edge. There is no requirement for an explicit Kutta Condition in the form of a singularity removal as this Condition is inherently satisfied through an appropriate balance of normal momentum with the pressure jump across the sheet. A pressure jump at the edge results in the generation of new vorticity. The shedding angle is dictated by the normal impulse of the intrinsic flow inside the bound sheets as they merge to form the free sheet. When there is zero entrainment everywhere the model reduces to the conventional vortex sheet with no mass. Consequently, the pressure jump must be zero and the shedding angle must be tangential so that the sheet simply convects off the wedge face. Lastly, the vortex-entrainment sheet model was demonstrated on two shedding example problems.
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the vortex doublet as a generating mechanism of inviscid vortex sheets part i separation at a sharp edge
2018Co-Authors: Adam Devoria, Kamran MohseniAbstract:In two-dimensional flow, the vortex sheet corresponding to inviscid flow separation at a sharp edge is generated by a vortex doublet, which arises as a result of representing the solid surfaces as vortex doublet sheets. Namely, the inviscid limit of the attached boundary layer and its image layer inside the surface comprise the doublet sheet. The sheet strength represents the amount of circulation in the layer and the net strength of interacting layers at the sharp edge relates to the shed circulation. This net strength is a global quantity that represents communication of flow changes to the shedding point. The shedding of a vortex sheet is interpreted as the doublet sheet being `torn apart,' such that one layer is the vortex sheet shed into the fluid and the other layer is the image sheet. The unsteady Kutta Condition is manifested by requiring that the strength of the doublet induce a flow that instantaneously and mutually neutralizes itself with the singular pressure gradient of the flow attempting to navigate around the sharp edge. The neutralization of the pressure gradient is accomplished by the inviscid generation of vorticity at the edge and is also the mechanism that tears apart the doublet. These results are obtained at the level of the Euler equation (momentum) instead of Bernoulli's equation (energy). As such, there is a finite force exactly at the sharp edge that is associated with the inviscid generation of vorticity and is proportional to the change of the doublet strength. Furthermore, this force corresponds to an `acceleration reaction' of the fluid that is impulsively accelerated as it passes the sharp edge, which in turn communicates an instantaneous change in the total kinetic energy of the fluid to infinity. For a finite body, this force is finite even for an impulsive acceleration. Example simulations are presented for validation of the derived shedding equations.
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unsteady aerodynamics and vortex sheet formation of a two dimensional airfoil
2017Co-Authors: Xi Xia, Kamran MohseniAbstract:Unsteady inviscid flow models of wings and airfoils have been developed to study the aerodynamics of natural and man-made flyers. Vortex methods have been extensively applied to reduce the dimensionality of these aerodynamic models, based on the proper estimation of the strength and distribution of the vortices in the wake. In such modelling approaches, one of the most fundamental questions is how the vortex sheets are generated and released from sharp edges. To determine the formation of the trailing-edge vortex sheet, the classical steady Kutta Condition can be extended to unsteady situations by realizing that a flow cannot turn abruptly around a sharp edge. This Condition can be readily applied to a flat plate or an airfoil with cusped trailing edge since the direction of the forming vortex sheet is known to be tangential to the trailing edge. However, for a finite-angle trailing edge, or in the case of flow separation away from a sharp corner, the direction of the forming vortex sheet is ambiguous. To remove any ad hoc implementation, the unsteady Kutta Condition, the conservation of circulation as well as the conservation laws of mass and momentum are coupled to analytically solve for the angle, strength and relative velocity of the trailing-edge vortex sheet. The two-dimensional aerodynamic model together with the proposed vortex-sheet formation Condition is verified by comparing flow structures and force calculations with experimental results for several airfoil motions in steady and unsteady background flows.
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on the mechanism of high incidence lift generation for steadily translating low aspect ratio wings
2017Co-Authors: Adam Devoria, Kamran MohseniAbstract:High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta Condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.
Adam Devoria - One of the best experts on this subject based on the ideXlab platform.
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the vortex entrainment sheet in an inviscid fluid theory and separation at a sharp edge
2019Co-Authors: Adam Devoria, Kamran MohseniAbstract:In this paper a model for viscous boundary and shear layers in three dimensions is introduced and termed a vortex-entrainment sheet. The vorticity in the layer is accounted for by a conventional vortex sheet. The mass and momentum in the layer are represented by a two-dimensional surface having its own internal tangential flow. Namely, the sheet has a mass density per-unit-area making it dynamically distinct from the surrounding outer fluid and allowing the sheet to support a pressure jump. The mechanism of entrainment is represented by a discontinuity in the normal component of the velocity across the sheet. The velocity field induced by the vortex-entrainment sheet is given by a generalized Birkhoff–Rott equation with a complex sheet strength. The model was applied to the case of separation at a sharp edge. No supplementary Kutta Condition in the form of a singularity removal is required as the flow remains bounded through an appropriate balance of normal momentum with the pressure jump across the sheet. A pressure jump at the edge results in the generation of new vorticity. The shedding angle is dictated by the normal impulse of the intrinsic flow inside the bound sheets as they merge to form the free sheet. When there is zero entrainment everywhere the model reduces to the conventional vortex sheet with no mass. Consequently, the pressure jump must be zero and the shedding angle must be tangential so that the sheet simply convects off the wedge face. Lastly, the vortex-entrainment sheet model is demonstrated on several example problems.
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the vortex entrainment sheet in an inviscid fluid theory and separation at a sharp edge
2018Co-Authors: Adam Devoria, Kamran MohseniAbstract:In this paper a model for viscous boundary and shear layers in three-dimensions is introduced and termed a vortex-entrainment sheet. The vorticity in the layer is accounted for by a conventional vortex sheet. The mass and momentum in the layer are represented by a two dimensional surface having its own internal tangential flow. Namely, the sheet has a mass density per-unit-area making it dynamically distinct from the surrounding outer fluid. The mechanism of entrainment is represented by a discontinuity in the normal component of velocity across the sheet. The sheet mass is able to support a pressure jump, which in turn may cause additional entrainment. This feature was confirmed when the model was used to represent the Falkner-Skan boundary layers. The velocity field induced by the vortex-entrianment sheet is given by a generalized Birkhoff-Rott equation with a complex sheet strength. The model was also applied to the case of separation at a sharp edge. There is no requirement for an explicit Kutta Condition in the form of a singularity removal as this Condition is inherently satisfied through an appropriate balance of normal momentum with the pressure jump across the sheet. A pressure jump at the edge results in the generation of new vorticity. The shedding angle is dictated by the normal impulse of the intrinsic flow inside the bound sheets as they merge to form the free sheet. When there is zero entrainment everywhere the model reduces to the conventional vortex sheet with no mass. Consequently, the pressure jump must be zero and the shedding angle must be tangential so that the sheet simply convects off the wedge face. Lastly, the vortex-entrainment sheet model was demonstrated on two shedding example problems.
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the vortex doublet as a generating mechanism of inviscid vortex sheets part i separation at a sharp edge
2018Co-Authors: Adam Devoria, Kamran MohseniAbstract:In two-dimensional flow, the vortex sheet corresponding to inviscid flow separation at a sharp edge is generated by a vortex doublet, which arises as a result of representing the solid surfaces as vortex doublet sheets. Namely, the inviscid limit of the attached boundary layer and its image layer inside the surface comprise the doublet sheet. The sheet strength represents the amount of circulation in the layer and the net strength of interacting layers at the sharp edge relates to the shed circulation. This net strength is a global quantity that represents communication of flow changes to the shedding point. The shedding of a vortex sheet is interpreted as the doublet sheet being `torn apart,' such that one layer is the vortex sheet shed into the fluid and the other layer is the image sheet. The unsteady Kutta Condition is manifested by requiring that the strength of the doublet induce a flow that instantaneously and mutually neutralizes itself with the singular pressure gradient of the flow attempting to navigate around the sharp edge. The neutralization of the pressure gradient is accomplished by the inviscid generation of vorticity at the edge and is also the mechanism that tears apart the doublet. These results are obtained at the level of the Euler equation (momentum) instead of Bernoulli's equation (energy). As such, there is a finite force exactly at the sharp edge that is associated with the inviscid generation of vorticity and is proportional to the change of the doublet strength. Furthermore, this force corresponds to an `acceleration reaction' of the fluid that is impulsively accelerated as it passes the sharp edge, which in turn communicates an instantaneous change in the total kinetic energy of the fluid to infinity. For a finite body, this force is finite even for an impulsive acceleration. Example simulations are presented for validation of the derived shedding equations.
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on the mechanism of high incidence lift generation for steadily translating low aspect ratio wings
2017Co-Authors: Adam Devoria, Kamran MohseniAbstract:High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta Condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.
Christophe Eloy - One of the best experts on this subject based on the ideXlab platform.
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extension of lighthill s slender body theory to moderate aspect ratios
2018Co-Authors: Zhanle Yu, Christophe EloyAbstract:Abstract Calculating the fluid forces acting on a moving body at high Reynolds number is crucial in many fluid–structure interaction problems, such as fish swimming or flutter instabilities. To estimate these forces, Lighthill developed the slender-body theory, which assumes a potential flow and an asymptotically small aspect ratio. Yet, it is still unclear whether Lighthill’s theory is still valid for aspect ratios of order one. To address this question, we solve numerically with a panel method the full three-dimensional problem of a rectangular plate deforming periodically in a potential flow. These numerical simulations are used to calculate the pressure jump distribution across the plate for different aspect ratios. We find that numerical simulations and slender-body theory give similar results far from trailing edge. Close to the trailing edge however, there is a discrepancy, which is due to the use of a Kutta Condition in the simulations (i.e. no pressure jump at the trailing edge), while, in the slender-body theory, the pressure jump is non zero. We propose a simple extension of Lighthill’s slender-body theory that accounts for this discrepancy. The usefulness of this extension is then discussed and illustrated with a generic fluid–structure interaction problem and with the flag instability problem.
Pengfei Liu - One of the best experts on this subject based on the ideXlab platform.
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A Broyden numerical Kutta Condition for an unsteady panel method
2002Co-Authors: Pengfei Liu, Neil Bose, Bruce ColbourneAbstract:In panel methods, numerical Kutta Conditions are applied in order to ensure that pressure differences between the surfaces at the trailing edges of lifting surface elements are close to zero. Previous numerical Kutta Conditions for 3-D panel methods have focused on use of the Newton-Raphson iterative procedure. For extreme unsteady motions, such as for oscillating hydrofoils or for a propeller behind a blockage, the Newton-Raphson procedure can have severe convergence difficulties. The Broyden iteration, a modified Newton-Raphson iteration procedure, is applied here to obtain improved convergence behavior. Using the Broyden iteration increases the reliability, robustness and in many cases computing efficiency for unsteady, multi-body interactive flows. This method was tested in a time domain code for an ice class propeller in both open water flow and during interaction with a nearby ice blockage. Predictions showed that the method was effective in these extreme flows.
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automated marine propeller geometry generation of arbitrary configurations and a wake model for far field momentum prediction
2001Co-Authors: Pengfei Liu, N Ose, Uce ColbourneAbstract:This paper first describes procedures and methodologies to automatically produce marine propeller geometry with optional auxiliary bodies such as nozzles, blockages and rudders. This process is designed and implemented for a general boundary element method (the panel method) to deal with both lifting body and non-lifting body flows. The generated geometry is represented by quadrilateral and triangular panels that can be used by other mesh generation codes to produce 3D volumetric mesh for CFD work. The vertices of these generated panels are set so that the normal of the surfaces points inside the body. The order of the panels and their side indices are aligned for numerical procedures such as differentiation of the perturbation doublet potential for surface tangential velocities and Kutta Condition at the trailing edge. A DXF file format was also implemented as one of the output files that can be used for propeller manufacturing via CNC and for commercial CFD codes that use geometry data input. Based on the near field wake modeling studies performed by the authors and previous experimental investigations on far wake turbulent jet measurements, a far wake model for a propeller panel method is implemented to enhance the capability of predicting the velocities and momentum impact on the risers under a floating production storage off-loading (FPSO) system during operation. This far wake model consists of contraction wake (within one propeller diameter downstream), transition wake (one to two diameters downstream), and inflation wake (two diameters beyond). Near field velocity prediction of this far wake model is validated using previous LDV measurement. Further experimental studies are scheduled for LDV/PIV measurement up to 20-diameter downstream.
Bruce Colbourne - One of the best experts on this subject based on the ideXlab platform.
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A Broyden numerical Kutta Condition for an unsteady panel method
2002Co-Authors: Pengfei Liu, Neil Bose, Bruce ColbourneAbstract:In panel methods, numerical Kutta Conditions are applied in order to ensure that pressure differences between the surfaces at the trailing edges of lifting surface elements are close to zero. Previous numerical Kutta Conditions for 3-D panel methods have focused on use of the Newton-Raphson iterative procedure. For extreme unsteady motions, such as for oscillating hydrofoils or for a propeller behind a blockage, the Newton-Raphson procedure can have severe convergence difficulties. The Broyden iteration, a modified Newton-Raphson iteration procedure, is applied here to obtain improved convergence behavior. Using the Broyden iteration increases the reliability, robustness and in many cases computing efficiency for unsteady, multi-body interactive flows. This method was tested in a time domain code for an ice class propeller in both open water flow and during interaction with a nearby ice blockage. Predictions showed that the method was effective in these extreme flows.
