Author: W. Geissler
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ISBN:
Category :
Languages : en
Pages : 12
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Book Description
Experimental investigations have exposed a strong dependency of unsteady separation characteristics in the regime of dynamic stall on airfoil shape, Reynolds and Mach number, frequency and time-dependent incidence. A suitable prediction method should therefore be able to account for these various parameters. Coupling procedures between two-dimensional unsteady boundary layers and inviscid surface singularly methods (panel methods) have been developed for analytical investigation of unsteady turbulent separation. In the present study, the influence of various parameters on unsteady separation is discussed in detail and comparison with experimental data is made. The results show that, even for weak coupling between boundary layer and inviscid boundary conditions, good correspondence exists between theory and experiment with respect to the development of unsteady separation.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 64
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Author: Tuncer Cebeci
Publisher:
ISBN:
Category :
Languages : en
Pages : 22
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Book Description
The results of an investigation of boundary layers close to the stagnation point of an oscillating airfoil are reported. Two procedures for generating initial conditions - the characteristics-box scheme and a quasi-static approach - were investigated, and the quasi-static approach was shown to be appropriate provided the initial region was far from any flow separation. With initial conditions generated in this way, the unsteady boundary-layer equations were solved for the flow in the leading-edge region of a NACA 0012 airfoil oscillating from 0 degrees to 5 degrees. Results were obtained for both laminar and turbulent flow, and, in the latter case, the effect of transition was assessed by specifying its occurrence at different locations. The results demonstrate the validity of the numerical scheme and suggest that the procedures should be applied to calculation of the entire flow around oscillating airfoils. (Author).
Author:
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ISBN:
Category :
Languages : en
Pages : 33
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This research program investigated the physical mechanisms involved in the onset of leading edge separation when airfoils pitch to high angles of attack. Both constant pitch rate and variable pitch rate motions were considered. The highlights of results from a combined experimental and computational effort are described in this report. The conclusions from this research indicate the need for boundary-layer resolved measurements of the flow behavior near the leading edge and the evolution of the reverse flow regions on the suction surface. Furthermore, the deliberate shaping of the pitch trajectory for the purpose of optimization of separation delay is suggested as one way to manage the flow and aerodynamic behavior of an airfoil. Dynamic Stall, Unsteady Separation.
Author: John Powers Retelle (Jr)
Publisher:
ISBN:
Category :
Languages : en
Pages : 140
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Book Description
Measurements of boundary layer flow reversal made in a wind tunnel on a stationary NACA 0012 airfoil immersed in a longitudinally oscillating subsonic flow are described. A split-film sensor provided flow reversal data in regions above the airfoil surface throughout the period of flow oscillation. At angles of attack below the steady-flow stall angle of attack and for small amplitude oscillations, regions of boundary layer flow reversal near the leading edge were observed to vay nominally in phase with e time-varying external velocity. However, an increase in the oscillation amplitude produced a phase lead and an increased downstream extent of the flow reversal region. It is shown that boundary layer flow reversal on a stationary airfoil in an oscillating free stream responds to changes in the flow unsteadiness in a manner similar to the flow reversal observed during the onset of dynamic stall on an airfoil oscillating in pitch.
Author: Wolfgang Geissler
Publisher:
ISBN:
Category :
Languages : en
Pages : 15
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Book Description
Unsteady viscous/inviscid interaction phenomena were investigated on airfoils operating under dynamic stall conditions. It is well known from experiments that, in the upstroke region of a sinusoidally oscillating airfoil, the flow remains attached up to incidences considerably larger than the static stall angle. Reversed flow areas develop close to the airfoil surface without boundary layer separation. These complicated flow phenomena are investigated in the present study on the basis of coupling procedures between a time-dependent inviscid panel method and 2-d unsteady boundary layer codes. Two strategies are pursued: 1. Coupling of inviscid panel method with boundary layer code-direct mode; and 2. Strong coupling of inviscid panel method with boundary layer code-inverse mode. The main features of the unsteady time-marching panel method and boundary layer codes are discussed. Emphasis is placed on the investigation of numerical stability and the phenomenon of unsteady separation.
Author: John Powers Retelle
Publisher:
ISBN:
Category : Air flow
Languages : en
Pages : 262
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Author: T. Cebeci
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ISBN:
Category :
Languages : en
Pages : 19
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Book Description
Preliminary results are presented for the analysis of unsteady laminar boundary layers on oscillating airfoils. An examination of the evolution of the boundary layer near the nose of an oscillating airfoil has revealed that, when the reduced frequency is of the same order as in experiments on dynamic stall, the unsteady boundary layer ceases to behave in a smooth manner just downstream of separation and before one cycle is completed. As for the case of the implusively started circular cylinder, the irregular behavior signals the onset of a singularity in the solution of the boundary layer equations. Numerical results for the method are compared with the numerical results of van Dommelen and Shen for the impulsively started circular cylinder, the irregular behavior signals the onset of a singularity in the solution of the boundary layer equations. Numerical results for the method are compared with the numerical results of van Dommelen and Shen for the impulsively started circular cylinder.
Author:
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ISBN:
Category : Aeronautics
Languages : en
Pages : 702
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Author: Tuncer Cebeci
Publisher:
ISBN:
Category :
Languages : en
Pages : 29
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Book Description
An interactive boundary-layer method previously developed and tested for steady flows is used here in a quasi-steady manner to examine the evolution of the flow behavior of airfoils subject to harmonic oscillation and ramp-type motions. The calculations encompass the airfoil and wake flows at angles of attack which lead to separation. The results quantify the effects of the viscous boundary layer and wake on the variation of lift coefficient with angle of attack and reduced frequency. These effects are shown to be large at angles of attack which involve boundary-layer separation. Keywords: Boundary layer flow; Flow separation; Fluid mechanics; Oscillating airfoils; Stalling; Lift coefficients; Unsteady motion. (edc).
