Unsteady Separation Characteristics of Airfoils Operating Under Dynamic Stall Conditions PDF Download
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Author: Wolfgang Geissler
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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: 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: 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: Bernardo Oliveira Vieira
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ISBN:
Category :
Languages : en
Pages :
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Book Description
Traditional rotorcraft airfoil design is based on steady-state aerodynamics, despite the many sources of unsteady-flow in forward flight. At high-thrust and high-speed conditions, the rotor may be susceptible to dynamic stall; consequently, large margins are necessary to prevent fatigue loads on the blades and pitch links, limiting operation under high altitudes, payload, and temperatures, as well as during maneuvers.This work revises typical design requirements and proposes new ways to qualify airfoils in dynamic stall. A number of design studies are conducted to better understand the relation between airfoil shape and dynamic stall behavior. The design manipulations are handled by an inverse-design, conformal mapping method, and unsteady Reynolds-averaged Navier-Stokes equations are used to predict the unsteady aerodynamic performance. In unsteady flow, the occurrence of aerodynamic lags in the development of pressures, boundary-layer separation, and viscous-inviscid interactions suggest more strict requirements than in steady flow. In order to postpone the onset of dynamic stall, the design needs to handle competing leading- and trailing-edge separation mechanisms, which are heavily influenced by shock waves and laminar-turbulent transition effects. It is found that a particular tailoring of the trailing-edge separation development can provide adequate dynamic stall characteristics and minimize penalties in drag and nose-down pitching moments. At the same time, a proper design of the nose shape is required to avoid strong shock waves and prevent premature stall. A proof-of-concept airfoil is developed to improve dynamic stall behavior, while meeting other stringent requirements. Performance calculations using information obtained from comprehensive analysis (RCAS) based on a UH-60A helicopter suggest that an expansion of the operational envelope is possible, while also reducing hover drag, maintaining low pitching moments, and providing reasonable margins to drag rise at the maximum speed of the UH-60A helicopter.Finally, pitching wing calculations are conducted to demonstrate the proposed concepts in three-dimensional flow. The new wing experiences a more favorable dynamic stall inception and considerable decreases in the integrated peak pitching moments compared to traditional designs.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 156
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Book Description
Author: Phillip B. Davidson
Publisher:
ISBN: 9781321516388
Category : Aerofoils
Languages : en
Pages : 386
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Book Description
In recent years, the blade geometry on wind turbines and helicopters has been optimized for a particular span location. Unsteady flow phenomena like dynamic stall limit these designs and need to be better understood and correctly simulated. Currently, empirical and computational fluid dynamics (CFD) methods are used to simulate rotating wind turbine or helicopter blades, but each of these methods has limitations in predicting unsteady separated flows. To address these needs, the present work investigated oscillating airfoils over a range of conditions with an approach that provided fast, low-cost unsteady pressure data combined with a highly resolved flow field to better understand the physics of dynamic stall. An additional objective was to show how such data may be used to assess CFD simulations. This research has yielded interesting results showing characteristics of thin airfoil stall, leading edge stall, and trailing edge stall that were sorted and classified. Classification of the oscillating airfoil behavior with or without dynamic stall was performed using previous definitions for stall regime, separation characteristics, and other qualitative differences in stall pattern. After classifying the unsteady flow for each of the cases, comparison of experimental results and results obtained using an unsteady Reynolds Averaged Navier-Stokes (URANS) solver was performed to assess the ability of the solver to produce the same unsteady effects. Although both experiment and computation produced similar flow features, the timing and magnitude of the features in the dynamic stall and re-attachment process of the pitching cycle exhibited some significant differences.
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: C. M. Wang
Publisher:
ISBN:
Category :
Languages : en
Pages : 11
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Book Description
A solution procedure is presented for the computation of dynamic stall phenomena encountered by arbitrary shaped airfoils under arbitrary flow conditions. This procedure solves the unsteady, incompressible Navier-Stokes and the unsteady boundary layer equations using an efficient, zonal approach. A number of results for a modified NACA 0012 airfoil experiencing dynamic stall are presented and compared with available numerical data. Qualitative comparisons with flow visualization experiments are also presented. (Author).
Author:
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Category :
Languages : en
Pages :
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Book Description
The effect of a Gurney flap on an unsteady airfoil flow is experimentally and computationally examined. In the experiment, the details of the unsteady boundary layer events on the forward portion of the airfoil are measured. In the computation, the features of the global unsteady flow are documented and correlated with the experimental observations. The experiments were conducted in the North Carolina State University subsonic wind tunnel on an oscillating airfoil at pitch rates of 65.45 degrees/sec and 130.9 degrees/sec. The airfoil has a NACA0012 cross-section and is equipped with a 1.5% or 2.5% chord Gurney flap. The airfoil is tested at Reynolds numbers of 96,000, 169,000 and 192,000 for attached and light dynamic stall conditions. An array of surface-mounted hot-film sensors on the forward 25% chord of the airfoil is used to measure the unsteady laminar boundary layer separation, transition-to-turbulence, and turbulent reattachment. In parallel with the experiments incompressible Navier-Stokes computations are conducted for the light dynamic stall conditions on the airfoil with a 2.5%c Gurney flap at a Reynolds number of 169,000. The experimental measurements show that the effect of the Gurney flap is to move the separation, transition and reattachment forward on the airfoil. This effect is more marked during the airfoil's pitch-down than during pitch-up. The computational results verify these observations, and also show that the shedding of the dynamic stall vortex is delayed. Thus the adverse effects of dynamic stall are mitigated by the Gurney flap.
Author: Julie Anne Lovato
Publisher:
ISBN:
Category : Aerofoils
Languages : en
Pages : 130
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Book Description
"This experimental analysis presents a comprehensive study of the separating boundary layer over a static airfoil under natural and actively controlled conditions. Near-surface hot-film and surface pressure measurements, as well as flow visualization are used to analyze the large-scale nature of the flow over a two-dimensional NACA-0015 airfoil and determine forcing effects. Results from the static study are then extended for an initial evaluation of unsteady airfoil control. Results show that the fundamental frequency associated with free shear layer instabilities for this case is an integral multiple of the frequency associated with wake structures. The static separating boundary layer response to active control confirms that it is a boundary layer transitioning to a free shear layer. Qualitative analyses show that significant reduction in overall static separation can be achieved under forcing conditions. Upper airfoil surface suction values are also significantly increased over the natural values. Applying tangential pulsed air control at static fundamental frequencies to a dynamic airfoil results in delay of the dynamic stall vortex formation and a delay of dynamic stall. These discoveries indicate that the developed control methodology may prove successful in increasing unsteady aircraft maneuverability. Subject terms: Static Airfoil Control; Separation Control; Unsteady Aerodynamics; Boundary Layer Control."--Report documentation page.
Author: Stephen Arthur Patay
Publisher:
ISBN:
Category : Aerofoils
Languages : en
Pages : 9
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Book Description
To obtain a better understanding of the leading edge separation mechanism on a wing pitching upward with a constant angular velocity, an unsteady boundary layer calculation is performed. The analysis considers a potential flow about a Joukowsky airfoil, which then forms a basis for the boundary layer calculations. An integral technique which is similar to the Karman-Pohlhausen method is used to determine the viscous flow. The movement of the separation point is calculated for several pitching rates. A comparison with some experimental data was made for both the static and dynamic case. It was determined that the point of boundary layer separation coincides with the formation of a separation bubble for the dynamic cases considered. (Author).
