Modeling Aerodynamics, Including Dynamic Stall, for Comprehensive Analysis of Helicopter Rotors

Abstract

Abstract:To fulfill the objective of a predictive tool for rotorcraft, comprehensive analysis (CA) needs to be capable of providing both accurate and time-efficient predictions of rotor air loads and structural loads. The more recent methodology based on comprehensive analysis coupled with high-fidelity computational fluid dynamics (CFD) has shown improved predictions of air loads, but it has not the strength of computational efficiency and the versatility of stand-alone CA. The present article is concerned with modeling aerodynamics about helicopter rotors for CA. The aerodynamics about rotors are very complex, encompassing subsonic to transonic flow with unsteady, stalled behavior and 3D effects. CA treats aerodynamics as separated into local and global flows. Semi-empirical models of dynamic stall were created in the 1970s-1990s for modeling unsteady local aerodynamics, including stalled flow. Most of them fail to provide good predictions of experimental results and also suffer problems of numerical convergence. The main effort in this study is about modeling local aerodynamics based on the revised "ONERA-Hopf bifurcation model". It is implemented in the comprehensive analysis code of ONERA according to a scheme that ensures numerical convergence. The experimental results obtained in the Wind Tunnel S1 of Modane (France) in 1991 on the Rotor 7A are considered for validation of the analysis under three flight test conditions: high-speed test, high-thrust tests with light stall and deep stall, respectively. There is a reasonable agreement between the predictions of CA with experimental results. The distinct features of the stall model are the modeling of the boundary-layer effects and the vortex-shedding phenomenon.

Numerical Investigation of Jet Angle Effect on Airfoil Stall Control

“…where ν t is defined as ν t = µ t ρ =ν f v1 . The working variableν is obtained from the SA transport Equation (6). In addition, the following model coefficients are used:…”

“…Stall phenomena, typically dynamic stall, impact on designing and analyzing lift surfaces. For example, conventional helicopter main rotor blades frequently encounter dynamic stall in high-speed forward flight conditions [2][3][4][5][6][7]. The blade stall can occur in the retreating side because the blade needs to pitch up in order to balance aerodynamic forces.…”

Predicting Non-Linear Flow Phenomena through Different Characteristics-Based Schemes

“…The equations are iterated in pseudo-time due to the non-physical time derivative of pressure. In order to advance the system of Equations (1) and (2) in real time, a dual-time stepping procedure needs to be employed for which a real time derivative is added to the momentum equation. The time-step has to satisfy the following condition as…”

“…Despite the advances made in the field of computational fluid dynamics over the past decades, predicting flow patterns around aerodynamic shapes remains a challenge for aerospace applications. The flow around a wing can have a transonic behaviour which, at high angles of attack, may be supplemented by flow separation, strong crossflow gradients as well as a hysteresis in the lift slope [1,2]. Traub [3] highlighted further that at low Reynolds number flows, laminar separation bubbles exist which have an inherently unsteady behaviour.…”

Development of a second-order dynamic stall model

“…However, a clear single best model was not found. Gonzalez and Munduate (2007) and Wala et al (2018) both showed promising results using modified and optimized Beddoes-Leishman models compared with experimental data. Inaccuracies in dynamic stall models may be due to the abovedescribed fact that they are not properly designed for high angles of attack and that there are some of them that do not specifically describe vortex shedding behaviour.…”