Weixing Yuan

An Investigation of Low-Reynolds-number Flows past Airfoils

This paper presents an investigation of low-Reynolds-number flows past an SD7003 airfoil at Re = 60K, where transition takes place across a laminar separation bubble (LSB). Results of experimental measurements and numerical calculations are analyzed and discussed. In particular, reasonably good results were obtained using three different numerical approaches: Large-eddy simulation (LES) that demonstrated vortical structures at different transition stages, and where the transition occurred naturally; unsteady Reynolds-averaged Navier-Stokes (URANS) simulations for several turbulence models based on the ω-length-scale equation, coupled to a linear stability solver to predict transition point; URANS simulations using a low-Re k-e turbulence model, where eddy viscosity was suppressed in the near-wall laminar region by damping functions.

Computational and experimental investigations of low-Reynolds-number flows past an aerofoil

This paper presents investigations of low-Reynolds-number flows past an SD7003 aerofoil at Re = 60k, where transition takes place across a laminar separation bubble (LSB). Results of experimental measurements and numerical calculations are analysed and discussed. In particular reasonably good results were obtained using two different numerical approaches: Large-eddy simulation (LES) that demonstrated vortical structures at different transition stages, and where the transition occurred naturally; unsteady Reynolds-averaged Navier-Stokes (URANS) simulations for several turbulence models based on the ω-length-scale equation, coupled to a linear stability solver to predict the transition position.

A parametric study of LES on laminar-turbulent transitional flows past an airfoil

Low-Reynolds-number aerodynamic performance of small-sized air vehicles is an area of increasing interest. In this study, low-Reynolds-number flows past an SD7003 airfoil are investigated to understand important viscous features of laminar separation and transitional flow followed by the complicated behavior of the flow reattachment process. In order to satisfy the three-dimensional (3D) requirement of the code, a simple “3D wing” is constructed from a two-dimensional (2D) airfoil. A parametric study of large eddy simulation (LES) on the airfoil flows at Re = 60,000 is performed. Effects of grid resolution and sub-grid scale (SGS) models are investigated. Although 3D effects cannot be accurately captured owing to the limitation of the grid resolution in the spanwise direction, the preliminary LES calculations do reveal some important flow characteristics such as leading-edge laminar separation and vortex shedding from the primary laminar separation bubble on the low-Reynolds-number airfoil.

Numerical and Experimental Simulations of Flapping Wings

This paper presents recent progress in a continuing investigation of the aeromechanical aspects of unsteady flapping wings for micro air vehicles (MAV). Numerical simulations were performed for two-dimensional (2D) pitching-plunging airfoils and three-dimensional (3D) flapping wings, mainly at hover conditions, using an in-house code called INSflow. The results were compared with available experimental data obtained in the water tunnel at the NRC-IAR. The investigation revealed that, at hover conditions, the vortices formed during the airfoil plunging motion may remain near the airfoil and affect new vortex formations, and thus the integral aerodynamic performance. In addition, the flow around the 3D insect-like wing is fully three-dimensional. The tip flow affects the flow separation, reducing the separation intensity. Two-dimensional calculations may over-predict the separation and the shedding vortices, thus affecting the generation of aerodynamic forces.

Aerodynamic Study of a Flapping-Wing NAV Using a Combination of Numerical and Experimental Methods

The development and acquisition of a new class of military systems known as Nano Air Vehicle (NAV) is possible in not too distant a future as a result of technological progress in a number of areas. These vehicles will likely use flapping wings as there is strong evidence that for very small craft, flapping-wing performance is superior to other options due to dynamic effects that create much higher average lift at low Reynolds numbers. An essential step towards engineering realization of flapping-wing flight is the understanding of the issues for the fully 3-D motion. In this paper, we present an approach adopted towards that objective. Based on system considerations, the general characteristics of a notional NAV to be studied such as size, mass, and wing motions were established. Then, three methods were developed and applied to the notional NAV. The detailed flow physics is captured using accurate Navier-Stokes CFD solutions and a tailored designed instrumented rig in a water tunnel. A vortex-lattice based engineering-type model, refined with the higher accuracy CFD and experimental results, was developed and will be used to efficiently study a variety of prescribed wing shapes and flapping patterns.

Transition Prediction in Low Reynolds Airfoil Flows using Finite Element/Difference Solvers Coupled with the e n Method: a Comparative Study

The Institute for Aerospace Research (IAR), of the National Research council (NRC) of Canada, and the Technical University of Braunschweig (TUBS), in Germany, have been collaborating, over the past three years, on low Reynolds airfoil flows research using computational flow dynamics and wind tunnel experiments. The present paper addresses one of the research aspects which focused mainly on the development of a finite element algorithm at IAR, FEAT2D, for transition prediction in two-dimensional incompressible laminar flows. The FEAT2D code performance was accessed by conducting comparisons with the available COAST3 code results that was used by TUBS for the prediction of the transition location. The numerical solutions of flows past an experimental airfoil, HGR-01, were considered for stability analysis. The solutions were obtained using RANS simulations with different turbulence models. A coupling technique between the RANS solvers and the transition modules was performed. The finite element algorithm was based on the high precision Hermite quintic element using a non-uniform grid distribution. Temporal and spatial stability theories were considered and the e n method was implemented to compute the global amplification rate in the streamwise direction. Validations were conducted for the Blasius boundary layer and for the HGR-01 airfoil flows at various angles of attack. The present finite element results for transition location prediction were found to be in excellent agreement with those predicted by the COAST3 code. The FEAT2D code was also validated with an LES solution of low Reynolds incompressible flows past the SD7003 airfoil by comparing the n-factor with the amplitude of the Reynolds shear stress. The transition location was determined by FEAT2D code using the time-averaged LES solution as input data. For the LES solution the transition location was assumed to be where the magnitude of the Reynolds shear stress reaches 0.1%.

Experimental Simulation of Flapping Wings for Nano-Air-Vehicles

apping wings operating at high reduced frequency with a complex two-dimensional or three-dimensional pattern. The experimental measurements are demonstrated by the test cases of two-dimensional and three-dimensional apping wings, designed according to the proposed notional nano-air-vehicle at a hovering condition. The features of the water tunnel, the geometric and kinematic parameters of the airfoils/wings, and the setups of the motion rigs for each test case are described in detail. The measured forces and particle image velocimetry data are analyzed and cross-checked with the numerical results obtained from an in-house code. The analyses and comparisons of the experimental and numerical results conrm that the established experimental approach obtained a quantitatively reliable solution for the development of apping wings and can serve for numerical validation. This is an essential step towards engineering realization of the functionality of insect ight.

Experimental and computational investigations of flapping wings for Nano-air-vehicles

This paper presents the finalized results of a recent project which investigated the aeromechanical aspects of aerodynamic force generation by making use of flapping wings. Flapping-wing experiments using small wings have some unique challenges posed by the low force level (∼1 N) and the cyclic wing motion. A tailored experimental water tunnel facility was developed for flapping wings operating at high reduced frequency with a complex two-dimensional and a three-dimensional motion profile. The experimental capability is demonstrated by the test cases of two-dimensional and three-dimensional flapping wings, designed according to a proposed notional nano-air-vehicle at a hovering condition. The features of the water tunnel, the geometric and kinematic parameters of the airfoils/wings, and the setups of the motion rigs for each test case are described. Measured forces and particle image velocimetry data are analyzed and cross-checked with the numerical results obtained from a code developed in-house. The comparisons of the experimental and numerical results show that the established experimental approach obtained a quantitatively reliable solution for the development of flapping wings and can serve for numerical validation of engineering tool developments. The investigation reveals that the kinematics of a rigid airfoil or wing is the dominant influence in the generation of aerodynamic forces, while the cross-section profile plays a secondary role. An asymmetric-wake-in-time is found behind the single airfoils and wings, which contributes to an asymmetry behavior of the resulting aerodynamic forces. In addition to the findings of single airfoils and wings, further analyses of the numerical and experimental results confirm that wing-wing interaction through the clap-fling mechanism can intensify the generation of the thrust force while accompanied by a small reduction in the overall propulsion efficiency.

Simulations of Airfoil Limit-cycle Oscillations at Transitional Reynolds Numbers

Structural response models addressing one-degree-of-freedom (1DOF) and twodegree-of-freedom (2DOF) aeroelastic oscillations were coupled with an in-house incompressible CFD code to perform large-eddy-based simulations (LES) for flows past rigid airfoils in free-to-rotate and free-to-rotate-and-heave conditions at low Reynolds numbers. As observed in experiments, the numerical simulations confirmed the presence of the self-sustained low-amplitude limit-cycle oscillations (LCOs) of the airfoils. It is understood that this behavior in the transitional Reynolds number regime results from the unsteadiness of the laminar boundary layer separation and its delayed recovery when compared to the corresponding static conditions.

Large-eddy simulation of curved-geometry flows using contravariant components of velocity

The current large-eddy simulation (LES) research makes use of the contravariant components as the dependent variables on a staggered grid system for the discretisation of the governing equations in curvilinear coordinates. This technology provides a possibility to investigate efficiently turbulent flows in complex geometries. To test and validate the recently developed in-house LES code, LESSGA (Large-Eddy Simulation on a Staggered Grid Arrangement), numerical simulations were performed for turbulent flows in a concentric annular pipe and transitional flows past an airfoil. In this article, the computed results of flows in a concentric annular pipe with a radius ratio of a = R inner/Router = 0.5 at and flows past an SD7003 airfoil at Rec = 60,000 and angle of attack α = 4° are compared with available experimental and DNS data. Technical difficulties experienced are also discussed.