Ralf Wokoeck

RANS Simulation and Experiments on the Stall Behaviour of an Airfoil with Laminar Separation Bubbles

Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carried out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving laminar separation bubbles. One of the methods computes transition locations across laminar separation bubbles whereas the other assumes transition onset where laminar separations occur. For validation of the numerical methods an extensive measurement campaign has been carried out. It is shown, that the methodology mentioned first can simulate the size of laminar separation bubbles for angles of attack up to where the separation bubble and the turbulent separation at the trailing edge are well behaved and steady in the mean. With trailing edge separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Cases with large 3D flow structures inside the turbulent trailing edge separations in windtunnel experiments for high angles of attack are compared and analysed along with 2D and 3D steady RANS calculations that model the measurement section of the windtunnel.

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%.

RANS Simulation and Experiments on the Stall Behaviour of a Tailplane Airfoil

Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carded out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving laminar separation bubbles. One of the methods computes transition locations across laminar separation bubbles whereas the other assumes transition onset where laminar separations occur. For validation of the numerical methods an extensive measurement campaign has been carded out. It is shown, that the methodology mentioned first can simulate the size of laminar separation bubbles for angles of attack up to where the separation bubble and the turbulent separation at the trailing edge are well behaved and steady in the mean. With trailing edge separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Finally, for flows with increased unsteady behaviour of both, separation bubble and turbulent separation, which were observed at higher angles of attack in the experiment between maximum lift and leading-edge stall, steady state prediction methods for transition can no longer be applied and time-accurate methods have to be developed in a further step.

Measurements of the Aerodynamic Characteristics of Low Aspect Ratio Wings in Complex Motions

This paper describes aerodynamic measurements of micro aerial vehicle sized wings in perching motion. The perching motion is modelled according to the wing motions of birds to rapidly reduce velocity when landing on a small spot i.e. a perch. The motion is defined as a discontinuous pitching of a wing from low starting angles of attack to high final angles. The study was carried out with two rectangular flat plate wings performing the perching motion from 0° to 45° angle of attack. Force measurements with an internal balance were taken for various combinations of Reynolds numbers and reduced frequencies. 2D particle image velocimetry of the flow field was carried out for different time phases of one particular combination, where the force measurements indicate changes in the flow field to occur, to give more insight to the complex unsteady aerodynamic response to this motion type.

Simultaneous Measurements of Unsteady Aerodynamic Loads, Flow Velocity Fields, Position and Wing Deformations of MAVs in Plunging Motion

A new wind tunnel environment for low Reynolds number testing of Micro Air Vehicles (MAV) is introduced, providing a test rig for plunge and pitch motions and a 6-component force balance. In this study a rigid as well as a flexible version of a typical MAV wing is investigated. Optical measurements techniques are adapted to measure simultaneously the instantaneous model position, orientation, wing deformations and flow fields.

3D Flow Field Investigations on Flapping Wing Aerodynamics

Three-dimensional, unsteady flow fields of a flapping low aspect ratio wing are investigated by means of tomographic PIV (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore force measurements have been done. Tomo-PIV was applied to the flow above the flat-plate-wing during the down-stroke. A high spatial resolution and a large volume thickness could be achieved by using sensitive sCMOS cameras and a traversable set-up. The CFD calculations cover the complete period of motion. Analyzing the vortex dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number (Re = 18,000 and Re = 36,000) flows. Two different Strouhal numbers (St = 0.06 and St = 0.13) are considered and their effects on the development of a leading edge and tip vortex are discussed.

Experimental and numerical investigations of pitch-plunging wing aerodynamics at low Reynolds number

In the present paper, the first results of a broad study of static and moving flat plate wings for micro air vehicles (MAVs) are presented. Wind tunnel experiments with force measurements and particle image velocimetry (PIV) were performed in a specially designed wind tunnel environment for low Reynolds number investigations. Corresponding numerical flow simulations were also carried out. Effects of the wing shape (elliptical and rectangular), the Reynolds number (18,000, 36,000 and 108,000) and the kinematics using different motion forms of pure plunge and combined pitch/plunge with a reduced frequency of k = 0.2 are discussed.

Three-Dimensional Flow Field Investigations of Flapping Wing Aerodynamics

Three-dimensional unsteady flow fields of a flapping, low-aspect-ratio wing have been investigated by means of highly resolved tomographic particle image velocimetry (Tomo-PIV) measurements and computational fluid dynamics (CFD). Furthermore, force measurements have been carried out. Tomo-PIV was applied to the flow above a flat plate wing during the downstroke. High spatial resolution and large volume thickness could be achieved by using sensitive sCMOS cameras and a traversing setup. The CFD calculations covered the complete period of motion. The analysis of the vortex-dominated flow fields provides a deeper understanding of vortex interaction and three-dimensionality of low Reynolds number (Re=18,000 and Re=36,000) flows. Two different Strouhal numbers (St=0.06 and St=0.13) are considered and their effects on the development of a leading edge and tip vortex are discussed. The PIV results show instantaneous flow fields after a leading edge separation that are dominated by small-scale turbulent vortex st...

LES/RANS Simulations of Airfoil Flows near Combined Leading-Edge Trailing-Edge Stall

This paper presents a numerical investigation of low-Reynolds-number flows past a research airfoil, HGR-01, at a high angle of attack near combined leading-edge trailing-edge stall. At high angles of attack, a laminar separation bubble exists at the airfoil leading edge while turbulent separation appears at the trailing edge. Laminar-turbulent transition occurs over or across the laminar separation bubble. Numerical experiments for the flow past the airfoil at Re = 0.65 × 10 6 and α = 12° show that both a conventional Reynolds -averaged Navier-Stokes (RANS) approach, coupled with a transition prediction module, and comprehensive large-eddy simulation (LES) are able to capture both laminar and turbulent separations at the leading and trailing edges, respectively . Challenges and difficulties experienced in the numerical exercises and further investigation efforts needed to reduce the discrepancies between computational fluid dynamics (CFD) and experimental results are discussed.

Time Resolved Flow Field Investigations of Low Reynolds number Transient Maneuvers

Transient pitch and plunge maneuvers of low aspect ratio wings in a special low Reynolds number wind tunnel have been investigated by means of high speed Particle Image Velocimetry to analyze the leading edge vortex dynamics quantitatively. A specific requirement of the measurement was to design and apply an optical arrangement for the laser light sheet which moves with the wing so as to keep the light sheet in fixed position on the upper side of the model. This became necessary because of the low pulse energy of high speed lasers and the large amplitudes of motion. This paper shows the principle and the implementation of this particular set-up. The evaluation procedure shows some specific steps concerning the preprocessing, for example, mask generation and disparity correction. The temporally-resolved data sets enable a tracking of vortices and a detailed analysis of the leading edge vortex dynamics in terms of circulation build-up and convection velocity.