Anthony Donald Gardner

Improved Two-Dimensional Dynamic Stall Prediction with Structured and Hybrid Numerical Methods

A joint comprehensive validation activity on the structured numerical method elsA and the hybrid numerical method TAU was conducted with respect to dynamic stall applications. In order to improve two-dimensional prediction, the influence of several factors on the dynamic stall prediction were investigated. The validation was performed for three deep dynamic stall test cases of the rotor blade airfoil OA209 against experimental data from two-dimensional pitching airfoil experiments, covering low speed and high speed conditions. The requirements for spatial discretization and for temporal resolution in elsA and TAU are shown. The impact of turbulence modeling is discussed for a variety of turbulence models ranging from one-equation Spalart-Allmaras-type models to state-of-the-art seven-equation Reynolds stress models. The influence of the prediction of laminar/turbulent boundary layer transition on the numerical dynamic stall simulation is described. Results of both numerical methods are compared to allow conclusions to be drawn with respect to an improved prediction of dynamic stall.

Ground Testing of the HyShot Supersonic Combustion Flight Experiment in HEG and Comparison with Flight Data

The first phase of the HyShot supersonic combustion ramjet (scramjet) flight experiment program of The University of Queensland in Australia was designed to provide benchmark data on supersonic combustion for a flight Mach number of approximately M=8. The second flight of the HyShot program, performed on July 30th 2002, was successful and supersonic combustion was observed along the specified trajectory range. The operating range of the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Centre (DLR) was recently extended. The facility now has the capability of testing a complete scramjet engine with internal combustion and external aerodynamics at M=7.8 flight con-ditions in altitudes of about 30 km. A post-flight analysis of the HyShot flight experiment was performed using an operational scramjet wind tunnel model with a geometry which is identical to that of the flight configuration.

Experimental investigation of dynamic stall performance for the EDI-M109 and EDI-M112 airfoils

An experimental investigation of the dynamic performance of two new rotor blade airfoils was undertaken in a transonic wind tunnel. The EDI-M109 and EDI-M112 airfoils were tested at 0.3<M<0.5 for pitching motions with amplitude 0.5° to 8° and frequencies 3.3Hz<f<45Hz. The results show that both new airfoils have acceptable dynamic stall performance, and the effect of frequency, amplitude, and higher order pitching motion on these results is described. The pitching moment peak size was found to have an approximately linear correlation to the normalised mean angular velocity, and thus test cases with the same maximum angle of attack and oscillation frequency had similar dynamic stall qualities. The correlation between low aerodynamic damping for high frequency, low amplitude pitching motion and poor dynamic stall performance is shown to be low. The dynamic stall response of the EDI-M112 airfoil is shown to be better for M=0.3 and M=0.4, and the response of the EDI-M109 airfoil is better at M=0.5. The dynamic performance of the airfoils is compared to the OA209.

Analysis of dynamic stall using dynamic mode decomposition technique

Dynamic mode decomposition is applied to investigate the unsteady flowfield around a pitching airfoil. The extracted flow structures are termed as dynamic mode decomposition modes. Analyses are performed for both attached flow and dynamic stall cases. Initially, flowfield data generated from numerical simulations are investigated. The effect of exclusion of the flowfield near the surface of the airfoil on the structure of the dynamic mode decomposition modes is examined. This is of vital importance for the experimental measurements, as the flowfield near the airfoil’s surface is difficult to measure using particle image velocimetry. In the latter part of the paper, dynamic mode decomposition modes extracted from the experimental data are analyzed. The structure of these modes are compared with the modes obtained from the proper orthogonal decomposition technique. Finally, effects of phase averaging on the modes are discussed.

Ground testing of the HyShot supersonic combustion flight experiment in HEG

The first phase of the HyShot supersonic combustion ramjet (scramjet) flight exper- iment program of The University of Queensland in Australia was designed to provide benchmark data on supersonic combustion for a flight Mach number of approximately M=8. The second flight of the HyShot program, performed on July 30th 2002, was successful and supersonic com- bustion was observed along the specified trajectory range. The operating range of the High Enthalpy Shock Tunnel Gottingen (HEG) of the German Aerospace Centre (DLR) was recently extended. The facility has now the capability of testing a complete scramjet engine with internal combustion and external aerodynamics at M=7.8 flight conditions in altitudes of about 30 km. A post flight analysis of the HyShot flight experiment was performed using an operational scramjet wind tunnel model with a geometry which is identical to that of the flight configuration.

Experimental Investigation of Unsteady Transition on a Pitching Rotor Blade Airfoil

The unsteady flow around the pitching helicopter main rotor blade airfoil EDI-M109 was experimentally investigated at conditions similar to those existing on a retreating rotor blade in forward flight. High speed pressure measurements and hot film anemometry were used to investigate the unsteady transition characteristics of the airfoil. Results are presented for dynamic test points with attached flow, light dynamic stall and deep dynamic stall at M = 0.3 and Re = 1.8 x 10^6. The results include the discussion of the periodicity of the hot film signals for different flow states. The transition process of the pitching airfoil is analysed and the significance of the intermittent region is described. A time delay between the transition and the model motion is discussed and a linear relationship between the transition position and the time is observed. The influences of the pitching amplitude on the transition characteristics are discussed and the flow separation initiating dynamic stall is analysed.

Comparison of Supersonic Combustion Tests with Shock Tunnels Flight and CFD

In 2002, the HyShot supersonic flight experiment was successfully launched and allowed to access experimental flight data for supersonic combustion. Ground based testing performed in the High Enthalpy Shock Tunnel (HEG) of the German Aerospace Center (DLR) allowed to reproduce the Mach 7.8 flight conditions. The present numerical work focuses on the computation of the experiments in HEG considering flows with and without combustion and fuel injection. Further, variations of the equivalence ratio and the resulting influence on the flow topology are studied.

Influence of rotation on dynamic stall

A computational investigation of the effect of rotation on two-dimensional (2D) deep dynamic stall has been undertaken, showing that the effect of rotation is to reduce the severity of the pitching moment peak and cause earlier reattachment of the flow. A generic single blade rotor geometry was investigated, which had a pitching oscillation around the quarter chord axis while in hover, causing angle-driven dynamic stall. The results at the midpoint of the blade have the same Mach number (0.31), Reynolds number (1.15×106), and pitching motion (α =13◦±7◦) as a dynamic stall test case for which significant experimental wind tunnel data and 2D computations exist. The rotating blade is compared with 2D computations and computations using the same blade without rotation at Mach 0.31 and with the same pitching motion. All test cases involve geometries propagating into undisturbed flow with no downwash. The three-dimensional (3D) grid computed without rotation had lower lift at the reference section than for a 2D computation with the same geometric angle of attack time history, and the lift overshoot classically observed for Spalart–Allmaras turbulence models during 2D dynamic stall was significantly reduced in the 3D case. Rotation reduced the strength of the dynamic stall vortex, which reduced the accompanying pitching moment peak by 25%.

Numerical Comparison of Dynamic Stall for Two-Dimensional Airfoils and an Airfoil Model in the DNW–TWG

The airfoil sections of helicopter rotors experience a wide range of flow conditions in forward flight from transonic flow on the advancing blade to subsonic flow and high angles of attack on the retreating blade. Most notably, the dynamic stall phenomenon has been a research topic for decades and various models have been introduced to predict the unsteady characteristics of the rotor blade undergoing unsteady separation. The objective of the present paper is to compare two dimensional (2D) dynamic stall computations, suitable for airfoil design studies considering unsteady characteristics, with computational fluid dynamics simulations of the wind tunnel environment taking into account three dimensionality and wall effects. Differences between experiment and 2D computations can be partly attributed to sidewall effects, which alter the effective angle of attack at the midsection pressure measurement plane. To gain more insight into these effects, investigations are presented, which show the wind tunnel wall boundary layers and separation effects at the sidewall–airfoil junction.

Numerical Investigation of Three-Dimensional Static and Dynamic Stall on a Finite Wing

Three-dimensional numerical computations using ONERA’s structured elsA code and the unstructured DLR-TAU code are compared with the OA209 finite wing experiments in static stall and dynamic stall conditions at a Mach number of 0.16 and a Reynolds number of 1 × 10^6 . The DLR-TAU computations were run with the Spalart–Allmaras and Menter shear stress transport (SST) turbulence models, and the elsA computations were carried out using the Spalart–Allmaras and the k–ω Kok + SST turbulence models. Although comparable grids were used, the static simulations show large discrepancies in the stall region between the structured and unstructured approaches. Large differences for the three-dimensional dynamic stall case are obtained with the computations using the Spalart–Allmaras turbulence model showing trailing edge separation only in contrast to the leading edge stall in the experiment. The three-dimensional dynamic stall computations with the two- equation turbulence models are in good agreement with the unsteady pressure measurements and flow field visualizations of the experiment, but also show a shift in the stall angle compared to the experiment. The analysis of the flow field around the finite wing using the numerical simulations reveals the evolution of the -shaped vortex, generated by the interaction of the blade tip vortex.