Pénélope Leyland

Effects of high-speed airflows on a surface dielectric barrier discharge

A detailed study of the interaction between high-speed gas flows and surface dielectric barrier discharges (DBD) is presented. In the present paper, it is demonstrated that a DBD can be sustained in transonic airflows, up to isentropic Mach numbers of 1.1. The plasma is characterized electrically, as well as optically with a CCD camera and a photomultiplier tube. Different airflow velocities, plasma excitation frequencies and voltages are investigated. The airflow has a significant influence on the plasma characteristics: the glow component is reduced, the discharge becomes more filamentary and most importantly, the light emission duration from individual microdischarges is reduced by more than a factor of ten at high flow velocities. Large edge effects play a key role in the interaction between the flow and the plasma. These results offer new perspectives for the use of dielectric barrier discharges in transonic and supersonic gas flows and their applications to airflow control and to plasma-assisted combustion.

Analysis of Fluid-Structu re Interaction on Moving Airfoils by Means of an Improved ALE-method

This paper presents a computational analysis of fluid-structure interaction. A staggered approach is adopted where the fluid and structure are integrated in time by separate solvers. The interaction is taken into account by the boundary conditions. The unsteady compressible flow is calculated by a two dimensional Euler and Navier-Stokes solver on unstructured grids. This solver is extended to moving domain problems by the Arbitrary Euler-Lagrange (ALE) method. The mesh is then deformed by an improved spring analogy which allows for large deformations of the computational grid. The influence of the structural time integration is investigated by several methods. The staggered approach is modified by a subcycling algorithm where the time steps for the fluid and structure are different. The capabilities of the improved spring analogy are shown by a large rotation of the NACA0012 airfoil Furthermore, numerical results are shown for the forced pitching vibration of the NACA0012 airfoil and aero-elasticity of the same airfoil supported by elastic springs.

Experimental Investigation of Pulsed Dielectric Barrier Discharge Actuators in Sub- and Transonic Flow

Experiments were conducted in order to investigate the ability of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators to control flow on airfoils in subsonic and transonic flow (up to Ma1 = 0.75, Re = 1.35 • 10^6). A NACA 0015 profile equipped with a leading edge mounted ns-DBD actuator was investigated up to Re = 2.3 • 10^5 (u1 = 24 m/s). Measurements of the surface pressure distribution clearly confirm the actuator’s potential to delay leading edge separation. In the following the impact on the control authority of different voltage pulse parameters, such as voltage amplitude, actuation frequency and rise/fall time of the pulse were investigated. The experiments in transonic flow were conducted on a NACA 3506 compressor blade profile. A ns-DBD actuator was placed at x/c = 0.33 where the foot of the shock-wave and boundary-layer separation was observed. Schlieren flow visualization showed the shock-wave boundary-layer interaction and was used to investigate the actuator’s effect on the shock position and shape. A high-speed camera allowed to acquire schlieren images at high acquisition rates and investigate as well the movement of the shock in the frequency domain. These results were verified with measurements of the static pressure at the side wall using unsteady pressure transducers.

Fully Coupled Fluid-Structure Algorithms for Aeroelasticity and Forced Vibration Induced Flutter: Applications to a Compressor Cascade

ABSTRACT Algorithms for Fluid-Structure Coupling techniques are investigated in the lime domain. The accurate prediction of the interaction requires consistency of the interface boundary conditions with the time levels of integration of the fluid and the structure equations. If staggered algorithms are used, the time delay causes non-physical energy dissipation in the system which modifies the calculated aeroelastic behaviour. In order to be compatible, the equations must be integrated simultaneously and implicitly. These techniques are tested on a standard aeroelastic airfoil problem, and then applied to the direct coupling of an assembly of 20 compressor blades performing torsional vibrations.

Subscale testing of the Fire II vehicle in a superorbital expansion tube

Testing of the first heatshield of the Fire II reentry vehicle was performed in the X1 superorbital expansion tube at The University of Queensland. The test model was a 1:28 scale replica of the flight vehicle, and incorporated three thermocouples: stagnation and two radial. A trajectory point towards the end of the first experimental testing period, at a total flight time of 1639.5s, an altitude of 61.5Km and velocity 11.1km/s was simulated in the expansion tube. Stagnation point heat transfer was obtained using a fast response coaxial type E thermocouple. In the current analysis the convective and radiative heating components were treated independently, where the convective component was scaled with the length scale and the absolute value of the radiative heat transfer was held constant. From this, the overall contribution of the radiative heat transfer to the total heat rate is decreased in the expansion tubes from an 18% contribution in flight to less than 1%, whereas the convective component was increased by a factor of 28. This results in the convective heat transfer being the major mode of heat transfer in the experimental models. From the Fay and Riddell empirical convective heat transfer correlation it was shown that the parameter Ch√Re should remain constant between the flight and experimental tests provided ρL scaling is maintained. Results from the current study show good agreement with the convective heating component of the flight vehicle and the Ch√Re values are in agreement to within 20% of the flight results. The results obtained in this study give a strong indication that the relative radiative heat transfer contribution in the expansion tube tests is less than that in flight, supporting the analysis that the absolute value remains constant with ρL scaling.

Stability Analysis of a Partitioned Fluid–Structure Thermal Coupling Algorithm

Numerical multiphysics coupling is gaining more and more importance in engineering. Partitioned algorithms are among the most efficient methods to solve the coupled problem. However, they present some drawbacks, with the stability of the algorithm itself being the crucial one. The principal factors that lead to instability are the physical and numerical properties of the system. In this paper, a thermal coupling between a fluid and a solid, performed through a parallel partitioned algorithm, is considered. A condition that ensures the stability of the algorithm is derived.

Completely parallel compressible flow simulations using adaptive unstructured meshes

New and efficient completely parallel strategies for unstructured mesh solvers with dynamically adaptative meshes are developed. From highly complex academic research problems of unsteady compressible flow simulation to full scale aircraft problems, all are simulated with high performance results. Dynamic load balancing is obtained for simple but robust domain partitioners, who work directly on the network of processors. The code is hybrid as it allows elements of different types to co-exist in the same mesh.

Investigation of Nanosecond Pulse Dielectric Barrier Discharges in Still Air and in Transonic Flow by Optical Methods

In the present study the interaction of nanosecond pulsed dielectric barrier discharge (ns-DBD) actuators with aerodynamic flow up to transonic velocities was investigated. The primary focus was on the influence of the flow on the discharge and the effects of the discharge itself. In addition, the influence of the ns-DBD on a shock-wave was studied. The aim was to improve the understanding of the plasma-flow interaction, a topic that is not yet fully understood, in particular for ns-DBD. The actuator was integrated in two different models, a NACA 3506 compressor blade profile and a bump geometry at the bottom of the wind tunnel. The effect of the rapid energy deposition close to the discharge was examined with the phase-locked schlieren visualisation technique. Images of the plasma acquired with short exposure times revealed information on the discharge evolution. The results show a significant effect of the flow on the discharge characteristics, in particular due to the drop of static pressure. On the other hand, no significant effect of the ns-DBD on the flow was observed due to unfavourable flow conditions, which underlines the importance of the actuator’s placement.

Master-slave performance of unstructured flow solvers on the cray T3D

Publisher Summary This chapter discusses performance issues of unstructured mesh solvers with mesh adaptation, for compressible flows, using different programming models and communication libraries. The partitioning of the grid into sub-domains is performed on a master (Cray YMP), as well as the subsequent adaptations, the solution process is executed on the slave (Cray Torus, 3-Dimensional (T3D)), with up to 128 processors. The chapter uses three flow solvers of different degrees of complexity to illustrate the trade-off between synchronization and redundant calculation necessary at sub-domain interfaces. Amongst the test cases presented, a transient regime effect in optimal design is calculated, which requires a high number of runs of complete convergence, illustrating the practical utility of parallel computational fluid dynamics (CFD), where central processing unit (CPU) time becomes the elapsed time and allows these series of runs to be made in a very short period of real time. The chapter demonstrates that a master-slave environment for solving steady state flows with auto-adaptive mesh adaptation is feasible as long as the number of refinements is low.

Analysis and rebuilding of experiments on a heated carbon graphite model in the X2 expansion tube

Numerical simulations have been performed to study radiative and ablative processes experienced by a space vehicle entering a planetary atmosphere. The simulations are compared with spectroscopic measurements of a model tested at hypersonic entry conditions in the X2 expansion tube at the University of Queensland. The tests used a hemicylin-drical graphite model to reproduce the ejection of carbon-based species into the flow due to surface reactions of an ablative material. Measurements were performed for flow over a cold (room temperature) model, and over an electrically pre-heated model, to study the influence of the model temperature on the recorded spectra. The experiments were rebuilt using numerical and physical analysis, the hot wall case. The flow configuration is fully three-dimensional, hence simulations are performed accordingly.