Generalized Expression of Chorochronic Periodicity in Turbomachinery Blade-Row Interaction

The unsteady flow which is generated when 2 turbomachinery blade-rows are in relative angular motion is periodic in time with a different period in the frame of reference associated with each blade-row, and is characterized by a pitchwise traveling wave chorochronic periodicity. This periodicity is studied for arbitrary angular velocities and pitch-ratio of the 2 blade-row and simple formulae for the corresponding interblade-phase angles are given

Global self-excited oscillations in a two-dimensional heated jet : a numerical simulation

The aim of this work was to develop a numerical methodology to gain insight in the low-density jet behaviour with a nonlinear approach. Numerical simulations are shown to differentiate convective and absolute instability regimes and to capture a self-excited global mode in an open flow : the 2D hot jet. The first part is devoted to numerical methodology and its validation on this unsteady problem which is known to be noise sensitive. The second part presents numerical results. They confirm theoretical and experimental results on the development of self-exited global oscillations of the jet column when the density ratio is lower than its critical value. The global mode and its associated Hopf bifurcation are identified

Le contrôle d'un jet d'air rectangulaire via l'introduction de cylindres animés d'un mouvement de rotation

L'idée à l'origine de l'étude est de modifier la structure d'un écoulement pour transformer cet écoulement convectivement instable en un écoulement globalement instable. Les jets globalement instables montrent un taux d'expansion plus élevé que les jets homogènes convectivement instables. Une simulation numérique a montré que le forçage, d'un jet convectivement instable, au moyen de sources de vorticité contrôlées, conduisait à une augmentation du mélange. Notre étude vise à réaliser un montage expérimental équivalent afin de confirmer les premiers résultats observés

Loi d'échelle des modes globaux dans les écoulements cisaillés ouverts: le jet plan chauffé

Loi d'échelle des modes globaux dans les écoulements cisaillés ouverts: le jet plan chauff

Sails Aerodynamics - A Seatrip around sail shapes

The Aerodynamics of sails and sails interaction problem are of high interest for many decades of racing yachts. Soft sails have a long history for sailing yachts. Rigs have changed over time and regions of the world. The square rig has been used for a long time but was mainly a downwind rig without positive interaction between sails. The Latin rig use triangular sails and has better performances than the square rig in upwind conditions and is more manoeuvrable. The Bermuda or Marconi rig with a mainsail fixed on a mast and a headsail or jib have higher performances in upwind conditions and allows more easy tackle. It is largely used today on racing yachts and is well adapted to a large range of boat size. It is the most known rig configuration for sails interaction. It will be one focus point of this paper. For simplicity, we will not consider the high length boats using multi masts system, as it is a variation on the same theme. Knowing this restriction, it is a complex subject if we think about the slow evolution and the many variations used in a given period of time. This may seems a paradox knowing that racing boats are driven by a unique objective which is speed on sea water in given wind conditions. But as every sailor know, the way to sail fast on water are numerous depending on sea state, wind conditions and boat architecture and is a subject of many contradictory stories. It will be seen how the sail and rig evolution may be related to aerodynamic research and tools evolution over years to help designing advanced sails and rigs for racing sailing yachts. The fact that aeronautic research has frequently played a role of precursor in maritime new findings will be emphasized. Interacting sails (mainsail + jib) is a solution, which is the result of a long evolution over years of practice, and trial and errors on various boats. It has been used for its ability to generate a high driving force without too much heeling force and its consequences leeway and heeling moment. It has been described and explained in a long list of publications. We will go through selected ones to focus on main parameters affecting sail interaction before to go further toward recent evolutions of this classic and largely used rig. It will be shown how tools evolution has play a key role in the understanding of the sail interaction problem with Gentry, Marchaj and Milgram publications. Later, a well-known evolution has been proposed on Star & Stripes in 1988 with a semi-rigid mainsail. Then, more recently a transition occurs by replacing softs or semi-rigid mainsails by fully rigid ones, as it has been popularized by the famous multi-elements wingsail of the giant BOR 90, and later, by the two last editions of the America’s cup yachts (AC72, AC45, AC48, AC50). All these evolutions are based on a symmetric rigid mainsail and a classic soft jib or gennaker. The aerodynamic basics and performances of these new configurations will be described from recent publications. Explaining the rational behind this evolution, and anticipate possible future evolution will be proposed. Putting this sail design evolution story in perspective, we will show how design tools for sails and rigs have largely contributed to the understanding of sails aerodynamics and to these evolutions from soft to rigid sails. Few figures illustrating these elements are given below and will be completed

Aerodynamic analysis of 3D multi-elements wings : an application to wingsails of flying boats

International audienceA study of multi-elements wings applied to a wingsail of a america's cup yacht

Integration of CFD tools in aerodynamic design of contra-rotating propellers blades

A strategy for Contra-Rotating Open Rotors blades design is presented. It is based on several analytical or CFD tools with increasing levels of accuracy. A preliminary design is made with a lifting-line code. These first geometries are then validated and improved with more advanced CFD methods. Steady CFD computations on a blade passage using the mixing-plane technique permit to perform an accurate sensitivity analysis of the main blade geometrical parameters. The key parameters driving the propulsion efficiency are thus identified. Modeling rotor-rotor interactions require more costly unsteady simulations such as chorochronic computations on a blade passage, or chimera/sliding-mesh techniques on the full model for assessing even the installation effects. These methods are performed at the end of the process to validate the robustness of advanced designs to those interactions. The relevance of this strategy is discussed highlighting the results and limitations

Route to chaos on a dragonfly wing cross section in gliding flight

The route from linear towards nonlinear and chaotic aerodynamic regimes of a fixed dragonfly wing cross section in gliding flight is investigated numerically using direct Navier-Stokes simulations (DNSs). The dragonfly wing consists of two corrugations combined with a rear arc, which is known to provide overall good aerodynamic mean performance at low Reynolds numbers. First, the three regimes (linear, nonlinear, and chaotic) are characterized, and validated using two different fluid solvers. In particular, a peculiar transition to chaos when changing the angle of attack is observed for both solvers: The system undergoes a sudden transition to chaos in less than 0.1 degree. Second, a physical insight is given on the flow interaction between the corrugations and the rear arc, which is shown as the key phenomenon controlling the unsteady vortex dynamics and the sudden transition to chaos. Additionally, aerodynamic performances in the three regimes are given, showing that optimal performances are closely connected to the transition to chaos

Active control of a stalled airfoil through steady or unsteady actuation jets

The active control of the leading-edge (LE) separation on the suction surface of a stalled airfoil (NACA 0012) at a Reynolds number of 106 based on the chord length is investigated through a computational study. The actuator is a steady or unsteady jet located on the suction surface of the airfoil. Unsteady Reynolds-Averaged Navier–Stokes (URANS) equations are solved on hybrid meshes with the Spalart–Allmaras turbulence model. Simulations are used to characterize the effects of the steady and unsteady actuation on the separated flows for a large range of angle of attack (0<a<28 deg). Parametric studies are carried out in the actuator design-space to investigate the control effectiveness and robustness. An optimal actuator position, angle, and frequency for the stalled angle of attack a¼19 deg are found. A significant increase of the lift coefficient is obtained (+84% with respect to the uncontrolled reference flow), and the stall is delayed from angle of attack of 18 deg to more than 25 deg. The physical nonlinear coupling between the actuator position, velocity angle, and frequency is investigated. The critical influence of the actuator location relative to the separation location is emphasized

Viscous computational fluid dynamics as a relevant decision-making tool for mast-sail aerodynamics

Viscous computational fluid dynamics based on Reynolds averaged Navier-Stokes (RANS) equations have been used to simulate flow around typical mast-sail geometries. It is shown how these advanced numerical methods are relevant to investigate the complexity of such strongly separated flows. Detailed numerical results have been obtained and compared to experimental ones. Comparative analysis has shown that RANS methods are able to capture the main flow features, such as mast-flow separation, recirculation bubble, bubble reattachment through a laminar-turbulent transition process, and trailing-edge separation. A second part has been devoted to the comparative behavior of these flow features through parameters variations to evaluate the qualitative and quantitative capabilities of RANS methods in mastsail design optimization. The last part illustrates through two examples how RANS methods may be used to optimize the design of mast-sail geometries and evaluate their relative performances