First Instability and Structural Sensitivity of the Flow Past Two Side-by-Side Cylinders

The onset of two-dimensional instabilities in the flow past two side-by-side circular cylinders is numerically investigated in the ranges 0.1 <= 6 <= 3 and Re < 100, with g being the non-dimensional gap spacing between the surfaces of the two cylinders and Re the Reynolds number. A comprehensive, global stability analysis of the symmetric base flow is carried out, indicating that three harmonic modes and one steady antisymmetric mode become unstable at different values of g and Re. These modes are known to promote distinct flow regimes at increasing values of g: single bluff-body, asymmetric, in-phase and antiphase synchronized vortex shedding. For each mode, the inherent structural sensitivity is examined in order to identify the core region of the related instability mechanism. In addition, by exploiting the structural sensitivity analysis to base flow modifications, a passive control strategy is proposed for the simultaneous suppression of the two synchronized shedding modes using two small secondary cylinders. Its effectiveness is then validated a posteriori by means of direct numerical simulations

Centre-Manifold Reduction of Bifurcating Flows

In this paper we describe a general and systematic approach to the centre-manifold reduction and normal form computation of flows undergoing complicated bifurcations. The proposed algorithm is based on the theoretical work of Coullet & Spiegel (SIAM J. Appl. Maths, vol. 43(4), 1983, pp. 776821) and can be used to approximate centre manifolds of arbitrary dimension for large-scale dynamical systems depending on a scalar parameter. Compared with the classical multiple-scale technique frequently employed in hydrodynamic stability, the proposed method can be coded in a rather general way without any need to resort to the introduction and tuning of additional time scales. The method is applied to the dynamical system described by the incompressible NavierStokes equations showing that high-order, weakly nonlinear models of bifurcating flows can be derived automatically, even for multiple codimension bifurcations. We first validate the method on the primary Hopf bifurcation of the flow past a circular cylinderand after we illustrate its application to a codimension-two bifurcation arising in the flow past two side-by-side circular cylinder

On the Origin of the Flip-Flop Instability of two Side-by-Side Cylinder Wakes

In this work the flip-flop instability occurring in the flow past two side-by-side circular cylinders is numerically investigated within the range of non-dimensional gap spacing 0.6<g<1.4 and Reynolds number 50<Re\leq 90. The inherent two-dimensional flow pattern is characterized by an asymmetric unsteady wake (with respect to the horizontal axis of symmetry) with the gap flow being deflected alternatively toward one of the cylinders. Such behaviour has been ascribed by other authors to a bistability of the flow, and therefore termed flip-flop. In contrast, the simulations performed herein provide new evidence that at low Reynolds numbers the flip-flopping state develops through an instability of the in-phase synchronized vortex shedding between the two cylinder wakes. This new scenario is confirmed and explained by means of a linear global stability investigation of the in-phase periodic base flow. The Floquet analysis reveals indeed that a pair of complex-conjugate multipliers becomes unstable having the same low frequency as the gap flow flip-over. The neutral curve of this secondary instability is tracked within the above range of gap spacing. The spatiotemporal shape of the unstable Floquet mode is then analysed and its structural sensitivity is considered in order to identify the 'core' region of the flip-flop instability mechanism

Experimental investigation of a helicopter rotor with Gurney flaps

The present work describes an experimental activity carried out to investigate the performance of Gurney flaps on a helicopter rotor model in hovering. The four blades of the articulated rotor model were equipped with Gurney flaps positioned at 95% of the aerofoil chord, spanning 14% of the rotor radius. The global aerodynamic loads and torque were measured for three Gurney flap configurations characterised by different heights. The global measurements showed an apparent benefit produced by Gurney flaps in terms of rotor performance with respect to the clean blade configuration. Particle image velocimetry surveys were also performed on the blade section at 65% of the rotor radius with and without the Gurney flaps. The local velocity data was used to complete the characterisation of the blade aerodynamic performance through the evaluation of the sectional aerodynamic loads using the the control volume approach

Wind-Tunnel Tests of a Tilt-Rotor Aircraft

A wide aerodynamic test campaign has been carried out on the tiltrotor aircraft ERICA at the Large Wind Tunnel of Politecnico di Milano by means of a modular 1:8 scale model in order to produce a dataset necessary to better understand the aerodynamic behaviour of the aircraft and to state its definitive design. The target of the tests was the measurement of the aerodynamic forces and moments in several different configurations and different attitudes. The test program included some conditions at very high incidence and sideslip angles that typically belong to the helicoptermode flight envelope and measurements of forces on the tail and on the tilting wings. A large amount of data has been collected that will be very useful to refine the aircraft design. In general the aircraft aerodynamics do not present any critical problems, but further optimisation is still possible. From the viewpoint of drag in the cruise configuration, the sponsons of the landing gear seem to be worth some further design refinement since they are responsible for a 20% drag increase with respect to the pure fuselage configuration. On the contrary, the wing fairing has proved to work well when the aircraft longitudinal axis is aligned with the wind, providing just a slight drag increase. Two other interesting aspects are the quite nonlinear behaviour of the side force for the intermediate sideslip angles as well as the noticeable hysteresis in the moment coefficient at very high incidence angles

Reconstruction de la pression à partir de mesures PIV par une méthode de Poisson discrète

International audienceCe papier présente une méthode de reconstruction de la pression pour les écoulements incompressibles qui se propose d’améliorer l’approche commune aux techniques fondées sur l’équation de Poisson. Cette méthode s’affranchit du choix, souvent heuristique et délicat, des conditions aux limites pour la pression, tout en gardant les propriétés avantageuses de filtrage spatial des erreurs de mesure par des opérateurs elliptiques. Le principe de cette méthode repose sur la ré-écriture des équations de Navier–Stokes sous la forme d’un problème de Stokes forcé, dont le terme de forçage est alimenté par les données PIV, et pour lequel les conditions aux limites sont posées naturellement sur le champ de vitesse, connu par défaut dans tout le volume de mesure. Suite à une discrétisation aux éléments finis, la pression est calculée par la résolution du complément du Schur associé. Après avoir donné les détails mathématiques et numériques de la méthode, ainsi que sa validation, nous présentons les résultats obtenus par application à des données PIV synthétiques issue de simulations numériques 3D IDDES (Improved Delayed Detached Eddy Simulation) autour d’une profile NACA0015 à forte incidence

Wind-tunnel tests of a heavy-class helicopter optimised for drag reduction

Wind-tunnel tests of a heavy-class helicopter model were carried out to evaluate the effectiveness of several components optimised for drag reduction by computational fluid dynamics analysis. The optimised components included different hub-cap configurations, a fairing for blade attachments and the sponsons. Moreover, the effects of vortex generators positioned on the back ramp were investigated. The optimisation effect was evaluated by comparison of the drag measurements carried out for both the original and the optimised helicopter configurations. The comprehensive experimental campaign involved the use of different measurement techniques. Indeed, pressure measurements and stereo particle image velocimetry surveys were performed to achieve a physical insight about the results of load measurements. The test activity confirms the achievement of an overall reduction of about 6% of the original model drag at cruise attitude

Linear global and asymptotic stability analysis of the flow past rectangular cylinders moving along a wall

The primary instability of the steady two-dimensional flow past rectangular cylinders moving parallel to a solid wall is studied, as a function of the cylinder length-to-thickness aspect ratio AR = L/D and the dimensionless distance from the wall g = G/D. For all A, two kinds of primary instability are found: a Hopf bifurcation leading to an unsteady two-dimensional flow for g &gt;= 0.5, and a regular bifurcation leading to a steady three-dimensional flow for g &lt; 0.5. The critical Reynolds number Re-c,Re- (2-D) of the Hopf bifurcation (Re = U infinity D/nu, where U-infinity is the free stream velocity, D the cylinder thickness and. the kinematic viscosity) changes with the gap height and the aspect ratio. For AR &lt;= 1, Re-c, (2-D) increases monotonically when the gap height is reduced. For AR &gt; 1, Re-c, (2-D) decreases when the gap is reduced until g approximate to 1.5, and then it increases. The critical Reynolds number Re-c, (3-D) of the three-dimensional regular bifurcation decreases monotonically for all AR, when the gap height is reduced below g &lt; 0.5. For small gaps, g &lt; 0.5, the hyperbolic/elliptic/centrifugal character of the regular instability is investigated by means of a short-wavelength approximation considering pressureless inviscid modes. For elongated cylinders, AR &gt; 3, the closed streamline related to the maximum growth rate is located within the top recirculating region of the wake, and includes the flow region with maximum structural sensitivity; the asymptotic analysis is in very good agreement with the global stability analysis, assessing the inviscid character of the instability. For cylinders with AR &lt;= 3, however, the local analysis fails to predict the three-dimensional regular bifurcation