Control of turbulent incompressible flows around bluff bodies using Large Eddy Simulations

The turbulent separated flows past several bluff bodies, i.e. a circular cylinder, the Malavard-Cousteau's Turbosail and a NACA0012 airfoil, are computed by using Large Eddy Simulations (LES). In order to modify the aerodynamic coefficients of the circular cylinder, three different control techniques are applied: rotation, boundary layer suction and synthetic jets. For the Turbosail and the NACA0012 profile, only a control by boundary layer suction is used. Moderate Reynolds numbers from Re = 2,000 to Re = 100,000 are considered in this study, corresponding to the subcritical regime (laminar separation of the boundary layer). The control efficiency on both lift enhancement and drag reduction is analyzed as function of the control parameters (position, intensity...). Finally, comparisons with experimental studies carried on in LEA/ENSMA (Poitiers, France) are shown for the flow around a circular cylinder, without and with control by boundary layer suction, at a high Reynolds number Re = 100,000.

The LES code used has been developed in LIMSI for a decade [1]. It solves the spatially-filtered incompressible Navier-Stokes equations written in their velocity-vorticity formulation. The LES technique is based on the fact that most of the turbulent kinetic energy is contained in the largest structures of the flow. Therefore, in LES, only these structures are directly computed. The smallest scales, typically smaller than the size of the grid cell, are modeled using a subgrid-scale model. The mixed scale model [2], developed jointly in LIMSI and ONERA which considers both energetic and dynamical aspects of the interaction between the large eddies and the smallest structure, is used in this study.

The flow around a circular cylinder at a high Reynolds number Re = 100,000 is first considered, without and with control by boundary layer suction. In order to show the ability of the LES to compute high Reynolds numbers separated turbulent flows, the results are compared with experimental results obtained by J. TENSI and S. BOURGOIS (LEA/ENSMA). The mean pressure distribution are presented on Figure 1 without control (left) and with control by suction (right). Despite the high Reynolds number and the related turbulent behavior with a large spectrum of spatial scales, a good agreement is recovered between the two different approaches, either for the uncontrolled or the controlled flow, proving that LES is a good tool for studying turbulent flows.
The second part of this study deals with the control of flow around the Turbosail at Re = 20,000. This bluff body was created by Malavard and Cousteau in order to create a new wind-powered boat, named Alcyone, to replace the old Calypso. The Turbosail, an ovoid-section cylinder acts exactly like a sail, the lift force being created by a huge fan, located in the cylinder, and sucking the flow through a permeable wall (Figure 2). In this study, the suction velocity is ten times smaller than the upstream velocity and the suction surface, beginning at about 45 % of the profile chord, spreads over 26 % of the chord.
When the suction is not applied, the Turbosail has really poor aerodynamical performances as a sail. Indeed, a high drag force appears and no lift force is observed, for the three considered angles of incidence 0°, 10° and 20°. However, when the suction is switched on, and whatever the angle of incidence is, the performances are sharply improved and the Turbosail may then be considered as a very efficient way to move a boat. The influence of the suction on aerodynamic coefficients can be shown on Figure 3 (left).
Finally, the flow around a thin wing profile (NACA0012) for Re = 10,000 is considered. The suction is here applied on a surface of 12 % of the chord, starting close to the upstream stagnation point, with a suction velocity ten times smaller than the upstream velocity. On the opposite of the Turbosail, the NACA0012 has a great lift-on-drag ratio even when no suction occurs. Nevertheless, the suction is able to raise this ratio up to very high values, proving that this kind of control techniques is not only able to create a lift force, but also to enhance it (Figure 3(right)). In addition to the efficiency improvement, the suction can also greatly modify the flow patterns, such as the vortex shedding. Indeed, it can be observed on Figure 4 that when a suction is applied, the shed vortex are smaller than they would be in the uncontrolled case. The main reason is that the boundary layer separation is delayed by the suction. The wake thickness and consequently the typical size of the vortex are then reduced.
Two main conclusions can be drawn about this study. First, the ability of Large Eddy Simulations to predict high Reynolds number separated turbulent flows has been proved by means of comparisons with experimental results (LEA/ENSMA), even if a control by suction is applied. The control by boundary layer suction has then been shown to be of great interest to modify the aerodynamic coefficients, either for a bluff body or for an airfoil. Moreover, the suction can also strongly modify the vortex shedding pattern and appear to be a good method to reduce Vortex-induced Vibrations on airplane wings for instance. Several other means of control have to be checked in the future, mainly synthetic jets which are known to be rather efficient as a flow control technique, consisting of a half-period suction followed by a half-period blowing to ensure that there will be no net mass injection in the flow. In addition, because of the high Reynolds number considered here, the influence of the suction in the spanwise direction might also be investigated. This study is part of Guillaume Fournier's PhD Thesis, defended on May 26th [3].

References

  [1]  Lardat R.: Simulations numeriques d'ecoulements externes instationnaires decolles autour d'une aile avec des modeles de sous-maille. PhD Thesis, Universite Pierre et Marie Curie - Paris VI, 1997.
  [2]   Ta Phuoc L.: Modèles de sous-maille appliqués aux écoulements instationnaires et décollés., Journée Thématique DRET, Paris, 1994.
  [3]   Fournier G.: Controle de l'ecoulement decolle autour de profils epais par la Simulation des Grandes Echelles. PhD Thesis, Universite Pierre et Marie Curie - Paris VI, 2005.