Study On The Flow Control And Aerodynamic Enhancement Of Lifting Surfaces

In aircrafts'manufacturing,focus is on getting continuously to the highest performances ever.However,several constraints may overlap such as,developing a more efficient vehicles while being as greener as the growing environmental awareness require.Aiming to overcome these limitations,several fields have been developed and proven their effectiveness in the real applications such as,the flow control.The different strategies of flow control aim to manage the flow from its natural development to a desirable state.Reducing the drag,delaying the stall,or enhancing the aircraft maneuverability are all reachable owing to this field of engineering.It constitutes an open research topic in which we still lack fundamental understanding.This thesis tackles challenges related to the enhancement of the aerodynamic performances of an aircraft or a lifting surface by introducing a novel active flow control strategy.It aims principally to develop a strategy to prevent the flow separation at high Angles of Attack(AOA)and therefore,delay the stall occurrence.Two flow control methods have been investigated in the present thesis.The first one numerically using the commercial software ANASYS Fluent and the second one,experimentally in the Joint Laboratory of Wind Tunnel and Wave Flume,Harbin Institute of Technology,Harbin,China.The dissertation begins with a discussion of some aerodynamic features necessary to understand the required fundamentals.It covers the flow separation physics and their impacts on a lifting surface aerodynamic performance.Regarding to the airfoil thicknesses and Re number,the different kind of stall were mentioned.Then,owing to the growing interest on Micro Aerial Vehicles(MAVs),particular attention was given to the low Reynold(Re)aerodynamic over Low Aspect Ratio(LAR)wings.This field shows new aerodynamic aspects(such as,the vortex lift and the laminar separation bubble)still requiring further investigations.As all flyers categories(MAV,transport or fighter aircraft)under specific conditions,may lose their aerodynamic performances,their flight safe management requires further flow-control assistance to overcome any eventual need.Different flow control strategies such as,fluidic,moving wall,and morphing wing have been surveyed herein.The second part focuses on a pointwise fluidic control known as the Synthetic Jet Actuators(SJA).It investigates numerically the effectiveness of this kind of control over a bi-dimensional NACA0015 airfoil.However,Results showed a good agreement with the experimental data at low to moderate AOAs,both used turbulent models presented a weakness to predict the stall angle effectively.The synthetic jet control exhibits an extraordinary lift coefficient enhancement at high AOA but seems to be less obvious at low AOA,where the flow is still attached.A synthetic jet of a normalized frequency(F+=2)and a momentum(C?of 0.56%),delays the stall onset from 15 to19deg with enhancing the lift coefficient by 40%.Moreover,the actual part has been enriched by studying the effect of the jet's frequency and momentum on the lift temporal signal.Also,the interaction between the mean flow and the synthetic jet structures topology was undertaken.The third part is devoted to experimentally assess the aerodynamic effectiveness of the new flow control method designed,i.e.,the translating belt flow control.The novel technic has been tested over a LAR(0.57 AR)thick airfoil(NACA0015 shaped)performing at a Low Res.Different mounting allowed to quantify the aerodynamic forces generated at the different control actuation speeds.The moving belt control succeeded to enhance the aerodynamic performances considerably.This kind of control postponed the stall onset by 25 deg and produced a 103%gain in the lift force without any saturation signs at a control speed ratio of U_b/U=6.Particle image velocimetry(PIV)measurements confirmed the effectiveness of the moving-wall control strategy on the upper surface flow reattachment.Moreover,other quantities such as the vortices and the swirling strength are investigated.Finally,in the aim to understand the control mechanisms,the last part verifies the LAR thick airfoils'conformity with the nonlinear lift approximation equation by calculating the different(potential and vortex)lift coefficients and comparing them with those of thin airfoils.Then,the moving-wall flow control effects on the tri-dimensional character of a LAR wing has been emphasized.Series of PIV measurements and flow visualization of the near wing-tip field at different chord-wise locations show the control interaction with the span-wise flow in matter of enhancing the momentum toward the wing-root and the wing-tip vortex strength.