Aerodynamic Hysteresis Loop And Flow Control On A Rectangular Wing At Low Reynolds Numbers

Reynolds number based on the airfoil chord length between 10~4 and 10~6 is considered to be the low Reynolds number,in which the airfoil shows some worse aerodynamic characteristics such as the static aerodynamic hysteresis loop for the angles of attack close to the static stall angle for the airfoil.The investigation on the Static aerodynamic hysteresis of the NACA633-421 airfoil at low Reynolds number is carried out in NUAA low speed wind tunnel with flow visualization,force measurement,pressure measurement,PIV(particle image velocimetry)technique and hotwire probe.Results show that laminar separation bubble,which cannot resist adverse pressure gradient,disappearing and rupturing is a major cause leading to static stall occurs.The flow obtained along the decreasing angle branch be unable to form a laminar flow separation bubble at static stall angle to stabilize flow on the upper airfoil is the key to the static hysteresis of the wing.By using active flow control to reform the laminar flow separation bubble can weaken and restrain static aerodynamic hysteresis.The influence on the aerodynamic hysteresis caused by synthetic jet slots,which are on the leading edge of the wing,is investigated.Synthetic jet slots make the laminar separation bubble“strong”at sensitive Reynolds number.And the bubble can exist stably through the sensitive angles of attack at sensitive Reynolds number.The hysteresis at the medium angle of attack disappears.However,the hysteresis at the high angle of attack still exists,the stall angle and the flow reattach angle are higher and the hysteresis area is slightly increased.The exist of synthetic jet slots on the leading edge of the wing making the leading edge laminar separation bubble smaller is the reason for suspending the stall.Using hot wire probe technique and particle image velocimetry to investigate the outlet flow field characteristics of synthetic jet slots.And synthetic jet technology was applied to controlling aerodynamic hysteresis loop at low Reynolds number:Under the condition of low Reynolds number flow(Re=1.24E5),the original hysteresis loop area of 21°~27°has been completely eliminated.Increasing the flow Reynolds number to Re=1.42E5 and Re=1.59E5,the original hysteresis loop area of 22°~29°narrow down to 27°~29°,which gets a 71.4% shrinkage.