低雷诺数翼型蒙皮主动振动气动特性及流场结构数值研究

NUMERICAL STUDY ABOUT AERODYNAMIC CHARACTERISTICS AND FLOW FIELD STRUCTURES FOR A SKIN OF AIRFOIL WITH ACTIVE OSCILLATION AT LOW REYNOLDS NUMBER

针对低雷诺数(Re)翼型气动性能差的特点,文章通过对翼型柔性蒙皮施加主动振动的方法,提高翼型低Re下的气动特性,改善其流场结构.采用带预处理技术的Roe方法求解非定常可压缩Navier--Stokes方程,对NACA4415翼型低Re流动展开数值模拟.通过时均化和非定常方法对比柔性蒙皮固定和振动两种状态下的升阻力气动特性和层流分离流动结构.初步研究工作表明在低Re下柔性蒙皮采用合适的振幅和频率,时均化升阻力特性显著提高,分离泡结构由后缘层流分离泡转变为近似的经典长层流分离泡,分离点后移,分离区缩小.在此基础上,文章更加细致研究了柔性蒙皮两种状态下单周期内的层流分离结构及壁面压力系数分布非定常特性和演化规律.蒙皮固定状态下分离区前部流场结构和压力分布基本保持稳定,表现为近似定常分离,仅在后缘位置出现类似于卡门涡街的非定常流动现象.柔性蒙皮振动时从分离点附近开始便产生分离涡,并不断向下游移动、脱落,表现为非定常分离并出现大范围的压力脉动.蒙皮振动使流体更加靠近壁面运动,大尺度的层流分离现象得到有效抑制.

For the poor aerodynamic performance of airfoil at low Reynolds number, the paper targets the active oscillation for flexible skin of airfoil in order to improve its aerodynamic characteristics and flow field structures. Roe method with preconditioning technique was used to solve the unsteady compressible N--S equations and simulate the flow for NACA4415 airfoil at low Reynolds number. The aerodynamic force characteristics and laminar flow separation structures were compared by time-average and unsteady methods when the skin is static or oscillating. Preliminary studies indicate when the flexible skin actively oscillates with appropriate amplitude and frequency, the time-average lift and drag characteristics increase significantly. The separation bubble structures transform from trailing-edge laminar separation bubble to classic long laminar separation bubble. The separation position moves downstream and the separation region reduces.On this basis, the paper studied the alterations of the unsteady flow structures and pressure coefficient distributions more detailed during one period with the two states of the skin. The flow field structures and the pressure distributions keep steady in the front part of separation region at a static skin. The flow approximates steady separation and the unsteady flow phenomena like Karman vortex streets only appear at the trailing edge. Otherwise, for the oscillating skin, the unsteady vortexes generate near the separation position, subsequently move, and then shed along the airfoil surface. The flow presents unsteady separation and displays a wide range of pressure oscillation. Owing to the oscillating skin, the fluid movements are closer to the wall. The large scale laminar flow separation phenomenon is suppressed apparently.