Study on Prolate Spheroid Pitching Oscillation in Viscous Stratified Flow
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摘要: 以黏性密度分层流下椭球体自由俯仰振荡衰减过程为研究内容，建立了密度连续分层流数值计算模型.通过对经典小球黏性绕流场的数值模拟和增阻系数的计算验证了数值模型的正确性.以初始45°攻角下的椭球体俯仰振荡过程为研究对象，采用基于Aitken亚松弛适应算法的双向流固耦合方法，数值模拟了不同内Froude(弗汝德)数Fri下椭球体俯仰衰减振荡的动态绕流场.数值研究结果表明，俯仰振荡将上下搅动周围流体，在椭球体上下两侧对称形成四个密度涡环，密度的垂向分层限制了涡环的垂向传播，也加速了涡环的消失，这种限制助长了水平运动的发展，远场尾涡流场将以水平波动的形式传播.在较高的内Froude数Fri和Reynolds(雷诺)数Re下，双向耦合抑制了数值震荡.研究还发现，随着来流速度的增加，阻力系数不增反降，这说明，对于自由俯仰振荡的椭球体，负阻尼现象仍然会出现.Abstract: The numerical calculation model for the continuous stratified flow was established through the study on the decaying process of the free pitching oscillation of the prolate spheroid in viscous stratified flow. The correctness of the numerical model was verified through numerical simulation of the viscous flow field of a sphere and calculation of its increasing drag coefficient. Under the 45° prolate spheroid initial angle the pitching oscillation process was investigated, with the Aitken sub-relaxation self-adaptive algorithm-based 2-way fluid-solid coupling method, and numerical simulation of the flow field around the prolate spheroid in pitching decaying oscillation at different values of Froude number Fri was performed. The numerical results show that, the pitching up and down agitates the surrounding fluid, and forms 4 symmetric density vortex rings on both sides of the prolate spheroid; the vertical density stratification limits vertical propagation of the vortex rings and accelerates disappearance of the vortex rings, and this limitation contributes to the development of horizontal motion. At higher Fri and Reynolds number Re,the 2-way coupling method suppresses numerical oscillations. The research also finds that, with the increase of the incoming velocity, the drag coefficient decreases, which means that for the prolate spheroid with free pitching oscillation, the phenomenon of negative drag still appears.
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Key words:
- viscous stratified flow /
- prolate spheroid /
- free decaying /
- pitching oscillation
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