Mechanism of natural transition delay by hydrogel metamaterial over a body of revolution
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摘要:
水下航行体首部声纳探测能力与边界层转捩密切相关。本文开展了水凝胶超材料在延迟水下航行体边界层自然转捩方面的应用基础研究,并探究了其内在机理。在高速水洞中分别开展了刚性和水凝胶表面的SUBOFF模型总阻力系数与二维瞬时速度场大视场PIV实验测试,采用多路径积分算法对SUBOFF模型周围脉动压力场进行估算;结合刚性SUBOFF模型边界层流动线性稳定性分析与PIV流场测试结果,获得刚性SUBOFF模型边界层自然转捩特性;采用“比光强”算法,对低速水洞中水凝胶表面SUBOFF模型局部形变与近壁区速度场进行频谱分析,揭示水凝胶表面流−固耦合作用机制;基于法向瞬时速度分量的连续子波变换,对边界层瞬时流场间歇性进行分析,揭示水凝胶在流−固耦合作用下延迟SUBOFF模型边界层自然转捩的机理。
Abstract:Laminar-turbulence transition in a bow boundary layer is crucial for the performance of the bow sonar of an underwater vehicle. An investigation on the application of the hydrogel metamaterial on natural transition delay is conducted and the underlying mechanism is analyzed. The two-dimensional velocity fields around both rigid and hydrogel SUBOFF models are acquired by a long-distance PIV in a high-speed water tunnel, and drag coefficients are measured as well. Based on the velocity fields, the pressure fluctuations around the SUBOFF models are calculated through the multi-path integration algorithm. Combining the linear instability analysis and PIV measurement, the characteristics of natural transition of the boundary layer over the rigid SUBOFF model are analyzed. The flow-hydrogel coupling is figured out, according to the spectral analysis of local deformation of the hydrogel surface and the velocity above the hydrogel SUBOFF model in a low-speed water tunnel. The intermittency of the instantaneous boundary layer flows are denoted by continuous wavelet transform of the wall-normal velocity components. Eventually, compared with the flow intermittency of the rigid SUBOFF model, the mechanisms of natural transition delay by the hydrogel metamaterial are discussed, under the influence of the flow-hydrogel coupling.
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表 1 测试仪器及主要参数
Table 1 Test facilities and major parameters
仪器 型号参数 测力天平 型号:L6H5‒4kg 量程:4 kg 精度:0.03% 应变放大器 型号:DH3840 输出电压:10V 增益:300 高频激光器 型号:Beamtech Vlite‒Hi‒35型 量程:200 Hz~10 kHz 精度:能量不稳定性$\leqslant1\% $ 高速相机 型号:Phantom LAB310 像素:1280 像素 × 800 像素 帧速:满幅帧率3260 Hz -
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