Letter: The effects of streamwise system rotation on pressure fluctuations in a turbulent channel flow

In this letter, we report the modulating effects of streamwise system rotation on both the amplitude and the wavenumber of pressure fluctuations in a plane channel flow. The analysis of the pressure field is conducted based on a set of comprehensive direct numerical simulation data of six rotation numbers. It is observed that high pressure fluctuation regions collocate with the Taylor-Gortler-like (TGL) vortex cores. By decomposing the pressure field into rotation-induced and convection-induced parts, it is observed that the rotation-induced part dominates the total pressure fluctuations and facilitates the growth of TGL vortices. Furthermore, through a spectral analysis, it is discovered that the system rotation acts as a "linear amplifier," which converts high-wavenumber low-amplitude streamwise vorticity fluctuations into low-wavenumber high-amplitude pressure fluctuations. Published by AIP Publishing

一种风力发电机的尾迹识别方法

本发明提供一种风力发电机的尾迹识别方法,先采集试验风力发电机的所有流场数据,然后对流场数据通过差分方法进行求导计算,得到速度梯度张量,然后再计算出流场每个点上的流动不变量数据；对已知的数据进行分析,并按预定标准将收集的数据分为强湍流和弱湍流；以流动不变量数据作为输入量,以强湍流和弱湍流数据作为学习对象,通过机器学习算法软件生成一个识别器；将需要识别流场的不变量数据输入识别器,然后根据预定标准将符合强湍流的数据区域绘出,即得到产生当前需要识别流场的风力发电机的尾迹区域。本发明使用了现代计算机科学中的大数据分析方法,只需要提供充分的数据,而不需要加入其他人为干涉,可以保证计算结果的客观性

海上大气湍流的数值模拟

为了研究海上大气湍流,我们发展了一套多功能的数值计算程序。利用该程序,进行了海面温度间断上大气湍流的研究。研究发现当空气从低温水面流向高温水面时,出现低空射流现象,风速在0.3倍边界层厚度处达到最大,在此高度之上,风速随着高度增加而减小,这样的风速分布和传统边界层中有显著不同。进一步研究发现由温差引起的密度差是引起低空射流的主要因素

On the wavenumber-frequency spectrum of the wall pressure fluctuations in turbulent channel flow

Direct numerical simulations (DNS) of turbulent channel flows up to Re-tau approximate to 1000 are conducted to investigate the three-dimensional (consisting of streamwise wavenumber, spanwise wavenumber and frequency) spectrum of wall pressure fluctuations. To develop a predictive model of the wavenumber-frequency spectrum from the wavenumber spectrum, the time decorrelation mechanisms of wall pressure fluctuations are investigated. It is discovered that the energy-containing part of the wavenumber-frequency spectrum of wall pressure fluctuations can be well predicted using a similar random sweeping model for streamwise velocity fluctuations. To refine the investigation, we further decompose the spectrum of the total wall pressure fluctuations into the autospectra of rapid and slow pressure fluctuations, and the cross-spectrum between them. We focus on evaluating the assumption applied in many predictive models, that is, the magnitude of the cross-spectrum is negligibly small. The present DNS shows that neglecting the cross-spectrum causes a maximum error up to 4.7 dB in the subconvective region for all Reynolds numbers under test. Our analyses indicate that the approximation of neglecting the cross-spectrum needs to be applied carefully in the investigations of acoustics at low Mach numbers, in which the subconvective components of wall pressure fluctuations make important contributions to the radiated acoustic power

俯冲射流引起的自由表面湍流中气泡夹带的数值研究

气泡的产生和夹带是气液两相湍流的重要特征。船舶在水面航行时,激起的俯冲射流和自由表面相互作用是空气夹带的主要来源。为研究俯冲射流引起的气泡夹带和两相湍流的力学机理,我们设计了俯冲射流入水的数值实验,用我们内部的CAS-TANK两相流求解器进行了数值模拟,并对密度加权的平均速度和气泡的平均体积分数进行了统计,初步结果与同工况下的实验观测一致,我们还对入水口附近的气泡较为集中的区域进行了气泡半径密度谱进行了评估,结果符合-10/3幂律。已有进展为后续进行两相湍流机理的研究奠定了基础

使用机器学习识别圆柱尾迹中的湍流区域

探测湍流/非湍流界面是湍流研究中一个具有挑战性的课题。本文使用了机器学习方法训练探测器,用于识别圆柱绕流尾迹中的湍流区域。为保证所得到的湍流区域不依赖于坐标系的选取,我们提出了使用流动不变量作为探测器的输入变量,包括出现在脉动速度、脉动应变率张量、脉动涡张量的输运方程中的张量的不变量。训练所使用的数据为使用直接数值模拟/大涡模拟生成的Re=100和3900的圆柱绕流流场,使用的模型是Extreme Gradient Boosting(XGBoost),该方法属于监督学习方法,训练时需要给出流场对应的标签,即流动状态为湍流或非湍流,因此训练数据需选自流动状态已知的区域,例如湍流样本选自Re=3900的流动中远离湍流/非湍流界面的尾迹核心区域,非湍流样本则选自Re=3900的流动中的圆柱上游区域以及Re=100的流动中的圆柱尾迹区域。在训练完成之后,将流动中任意一点的不变量输入到探测器中,可以得到该点的流动状态,遍历全场即可得到流场中的湍流/非湍流界面。为了保证探测器结果的客观性,我们检验了训练过程中人为因素的影响,包括湍流样本的选取区域以及作为输入的不变量的个数。测试结果表明,扩大或缩小湍流样本的选取区域不影响湍流/非湍流界面的探测结果,增加作为输入的不变量的个数也不影响探测结果,从而保证了结果的客观性。和传统的湍流/非湍流界面的识别方法相比,客观性是机器学习方法的主要优势,具体体现在两个方面:1.传统方法需要人为指定输入变量的阈值作为区分湍流和非湍流的判据,而阈值的选取具有主观性,需要根据流动参数人为调整才能取得理想的识别效果,而机器学习方法不需要指定阈值,排除了这一人为因素的影响;2.传统方法通常只能选择一个或者两个变量作为输入量,而机器学习方法可以同时处理多个输入变量,从而反应湍流的不同特点,以本文的研究为例,输入的变量中对探测结果起决定性作用的不变量分别表征了湍流运动的非定常特性、涡拉伸现象、以及三维性等重要性质,并且不同的不变量具有不同的特征尺度,因此也反应了湍流的多尺度特性

Sharp interface immersed boundary method for simulating three-dimensional swimming fish

A second order finite-difference numerical method is used to solve the Navier-Stokes equations of incompressible flow, in which the solid body with complex geometry is immersed into the fluid domain with orthogonal Cartesian meshes. To account for influences of the solid body, interactive forces are applied as boundary conditions at Cartesian grid nodes located in the exterior but in the immediate vicinity of the solid body. Fluid flow velocities in these nodes are reconstructed to track and control the deformation of the solid body, in which the local direction normal to the body surface is employed using the level-set function. The capabilities of this method are demonstrated by the application to fish swimming, and the computed behaviors of swimming fish agree well with experimental ones. The results elucidate that the ability of swimming fish to produce more thrust and high efficiency is closely related to the Reynolds number. The single reverse Karman street tends to appear when both the Strouhal number and tail-beating frequency are small, otherwise the double-row reverse Karman street appears. The algorithm can capture the geometry of a deformable solid body accurately, and performs well in simulating interactions between fluid flow and the deforming and moving body

Influence of domain size on direct numerical simulation of turbulent flow in a moderately curved concentric annular pipe

Direct numerical simulation (DNS) of turbulent flow in a concentric annular pipe was performed using a pseudo-spectral method computer code. In order to study the effects of computational domain size on the turbulence statistics, 12 test cases of different domain sizes are compared. The characteristics of the velocity field are examined at two different Reynolds numbers. It is observed that the predictive accuracy of the first- and second-order statistics is sensitive to the axial and azimuthal domain sizes. It is also found that the scales and dynamics of turbulence structures vary with the surface curvature of the concave and convex walls. The characteristic length scales of the turbulence structures are identified through a spectral analysis, and it is observed that a minimum computational domain is required in order to accurately capture the near-wall streaky and hairpin structures of a concentric annular pipe flow using DNS