Numerical investigation of turbulent channel flow controlled by spatially oscillating spanwise Lorentz force

A formulation of the skin-friction drag related to the Reynolds shear stress in a turbulent channel flow is derived. A direct numerical simulation （DNS） of the turbulent control is performed by imposing the spatially oscillating spanwise Lorentz force. Under the action of the Lorentz force with several proper control parameters, only the periodi- cally well-organized streamwise vortices are finally observed in the near-wall region. The Reynolds shear stress decreases dramatically, especially in the near-wall area, resulting in a drag reduction.

Mechanism of controlling turbulent channel flow with the effect of spanwise Lorentz force distribution

A direct numerical simulation（DNS） is performed to investigate the control effect and mechanism of turbulent channel flow with the distribution of spanwise Lorentz force. A sinusoidal distribution of constant spanwise Lorentz force is selected, of which the control effects, such as flow characters, mean Reynolds stress, and drag reductions, at different parameters of amplitude A and wave number k_x are discussed. The results indicate that the control effects vary with the parameter A and k_x. With the increase of A, the drag reduction rate D_r first increases and then decreases rapidly at low k_x,and slowly at high k_x. The low drag reduction（or even drag increase） is due to a weak suppression or even the enhancements of the random velocity fluctuation and mean Reynolds stress. The efficient drag reduction is due to the quasi-streamwise vortex structure induced by Lorentz force, which contributes to suppressing the random velocity fluctuation and mean Reynolds stress, and the negative vorticity improves the distribution of streamwise velocity. Therefore, the optimal control effect with a drag reduction of up to 58% can be obtained.

Mechanism of drag reduction by spanwise oscillating Lorentz force in turbulent channel flow

Numerical simulations and experimental research are both carried out to investigate the controlled effect of spanwise oscillating Lorentz force on a turbulent channel flow. The variations of the streaks and the skin friction drag are obtained through the PIV system and the drag measurement system, respectively. The flow field in the near-wall region is shown through direct numerical simulations utilizing spectral method. The experimental results are consistent with the numerical simulation results qualitatively, and both the results indicate that the streaks are tilted into the spanwise direction and the drag reduction utilizing spanwise oscillating Lorentz forces can be realized. The numerical simulation results reveal more detail of the drag reduction mechanism which can be explained, since the spanwise vorticity generated from the interaction between the induced Stokes layer and intrinsic turbulent flow in the near-wall region can make the longitudinal vortices tilt and oscillate, and leads to turbulence suppression and drag reduction.

Direct numerical simulation on relevance of fluctuating velocities and drag reduction in turbulent channel flow with spanwise space-dependent electromagnetic force

Based on the Fourier–Chebyshev spectral method, the control of turbulent channel flow by space-dependent electromagnetic force and the mechanism of drag reduction are investigated with direct numerical simulation(DNS) methods for different Reynolds numbers. A formula is derived to express the relation between fluctuating velocities and the friction drag coefficient. With the application of electromagnetic force, the in-depth relations among the fluctuating velocities near the wall, Reynolds stress, and the effect of drag reduction for different Reynolds numbers are discussed. The results indicate that the maximum drag reductions can be obtained with an optimal combination of parameters for each case of different Reynolds numbers. The fluctuating velocities along the streamwise and normal directions are suppressed significantly,while the fluctuating velocity along the spanwise direction is enhanced dramatically due to the spanwise electromagnetic force. However, the values of Reynolds stress depend on the fluctuating velocities along the streamwise and normal directions rather than that along the spanwise direction. Therefore, the significant effect of drag reduction is obtained. Moreover,the maximum drag reduction is weakened due to the decay of control effect for fluctuating velocities as the Reynolds number increases.

确定空间分布的展向Lorentz力作用下槽道湍流的壁面减阻

文章采用直接数值模拟方法,对具有确定空间分布的展向Lorentz力作用下槽道湍流的壁面减阻效果和减阻机理进行了研究,讨论了电磁力强度A和流向波数kx对湍流流场的展向速度、条带、流向涡、Reynolds应力分布和壁面减阻率的影响。结果表明,湍流固有流场和电磁力诱导流场之间相互调制,存在最优参数A、kx,使得诱导流场主宰壁面边界层,减阻效果最好。

基于壁面主动变形的湍流减阻控制研究

采用谱方法,对反向控制下壁面主动变形的槽道湍流进行了直接数值模拟研究.结果表明,在壁面最大变形量小于5倍黏性尺度条件下,压差阻力可略,摩擦阻力降低7.6%.施加控制后,湍流强度和雷诺应力受到明显抑制,平均速度剖面对数区上移.受壁面法向运动的影响,条带结构强度减弱、尺度变大;流向涡外移且强度减弱,其倾斜和抬起的角度均有不同程度的减小.壁面变形呈现流向拉长的凹槽结构,其平均间距为90倍黏性尺度.

槽道湍流展向振荡电磁力减阻的机理研究

对槽道湍流的展向振荡电磁力控制进行了实验和数值研究.实验通过PIV系统和浮动床阻力测试系统记录近壁区的条带变化和壁面阻力变化.计算时,利用谱方法直接模拟电磁力控制下的近壁流场.实验和计算结果定性一致,皆表明展向振荡电磁力可以减少壁面阻力,并使条带倾斜.计算结果还进一步揭示了电磁力减阻的机理.电磁力诱导产生的流向涡与壁湍流的相互作用,在近壁处形成负的脉动展向涡,该涡将导致流向涡的倾斜和振荡,从而抑制湍流,减少壁面阻力.

槽道湍流中拟涡能输运对反向控制的瞬时响应

利用直接数值模拟研究了槽道湍流中脉动拟涡能输运对反向控制的瞬时响应.发现流向和展向拟涡能的衰减首先由拉伸产生项的抑制引起,而法向拟涡能的减小是因为控制阻碍了平均剪切的倾斜.在控制的初始阶段,流向拟涡能的演化远远落后于其它两个分量的变化.法向涡量快速单调减小,并对其它两个分量的减弱起到了重要作用.

槽道湍流的法向与展向电磁力壁面减阻

法向与展向电磁力可以有效调制湍流近壁流动及减少壁面摩擦阻力。为进一步揭示该方法的减阻机理,利用Fourier-Chebyshev谱方法,通过直接数值模拟(DNS),对槽道湍流的法向与展向电磁力控制和减阻问题进行了研究。结果表明,对于确定的流向波长λx+,存在最佳的电磁力强度St,使阻力降最大,最佳St与λx+成反比。法向与展向电磁力对湍流的控制过程实质上是一种由电磁力诱导的调制波对壁湍流的调制过程。在优化参数控制下,当法向与展向电磁力诱导的流场被用来调制固有的近壁湍流流场时,固有流场和诱导流场同时受到调制。在这种调制波作用下,调制流场逐渐主宰壁面边界层,这导致了壁面阻力的下降,平均减阻率最高可达8%。

槽道湍流在确定分布的展向电磁力调制下的壁面减阻机理

为了研究槽道湍流中确定分布的展向电磁力的流动控制与减阻问题,采用直接数值模拟的方法,对槽道湍流的确定分布展向电磁力控制后的流场中流向脉动速度、法向脉动速度以及雷诺应力分布的变化规律进行了研究。研究结果表明,槽道湍流流场存在大量分布紊乱的准流向涡和发夹涡结构,经过确定分布的展向电磁力调制之后,该流场的发夹涡结构基本消失,仅存在规律分布的准流向涡结构。控制后的流场相比于湍流流场,其流向脉动速度和法向脉动速度均受到了束缚作用,尤其是近壁区域更为明显。整个控制过程的实质是雷诺应力离散点向原点集中的过程,这个过程导致了流场雷诺应力和壁面阻力的下降。

展向行波电磁力控制下的槽道湍流雷诺应力

为研究槽道湍流的电磁力减阻控制问题,采用直接数值模拟(DNS)方法对槽道湍流的展向行波电磁力控制问题进行数值研究.讨论了由展向行波诱导的诱导流场对槽道湍流流场的调制过程,分析了控制前后雷诺应力离散点分布、平均雷诺应力分布以及雷诺应力波谱分布的规律.研究表明,在合适参数条件控制下,展向行波电磁力的控制不仅使近壁条带结构基本消失,形成加宽的低速条带,而且能使控制后的流场中涡结构几乎消失,而仅剩下由展向行波电磁力诱导产生的带状负涡结构,同时展向行波电磁力在一定程度上抑制了湍流的脉动,减小了雷诺应力,最终使壁面阻力下降.

确定分布的展向Lorentz力调制下的槽道湍流涡结构

采用直接数值模拟方法,对槽道湍流中确定分布的Lorentz力的流动控制与减阻问题进行研究.讨论了Lorentz力作用于槽道湍流后,流场的特性和涡结构的特性,并对此类Lorentz力对槽道湍流的控制与减阻机理进行了讨论.研究发现:1)Lorentz力诱导的层流流场壁面附近存在梯度极大的展向速度剪切层,该剪切层容易形成流向涡结构;2)在给定合适参数的确定分布的Lorentz力作用下,湍流流场仅剩周期分布的准流向涡;3)与未控制流场相比,控制后的流场中,准流向涡的抬升高度大大降低,从而减小猝发强度,使壁面阻力下降.

基于空间变化展向电磁力的槽道湍流壁面减阻特性研究

槽道湍流空间变化的展向Lorentz力可以有效干扰湍流拟序结构,抑制壁湍流的产生,达到良好的减阻效果。该文以槽道流形成的充分发展的湍流边界层为基本研究对象,利用Fourier-Chebyshev谱方法,通过直接数值模拟(DNS),对槽道湍流空间变化的展向Lorentz力控制和减阻问题进行了研究。传统方法往往直接对湍流进行研究,但由于湍流结构的复杂性,减阻机理不能完全解释清楚。为解决上述问题,提出了Lorentz力对湍流的控制过程实质上是诱导流场与固有流场相互作用。首先分析了诱导流场展向速度分布特点,然后介绍了固有流场典型特征,进而把调制流场和诱导流场、固有流场进行对比,发现调制流场是通过诱导流场与固有流场相互作用实现的。研究结果表明,在优化参数控制下,诱导流场逐渐主宰壁面边界层,这导致了壁面阻力的下降,平均减阻率最高可达39%。