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首页> 外文期刊>Journal of Fluid Mechanics >Drag reduction in turbulent flows along a cylinder by streamwise-travelling waves of circumferential wall velocity
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Drag reduction in turbulent flows along a cylinder by streamwise-travelling waves of circumferential wall velocity

机译：通过圆柱形的圆周壁速度的波动沿圆筒沿缸体减小湍流减少

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Drag reduction at the external surface of a cylinder in turbulent flows along the axial direction by circumferential wall motion is studied by direct numerical simulations. The circumferential wall oscillation can lead to drag reduction due to the formation of a Stokes layer, but it may also result in centrifugal instability, which can enhance turbulence and increase drag. In the present work, the Reynolds number based on the reference friction velocity and the nominal thickness of the boundary layer is 272. A map describing the relationship between the drag-reduction rate and the control parameters, namely, the angular frequency omega(+) = omega nu/u(tau 0)(2) and the streamwise wavenumber k(x)(+) = k(x)nu/u(tau 0), is obtained at the oscillation amplitude of A(+) = A/u(tau 0) = 16, where u(tau 0) is the friction velocity of the uncontrolled flow and nu is the kinematic viscosity of the fluid. The maximum drag-reduction rate and the maximum drag-increase rate are both approximately 48 %, which are respectively attained at (omega(+), k(x)(+)) = (0.0126, 0.0148) and (0.0246, 0.0018). The drag-reduction rate can be scaled well with the help of the effective thickness of the Stokes layer. The drag increase is observed in a narrow triangular region in the frequency-wavenumber plane. The vortices induced by the centrifugal instability become the primary coherent structure in the near-wall region, and they are closely correlated with the high skin friction. In these drag-increase cases, the effective control frequency or wavenumber is crucial in scaling the drag-increase rate. As the wall curvature normalised by the boundary layer thickness becomes larger, the drag-increase region in the (omega(+), k(x)(+)) plane as well as the maximum drag-increase rate also become larger. Net energy saving with a considerable drag-reduction rate is possible when reducing the oscillation amplitude. At A(+) = 4, a net energy saving of 18% can be achieved with a drag-reduction rate of 25%
机译：通过直接数值模拟研究沿轴向沿轴向湍流流动在湍流中的外表面上的减小。由于斯托克斯层的形成，圆周壁振荡可能导致减少减少，但也可能导致离心不稳定性，这可以增强湍流并增加阻力。在本作工作中，基于参考摩擦速度和边界层的标称厚度的雷诺数是272.描述了减压率与控制参数之间的关系，即角频率ω（+） = OMEGA NU / U（TAU 0）（2）和流动波数k（x）（+）= k（x）nu ​​/ u（tau 0）在振荡幅度的a（+）= a / U（TAU 0）= 16，其中U（TAU 0）是不受控制的流动的摩擦速度，NU是流体的运动粘度。最大阻力率和最大阻力增加率约为48％，分别在（OMEGA（+），K（x）（+））=（0.0126,0.0148）和（0.0246,0018）时获得。在斯托克斯层的有效厚度的帮助下，可以缩放减速率。在频率波数平面中的窄三角区域中观察到拖拉增加。离心不稳定性引起的涡流成为近壁区域中的主要相干结构，它们与高皮肤摩擦密切相关。在这些阻力增加的情况下，有效的控制频率或波数在缩放拖延率方面是至关重要的。由于边界层厚度归一化的壁曲率变大，因此（ω（+），k（x）（+））平面中的拖延区域以及最大拖延率也变大。在减小振荡幅度时，可以实现具有相当大的阻力率的净节能。在（+）= 4时，可以通过减少25％的阻力率来实现18％的净节能

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