2 results
Weakly sheared turbulent flows generated by multiscale inhomogeneous grids
- Shaokai Zheng, P. J. K. Bruce, J. M. R. Graham, J. C. Vassilicos
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- Journal:
- Journal of Fluid Mechanics / Volume 848 / 10 August 2018
- Published online by Cambridge University Press:
- 13 June 2018, pp. 788-820
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A group of three multiscale inhomogeneous grids have been tested to generate different types of turbulent shear flows with different mean shear rate and turbulence intensity profiles. Cross hot-wire measurements were taken in a wind tunnel with Reynolds number $Re_{D}$ of 6000–20 000, based on the width of the vertical bars of the grid and the incoming flow velocity. The effect of local drag coefficient $C_{D}$ on the mean velocity profile is discussed first, and then by modifying the vertical bars to obtain a uniform aspect ratio the mean velocity profile is shown to be predictable using the local blockage ratio profile. It is also shown that, at a streamwise location $x=x_{m}$, the turbulence intensity profile along the vertical direction $u^{\prime }(y)$ scales with the wake interaction length $x_{\ast ,n}^{peak}=0.21g_{n}^{2}/(\unicode[STIX]{x1D6FC}C_{D}w_{n})$ ($\unicode[STIX]{x1D6FC}$ is a constant characterizing the incoming flow condition, and $g_{n}$, $w_{n}$ are the gap and width of the vertical bars, respectively, at layer $n$) such that $(u^{\prime }/U_{n})^{2}\unicode[STIX]{x1D6FD}^{2}(C_{D}w_{n}/x_{\ast ,n}^{peak})^{-1}\sim (x_{m}/x_{\ast ,n}^{peak})^{b}$, where $\unicode[STIX]{x1D6FD}$ is a constant determined by the free-stream turbulence level, $U_{n}$ is the local mean velocity and $b$ is a dimensionless power law constant. A general framework of grid design method based on these scalings is proposed and discussed. From the evolution of the shear stress coefficient $\unicode[STIX]{x1D70C}(x)$, integral length scale $L(x)$ and the dissipation coefficient $C_{\unicode[STIX]{x1D716}}(x)$, a simple turbulent kinetic energy model is proposed that describes the evolution of our grid generated turbulence field using one centreline measurement and one vertical profile of $u^{\prime }(y)$ at the beginning of the evolution. The results calculated from our model agree well with our measurements in the streamwise extent up to $x/H\approx 2.5$, where $H$ is the height of the grid, suggesting that it might be possible to design some shear flows with desired mean velocity and turbulence intensity profiles by designing the geometry of a passive grid.
Perturbing vortex packets in a turbulent boundary layer
- Shaokai Zheng, Ellen K. Longmire
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- Journal:
- Journal of Fluid Mechanics / Volume 748 / 10 June 2014
- Published online by Cambridge University Press:
- 29 April 2014, pp. 368-398
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A zero pressure gradient turbulent boundary layer of $\textit {Re}_{\tau }=2500$ was perturbed by a single spanwise array of finite cylinders mounted on the bounding surface and extending through the logarithmic region. The cylinder height was $H/\delta =0.2$ ($H^{+}=500$), where $\delta $ is the boundary layer thickness, with an aspect ratio ($AR$) (height/diameter) of four. Streamwise–spanwise ($x\text {--}y$) planes of the flow were examined by particle image velocimetry (PIV) up to $7\delta $ downstream at a wall-normal location of $z^{+}=300$ for cylinder array spacings ranging from $0.2\delta $ to $0.8\delta $. Average streamwise velocity fields showed a splitting, then merging pattern of cylinder wakes which occurred further downstream as the cylinder spacing increased. Based on measurements at the furthest downstream location, both the spanwise variation of average streamwise velocity and the Fourier content in the instantaneous fields suggested that the case with $0.6\delta $ cylinder spacing, which matched the dominant spanwise scale in the unperturbed flow, yielded the most persistent downstream flow organization. A flying PIV method was implemented to track specific packet structures over a range $-2<x/\delta <7$ with respect to the cylinder array, corresponding to a time scale of $12.4\delta /U_{\infty }$. Packets approaching the $0.2\delta $ spacing array first lost their organization but then regained it a distance $2\delta $ downstream, suggesting that a persistent outer layer organization propagated inwards into the log region. For arrays with larger spanwise spacing, approaching packets were generally redirected into the spanwise location midway between cylinders and sometimes enhanced.