Search

In this paper, we assess the local isotropy of higher-order statistics in the intermediate wake region. We focus on normalized odd moments of the transverse velocity derivatives, $${M_{2n + 1}}(\partial u/\partial z) = {{\overline{{{(\partial u/\partial z)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial z)}^2}} }^{(2n + 1)/2}}}}$$ and $${N_{2n + 1}}(\partial u/\partial y) = {{\overline{{{(\partial u/\partial y)}^{2n + 1}}} }}/{{{{\overline{{{(\partial u/\partial y)}^2}} }^{(2n + 1)/2}}}}$$ , which should be zero if local isotropy is satisfied ( n is a positive integer). It is found that the relation $$M_{2n+1}(\partial u/\partial z) \sim R_\lambda ^{-1}$$ is supported reasonably well by hot-wire data up to the seventh order ( $$n=3$$ ) on the wake centreline, although it is also dependent on the initial conditions. The present relation $$N_{3}(\partial u/\partial y) \sim R_\lambda ^{-1}$$ is obtained more rigorously than that proposed by Lumley (Phys Fluids 10:855-858, 1967) via dimensional arguments. The effect of the mean shear at locations away from the wake centreline on $$M_{2n+1}(\partial u/\partial z)$$ and $$N_{2n+1}(\partial u/\partial y)$$ is addressed and reveals that, although the non-dimensional shear parameter is much smaller in wakes than in a homogeneous shear flow, it has a significant effect on the evolution of $$N_{2n+1}(\partial u/\partial y)$$ in the direction of the mean shear; its effect on $$M_{2n+1}(\partial u/\partial z)$$ (in the non-shear direction) is negligible. [ABSTRACT FROM AUTHOR]

The near wake of a wall-mounted finite-length square cylinder with an aspect ratio of 7 is investigated based on the proper orthogonal decomposition (POD) of the PIV data measured in three spanwise planes, i.e., z/ d = 6, 3.5 and 1.0, near the cylinder free end, mid-span and fixed end (wall), respectively. The Reynolds number based on free-stream velocity ( U) and cylinder width ( d) is 9,300. A two-dimensional (2D) square cylinder wake is also measured and analyzed at the same Reynolds number for the purpose of comparison. The structures of various POD modes show marked differences between the two flows. While the coefficients, a and a, of the POD modes 1 and 2 occur within an annular area centered at a = a = 0 in the 2D wake, their counterparts are scattered all over the entire circular plane at z/ d = 1.0 and 3.5 of the finite-length cylinder wake. Flow at z/ d = 6 is dominated by POD mode 1, which corresponds to symmetrical vortex shedding and accounts for 54.0 % of the total turbulent kinetic energy (TKE). On the other hand, the POD modes 1 and 2, corresponding to anti-symmetrical vortex shedding, are predominant, accounting for about 45.0 % of the total TKE, at z/ d = 3.5 and 1. It has been found that the flow structure may be qualitatively and quantitatively characterized by the POD coefficients. For example, at z/ d = 6, a larger a corresponds to a smaller length of flow reversal zone and a stronger downwash flow. At z/ d = 3.5 and 1, two typical flow modes can be identified from a and a. While large a and/or a correspond to anti-symmetrical vortex shedding, as in a 2D cylinder wake, small a and a lead to symmetrical vortex shedding. Any values between the large and small a and/or a correspond to the flow structure between these two typical flow modes. As such, the probability of occurrence of a flow structure may be determined from the distribution of the POD coefficients. [ABSTRACT FROM AUTHOR]

The lasting high fuel cost has recently inspired resurgence in drag reduction research for vehicles, which calls for a thorough understanding of the vehicle wake. The simplified Ahmed vehicle model is characterized by controllable flow separation, thus especially suitable for this purpose. In spite of a considerable number of previous investigations, our knowledge of flow around this model remains incomplete. This work aims to revisit turbulent flow structure behind this model. Two rear slant angles, i.e., α = 25º and 35º, of the model were examined, representing two distinct flow regimes. The Reynolds number was 5.26 × 10 based on the model height ( H) and incident flow velocity. Using particle image velocimetry (PIV), flow was measured with and without a gap ( g/H = 0.174) between the vehicle underside and ground in three orthogonal planes, viz. the x- z, x- y and y- z planes, where x, y, and z are the coordinates along longitudinal, transverse, and spanwise directions, respectively. The flow at g/H = 0 serves as an important reference for the understanding of the highly complicated vehicle wake ( g/H ≠ 0). While reconfirming the well-documented major characteristics of the mean flow structure, both instantaneous and time-averaged PIV data unveil a number of important features of the flow structure, which have not been previously reported. As such, considerably modified flow structure models are proposed for both regimes. The time-averaged velocities, second moments of fluctuating velocities, and vorticity components are presented and discussed, along with their dependence on g/H in the two distinct flow regimes. [ABSTRACT FROM AUTHOR]

The effect of cylinder aspect ratio (≡ H/ d, where H is the cylinder height or length, and d is the cylinder diameter) on the drag of a wall-mounted finite-length circular cylinder in both subcritical and critical regimes is experimentally investigated. Two cases are considered: a smooth cylinder submerged in a turbulent boundary layer and a roughened cylinder immersed in a laminar uniform flow. In the former case, the Reynolds number Re (≡ U d/ ν, with U being the free-stream velocity and ν the fluid viscosity) was varied from 2.61 × 10 to 2.87 × 10, and two values of H/ d (2.65 and 5) were examined; in the latter case, Re = 1.24 × 10-1.73 × 10 and H/ d = 3, 5 and 7. In the subcritical regime, both the drag coefficient C and the Strouhal number St are smaller than their counterparts for a two-dimensional cylinder and reduce monotonously with decreasing H/ d. The presence of a turbulent boundary layer causes an early transition from the subcritical to critical regime and considerably enlarges the Re range of the critical regime. No laminar separation bubble occurs on the finite-length cylinder immersed in the turbulent boundary layer, and consequently, the discontinuity is not observed in the C- Re and St- Re curves. In the roughened cylinder case, the Re range of the critical regime grows gradually with decreasing H/ d, while the C crisis becomes less obvious. In both cases, H/ d has a negligible effect on the critical value of Re at which transition occurs from the subcritical to critical regime. [ABSTRACT FROM AUTHOR]

The streamwise evolution of an inclined circular cylinder wake was investigated by measuring all three velocity and vorticity components using an eight-hotwire vorticity probe in a wind tunnel at a Reynolds number Re of 7,200 based on free stream velocity ( U) and cylinder diameter ( d). The measurements were conducted at four different inclination angles (α), namely 0°, 15°, 30°, and 45° and at three downstream locations, i.e., x/ d = 10, 20, and 40 from the cylinder. At x/d = 10, the effects of α on the three coherent vorticity components are negligibly small for α ≤ 15°. When α increases further to 45°, the maximum of coherent spanwise vorticity reduces by about 50%, while that of the streamwise vorticity increases by about 70%. Similar results are found at x/ d = 20, indicating the impaired spanwise vortices and the enhancement of the three-dimensionality of the wake with increasing α. The streamwise decay rate of the coherent spanwise vorticity is smaller for a larger α. This is because the streamwise spacing between the spanwise vortices is bigger for a larger α, resulting in a weak interaction between the vortices and hence slower decaying rate in the streamwise direction. For all tested α, the coherent contribution to $$ \overline{{v^{2}}} $$ is remarkable at x/ d = 10 and 20 and significantly larger than that to $$ \overline{{u^{2}}} $$ and $$ \overline{{w^{2}}}. $$ This contribution to all three Reynolds normal stresses becomes negligibly small at x/ d = 40. The coherent contribution to $$ \overline{{u^{2}}} $$ and $$ \overline{{v^{2}}} $$ decays slower as moving downstream for a larger α, consistent with the slow decay of the coherent spanwise vorticity for a larger α. [ABSTRACT FROM AUTHOR]