Abstract:The transport of momentum and heat in the turbulent intermediate wake of a circular cylinder is inherently three-dimensional (3-D). This work aims to gain new insight into the 3-D vorticity structure, momentum and heat transport in this flow. All three components of the velocity and vorticity vectors, along with the fluctuating temperature, are measured simultaneously, at nominally the same point in the flow, with a probe consisting of four X-wires and four cold wires. Measurements are made in the ($x$, $y$) o… Show more
“…2003; Chen et al. 2016; and also the visualisation in figure 2). After this turning point, the growth of the wake likely transitions from being large-scale engulfment-driven entrainment to small-scale nibbling-driven entrainment Figure 7.Streamwise distribution of the mean interface positions ( a ) and ( b ) of all cases.…”
Section: Resultsmentioning
confidence: 83%
“…2003; Chen et al. 2016). After this region, the mean positions of the interfaces, including both TNTI and all TTI cases, display a reasonable scaling with the wake half-width (figure 7 b ).…”
Section: Discussionmentioning
confidence: 99%
“…In such a region of the flow, the coherent vortices in the wake emanating from the shear layers shed from the cylinder experienced a significant decay (Matsumura & Antonia 1993; Chen et al. 2016), which allows us to investigate the streamwise evolution of both the TTI and TNTI position/geometry concerning the questions raised above. The paper is organised as follows.…”
This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are varied independently, and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high Schmidt number (
$Sc$
) scalar, and a planar laser induced fluorescence experiment was carried out to capture the interface between the wake and the ambient flow from
$x/d = 5$
to 40, where
$x$
is the streamwise coordinate from the centre of the cylinder, and
$d$
is the cylinder's diameter. It is found that the TTI generally spreads faster towards the ambient flow than the TNTI. A transition region of the interfaces’ spreading is found at
$x/d \approx 15$
, after which the interfaces propagate at a slower rate than previously (upstream), and the mean interface positions of both the TNTI and TTI scale with the local wake half-width. The locations of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time, we explore the spatial variation of the ‘roughness’ of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity becomes important only for the fractal dimension farther downstream.
“…2003; Chen et al. 2016; and also the visualisation in figure 2). After this turning point, the growth of the wake likely transitions from being large-scale engulfment-driven entrainment to small-scale nibbling-driven entrainment Figure 7.Streamwise distribution of the mean interface positions ( a ) and ( b ) of all cases.…”
Section: Resultsmentioning
confidence: 83%
“…2003; Chen et al. 2016). After this region, the mean positions of the interfaces, including both TNTI and all TTI cases, display a reasonable scaling with the wake half-width (figure 7 b ).…”
Section: Discussionmentioning
confidence: 99%
“…In such a region of the flow, the coherent vortices in the wake emanating from the shear layers shed from the cylinder experienced a significant decay (Matsumura & Antonia 1993; Chen et al. 2016), which allows us to investigate the streamwise evolution of both the TTI and TNTI position/geometry concerning the questions raised above. The paper is organised as follows.…”
This study aims to examine the spatial evolution of the geometrical features of the turbulent/turbulent interface (TTI) in a cylinder wake. The wake is exposed to various turbulent backgrounds in which the turbulence intensity and the integral length scale are varied independently, and comparisons to a turbulent/non-turbulent interface (TNTI) are drawn. The turbulent wake was marked with a high Schmidt number (
$Sc$
) scalar, and a planar laser induced fluorescence experiment was carried out to capture the interface between the wake and the ambient flow from
$x/d = 5$
to 40, where
$x$
is the streamwise coordinate from the centre of the cylinder, and
$d$
is the cylinder's diameter. It is found that the TTI generally spreads faster towards the ambient flow than the TNTI. A transition region of the interfaces’ spreading is found at
$x/d \approx 15$
, after which the interfaces propagate at a slower rate than previously (upstream), and the mean interface positions of both the TNTI and TTI scale with the local wake half-width. The locations of both the TNTI and TTI have non-Gaussian probability density functions (PDFs) in the near wake because of the influence of the large-scale coherent motions present within the flow. Further downstream, after the large-scale coherent motions have dissipated, the TNTI position PDF does become Gaussian. For the first time, we explore the spatial variation of the ‘roughness’ of the TTI, quantified via the fractal dimension, from near field to far field. The length scale in the background flow has a profound effect on the TTI fractal dimension in the near wake, whilst the turbulence intensity becomes important only for the fractal dimension farther downstream.
“…Chen et al. (2016) also observed a dominance of the component at x / D = 40 of the turbulent wake at Re = 2500, and attributed this phenomenon to the dominance of the strong mean flow in the streamwise direction. Furthermore, Zhou & Antonia (2000) showed that in the turbulent wake generated by a grid (without large-scale coherent primary vortices which develop in the wake of a cylinder), the component in the direction of the mean flow was also noticeably larger than the component.…”
The turbulent kinetic energy and energy dissipation rate in the wake of a circular cylinder are examined at a Reynolds number of 1000. The turbulence characteristics are quantified using direct numerical simulation, which provides a comprehensive dataset that is almost impossible to acquire from physical experiments. The energy dissipation rate is decomposed into the components due to the mean flow, the coherent primary vortices and the remainder. It is found that the remainder component, which develops only in a three-dimensional turbulent wake and resides mainly in the regions of vortices, accounts for 95 % and 97 % of the total dissipation rate for 10 and 20 cylinder diameters downstream of the cylinder, respectively (while the remainder accounts for 62 % and 83 % of the total turbulent kinetic energy). Based on the remainder component, the validity of local isotropy, local axisymmetry, local homogeneity and homogeneity in the y–z plane for the turbulent dissipation in the wake is examined. The analysis reveals that the turbulent dissipation is largely locally homogeneous, but not locally isotropic or axisymmetric, even after the annihilation of the primary vortex street. In addition, the performances of the four corresponding surrogates to the true dissipation rate are evaluated. Owing to the general validity of local homogeneity, the surrogates of local homogeneity and homogeneity in the y–z plane perform well. Although local axisymmetry does not hold, the corresponding surrogate performs well, because errors from different terms largely cancel out. However, the surrogate of local isotropy generally underestimates the true dissipation rate.
“…A subgrid-scale model with the invariants of the strain rate tensor better predicted the pressure profile near pins, while the surface-adjusting local eddy viscosity modelling gained the velocity field in better accordance with experimental data. Chen et al [33] developed a phase-averaging approach to study momentum, vorticity structure, and turbulent heat transfer behind a round cylinder. The presented model described the structures similar to rib in taking away heat in detail and highlighted the half of the spanwisely expanded vortex.…”
This study analyzes transient turbulent modeling of three-dimensional multiple dimpled fin array using large eddy simulation (LES). The Navier–Stokes equations as well as the energy equation were constructed by the finite volume method and then discretized to form algebraic equations, which were solved by semi-implicit method for pressure-linked equation (SIMPLE). The solutions of temperature and velocity were obtained by iterating computation until it converged within each step. This simulation places nine fins on the bottom surface of a channel and changes the height of the dimple (0.4, 0.8, and 1.2 mm) with three different levels of Reynolds number (Re) (3500, 5000, and 6500) to investigate the temperature and flow field without gravity in forced convection. The results indicate that the dimpled fin array can generate vortices between the convex/concave dimples and the fin base and increase the influences of the height of the dimple on the flow field around the fin array. The averaged time-mean of the Nusselt number (Nu) for the dimple height of 0.8 mm is higher than that of the no-dimple case up to 14.4%, while the averaged time-mean Nu for the dimple height of 1.2 mm is lower than that of the no-dimple case up to 11.6%.
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