Influence of PIV interrogation area on turbulent statistics up to 4th order moments in smooth and rough wall turbulent flows

Shah, Mohammad K.; Agelin‐Chaab, Martin; Tachie, Mark F.

doi:10.1016/j.expthermflusci.2007.09.004

Cited by 24 publications

(10 citation statements)

References 40 publications

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“…Near the roughness, u u u increases slightly with decreasing resolution and distance from the wall, reaching a maximum difference of ∼15 %. Both trends are consistent with recent observations by Shah, Agelinchaab & Tachie (2008). The present trends also suggest that it is likely that even our present high resolution is still insufficient to capture all the relevant energetic small-scale turbulent motions.…”

Section: Resultssupporting

confidence: 92%

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

2011

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Utilizing an optically index-matched facility and high-resolution particle image velocimetry measurements, this paper examines flow structure and turbulence in a rough-wall channel flow for Re τ in the 3520-5360 range. The scales of pyramidal roughness elements satisfy the 'well-characterized' flow conditions, with h/k ≈ 50 and k + = 60 ∼ 100, where h is half height of the channel and k is the roughness height. The near-wall turbulence measurements are sensitive to spatial resolution, and vary with Reynolds number. Spatial variations in the mean flow, Reynolds stresses, as well as the turbulent kinetic energy (TKE) production and dissipation rates are confined to y < 2k. All the Reynolds stress components have local maxima at slightly higher elevations, but the streamwise-normal component increases rapidly at y < k, peaking at the top of the pyramids. The TKE production and dissipation rates along with turbulence transport also peak near the wall. The spatial energy and shear spectra show an increasing contribution of large-scale motions and a diminishing role of small motions with increasing distance from the wall. As the spectra steepen at low wavenumbers, they flatten and develop bumps in wavenumbers corresponding to k − 3k, which fall in the dissipation range. Instantaneous realizations show that roughness-scale eddies are generated near the wall, and lifted up rapidly by large-scale structures that populate the outer layer. A linear stochastic estimation-based analysis shows that the latter share common features with hairpin packets. This process floods the outer layer with roughness-scale eddies, in addition to those generated by the energy-cascading process. Consequently, although the imprints of roughness diminish in the outer-layer Reynolds stresses, consistent with the wall similarity hypothesis, the small-scale turbulence contains a clear roughness signature across the entire channel.

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Section: Resultssupporting

confidence: 92%

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

2011

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“…The limited resolution affects the near-wall trends, but not those in the outer layer, consistent with prior studies (e.g. Ligrani & Moffat 1986;Shah, Agelinchaab & Tachie 2008). Figures 8 and 9 indicate that all the locally normalized stresses in the FPG boundary layer are much weaker than those in ZPG conditions, in agreement with previous …”

Section: Results: Mean Flow and Turbulence Statisticssupporting

confidence: 89%

Effect of mean and fluctuating pressure gradients on boundary layer turbulence

2014

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This study focuses on the effects of mean (favourable) and large-scale fluctuating pressure gradients on boundary layer turbulence. Two-dimensional (2D) particle image velocimetry (PIV) measurements, some of which are time-resolved, have been performed upstream of and within a sink flow for two inlet Reynolds numbers, Re θ (x 1 ) = 3360 and 5285. The corresponding acceleration parameters, K, are 1.3 × 10 −6 and 0.6 × 10 −6 . The time-resolved data at Re θ (x 1 ) = 3360 enables us to calculate the instantaneous pressure distributions by integrating the planar projection of the fluid material acceleration. As expected, all the locally normalized Reynolds stresses in the favourable pressure gradient (FPG) boundary layer are lower than those in the zero pressure gradient (ZPG) domain. However, the un-scaled stresses in the FPG region increase close to the wall and decay in the outer layer, indicating slow diffusion of near-wall turbulence into the outer region. Indeed, newly generated vortical structures remain confined to the near-wall region. An approximate analysis shows that this trend is caused by higher values of the streamwise and wall-normal gradients of mean streamwise velocity, combined with a slightly weaker strength of vortices in the FPG region. In both boundary layers, adverse pressure gradient fluctuations are mostly associated with sweeps, as the fluid approaching the wall decelerates. Conversely, FPG fluctuations are more likely to accompany ejections. In the ZPG boundary layer, loss of momentum near the wall during periods of strong large-scale adverse pressure gradient fluctuations and sweeps causes a phenomenon resembling local 3D flow separation. It is followed by a growing region of ejection. The flow deceleration before separation causes elevated near-wall small-scale turbulence, while high wall-normal momentum transfer occurs in the ejection region underneath the sweeps. In the FPG boundary layer, the instantaneous near-wall large-scale pressure gradient rarely becomes positive, as the pressure gradient fluctuations are weaker than the mean FPG. As a result, the separation-like phenomenon is markedly less pronounced and the sweeps do not show elevated small-scale turbulence and momentum transfer underneath them. In both boundary layers, periods of acceleration accompanying large-scale ejections involve near-wall spanwise contraction, and a high wall-normal momentum flux at all elevations. In the ZPG boundary layer, although some of the ejections are preceded, and presumably initiated, by regions of adverse pressure gradients and sweeps upstream, others are not. Conversely, in the FPG boundary layer, there is no evidence of sweeps or adverse pressure gradients immediately upstream of ejections. Apparently, the mechanisms initiating these ejections are either different from those involving large-scale sweeps or occur far upstream of the peak in FPG fluctuations.

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“…They concluded that the hypothesis can be contradicted in channels roughened on one side only, due to the asymmetry in the boundary conditions, while it holds in symmetric channels. More recently, Shah et al (2008) compared the effects of twodimensional (transverse square bars) and three-dimensional (wire mesh) roughness, through the particle image velocimetry (PIV) technique. The authors found that wire mesh (in the transitional regime) effects are confined to the buffer region, while the rib roughness (in fully rough regime) effects extend to the entire boundary layer.…”

Section: Introductionmentioning

confidence: 99%

Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

Marchis

Napoli

2012

International Journal of Heat and Fluid Flow

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a b s t r a c tWall-resolved Large Eddy Simulation of fully developed turbulent channel flows over two different rough surfaces is performed to investigate on the effects of irregular 2D and 3D roughness on the turbulence. The two geometries are obtained through the superimposition of sinusoidal functions having random amplitudes and different wave lengths. In the 2D configuration the irregular shape in the longitudinal direction is replicated in the transverse one, while in the 3D case the sinusoidal functions are generated both in streamwise and spanwise directions. Both channel walls are roughened in such a way as to obtain surfaces with statistically equivalent roughness height, but different shapes. In order to compare the turbulence properties over the two rough walls and to analyse the differences with a smooth wall, the simulations are performed at the same Reynolds number Re s = 395. The same mean roughness height h = 0.05d (d the half channel height) is used for the rough walls.The roughness function obtained with the 3D roughness is larger than in the 2D case, although the two walls share the same mean height. Thus, the considered irregular 3D roughness is more effective in reducing the flow velocity with respect to the 2D roughness, coherently with the literature results that identified a clear dependence of the roughness function on the effective slope (see Napoli et al. (2008)), higher in the generated 3D rough wall. The analysis of higher-order statistics shows that the effects of the roughness, independently on its two-or three-dimensional shape, are mainly confined in the inner region, supporting the Townsend's wall similarity hypothesis. The tendency towards the isotropization is investigated through the ratio between the resolved Reynolds stress components, putting in light that the 3D irregular rough wall induces an higher reduction of the anisotropy, with respect to the 2D case.

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Influence of PIV interrogation area on turbulent statistics up to 4th order moments in smooth and rough wall turbulent flows

Cited by 24 publications

References 40 publications

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

Near-wall turbulence statistics and flow structures over three-dimensional roughness in a turbulent channel flow

Effect of mean and fluctuating pressure gradients on boundary layer turbulence

Effects of irregular two-dimensional and three-dimensional surface roughness in turbulent channel flows

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