Spatial resolution issues in rough wall turbulence

Gatti, Davide; Stroh, Alexander

doi:10.1007/s00348-022-03412-x

Cited by 9 publications

(4 citation statements)

References 34 publications

(57 reference statements)

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“…Although the spanwise spatial resolution may not pose a concern in terms of the dispersive component Ũ for surfaces characterised by infinitely spanwise features (Λ = ∞, such as the spanwise bars investigated by Ghanadi and Djenidi (2021), where the ratio of l/Λ ≈ 0), the attenuation of turbulent fluctuation u ′ will always remain an issue since l/⟨η⟩ k ̸ = 0. As such, from the findings of Gatti et al (2022) and bolstered by our own results, we reaffirm the importance of consistently and carefully considering spanwise spatial resolution over rough walls.…”

Section: Discussionsupporting

confidence: 82%

“…spanwise resolution). However, an assessment of spatial resolution using a DNS database of three-dimensional irregular roughness by Gatti et al (2022) reported a notable attenuation of streamwise velocity fluctuations with increasing spanwise averaging size, in apparent disagreement with Ghanadi and Djenidi (2021). Consequently, Gatti et al (2022) recommended adhering to the spatial resolution guidelines for smooth-wall flows until further evidence emerges regarding flow over rough walls.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of Spatial Resolution Effects in Rough Wall-bounded Turbulence

Xia,

Chung,

Marusic

et al. 2024

Preprint

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This study investigates the impact of insufficient spanwise spatial resolution on the measurement accuracy of streamwise velocity fluctuations over rough walls. We use a direct numerical simulation (DNS) database of turbulent open channel flow over three-dimensional sinusoidal roughness with varied wavelengths and roughness heights. Employing a triple decomposition, we investigate both the attenuation of the turbulent fluctuations (about the local mean), $u^\prime$ and the dispersive stresses (roughness-induced fluctuations of the time-averaged mean about the global mean), $\tilde{U}$. A boxcar filter on DNS data is applied to investigate the effects of spanwise spatial filtering on these quantities. Our analysis reveals the significance of two key length-scale ratios for velocity measurements over rough walls: the wire length relative to the spatially and temporally plane-averaged Kolmogorov scale at the roughness crest ($l/\langle \eta\rangle_k$), and the wire length relative to the roughness spanwise wavelength ($l/\Lambda$). We observe that maintaining $l/\langle \eta\rangle_k$ constant while increasing $l/\Lambda$ attenuates the variance of $\tilde{U}$ and $u^\prime$ within the roughness sublayer. When fixing $l/\Lambda$, an increase in $l/\langle \eta\rangle_k$ influences the turbulent fluctuations across all wall-normal locations. These findings highlight the necessity of considering both length scales when evaluating spanwise spatial resolution in turbulence measurements over rough walls.

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Section: Discussionsupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Evaluation of Spatial Resolution Effects in Rough Wall-bounded Turbulence

Xia,

Chung,

Marusic

et al. 2024

Preprint

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“…The one by Segalini et al (2011), however, might be more generally applicable, although it relies on a correlation between the attenuation and λ g , taking only ZPG TBL data as reference. As discussed in Sanmiguel Vila et al (2020), this could be a potentially erroneous assumption in non-canonical flows, such as TBLs subjected to adverse pressure gradients (APGs) or flows along rough walls (Gatti et al, 2022). When the flow is decelerated due to the presence of an APG, the overall structure and dynamics of the TBL are greatly affected: internal shear layers appear, with large scale structures leaving a larger imprint at the wall, and small-scale energy becoming more relevant in the outer region of the TBL as discussed in Sanmiguel Vila et al (2020) and Pozuelo et al (2022).…”

Section: Introductionmentioning

confidence: 99%

Spatial averaging effects in adverse pressure gradient turbulent boundary layers

Mallor,

Örlü,

Schlatter

2024

Preprint

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Thermal anemometry sensors for time-resolved velocity measurements average the measured signal over the length of their sensor, thereby attenuating fluctuations stemming from scales smaller than the wire length. Several compensation methods have emerged for wall turbulence, the most prominent ones relying on the small-scale universality in canonical flows or on the reconstruction based on two attenuated variance profiles obtained with sensors of different length. To extend these methods to non-canonical flows, the present work considers various adverse-pressure gradient (APG) turbulent boundary layer (TBL) flows in order to explore how the small-scale energy is affected in the inner and outer layer and how the two prominent correction methods perform as function of wall-distance, wire length and flow condition. Our findings show that the increased levels of small-scale energy in the inner, but also outer layer associated with APG TBLs reduces the applicability of empirical methods based on the universality of small-scale energy. On the other hand, a correction based on the relationship between the spanwise Taylor microscale and the two-point streamwise velocity correlation function, is able to correct the attenuated profiles of non-canonical cases. Combining the strength of both methods, a composite profile for the spanwise Taylor microscale is suggested, which then is used for the correction of probe-length attenuation effects across a multitude of flow conditions.

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“…However, these corrections often only address the large-scale component of velocity in zero or constant pressure gradient flows, which although useful, do not directly translate to the vast majority of non-equilibrium, anisotropic, and rough wall turbulence studies. In fact, very few investigations of spatial resolution effects on measuring rough wall flows exist, most recently by Gatti et al (2022), who applied spatial filtering to mimic instrument resolution limitations to their DNS of a rough wall flow [11]. Although they did not explore the impacts to all stress components, the authors observed an attenuation of the streamwise Reynolds normal stress component when simulated with a coarser grid and concluded that existing methods for post-measurement correction are best applied for smooth wall flows [11].…”

Section: Introductionmentioning

confidence: 99%

Spatial Resolution Limitations of Particle Image Velocimetry in High Reynolds Number Turbulent Boundary Layers

Vishwanathan

Fritsch

Devenport

et al. 2022

Preprint

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Particle image velocimetry (PIV) has become a standard technique in turbulence experiments for its non-intrusive ability to simultaneously measure multiple points in a flow field. However, inherent volume averaging of the interrogation windows as well as fixed limitations of camera spatial resolution and related hardware set the minimum and maximum scales of structures that can be resolved with this measurement technique. Resolution limitations present themselves as an attenuation of the measured turbulence statistics, seen particularly in the Reynolds wall-normal stress components. This experimental study investigates the impact of hardware-level resolution limitations on the resolved turbulence statistics of PIV measurements made of both rough and smooth wall turbulent boundary layers influenced by external pressure gradients. Velocity fields are measured with PIV using two lens magnifications; one with a baseline lens and one with the addition of a teleconverter lens to view the same flow-field with double magnification (i.e. a finer spatial resolution). A comparison of the turbulence statistics between the rough and smooth wall, components of velocity, and pressure gradient magnitudes are analyzed. The smooth wall turbulence data consistently show a much higher sensitivity to the spatial resolution, particularly for the wall-normal component, due to the smaller eddy length-scales. A fixed camera resolution limits the measured range of scales which affects comparative magnitudes of turbulence for flows with different local Reynolds numbers. The rough wall data measurements show a variable sensitivity to the camera resolution depending on local Reynolds number and streamwise flow history. A statistical convergence study based on the anistropy of turbulence scales is necessary in experimental design for the study of turbulent boundary layers.

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Spatial resolution issues in rough wall turbulence

Cited by 9 publications

References 34 publications

Evaluation of Spatial Resolution Effects in Rough Wall-bounded Turbulence

Evaluation of Spatial Resolution Effects in Rough Wall-bounded Turbulence

Spatial averaging effects in adverse pressure gradient turbulent boundary layers

Spatial Resolution Limitations of Particle Image Velocimetry in High Reynolds Number Turbulent Boundary Layers

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