Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session E08: Boundary Layers: Turbulent Boundary Layers (3:10pm - 3:55pm CST)Interactive On Demand
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E08.00001: A Minimal Flow Unit of the Logarithmic Layer in the Absence of Near-Wall Eddies and Large Scales Hyunji Jane Bae, Adrian Lozano-Duran The observation that the buffer and viscous layers of wall-bounded flows can be simulated in periodic boxes of minimal dimensions has been useful in understanding wall turbulence since it enables the study of individual flow features in isolation from their mutual interactions. In this talk, we present and discuss a flow configuration that isolates a portion of the log layer by limiting the formation of larger outer-layer structures while suppressing the formation of the near-wall eddies. We achieve this by applying the Robin boundary condition in a minimal flow unit tailored for the log region. This method, in addition to isolating the log-layer eddies, can utilize the scale separation in the entirety of the domain such that large-eddy simulation (LES) can be performed without the restrictive grid-resolution requirements near the wall for no-slip walls. We perform direct numerical simulation and LES of the proposed setup to demonstrate that the log-layer structures can be isolated using one-point statistics and energy spectra. [Preview Abstract] |
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E08.00002: A comparison of objective momentum transport barriers and uniform momentum zone interfaces Nikolas Aksamit, George Haller Uniform momentum zones (UMZs) have become a prominent avenue with which to investigate and explain fluid behavior in turbulent boundary layers. Definitions of UMZs have, however, relied on non-material observer-dependent characterizations of one-dimensional momentum projections that are dependent on the size and resolution of the measurement domain, as well as the choice of statistical methods employed. This research harnesses recent mathematically-proven definitions of momentum-flux-minimizing surfaces to identify three-dimensional momentum transport barriers in turbulent boundary layer flows. Our definition has the unique advantages of being frame-invariant, based on physics, and is not sensitive to UMZ definition ambiguities. A comparison of these momentum-transport limiting structures, common frame-dependent vortices, and UMZ interfaces has been evaluated for these flows. The organization of boundary layer turbulence via objective momentum transport barriers shows great promise. [Preview Abstract] |
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E08.00003: Structures in the viscous sublayer and the prediction of wall shear stress. Santosh kumar Sankar, Xinyi Huang, Xiang Yang, Jiarong Hong The study of wall-bounded turbulence has largely focused on the buffer and logarithmic regions above the wall. However, the need to unravel the effects of sublayer roughness structures on turbulence (Evans et al. \textit{PNAS} 2018, 115, 1210-1214; Sirovich {\&} Karlsson \textit{Nature} 1997, 6644, 753-755) requires measurements that can fully resolve the flow within the viscous sublayer. Hot-film anemometry and particle image velocimetry, though widely used for such studies, introduce spatial averaging either along the length of probes for the former or the thickness of light sheet, for the latter. In contrast, by capturing holograms from backscattered light from tracer particles through Digital Fresnel Reflection Holography (DFRH), introduced in Kumar {\&} Hong \textit{Optics Express} 2018, 26(10), 12779-12789, we capture 3D flow within the viscous sublayer at high resolution. Our technique is able to resolve sub-viscous scale meandering motions which are typically not resolved by state-of-art direct numerical simulations (DNS). Furthermore, our experiment also measures a higher frequency of extreme wall shear stress events when compared to DNS, which can be traced to the limited spatial resolution in the streamwise and spanwise directions in the simulation. [Preview Abstract] |
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E08.00004: On the enhancement of boundary layer skin friction by turbulence: a moment-of-momentum approach Perry Johnson, Ahmed Elnahhas Turbulence enhances the skin friction coefficient of a boundary layer by augmenting momentum transport, which increases the drag of streamlined bodies. Turbulence also increases the streamwise growth of the boundary layer, however, indirectly opposing the skin friction enhancement. Other phenomena such as freestream pressure gradients also influence the skin friction. In this presentation, a moment-of-momentum integral equation is introduced to quantify these effects, decomposing the skin friction coefficient into that of a baseline laminar zero pressure gradient (ZPG) boundary layer plus augmentations due to turbulence, freestream pressure gradient, and changes to the streamwise development of the boundary layer. While similar to the relations of Fukagata, Iwamoto, and Kasagi (FIK) as well as Renard and Deck (RD), our approach is unique in that it truly isolates the skin friction of a laminar ZPG boundary layer in a single term so that other terms may be fairly interpreted as augmentations relative to that baseline case. In the process, a clearer interpretation of the original FIK approach emerges. The moment-of-momentum approach is demonstrated for laminar boundary layers with non-zero pressure gradients, as well as transitional and turbulent boundary layers. [Preview Abstract] |
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E08.00005: Abstract Withdrawn
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E08.00006: Persistence analysis in convective turbulence Subharthi Chowdhuri, Tamas Kalmar-Nagy, Tirtha Banerjee We carry out a detailed analysis of the statistical characteristics of the persistence probability distribution functions (PDFs) of velocity and temperature fluctuations in the surface layer of a convective boundary layer, using a field-experimental dataset. Our results demonstrate that for the time scales smaller than the integral scales, the persistence PDFs of turbulent velocity and temperature fluctuations display a clear power-law behaviour, associated with self-similar eddy cascading mechanism. Moreover, we also show that the effects of non-Gaussian temperature fluctuations act only on those scales which are larger than the integral scales of temperature, where the persistence PDFs deviate from the power-law and drop exponentially. Furthermore, the mean time scales of the negative temperature fluctuation events persisting longer than the integral scales are found to be approximately equal to twice the integral scale in highly convective conditions. However, with stability this mean time scale gradually decreases to almost being equal to the integral scale in the near neutral conditions. Contrarily, for the long positive temperature fluctuation events, the mean time scales remain roughly equal to the integral scales, irrespective of stability. [Preview Abstract] |
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E08.00007: The Wall-Pressure Footprint of the Largest Motions in a Turbulent Boundary Layer Bradley Gibeau, Sina Ghaemi Few investigations have resolved the low-frequency region of the wall-pressure spectrum beneath a turbulent boundary layer (TBL). As a result, we currently cannot reliably describe the coupling between wall-pressure and the largest motions in the flow, namely the large- (LSMs) and very-large-scale motions (VLSMs). We probe the relationship between wall-pressure and the largest motions in a TBL at $Re_\tau~=~2600$ using simultaneous time-resolved particle image velocimetry and wall-pressure measurements. The latter have been post-processed to remove wind tunnel background noise and Helmholtz resonance to ensure reliability across the relevant frequencies. Filtering and correlation techniques reveal that the lowest frequencies of the wall-pressure spectrum are associated with the modulation of the VLSMs. Positive (negative) VLSMs are found to cause positive (negative) wall-pressure. An adjacent band of higher frequencies is found to be associated with the ejection and sweeping patterns of the LSMs. The demarcation between these two frequency bands coincides with the peak of the wall-pressure spectrum, suggesting that the peak may be a result of the transition between pressure sources that occurs at this point in the frequency domain. [Preview Abstract] |
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E08.00008: On the Spectral Decomposition of Skewness in the Turbulent Boundary Layer. Flint Thomas, Samaresh Midya, Stanislav Gordeyev, Mitchell Lozier It is now widely accepted that large-scale vortical structures are an important and universal feature of the outer region of the high Reynolds number turbulent boundary layer (TBL). It has also been demonstrated that these outer layer structures impose their imprint on the near-wall region in the form of the amplitude and phase modulation of near-wall velocity fluctuations. This effect has previously been quantified by an amplitude modulation correlation coefficient between lower frequency, outer layer fluctuations and an envelope function characterizing the amplitude modulation of near-wall, higher frequency fluctuations. It has also been clearly demonstrated that the amplitude modulation of near-wall fluctuations is closely related to the skewness. This provides the motivation for the current work which is to examine the spectral decomposition (in frequency domain) of the skewness by means of the real part of the bispectrum. In this paper, the spectral decomposition of skewness is presented for both a zero-pressure gradient high Reynolds number (Ret $=$ 3,200) TBL as well as one at a much lower Reynolds number (Ret $=$ 700) in which the outer layer structure is artificially imposed via a plasma-based active flow control device. This provides a periodic outer layer structure which makes the modal content of the skewness more apparent and aids in interpretation of the spectral decomposition of skewness in the higher Reynolds number TBL. [Preview Abstract] |
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E08.00009: Towards data-driven modelling of coherent motions in wall turbulence Rahul Deshpande, Dileep Chandran, Jason Monty, Ivan Marusic The present work reports a set of unique multi-point hotwire measurements conducted in a high Reynolds number turbulent boundary layer, aimed at facilitating coherent structure-based modelling of a wall-bounded flow. The synchronous hotwire signals are used to compute the two-dimensional (2-D) cross-spectra of the streamwise velocity as a function of streamwise wavelength, spanwise wavelength and wall-normal separation. This 2-D cross-spectrum isolates the structures that are coherent across the chosen wall-normal separation, and indicates their scale-specific energy contributions as well as 3-D geometrical characteristics. Therefore, with a careful selection of the measurement locations, we are able to directly isolate contributions from the statistically relevant coherent motions in the logarithmic region, i.e., (i) the self-similar wall-coherent motions associated with Townsend's attached eddies, and (ii) the tall non-self-similar but wall-coherent superstructures. The 3-D geometrical interpretations of these coherent motions can be utilized in coherent structure-based models, such as the attached eddy model, and used to predict turbulence statistics at very high Reynolds numbers, relevant to atmospheric boundary layer investigations. [Preview Abstract] |
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E08.00010: Anisotropic Eddy Viscosity in a Separated Turbulent Boundary Layer Danah Park, Ali Mani We provide a direct measurement of the eddy viscosity in a separated turbulent boundary layer where a separation bubble is induced in a fully turbulent boundary layer over a flat plate. The study is conducted using a statistical technique called the macroscopic forcing method (MFM), with data gathered from direct numerical simulations. In this work, we employed MFM to reveal the leading-order eddy viscosity operator which can be expressed as a fourth-order tensor acting on the local mean velocity gradient. Our result indicates a highly anisotropic eddy viscosity. We contrast the computed eddy viscosity against the standard Spalart-Allmaras model to identify the key anisotropic directions influencing the momentum budget in the Reynolds-Averaged Navier-Stokes equation. [Preview Abstract] |
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E08.00011: Analysis of the contribution of velocity-vorticity correlations to skin friction coefficient in adverse pressure gradient turbulent boundary layers (APG-TBLs) Shevarjun Senthil, Callum Atkinson, Julio Soria DNSs are used to analyse the contribution of velocity-vorticity correlations to skin friction in incompressible TBL flows with different pressure gradients, namely a zero pressure gradient (ZPG), a mild APG, and a strong APG. Their contributions are computed based on the decomposition presented by Yoon et. al. (2016). The contribution of the molecular transfer due to the mean vorticity is negligible and does not change with the pressure gradient. For all the pressure gradient cases, the contribution of the advective vorticity transport term is negative, whereas the vortex stretching term provides a positive contribution to the skin friction coefficient. It is shown that the combined contribution of these two terms can be considered as the contribution from the Reynolds shear stress with a constant weight for all the pressure gradient cases. The contributions from the molecular diffusion at the wall and the streamwise inhomogeneity effects resulting from the spatial development of the flow increase with the pressure gradient and become dominant contributors when the flow reaches the verge of separation. An alternate method, based on the identity of Renard and Deck (2016), to compute the contribution of velocity-vorticity correlations is also presented. [Preview Abstract] |
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E08.00012: Experimental Studies of the Response of the Turbulent Boundary Layer to Periodic Actuation Mitchell Lozier, Stanislav Gordeyev, Flint Thomas It has been established that the dynamics of large-scale structures (LSS) in outer region of turbulent boundary layers (TBL) and the near-wall small-scale turbulence are correlated. In the study reported here, a plasma-based active flow control device was placed at various heights within a TBL in order to introduce periodic disturbances into the wake region. The boundary layer Reynolds number was Re$_{\mathrm{\tau }}=$700, that no naturally occurring large-scale structure was present. Via actuation, a periodic large-scale structure was introduced into TBL, and the TBL's near-wall response to this structure was studied using a single hot-wire. In previous experiments, it was shown that this large-scale structure had a strong modulating effect on the near-wall turbulence downstream of the actuator. Here the TBL response was tested at different actuator wall normal positions and different actuation frequencies in order to characterize the near-wall modulating effect. Results showed the largest modulation effect when the actuation frequency is equal to the burst/sweep frequency of the near-wall TBL structure. Results also showed that by moving the actuator closer to the wall, the near-wall turbulence intensity was reduced and the modulation effect became more localized to the near-wall region. [Preview Abstract] |
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E08.00013: Spatial correlation between large scales and height of small scales in a turbulent boundary layer Shaurya Shrivastava, Theresa Ann Saxton-Fox In this study, the effect of large scales on the spatial organisation of small-scales in a turbulent boundary layer is investigated. Span-wise vortices in the outer layer are identified using the Triple Decomposition Method (Kolář, V., 2007) from 2D particle image velocimetry (PIV) data. A novel curve-fitting technique was employed to capture the path followed by hairpin packets over an extended spatial domain, along which the small scales are organised. Using spectral analysis and conditional averaging, the height of small scales was found to be correlated with the large scale fluctuating velocity field, which is consistent with previous results. (Mathis et al., 2009; Chung and McKeon, 2010). Additionally, the distance between large scale velocity iso-contours and the path followed by hairpin packets is quantitatively found to be small. [Preview Abstract] |
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E08.00014: Scale interactions in a turbulent boundary layer overlying roughness with spanwise heterogeneity using high speed PIV. Rongnan Yao, Gokul Pathikonda, Kenneth T. Christensen Recent studies have shown that, similar to smooth-wall turbulence, boundary layers overlying roughness embody strong inner--outer interactions even though the near-wall region is significantly perturbed by the roughness. The intensity of these interactions can be even stronger in rough wall flow as reflected in amplitude and frequency modulation correlations. In this study, we investigate these scale interactions for the case of flow overlying roughness with spanwise heterogeneity using high-speed particle image velocimetry. Spanwise-heterogeneous surfaces are common in both nature and engineering applications, and these topographies give rise to unique flow physics that requires further study for a more comprehensive understanding. Here, two spanwise heterogeneous surfaces are used: a complex heterogeneous roughness and an idealized ridge-type roughness. The PIV measurements are performed in a refractive-index matched flow facility to enable high quality data yield in the near-wall region. For both topographies, streamwise--wall-normal flow fields are measured at high-momentum and low-momentum pathways associated with roughness-induced secondary flows owing to topographical spanwise heterogeneity. These data provide a basis for exploring the spatio-temporal nature of inner--outer interactions. [Preview Abstract] |
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E08.00015: Are measurements of fluctuating wall-shear stress with ̄flush-mounted cavity hot-wires possible? Ramis \"Orl\"u, Adalberto Perez, Alex Alvisi, Alessandro Talamelli, Philipp Schlatter Flush-mounted cavity hot-wire probes have been around since two decades, but have typically not been applied as often compared to the traditional wall hot-wires mounted several wire diameters above the surface. The former is believed to significantly enhance the frequency response of the sensor. The recent work using a cavity hot-wire by Gubian et al. (Phys. Rev. Fluids 2019) came to the surprising conclusion that the magnitude of the fluctuating wall-shear stress $\tau_{w,rms}^+$ reaches an asymptotic value of 0.44 beyond the friction Reynolds number $Re_\tau\sim 600$. In an effort to explain this result, which is at odds with the majority of the literature, the present work combines direct numerical simulations (DNS) of a turbulent channel flow with a cavity modelled using the immersed boundary method, and an experimental replication of the study of Gubian et al. in a turbulent boundary layer. It is shown that the measurements of Gubian et al. can be replicated qualitatively as a result of measurement issues. Based on our results, we will discuss why cavity hot-wire probes should neither be used for quantitative nor qualitative measurements of wall-bounded flows, and that several experimental shortcomings can interact to sometimes falsely yield seemingly correct results. [Preview Abstract] |
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