Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
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 NearWall Eddies and Large Scales Hyunji Jane Bae, Adrian LozanoDuran The observation that the buffer and viscous layers of wallbounded 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 outerlayer structures while suppressing the formation of the nearwall 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 loglayer eddies, can utilize the scale separation in the entirety of the domain such that largeeddy simulation (LES) can be performed without the restrictive gridresolution requirements near the wall for noslip walls. We perform direct numerical simulation and LES of the proposed setup to demonstrate that the loglayer structures can be isolated using onepoint statistics and energy spectra. [Preview Abstract] 

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 nonmaterial observerdependent characterizations of onedimensional 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 mathematicallyproven definitions of momentumfluxminimizing surfaces to identify threedimensional momentum transport barriers in turbulent boundary layer flows. Our definition has the unique advantages of being frameinvariant, based on physics, and is not sensitive to UMZ definition ambiguities. A comparison of these momentumtransport limiting structures, common framedependent 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] 

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 wallbounded 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, 12101214; Sirovich {\&} Karlsson \textit{Nature} 1997, 6644, 753755) requires measurements that can fully resolve the flow within the viscous sublayer. Hotfilm 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), 1277912789, we capture 3D flow within the viscous sublayer at high resolution. Our technique is able to resolve subviscous scale meandering motions which are typically not resolved by stateofart 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] 

E08.00004: On the enhancement of boundary layer skin friction by turbulence: a momentofmomentum approach Perry Johnson, Ahmed Elnahhas 

E08.00005: Abstract Withdrawn Highspeed turbulent boundary layers play an important role in many engineering applications such as highspeed aircrafts. Pressure and thermal loadings on aerodynamic surfaces are determined primarily by turbulent boundary layers. Direct numerical simulation (DNS) of supersonic boundary layers provides a sufficiently accurate prediction of turbulence and wall pressure fluctuations. However, its computational cost increases significantly at realistic Reynolds numbers. Largeeddy simulation (LES) is capable of describing turbulent motions at a much lower computational cost by modeling subgridscale motions. In this study, LES calculations on unstructured grids are validated for the DNS of Mach 2.5 spatiallydeveloping turbulent boundary layers (Duan et al., J. Fluid Mech. 2014). The dynamic Smagorinsky model is used to model subgridscale fluctuations, and both wallresolved and wallmodeled LES are performed, respectively. Turbulence statistics compare well with the corresponding DNS results, and wallpressure spectra are examined in detail. 

E08.00006: Persistence analysis in convective turbulence Subharthi Chowdhuri, Tamas KalmarNagy, 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 fieldexperimental 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 powerlaw behaviour, associated with selfsimilar eddy cascading mechanism. Moreover, we also show that the effects of nonGaussian temperature fluctuations act only on those scales which are larger than the integral scales of temperature, where the persistence PDFs deviate from the powerlaw 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] 

E08.00007: The WallPressure Footprint of the Largest Motions in a Turbulent Boundary Layer Bradley Gibeau, Sina Ghaemi Few investigations have resolved the lowfrequency region of the wallpressure spectrum beneath a turbulent boundary layer (TBL). As a result, we currently cannot reliably describe the coupling between wallpressure and the largest motions in the flow, namely the large (LSMs) and verylargescale motions (VLSMs). We probe the relationship between wallpressure and the largest motions in a TBL at $Re_\tau~=~2600$ using simultaneous timeresolved particle image velocimetry and wallpressure measurements. The latter have been postprocessed 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 wallpressure spectrum are associated with the modulation of the VLSMs. Positive (negative) VLSMs are found to cause positive (negative) wallpressure. 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 wallpressure 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] 

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 largescale 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 nearwall region in the form of the amplitude and phase modulation of nearwall 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 nearwall, higher frequency fluctuations. It has also been clearly demonstrated that the amplitude modulation of nearwall 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 zeropressure 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 plasmabased 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] 

E08.00009: Towards datadriven modelling of coherent motions in wall turbulence Rahul Deshpande, Dileep Chandran, Jason Monty, Ivan Marusic The present work reports a set of unique multipoint hotwire measurements conducted in a high Reynolds number turbulent boundary layer, aimed at facilitating coherent structurebased modelling of a wallbounded flow. The synchronous hotwire signals are used to compute the twodimensional (2D) crossspectra of the streamwise velocity as a function of streamwise wavelength, spanwise wavelength and wallnormal separation. This 2D crossspectrum isolates the structures that are coherent across the chosen wallnormal separation, and indicates their scalespecific energy contributions as well as 3D 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 selfsimilar wallcoherent motions associated with Townsend's attached eddies, and (ii) the tall nonselfsimilar but wallcoherent superstructures. The 3D geometrical interpretations of these coherent motions can be utilized in coherent structurebased 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] 

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 leadingorder eddy viscosity operator which can be expressed as a fourthorder 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 SpalartAllmaras model to identify the key anisotropic directions influencing the momentum budget in the ReynoldsAveraged NavierStokes equation. [Preview Abstract] 

E08.00011: Analysis of the contribution of velocityvorticity correlations to skin friction coefficient in adverse pressure gradient turbulent boundary layers (APGTBLs) Shevarjun Senthil, Callum Atkinson, Julio Soria DNSs are used to analyse the contribution of velocityvorticity 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 velocityvorticity correlations is also presented. [Preview Abstract] 

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 largescale structures (LSS) in outer region of turbulent boundary layers (TBL) and the nearwall smallscale turbulence are correlated. In the study reported here, a plasmabased 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 largescale structure was present. Via actuation, a periodic largescale structure was introduced into TBL, and the TBL's nearwall response to this structure was studied using a single hotwire. In previous experiments, it was shown that this largescale structure had a strong modulating effect on the nearwall 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 nearwall modulating effect. Results showed the largest modulation effect when the actuation frequency is equal to the burst/sweep frequency of the nearwall TBL structure. Results also showed that by moving the actuator closer to the wall, the nearwall turbulence intensity was reduced and the modulation effect became more localized to the nearwall region. [Preview Abstract] 

E08.00013: Spatial correlation between large scales and height of small scales in a turbulent boundary layer Shaurya Shrivastava, Theresa Ann SaxtonFox In this study, the effect of large scales on the spatial organisation of smallscales in a turbulent boundary layer is investigated. Spanwise vortices in the outer layer are identified using the Triple Decomposition Method (Kolář, V., 2007) from 2D particle image velocimetry (PIV) data. A novel curvefitting 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 isocontours and the path followed by hairpin packets is quantitatively found to be small. [Preview Abstract] 

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 smoothwall turbulence, boundary layers overlying roughness embody strong innerouter interactions even though the nearwall 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 highspeed particle image velocimetry. Spanwiseheterogeneous 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 ridgetype roughness. The PIV measurements are performed in a refractiveindex matched flow facility to enable high quality data yield in the nearwall region. For both topographies, streamwisewallnormal flow fields are measured at highmomentum and lowmomentum pathways associated with roughnessinduced secondary flows owing to topographical spanwise heterogeneity. These data provide a basis for exploring the spatiotemporal nature of innerouter interactions. [Preview Abstract] 

E08.00015: Are measurements of fluctuating wallshear stress with ̄flushmounted cavity hotwires possible? Ramis \"Orl\"u, Adalberto Perez, Alex Alvisi, Alessandro Talamelli, Philipp Schlatter Flushmounted cavity hotwire probes have been around since two decades, but have typically not been applied as often compared to the traditional wall hotwires 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 hotwire by Gubian et al. (Phys. Rev. Fluids 2019) came to the surprising conclusion that the magnitude of the fluctuating wallshear 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 hotwire probes should neither be used for quantitative nor qualitative measurements of wallbounded flows, and that several experimental shortcomings can interact to sometimes falsely yield seemingly correct results. [Preview Abstract] 
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