Bulletin of the American Physical Society
76th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2023; Washington, DC
Session X11: Boundary Layers: Roughness IV |
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Chair: Marc Calaf, University of Utah Room: 143A |
Tuesday, November 21, 2023 8:00AM - 8:13AM |
X11.00001: Turbulent boundary layers over surfaces with temperature and elevation heterogeneities Abdelhalim Abdeldayem, Johan Meyers, Raúl Bayoán B Cal Secondary motions associated with surface heterogeneities have been a hot research topic for the past decade. The big swirling motions greatly impact the transport in the turbulent boundary layer. Heterogeneity examples include surface elevation (Vanderwel & Ganapathisubramani 2015), roughness (Nugroho et al. 2013), temperature (Bon & Meyers 2022), and heat flux (Salesky et al. 2022). However, in nature, two or more types of heterogeneity coexist. One of the ubiquitous cases is the coincidence of surface elevation and temperature heterogeneity. A better understanding of this case is important for weather forecasting, urban planning, and wind farm planning, among others. To this end, a series of experiments have been performed in the wind tunnel at Portland State University. One line of heterogeneity in the middle of the test section was considered. The different cases covered heterogeneous elevation with homogeneous temperature, homogeneous elevation with heterogeneous temperature, and both heterogeneities in neutral and stable boundary layers. The spanwise vertical plane was captured using stereoscopic PIV. Despite the fact that the secondary motions are not clear in the in-plane velocities, preliminary results show that the extent of the low and high momentum pathways in the streamwise velocity is affected by both types of heterogeneity. Further analysis of the data is under way. The new data set will help understand the tangled phenomenon and provide a benchmark for numerical simulations. |
Tuesday, November 21, 2023 8:13AM - 8:26AM |
X11.00002: Surrogate Modeling of Urban Boundary-Layer Flow Gurpreet Singh Hora, Marco G Giometto Surrogate modeling is a valuable tool for applications that require repetitive evaluations of expensive simulations. This work compares two machine-learning surrogates to predict canopy flow statistics, accommodating any approaching mean wind angle. The first surrogate is based on the K-nearest neighbors, and the second on a multi-layer perceptron network. The surrogates are trained and evaluated using flow statistics from large-eddy simulations of open-channel flow over and within an array of surface-mounted cuboids spanning a range of approaching wind angles. The proposed surrogate models are highly efficient and able to reconstruct time-averaged three-dimensional flow statistics with a coefficient of determination > 0.96 when trained using many training samples. As the number of training samples decreases, the accuracy of both models deteriorates, with the MLP featuring an overall superior performance. It will be shown that the observed behavior can be attributed to the non-linearity of the MLP's activation function, which allows it to effectively capture non-linearities in the input-output mapping. Further, this study found that ML-specific metrics may lead to misplaced confidence in model accuracy. |
Tuesday, November 21, 2023 8:26AM - 8:39AM |
X11.00003: Direct Numerical Simulation of the Aerodynamically Rough Atmospheric Boundary Layer – Scaling Analysis of Rough Periodic Surfaces Jonathan Kostelecky, Cedrick Ansorge Virtually all atmospheric flows in nature are, to some extend, flows over rough surfaces with roughness sizes from the sub-millimeter to meter scale. Roughness as an inherent feature of the atmospheric boundary layer (ABL) enhances drag, mixing and transport properties of the flow (w.r.t. pollutants, moisture, temperature ...). To study the effect of roughness on the ABL, we perform direct numerical simulations (DNS) of the turbulent neutral Ekman flow over rough surfaces. The lower boundary of the simulation domain is covered with fully resolved three-dimensional cubical roughness elements of varying mean heights (small compared to the boundary layer thickness). The parameters of our simulations correspond to the transitionally and fully rough regime. The spatially fully resolved data allow precise estimation of the roughness parameters: friction velocity, z-nought for scalar and momentum as well as the displacement height. Those parameters are used to present a universal scaling of velocities and scalar profiles in the logarithmic layer for rough flows, which is studied in depth. |
Tuesday, November 21, 2023 8:39AM - 8:52AM |
X11.00004: Spatially Heterogeneous Vegetated Canopy Boundary Layers Ryan Scott, Abdelhalim Abdeldayem, Facundo Cabrera-Booman, Marc Calaf, Raúl Bayoán B Cal Wind dispersed mosses rely on a combination of canopy structure and favorable atmospheric conditions to eject spores into the atmospheric boundary layer for transport to new habitats. Here spatial heterogeneity within the canopy is considered in terms of its impacts on the vegetated canopy sublayer and its consequences for spore transport. In order to assess the role of heterogeneity within the canopy, a series of wind tunnel experiments were performed at Portland State University using a live moss canopy and two surrogates manufactured from reticulated foam. Instantaneous velocity measurements were obtained from two hot wire probes located above the canopy. The lower probe remained fixed while the second probe performed a vertical transect of the boundary layer. Canopy heterogeneity is quantified with a lacunarity based index and related to two-point correlations as well as the turbulent spectra above each canopy type. The natural moss canopy is more heterogeneous than its manufactured counterparts and introduces turbulence across a broader range of scales. Further comparisons between the boundary layer over each canopy based on two-point correlations will be detailed in presentation. |
Tuesday, November 21, 2023 8:52AM - 9:05AM |
X11.00005: Impact of the Numerical Domain on Turbulent Flow Statistics: Scalings and Considerations for Canopy Flows Atharva S Sathe, Marco G Giometto Motivated by the need to better predict weather and dispersion processes, the past decades have seen substantial efforts devoted to the study of turbulence within and above urban canopies. Vast majority of studies have relied on the open channel flow over arrays of surface mounted cuboids setup, which is a convenient surrogate of the urban boundary layer. However, as these simulations are an approximation of reality, special care is often required in the setup of the computational model. Many-a-times, findings are not in agreement with well established theories, experimental results, and with results from related numerical investigations. This is often the result of a poorly designed computational domain. This work examines the impact of domain size on selected flow statistics and turbulent flow structures, and discusses scaling relations that should be taken into account at the simulation-design stage. We will present an approach based on Buckingham Pi theorem to analyze the effect of scale separation in flow over surface-mounted cuboids, and elucidate fundamental mechanisms controlling the scaling of flow statistics in both densely and sparsely packed cuboid configurations. We will show how flow statistics scale differently for densely and sparsely packed cases, and why two different sets of pi groups are required to accurately isolate the impact of scale separation. Ultimately, this research offers valuable recommendations to help researchers tailor the domain size for their specific applications. |
Tuesday, November 21, 2023 9:05AM - 9:18AM |
X11.00006: Atmospheric Flows and Pollutant Transport over Real Urban Morphology: Empirical Mode Decomposition Chun-Ho Liu, Yixun Liu Momentum transport and pollutant dispersion in the atmospheric surface layer (ASL) are governed by broad-spectrum turbulence, which, however, have not been explicitly investigated in the context of real urban morphology. The building-resolved large-eddy simulation (LES) of winds and pollutants over the Kowloon downtown, Hong Kong, is decomposed into a few intrinsic mode functions (IMFs) through empirical mode decomposition (EMD). It is shown that four IMFs are sufficient to capture most of the turbulence kinetic energy (TKE) in real-urban ASL. The first and second IMFs, which are initiated by individual buildings, capture the small-scale vortex packets, populating within the irregular building clusters. On the contrary, the third and fourth IMFs capture the large-scale motions (LSMs) detached to the ground surface that largely govern the transport. LSMs are long, streaky structures that mainly consist of streamwise TKE components, enhancing the vertical transport processes. Besides, the streaky LSMs play crucial roles in near-field pollutant dilution while the small-scale vortex packets are more important to mid- and far-field processes. |
Tuesday, November 21, 2023 9:18AM - 9:31AM |
X11.00007: Evaluating the role of restricted nonlinear interactions in drag degradation in rough wall-bounded turbulence Bianca Viggiano, Christopher J Camobreco, Daniel Chung, Ricardo Garcia-Mayoral, Dennice F Gayme Understanding and characterizing the role of mechanisms that erode the drag-reduction performance of riblets is critical for optimizing design. Recent studies have found spanwise rollers to be associated with the break-down of drag reduction and contribute to the subsequent drag increase in certain geometries. Secondary flows have also been linked to this performance degradation but the relative importance of these mechanisms and others has yet to be understood. In this study, we investigate the role of nonlinearity through a combination of DNS data analysis and direct investigation of relevant scale interactions using the restricted nonlinear model (RNL). The scale decomposition of the RNL provides a means to directly analyze and simulate the flow with restricted nonlinear scale interactions (which are confined to those contributing to the dynamics of the large-scale streamwise averaged mean flow). Comparisons are also made with data obtained from an augmented RNL (ARNL) simulation which includes intermediate modes that nonlinearly interact with the mean flow while still retaining order reduction, thus extending the number of scale interactions. The reduced order models allow us to isolate certain terms to understand the specific contributions to the spatio-temporal interactions. Results obtained using an amplitude-modulated triple decomposition of the data provide insight into these nonlinear contributions to the drag degradation mechanisms. |
Tuesday, November 21, 2023 9:31AM - 9:44AM |
X11.00008: Interactions of Wall Turbulence with Patches of Flexible Roughness Pratik S Deshpande, Ebenezer P Gnanamanickam The interaction of a zero-pressure gradient boundary layer with patches of hair-like, high aspect ratio, flexible, micro-pillars was studied systematically. These micro-structures are inspired by naturally occurring sensory structures such as those seen in fishes, insects, etc. Experiments were conducted in a zero-pressure gradient boundary layer facility. Patches of these micro-pillars, each comprising micro-pillars of systematically varied parameters were manufactured and studied. The fetch of these patches was greater than the boundary layer thickness in both the streamwise and spanwise directions. The individual micro-pillar parameters that were varied were the inner normalized height, the aspect ratio, and the Youngs Modulus of the micro-pillar material. These parameters collectively varied the stiffness of each micro-pillar that comprised an array. In addition, the streamwise and spanwise spacing between micro-pillars, within an array, were varied. The flow developing over these arrays and the smooth wall case were compared with each other. In general, the turbulent intensity profiles showed a shift in the inner peak away from the wall. This inner peak had moved over the height of the micro-pillars. An increase in the small-scale energy in the near-wall region was also observed. This increase was more pronounced in the case of the micro-pillars with lower stiffness. |
Tuesday, November 21, 2023 9:44AM - 9:57AM |
X11.00009: Impact of Flow Pulsation on Roughness Effects in Turbulent Channel Flows John Miles, Benjamin S Savino, Wen Wu, Taiho Yeom Modulation of roughness effects by pulsating pressure gradients (PG) is explored by DNS of turbulent channel flows. Sinusoidal pulsation of PG is examined for ω+ = 0 (steady), 0.02, and 0.04. The magnitude of the pulsation is ~25% of the mean PG, which drives the flow at $Re_{τ,o} = 1500$. Comparisons are made between flows over the smooth wall and sandgrain roughness consisting of randomly distributed ellipsoids. The mean roughness height is $k/H=0.04$. A quintuple decomposition is proposed to isolate the pulsation- and roughness-induced fluctuations. Roughness appears to be the dominant factor characterizing the flow: Townsend's outer-layer similarity remain holds for the mean velocity and Reynolds stresses beyond a $2k$-thick roughness sublayer. Within the roughness sublayer, the mean wake (dispersive) flow stays the same regardless of the pulsation. However, pulsation creates a pulsating wake within the roughness sublayer which significantly amplifies the Reynolds stresses. The equivalent roughness height increases from $k_s^+ = $ 94 (steady) to 225 (ω+ = 0.04). Without the roughness, Stoke layers are limited below the buffer layer for the frequencies examined. Roughness offsets them by 0.2-0.3$k$ and thickens by a factor of five, leading to wall-normal variations in pulsation amplitude and phase up to the mid log-law region. |
Tuesday, November 21, 2023 9:57AM - 10:10AM |
X11.00010: The hydrodynamic properties of unconventional surface roughness Shyam Nair, VISHAL WADHAI, Robert F Kunz, Xiang Yang Surface roughness is characterized by its statistics such as the average height, ka, the root-mean-square, krms, the skewness, Sk, etc. A survey of the roughness in the existing literature shows limited work on roughness with low Sk and high krms/ka, which is the focus of this talk. By controlling the spacing, height, and arrangement of rectangular protrusions and pits, such rough walls are constructed. Direct numerical simulations (DNSs) are conducted to study their hydrodynamic properties. The data suggests that Sk has a minor effect on the equivalent sandgrain roughness height (ks) as pits, that contribute to negative Sk, do not significantly contribute to drag. Furthermore, the roughness arrangement, which is not captured by single-point roughness statistics, has a significant effect on ks. Attempts in developing correlation-type rough-wall models show that the predictive power of a rough-wall model depends on how well the input space distinguishes the rough walls in the calibration dataset. Moreover, the importance of roughness statistics depends on the rough walls under consideration. These findings suggest that a universal rough-wall model might not be possible, and models for specific types of roughness might be more practical. |
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