Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session H03: Boundary Layers: General II |
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Chair: Dennice Gayme, Johns Hopkins Room: North 121 A |
Monday, November 22, 2021 8:00AM - 8:13AM |
H03.00001: On the scaling and distribution of prograde vortices in wall turbulence Michele Guala, Michael Heisel, Charitha M De Silva, Nicholas Hutchins, Ivan Marusic Prograde vortex cores and internal shear layers were identified and analyzed across a set of smooth and rough wall turbulent boundary layer experiments extending up to the atmospheric surface layer. The main focus is on the scaling of vortex cores and of their azimuthal velocity. Evidence suggests that the diameter of the largest spanwise vortices identified in the velocity field is comparable to the Taylor microscale λ_{T}, while their azimuthal velocity is governed by the shear velocity u_{τ}. This emerging scaling is consistent with the average thickness and velocity jump across the internal shear layers, which tend to overlap with organized prograde vortices. Results highlight the relevance of the large-eddy turnover time and of the interaction between uniform momentum zones, internal shear layers and prograde vortices. A dynamic equilibrium is inferred to govern the stretching of the vortices at the rate proportional to the attached-eddy time scale associated with the uniform momentum zones confining them. The persistence of u_{τ} scaling in shear layers and vortex azimuthal velocity, and of the interaction with wall-distance-dependent uniform momentum zone provide phenomenological explanation of why these coherent features are key to the structure of the mean velocity profile. |
Monday, November 22, 2021 8:13AM - 8:26AM |
H03.00002: Restricted Nonlinear Scale Interactions at High Reynolds Numbers Benjamin A Minnick, Dennice F Gayme Restricted nonlinear (RNL) dynamics comprise a streamwise constant mean flow interacting with a dynamically restricted perturbation field. This model has been shown to capture key aspects of wall-turbulence including low order statistics and roll/streak interactions of the self-sustaining process at moderate Reynolds numbers. In this work we illustrate that the interpretation of the RNL dynamics as the evolution of a large-scale streamwise mean component interacting with small-scale perturbations is consistent with a similar scale decomposition of DNS data. This analysis allows us to evaluate the role of an additional streamwise-varying scale that captures additional scale interactions known to operate at higher Reynolds number turbulence. Augmenting the large-scale dynamics with this intermediate scale enables the recovery of accuracy of low-order statistics. The large-scale streamwise mean in this augmented RNL (ARNL) framework acts as a long train concatenated by turbulent bulges represented by the newly added streamwise varying large-scale. This ARNL model captures the foot-print of this large-scale on the small-scale structures near the wall. The results also show evidence of amplitude modulation of the small-scales, consistent with observations reported in the literature. |
Monday, November 22, 2021 8:26AM - 8:39AM |
H03.00003: Layered structure of turbulent plane wall jet Tie Wei, Yanxing Wang, Xiang Yang Based on the force balances in the mean momentum equation, a turbulent plane wall jet is divided into three regions: a boundary-layer-like region (BLR) adjacent to the wall, a half-free-jet-like region (HJR) away from the wall, and a plug-flow-like region (PFR) in between. In the PFR, the mean streamwise velocity is essentially the maximum velocity $U_\mathrm{max}$, and the simplified mean continuity and mean momentum equations result in a linear variation of the mean wall-normal velocity and Reynolds shear stress. In the HJR, a proper scale for the mean wall-normal flow is the mean wall-normal velocity far from the wall $|V_\infty|$ and a proper scale for the Reynolds shear stress is $U_\mathrm{max}|V_\infty|$, similar to those in a turbulent free jet. The BLR region can be further divided into four sub-layers, similar to those in a canonical pressure- or shear-driven wall-bounded turbulent flow. Building on the log law for the mean streamwise velocity in the BLR, a new skin friction law is proposed for a turbulent wall jet. The new prediction agrees well with the correlation of Bradshaw and Gee over moderate Reynolds numbers, but gives larger skin frictions at higher Reynolds numbers. |
Monday, November 22, 2021 8:39AM - 8:52AM |
H03.00004: Turbulent skin-friction drag reduction with superhydrophobic longitudinal microgrooves under dynamic wetting conditions Amirreza Rastegari, Rayhaneh Akhavan The effect of dynamic wetting conditions on turbulent skin-friction drag reduction (DR) with superhydrophobic (SH) longitudinal microgrooves is investigated by direct numerical simulation (DNS) using free-energy lattice Boltzmann (FELB) methods. The simulations were performed in turulent channel flows at a bulk Reynolds number of Re_{b}=7200 (Re_{τ0}≈222), at a viscosity ratio of N=μ_{liq}/μ_{vap}≈55, Weber number of We_{τ0}= ρ_{liq}u_{τ0}ν_{liq}/σ≈3.65x10^{-3}, with arrays of SH longitudinal micrgrooves on both walls at nominal solid fractions of φ_{s,n}=1/2 and φ_{s,n}=1/16, groove widths of 15≤g^{+0}≤64, groove aspect ratios of d/g=1, and advancing and receding contact angles of θ_{adv}=112^{○}and θ_{rec}=106^{○}. The presence of dynamic wetting conditions is found to result in drops of ≈3-17% and ≈11-35% in the magntude of DR at φ_{s,n}=1/2 and φ_{s,n=}1/16, respectively, compared to the results obtained with stationary, flat, shear-free interfaces. These drops in DR are found to arise primarily from the motion and displacement of the contact line. Motion of the contact line is found to have two effects: (i) it increases the effective solid fraction that the fluid is exposed to from the nominal solid fraction, φ_{s,n}, to a wetted solid fraction, φ_{s,w}〉φ_{s,n}, and (ii) it leads to the formation of corner vortices which can act as surface roughness. The former leads to drops of ≈10-35% in the effective streamwise slip velocity, while the latter leads to enhancements of up to 200% in the effective spanwise slip. The detailed mechanisms behind these phenomena and the scaling of SH DR in the presence of dynamic wetting conditions will be discussed. |
Monday, November 22, 2021 8:52AM - 9:05AM |
H03.00005: Drag reduction effects of dynamic superhydrophobic surface in turbulent channel Kimberly Liu, Ali Mani Superhydrophobic surfaces (SHS) have been widely studied for the purposes of drag reduction by reducing skin friction drag. Experiments of patterned SHS with pressure control have successfully sustained wall-attached air films and have additionally shown that dynamic modulation of air film height can lead to even further drag reduction. Under such conditions, rapid change in the height and shape of the air film induce a jet-like flow structure with substantial wall-normal velocities (Wang and Gharib, J. Fluid Mech. 2020). We numerically simulate both the transient and fully developed behavior of these jets, using parameters matching those of the experiments. We investigate the effects of different dynamic air film height profiles on the characterization of the jet. Finally, the impact of this air film oscillation on drag reduction in a turbulent channel will be presented. |
Monday, November 22, 2021 9:05AM - 9:18AM |
H03.00006: Large eddy simulation of helical- and straight-bladed vertical axis wind turbines in the atmospheric boundary layer flows Masoumeh Gharaati, Shuolin Xiao, Di Yang In this study, the turbulent wake flows behind helical- and straight-bladed VAWTs operating in the atmospheric boundary layer (ABL) are investigated using large-eddy simulation (LES). In the LES model, the aerodynamic forces induced by the turbine blades are applied to the flow field utilizing the actuator line model. The interaction between the VAWTs and the ABL flow is modeled using a concurrent precursor simulation method. In particular, a precursor LES is used to obtain the fully developed ABL turbulence, which provides the inflow condition for the main LES in which the VAWTs are modeled. A straight-bladed VAWT is used as the benchmark case, and several helical-bladed VAWTs with different blade twist angles are considered to explore the effect of the blade helicity on the wake flow characteristics. Based on the simulation results, systematic statistical analyses are performed to characterize the differences in the turbulent wake flows generated by the helical- and straight-shaped blades and their effects on wind energy harvesting. |
Monday, November 22, 2021 9:18AM - 9:31AM |
H03.00007: Effect of veer on a yawed wind turbine wake in neutral and stable atmospheric boundary layer Ghanesh Narasimhan, Dennice F Gayme, Charles Meneveau Yaw control of wind turbines can improve wind farm efficiency by wake steering. The spanwise forcing from yawing generates a counter-rotating vortex pair (CVP) behind the turbine. It induces a side-wash velocity that deflects and curls the wake. Simultaneously, wind veer occurs in atmospheric boundary layer due to Coriolis acceleration. We study the effects of veer on the properties of the wake of a yawed turbine under conventionally neutral (CNBL) and stable (SBL) atmospheric conditions. Large Eddy Simulations of a yawed turbine under these different atmospheric conditions is performed. The lateral shear rate of veer is much stronger in SBL than in CNBL. The veer in CNBL results in a streamwise vorticity which is a linear superposition of its vorticity and the vorticity associated with the CVP. Upon removing the veer vorticity, the resultant CVP agrees well with an analytical model proposed in Shapiro et al. (JFM 903, R2, 2020) which does not include wind veer. However, the stronger veer in SBL distorts the velocity deficit and vorticity structures which significantly affects the downstream evolution of the wake. We discuss the development of analytically tractable models to include veer effects on wakes in stable and neutral conditions. |
Monday, November 22, 2021 9:31AM - 9:44AM |
H03.00008: On the role of wind turbines in CO_{2} sequestration Clarice Nelson, Venkatesh Pulletikurthi, Diego Siguenza, Mirian Velay-Lizancos, Umberto Ciri, Luciano Castillo Carbon dioxide (CO_{2}) emissions from dispersed sources, such as ground vehicles and airplanes, are difficult to eliminate and their impact on human health in populous areas is a major challenge. A potential solution, Direct Air Capture (DAC) of CO_{2}, is currently uneconomical due to the low relative concentration in the air. It has been shown in previous studies that the turbulent energy fluxes play a major role in horizontal axis wind turbine's (HAWT) energy entrainment. Therefore in this study, we demonstrate that this mechanism can increase both power generation and local concentration of CO_{2} in the inner atmospheric boundary layer (ABL). Mass transport equations are incorporated into NREL's SOWFA LES solver to simulate a 1MW HAWT under three different ABL conditions; stable, unstable, and neutral. The entrainment of mass fluxes are calculated in the wake of the turbine to determine the viability of capturing the increased CO_{2} concentration. |
Monday, November 22, 2021 9:44AM - 9:57AM |
H03.00009: An Experimental Survey on the Interaction of Wind Turbines over Complex Terrain Diego Siguenza, Venkatesh Pulletikurthi, Josuenny ODonnell, Clarice Nelson, Jhon Quinones, Shyuan Cheng, Ali Doosttalab, Leonardo P Chamorro, Luciano Castillo This work uses particle image velocimetry and power measurements to reveal the wake characteristics of the interaction between in-line scaled-down wind turbine arrays with 2D hills. We focused on the streamwise momentum recovery mechanisms in the windward side of the hills, from gentle to steep-up slopes. Results show that the advection terms are more significant than turbulence as a wake recovery mechanism with steeper hill slopes. We also show that the turbulence reduces due to the flow acceleration leading to greater power availability at the top of the hills. Furthermore, the results presented here can be used for wind turbines layout optimization in complex terrains. |
Monday, November 22, 2021 9:57AM - 10:10AM |
H03.00010: Investigating the effect of thermal stratification on wind farm blockage using LES Jessica Strickland, Srinidhi Nagarada Gadde, Richard Stevens A wind farm acts as an obstruction, influencing incoming air flow as it approaches the turbine-array. Unfortunately, this blockage effect prevents the first row of a wind farm from acting as a perfect reference because the incoming flow is impacted by what follows downstream. Understanding how and to what extent blockage affects power production is gaining interest in the wind energy field. However, this effect remains relatively unaccounted for compared to other well-established physical phenomenon impacting wind farm performance. |
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