Laboratory experiments have yielded evidence suggestive of large-scale meandering motions in the wake of an axial flow hydrokinetic turbine in a turbulent open channel flow (Chamorro et al., J. Fluid ...Mech., vol. 716, 2013, pp. 658–670). We carry out a large-eddy simulation (LES) of the experimental flow to investigate the structure of turbulence in the wake of the turbine and elucidate the mechanism that gives rise to wake meandering. All geometrical details of the turbine structure are taken into account in the simulation using the curvilinear immersed boundary LES method with wall modelling (Kang et al., Adv. Water Resour., vol. 34(1), 2011, pp. 98–113). The simulated flow fields are in good agreement with the experimental measurements and confirm the theoretical model of turbine wakes (Joukowski, Tr. Otdel. Fizich. Nauk Obshch. Lyub. Estestv., vol. 16, 1912, no. 1), yielding a near-turbine wake that consists of two layers: the tip vortex (or outer) shear layer that rotates in the same direction as the rotor; and the inner layer counter-rotating hub vortex. Analysis of the calculated instantaneous flow fields reveals that the hub vortex undergoes spiral vortex breakdown and precesses slowly in the direction opposite to the turbine rotation. The precessing vortex core remains coherent for three to four rotor diameters, expands radially outwards, and intercepts the outer shear layer at approximately the location where wake meandering is initiated. The wake meandering manifests itself in terms of an elongated region of increased turbulence kinetic energy and Reynolds shear stress across the top tip wake boundary. The interaction of the outer region of the flow with the precessing hub vortex also causes the rotational component of the wake to decay completely at approximately the location where the wake begins to meander (four rotor diameters downstream of the turbine). To further investigate the importance of turbine geometry on far-wake dynamics, we carry out LES under the same flow conditions but using actuator disk and actuator line parametrizations of the turbine. While both actuator approaches yield a meandering wake, the actuator line model yields results that are in better overall agreement with the measurements. However, comparisons between the actuator line and the turbine-resolving LES reveal significant differences. Namely, in the actuator line LES model: (i) the hub vortex does not develop spiral instability and remains stable and columnar without ever intercepting the outer shear layer; (ii) wake rotation persists for much longer distance downstream than in the turbine-resolving LES; and (iii) the level of turbulence kinetic energy within and the overall size of the far-wake meandering region are considerably smaller (this discrepancy is even more pronounced for the actuator disk LES case) compared with the turbine-resolving LES. Our results identify for the first time the instability mechanism that amplified wake meandering in the experiment of Chamorro et al., show that computational models that do not take into account the geometrical details of the turbine cannot capture such phenomena, and point to the potential significance of the near-hub rotor design as a means for suppressing the instability of the hub vortex and diminishing the extent and intensity of the far-wake meandering region.
► Integrated laboratory flume experiments and numerical simulations. ► Investigate the capability of URANS models in prediction of local scour around different bridge piers. ► Extensive grid ...refinement studies. ► The results underscore the effect of pier leading-edge bluntness on scour-inducing mechanisms.
Experiments and numerical simulations are carried out to study clear-water scour around three bridge piers with cylindrical, square, and diamond cross-sectional shape, respectively. To handle movable-bed channels with embedded hydraulic structures, the fluid–structure interaction curvilinear immersed boundary (FSI-CURVIB) method is employed. The hydrodynamic model solves the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the k-ω turbulence model using a second-order accurate fractional step method. Bed erosion is simulated by solving the sediment continuity equation in the bed-load layer using a second-order accurate unstructured, finite-volume formulation with a sand-slide, bed-slope-limiting algorithm. Grid sensitivity studies are carried out to investigate the effect of grid resolution on the predictive capability of the model. Comparisons of the simulations with the experimental data show that for all three cases the agreement is reasonable. A major finding of this work, however, is that the predictive capability of the URANS morphodynamic model improves dramatically for the diamond shape pier for which sediment transport is driven primarily by the shear layers shed from the pier sharp edges. For piers with blunt leading edge, on the other hand, as the circular and square shapes, the URANS model cannot resolve the energetic horseshoe vortex system at the pier/bed junction and thus significantly underpredicts both the scour depth at the nose of the pier and the rate of scour growth. It is also shown that ad hoc empirical corrections that modify the calculated critical bed shear stress to enhance scour rate in the pier leading edge need to be applied with caution as their predictive capabilities are not universal but rather depend on the pier shape and the region of the flow.
The coronavirus disease outbreak of 2019 has been causing significant loss of life and unprecedented economic loss throughout the world. Social distancing and face masks are widely recommended around ...the globe to protect others and prevent the spread of the virus through breathing, coughing, and sneezing. To expand the scientific underpinnings of such recommendations, we carry out high-fidelity computational fluid dynamics simulations of unprecedented resolution and realism to elucidate the underlying physics of saliva particulate transport during human cough with and without facial masks. Our simulations (a) are carried out under both a stagnant ambient flow (indoor) and a mild unidirectional breeze (outdoor), (b) incorporate the effect of human anatomy on the flow, (c) account for both medical and non-medical grade masks, and (d) consider a wide spectrum of particulate sizes, ranging from 10 µm to 300 µm. We show that during indoor coughing some saliva particulates could travel up to 0.48 m, 0.73 m, and 2.62 m for the cases with medical grade, non-medical grade, and without facial masks, respectively. Thus, in indoor environments, either medical or non-medical grade facial masks can successfully limit the spreading of saliva particulates to others. Under outdoor conditions with a unidirectional mild breeze, however, leakage flow through the mask can cause saliva particulates to be entrained into the energetic shear layers around the body and transported very fast at large distances by the turbulent flow, thus limiting the effectiveness of facial masks.
► Novel numerical method for coupled hydro-morphodynamics simulations. ► Sediment/water interface is treated as a sharp immersed boundary in the background curvilinear mesh used to discretize the ...curved open channel. ► The Exner–Polya equation is solved on the unstructured bed mesh in a fully coupled manner with the hydrodynamic equations (URANS). ► Numerical sensitivity studies and comparisons with measurements demonstrate the predictive capabilities of the method.
The fluid–structure interaction curvilinear immersed boundary (FSI-CURVIB) numerical method of Borazjani et al.
3 is extended to simulate coupled flow and sediment transport phenomena in turbulent open-channel flows. The mobile channel bed is discretized with an unstructured triangular mesh and is treated as a sharp-interface immersed boundary embedded in a background curvilinear mesh used to discretize the general channel outline. The unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the
k
−
ω turbulence model are solved numerically on a hybrid staggered/non-staggered grid using a second-order accurate fractional step method. The bed deformation is calculated by solving the sediment continuity equation in the bed-load layer using an unstructured, finite-volume formulation that is consistent with the CURVIB framework. Both the first-order upwind and the higher-order hybrid GAMMA schemes
12 are implemented to discretize the bed-load flux gradients and their relative accuracy is evaluated through a systematic grid refinement study. The GAMMA scheme is employed in conjunction with a sand-slide algorithm for limiting the bed slope at locations where the material angle of repose condition is violated. The flow and bed deformation equations are coupled using the partitioned loose-coupling FSI-CURVIB approach
3. The hydrodynamic module of the method is validated by applying it to simulate the flow in an 180° open-channel bend with fixed bed. To demonstrate the ability of the model to simulate bed morphodynamics and evaluate its accuracy, we apply it to calculate turbulent flow through two mobile-bed open channels, with 90° and 135° bends, respectively, for which experimental measurements are available.
High-resolution large-eddy simulation (LES) is carried out for investigating three-dimensional flow fields around a rectangular spur dike installed in an open-channel flume. The LES showed good ...agreement with the measurement obtained using Acoustic Doppler Velocimetry. Analysis of the LES result shows that the flow structure around and in the wake of the spur dike is highly complex and three-dimensional. Namely, flow upstream of the spur dike is featured by a vortex system near the bed, another vortex system beneath the free surface, and a recirculation region in front of the spur dike. All of these flow structures are laterally oriented. Moreover, flow in the wake region consists of a large vertically oriented recirculation region and a smaller laterally oriented recirculation region near the bottom corner downstream of the spur dike.
► Investigate the predictive capability of URANS and LES hydro- and morpho-dynamic model. ► Numerical simulations of scour around real-life stream restoration structures. ► Laboratory experiments for ...model validation. ► LES is shown to yield better predictions compared to URANS on the same grid size.
Local scour of the streambed around three models of stream restoration rock structures, including a rock vane, a cross vane, and a J-hook vane, is investigated via laboratory experiments and numerical simulations. In the experimental study, a physical model of each rock structure is constructed via an assembly of rocks and installed in a straight mobile sand bed flume. Continuous bed topography measurements provide insight into the time evolution of the scour patterns downstream of the structures and yield comprehensive data sets for validating the numerical simulations in terms of scour patterns, maximum scour depths, and bar migration dynamics. The numerical simulations are carried out using the coupled, hydro-morphodynamic Curvilinear Immersed Boundary (CURVIB) method of Khosronejad et al. (2011) 17. The mobile channel bed and the individual rocks comprising a stream restoration structure are discretized with an unstructured triangular mesh and treated as sharp-interface immersed boundaries embedded in the background curvilinear mesh used to discretize the flow domain. For each case, simulations are carried out solving both the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the k–ω model and filtered Large-Eddy Simulation (LES) equations closed with the dynamic Smagorinski subgrid scale model. Both the URANS and LES models yield flow and scour patterns in reasonable agreement with the measurements with the LES results being consistently in better overall agreement with the measurements. To our knowledge, the present study is the first attempt to simulate local scour patterns around realistic model of stream restoration rock structures by taking into account and directly modeling their arbitrarily complex geometrical features.
We conducted flume experiments to study the flow behavior around a debris accumulation at a two-pier bridge. An Acoustic Doppler Velocimetry was utilized to measure velocity in cases with and without ...debris. We examined the impact of this accumulation on the velocity field and bed shear stress to better understand the flow and sediment transport mechanisms near the debris buildup at the bridge. Debris clusters were found to cause strong downward flows behind the bridge piers and to create turbulent shear layers emanating from the debris base. These flow patterns caused an increase in bed shear stress behind the piers by up to 40%. Downstream of the bridge, the debris-induced bed shear stress increase persisted for a distance equivalent to four times the spacing between the piers.
A Large-Eddy Simulation (LES) model is used to study flow dynamics of a flash flood event in a dry-bed, desert wash, the so-called Tex Wash, near the Tex Wash Bridge on Interstate 10 in the Mojave ...Desert of California. The evolving free surface of the flash flood is tracked using the level-set method. A bed morphodynamics module is coupled to the hydrodynamics model to calculate the erosion and bed evolution of the mobile bed of the wash under flash flood conditions. Flash floods in a desert wash can be characterized with a number of salient features such as the (1) existence of both the dry- and wet-cells on the bed surface of the wash that correspond to the air and water phases, respectively; (2) presence of various flow regimes, critical, sub-critical, and super-critical in the flow domain; and (3) occurrence of a highly transient and complicated flow field and, subsequently, sediment dynamics throughout the wash. A numerical modeling effort is presented to study a recorded flash flood and the corresponding scour processes in the Tex Wash. The flood event occurred in 2015 and lead to the collapse of the Tex Wash Bridge. The of the current study is to gain insight into the flood flow and sediment transport mechanisms, which resulted in the collapse of the bridge. To that end, a study area, which includes a 0.65 km-long reach of Tex Wash at its intersection with the Tex-Wash Bridge, was selected. The bathymetry of the wash was obtained using light-detection-and-ranging (LiDAR) technology and used to construct the computational domain of the wash and bridge foundations. The transient flow of the flash flood, in both air and water phases, and the evolving morphology of the wash are numerically simulated. The site-specific numerical simulation revealed the formation of deep scour regions adjacent to the right abutment of the upstream bridge, where significant erosion caused the collapse of the bridge. Moreover, the results show that most of the scour processes take place during the steady phase of the flash flood when the desert stream is filled with water. However, the transient phase of the flash flood is rather short and contributes to a very limited amount of erosion within the stream.
Large-eddy simulation (LES) model is used to study flow dynamics of a flash flood event in a dry-bed desert wash, the so-called Tex Wash, near the Tex Wash Bridge on Interstate 10 in the Mojave Desert of California. Our results show that most of the scour processes takes place during the steady phase of the flash flood when the stream is filled with the flood water. Figure 1: Computed instantaneous flow field of the flash flood in the Tex wash at the water surface. (A) shows the contour of the velocity magnitude (V) and the bed elevation (zb) of the wash. The zoomed-in view of (B) shows the contour of velocity magnitude around the bridge piers “P1” and “P2” within the narrow channel. In the zoomed-in window of (C), the contours of velocity magnitude is presented. Near the upper part of this window, one can see the hydraulic jump. The zoomed-in window of (D) shows the Fr number distribution in the wash. The flash flood flows from bottom to top. Display omitted
The wake dynamics of a wind turbine are influenced by the atmospheric turbulence and the wake of its upwind turbine. In this work, we investigate the wake characteristics of a waked wind turbine for ...four different downwind spacings and three different inflows using large-eddy simulation with a turbine parameterized using the actuator surface model. The wake statistics of the waked turbine are compared with those of the stand-alone wind turbine under the same inflow. The results show that the oncoming wake significantly affects the near wake of the waked turbine, where it accelerates the wake recovery by increasing the turbulent convection, and increases the turbulence kinetic energy. The velocity deficit and turbulence kinetic energy in the far wake, on the other hand, are fairly similar with each other for the considered different turbine spacings, and are close to those of the stand-alone wind turbine. As for the wake meandering of the waked wind turbines, it is initiated quickly and enhanced by the oncoming wake turbulence, as shown by the meandering amplitudes and the power spectral density of the instantaneous wake positions. The growth rates of the wake meandering from the waked wind turbines, on the other hand, are close to that of the stand-alone wind turbine, indicating the critical role of the atmospheric turbulence on wake meandering. The present work details how the oncoming wake influences the wake dynamics of the downwind turbine, and provides physical insights on developing engineering models to take into account such effects.
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