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Passive flow control devices, such as vortex generators (VGs), can effectively modulate the turbulent boundary layer flow near regions of adverse pressure gradients, but the interactions between the salient flow structures produced by VGs and those of the separated flow are not fully understood. In this study, a spatially evolving turbulent boundary layer interacting with a wall-mounted cube ahead of a backward-facing ramp is investigated using wall-resolved large-eddy simulations for a Reynolds number of 19,600, based on the inlet boundary layer thickness and freestream velocity. Different cube configurations are examined to isolate the effects of cube height and streamwise position. The large counter-rotating flow produced by the larger cubes generated more turbulent kinetic energy, thereby resulting in smaller separated regions over the ramp. Although significantly lower levels of turbulent kinetic energy dissipation than production are observed in the outer flow regions of the horseshoe vortex and in the induced counter-rotating flow, the spatial distribution of dissipation is similar to that of the turbulent kinetic energy. When the upstream position of an isolated single cube, relative to the leading ramp edge, is more than three cube heights, the streamwise decay of the counter-rotating flow results in lower levels of turbulent kinetic energy. [ABSTRACT FROM AUTHOR]

The geometric shape of the submarine sail has a significant effect on the flow uniformity amount in the submarine impeller plane. In the present study, using a Hotwire and a five-hole probe, the downstream flow field for the three sail in the SUBOFF submarine model was investigated. Two of the sails have the same NACA0018 cross-section and height, one of which has a fillet on the leading-edge. The third sail has the NACA0018 cross-section which is shaped like a dolphin's dorsal fin. In this study, the rotational field of the vortex downstream of the sail has been studied. The presence of fillets at the leading-edge of the sail significantly reduces the amount and intensity of the vortex. The sail with a dolphin's dorsal fin geometry also has a significant effect on improving the flow quality. In addition, as the distance from the sail increases downstream, the wake becomes weaker. [ABSTRACT FROM AUTHOR]

The development of pier scour in coastal environments severely affects the bridge's stability. Therefore, estimating pier scour around the vertical cylinder is important for the safety of the bridge structure. The estimation of pier scour depth in combined wave-current conditions has become a challenging task for researchers in recent times. The existing empirical formulations that calculate scour in the combined action of current and wave are scarce and may not always provide accurate results. Machine-learning (ML) techniques have become increasingly popular for their prediction capabilities in the fields of hydraulics and coastal engineering in recent years. Therefore, the present study aims to develop Boosting ML techniques (i.e., AdaBoost, XGBoost, CatBoost, and LightGBM) of ML to estimate pier scour in combined wave-current conditions. The non-dimensional parameters, such as Keulegan–Carpenter (KC) number, Relative flow velocity (Ucw), and Absolute Froude number (Fra), are used as input parameters, whereas scour depth (S/D) is the output parameter in Boosting ML models. The sensitivity analysis has been performed to demonstrate the relative importance of the input parameter on S/D. The performance metrics show that the XGBoost model with the input combination of Fra, KC, and Ucw provides the highest accuracy of 92.47% and outperforms SVM, CatBoost, AdaBoost, and LightGBM models. The XGBoost model also outperforms the existing empirical formulations. Therefore, it can be concluded that the XGBoost techniques can be used as a reliable, accurate, and alternative tool to estimate pier scour depth in the combined action of current and wave. [ABSTRACT FROM AUTHOR]

The efficiency of boundary-layer ingestion (BLI) propulsion systems is closely related to the amount of ingested boundary layer. A high BLI fraction requires low propulsor flow power. In this paper, the internal flow characteristics and performance of an S-shaped inlet with a high BLI, up to 83.8% of the inlet capture height, are explored in detail under a freestream Mach number of 0.75. The results reveal that the middle and lower parts of the aerodynamic interface plane (AIP) section are packed with low-energy flows dominated by a pair of vortices, formed by the coalescence of side and separation vortices. At MAIP=0.46, the side vortex, originating near the first inlet bend, is caused by a spanwise pressure gradient, while the side vortex is connected to the horseshoe vortex at the root of the inlet lip at MAIP=0.33. The vortex coalescence results in a sudden increase in the strength of the vortex core and a stepwise increase in circulation (ΓRx). The flow vortex intensity in the inlet is divided into three stages from the distribution of ΓRx. In addition, a reduction in MAIP increases the intensity of secondary flow along the section, particularly the intensity at the flow separation zone. With increasing MAIP, the swirl intensity decreases, whereas the total pressure distortion index DC60 rises, reaching 0.54, which is beyond the requirement of turbomachinery component flow uniformity. [ABSTRACT FROM AUTHOR]

One of the major problems associated with bridge piers is ensuring their safety against local scouring caused by the erosive action of flow. Numerous countermeasures have been developed and tested to solve this problem, among which sacrificial piles are highly recognized due to their high performance, economy, durability, and ease of construction. Several factors affect the performance of sacrificial piles, such as their number, size, degree of submergence, and geometric arrangement parameters. In this study, the performance of a group of linearly arranged cylindrical sacrificial piles in reducing local scour around a circular bridge pier was investigated by varying the number of piles (or sheltering area) and distance between piles and the pier under clear-water conditions. Three values of distance between piles and the pier and three values of sheltering area (or number of piles) were tested. The efficiencies of sacrificial piles in different configurations were presented in terms of the percentage reduction in maximum scour depth at an unprotected pier under the same hydraulic conditions. The results of this experiment show that when linearly arranged sacrificial piles are placed close to the pier (at distance D; D is the pier diameter), an increase in the number of piles (or sheltered area) results in an increased scour depth, and when placed far from the pier (2D and 3D), an increase in the number of piles results in a decrease in scour depth around the pier. In addition, for 40% and 60% sheltering conditions, scour depth increased with an increase in the spacing between piles and the pier, while for 80% sheltering conditions, optimum protection was observed at a distance of 2D. Overall, two piles placed at distance D provided optimum protection with a scour depth reduction of 41.6%, while minimum protection was recorded when the same were placed at a spacing of 3D from the pier (25.5%). [ABSTRACT FROM AUTHOR]

The complex vortex structure compressor leads to the problem that the transition model is insufficient in predicting the flow instability of the compressor. In this paper, the rectangular cascade of compressor of different turning-angle conditions is taken as the object, and the transition characteristics on the end wall and the blade surface of the compressor cascade are in comparison by the method of large eddy simulation/LES. The effects of the horseshoe vortex and the separation bubble over the compressor cascade on the transition process are emphatically discussed. By analyzing characteristic parameters of the vortex structure, it is found that the separated transitional flow corresponds to multiple separations-and reattachments of the shedding vortex, and is affected by the cross-flow transition and the separate-transition. Finally, by discussing the instability of the separation line, reattachment line and the cross-flow inflection point of the separated transitional flow, it reveals that the transient disturbance caused by the vortex motion is an important reason affecting the prediction accuracy of the transition model. [ABSTRACT FROM AUTHOR]

A three-dimensional large eddy simulation model is used to simulate the turbulent flow dynamics around a circular pier in live-bed and clear-water scour conditions. The Navier–Stokes equations are transformed into a σ -coordinate system and solved using a second-order unstructured triangular finite-volume method. We simulate the bed evolution by solving the Exner-Polya equation assisted by a sand-slide model as a correction method. The bedload transport rate is based on the model of Engelund and Fredsœ. The model was validated for live-bed conditions in a wide channel and clear-water conditions in a narrow channel against the experimental data found in the literature. The in-house model NSMP3D can successfully produce both the live-bed and clear-water scouring throughout a stable long-term simulation. The flow model was used to study the effects of the blockage ratio in the flow near the pier in clear-water conditions, particularly the contraction effect at the zone where the scour hole starts to form. The scour depth in the clear water simulations is generally deeper than the live-bed simulations. In clear-water, the results show that the present model is able to qualitatively and quantitatively capture the hydrodynamic and morphodynamic processes near the bed. In comparison to the wide channel situation, the simulations indicate that the scour rate is faster in the narrow channel case. [ABSTRACT FROM AUTHOR]

This article presents a review of the state of research on bridge pier scour under combined wave–current flow. The hydrodynamics and scour around the bridge pier under combined wave–current flow have been explained in detail based on the information available in the literature. The impact of relative flow velocity (Ucw), Keulegan–Carpenter number (KC), absolute Reynolds number (Rea), and sediment characteristics on bridge pier scour under combined wave–current flow is presented. This study includes physical modelling of scour with various formulations to predict scour depth and calculation procedures related to scour under combined wave–current flow in the coastal environment. In addition, this study also provides the development of numerical models to investigate bridge pier scour in detail. In the end, future prospects of hydrodynamics and scour around the bridge pier under combined wave–current flow are delineated. [ABSTRACT FROM AUTHOR]

Motivated by the advantageous performance of air-jet vortex generators (AJVGs) in controlling shock-induced flow separation, we aim at understanding the effects of relevant injection parameters on the physical mechanisms involved, as well as on the control effectiveness. We performed implicit large-eddy simulations to examine the influence of injection pressure on a spanwise-inclined jet in supersonic crossflow at Mach 2.5 and with momentum-thickness Reynolds number 7000. Four injection pressures were considered. The jet and jet/crossflow interactions strengthen with injection pressure, which is reflected in an enhanced obliqueness and asymmetry of the flow topology, strengthening the induced major counter-rotating vortex pair (CVP). This strengthening of the major CVP stagnates at a certain threshold, above which the flow blockage and momentum influx caused by the jets become so strong that they counter the positive influence on the CVP. Subsequently, we installed these jets in an AJVG control setup for a 24 deg compression-ramp interaction with separation to analyze the control effectiveness and interplay of the injection pressure and jet spacing. The strongest effects on control effectiveness are not related to the injection pressure itself, but to interactions between the jet-induced shocks and vortical structures. Therefore, there is a strong link between jet spacing and injection pressure regarding the control effectiveness. Based on these observations, we provide recommendations for designing efficient AJVG control setups. [ABSTRACT FROM AUTHOR]

[Display omitted] Hypothesis : Horseshoe vortices are known to emerge around large-scale obstacles, such as bridge pillars, due to an inertia-driven adverse pressure gradient forming on the upstream-side of the obstacle. We contend that a similar flow structure can arise in thin-film Stokes flow around micro-obstacles, such as used in textured surfaces to improve wettability. This could be exploited to enhance mixing in microfluidic devices, typically limited to creeping-flow regimes. Experiments : Numerical simulations based on the Navier–Stokes equations are carried out to elucidate the flow structure associated with the wetting dynamics of a liquid film spreading around a 50 μm diameter micro-pillar. The employed multiphase solver, which is based on the volume of fluid method, accurately reproduces the wetting dynamics observed in current and previous (Mu et al., Langmuir, 2019) experiments. Findings : The flow structure within the liquid meniscus forming at the foot of the micro-pillar evinces a horseshoe vortex wrapping around the obstacle, notwithstanding that the Reynolds number in our system is extremely low. Here, the adverse pressure gradient driving flow reversal near the bounding wall is caused by capillarity instead of inertia. The horseshoe vortex is entangled with other vortical structures, leading to an intricate flow system with high-potential mixing capabilities. [ABSTRACT FROM AUTHOR]

Several high-resolution scale-resolving simulations are carried out to examine the effect of riblets on the mean and turbulent statistics of a zero-pressure-gradient boundary layer. The Reynolds number is chosen such that the riblets are exposed to both the transitional and turbulent regimes of the boundary layer. This is in contrast to the turbulent channel flow or fully turbulent boundary layers studied in the literature. The boundary layer is subjected to freestream turbulence and roughness tripping. The transition process and the extent of the turbulent regime on the riblets are altered by tripping the boundary layer with an isolated hemispherical roughness element. The influence of the riblets on the transition onset and the viscous drag reduction is demonstrated through time-averaged, phase-averaged, and instantaneous flow quantities. The effect of the V-shaped riblets with both sharp and curved tips (and valleys) is also explored. With riblets, the viscous drag in the turbulent regime is reduced by 4-6% when compared to a smooth surface, and the efficacy of sharp V-shaped riblets is shown to be marginally higher than for the curved riblets. In the transitional flow regime, the coherent structures over riblets are predominantly spanwise oriented. In particular, a separated shear layer forms over the riblet leading edge as the flow encounters an abrupt surface transition from the smooth surface onto the riblets. A leading-edge ramp is shown to effectively minimize the additional spurt in the turbulent kinetic energy and the associated losses incurred due to this abrupt surface change. [ABSTRACT FROM AUTHOR]

High-fidelity computational fluid dynamics (CFD) flow simulations were carried out to assess the sources of aerodynamic support interference of an aircraft half-model with a T-tail empennage mounted in a wind tunnel in an attempt to improve how well a semispan model can represent the corresponding full-span aircraft. The Bombardier research aircraft configuration (BRAC) model was used, which consists of a fuselage, a fuselage to wing fairing, a cruise wing with flap fairings, an engine nacelle and pylon, a vertical tail, and a horizontal stabilizer (HSTAB). The standard half-model support used at the National Research Council of Canada's 1.5 m trisonic wind tunnel consists of a wall-mounted splitter plate to remove a sizable portion of the upstream tunnel wall boundary layer and a two-dimensional peniche sandwiched between the model symmetry plane and the splitter plate. CFD simulations of the BRAC model mounted in the wind tunnel were performed using the Menter shear stress transport turbulence model at a Mach number of 0.2. The CFD results for the standard (baseline) mounting arrangement compared favorably well with the half-model experimental data. The BRAC CFD model was then modified to provide reference "full-span" data and to examine several alternate model mounting arrangements. All simulations were compared with the ideal full-span reference simulations. Significant disturbances were found to be induced by the reflection plane boundary layer and primary horseshoe vortex on the flowfield surrounding the HSTAB, which had a direct impact on the measured overall model forces and moments. [ABSTRACT FROM AUTHOR]

A rotor blade radial loading profile is presented that aims to passively mitigate the formation of coherent tip vortex structures in the near-field of the rotor wake. The method leverages principles of Helmholtz's theorems, where the strength of the vortex wake is proportional to radial gradients in the blade loading distribution. By avoiding such radial gradients, the near-field roll-up of the wake vortex can be mitigated. A pair of two-bladed rotor designs configured with the same thrust coefficient was produced using a finite-vortex rotary lifting line framework coupled to a constrained power optimization problem. A baseline rotor was designed based on a power-optimized approach, and a wake-optimized rotor was designed based on a power-optimized approach with an additional blade-root bending moment constraint. Phase-averaged stereoscopic particle image velocimetry data were acquired for each rotor operating in hover. The wake-optimized rotor blade produced a wake characterized by conical vortex sheets of significantly lower peak vorticity than the compact, helical tip vortices produced by the baseline design. The wake-optimized design exhibited increased axial velocities near the root that caused radial distortions of the wake flow structures, which were also found to decay at a faster rate than that for the baseline design. [ABSTRACT FROM AUTHOR]

To reduce the local scour around pier-type structures, a "porous plate method" was proposed, aiming at effectiveness in multidirectional horizontal flows. The porous plates installed on the pier surface near the bed play two important roles in the reduction of the local scour. One is "breaking down or weakening" of the horseshoe vortex by a decline in vorticity. In the neighborhood (just upstream region), the local scour depth greatly decreases due to suppression of picking up sediment grains from the bed. Another is the resultant "flow regulation" in the boundary layer on the pier surface. In the downstream side, the wake separated from the pier surface behaves as "semi-homogeneous turbulence" without a Kármán vortex street, then the scour area drastically decreases. [ABSTRACT FROM AUTHOR]

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour. [ABSTRACT FROM AUTHOR]

A three-dimensional large eddy simulation model is used to simulate the turbulent flow dynamics around a circular pier in live-bed and clear-water scour conditions. The Navier–Stokes equations are transformed into a σ-coordinate system and solved using a second-order unstructured triangular finite-volume method. We simulate the bed evolution by solving the Exner-Polya equation assisted by a sand-slide model as a correction method. The bedload transport rate is based on the model of Engelund and Fredsœ. The model was validated for live-bed conditions in a wide channel and clear-water conditions in a narrow channel against the experimental data found in the literature. The in-house model NSMP3D can successfully produce both the live-bed and clear-water scouring throughout a stable long-term simulation. The flow model was used to study the effects of the blockage ratio in the flow near the pier in clear-water conditions, particularly the contraction effect at the zone where the scour hole starts to form. The scour depth in the clear water simulations is generally deeper than the live-bed simulations. In clear-water, the results show that the present model is able to qualitatively and quantitatively capture the hydrodynamic and morphodynamic processes near the bed. In comparison to the wide channel situation, the simulations indicate that the scour rate is faster in the narrow channel case.

The present work investigates turbulent flow structures and behavior near the leading edge of a longitudinal flow obstruction in an open channel with an inclined bank. A volumetric particle image velocimetry (VPIV) was employed to collect velocity data. The results indicate that a relatively moderate channel bank angle ( θ = 28 ∘ ) does not prevent the formation of a junction vortex (JV) system. Indeed, it is found that the JV system develops over the channel bank and extends to the leading edge of the flow obstruction. It is demonstrated that the main factor that strengthens the primary junction vortex (JV1) at the tip of the protrusion is the pronounced downward flow. The probability density functions (pdfs) of the velocity fluctuations specify that the JV1 oscillates aperiodically between the so-called zero-flow and back flow modes. This explains the amplification of TKE at its core. It is shown that the velocity difference between the peaks of the pdf increases from over the channel bank towards the tip of the flow obstruction. The aperiodic behavior of the JV1 is confirmed via the proper orthogonal decomposition (POD) technique. Specifically, it is demonstrated that the leading POD modes, associated with the JV system, contain lower energy content compared to periodic flows. The time-averaged vertical vorticity field verifies the presence of a tornado like vortex structure near the upstream face of the retaining wall. Finally, the results suggest that the largest bed shear stress values in the mean flow are located near the tip of the protrusion. [ABSTRACT FROM AUTHOR]

In this paper, details, and results of three-dimensional numerical modeling of flow around the semi-conical piers vertically mounted on the bed in a channel, are presented. For flow simulation, 3-D Navier–Stokes equations are solved numerically using the finite volume method and large eddy simulation. In this study, the semi-conical piers with different side slope angles (α ) are tested, and the flow around them is compared with the cylindrical reference pier. Flow structures, vortex shedding behind piers, horseshoe vortices, instantaneous and time-averaged flow structures are presented and discussed. Numerical model results show that the semi-conical piers are eventuated remarkable reduction (up to 25%) in downward flow velocity in the upstream side of the piers, and much more reduction (up to 46%) in bed shear stresses in comparison with the cylindrical pier. Moreover, the model results showed some decrease in vortex shedding frequency for the semi-conical piers compared to the cylindrical pier. Article highlights: We report on numerical results of large eddy simulation of the flow around semi-conical piers with different side slopes. This research is significant because of the effect of these piers on the: Reduction of the downward flow and the bed shear stress around the piers. Reduction of vortex shedding frequency for the semi-conical piers compared to the cylinder. Different behavior of the horseshoe vortices at the upstream compared to the cylinder. [ABSTRACT FROM AUTHOR]

A joint experimental and computational study is performed to quantify the effects of various expansion ratios on the flowfields and acoustic properties of a jet, issuing from a diamond-shaped nozzle. Three expansion ratios are studied corresponding to perfectly expanded, overexpanded, and underexpanded regimes, which progressively increase the far-field acoustic impact. Nozzle asymmetry leads to distinct mean flow properties across the two principal (major/minor) axes of the jets, with the potential core and shock-expansion cells exhibiting a faster collapse on the major axis plane. Asymmetry is also observed in the computed near-field pressure spectra, which exhibits higher amplitudes in the minor axis plane, consistent with the higher far-field sound levels experimentally measured on this plane for all three jets. Near-field spectra also identify three specific bands, corresponding to broadband shock-associated noise (BBSAN), mixing noise, and that associated with nonradiating subsonic convecting eddies. Further analysis of noise mechanisms through the turbulent kinetic energy (TKE) budget shows enhanced levels of TKE on the minor axis plane, resulting from correlations between fluctuations in streamwise and cross-stream velocities, and gradients across the shear layers. Fourier analysis of near-field coherent structures in pressure identified axisymmetric features in the most energetic modes, followed by the first helical mode, which was enhanced in the presence of shock-expansion cells in the imperfectly expanded jets. The most energetic modes in the overexpanded shock-expansion cells have a strong harmonic nature to it, corresponding to the peak frequencies of its near-field BBSAN component. [ABSTRACT FROM AUTHOR]

The flow past a simplified wing-body junction configuration is simulated using a shear layer adaptive improved delayed detached-eddy-simulation (SLA-IDDES) approach in which an appropriate subgrid lengthscale is adopted in the boundary region, where the grid may be strongly anisotropic, to prevent the excessive generation of subgrid-scale eddy viscosity. The numerical results show overall good agreement with the experiment carried out by Ölçmen and Simpson. The self-induced chaotic switching of the horseshoe vortex between two flow modes is simulated and analysed, and its effects on the vortex legs and their inherent oscillation are also investigated. It is found that the corner vortex also exhibits an alternating formation and breakdown process with basically the same frequency as the horseshoe vortex. Corner separation is found to be affected also by the upstream horseshoe vortex and the inherent oscillation of the vortex legs. [ABSTRACT FROM AUTHOR]

Interaction between the coolant and the secondary flow plays an important role in endwall cooling performance. For the leading-edge region, oncoming main flow inside the boundary layer impinges onto the vane leading edge and turns into the horseshoe vortex. Horseshoe vortex entrains coolant off the surface, thus posing severe challenges to the cooling design there. Based on analyses on the leading-edge vortex formation mechanism, a new kind of endwall film cooling design, vertical hole upstream of the saddle point, is proposed to obtain more uniform film coverage over the vane/endwall junction region. Coolant injected from the vertical hole can pass over the horseshoe vortex and impinge around the stagnation line on the vane leading edge. Uniform film coverage can be obtained around the vane leading edge where coolant clings to the endwall surface due to the span-wise pressure gradient of the stagnation region. Numerical simulations are conducted about the cooling performance of two main kinds of both isotropic and anisotropic hole geometries for the endwall and vane surface. Results come that the anisotropic hole shows significant advantages over the isotropic one because it suppresses the symmetrical kidney vortices thus weakening the mixture with high-temperature gas. Blowing ratio (M) effect is analyzed and conclusions are drawn that the cooling performance of the endwall around the leading edge is sensitive to M and adiabatic film cooling effectiveness peaks at about M = 2.0. Better cooling performance over the vane corner region can be obtained when M gets even higher while the effective film coverage area shrinks. Apart from that, the phenomenon of phantom cooling on the upper triangular region of the suction surface can be observed when coolant on the endwall is entrained by the vortex formed at the corner of the leading edge. [ABSTRACT FROM AUTHOR]

This study proposes a sweeping jet as a novel solution to control catastrophic flow-acoustic resonance through a combined experimental and scale-adaptive simulation (SAS) strategy. The relationships between resonance intensity and the geometry of the dual sweeping-jet controllers (SJCs) and mass ratio of the jet flow to mainstream flow were established by the wall-pressure measurements. The system with dual SJCs at two upstream channel-branch intersections with spanwise jet sweeping achieved the best control performance at the low mass ratio of Qr=1%. Through complementary simulations, self-sustained unsteady flow dynamics within dual SJCs, strong flow-acoustic resonance within the channel-branch system and complete suppression of the aeroacoustic field by the most efficient dual SJCs were demonstrated. The simulation results were first validated in terms of the frequency and amplitude information of the experimentally measured wall-pressure fluctuations. Subsequently, a proper orthogonal decomposition analysis was conducted and a large-scale synchronous sweeping behavior was observed when the jet flow proceeded from the fluidic oscillators to the free space. Thereafter, SAS results revealed the dual SJCs could remarkably attenuate velocity fluctuations, shear-layer momentum thickness, and Reynolds and Lighthill stresses. These improvements were achieved by completely suppressing the large-scale shear-layer vortex shedding into a series of horseshoe vortices. [ABSTRACT FROM AUTHOR]

Previous efforts determined a set of calibrated, optimal model parameter values for Reynolds-averaged Navier-Stokes (RANS) simulations of a compressible jet in crossflow (JIC) using a k-e turbulence model. These parameters were derived by comparing simulation results to particle image velocimetry (PIV) data of a complementary JIC experiment under a limited set of flow conditions. Here, a k-e model using both nominal and calibrated parameters is validated against PIV data acquired from a much wider variety of JIC cases, including a realistic flight vehicle. The results from the simulations using the calibrated model parameters showed considerable improvements over those using the nominal values, even for cases that were not used in the calibration procedure that defined the optimal parameters. This improvement is demonstrated using a number of quality metrics that test the spatial alignment of the jet core, the magnitudes of multiple flow variables, and the location and strengths of vortices in the counter-rotating vortex cores on the PIV planes. These results suggest that the calibrated parameters have applicability well outside the specific flow case used in defining them and that with the right model parameters, RANS solutions for the JIC can be improved significantly over those obtained from the nominal model. [ABSTRACT FROM AUTHOR]

In aero-engine compressors, corner separations cause blockage, loss generation, and flow instabilities. The aim of the current study is to gain better knowledge of the vortical structures in corner separation flows and their resultant blockage and loss. Numerical simulations at different incidences have been performed for a high-speed linear compressor cascade. First, vortical structures have been visualized with the help of the Q-criterion and eigenvector method, and those associated with the corner separation have been focused on. It was found that the corner separation occurring on either the pressure surface or suction surface corner region was characterized by a large-scale streamwise vortex, named as the corner separation vortex in this paper. The pressure-side corner separation vortex is observed at a great negative incidence initiated from a focus point on the pressure surface, while the suction-side corner separation vortex observed at all concerned incidences originated from a focus on the endwall near the suction surface. Then, the mechanism by which corner separation vortices resulted in blockage and loss has been uncovered. The results showed that low-momentum fluids inside the core region of corner separation vortices led to the flow blockage and the viscous friction between them, and mainstream was responsible for the entropy generation. One major contributor of low-momentum fluids in the suction-side corner region was an off-wall reversed-flow region which was attributed to the breakdown of the suction-side corner separation vortex. Finally, multi-vortex structures in the leading region of the suction-side corner separation at two high positive incidences have been analyzed. It was found that the horseshoe vortex influenced the formation of the suction-side corner separation vortex, which induced a multi-vortex structure, and prevented the onset location of suction-side corner separation vortex from moving upstream with the increased incidence. [ABSTRACT FROM AUTHOR]

We propose a novel Clebsch method to simulate the free-surface vortical flow. At the center of our approach lies a level-set method enhanced by a wave-function correction scheme and a wave-function extrapolation algorithm to tackle the Clebsch method's numerical instabilities near a dynamic interface. By combining the Clebsch wave function's expressiveness in representing vortical structures and the level-set function's ability on tracking interfacial dynamics, we can model complex vortex-interface interaction problems that exhibit rich free-surface flow details on a Cartesian grid. We showcase the efficacy of our approach by simulating a wide range of new free-surface flow phenomena that were impractical for previous methods, including horseshoe vortex, sink vortex, bubble rings, and free-surface wake vortices. [ABSTRACT FROM AUTHOR]

In this research activity numerical simulations are carried out to investigate the flow field upstream of a symmetric streamlined body mounted perpendicular to a flat plate with and without clearance gap between the tip of the streamlined body and the flat plate with laminar boundary layer. The developed numerical model successfully predicted the three-dimensional horseshoe vortex system upstream of the streamlined body with and without the tip gap. The resulting vortex system for the configuration with tip gap contains multiple vortices with characteristics similar to that of end-wall-flows of surface-mounted obstacles. The effects of varying tip gap clearance for various values of free stream Reynolds number are also investigated. It was found that the introduction of a gap between the streamlined body tip and flat surface caused shifting of the vortex structure system in the upstream direction. Moreover, it is observed that the free stream Reynolds number and the tip gap between the streamlined body and the flat plate substantially influences the unsteady character of the flow field and the vortex system structure. Results obtained from the numerical simulations are compared with experimental measurements of a blunt body configuration and have been found in good agreement. [ABSTRACT FROM AUTHOR]

Bridge pier's local scouring is known to be a destructive factor in river engineering science. This phenomenon is widespread in river intersecting structures such as bridge piers, spur dykes, and downstream river structures. Extensive research has been conducted to reduce and control destructive phenomena, and many solutions have been proposed. These solutions are divided into two parts, namely, direct and indirect protection. In this study, the direct method was studied by defining scenarios. Since many bridges are affected by scouring during the operation, in the present study, the collar method, which is known as a direct protection method, in the case of flexible and permeable, is suggested. The technique is presented an adjustable chain collar, three times bigger than the pier's width (w/d=3), and its effect is investigated in clear water conditions. In the defined scenarios, three different diameters of the chain as CI=5 mm, CII=10 mm, and CIII=15 mm were used to control chain shapes' effect, and three dimensionless flow parameters (U/Uc 0.73, 0.85, and 0.96) were selected to investigate the effect of flow conditions. According to the results, the scour depth is related to changes in the diameter of the collar chain, as the final scour depth decreases by increasing the diameter of the chain from CI to CIII. Therefore, in the best conditions, for CIII, the dimensionless ratio of scouring reduction ( ) is equal to 71% near to inception motion parameter (U/Uc=0.96).

In this study, the flow and heat transfer characteristics of a liquid jet impinging on cylindrical cavity heat sinks with a local body heat source were numerically investigated. The Transition SST turbulence model was validated and adopted. The parameters of structure and flow, including d/D = 2, 3, and 4, h/D = 0.05, 0.10, 0.15, and 0.20, and Re = 5,000, 10,000, and 23,000, were investigated. The results revealed that the adoption of a cylindrical cavity structure can improve the heat transfer capacity of the heat sink. A horseshoe vortex introduced by an inclined jet near the cavity edge region improved the heat transfer performance. The maximum enhancement of the cylindrical cavity heat sink was 11.8% compared with the flat plate heat sink when d/D = 3, h/D = 0.15, and Re = 23,000.

Numerical simulations were conducted to investigate steady current scour around rectangular and square subsea caissons. The caissons, which are representative of subsea structures, have a submerged height ranging from 0.5 to several times their longest base dimension. The flow model used is based on the Reynolds Averaged Navier-Stokes equations, and the scour model simulates bed load and suspended load transport to predict the evolution of the seabed profile. The aim of this study is to investigate the mechanisms of local scour, specifically the contribution of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour. The model is validated through comparisons with experimental measurements, indicating reasonable agreement. Based on the numerical model results, the mechanisms of local scour around square and rectangular caissons are found to be qualitatively similar. If the flow is directed perpendicular to one of the caisson faces, both the horseshoe vortex and the local streamline contraction contribute to scour. The maximum scour depth is located at the two upstream corners of the caisson due to the combined effects of the horseshoe vortex and streamline contraction. When the flow approaches the caisson with a diagonal attack angle of 45°, the two upstream faces act like a streamlined wedge that splits the incoming flow. The study found that the horseshoe vortex almost disappears and the scour is primarily caused by the local velocity amplification at the two side corners. If the flow attack angle is 45°, the scour does not initiate near the upstream corner without the contribution from the horseshoe vortex. These findings are expected to serve as a valuable theoretical foundation for the design of scour protection. • Scour around subsea caissons is studied by fully three-dimensional simulations. • The contributions of the horseshoe vortex and the local streamline contraction near the caisson corners to local scour are identified. [ABSTRACT FROM AUTHOR]

Wingtip-flow effects on the aerodynamic forces and moments of a wing become increasingly significant as the aspect ratio decreases. These effects are dominated by the tip vortex and an associated suction zone, which results in a supplementary "vortex lift" that is not captured by conventional vortex lattice methods (VLMs). Although many augmented VLMs have addressed such free-vortical flows on the leading edges of delta wings and even wingtips, they do not account for the portion of the vorticity that may be supported by the attached flow at the tip. The current work consists of an augmented VLM designed to capture the geometry and strength of the free tip vortex along the wingtip, and it is distinguished from prior models by the capability to allocate vorticity between the bound wingtip flow and the free tip-vortex flow at each chordwise position. This paper presents the model and its predictions of tip-vortex positions, as well as forces and moments on low-aspect-ratio wings. The model demonstrates lift distributions typical of vortex lift, promising agreement with forces and moments from experimental work as well as modulation of the forces and moments by selection of the maximum allowable bound vorticity along the tips. [ABSTRACT FROM AUTHOR]

Numerical simulations of the NASA Juncture Flow Experiment are carried out, employing a Reynolds-stress model. A transformation of the length-scale determining variable is suggested for improving the numerical robustness of the model. Results obtained with different unstructured flow solvers on different sets of grids show very good agreement, except for velocity and Reynolds-stress profiles in the vicinity of the predicted separation region. A recently developed length-scale correction and, particularly, prescribing transition on the fuselage and wing increase the predicted size of the separation region, where the variation is on the order of the experimental uncertainty. Detailed comparison of velocity and Reynolds-stress profiles with experimental data reveal a general deficiency of the model in predicting the peak of the streamwise normal stress and the distribution of all Reynolds stresses in the vicinity of the separation region. However, all predicted sizes of the separation region are in fair agreement with the experimental data. [ABSTRACT FROM AUTHOR]

Turbulent flow over a wall-mounted cube presents an important canonical problem with applications in circuit design, urban building design, or aerodynamics. Flow over a cube placed in a fully developed turbulent channel flow has long been the preferred problem of investigation for researchers. However, a more relevant practical scenario is flow over a cube placed in a spatially evolving turbulent boundary layer (SETBL). In this work, wall-resolved large-eddy simulations of flow over a wall-mounted cube placed in an SETBL on a flat plate are performed at a Reynolds number of 19,600 based on the mean velocity and the inlet boundary-layer thickness. The incompressible Navier-Stokes equations are solved using a second-order finite-volume approach with a dynamic k -equation eddy-viscosity model. At this Reynolds number, the key nondimensional parameter governing this problem is the ratio of cube height to boundary-layer thickness, h/δo. This paper investigates the dependence of the turbulence wake characteristics on h/δo. Specifically, this paper analyzes the vortical structures and the separation and reattachment lengths. It is further examined how the turbulent kinetic energy budget varies with h/δo. It is found that the size of the horseshoe vortex roughly scales with the size of the cube, and that the decay of turbulent kinetic energy per unit area over streamwise distance in the near wake exhibits a power-law behavior. [ABSTRACT FROM AUTHOR]

Formation flying techniques, demonstrated by migratory birds, have the potential to significantly save energy when two or more transport aircraft fly the same route at the same time. In fuel-saving formations, aircraft are flying straight trajectories for most of the time. For proof of concept, however, how vertical and lateral maneuvers can be flown has to be investigated. Those investigations require a flight simulator that is equipped with adequate flight mechanical models. Not much research exists on maneuvers of formation. The paper addresses the components that are needed to perform this research with real-time flight simulations. They are, first, a method to model wake vortices on curved trajectories; second, a formation flight control system, which comprises a method for the relative position determination, the design of control laws, and suitable cockpit displays; and third, techniques that allow vertical and lateral flight maneuvers. Two different strategies to position the following aircraft relative to the wake vortex during turns are described, and simulation results show their differences in terms of fuel savings and passenger comfort. Piloted flight simulator tests confirmed the usability of the proposed flight techniques for formation maneuvers, and the participating airline pilots indicated general consent to the concept. [ABSTRACT FROM AUTHOR]

This study introduced a new riprap placement design for the purpose of reducing bridge pier scour. The design consisted of embedding a layer of the riprap around the pier, flush with the surrounding bed, with a depression to help guide the horseshoe vortex. The new countermeasure design was tested among other conventional riprap placement designs in clear-water flow conditions and transition-flow conditions. The transition-flow condition is the evolution period from clear water to live bed where the bed has yet to reach an equilibrium state. The new countermeasure design was found to reduce greatly the depth and volume of scour in both flow conditions, up to 81.0% and 92.3%, respectively, which was more than any other riprap countermeasure tested. In addition, the new design used 18% less riprap material while also experiencing less shape deterioration when compared with a traditional flat riprap cover. Therefore, installing riprap in this specified shape yields an improved scour countermeasure that could help preserve infrastructure and increase public safety. [ABSTRACT FROM AUTHOR]

This study performed a bridge pier collar comparison for the purpose of reducing scour, while introducing a new collar design. The new collar, referred to as Collar Prototype Number 3, was designed based on an equilibrium scour hole and provides a method of controlling the horseshoe vortex. Numerical modelling was utilized to show the flow field and bed shear stress as a result of using Collar Prototype Number 3. A prototype model of Collar Prototype Number 3 and a flat plate collar were constructed for the purpose of comparing both experimentally under different flow conditions. Collar Prototype Number 3 proved to be an effective device as it reduced scour in terms of the maximum depth downstream of the pier and overall volume, in some instances, by up to 69.7% and 75.7%, respectively. Furthermore, Collar Prototype Number 3 outperformed the flat plate collar, especially because, unlike the flat plate collar, Collar Prototype Number 3 did not experience undermining. [ABSTRACT FROM AUTHOR]

The present study numerically describes the nature of the arising boundary layer along an isothermal vertical plate, on which is mounted a bluff body. The investigation parameter is the cross-section shape of the bluff body, which includes a square, circular, and prismatic cylinders. To entirely embedded cylinder in the incoming boundary layer, the simulations include short cylinders with an aspect ratio fixed to unity. The cylinder location related to the leading edge of the plate is identified by considering laminar Grashof number, which experimentally is investigated in parallel work. Besides this, the experimental study of a square cylinder, confirms the reliability and accuracy of numerical simulation. As a result, by providing steady-state simulation, the presence of a complex vortex structure is proved. Besides, the effects of the cross-section shape of the cylinder on the dynamics characteristics of the vortex system include vorticity strength, size, and position are categorized. Finally, the significant effects of formed juncture flow on the three-dimensional thermal characteristics of arising free convective boundary layers along with the heated vertical plate are emphasized by a description of heat transfer coefficients. In particular, the importance of the frontal bluntness of cylinder in the field of thermal management is claimed, where the maximum value of the heat transfer coefficient for circular, prismatic, and square cylinders, respectively appears at the angular coordinates of0, 45, and 90 degrees, which correspond with the location of cylinder edges.

Recent studies have shown that the presence of ice cover leads to an intensified local scour pattern in the vicinity of bridge piers. To investigate the local scour pattern in the vicinity of bridge pier under ice-covered flow condition comparing to that under open channel flow condition, it is essential to examine flow field around bridge piers under different flow conditions. In order to do so, after creation of smooth and rough ice covers, three-dimensional timeaveraged velocity components around four pairs of bridge piers were measured using an Acoustic Doppler velocimetry (ADV). The ADV measured velocity profiles describe the difference between the velocity distributions in the vicinity of bridge piers under different covered conditions. Experimental results show that the vertical velocity distribution which represents the strength of downfall velocity is the greatest under rough covered condition which leads to a greater scour depth. Besides, results show that the turbulent intensity increases with pier size regardless of flow cover, which implies that larger scour depth occurs around piers with larger diameter.

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.

The present work investigates turbulent flow structures and behavior near the leading edge of a longitudinal flow obstruction in an open channel with an inclined bank. A volumetric particle image velocimetry (VPIV) was employed to collect velocity data. The results indicate that a relatively moderate channel bank angle (θ=28∘) does not prevent the formation of a junction vortex (JV) system. Indeed, it is found that the JV system develops over the channel bank and extends to the leading edge of the flow obstruction. It is demonstrated that the main factor that strengthens the primary junction vortex (JV1) at the tip of the protrusion is the pronounced downward flow. The probability density functions (pdfs) of the velocity fluctuations specify that the JV1 oscillates aperiodically between the so-called zero-flow and back flow modes. This explains the amplification of TKE at its core. It is shown that the velocity difference between the peaks of the pdf increases from over the channel bank towards the tip of the flow obstruction. The aperiodic behavior of the JV1 is confirmed via the proper orthogonal decomposition (POD) technique. Specifically, it is demonstrated that the leading POD modes, associated with the JV system, contain lower energy content compared to periodic flows. The time-averaged vertical vorticity field verifies the presence of a tornado like vortex structure near the upstream face of the retaining wall. Finally, the results suggest that the largest bed shear stress values in the mean flow are located near the tip of the protrusion.

Featured Application: The (k-ω) turbulence model showed good results in capturing the flow structure at the wake zone downstream of the pier, with continuous oscillation matching the pier downstream turbulence pattern. Personal computer computational and storage resources were sufficient to run the computational fluid dynamics (CFD) simulations for the case in hand with the selected turbulence model. The required computational resources are cheap and affordable compared to those required for the Large Eddy Simulation (LES) and Detached Eddy Simulation (DES). This will lead to significant savings in time and resources in future CFD modeling for similar cases. In the current study, a three-dimensional CFD model is utilized to investigate the variation of the flow structure and bed shear stress at a single cylindrical pier during scour development. The scour development is presented by seven solidified geometries of the scour hole, collected during previous experimental work at different scour stages. Different turbulence models are evaluated and the (k-ω) model is chosen due to its relative accuracy in capturing the flow oscillation and vortex shedding at the pier downstream side with personal computer computational and storage resources. The numerical results are verified against dimensionless parameters from different previous experimental works. This research describes in detail the flow structure and bed shear stress variations through seven stages of the scour hole development. The dimensionless area-averaged circulation coefficient (Ψi) is developed to evaluate the changes in the vortex strength through the scouring process by eliminating the calculation area effect. It was concluded that the circulation in the (Y) direction is the main driving factor in the development of the scour hole more than the circulation in the (X) direction. The ratio between the horseshoe vortex (HV) mean size and the scouring depth (DV/dS) in addition to the location of the maximum bed shear stress are investigated during different stages of the scour development. [ABSTRACT FROM AUTHOR]

Bridge pier scour poses major concerns for public safety and riverine communities because of its ability to undermine pier foundations. A new collar design is introduced as an improvement on the existing Flat Plate Collar (FPC). Uniquely, the three-dimensional collar is specifically designed to contain the horseshoe vortex and guide it safely downstream, while protecting the riverbed around the pier base from the passing accelerated flow. Large shear stresses are thereby prevented from contacting the riverbed, and thus scour is considerably reduced. The collar design was achieved in an iterative hybrid manner through the use of OpenFOAM (CFD) software and by employing experimental tests. Collar designs were tested numerically and adjusted until an improved shape was reached. Then, a physical model of the new collar and an FPC were constructed and tested experimentally. The collar is demonstrated to reduce scour, more than that of an FPC; however further revisions are required for it to be an effective scour solution. [ABSTRACT FROM AUTHOR]