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In the context of hypersonic flows, shock-wave/turbulent boundary-layer interactions (SWTBLIs) can lead to substantial aerodynamic heating. The commonly used Reynolds-averaged Navier–Stokes (RANS) method in engineering applications faces challenges in accurately predicting heat transfer in these conditions, as the assumptions made by Morkovin's assumption in the RANS method are not applicable in hypersonic SWTBLI flows. This article addresses these challenges by introducing a variable turbulent Prandtl number (P r t ) model for non-adiabatic walls, established through field inversion and symbolic regression (SR) techniques. The methodology begins with field inversion for an oblique SWTBLI case at Mach 5. The corrected field, denoted as β (x) , obtained from this inversion, is integrated with selected local flow characteristics to derive a corrected expression using SR. Following the necessary adjustments, this expression is incorporated into the RANS solver. Various cases with different wall cooling ratios, Mach numbers, and Reynolds numbers are chosen to assess the generalization ability of the variable P r t model. The outcomes demonstrate a substantial improvement in the model's ability to predict heat transfer in hypersonic SWTBLI flows without compromising the baseline model's performance in predicting the undisturbed boundary layer. The correction effect remains notably enhanced even in complex three-dimensional cases. • Corrected heat flux and skin friction by adjusting Reynolds stress and turbulent Prandtl number. • Developed a variable Prandtl number model using symbolic regression. • Analyzed physical mechanisms of the modified SST model for separated flow. analyzed. • Nine hypersonic SWTBLI flows showed significant improvements in calculations. [ABSTRACT FROM AUTHOR]

In the pursuit of mitigating CO2 emissions, this study investigates the optimization of CO2 purification within a negative CO2 emission power plant using a spray ejector condenser (SEC) coupled with a separator. The approach involves direct‐contact condensation of vapor, primarily composed of an inert gas (CO2), facilitated by a subcooled liquid spray. A comprehensive analysis is presented, employing a numerical model to simulate a cyclone separator under various SEC outlet conditions. Methodologically, the simulation, conducted in Fluent, encompasses three‐dimensional, transient, and turbulent characteristics using the Reynolds stress model turbulent model and mixture model to replicate the turbulent two‐phase flow within a gas–liquid separator. Structural considerations are delved into, evaluating the efficacy of single‐ and dual‐inlet separators to enhance CO2 purification efficiency. The study reveals significant insights into the optimization process, highlighting a notable enhancement in separation efficiency within the dual‐inlet cyclone, compared to its single inlet counterpart. Specifically, a 90.7 % separation efficiency is observed in the former, characterized by symmetrical flow patterns devoid of wavering CO2 cores, whereas the latter exhibits less desirable velocity vectors. Furthermore, the investigation explores the influence of key parameters, such as liquid volume fraction (LVF) and water droplet diameter, on separation efficiency. It is ascertained that a 10 % LVF with a water droplet diameter of 10 µm yields the highest separation efficiency at 90.7 %, whereas a 20 % LVF with a water droplet diameter of 1 µm results in a reduced efficiency of 50.79 %. Moreover, the impact of structural modifications, such as the addition of vanes, on separation efficiency and pressure drop is explored. Remarkably, the incorporation of vanes leads to a 9.2 % improvement in separation efficiency and a 16.8 % reduction in pressure drop at a 10 % LVF. The findings underscore the significance of structural considerations and parameter optimization in advancing CO2 capture technologies, with implications for sustainable energy production and environmental conservation. [ABSTRACT FROM AUTHOR]

High-order accurate Large-eddy simulations (LES) have been carried out for compressible, turbulent flow through two bell-shaped convergent-divergent nozzles- one with a short divergent portion and the other with a longer one. An explicit filtering approach based on approximate deconvolution method is used for the LES. The nozzles have isothermal walls and circular cross-sections. The incoming flow is a turbulent, subsonic fully developed pipe flow at centreline Mach number of around 0.4 and friction Reynolds number 216. An adaptive filter is used to capture the shocks appearing in the divergent portion of the nozzles. The complicated behaviour of the mean flow, Reynolds stresses, production and pressure-strain terms of the axial Reynolds stress budget in the shock train region is shown. The broadband nature of the shock train oscillations having multiple dominant frequencies in these nozzles is observed in the present LES. [ABSTRACT FROM AUTHOR]

Understanding the ecological conditions of vegetation growth in water sources is vital to appraise the influence of vegetation on river engineering. Based on the experimental information that is accessible, the consequences of vegetation on flow resistance is described as an alteration in the drag coefficient and the planned area. The current study analytically estimates the vertical distribution of stream-wise velocity in open-channel flow while considering rigid and flexible vegetation. The flow is vertically separated into top free water layer and bottom vegetation layer using the projected deflection height of both vegetation. Related momentum calculations for each layer are then derived. Based on the gathered experimental data, a 3D numerical model with various simulation situations is used to model, calibrate, and evaluate the artificial cylinders. A considerable deflection analysis is utilised to calculate the velocity-dependent stem height. This has proven to be more precise compared to formerly deflection investigation. The estimated outcomes show that precise predictions may be made for the vertical contours of vertical Reynolds shear stress based on mean horizontal velocity. The numerical simulations demonstrate that plant flexibility reduces the vertical Reynolds shear stress and prompted flow resistance force of the vegetation. [ABSTRACT FROM AUTHOR]

This paper investigates the shape and hydrodynamic characteristics of the sand bed at the junction of urban pipelines and their relationship with the flow ratio. Various hydrodynamic and morphological features, including shear layers, spiral cells, and scour pits. The dataset used for analysis consists of a three-dimensional time-averaged velocity field, turbulence, bed morphology, and confluence morphology of equilibrium phases obtained under controlled laboratory conditions. For large flow ratios (q*), significant local erosion occurs near the downstream shear plane at the junction. When the flow of tributaries surpasses that of the main stream, the strength and downstream extension of the spiral cell blocks increase. The max Reynolds number shear stress predominantly concentrates in the middle region of the water depth, aligning with the turbulent kinetic energy representation of the shear layer. Keeping the flow ratio constant, the velocity, turbulent kinetic energy, and absolute Reynolds shear stress all escalate with increased flow. [ABSTRACT FROM AUTHOR]

Motivated by the saturation of drag reduction effectiveness at high non-dimensional riblet spacing in turbulent boundary layer flows, this study seeks to investigate the influence of a secondary blade riblet structure on flow statistics and friction drag reduction effectiveness in comparison to the widely explored single-scale blade riblet surface. The turbulent flow dynamics and drag reduction performance over single- and multi-scale blade riblet surfaces were experimentally examined in a flow visualization channel across various non-dimensional riblet spacings. The shear velocity was quantified by the streamwise velocity distributions from the logarithmic layer via planar Particle Image Velocimetry (PIV) measurements, whereas the near-wall flow dynamics were characterized by a Micro Particle Image Velocimetry (micro-PIV) system. The results highlighted that although both riblet surfaces exhibited similar drag reduction performances at low non-dimensional riblet spacings, the presence of a secondary riblet blade structure can effectively extend the drag reduction region with the non-dimensional riblet spacing up to 32 and achieve approximately 10% lower friction drag in comparison to the single-scale riblet surface when the non-dimensional riblet spacing increases to 44.2. The average number of uniform momentum zones (UMZs) on the multi-scaled blade riblet has also reduced by 9% compared to the single-scaled riblet which indicates the reduction of strong shear layers within a turbulent boundary layer. The inspection of near-wall flow statistics demonstrated that at high non-dimensional riblet spacings, the multi-scale riblet surface produces reduced wall-normal velocity fluctuations and Reynolds shear stresses. Quadrant analysis revealed that this design allows for the suppression of both the sweep and ejection events. This experimental result demonstrated that surfaces with spanwise variations of riblet heights have the potential to maintain drag reduction effectiveness across a wider range of flow speeds. [ABSTRACT FROM AUTHOR]

In order to reveal the modulation of particles on turbulent coherent structures and particle concentration variations induced by turbulence, particle image velocimetry experiments were performed in a dilute liquid–solid two-phase turbulent boundary layer. Large-scale coherent structures, including streaks and hairpin vortices, were extracted in the liquid–solid two-phase turbulent boundary layer by using multiple methods, such as proper orthogonal decomposition and finite-time Lyapunov exponent. The results showed that the addition of particles thickened the buffer layer of the average velocity profile, weakened the streamwise turbulence intensity at y+ > 10, strengthened the wall-normal turbulence in the near wall region (y+ < 52), and weakened the Reynolds stress in the logarithmic law region. A more thorough study of the turbulent structure revealed that the particles caused the mean spacing of the near-wall streak structure to increase, hairpin vortex migration to slow down, the uplift of hairpin vortex to be suppressed, and the overall size of the hairpin vortex packet to be compressed. This demonstrated that particles could modulate turbulence by inhibiting the development of hairpin vortices. [ABSTRACT FROM AUTHOR]

In the current experimental study, turbulent flow structures and Reynolds stress anisotropy in an asymmetric sinuous mobile channel are investigated. Experiments were conducted in a laboratory on an asymmetric plane-shaped sinuous model of a riverway with a sinuosity of 1.23. Three-dimensional instantaneous velocity data were measured using an acoustic Doppler vectrino profiler and processed to compute turbulent flow structures and Reynolds stress anisotropy. Spectral analysis of the streamwise fluctuating component of velocity shows the presence of "−1" and "−5/3" slope regions. Analysis of bursting events in the near-bed region reveals that the contribution of sweep and ejection events is more dominant than outward and inward interactions. The intermixing of events with depth shows the presence of helical flow. The probability of occurrence of bursting events indicated that the ejections are higher than other events. The Reynolds stress anisotropy tensor showed higher contribution in the streamwise followed by transverse and vertical directions, respectively, at all locations. The anisotropic invariant map indicated two-component turbulence in the near-bed region and one-component turbulence toward the surface at upstream and apex locations. At the bend downstream, the two-component turbulence diminished near the bed, and one-component turbulence increased with vertical depth. The eigenvalues of the non-dimensional anisotropy tensor indicated that the principal component was higher than the other two components in the near-bed region and started decreasing with increasing depth. The anisotropic invariant function showed that the near-bed region tended to approach an isotropic limit but departed from it in the outer flow region with increasing vertical depth. [ABSTRACT FROM AUTHOR]

This study employed numerical simulations to investigate the three-dimensional hydrodynamic structure of a channel obstructed by submerged rigid canopies. After validating the model using existing experimental data, a series of numerical experiments varying the canopy density and the blocking ratio were conducted. The results indicate that submerged canopy, different from emergent canopies, triggers the generation of three-dimensional coherent vortices. When canopies expand laterally, the scale, location, and evolution characteristics of cross-sectional secondary flows vary. The direction of secondary circulation at the canopy top of the fully covered vegetation channel is affected by the river width/depth ratio. Three-dimensional coherent vortices are jointly controlled by vertical and transverse Reynolds stresses. When the channel is seriously blocked, the degree of momentum exchange in the mixing layer does not change significantly. The boundary friction effect dominates the momentum exchange process near the wall. An equation coupling the drag length scale and blocking ratio with the vertical coherent vortex penetration length is proposed, indicating that the penetration length increases exponentially with both variables. Higher canopy density exhibits a stronger resistance to the vertical coherent vortex penetration length. In the process of canopy lateral expansion, the evolution time of the outer vortex to scale stability is longer than that of the inner vortex. The significance of research on the three-dimensional vortex structure of rigid submerged vegetation is established based on previous studies and existing conclusions. [ABSTRACT FROM AUTHOR]

The present study illustrates an experimental investigation of flow hydrodynamics in the vicinity of a submerged circular pier across various submergence levels under only current and wave–current combined flow conditions. The instantaneous velocity data are collected using particle image velocimetry for three distinct frequencies of waves to determine the influence of wave superimposition on the current-induced turbulence parameters. The distribution of phase averaged turbulence quantities, such as mean velocities, Reynolds shear stress, turbulent kinetic energy, flow patterns, and vorticity analysis by Q-criterion, are presented. The results provide insight into the impacts of wave frequency and submergence ratio on the formation of horseshoe vortices, trailing vortex, and reverse flow zones. From the results it is observed that a decrease in the submergence level of the structure causes the formation of strong horseshoe vortices and reverse flow zones in the absence of waves. Also, an increase in wave frequency intensifies the turbulence kinetic energy at the upstream of the pier and eddy generation behind the pier. The present findings highlight the effects of pier submergence and wave characteristics, such as frequency, wave height, and wave period, on flow patterns and turbulent flow characteristics and aid in the design of marine structures. Furthermore, experimental data serve as a valuable resource for validating theoretical or mathematical models related to combined wave–current environments. [ABSTRACT FROM AUTHOR]

Offshore floating wind turbines may undergo swaying motions, resulting in significant changes in the wake wind field characteristics of the wind turbine, which can seriously affect the applicability and accuracy of existing wake models. This paper systematically investigates the turbulent wake flows of floating wind turbine with swaying motion based on an improved actuator disk method (ADM). The improved ADM is introduced to reproduce the wake flows of wind turbines, and the characteristics of the turbulent wind fields (i.e., mean wind velocity, turbulence intensity and Reynolds stress) are verified by wind tunnel tests. Furthermore, the wind fields of a floating wind turbine with different swaying amplitudes under turbulent atmospheric boundary layer are simulated, and the mean wind fields and turbulent statistics are analyzed. The performance of various existing wake models (i.e., Jensen model, modified Jensen model and Gaussian model) are compared, and a Gaussian-Shear wake model is proposed for floating wind turbines, which can account for non-uniform inflow and accommodate different swaying amplitudes. The results indicate that the proposed Gaussian-Shear wake model outperforms the other three models in describing the wake flows of floating wind turbines with swaying motions, which can be used for layout optimization and yaw control of offshore floating wind turbines. [ABSTRACT FROM AUTHOR]

In viscoelastic turbulent wall-bounded flows, the suppression of near-wall vortical structures due to viscoelastic stress significantly reduces both the frictional drag and heat transfer. To investigate the effect of the Prandtl number (P r) on the heat transfer reduction rate (HTR), we conducted a series of direct numerical simulations of passive scalar transport using the finitely extensible nonlinear elastic-Peterlin (FENE-P) model for a viscoelastic turbulent channel flow. Various values of P r from 0.1 to 5.0 were tested at a frictional Reynolds number of 125. The results revealed that the HTR was almost constant for P r ≥ 2.0 at a given drag-reduced flow and was higher than the drag reduction rate, aligning with previous experimental observations. However, in the case of lower- P r fluids (P r ≤ 0.7), the HTR decreased as P r decreased. The variation in the Nusselt number (N u) for P r was examined by decomposing N u into three components: laminar flow contribution, turbulent heat flux contribution, and contribution owing to the deviation in the mean velocity profile from the laminar profile. For lower- P r fluids (P r ≤ 0.7), the contribution of the wall-normal turbulent heat flux was insufficient to achieve the same HTR as that observed for Pr = 5.0. Despite the reduced wall-normal turbulent heat flux in the viscoelastic flows, the instantaneous flow fields showed a substantial similarity in the turbulent structures of the Reynolds shear stress compared to those of the wall-normal turbulent heat flux, which was maintained at various P r values. This was also statistically confirmed through the weighted joint probability density function. [ABSTRACT FROM AUTHOR]

Mechanisms that ultimately lead to the enhanced wall shear stress toward the end of transition to turbulence in a zero-pressure gradient boundary layer are examined for two different transition routes using direct numerical simulations. This paper examines, using a vorticity point of view, late-stage transition mechanisms in roughness induced transition produced by distributed roughness, and a classical transition caused by a large amplitude Tollmien–Schlichting (TS) wave interacting with free stream disturbances adding to the recent insights on discrete roughness induced transition [Suryanarayanan et al., "Roughness induced transition: A vorticity point of view," Phys. Fluids 31(2), 024101 (2019)]. The Reynolds stress is written in terms of vorticity fluxes, and large negative values of the vorticity flux term associated with the correlation of the spanwise velocity and wall-normal vorticity, w ′ ω y ′ ¯ , are observed in the late-stage transition in all cases. A decrease in wall shear stress is observed when near-wall spanwise motion is suppressed, whereas suppression of spanwise motion far away from the wall does not immediately alter wall shear stress; this observation further supports the finding that w ′ ω y ′ ¯ is the dominant term that increases wall shear stress during transition. w ′ ω y ′ ¯ is demonstrated to be correlated with streamwise vorticity near the wall, and this mechanism is illustrated by studying the evolution of a streamwise vortex in a Couette flow. [ABSTRACT FROM AUTHOR]

The precise simulation of full-size wing-body configuration in real flight conditions is still a challenge in computational fluid dynamics in which transition and flow separation are the most crucial issues. To predict these problems robustly by high-order numerical methods, this paper proposes a new transitional Reynolds-stress model, which combines λ -scale (λ = τ 8 )-based SSG (Speziale, Sarkar, and Gatski)/LRR (Launder, Reece, and Rodi) model with γ − R e θ t transition model. Compared with the ω -scale, the λ -scale variable has a natural boundary condition on the wall (helpful for numerical stability) and avoids an additional modification during the transition from laminar to turbulent flow. The T3 series plates with/without pressure gradient, 30P-30N multi-element airfoil, and DLR (German Aerospace Center) 6:1 prolate spheroid are carried out to validate the reliability of the new nine-equation transition model. Furthermore, the new model is applied to the analysis of National Aeronautics and Space Administration juncture flow. Numerical results show that the new transitional model has an obvious advantage in the prediction of Reynolds stresses over the traditional γ − R e θ t SST (shear stress transport) k − ω (k is the turbulence kinetic energy, ω is the specific dissipation rate) model, and then, more physical junction separation as well as transition onset can be obtained. [ABSTRACT FROM AUTHOR]

The particle plume, a ubiquitous particle–fluid coupled phenomenon in tailing discharge from deep-sea mining, undergoes suspension and diffusion over distances transportation. Our study is motivated by predicting plume dispersion patterns driven by different initial momentums, relying on understanding complex fluid–particle interaction mechanics. To consider irregular particle shapes and discrete effects, a discrete element method and large-eddy simulation coupled model is established in our in-house solver to simulate particle plumes and investigate flow characteristics from a Lagrangian perspective. The influence of the initial incident velocity W0 on particle flow regimes, movement patterns, velocity, concentration, Reynold shear stress, fluid–particle interactions, and energy budget is explored. The results show that a counter-rotating vortex pair forms in the initial stage, with ambient fluid entrainment inducing coherent vortex splitting into numerous vortex filaments, causing significant radial diffusion. Plume transportation begins with rapid settling, followed by a decrease to a roughly constant level. Increasing W0 enhances the particle velocity, allowing plumes to advance faster. This results in particle diffusion rate and concentration dilution rate increasing with decreasing W0. Away from the nozzle centerline, negative axial velocity magnitudes increase as W0 decreases, prompting particle radial diffusion. Additionally, for cases with low W0, significant particle concentration in regions far from the nozzle dampens pulsatile velocity, resulting in decreased Reynolds stress with decreasing W0. Notably, despite the complexity of particle–fluid interactions in plumes, the conversion of initial gravitational potential energy into particle and fluid kinetic energy is limited across all W0. [ABSTRACT FROM AUTHOR]

The effect of surface wave on the double-averaged (DA) current-only flow properties is examined over two different types of rough beds comprising hemispherical elements with regular and staggered arrangements. For each case, the relative roughness spacing of p/r = 4 is maintained, where p = pitch distance and r = hemisphere height. The changes in turbulence characteristics against the water depth are analysed for superimposed surface waves propagating along the current with 1 and 2 Hz frequencies. The effect of regular and staggered roughness arrangements on different turbulence characteristics is explored. Results demonstrate that the DA stream-wise velocity decreases and increases at the outer region and within the roughness region, respectively, due to wave propagation. Owing to the surface wave, the DA intensity and form-induced intensity increase above the roughness layer and decrease within the bottom boundary layer. The values of form-induced intensities are smaller (greater) than the DA turbulence intensities at each considered flow depth within the roughness (outer) layer. For the entire flow depth, the values of DA Reynolds stress decrease due to wave interactions along a steady current. Again, the turbulence is observed to be close to isotropic for the current-only flow at the outer zone, while the flow transits to an anisotropic state with wave imposition on current. Further, the computed values of the turbulence indicator and dispersion functions are used for the identification of the level of turbulence and dispersion at each flow depth for the locations upstream and downstream of a hemisphere in the hemispherical bed. [ABSTRACT FROM AUTHOR]

River meanders and channel curvatures play a significant role in sediment motion, making it crucial to predict incipient sediment motion for effective river restoration projects. This study utilized an artificial intelligence method, multiple linear regression (MLR), to investigate the impact of channel curvature on sediment incipient motion at a 180-degree bend. We analyzed 42 velocity profiles for flow depths of 13, 15, and 17 cm in a laboratory flume. The results indicate that the velocity distribution was influenced by the sediment movement threshold conditions due to channel curvature, creating a distinct convex shape based on the bend's position and flow characteristics. Reynolds stress distribution was concave in the upstream bend and convex in the downstream bend, underscoring the bend's impact on incipient motion. Bed Reynolds stress was highest in the first half of the bend (0 to 90 degrees) and lowest in the second half (90 to 180 degrees). The critical Shields parameter at the bend was approximately 8–61% lower than the values suggested by the Shields diagram, decreasing from 0.042 at the beginning to 0.016 at the end of the bend. Furthermore, our findings suggest that the MLR method does not significantly enhance the understanding of sediment movement, highlighting the need for a more comprehensive physical rationale and an expanded dataset for studying sediment dynamics in curved channels. [ABSTRACT FROM AUTHOR]

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A large-eddy simulation was implemented for the flow around a cylindrical observation tower to investigate the effects of the tower structure on wind speed and drag coefficient. The mean wind velocity accelerates above the tower because flow separation occurs at the leading edge of the top of the tower. The drag coefficient is strongly linked to the Reynolds shear stress. Above the tower, the Reynolds shear stresses change from negative to positive within the recirculation zone and return to a negative value in the latter half of the tower because of the steep velocity gradients near the top of the tower. The change in the Reynolds shear stress results in an inaccurate drag coefficient. When one anemometer is used, a location at over 10 m above the top of the tower is suitable for measuring the drag coefficient accurately. When two anemometers are used, the Reynolds shear stress can be measured more accurately. Although the effects of the tower on the drag coefficient are not entirely removed, the use of two anemometers is a promising approach to estimate the drag coefficient in a tower. [ABSTRACT FROM AUTHOR]

AbstractThe present work reports the manipulation of the flow field over different shapes of surface-mounted obstacles using a synthetic jet. A round synthetic jet is placed upstream and downstream of surface-mounted ribs on a flat surface. Particle image velocimetry and hotwire anemometry are used to study the flow field characteristics and features. Heat transfer rate was obtained using transient liquid crystal thermography technique. Experiments are carried out at Reynolds number of 32,000 in a low-speed wind tunnel. Flow field features behind ribs are analyzed using time-averaged velocity, streamline, vorticity Reynolds stress and turbulent intensity. The spanwise average Nusselt number distribution for the downstream rib location is compared for two optimum locations of the synthetic jet relative to the rib. The recirculation bubble length formed downstream of a rib varies with the frequency of the synthetic jet, its location, and the shape of the rib. A drastic reduction in reattachment length is observed at 50 and 80 Hz frequencies for square rib. The highest heat transfer rate is obtained for the square rib at 50 Hz and 4 voltage peak-to-peak excitation amplitude for the upstream location of the synthetic jet relative to the rib. [ABSTRACT FROM AUTHOR]

To provide computational fluid dynamics (CFD)–grade experimental data for studying stratification, measurements on the High-Resolution Jet (HiRJet) facility at the University of Michigan have been conducted with density differences of $1.5\% $ 1.5 % and $ - 1.5\% $ − 1.5 % , respectively. Fluid with a density different from the fluid initially present in the HiRJet tank was injected, and the propagation of the time-dependent density stratification was captured on a two-dimensional plane with the aid of the wire-mesh sensor technique for Reynolds numbers near 5000 and Richardson numbers near 0.29. Direct numerical simulations (DNSs) of the two cases have also been conducted to expand the multifidelity database. The novel experimental and DNS data were then used to assess the predictive capabilities of the Standard $k - \varepsilon $ k − ε (SKE) model and the Reynolds Stress Transport (RST) model. In particular, the propagation speed and thickness of the stratification fronts were assessed by comparing the CFD results against the experimental and DNS data. It was found that the general trends of the stratified density fronts were well predicted by the CFD simulations; however, slight overprediction of the thickness of the stratification layer was found with the SKE model while the RST model gave a larger overprediction of the mixing. Examination of the turbulent statistics showed that the turbulent viscosity was largely overpredicted by the RST model compared to the SKE model. [ABSTRACT FROM AUTHOR]

The present study focuses on the experimental investigation and numerical study using Large Eddy Simulation (LES) for an air-impinging jet. The investigation is carried out for slot jet impingement on the heated cylinder with an adiabatic flat plate beneath the heated cylinder. The parametric investigation is conducted for parameters such as Reynolds number (Re), jet-to-cylinder spacing (h/S), surface curvature (S/D), and plate length (P/D). They are varied as Re = 10,000–25,000; h/S = 4–12, S/D = 0.072–0.108 and P/D = 0.5–2. The Nusselt number distribution over a cylinder is experimentally obtained, and to understand the impact of parameters such as Re, h/S, S/D, and P/D on it, the numerical results are extracted from LES. The Nusselt number improves with increasing Re and decreasing h/S however their impacts are different. The major influence of Re and h/S is noticeable from θ = 0°–180° and θ = 0°–90°; however, the impact of h/S is negligible from θ = 90°–180°. The Nusselt number gets enhanced with increasing S/D due to more air entrainment, which produces higher Reynolds stress and turbulence kinetic energy. The increasing plate length (P/D) shifts flow separation at a lower angular position (θ), thereby increasing the recirculation zone and reducing the attached flow region. The recirculation zone contributes to heat transfer improvement in that region. Hence, a longer plate length P/D = 2 is an ideal selection compared to P/D = 0.5 and 1 as it better facilitates the positioning of cylindrical food products with optimum heat transfer. Further, correlations are proposed for stagnation and average Nusselt numbers in terms of Re, h/S, S/D and P/D. [ABSTRACT FROM AUTHOR]

Experimental investigations of flow past two (one above other) horizontal cylinders have been presented in this paper. The experimental data captured over a rough-bed with double cylinders having three different diameters, were used for the analysis. Firstly, general characteristics such as streamwise velocity, Reynolds shear stress and turbulence intensity profiles have been elucidated at different downstream locations from cylinder position; then some advanced analysis such as length scales, turbulent kinetic energy (TKE) fluxes and budget have been investigated to exhibit their variations. Length scale profiles exhibit higher values in near bed; indicating comparatively larger eddies' downstream of both cylinders. TKE budget profiles indicate highly negative pressure energy diffusion rate together with a sufficient TKE production rate, stabilized by the amplified TKE diffusion and dissipation rates. To determine one of the most prime behaviors of turbulence characteristics, namely TKE dissipation rate more accurately, the concept of structure function has been employed. Primarily velocity gradient method elucidates TKE dissipation rate, then it has been verified by Kolmogorov's two-thirds law using second order structure function and Kolmogorov's − 5/3 law using the method of power spectra. [ABSTRACT FROM AUTHOR]

This study numerically investigates and compares the secondary currents, velocity dips, turbulence properties, and boundary shear stresses in supercritical narrow open channel flows (OCFs) and in narrow duct flows (DFs) using an updated Launder–Reece–Rodi Reynolds stress model in OpenFOAM, which was validated previously for supercritical flows using experimental data. Six steady state simulations were performed at a bulk velocity of 2.31 m/s covering Reynolds numbers from 3.08×105 to 6.16×105 and aspect ratios (width to flow depth) of 1.0. 1.25, and 2.0, which in combination with the observed Froude numbers from 1.65 to 2.33 for OCFs are comparable to sediment bypass tunnel flows. Although free surface produces greater maximum secondary flows, the top wall in DFs creates stronger bulging of the longitudinal velocity above the velocity dips, which generates marginally higher maximum longitudinal velocity and significantly higher velocity fluctuations compared to OCFs. Two pairs of corner vortices are observed in each half width for DFs. However, such vortices differ in OCFs, in which the reduction of aspect ratio develops intermediate vortices. Such differences in the secondary currents are interrelated to the observed variations in the distributions of longitudinal velocity and Reynolds stresses. Higher average bed and sidewall shear stresses are obtained for DFs than for OCFs. The bottom vortices undulate the bed shear stress distributions. Similarly, the sidewall corner vortices (for DFs) or intermediate vortices and inner secondary vortices (for OCFs) undulate the wall shear stress distributions. These undulations are further influenced by the aspect ratio. Moreover, the flow characteristics below the mid depth observed for OCFs are comparable to those obtained for DFs, especially for the square cross sections with aspect ratio of 1.0. [ABSTRACT FROM AUTHOR]

Bioroughness plays an important role in modifying the velocity and sediment flux near the riverbed. It is therefore pertinent to study the influence of benthic fauna on the bed forms. To this end, large-eddy simulations are performed to investigate the influence of the arrays of mounds and their density on the turbulence characteristics in an open-channel flow. The simulated distributions of the time-averaged streamwise velocity and the turbulence intensity are in good agreement with the experimental data. Four numerical simulations are performed with varying streamwise spacings of mounds. Details of the time-averaged and instantaneous flow velocities are analyzed by multiple visualization methods, and the effects of the bioroughness density on the equivalent roughness height and the Darcy–Weisbach friction factor are quantified. The time-averaged flow in the wake of the mounds is characterized by a symmetric pair of vortices. The mounds behave like bluff bodies, increasing the riverbed roughness and heterogeneity in the flow environment. An increase in mound density is to promote the development of secondary currents and to increase the dispersive stress near the bed. The peaks of the Reynolds shear stress distributions decrease in both the streamwise and vertical directions for the high-density case due to a blockage effect. The instantaneous flow features, in the form of various turbulence structures, are generated near the top edge and the wake zone of mounds. The spacing between low-speed streaks decreases with an increase in equivalent roughness height. Multifrequency behavior that is observed is a result of shear layer roll-up from the edges of mounds and the flapping of wake. Finally, two formulas for equivalent roughness height and Darcy–Weisbach friction factor are proposed involving the bioroughness density and height. The findings demonstrate the effects of the bioroughness on the near-bed turbulence characteristics and sediment stability. Practical Applications: Bed forms produced by aquatic benthos are usually of small scale but with a large abundance. We performed a flume experiment as well as large-eddy simulations (a high-fidelity numerical simulation approach) to answer whether these microtopographies are important when studying flow and transport processes near the riverbeds. Four numerical simulations are performed with varying mound density. It was found that the microtopographies created by benthos indeed change the near-bed turbulence characteristics and sediment stability. Specifically at high abundances, benthos-induced bed forms could raise the bed roughness and the friction factor eightfold and threefold, respectively. This suggests the necessity of a modified roughness height formula and a friction coefficient formula based on the benthos abundance, which has been carried out in this study. Thus, engineers who are concerned with the flow and sediment transport in habitats of high-abundance benthos (e.g., wetlands and estuaries) may prefer to use these modified formulas and to develop a more accurate model of sediment transport. [ABSTRACT FROM AUTHOR]

It is known that examining turbulence effects on medical devices has an important effect in design and optimization of blood-contacting devices. CFD has been commonly used on prosthetic heart valves, stents, and Ventricular Assist Devices (VADs) in both the design process and also on hemodynamics of the flow characteristics. In this study, flows in the FDA nozzle were modeled to examine Reynolds stresses in the whole domain. The flow behavior was determined by applying the Reynolds-Averaged Navier-Stokes model of turbulence (k-ω SST) to simulate five distinctive experimental cases in the nozzle taken from the literature. The Reynolds stress frequencies are determined for the five different experimental conditions. Results showed that the highest velocity case (corresponding throat Reynolds number of 6500) has much higher Reynolds stresses with a high number of frequencies. However, the lowest velocity case has very small Reynolds numbers in a very high frequency. When different parts of the nozzle were examined, the Reynolds stress values showed more fluctuations for the higher velocities and more regular profiles for the lower velocity cases. [ABSTRACT FROM AUTHOR]

The boundary layer's separation loss in compressor cascades constitutes a significant portion of profile loss, critically influencing aerodynamic performance optimization and control. This study employs Large Eddy Simulation (LES) to examine separation losses at varying attack angles, focusing on a rectangular compressor cascade. Specifically, it explores the long separation bubble at a 45% blade height cross-section under designed incidence. Analysis of the separation bubble's transition process revealed a notable surge in total pressure loss rate prior to transition, which stabilized following reattachment. The study thoroughly investigates the evolution of long bubbles, employing quadrant analysis of Reynolds stress, critical point theory, and an in-depth examination of individual vortex dynamics. The findings indicate that the peak of cross-flow within the separation bubble acts as the primary mechanism initiating the transition. This insight is corroborated by DNS calculations of natural transitions on flat plates. Building upon these findings, the study discusses the effects of varying attack angles on transition processes. Notably, increased incidence prompted the upstream migration of the long separation bubble, transforming it into a short bubble at the leading edge. This shift led to a fivefold increase in separation loss and doubled the frequency of transverse flow fluctuations. [ABSTRACT FROM AUTHOR]

Sediment dredging has been an ever‐growing issue, especially in developing nations with high demand for concrete filler material. River systems are adversely affected by sediment mining, resulting in decreased stability of the riverbed and riverbanks. Nature‐based solutions for riverbank erosion have been a research topic that has led to the proposal of vegetation on the riverbanks. However, little is known about the extent of riverbank vegetation required to negate the devastating effects of sediment mining because dense vegetation severely affects the flow structure and becomes a waste trap. This experimental study uses sparsely dense, flexible, and bladed vegetation to study the annulment effects of vegetation against the existing mining pit. Near‐bed turbulence and sediment transport have increased in the test section in the presence of a mining pit. The increase in near‐bed streamwise and transverse Reynolds shear stresses helped us understand the increased sediment movement in streamwise and lateral directions. The morphology of the test section showed increased riverbed erosion at the beginning of the test segment. The entire cross‐section was levelled at the end of the test section, and aggradation was downstream of the test segment. In contrast, in the vegetated riverbank case, the initial profile of the bank was almost unchanged for the same discharge of flow and upstream sand pit. The sparse vegetation overperformed the intended negation effects. This study establishes that sparse vegetation would perform better in maintaining the channel morphology, which otherwise in dense vegetation would have faced a high erosion rate in the main channel while giving the same protection to the riverbanks. [ABSTRACT FROM AUTHOR]

Being a direct computational aeroacoustics method, Lattice Boltzmann method (LBM) has great potential and broad application perspective in the field of numerical simulation of aerodynamic noise due to its low dispersion and low dissipation. A series of numerical algorithms and the related improvements based on the standard LBM method are proposed and developed in this paper to adapt to the airfoil noise calculation with complex grid at middle‐high Reynolds number. First, a new mathematical‐boundary‐recognition algorithm based on Green's formula is proposed to deal with complex curved geometric models, which is validated by three‐element airfoil 30P30N benchmark. Then, in order to reduce grid redundancy and improve computing efficiency, the grid refinement technique of domain decomposition model (DDM) is adopted and also improved, which is verified by calculating the flow and sound fields around 2D and 3D cylinders at Reynolds number equal to 90,000. Finally, three different LES turbulence models are combined with the standard MRT‐LBM method, where different finite difference schemes are used to solve Reynolds stress tensor which is different from the traditional one. Through the direct acoustic numerical simulation of NACA0012 airfoil at Reynolds number equal to 200,000, the effects of Smagorinsky models and Wall‐adapting local eddy‐viscosity (WALE) model on aerodynamic noise prediction are compared and analyzed. Overall, the proposed methodology is shown to be appropriate for predicting the aerodynamic noise at low Mach number and can successfully simulate the generation and propagation of far field acoustics. [ABSTRACT FROM AUTHOR]

The long lasting demand for better turbulence models and the still prohibitively computational cost of high‐fidelity fluid dynamics simulations, like direct numerical simulations and large eddy simulations, have led to a rising interest in coupling available high‐fidelity datasets and popular, yet limited, Reynolds averaged Navier–Stokes simulations through machine learning (ML) techniques. Many of the recent advances used the Reynolds stress tensor or, less frequently, the Reynolds force vector as the target for these corrections. In the present work, we considered an unexplored strategy, namely to use the modeled terms of the Reynolds stress transport equation as the target for the ML predictions, employing a neural network approach. After that, we solve the coupled set of governing equations to obtain the mean velocity field. We apply this strategy to solve the flow through a square duct. The obtained results consistently recover the secondary flow, which is not present in the baseline simulations that used the κ−ϵ$$ \kappa -\epsilon $$ model. The results were compared with other approaches of the literature, showing a path that can be useful in the seek of more universal models in turbulence. [ABSTRACT FROM AUTHOR]

Summary: To robustly and accurately simulate wall‐bounded turbulent flows at high Reynolds numbers, we propose suitable boundary treatments for wall‐modeled large‐eddy simulation (WMLES) coupled with a high‐order flux reconstruction (FR) method. First, we show the need to impose an auxiliary boundary condition on auxiliary variables (solution gradients) that are commonly introduced in high‐order discontinuous finite element methods (DFEMs). Auxiliary boundary conditions are introduced in WMLES, where the grid resolution is too coarse to resolve the inner layer of a turbulent boundary layer. Another boundary treatment to further enhance stability with under‐resolved grids, is the use of a modal filter only in the wall‐normal direction of wall‐adjacent cells to remove the oscillations. A grid convergence study of turbulent channel flow with a high Reynolds number (Reτ≈5200$$ R{e}_{\tau}\approx 5200 $$) shows that the present WMLES framework accurately predicts velocity profiles, Reynolds shear stress, and skin friction coefficients at the grid resolutions recommended in the literature. It was confirmed that a small amount of filtering is sufficient to stabilize computation, with negligible influence on prediction accuracy. In addition, non‐equilibrium periodic hill flow with a curved wall, including flow separation, reattachment, and acceleration at a high Reynolds number (Reh≈37,000$$ R{e}_h\approx 37,000 $$), is reported. Considering stability and the prediction accuracy, we recommend a loose auxiliary wall boundary conditions with a less steep velocity gradient for WMLES using high‐order DFEMs. [ABSTRACT FROM AUTHOR]

Blind tees, as important junctions, are widely used in offshore oil and gas transportation systems to improve mixing flow conditions and measurement accuracies in curved pipes. Despite the significance of blind tees, their unsteady flow characteristics and mixing mechanisms in turbulent flow regimes are not clearly established. Therefore, in this study, Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations, coupled with Explicit Algebraic Reynolds Stress Model (EARSM), are employed to explore the complex turbulent flow characteristics within blind-tee pipes. Firstly, the statistical flow features are investigated based on the time-averaged results, and the swirl dissipation analysis reveals an intense dissipative process occurring within blind tees, surpassing conventional elbows in swirling intensity. Then, the instantaneous flow characteristics are investigated through time and frequency domain analysis, uncovering the oscillatory patterns and elucidating the mechanisms behind unsteady secondary flow motions. In a 2D-length blind tee, a nondimensional dominant frequency of oscillation (Stbt = 0.0361) is identified, highlighting the significant correlation between dominant frequencies inside and downstream of the plugged section, which emphasizes the critical role of the plugged structure in these unsteady motions. Finally, a power spectra analysis is conducted to explore the influence of blind-tee structures, indicating that the blind-tee length of lbt = 2D enhances the flow-mixing conditions by amplifying the oscillation intensities of secondary flow motions. [ABSTRACT FROM AUTHOR]

In this study, both the optical and piezoelectric properties of polyvinylidene fluoride (PVDF) electrospun nanofibers were investigated at different needle-to-collector distances of the electrospinning process at constant applied high voltage. For piezoelectric characterization, the fabricated nanofiber mats were subjected to applied forces, including cyclic force, variable frequency-based loads, and free-falling masses (impulse loading), along with power density analysis for different load resistance values. In addition, both optical absorbance and transmittance measurements were conducted to evaluate the optical properties of the fabricated nanofibers. The piezoelectric analysis demonstrated the best piezoresponse of the fabricated nanomats at a needle-to-collector distance of 15 cm and high voltage of 22 kV. However, a trade-off between piezoelectric response and optical transmissivity was observed based on the electrospinning distance parameter. The relatively higher optically transparent sample exhibited only moderate piezoelectric response, while the less transparent sample displayed the highest piezoelectric activity. Based on the optimized sample and piezoelectric analysis, the synthesized nanofiber mat was subjected to applied mechanical stress in the form of variable velocity and momentum loads. A maximum potential of approximately 16 V was harvested through velocity and momentum impact, especially with the addition of a double-layer PVDF membrane. Furthermore, the sensing effect of airflow pressure on single/double-layer PVDF was studied. The single-layer PVDF membrane generated 79 mV under an airflow speed of 21 km/h, while the double-layer membrane produced 114 mV potential under the same airflow. This study highlights the diverse applications of PVDF nanofiber mats as multifunctional sensors and energy harvesting applications from mechanical shocks and airflow impact. [ABSTRACT FROM AUTHOR]

The internal flow scale of a Pelton turbine is variable, and the interaction between the jet and the bucket has strong transient characteristics, resulting in an incomplete understanding of its internal vortex structure evolution and energy dissipation mechanisms. In order to reveal the influence of vortices on the flow regime and energy dissipation mechanisms in the turbine, this paper establishes the correlation between the unsteady flow characteristics and energy dissipation inside the Pelton turbine based on the energy balance equation and quantifies the energy losses inside the turbine. The results indicate that vortex structures present a non-uniform distribution inside the jet, which disrupts the uniformity of the jet velocity distribution, resulting in an uneven distribution of high-vorticity zones on the bucket surfaces and intensifying flow interference among the buckets. The strong shear flow caused by the downstream flow detachment of the needle guide, the turbulent boundary layer on the jet surface, and the wake effect downstream of the needle tip are the main reasons for the dissipation of fluid kinetic energy. The distribution range of energy loss on the rotating bucket closely corresponds to the position of the high-vorticity zone. Energy dissipation inside the turbine is primarily in the form of turbulent kinetic energy, and the injectors and runner are the primary energy dissipation components. Moreover, inside the injectors, each form of energy loss remains relatively constant, whereas inside the runner, its rotation induces fluctuating energy losses attributed to Reynolds stress work. The results contribute to an enhanced understanding of the energy dissipation characteristics and complex flow mechanisms within the turbine, providing reference for the optimisation and efficient operation of the multi-nozzle Pelton turbine. [ABSTRACT FROM AUTHOR]

Iron ore pellet production processes produce large amounts of CO2 emissions when burning fossil fuels. One example is the grate-kiln process, where normally a coal flame is used to indurate pellets. To mitigate the emissions, coal used in rotary kilns can be replaced with for example hydrogen. However, the fuel is mixed with secondary process air, and replacing a solid fuel with a gaseous one changes the mixing characteristics. This demands different means of injecting the fuel. A coaxial jet can be used to control the mixing of fuel and secondary air, as well as the flow field. The aim is to control the mixing of hydrogen and secondary air to achieve a hydrogen flame that is similar to the reference coal flame. This study numerically investigates different coaxial jet configurations. Steady-state simulations of a simplified model of the real kiln are performed using a Reynolds Stress turbulence model. Results show that decreasing the momentum flow ratio between the outer and inner jet, Mjet, to a certain value delays the spread of the hydrogen jet and thus gives a longer jet. Further decreasing this ratio gives an even longer jet, but has the side effect of producing recirculation of hydrogen. [ABSTRACT FROM AUTHOR]

Direct numerical simulations of spiral Poiseuille flows in a narrow gap geometry are performed with the aim of identifying the mechanisms governing the dynamics of the axial friction coefficient. The investigation has explored a small portion of the Reynolds number–Taylor number phase space ($600 \leq Re \leq 5766$ and $1500 \leq Ta \leq 5000$), for which reference experimental results are available. The study is focused on the mechanism leading to the enhancement of the axial friction coefficient with the Taylor number when the Reynolds number is kept constant. The analysis of the spatial distribution of the Reynolds stress tensor and of the turbulent energy budget has evidenced the key role of the pressure–strain correlation in the energy transfer from the azimuthal to the axial component. The latter eventually determines the increase of the axial friction coefficient through the enhanced radial mixing of axial momentum. Data have also shown that the flow dynamics is heavily dependent on the $Ta/Re$ ratio, and different regimes develop (ranging from laminar to turbulent), each with peculiar behaviours. [ABSTRACT FROM AUTHOR]

Zero-plane is an important reference level included in the logarithmic velocity profile. The prediction of the zero-plane displacement remains challenging, especially in open-channel flows with submerged vegetation. In this study, we first review the existing formulas for estimating the zero-plane displacement. Then, we conducted a series of laboratory experiments of open-channel flows with submerged rigid vegetation, by varying flow rate, water depth, vegetation density, and stem diameter. By utilizing particle image velocimetry (PIV) technique, the spatio-temporal averaged (double-averaged) flow velocity and Reynolds shear stress profiles were obtained. The data analysis shows that the zero-plane displacement is almost proportional to the surface layer depth for the shallow flow cases. As the flow depth increases, the zero-plane displacement tends to be constant and increases with decreasing vegetation density. This result substantiates the hypothesis that the zero-plane displacement is related to large-scale eddies. Lastly, a heuristic model is developed to explain possible variations of the large-scale eddies, which yields a formula for predicting the zero-plane displacement for flows subjected to submerged rigid vegetation. [ABSTRACT FROM AUTHOR]

Flexible canopy flows are often encountered in natural scenarios, e.g. when crops sway in the wind or when submerged kelp forests are agitated by marine currents. Here, we provide a detailed characterisation of the turbulent flow developed above and between the flexible filaments of a fully submerged dense canopy and we describe their dynamical response to the turbulent forcing. We investigate a wide range of flexibilities, encompassing the case in which the filaments are completely rigid and standing upright as well as that where they are fully compliant to the flow and deflected in the streamwise direction. We are thus able to isolate the effect of the canopy flexibility on the drag and on the inner–outer flow interactions, as well as the two flapping regimes of the filaments already identified for a single fibre. Furthermore, we offer a detailed description of the Reynolds stresses throughout the wall-normal direction resorting to the Lumley triangle formalism, and we show the multi-layer nature of turbulence inside and above the canopy. The relevance of our investigation is thus twofold: the fundamental physical understanding developed here paves the way towards the investigation of more complex and realistic scenarios, while we also provide a thorough characterisation of the turbulent state that can prove useful in the development of accurate turbulence models for RANS and LES. [ABSTRACT FROM AUTHOR]

We investigate Reynolds number effects in strong shock-wave/turbulent boundary-layer interactions (STBLI) by leveraging a new database of wall-resolved and long-integrated large-eddy simulations. The database encompasses STBLI with massive boundary-layer separation at Mach $2.0$ , impinging-shock angle $40^{\circ }$ and friction Reynolds numbers ${\textit {Re}}_\tau$ $355$ , $1226$ and $5118$. Our analysis shows that the shape of the reverse-flow bubble is notably different at low and high Reynolds number, while the mean-flow separation length, separation-shock angle and incipient plateau pressure are rather insensitive to Reynolds number variations. Velocity statistics reveal a shift in the peak location of the streamwise Reynolds stress from the separation-shock foot to the core of the detached shear layer at high Reynolds number, which we attribute to increased pressure transport in the separation-shock excursion domain. Additionally, in the high Reynolds case, the separation shock originates deep within the turbulent boundary, resulting in intensified wall-pressure fluctuations and spanwise variations associated with the passage of coherent velocity structures. Temporal spectra of various signals show energetic low-frequency content in all cases, along with a distinct peak in the bubble-volume spectra at a separation-length-based Strouhal number $St_{L_{sep}}\approx 0.1$. The separation shock is also found to lag behind bubble-volume variations, consistent with the acoustic propagation time from reattachment to separation and a downstream mechanism driving the shock motion. Finally, dynamic mode decomposition of three-dimensional fields suggests a Reynolds-independent statistical link among separation-shock excursions, velocity streaks and large-scale vortices at low frequencies. [ABSTRACT FROM AUTHOR]

AbstractIn order to grasp the influence of exhaust pipe size on cyclone flow fields and improve the recovery and utilization rate of coal powder in power plant, the flow characteristics and the separation capacity of the fine coal powder separator in power plant were investigated through numerical simulation. The Reynolds stress model was employed to model the turbulent flow. The Eulerian-Lagrangian computational procedure was used to predict particles tracking in the cyclone. The particle trajectories were simulated using the discrete random walk. Five cyclone types with different insertion lengths of the exhaust pipe at the same other geometric dimensions were considered. It was found that tangential velocity played a dominant role in the radial distribution of particles, which was mainly represented by Rankine vortex. When the insertion length of the exhaust pipe was increased from 1638 mm to 5100 mm, the grade efficiency of 5–12 μm particles was increased by 19%, and the pressure drop reduced by 8%. [ABSTRACT FROM AUTHOR]

This study investigating the accuracy of turbulence model simulations of the exhaust manifold using computational fluid dynamics (CFD) carries significant implications. By modeling and analyzing the flow of emissions, we aim to identify areas of high stress and pressure, minimize the pressure drop, and maximize the flow of exhaust gases. This not only enhances engine performance, reduces emissions, and improves the durability of the manifold but also provides a unique opportunity to predict and analyze the flow and performance of the exhaust manifold, both quantitatively and qualitatively. This paper aims to provide a detailed comparison of five turbulence models that are commonly used in CFD to offer valuable insights into their accuracy and reliability in predicting the flow characteristics of exhaust gases. The results show that the k-kl-ω model showed the highest maximum velocity and the most comprehensive temperature range, efficiently capturing the transitional flow effects. The K-ω STD and SST transition models displayed significantly higher turbulent kinetic energy (TKE) values, indicating their enhanced effectiveness in modeling complex turbulent and transitional flows. Conversely, the Reynolds stress and RNG k-epsilon models displayed lower TKE values, suggesting more subdued turbulence predictions. Despite this, all models exhibited similar pressure drop trends, with a noticeable increase near the midpoint of the manifold. These quantitative findings provide valuable insights into the suitability of different turbulence models for optimizing exhaust manifold design. [ABSTRACT FROM AUTHOR]

Featured Application: This study demonstrates the application of high-order WENO-ZQ schemes in aerodynamic simulations of aircraft engine components. By integrating WENO-ZQ into NUAA-Turbo2.0 and employing RANS and hybrid RANS/LES turbulence models, we conducted detailed computations and simulations of aircraft engine compressors, high-pressure turbines, and turbine film cooling. Using WENO-ZQ enhances simulation accuracy and efficiency, effectively capturing critical flow details such as turbine wakes and film cooling distribution. This work highlights the importance of advanced numerical schemes in guiding the design and analysis of modern turbomachinery. In numerical simulations, achieving high accuracy without significantly increasing computational cost is often a challenge. To address this issue, this paper proposes an improved finite volume Weighted Essentially Non-Oscillatory (WENO) scheme for structured grids. By employing a single-point quadrature rule to perform flux integration on the control volume faces, this scheme is designed for use in NUAA-Turbo three-dimensional fluid solvers based on structured grids, utilizing RANS and RANS/LES coupling to simulate turbomachinery flows. Firstly, the new WENO scheme is validated against classical numerical test cases to evaluate its stability and reliability in handling discontinuities, double Mach reflection problems, and Rayleigh–Taylor (RT) instability. Compared to the original scheme, this improved finite-volume WENO scheme demonstrates better stability near discontinuities and more effectively resolves flow features at the same grid resolution. Next, for engineering applications related to turbomachinery, such as compressor and turbine characteristics, calculations using RANS are performed and the results obtained with WENO-ZQ3 and WENO-JS3 are compared. Finally, the new fifth-order WENO scheme is applied to RANS/LES coupling simulations of turbine wake and film cooling. The results indicate that the improved finite-volume WENO scheme provides better stability and accuracy in engineering applications. For instance, the average error in calculating compressor efficiency characteristics is reduced from 0.76% to 0.05%, the error in turbine vane pressure distribution compared to the experimental values is within 1%, and the error in film cooling efficiency centerline distribution compared to the experimental values is within 3%. Additionally, the qualitative results of turbine wake and film cooling show that even with a small number of grid points, more detailed flow physics can be captured, thereby reducing computational costs in aerodynamic applications. [ABSTRACT FROM AUTHOR]

In the present study, we performed direct numerical simulations for a hypersonic turbulent boundary layer over the windward side of a lifting body, the HyTRV model, at Mach number 6 and attack angle 2° to investigate the global and local turbulent features, and evaluate its difference from canonical turbulent boundary layers. By scrutinizing the instantaneous and averaged flow fields, we found that the transverse curvature on the windward side of the HyTRV model induces the transverse opposing pressure gradients that push the flow on both sides towards the windward symmetry plane, yielding significant effects of the azimuthal inhomogeneity and large-scale cross-stream circulations, moderate and azimuthal independent influences of adverse pressure gradient, and negligible impact of the mean flow three-dimensionality. Further inspecting the local turbulent statistics, we identified that the mean and fluctuating velocity become increasingly similar to the highly decelerated turbulent boundary layers over flat plates in that the mean velocity deficit is enhanced, and the outer layer Reynolds stresses are amplified as it approaches the windward symmetry plane, and prove to be self-similar under the scaling of Wei & Knopp (J. Fluid Mech., vol. 958, 2023, A9) for adverse-pressure-gradient turbulent boundary layers. Conditionally averaged Reynolds stresses based on strong sweeping and ejection events demonstrated that the Kelvin-Helmholtz instability of the strong embedded shear layer induced by the large-scale cross-stream circulations is responsible for the turbulence amplification in the outer layer. The strong Reynolds analogy that relates the mean velocity and temperature was refined to incorporate the non-canonical effects, showing considerable improvements in the accuracy of such a formula. On the other hand, the temperature fluctuations are still transported passively, as indicated by their resemblance to the velocity. The conclusions obtained in the present study provide potentially profitable information for turbulent modelling modification for the accurate predictions of skin friction and wall heat transfer. [ABSTRACT FROM AUTHOR]

Although the roll-streak (R-S) is fundamentally involved in the dynamics of wall turbulence, the physical mechanism responsible for its formation and maintenance remains controversial. In this work we investigate the dynamics maintaining the R-S in turbulent Poiseuille flow at R = 1650. Spanwise collocation is used to remove spanwise displacement of the streaks and associated flow components, which isolates the streamwise-mean flow R-S component and the second-order statistics of the streamwise-varying fluctuations that are collocated with the R-S. This partition of the dynamics into streamwise-mean and fluctuation components facilitates exploiting insights gained from the analytic characterization of turbulence in the second-order statistical state dynamics (SSD), referred to as S3T, and its closely associated restricted nonlinear dynamics (RNL) approximation. Symmetry of the statistics about the streak centreline permits separation of the fluctuations into sinuous and varicose components. The Reynolds stress forcing induced by the sinuous and varicose fluctuations acting on the R-S is shown to reinforce low- and high-speed streaks, respectively. This targeted reinforcement of streaks by the Reynolds stresses occurs continuously as the fluctuation field is strained by the streamwise-mean streak and not intermittently as would be associated with streak-breakdown events. The Reynolds stresses maintaining the streamwise-mean roll arise primarily from the dominant proper orthogonal decomposition (POD) modes of the fluctuations, which can be identified with the time average structure of optimal perturbations growing on the streak. These results are consistent with a universal process of R-S growth and maintenance in turbulent shear flow arising from roll forcing generated by straining turbulent fluctuations, which was identified using the S3T SSD. [ABSTRACT FROM AUTHOR]

We present data from direct numerical simulation (DNS) of the fully turbulent flow through nominally two-dimensional channels containing longitudinal, surface-mounted, rectangular ribs whose widths (W) are either one third of or equal to the gap (S - W) between consecutive ribs across the domain, where S is the span (centre-to-centre spacing) of the ribs. A range of the ratio of channel half-height (H) to span (S) is considered, covering 0.25 = H/S = 2.5. In each case, a fixed rib height (h) of 0.1H was used, but a number of cases with much smaller heights, h/H = 0.025 or 0.05, were also studied. The secondary flows resulting from the presence of the ribs are examined, along with their sources in terms of the axial vorticity transport equation, which highlights the effects of spanwise inhomogeneity in the Reynolds stresses. We show that the strength of the secondary flows depends strongly on H/S (and, correspondingly, on W/S) and that the major sources of axial vorticity arise near the top corners of the ribs, with convection of that vorticity dominating its spread. We also show that for smaller ribs, the secondary flow strengths are similar to those predicted by Zampino et al. (J. Fluid Mech., vol. 944, 2022, A4) using a linearised model of the Reynolds-averaged equations, which does not include the vorticity convection process; the behaviour of secondary flow topology and strength with varying W/H is thus noticeably different. [ABSTRACT FROM AUTHOR]

The presentation and modeling of turbulence anisotropy are crucial for studying large-scale turbulence structures and constructing turbulence models. However, accurately capturing anisotropic Reynolds stresses often relies on expensive direct numerical simulations (DNS). Recently, a hot topic in data-driven turbulence modeling is how to acquire accurate Reynolds stresses by the Reynolds-averaged Navier-Stokes (RANS) simulation and a limited amount of data from DNS. Many existing studies use mean flow characteristics as the input features of machine learning models to predict high-fidelity Reynolds stresses, but these approaches still lack robust generalization capabilities. In this paper, a deep neural network (DNN) is employed to build a model, mapping from tensor invariants of RANS mean flow features to the anisotropy invariants of high-fidelity Reynolds stresses. From the aspects of tensor analysis and input-output feature design, we try to enhance the generalization of the model while preserving invariance. A functional framework of Reynolds stress anisotropy invariants is derived theoretically. Complete irreducible invariants are then constructed from a tensor group, serving as alternative input features for DNN. Additionally, we propose a feature selection method based on the Fourier transform of periodic flows. The results demonstrate that the data-driven model achieves a high level of accuracy in predicting turbulence anisotropy of flows over periodic hills and converging-diverging channels. Moreover, the well-trained model exhibits strong generalization capabilities concerning various shapes and higher Reynolds numbers. This approach can also provide valuable insights for feature selection and data generation for data-driven turbulence models. [ABSTRACT FROM AUTHOR]

The massive flow separation in the flow around a circular cylinder is challenging for the large-eddy simulation (LES) using the traditional equilibrium wall model (EQWM) for accurate prediction. To address this problem, a data-driven-non-equilibrium wall model (DNEQWM) was developed based on the result of the high-fidelity wall-resolved LES (WRLES) and the theoretical analysis. A hybrid modeling strategy was adopted in DNEQWM to deal with different flow regions. An empirical formula based on the analysis of the WRLES result was used to compute the wall shear stress in the attached region while the integration of the Navier–Stokes (N–S) equation was used in the separated region. Both EQWM and DNEQWM were applied to the LES of the flow around a circular cylinder at a classical Reynolds number of 3900 to evaluate the performance of the new model. It was found that DNEQWM was significantly superior to EQWM based on the analyses of the results of global flow quantities, surface pressure distributions, and flow details of mean and fluctuation velocities and the Reynolds stress in the wake. Flow visualizations indicated that DNEQWM can effectively reproduce the phenomenon of alternative periodic vortex shedding in the wake. The computational cost of DNEQWM was slightly lower than that of EQWM and significantly less than that of WRLES. This study presents a practical methodology for the wall model for the LES of the flow around the bluff body with smooth curved surfaces. [ABSTRACT FROM AUTHOR]

This paper presents an outer velocity scale and an inner scaling law of Reynolds shear stresses that are valid for the entire adverse pressure gradient (APG) region before separation, based on asymptotic expansions in turbulent boundary layers (TBLs). By reexamining the velocity defect and Reynolds shear stress in the outer region of TBLs, it is demonstrated that the pressure gradient effect can be included in asymptotic expansions. The mathematical requirements to be satisfied by the outer velocity scale are given. On this basis, we define a new outer velocity scale Uo, which is used in the mean momentum balance analysis over the entire APG region. The results show that there is a slowly evolving region of the Reynolds shear stress gradient in APG TBLs. We obtain a logarithmic decay law for the Reynolds stress gradient by asymptotic matching in this region, thus providing functional expressions for the total shear stress and total shear stress gradient in the inner region of TBLs. These expressions are valid for the entire APG region. Furthermore, we use the new velocity scale to determine three self-similar parameters in the momentum equation to discuss the conditions for approximate equilibrium. The changes in parameters point to changes in two physical properties of APG TBLs, the inertial force and turbulence production. [ABSTRACT FROM AUTHOR]