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The primary aim of this work is to investigate the impact of the offset jet width on the unsteady flow characteristics of a turbulent dual jet, which consists of a wall jet and an offset jet. A computational fluid dynamics code is developed to solve the unsteady Reynolds-averaged Navier–Stokes (URANS) equations. The width of the offset jet is varied while keeping the width of the wall jet constant at the separation distance between the two jets. When the ratio of the offset jet width (w) to the separation distance (d) is w / d = 0. 5 , the flow field exhibits a periodic vortex shedding phenomenon. Conversely, when w / d = 0. 4 , the flow field remains steady. The shedding phenomenon is discernible even when w / d = 2. The instantaneous velocity components display sinusoidal oscillations at 0. 5 ≤ w / d ≤ 2. Applying the fast Fourier transform to these sinusoidal signals yields a distinct frequency peak at the vortex shedding frequency. Within the range of 0. 5 ≤ w / d ≤ 2 , the shedding frequency decreases as the width of the offset jet increases. This trend continues until it reaches a constant value at w / d = 1. 4. This indicates that the width of the offset jet has a notable influence on the shedding phenomenon within the range of 0. 5 ≤ w / d ≤ 1. 4. For 1. 4 < w / d ≤ 2 , the shedding frequency remains unaffected by the offset jet width variation. Depending on the value of w / d , the shedding phenomenon is characterized by three flow regimes: a steady flow regime (for w / d ≤ 0. 4), an outer share layer-influenced shedding regime (for w / d = 0. 5 − 1. 4), and an outer shear layer-free shedding regime (for w / d > 1. 4). [ABSTRACT FROM AUTHOR]

Accurate prediction of wind pressures on buildings is crucial for designing safe and efficient structures. Existing computational methods, like Reynoldsaveraged Navier-Stokes (RANS) simulations, often fail to predict pressures accurately in separation zones. This study proposes a novel deep-learning methodology to enhance the accuracy and performance of eddy viscosity modeling within RANS turbulence closures, particularly improving predictions for bluff body aerodynamics. A deep learning model, trained on large eddy simulation (LES) data for various bluff body geometries, including a flat-roof building and forward/backward facing steps, was used to adjust eddy viscosity in RANS equations. The results show that incorporating the machine learningpredicted eddy viscosity significantly improves agreement with LES results and experimental data, particularly in the separation bubble and shear layer. The deep learning model employed a neural network architecture with four hidden layers, 32 neurons, and tanh activation functions, trained using the Adam optimizer with a learning rate of 0.001. The training data consisted of LES simulations for forward/backward facing steps with width-to-height ratios ranging from 0.2 to 6. The study reveals that the machine learning model achieves a balance in eddy viscosity that delays flow reattachment, leading to more accurate pressure and velocity predictions than traditional turbulence closures like k-ω SST and k-ε. A sensitivity analysis demonstrated the pivotal role of eddy viscosity in governing flow separation, reattachment, and pressure distributions. Additionally, the investigation underscores the disparity in eddy viscosity values between RANS and LES models, highlighting the need for enhanced turbulence modeling. The findings presented in this paper offer substantive insights that can inform the advancement of more dependable computational methodologies tailored for engineering applications, encompassing wind load considerations for structural design and the intricate dynamics of unsteady aerodynamic phenomena. [ABSTRACT FROM AUTHOR]

River confluences contribute to the outflux of saturated dissolved gases in the water resulting from high dam discharges. This process is related to gas transfer across the water–air interface, which is primarily controlled by turbulent dissipation near the water surface. However, the near-surface turbulence dissipation is rarely reported in confluence hydrodynamics studies. This study conducted experiments with different discharge ratios to investigate near-surface turbulent motions at a laboratory-scale confluence. The higher dissipation rate ε H / U m 3 of near-surface turbulence was mainly located inside the interfacial shear layer between the two incoming streams (~ 10–4) and the bank separation zone (10–4–10–3) where high shear was found in the mean flow. By contrast, the dissipation rates were much lower inside the incoming flows and outside the two regions of high shear (~ 10–5). The magnitudes of the dissipation rate inside the shear layer were comparable in experiments where the mixing interface was in the Kelvin–Helmholtz mode or in the wake mode. The dissipation rate was found to increase away from the free surface outside the shear layer, while it was more uniformly distributed over the depth inside the layer possibly due to the presence of strongly-coherent, vertically-orientated vortices. In the far field, the mean shear within the shear layer was largely weakened. Nonetheless, the effects of flow separation persisted and laterally expanded to occupy the entire cross section. The dissipation rate ε H / U m 3 of the confluent flow was more than 10–4 even at a distance of 10 times the channel width in the post-confluence channel. Article Highlights: Near-surface dissipation rate largely increased in the post-confluence, and its effects persisted at a distance of > 10W. Flow separation and flow mixing interface were the dominant flow features that increase the near-surface dissipation. The magnitudes of the dissipation rate inside the shear layer were comparable in spite of different shear layer modes. [ABSTRACT FROM AUTHOR]

A mixing layer (ML) forms when two streams of different speeds or densities merge. MLs are ubiquitous in nature and can be often observed in the atmosphere, ocean, rivers, canals, lakes and reservoirs. This review paper focuses on the turbulent MLs developing in open-channel flows when the vertical size of the ML is smaller than its streamwise and spanwise dimensions. Such MLs are referred to as shallow MLs (SMLs). The SMLs often involve large-scale features such as quasi-two-dimensional coherent structures with a bed-normal axis, streamwise-oriented vortices, secondary currents, gravity currents, and bed-induced turbulent structures such as large- and very-large-scale motions. Considering various types of SMLs, we distinguish SMLs driven by (i) spanwise inhomogeneity of hydrodynamic parameters, (ii) lateral changes in flow resistance, and (iii) spanwise heterogeneity in fluid density. As SMLs and associated flow structures largely control transverse exchanges of various substances (e.g., sediments, pollutants, nutrients) and heat, the mixing of substances and thermal mixing are also addressed. Then, commonalities and differences among the various types of SMLs are identified. The paper is concluded with suggestions on future research efforts for advancing the knowledge on SMLs and capabilities for their predictions and control. [ABSTRACT FROM AUTHOR]

Accurate prediction of wind pressures on buildings is crucial for designing safe and efficient structures. Existing computational methods, like Reynolds-averaged Navier-Stokes (RANS) simulations, often fail to predict pressures accurately in separation zones. This study proposes a novel deep-learning methodology to enhance the accuracy and performance of eddy viscosity modeling within RANS turbulence closures, particularly improving predictions for bluff body aerodynamics. A deep learning model, trained on large eddy simulation (LES) data for various bluff body geometries, including a flat-roof building and forward/backward facing steps, was used to adjust eddy viscosity in RANS equations. The results show that incorporating the machine learning-predicted eddy viscosity significantly improves agreement with LES results and experimental data, particularly in the separation bubble and shear layer. The deep learning model employed a neural network architecture with four hidden layers, 32 neurons, and tanh activation functions, trained using the Adam optimizer with a learning rate of 0.001. The training data consisted of LES simulations for forward/backward facing steps with width-to-height ratios ranging from 0.2 to 6. The study reveals that the machine learning model achieves a balance in eddy viscosity that delays flow reattachment, leading to more accurate pressure and velocity predictions than traditional turbulence closures like k-ω SST and k-ε. A sensitivity analysis demonstrated the pivotal role of eddy viscosity in governing flow separation, reattachment, and pressure distributions. Additionally, the investigation underscores the disparity in eddy viscosity values between RANS and LES models, highlighting the need for enhanced turbulence modeling. The findings presented in this paper offer substantive insights that can inform the advancement of more dependable computational methodologies tailored for engineering applications, encompassing wind load considerations for structural design and the intricate dynamics of unsteady aerodynamic phenomena.

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]

We examine the genesis of coherent vortices in submerged vegetated flows by means of a linear stability analysis. The mathematical framework is comprised of the conservation equations of fluid mass and momentum. The problem is tackled by imposing normal mode perturbations over an underlying undisturbed flow. We find that the growth rate of perturbations takes maximum magnitude for a specific wavenumber, termed as the critical wavenumber. The critical wavenumber indicates the most favorable wavenumber of coherent vortices emerging in submerged vegetated flows. The critical wavenumber amplifies as the flow Reynolds number, and vegetation height and density augment. The migration velocity of incipient coherent vortices characterizes minimum magnitude for a selected value of the vegetation height. The unstable zone in the stability diagram embarks beyond a critical Reynolds number. The critical Reynolds number designates the onset of coherent vortex appearance in submerged vegetated flows. The predictions of the present study are congruent with the existing theoretical and experimental works. [ABSTRACT FROM AUTHOR]

The unsteady pressure field over an axisymmetric backward-facing step was investigated experimentally at transonic freestream Mach numbers of 1.05, 1.2, and 1.4. The study was aimed at examining the influence of transonic freestream Mach numbers on the spatio-temporal character of the unsteady pressure field and on the dominant modes/mechanisms driving it. Surface flow visualization, schlieren, and unsteady pressure measurements were carried out as part of the experimental investigation. From oil flow visualization and schlieren, the reattachment region was identified, and consequently, the mean reattachment length was estimated. The mean reattachment length shows an increase with the increase in freestream Mach number. The coefficient of mean pressure along the rearbody imitates a classical backward-facing step flow profile and can be divided into three distinct regions. The peak values of the coefficient of mean pressure and the coefficient of root mean square of the fluctuation are seen to decrease with an increase in the freestream Mach number. Conventional spectral analysis reveals that as the freestream Mach number increases, the dominant peak in the spectra shifts to lower frequencies. From the spectra, three dominant fluid dynamic mechanisms depending on the freestream Mach number have been identified. Proper Orthogonal Decomposition (POD) analysis shows that 79–84 % of the total energy contribution comes from the first six modes. The temporal dynamics of the POD modes indicate three prominent mechanisms are responsible for the unsteady pressure field. Spectral analysis of POD modes indicates that the spectra are primarily driven by the first three POD modes for freestream Mach number of 1.05 and the first two modes for freestream Mach numbers of 1.2 and 1.4. Moreover, it reveals the presence of three dominant modes, and the freestream Mach number strongly dictates the dominant mode that is driving the pressure field. [ABSTRACT FROM AUTHOR]

The effect of nozzle lip thickness and velocity ratio on coaxial subsonic jet mixing, at different Mach numbers, has been studied experimentally and numerically. Decay of coaxial subsonic jets emanating from coaxial nozzles of lip thickness 0.7, 1.7 and 2.65 mm with velocity ratio (VR) from 0.2 to 1.0 at primary jet exit Mach numbers of 0.6, 0.8 and 1.0 has been studied. Free jet without co-flow (VR0) was also studied for comparison. Jet centerline Mach number decay, turbulence and velocity variation in the radial direction are analyzed. The results show that mixing the coaxial jet at a low-velocity ratio is better than a high-velocity ratio, at all Mach numbers of the present study. The nozzle lip thickness has a significant influence on the secondary jet. Mixing of the jet in the presence of VR0.2 coaxial jet is found to be the highest. Characteristic decay of Mach 0.8 and 1.0 jet for lip thickness 1.7 and 2.65 mm is faster than lip thickness 0.7 mm. For a given lip thickness, increasing of velocity ratio is found to retard the mixing between primary and secondary jets. [ABSTRACT FROM AUTHOR]

The confluence between a pipe and an open channel is a common pattern. This study analyses the flow characteristics of pipe and channel confluences by examining the bed morphology and turbulent structure. A general process-response model of the hydro-morpho-sedimentary processes is proposed. The study also investigates the transport of contaminated sediment carried in the drainage pipe at the discharge outlet. The bed morphology at the confluence of the pipe and channel is characterized by a scour hole at the pipe outlet, a scour hole bar and a deposition zone. Above the scour hole and deposition zone, the inner shear layer exists with intense turbulence and low velocities between them. Contaminated sediments accumulate on the inner side of the channel, with coarser particles (D50 > 2.28 × 10–4 m) near the pipe outlet and finer particles carried downstream. The findings of this study could be helpful to pollutant transport management in natural confluences. [ABSTRACT FROM AUTHOR]

This paper presents a solution of the two-dimensional unsteady problem of the development of wave motion in a two-layer fluid of finite depth under ice cover modeled by a thin elastic plate taking into account longitudinal compression forces. The cases are considered where, in the unperturbed state, one of the layers is at rest and in the other (top or bottom) layer, the horizontal flow velocity varies linearly over the thickness. Dispersion relations are derived for three wave modes arising in the presence of shear flow. Vertical deflections of the ice cover caused by the presence of a pulsating source of perturbations in the initially motionless fluid layer are calculated. A special case is also considered where the fluid is bounded at the top by a solid lid. The problem is considered in a linear formulation, and the fluid is assumed to be ideal and incompressible. [ABSTRACT FROM AUTHOR]

Shock/shock interactions (SSIs) often lead to high thermal loads. To understand the influences of thermochemical non-equilibrium effects on the Type III SSI, four gas models based on the assumption of thermal perfect gas (TPG), thermal equilibrium chemical non-equilibrium gas (TECNG), thermal non-equilibrium chemical frozen gas (TNCFG), and thermochemical non-equilibrium gas (TCNEG) are employed to simulate the Type III SSI at Mach 10. The intersection of incident shock and bow shock is determined by dimensionless intercept of 0.05 and 0.1. It is found that the chemical non-equilibrium effects significantly lift up the impingement position of shear layer by reducing the standoff distance of bow shock. As a result, the peaks of wall pressure and heat flux calculated by TECNG and TCNEG model are higher than TPG and TNCFG model, respectively. Type IIIa SSI occur in the flows calculated by the TECNG and TCNEG model for the case with a dimensionless intercept of 0.1. The peaks of wall pressure and heat flux calculated by TECNG model are both over four times higher than those of TPG model. The thermal non-equilibrium effects slightly increase the standoff distance of bow shock and lower the impingement position of shear layer. In addition, the thermal non-equilibrium reduces the angle to the horizontal direction of shear layer by increasing the specific heat ratio. • Type III shock/shock interaction at Mach 10 are simulated by four gas models. • The influences of non-equilibrium effects on flow fields are analyzed. • The chemical non-equilibrium effects significantly increase the peak values of heat flux and pressure. • The mechanisms of non-equilibrium effects on peak heat flux and pressure are revealed. [ABSTRACT FROM AUTHOR]

This article presents the jet mixing characteristics of sonic under expanded co-flowing jet by means of varying the lip thickness between the primary and secondary nozzle. The lip thicknesses between the nozzles chosen were 0.3Dp and 1.5Dp and their jet mixing characteristics is studied numerically for the nozzle pressure ratio of 5. The jet mixing effectiveness in the axial and radial directions were analysed by the total pressure decay plots and contours of Mach number, density gradient and turbulent kinetic energy. The results exhibit that, for the reduced lip thickness of 0.3Dp, the core of the primary jet is increased due to the encasing given by the surrounding jet. For the dominant lip thickness of 1.5Dp, the primary jet core is reduced due to the strong interaction between the primary and co-flowing jet. The various jet mixing phenomenon like shear layer between surrounding jet and atmosphere, recirculation zone and Mach disk were also analysed from the contour plots. [ABSTRACT FROM AUTHOR]

The complex unsteady flow in cavities leads to the formation of large-scale disturbances in the shear layer. Natural closed-loop mechanisms provoke a dramatic increase in pressure pulsations and aerodynamic noise. This paper presents the experimental study of pressure fluctuations in closed-loop control in rectangular cavities using plasma dielectric barrier discharge. The flow velocity was 37 m/s, and the Reynolds number based on a cavity depth was approximately 120,000. The discharge ignition near the leading edge of the cavity provoked the shear layer restructuring. It was found that pressure fluctuations with an amplitude of 120 dB occur at frequencies 480 and 820 Hz. Frequency modulation of the discharge at resonant peaks was carried out by changing the phase shift of the power supply. The peak amplitude was reduced or increased by phase shifts from natural disturbances to forced ones. The optimum energy input was 50 W/m. This was three times less than the power consumption of the open-loop mode. The PIV visualization was organized in the phase-locked mode. The pressure spectrum corresponds to the magnitude of coherent structures in the shear layer of the cavity. [ABSTRACT FROM AUTHOR]

A set of 2-inch diameter nozzles is used to investigate the effect of varying exit boundary layer (BL) states on the radiated noise from high-subsonic jets. It is confirmed that nozzles involving turbulent boundary layers are the quietest while others, involving nominally-laminar BLs, are noisier. A turbulent BL is thicker and there is simply an effect of thickness on noise. A thicker BL results in a decrease in the sound pressure spectral amplitudes due to a less vigorous growth of instability waves in the jet's shear layer. A nominally-laminar BL, besides being thinner, may also involve significantly higher turbulence intensities, much higher than that in a turbulent BL. Such a BL state, referred to as 'highly disturbed laminar', results in the largest noise amplitudes especially on the high-frequency side of the spectrum. This transitional state, often encountered with model scale nozzles, involves a 'Blasius-like' mean velocity profile but large velocity fluctuation intensities and intermittency. The higher initial turbulence adds to the increase in high-frequency noise. The results leave little doubt that an anomaly noted with subsonic jet noise databases in the literature is due to similar effects of differences in the initial boundary layer state. [ABSTRACT FROM AUTHOR]

Confluences are common components of river networks and are characterized by a highly complex flow structure. Two confluence geometries, without and with the floodplain in the tributary, were comparatively investigated to highlight the effects of floodplain on confluence hydrodynamics. The three-dimensional velocity field and the spatial distribution of turbulent kinetic energy and Reynolds shear stresses were analysed. In the second geometry, a tilted shear layer was observed, which was related to the flow expansion from the main channel into the tributary. Significant secondary motions are mainly related to the fluid upwelling in the lee of the floodplain step and streamline curvature. A wider flow separation zone was found, while the length of the separation zone was not affected by the floodplain flow. The results could be useful to understand the complex hydrodynamics of the large confluence between the Yangtze River and the Poyang Lake, characterized from a floodplain under high flow conditions. [ABSTRACT FROM AUTHOR]

Sheared flow can profoundly influence the aeroacoustic performance of near-wall structures like metasurfaces on moving vehicles. In this work, a refractive generalized Snell's law is developed by combining Snell's law with the generalized Snell's law to manipulate anomalous reflections in an idealized sheared flow, comprising a uniform flow region and a stationary air region with velocity and temperature gradients. Theoretical analysis and numerical simulations of planar wave reflection by metasurface impedance under non-isothermal sheared flows are conducted, yielding favorable predictions of the zone of silence, as well as anomalous and total reflection behaviors. The total reflection between the shear layer and the metasurface is analyzed with the objective of confining incident wave below the shear layer. Remarkably, a specific configuration of hot-sheared flow and metasurface length can achieve total absorption over a range of incident wave angles from 0° to almost 90°. Moreover, the refractive generalized Snell's law, extended to account for directional deflection induced by flow convection, is utilized to regulate sound beam reflection, resulting in satisfactory outcomes under sheared conditions. This study offers a theoretical framework for wave control under non-isothermal and sheared flow conditions, which cannot be resolved by the traditional generalized Snell's law, and can be implemented in the design of metasurfaces for moving applications. • A refractive generalized Snell's law is developed. • Thermal effects on wavefront manipulation are evaluated theoretically and numerically. • The phenomenon of total reflection is found to confine sound waves below shear layer. • A specific configuration of hot shear layer and metasurface can achieve an incident angle interval of 90°for total reflection. [ABSTRACT FROM AUTHOR]

• A method for modeling the lateral distributions of depth-averaged velocity behind an emergent vegetation patch is provided. • The model incorporates bed friction, vegetation-induced resistance, secondary flow effects, and lateral momentum exchange. • The influence of secondary flow term on the lateral distributions of depth-averaged velocity is evaluated. • Some modeling insights for those with limited experience are offered. Aquatic vegetation provides ecological, hydrological, and aesthetic functions for rivers, and measuring velocity in vegetated channels is essential for river management. The paper presents a method for modeling the lateral distributions of depth-averaged velocities behind an emergent vegetation patch. Based on SKM, this approach divides the channel behind an emergent vegetation patch into pseudo-vegetation and free-flow regions, offering analytical solutions for the depth-averaged velocities in these two regions. The model incorporates several critical parameters, including the Darcy–Weisbach coefficient, lateral dimensionless eddy viscosity, drag force coefficient, and secondary flow coefficients. These coefficients are associated with bed friction, vegetation-induced resistance, secondary flow effects, and lateral momentum exchange, respectively, affecting the depth-averaged velocities. A comparison with published experimental data validates that the proposed model can predict the lateral distributions of depth-averaged velocities behind an emergent vegetation patch. A steady wake section exists behind an emergent vegetation patch for low-flow blockage. In the steady wake section, the secondary flow coefficients in pseudo-vegetation and free-flow regions remain relatively constant. Upon exiting the stable wake section, the secondary flow coefficient in the pseudo-vegetation region decreases with increasing distance from an emergent vegetation patch, and it in the free-flow region increases with increasing distance from an emergent vegetation patch. A sensitivity analysis of drag coefficient and secondary flow coefficients suggests that secondary flow coefficient in the free-flow region has a more significant effect on the lateral distributions of depth-averaged velocities compared to drag coefficient and secondary flow coefficient in the pseudo-vegetation region. [ABSTRACT FROM AUTHOR]

Advances in measuring and understanding separated, nominally two-dimensional (2D) shock-wave/turbulent-boundary-layer interactions (STBLI) have triggered recent campaigns focused on three-dimensional (3D) STBLI, which display far greater configuration diversity. Nonetheless, unifying properties emerge for semi-infinite interactions, taking the form of conical asymptotic behavior where shock-generator specifics become insignificant. The contrast between 2D and 3D separation is substantial; the skewed vortical structure of 3D STBLI reflects the essentially 2D influence of the boundary layer on the 3D character of the swept shock. As with 2D STBLI, conical interactions engender prominent spectral content below that of the turbulent boundary layer. However, the uniform separation length scale, which is crucial to normalizing the lowest-frequency dynamics in 2D STBLI, is absent. Comparatively, the spectra of 3D STBLI are more representative of the mid-frequency, convective, shear-layer dynamics in 2D, while phenomena associated with 2D separation-shock breathing are muted. Asymptotic behavior breaks down in many regions important to 3D-STBLI dynamics, occurring in a configuration-dependent manner. Aspects of inceptive regions near shock generators and symmetry planes are reviewed. Focused efforts toward 3D modal and nonmodalanalyses, moving-shock/boundary-layer interactions, fluid/structure interactions, and flow control are suggested as directions for future work. [ABSTRACT FROM AUTHOR]

Effects of three-dimensional (3-D) distributed roughness elements on the flow characteristics within a cavity are investigated using a series of high-fidelity eddy-resolving simulations. The cavity flows generate undesirable low-frequency pressure fluctuations due to the vortex impingement over the trailing edge of the cavity. We explore the possibility of employing distributed hemispherical roughness elements as a passive flow control strategy towards suppressing these pressure fluctuations. A rectangular cavity with a length to depth ratio, L/D, of 3 is considered. Simulations are carried out at a Mach number of 0.2 and Reynolds numbers of 7000 and 19300, based on the free-stream velocity and the depth of the cavity. The effect of sparsely and densely packed roughness elements on the stability of shear layer separating from the cavity are brought out. Pre-transitional fluctuations generated by the roughness elements (a) resulted in transitional/turbulent flow at the cavity leading edge for low/high Reynolds numbers (b) promoted an earlier breakdown of the large-scale coherent structures in the shear layer (c) are beneficial in decreasing the 'cavity tones' and the associated sound pressure levels (SPL) by 5-13 dB. Reduction in SPL is observed to be prominent at higher Reynolds numbers and with dense spacing between the roughness elements. At low Reynolds numbers, the benefit obtained by suppressing the 'cavity tones' can be eclipsed with an increase in the broadband noise. [ABSTRACT FROM AUTHOR]

To improve the accuracy of numerical simulation of friction stir welding (FSW) process, the tool tilt angle must be considered as a significant parameter. In this study, specific considerations for mechanical boundary conditions in Eulerian domain is employed to investigate the tool tilt angle influence on the thermomechanical behavior in FSW. Aluminum 6061-T6 with a thickness of 6 mm under a rotational speed of 800 RPM, a transverse speed of 120 mm/min, and a plunging depth of 0.1 mm were employed for the simulations. Results showed an almost symmetric temperature profile predicted by the model without considering the tool tilt angle, while after incorporating the tool tilt angle, the peak temperature point is moved to the tool backside (around 400 °C), resulting in better material bonding, enhancing the weld joint quality. Without accounting for the tool tilt angle, the highest temperature of 389 °C is observed, while with the tilt angle the maximum temperature of 413 °C is achieved. The temperature variations at different points of the leading (around 360 °C) and the trailing sides (around 400 °C) of the welding tool were measured. It was observed that, after considering the tilt angle, as the tool moves, a smooth and quick increase for the temperature at the tool trailing side is achieved. This smooth and quick increasing of the temperature at the trailing side results in reducing the possibility of the formation of defects, cracks, and voids. Finally, comparisons showed that the model computational time is acceptable, and using Eulerian formulation leads to achieving a remarkable accuracy. [ABSTRACT FROM AUTHOR]

The influence of circular ring on the dynamics of the flow on the developing region of axisymmetric round jet is examined. The jet is produced from a smoothly contracting round nozzle and the flow structure is controlled by varying the air blower speed in order to obtain various Reynolds numbers. Ring made of 0.5 mm diameter and positioned at x/d=0.12 is used to perturb the shear layer of round nozzle of diameter 25.4 mm. Hot wire measurement is carried out at the near and intermediate fields (0.5=x/d=30) for both perturbed (disturbed) and unperturbed (undisturbed) jet. Measurements were also made in the confined flow for both cases. The results show increase in potential core and further reduction in centerline velocity at the end of potential core for disturbed jet when compared with undisturbed jet for the Reynolds numbers considered (5210, 7000 and 9117). Spread rate reduces in perturbed jet due to the shear layer compression in the developing region. However, velocity profile seems not to be affected by the perturbation in the near field but a compression of the profile is noticed further away from near field. Also, the flow measured in the pipe attached to the nozzle for both cases show almost constant value of normalized centerline velocity and has reduced values when compared with undisturbed jet. The foregoing clearly indicates mass redistribution in the confined region. Overall, while the wavelength of the oscillation is unchanged, the magnitude is altered suggesting a change in the dynamics of the layer. [ABSTRACT FROM AUTHOR]

To improve the efficiency of harvesting energy, a cut-corner prism piezoelectric energy harvester based on galloping enhancement is designed in this study. Galloping is caused by the pressure difference between the upper and lower surfaces of the prism. Numerical simulations indicate that the cut-corner can reduce the occurrence of the shear layer reattachment phenomenon and also increase the strength of the secondary backflow between the shear layer and the lateral side of the prism, thereby raising the pressure difference between the upper and lower surfaces of the prism. The effect of cut-corner size on the performance of cut-corner prism energy harvesters is investigated experimentally. The results show that the optimal range for the prism is when the lengths of the cut-corner windward side and parallel side are 0.2B–0.4B (B is the length of the prism side) and 0.5B–0.6B, respectively. At a wind speed of 6.24 m/s and a resistance of 1 × 105 Ω, the maximum output power of the cut-corner prism energy harvester with a windward side length of 0.4B and a parallel side length of 0.6B can reach 47.5 mW, which is 261% higher than that of the reference square prism, effectively raising the performance of the piezoelectric energy harvester.

Flow past a bluff body in freestream turbulence can substantially change the flow behaviour compared to that in smooth inflow. This paper presents the study of wake flow and aerodynamics of an oscillating square cylinder at the resonant frequency in freestream turbulence, with the integral length not greater than the cylinder side and the turbulence intensity not greater than 10%. Large eddy simulations (LES) in the Cartesian grid using the Immersed Boundary Method (IBM) technique embedded in a FVM solver, together with an efficient synthetic turbulent inflow generator implemented in an in-house parallel FORTRAN code are used for the study. The results are compared with those for smooth inflow, and relevant data published in the literature. The key findings are: the freestream turbulence conditions evidently reduces the local turbulent scales and fluctuations in the shear layer compared to in smooth flow, as small scale freestream turbulence breaks down cylinder-generated larger scale eddies and weakens them; but does not evidently affect the vortex shedding frequency, or the length of the recirculation region behind the cylinder. This suggests negligible change of drag coefficient compared to in smooth inflow. Moreover, this is because the vortex shedding is dominated by the forced oscillation at the resonance frequency, and the turbulence intensity is small. [ABSTRACT FROM AUTHOR]

The internal weapon bay is widely used in modern aircraft; however, because the unsteady flows of the cavity would cause dangerous store separation and intense aerodynamic noise, the leading-edge spoiler is an easy and efficient passive flow-control method. The flow control of the leading-edge flat spoiler before the cavity of a low-aspect-ratio flying-wing aircraft is investigated based on numerical simulation. Numerical results show that the leading-edge flat spoiler completely changes the cavity flow; it obviously lifts up the shear layer and reduces the pressure inside the cavity. For the store separation from the weapon bay, the leading-edge flat spoiler is a very good passive flow-control method that curbs the nose-up trend of the store and produces a safe and stable store separation. Besides, the leading-edge spoiler reduces the noise in the rear of the cavity (max 8.2 dB), but increases the noise in the middle of the cavity (max 11.3 dB). In addition, the leading-edge spoiler brings in a large drag increase to the aircraft (39.41% when the height of spoiler is 0.2 m), which would affect the operational stability of the aircraft. The results of this paper could provide a reference for the flow control of weapon bays and the design of aircraft. [ABSTRACT FROM AUTHOR]

Turbulence is governed by various mechanisms, such as production, dissipation, diffusion, dilatation and convection, which lead to its evolution and decay. In high-speed flows, turbulence becomes complicated due to compressibility effects. Therefore, the goal of the current work is to characterize these mechanisms in rectangular supersonic jets by directly evaluating their contributions in turbulent kinetic energy (TKE) budget equation. The budgets are obtained using high-fidelity Large Eddy Simulations that employ WALE subgrid-scale model. Jet nearfield data are validated with PIV experimental measurements, available from the literature, which include mean flow and second-order statistics. To ensure spatial resolution and temporal convergence of higher-order statistics, qualitative performance metrics are presented. The results indicate that TKE production is the major source term, while pressure-dilatation term acts as a sink throughout the development of the jet. The diffusion term has the highest contribution from triple-velocity correlations, followed by pressure diffusion and molecular diffusion. Subgrid-scale diffusion and dissipation are also evaluated and their contributions are minimal. Each term is presented on both minor and major axis plane and reveals asymmetry in the statistics. A detailed explanation of budget contributions is provided, leading to the mechanisms responsible for the anisotropy of TKE. [ABSTRACT FROM AUTHOR]

• For L/D ratio of 13, the flow type transforms from a closed one to a transitional one. • The critical wall temperature is approximately 775 K. • As the incoming static temperature drops, the critical wall temperature declines. • Transition caused by the competition between the shear layer and recirculation zone. • Heat transfer is affected by the thickness of boundary layer or recirculation zone. The wall temperature of the high-speed aircraft increases quickly under hypersonic/supersonic incoming flow, which will cause a significant change in the flow structure. To study the transition of the flow type in the supersonic cavity controlled by the wall temperature, numerical simulations are conducted. The cavity length-to-depth ratio (L/D) is varied from 10 to 15, and the wall temperature ranges from 300 K to 1300 K. The results indicate that the type of cavity flow with an L/D ratio of 13 transforms from a closed cavity flow to a transitional cavity flow, when the temperature reaches approximately 775 K. And the transitional temperature rises with the elevated total temperature of the incoming flow. Furthermore, the mechanism of the cavity flow change with wall temperature could be the competition between the recirculation zone and the shear layer in the cavity. The rising pressure with higher wall temperature in the recirculation zone weakens the downward development of the cavity shear layer, preventing it from hitting the cavity floor. As a result, the mass exchange of cavity lip surface, pressure distribution, total pressure recovery coefficient, and heat transfer distribution in the supersonic cavity change dramatically. The critical wall temperature also affected by the sidewall effects and the inflow Mach number. [ABSTRACT FROM AUTHOR]

This paper presents a novel pressure-dependent velocity boundary condition based on wear theory according to our former work, which provides an explainable perspective in understanding the physics of periodic time scale sliding-to-sticking transition condition and shear layer formation, as well as quality weld/void defects formation, during friction stir welding/self-reacting friction stir welding using computational fluid dynamics model. With this newly developed boundary condition, effects of welding speed and rotation speed on the weld cross-section geometry can be robustly captured. Furthermore, different cases of quality weld and void defects formation are validated with shear localization model. In both shear localization model and computational fluid dynamics model with pressure-dependent velocity boundary condition, the rationale and physical explanation of quality weld/void defects formation, as well as tool-workpiece sliding-to-sticking transition behavior, are investigated and compared: The pressure-dependent velocity boundary condition describes the relationship between weld pitch and applied velocity boundary from geometrical restrictions, and shear localization model captures transient shear layer formation within one pin revolution from one-dimensional standpoint; the pressure-dependent velocity boundary condition accounts for the time-averaged transitioning from pure sliding to full sticking during each shear layer generation and deposition cycle, whereas the shear localization model directly describes the transition from sliding to full sticking at pin/workpiece interface during each tool revolution. It is worth noting that the shear layer itself is not explicitly captured in computational fluid dynamics model due to continuity, instead the equivalent effects and movement of periodic shear layer are described and modeled. The importance of shear layer that forms during each tool revolution in periodic weld formation mechanism is emphasized in both pressure-dependent velocity boundary condition of computational fluid dynamics model and shear localization model. [ABSTRACT FROM AUTHOR]

Free-surface flows with riparian corridors are known to develop large eddies resulting from the instability associated to the inflectional profile of the longitudinal velocity in the spanwise direction. They periodically generate strong momentum exchanges inside the vegetation corridor, triggering a wave-like motion, detectable as free-surface oscillations and out-of-phase velocity components. We propose a characterization of the flow inside the vegetation corridor, focusing on the wave-like motion and its influence on secondary currents. We conditionally sample the fluid motion to highlight the structure of the phase-averaged coherent structure. Quadrant analysis shows that there is a strong variation of Reynolds stress anisotropy in the spanwise direction, which is one of the key generation mechanisms of secondary currents. Spectrograms of longitudinal and lateral velocity fluctuations reveal that the oscillatory motion is imposed on the whole of the vegetated layer, because of continuity. The analysis of the phase-averaged 2D vertical-longitudinal flow reveals that there is a complex 3D pattern of mass fluxes associated to each large eddy. In particular there is an anti-symmetric net mass imbalance which, by mass conservation, generates a mass flux directed outwards, to the main channel, near the bottom of the channel. The Eulerian expression of this motion is obtained as the spatial average of the flow over the length of the large eddy, resulting in the pattern of the secondary current in the vertical-spanwise plane. It is shown that the secondary motion is a necessary feature of free-surface turbulent flows that develop large-scale inflectional instabilities. Article highlights: The flow presents a marked anisotropy in the core of the large-scale eddies resulting from high shear stress associated with the passage of this, and the spanwise variation of Reynolds stress anisotropy is related to the formation and growth of secondary flow. Analysis of the phase-averaged 2D vertical-longitudinal flow revealed that there is a complex 3D pattern of mass fluxes associated to large eddy, and by spatially-averaging this flow along the length of the eddy, it is obtained the Eulerian vertical-spanwise velocity field where the secondary currents are visible. A deterministic model of streamlines trends is proposed, based on the continuity equation, which explains the secondary current as a function of inflectional instability. [ABSTRACT FROM AUTHOR]

Shrubby and woody vegetation growing on floodplains profoundly influences hydrodynamic and transport processes in riverine systems. Existing hydrodynamic research is mostly focused on conditions with aquatic plants and rigid model vegetation. To appreciate the different hydrodynamic impacts of submerged floodplain and riverbank vegetation, a novel flume investigation was carried out. We simulated conditions found in riparian environments in terms of vegetation density, plant structure and flexibility, and presence of a grassy understory. Four experimental cases were defined so that vegetation exhibited different degrees of bending and streamlining. Extensive set of velocity measurements allowed reliable description of the double averaged flow. Vegetation morphology, with the flexibility-induced streamlining and dynamic motion controlled the magnitude and distribution of the vegetative drag, shaping the shear penetration within the canopy. The flows were highly heterogeneous, thus calling for spatially averaged approaches for the flow field investigation. The relative importance of dispersive momentum fluxes was high in the canopy bottom region where both Reynolds and dispersive stresses were small. The contribution of dispersive fluxes to momentum transport decreased with increasing reconfiguration. The results revealed the shear layers over floodplain vegetation to be dynamically similar to other environmental flows over porous obstructions. However, the velocity-dependent vegetative drag and deflected height introduced additional complexity in the flow simulation. Altogether our findings implied that accurate description of vegetated floodplain flows can be achieved only when plant morphology and flexibility are appropriately described in drag models. Article highlights: A novel experimental setup with flexible woody plants and grasses was used to model the hydrodynamics of vegetated floodplains. Plant morphology and flexibility controlled the vegetative drag, affecting key shear layer features, including the shear penetration. The spatially heterogeneous flows had higher dispersive stresses at the canopy bottom, where the total fluid stress was small. [ABSTRACT FROM AUTHOR]

The transonic flow around a square cylinder at Ma= 0.71 and Re= 4×105 has been studied by using the scale-adaptive simulation (SAS) method, and the characteristics of separated shear layer and wake have been analyzed in depth. To validate the SAS approach, the SAS results are compared with the existing numerical and experimental results. In the present transonic flow, the convective Mach number inside the shear layer is about 0.6. This indicates that the initial evolution of the separated shear layer is dominated by Kelvin-Helmholtz instability, and the roller spanwise eddies in the initial stage of the shear layer can be observed. In the regions near the shear layer and the wake, the doubling frequencies can be obtained indicative of the harmonic phenomenon inside the separated shear layer, which is closely related to the obvious merging of the vortices in the shear layer. Proper orthogonal decomposition of the pressure field shows that the transonic flow field of square cylinder is dominated by the antisymmetric mode, which is associated with the vortex shedding in the wake and the propagation of compression waves induced by the shear layer.

Supersonic steam injection from underwater vehicles into surrounding bulk water exhibits the formation of coherent structures due to the interfacial interaction between the steam and water. The mixing between the two is a function of the rate of growth of shear layer. In present work, experimental study is conducted with minor contribution from the CFD, to highlight the phenomena associated to the high-pressure steam injection into a pool of water under the influence of periodic cavitation which occurs near the steam's nozzle exit with its opening being at right angle to the opening of the exit nozzle. PIV setup along with piezoelectric acoustic emission sensors as well as LM35 temperature sensors and pitot tubes were applied to characterize the growth of the shear layer as a function of periodic cavitation with a range of steam's operating pressure. Based on the normalized shear growth rate as well as the Strouhal number and the normalized pitot thickness, the effect of rising in the cavitation on the variations of the thickness of the shear layer was studied. It was observed that higher area under the influence of the shear layer was due to the domination of the coherent flow structures, which influenced improved mixing between the steam and water. Comparison of our data with the available shear growth rate in literature shows good agreement when compared as a function of Mach number. [ABSTRACT FROM AUTHOR]

In order to obtain the flow characteristics of sub-super-sonic mixing layer including velocity distribution, pressure distribution and development of mixing layer, experimental and numerical investigations were conducted. PIV technique was employed to measure the two-dimensional velocity distribution in the experiment while the standard k-ω turbulent considering the effect of compressibility was adopted to simulate the flow characteristic of mixing layer. The Mach number of subsonic stream and supersonic one was 0.11 and 1.32, respectively. The results show the flow of mixing layer is temporally transient. The interface between two streams lies initially as an approximately line segment; afterward, it becomes wrinkled and distorted; finally, it breaks up. The mixing layer develops linearly along streamwise direction in the time averaged velocity field with a growth rate of 0.135. The velocity and total pressure distributions in the mixing layer are self-similar.

ABSTRACT: High-order accurate schemes are employed to numerically simulate the interaction of a supersonic jet and a co-directional supersonic inflow. A double backward-facing step model is proposed to investigate the interaction between the jet shear layer and the supersonic inflow shear layer. It is found that due to the interaction of the shear layer, a secondary jet is injected into the recirculation zone at the intersection of the two shear layers. The secondary jet produced by the interaction of the two shear layers has a periodicity because of shear layers interaction. The distinction in the shape of double backward-facing steps will induce changes in the period of the secondary jet. The analysis and discussion of the periodicity of the secondary jet are mainly focused in this letter.

A pressure-dependent velocity boundary condition is developed based on wear theory for modeling self-reacting friction stir welding using computational fluid dynamics approach, which provides a new perspective in understanding the physics of sliding/sticking transition condition. The importance of shear layer in weld formation is emphasized. Effects of welding speed on the weld cross-section geometry can be robustly captured with this newly developed boundary condition. Computational results show that at higher welding speed, the TMAZ boundary moves towards the pin periphery at the advancing side, which corresponds to the experimental observations. This tendency could serve as a numerical criterion to predict void defect formation. [ABSTRACT FROM AUTHOR]

Based on the theoretical approach developed in the article by G.A. Turichin et al. (High Temperature. 2006. Vol. 44. No. 5. Pp. 647–655), the characteristics of the plasma plume forming in the keyhole laser welding have been investigated. A condition corresponding to the existence of the Crocco integral was defined, making it possible to simplify the system of gas dynamics equations and obtain analytical solutions for a plasma plume in the form of a classical submerged jet. These solutions were analyzed for a wide range of metal vapor velocities and temperatures at the keyhole top. For some of the selected values, an agreement with the numerical calculations of other authors was found.