Extra Dissipation Research Articles

A generalized phase-field cohesive zone model ([formula omitted]PF-CZM) for fracture

In this work a generalized phase-field cohesive zone model (μPF-CZM) is proposed within the framework of the unified phase-field theory for brittle and cohesive fracture. With the introduction of an extra dissipation function for the crack driving force, in addition to the geometric function for the phase-field regularization and the degradation function for the constitutive relation, theoretical and application scopes of the original PF-CZM are broadened greatly. These characteristic functions are analytically determined from the conditions for the length scale insensitivity and a non-shrinking crack band in a universal, optimal and rationalized manner, for almost any specific traction–separation law. In particular, with an optimal geometric function, the crack irreversibility can be considered without affecting the target traction–separation softening law. Not only concave softening behavior but also high-order cohesive traction, both being limitations of the previous works, can be properly dealt with. The global fracture responses are insensitive not only to the phase-field length scale but also to the traction order parameter, though the crack bandwidth might be affected by both. Despite the loss of variational consistency in general cases, the resulting μPF-CZM is still thermodynamically consistent. Moreover, the existing numerical implementation can be adopted straightforwardly with minor modifications. Representative numerical examples are presented to validate the proposed μPF-CZM and to demonstrate its capabilities in capturing brittle and cohesive fracture with general softening behavior. The insensitivity to both the phase-field length scale and the traction order parameter is also sufficiently verified.

Quenched disorder and instability control dynamic fracture in three dimensions

Materials failure in 3D still poses basic challenges. We study 3D brittle crack dynamics using a phase-field approach, where Gaussian quenched disorder in the fracture energy is incorporated. Disorder is characterized by a correlation length R and strength σ. We find that the mean crack velocity v is bounded by a limiting velocity, which is smaller than the homogeneous material’s prediction and decreases with σ. It emerges from a dynamic renormalization of the fracture energy with increasing crack driving force G, resembling a critical point, due to an interplay between a 2D branching instability and disorder. At small G, the probability of localized branching on a scale R is super-exponentially small. With increasing G, this probability quickly increases, leading to misty fracture surfaces, yet the associated extra dissipation remains small. As G is further increased, branching-related lengthscales become dynamic and persistently increase, leading to hackle-like structures and a macroscopic contribution to the fracture surface. The latter dynamically renormalizes the actual fracture energy until, eventually, any increase in G is balanced by extra fracture surface, with no accompanying increase in v. Finally, branching width reaches the system’s thickness such that 2D symmetry is statistically restored. Our findings are consistent with a broad range of experimental observations.

Turbulence modulation by charged inertial particles in channel flow

Large amounts of small inertial particles embedded in a turbulent flow are known to modify the turbulent statistics and structures, a phenomenon referred to as turbulence modulation. While particle electrification is ubiquitous in particle-laden turbulence and significantly alters particle behaviour, the effects of inter-particle electrostatic forces on turbulence modulation and the underlying physical mechanisms remain unclear. To fill this gap, we perform a series of point-particle direct numerical simulations of turbulent channel flows at a friction Reynolds number of approximately 540, laden with uncharged and charged bidisperse particles. The results demonstrate that, compared to flows laden with uncharged particles, the presence of inter-particle electrostatic forces leads to substantial changes in both turbulent intensities and structures. In particular, the inner-scaled mean streamwise fluid velocity is found to shift towards lower values, indicating a noticeable increase in fluid friction velocity. Turbulent intensities appear to be further suppressed through facilitating the particles to extract momentum from the fluid phase and increasing extra turbulent kinetic dissipation by particles. Importantly, the overall drag is enhanced by indirectly strengthening the contribution of particle stress, even though the contribution of the total fluid stress is decreased. On the other hand, the magnitude of the large-scale motions is weakened by simultaneously reducing turbulent production and increasing particle feedback around the scales of the large-scale motions. Meanwhile, the average streaky fluid structures in the streamwise–spanwise planes and inclined fluid structures in the streamwise–wall-normal planes become expanded and flattened, respectively.

Model of bores interaction in the swash

This paper examines the impact of wave interactions in the inner-surf zone, including the swash, on shoreline excursion dynamics. Specifically, the study focuses on how the time lag between consecutive waves and the slope of the beachface affect shoreline dynamics. To investigate this, a laboratory experimental setup is developed to study the behaviour of two consecutive bore-type waves travelling over a fluid layer with constant depth, representing an idealized inner-surf zone, before impacting an inclined solid plane with slope β, representing the beachface. Bore waves are generated using two dam-break flow devices, with a controlled time lag Δt between them. This simplified physical model allows for the assessment of semi-theoretical predictive models based on shallow water approximation, resulting in ballistic-type models for shoreline motion. By combining these approaches, the study characterizes different flow regimes in the (Δt,β) parameter space. It is observed that varying Δt at a fixed β distinguishes four interaction regimes, either wave–wave interaction in the inner-surf zone or wave-swash interaction in the swash, with possibilities of merging or collision depending on wave orientation. In particular, increasing Δt leads to either bore–bore merging in the inner-surf, bore-runup merging in the swash, bore-backwash collision in the swash or bore–bore collision in the inner-surf. Bore–bore merging and bore-runup merging enhance runup, peaking when Δt is such that merging occurs near the transition from the inner-surf to the swash. On the contrary, bore-backwash collision results in additional dissipation of the second bore’s dynamics, leading to a reduced shoreline extension compared to that induced by the first bore. All processes within the swash, including the transition between bore-runup and bore-backwash regimes, as well as the enhancement and extra dissipation of the second bore’s swash excursion, exhibit some level of dependence on β. Overall, the results from this physical model help characterize and explain local mechanisms triggered by wave interaction near the shoreline. Validating this model’s relevance warrants specific attention through comparisons with field data. As an initial validation attempt, field data extracted from the literature are compared to the physical model, showing promising agreement.

Changes in body surface temperature reveal the thermal challenge associated with catastrophic moult in captive gentoo penguins.

Once a year, penguins undergo a catastrophic moult, replacing their entire plumage during a fasting period on land or on sea-ice during which time individuals can lose 45% of their body mass. In penguins, new feather synthesis precedes the loss of old feathers, leading to an accumulation of two feather layers (double coat) before the old plumage is shed. We hypothesized that the combination of the high metabolism required for new feather synthesis and the potentially high thermal insulation linked to the double coat could lead to a thermal challenge requiring additional peripheral circulation to thermal windows to dissipate the extra heat. To test this hypothesis, we measured the surface temperature of different body regions of captive gentoo penguins (Pygoscelis papua) throughout the moult under constant environmental conditions. The surface temperature of the main body trunk decreased during the initial stages of the moult, suggesting greater thermal insulation. In contrast, the periorbital region, a potential proxy of core temperature in birds, increased during these same early moulting stages. The surface temperature of the bill, flipper and foot (thermal windows) tended to initially increase during the moult, highlighting the likely need for extra heat dissipation in moulting penguins. These results raise questions regarding the thermoregulatory capacities of penguins in the wild during the challenging period of moulting on land in the current context of global warming.

Flow resistance enhancement for the leakage decrease of labyrinth seals using teeth tip winglets

A novel flow resistance enhancement technology is proposed to decrease the leakage flow for labyrinth seals adopting teeth tip winglets (TTW). The specially designed TTW is utilized to double the throttling effect at each teeth tip. In addition, the annular slots on the TTW can also generate teeth tip vertexes. These vertexes disrupt the direction of teeth tip leakage flow, enhancing the local flow resistance. To validate this concept, a three-dimension computational fluid dynamic model (CFD) is established. The flow field and leakage performance of the seal with TTWs are calculated with compassion to the baseline seal. Results show that the TTW seals increase flow resistance at the teeth tips, resulting in an extra dissipation effect. As a result, the maximum leakage decrease ratio reaches up to 15.1%

Wall-modeled large eddy simulation in the immersed boundary-lattice Boltzmann method

This work presents the wall-modeled large eddy simulation (WMLES) in the immersed boundary-lattice Boltzmann method (IB-LBM). Here, the wall model with both diffusive- and sharp-interface immersed boundary methods (IBMs) is incorporated into the IB-LBM to handle the turbulent boundary layer in high Reynolds number turbulent flows. To maintain the numerical stability, two collision models, i.e., multiple-relaxation-time (MRT) and recursive regularized (RR), are implemented. The performance of these models in the WMLES is examined and compared in the simulation of internal and external flows by considering four benchmarks, i.e., turbulent flow in a channel, flow around a hull of submarine, flow around an Ahmed car model, and flow around a circular cylinder. It is found that a diffusive-interface IBM with wall model is capable to achieve excellent results for the simulation of external flows around bluff objects but fails in the simulation of internal flows of underestimating the wall shear stress due to its extra dissipation. The sharp-interface IBM with the wall model predicts the internal flow very well but fails in some simulations of external flow around bluff bodies due to the failure in the separation flow modeling. It is also found that the MRT-LBM is less dissipative than the RR-LBM, but it generates spurious nonphysical noise in the turbulent flows and tends to be unstable at high Reynolds numbers. Therefore, the diffusive-interface IBM with the wall model is more suitable for the external turbulent flow modeling, while its sharp-interface counterpart is more suitable for the internal turbulent flow modeling. The RR-LBM outperforms the MRT-LBM for the better stability and less nonphysical noise.

On the coupling effect in the RF-biased inductively coupled plasma with the synchronous control

The coupling effects between the bias power and the inductive power in the RF-biased inductively coupled plasma with synchronous control are investigated by measuring electron energy distribution function using a compensated Langmuir probe. With synchronous control, the inductive power and the bias power are driven at an identical phase and frequency. The experimental results show that the inductive power lowers the self-bias voltage, while the bias power changes the plasma density by introducing extra power absorption and dissipation. The bias power also enhances the electron beam confinement, leading to an increase in electron density at a low pressure. Furthermore, in the E and H mode transition, with the bias power increasing, the hysteresis power reduces, and the electron density jump decreases.

Thermal Analysis of Al and Cu Metals Heat Sinks with Different Geometries at Raspberry Pi Control Cards Used for Image Analysis-Based Drone Control in Smart Agriculture Drones

A heat sink is a tool for dissipating the heat generated by electronic parts. The equipment's specific operating conditions necessitate the equipment's extra heat dissipation. This research compared and optimized the temperature and heat flux parameters based on the results of the design of a heat sink for CPU, RAM, and PCLe to a USB 3.0 bridge. It is aimed at an examination of the advantages and disadvantages of using square, rectangular, and circular shapes in the design of a heat sink. Copper and aluminum (Al) are the most common heat sink materials (Cu). Autodesk Inventor Pro software with Nastran module is used for design and thermal analysis.
 According to Inventor Nastran's thermal analysis results it is found that there is no significant difference between Al and Cu materials based on cooling capacity at designed models. Also, it is found that the geometry of the heat sinks directly affects the cooling capacity of a heat sink.
 The application results show that the cooling achievement is directly related to the correct heatsink design and enough surface area.
 According to Inventor Nastran's thermal analysis results it is found that there is no important difference between Al and Cu materials based on cooling capacity at designed models. Also, it is found that the geometry of the heat sinks directly affects the cooling capacity of a heat sink.
 The application results show that the cooling achievement is directly related to the correct heatsink design and enough surface area.

Extraction of Fringing-Effect Power Loss from Total Dissipation in Magnetic Component

Power magnetics in the energy storage configuration are not able to handle a significant amount of power without the introduction of a physical discontinuity in their magnetic path. This frequently takes the form of a discrete air gap giving rise to certain consequences such as extra power dissipation in the coils mounted on gapped cores. The ascertainment of the impact of the fringing magnetic field at the air gap on the efficiency of power conversion is highly problematic due to the complex nature of the phenomenon. The fringing-effect power loss typically coexists and is combined with all the other power-dissipation mechanisms, which greatly complicates the extraction of losses brought about solely by the fringing flux at the air gap from the total amount of dissipation in a given magnetic component. Magnetic cores of composite materials do not require a discrete air gap, as the air gap in them is distributed throughout the entire material, thus preventing the fringing magnetic flux from forming. However, there is a downside to this approach, as power loss in the material is comparably greater and so are the manufacturing costs. As shown here, distributed-gap-type core materials, due to the absence of physical discontinuity, and hence the lack of registerable fringing-effect power loss, can be utilized to comparatively ascertain and extract the extra power dissipation due to the fringing effect phenomenon in gapped magnetic components.

Numerical analysis of convective transport mechanisms in two-layer ternary (TiO 2 − SiO 2 − Al 2 O 3) Casson hybrid nanofluid flow in a vertical channel with heat generation effects

In this article, we investigate the flow dynamics and convective transport of nanofluid and ternary hybrid nanofluid in a two-layer vertical channel. Employing a Casson model with blood as the base fluid in both layers, the first layer incorporates nanofluid while the second layer comprises ternary hybrid nanofluid Laminar, incompressible flow with extra viscous dissipation effects is included in the formulation of this problem. The velocity and temperature are assumed to be continuous at the interface. The governing equations are converted into non-dimensional equations by using variables in the dimensionless form. The effective method known as homotopy analysis method (HAM) scheme is used to deal with the mathematical formulations. The behavior of velocity and temperature profile is sketched for various physical parameters and Nusselt number variations are tabulated. The results reveal that increasing the value of Eckert number raises the temperature. Temperature rises as and – – nanoparticles volume percentage rises. Nusselt number increases as Eckert number and Prandtl number increases while decreases as Casson parameters increase at the right wall. Rising values of and exhibit dissimilar behavior in the velocity profile. This study could be useful for medical purposes, like making better ways to deliver medicine through our blood vessels to treat cardiovascular diseases.

A generalized k−ϵ model for turbulence modulation in dispersion and suspension flows

A large amount of published data show that particles with diameter above 10% of the turbulence integral length scale (D/l>0.1) tend to increase the turbulent kinetic energy of the carrier fluid above the single-phase value, and smaller particles tend to suppress it. We attempted to remove limitations in earlier modelling efforts for solids on the coupling between the particles and turbulence, and better fits to the turbulence modulation amplitude as function of D/l was achieved for a number of data sets. Explicit algebraic forms of the full model were derived using asymptotic analysis, and these are general enough for application to emulsions, bubbles and solids in bulk regions of multiphase turbulent flow. Rigorous particle-kinetic theory was used to derive the work exchanged between the particles and the fluid due to both drag and added mass forces, where the latter is essential for low or moderate particle/fluid density ratios, enabling a well justified model also for emulsions and bubbles. A novel sub-model for turbulence production by vortex shedding due to turbulence-generated slip velocity was incorporated, where earlier models took the slip velocity as an input parameter. The correct asymptotic limit of vanishing turbulence modulation for small tracer particles was also provided, giving better fit to the data for small particles. We found that turbulence augmentation for large diameter solids is due to vortex shedding, and turbulence suppression for small diameters is due to mainly to turbulent drag forces and extra fluid dissipation – a conclusion that agrees with earlier models for solids, despite their possible shortcomings. An important finding is that the mechanisms for turbulence suppression for bubbles and emulsion droplets are similar to those of solids, but with the addition of added mass scaling factors. Another important observation is that augmentation may not occur at all for bubbles or emulsion droplets since the larger diameters require moderate turbulence levels to prevent breakup, so that vortex shedding may be insignificant.

Self-Adaptive Turbulence Eddy Simulation with a high-order finite differencing method for high Reynolds number complex flows

Accurate prediction of high Reynolds number wall-bounded flows is a challenge for turbulence modeling and numerical discretization. Moreover, complex engineering applications increase the difficulties in implementing robust and low-dissipation numerical schemes. A newly developed unified turbulence approach, Self-Adaptive Turbulence Eddy Simulation (SATES), is assessed in the simulation of a multi-element 30P30N airfoil at a high Reynolds number of 1.71 × 106. To reduce the numerical error, high-order finite differencing-based weighted compact nonlinear schemes (WCNSs) are successfully applied. The effects of grid resolution, numerical schemes and turbulence model are investigated by grouping studies. The SATES shows low sensitivity to grid resolution and predicts satisfactory results even on a coarse grid containing approximately 7.2 million cells. Compared with second-order accurate central differencing, the high-order accurate WCNS has less numerical dissipation and significantly improves the resolution of turbulence structures and the accuracy of numerical results. Z-type nonlinear weighted averaging interpolation, designed for shock capturing property in the WCNS scheme, leads to extra numerical dissipation and affects the accuracy of the results. The numerical results of the SATES are more accurate than those of the Smagorinsky Large Eddy Simulation (LES) and the Improved Delayed Detached Eddy Simulation (IDDES). The SATES provides sufficient modeling of the boundary layer on the coarse grid and a fast ‘transition’ from the RANS to the LES mode for accurate prediction of the separated shear layer and flap boundary layer separation. These results confirm that the SATES turbulence modeling approach is efficient for high Reynolds number flows and that the high-order accurate WCNS schemes show sufficient robustness and accuracy for engineering applications.

A flow control method for the leakage reduction of annular gas seals using the helical gas curtain generator

Advanced sealing technology is crucial for turbine efficiency. To reduce seal leakage, the flow control method based on gas injection is an attractive solution. The traditional flow control methods primarily focus on blocking the axial leakage flow, neglecting the helical flow effect of the leakage flow at the design stage. In this paper, a novel flow control method is proposed for the leakage reduction of annular gas seals using the helical gas curtain generator (HGCG). The helical flow effect of the leakage flow is first considered for further sealing enhancement. The design conception of the HGCG involves connecting seal cavities and teeth tips using the specially designed pad with helical injection slots. Driven by the intrinsic pressure difference between the connected areas, helical gas curtains are passively generated, producing multi-row fluidic barriers against the helical leakage flow at each teeth tip. Numerical investigation of a labyrinth seal with HGCGs shows that the proposed method effectively enforces flow resistance at teeth tips, resulting in an extra dissipation effect. The performance of the HGCG is enhanced as the helical flow effect increases. The maximum leakage reduction ratio reaches 59.2%, over eight times higher than that of the traditional air curtain technology (ACT). In addition, the HGCG increases the effective damping coefficient by 45% compared to the ACT. The proposed HGCG presents significant superiority in sealing improvement and rotordynamic stability enhancement.

Theoretical investigation on the impact effect in dynamic analysis of self‐centering pier system based on the Hertz contact‐pounding model

AbstractThe self‐centering (SC) pier system exhibits different mechanical behaviors under an earthquake compared with that in quasi‐static experiments, especially the impact effect on the rocking interface. Published theoretical analysis and experiments had indicated that the impact events significantly affected the rocking motion, including extra energy dissipation, greater contact stress, and even more serious damage. To describe the impact mechanism more clearly and obtain the impact responses more directly, this study investigates a dynamic theoretical model considering the impact effect and its influences on the seismic responses and rocking interface. First, a dynamic theoretical model based on the Hertz contact‐pounding model specific to the SC pier system is proposed, and the impact mechanism is described in detail. Subsequently, dynamic motion equations considering the impact effect are derived. The key parameters of the contact‐pounding element (stiffness KHertz and damping CHertz) that significantly affect the impact process are presented. The accuracy of the proposed theoretical model is verified by test data and a finite element model (FEM). Finally, the influences of the key parameters (KHertz and CHertz) on the impact force are analyzed, and the impact effect on the rocking interface is investigated. The results indicate that the impact effect significantly increases the vertical acceleration and contact stress, particularly under near‐fault earthquakes. This may cause unexpected damage on the rocking interface and should be an important consideration in the optimize design of SC pier system.

Small Knudsen rate of convergence to contact wave for the Landau equation

In this paper, we study the hydrodynamic limit to contact discontinuity of the compressible Euler system for the Landau equation with the physical Coulomb interaction as the Knudsen number ε>0 is vanishing. Our results are twofold. First, we construct the unique global-in-time solution to the Landau equation with suitably small initial data as ε>0 is sufficiently small. Second, we prove that the solution of the Landau equation converges to a local Maxwellian whose fluid quantities are the contact discontinuity of the corresponding Euler system uniformly away from the discontinuity as ε→0. Moreover, the uniform convergence rate is also obtained. The main idea is to introduce a new time-velocity weigh function to induce an extra quartic dissipation term for treating the large-velocity growth in the nonlinear estimates due to degeneration of the linearized Landau operator in the Coulomb case.

Experimental study on seismic behavior of concrete walls with external magnetorheological dampers

A reinforcing method for developing a kind of shear wall with both excellent self-centering capacity and high energy dissipation was presented, in which carbon fiber reinforced polymer (CFRP) bars or steel strands were longitudinally placed to provide a restoring effect, while magnetorheological (MR) dampers were externally coupled with wall panels to provide extra energy dissipation. To verify the feasibility of this method, three slender concrete walls were tested under reversed cyclic lateral loading while subjected to constant axial compression with an axial load ratio of 0.26, of which one was reinforced with CFRP bars, the second one was reinforced with CFRP bars and two external MR dampers, and the third one was reinforced with steel strands and two external MR dampers. The results indicated that the test walls possessed remarkable load-resisting capacities even when the lateral drift was as large as 1.8% as well as a lateral deformation recovery rate of 74%–89% after unloading. Owing to external MR dampers, the initial stiffness and ductility were increased by 28.3% and 29.8%, respectively. In addition, a specific quantitative analysis showed that the dampers could increase the cumulative energy dissipation of shear walls by 16.6%, of which the contribution provided by MR damper was 64.8%, confirming the effectiveness of MR dampers in improving energy dissipation while preserving the self-centering capacity.

Nonlinear Optical Radiation of a Lithium Niobate Microcavity

The nonlinear optical radiation of an integrated lithium niobate microcavity is demonstrated, which has been neglected in previous studies of nonlinear photonic devices. We find that the nonlinear coupling between confined optical modes on the chip and continuum modes in free space can be greatly enhanced on the platform of integrated microcavity, with feasible relaxation of the phase-matching condition. With an infrared pump laser, we observe the vertical radiation of second-harmonic wave at the visible band, which indicates a robust phase-matching-free chip-to-free-space frequency converter and also unveils an extra energy dissipation channel for integrated devices. Such an unexpected coherent nonlinear interaction between the free-space beam and the confined mode is also validated by the different frequency generation. Furthermore, based on the phase-matching-free nature of the nonlinear radiation, we build an integrated atomic gas sensor to characterize Rb isotopes with a single telecom laser. The unveiled mechanism of nonlinear optical radiation is universal for all dielectric photonic integrated devices, and provides a simple and robust chip-to-free-space as well as visible-to-telecom interface.

Noise reduction by perforated cascades in annular ducts

One of the latest trends in noise control relating to aeroacoustics is to mimic the silent flight capability of owls. Particularly, porosity is most often applied on cascades in ducts with axial flows, such as stator structures in an aero-engine. However, current acoustic scattering models of perforated cascades are based on two-dimensional methods without including the three-dimensional effects. In this paper, we present a fully three-dimensional acoustic scattering model for perforated cascades based on the lifting surface theory in which the dominant sound source reduces to dipoles alone under the thin airfoil assumption. Accordingly, the acoustic scattering of perforated cascades with single-mode incident wave was studied and obvious noise reduction was observed. The optimum Rayleigh conductivity and the maximum noise-reducing capability of the porosity varied substantially with different incident duct modes, whilst a larger cascade chord-length could achieve more noise reduction at optimum porosity. With a background flow, the Kutta condition can greatly influence the overall distribution of the unsteady loading on vanes. Additionally, the unsteady vortex shedding at the trailing edge offers extra sound energy dissipation mechanism. Therefore, the implementation of porosity on cascades is much different to the design of a traditional acoustic liner.

Spectral Gap Formation to Kinetic Equations with Soft Potentials in Bounded Domain

It has been unknown in kinetic theory whether the linearized Boltzmann or Landau equation with soft potentials admits a spectral gap in the spatially inhomogeneous setting. Most of existing works indicate a negative answer because the spectrum of two linearized self-adjoint collision operators is accumulated to the origin in case of soft interactions. In the paper we rather prove it in an affirmative way when the space domain is bounded with an inflow boundary condition. The key strategy is to introduce a new Hilbert space with an exponential weight function that involves the inner product of space and velocity variables and also has the strictly positive upper and lower bounds. The action of the transport operator on such space-velocity dependent weight function induces an extra non-degenerate relaxation dissipation in large velocity that can be employed to compensate the degenerate spectral gap and hence give the exponential decay for solutions in contrast with the sub-exponential decay in either the spatially homogeneous case or the case of torus domain. The result reveals a new insight of hypocoercivity for kinetic equations with soft potentials in the specified situation.