Stability analysis of single-phase thermosyphon loops by finite-difference numerical methods

Tensegrity system dynamics based on finite element method

Eulerian-Lagrangian localized adjoint methods for a nonlinear advection-diffusion equation

Modern fiber composites have extremely high specific strength and stiffness, significantly exceeding the corresponding characteristics of traditional metal structural materials. In the present work, the effect of the elastic modulus of circular and spiral ribs on the stress-strain state of a composite mesh compartment with cutouts reinforced with bezels investigated. For the calculations, the finite element method is used, which makes it possible to naturally consider the cutouts, stepwise varying thicknesses, various kinds of boundary conditions, and loads. The computational model of the composite lattice structure consisting of ribs is a finite element model constructed from beam elements. The parameters of the stress-strain state of a composite mesh compartment with notches reinforced by edgings are determined. The graphical dependences of the values of maximum stresses in the ribs and sheathing on the value of the modulus of elasticity are presented for the first time. The analysis of the obtained results carried out, which makes it possible to develop recommendations for an informed choice of carbon fabric types with different physical and mechanical characteristics for manufacturing the spiral and ring ribs, as well as the skin of the designed composite mesh compartments.

Two-scale model for quantifying the effects of laminar and turbulent mixing on algal growth in loop photobioreactors

Local heat transfer to impinging liquid jets in the initially laminar, transitional, and turbulent regimes

Low Reynolds number k-ϵ model is employed to demonstrate the Bejan's numerical heat and mass flow visualization in turbulent boundary layer regime. The governing turbulence kinetic energy and its dissipation rate equations are included along with conservation equations. The turbulent flow equations are highly nonlinear and coupled in nature, hence, an unconditionally stable Crank–Nicolson finite difference approach is deployed. The computer-generated heatlines, masslines, streamlines, and isotherms for different control parameters are analyzed in the turbulent flow regime. Average momentum, thermal, and concentration diffusion rates are increased upon the increment in in laminar regime and are decreasing with rising in turbulent regime. The stream, heat and masslines deviate more in turbulent regime when compared to laminar regime. Further, the kinetic energy and dissipation rate contours are deviated more in turbulent heat and mass flow regime for different values of . Mainly, in this research paper, the authors made an attempt to demonstrate the turbulent heat and mass flow visualization about a vertical plate using boundary layer approximations through LRN k-ϵ turbulence model.

Entropy generation due to natural convection has been calculated for a wide range of Rayleigh number (Ra) in both laminar (104 ≤ Ra ≤ 108) and turbulent (1010 ≤ Ra ≤ 1012) flow regimes, for diameter ratio of 2 ≤ D/d ≤ 5, for an isothermal vertical cylinder fitted with annular fins. In the laminar regime, the entropy generation was predominantly caused by heat transfer (conduction and convection) and the viscous contribution was negligible with respect to heat transfer. But in the turbulent regime, entropy generation due to fluid friction is significant enough although heat transfer entropy generation is still dominant. The results demonstrate that the degree of irreversibility is higher in case of finned configuration when compared with unfinned one. With the deployment of a merit function combining the first and second laws of thermodynamics, we have tried to delineate the thermodynamic performance of finned cylinder with natural convection. So, we have defined the ratio (I/Q)finned/(I/Q)unfinned. The ratio (I/Q)finned/(I/Q)unfinned gets its minimum value at optimum fin spacing where maximum heat transfer occurs in turbulent flow, whereas in laminar flow the ratio (I/Q)finned/(I/Q)unfinned decreases continuously with the increase in number of fins.

On the flow over cavities of large aspect ratio: A physical analysis

We study the radiation build-up in laminar and turbulent generation regimes in quasi-CW Raman fiber laser. We found the resulted spectral shape and generation type is defined by the total spectral broadening/narrowing balance over laser cavity round-trip, which is substantially different in different regimes starting from first round-trips of the radiation build-up. In turbulent regime, the steady-state is reached only after a few round-trips, while in the laminar regime the laser approaches the equilibrium spectrum shape asymptotically.

Technologies for reducing hydrodynamic skin-friction drag have a huge potential for energy-savings in applications ranging from propulsion of marine vessels to transporting liquids through pipes. The majority of previous experimental studies using hydrophobic surfaces have successfully shown skin-friction drag reduction in the laminar and transitional flow regimes (typically Reynolds numbers less than ≃106 for external flows). However, this hydrophobicity induced drag reduction is known to diminish with increasing Reynolds numbers in experiments involving wall bounded turbulent flows. Using random-textured hydrophobic surfaces (fabricated using large-length scalable thermal spray processes) on a flat plate geometry, we present water-tunnel test data with Reynolds numbers ranging from 106 to 9 × 106 that show sustained skin-friction drag reduction of 20%–30% in such turbulent flow regimes. Furthermore, we provide evidence that apart from the formation of a Cassie state and hydrophobicity, we also need a low surface roughness and an enhanced ability of the textured surface to retain trapped air, for sustained drag reduction in turbulent flow regimes. Specifically, for the hydrophobic test surfaces of the present and previous studies, we show that drag reduction seen at lower Reynolds numbers diminishes with increasing Reynolds number when the surface roughness of the underlying texture becomes comparable to the viscous sublayer thickness. Conversely, test data show that textures with surface roughness significantly smaller than the viscous sublayer thickness and textures with high porosity show sustained drag reduction in the turbulent flow regime. The present experiments represent a significant technological advancement and one of the very few demonstrations of skin-friction reduction in the turbulent regime using random-textured hydrophobic surfaces in an external flow configuration. The scalability of the fabrication method, the passive nature of this surface technology, and the obtained results in the turbulent regime make such hydrophobic surfaces a potentially attractive option for hydrodynamic skin-friction drag reduction.

Electrical effects are known to play an important role in particle-laden flows, yet a holistic view of how they modulate turbulence remains elusive due to the complexity of multifield coupling. Here, we present a total of 119 direct numerical simulations of particle-laden turbulent channel flow that reveal a striking ability of electrical effects to induce turbulence relaminarisation and markedly alter wall drag. As expected, the transition from turbulence to laminar flow is accompanied by abrupt changes in the statistical properties of both the fluid and particulate phases. Nevertheless, with increasing electrical effects, the wall-normal profiles of the mean streamwise fluid velocity and mean local particle mass loading exhibit opposite trends in the turbulent and laminar regimes, arising from the competition between turbophoresis and electrostatic drift. We identify three distinct flow regimes resulting from the electrical effects: a drag-reduced turbulent regime, a drag-reduced laminar regime, and a drag-enhanced laminar regime. It is revealed that relaminarization originates from the complete suppression of the streak breakdown in the near-wall self-sustaining cycle, followed by the sequential inhibition of other subprocesses in the cycle. In the turbulent regime, increasing electrical effects induce opposing trends in Reynolds and particle stress contributions to drag, yielding a non-monotonic drag response. In laminar regimes, by contrast, the drag coefficient increases monotonically as the Reynolds stress vanishes and particle-induced stress becomes dominant.

This study investigated the heat transfer and fluid flow characteristics of laminar and turbulent regimes in microchannels using a combined experimental and numerical approach. The objective of the study was to evaluate the influence of Reynolds number on thermal and hydraulic performance and to assess the applicability of conventional heat transfer correlations at microscale dimensions. An experimental setup consisting of a heated microchannel test section, controlled flow delivery system, and precision temperature and pressure measurement instruments was employed to obtain data for a wide range of Reynolds numbers covering both laminar and turbulent flow regimes. Complementary computational fluid dynamics simulations were conducted using appropriate governing equations and boundary conditions to predict velocity, temperature, and pressure fields within the microchannel. The results showed that the Nusselt number increased with Reynolds number in both flow regimes, with turbulent flow exhibiting significantly higher heat transfer rates than laminar flow. However, the turbulent regime was also associated with substantially higher friction factors and pressure drops, indicating increased pumping power requirements. The experimental and numerical results demonstrated close agreement, validating the reliability of the numerical model. The study further revealed that microscale effects such as entrance length influence and enhanced convection caused deviations from classical theoretical predictions developed for macroscale channels. Overall, the findings established that optimal microchannel thermal performance was achieved at moderate turbulent Reynolds numbers, where heat transfer enhancement outweighed hydraulic penalties. The study provided valuable insights for the design and optimisation of microchannel heat exchangers used in electronics cooling, energy systems, and other high heat flux applications.

On predicting the length, width, and volume of the jet diffusion flame

Convection in a vertical duct under the chemical reaction influence using Robin boundary conditions

In this manuscript, a new model of the time-fractional Galilei-invariant advection-diffusion model of distributed order is studied. An efficient hybrid numerical approach with high accuracy is used to estimate this equation. The finite difference numerical method is used to approximate the fractional operator in terms of the time variable and to approximate the integral term of distributed order, the Gaussian–Legendre integration is applied. To obtain a fully discrete numerical approach, we used a spectral element numerical approach, in which Legendre polynomials are used as the basis function. For the proposed numerical approach, the error and stability analysis are studied. For the efficiency of the numerical approach, some numerical examples are presented with graphs and tables.