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The use of additives, generally called DRAs (Drag Reducing Additives), has been proposed to re-duce the energy consumption in pressurized pipes. Although many research works have been conducted to analyze the effect of these additives, less attention have been devoted to the application of coatings to the pipe wall. This paper demonstrates that the application of a hydrophobic coating to the pipe can lead to a head loss reduction for a transition flow regime with moderate Reynolds number values (Re). For this purpose, an experiment was conducted to compare the performance of both coated and uncoated pipes by measuring the head losses and assessing the Drag Reduction Percentage (%DR) and the pipe friction factor (f). This was done for two Polyvinylchloride (PVC) pipes with different nominal diameters (PVC90 and PVC63). In addition, the flow velocity distribution was also measured in all these tests. The %DR decreased as the Re values increased, with the reduction being notably less pronounced for higher Re values. This could be explained by the fact that a partial slip condition is induced by the hydrophobic product. Its effect is significant for a transition regime where the effect of viscosity is important, but it becomes negligible for increasing levels of turbulence. No significant differences were observed in the flow distribution between coated and uncoated pipes, which seems to indicate that the velocity change could be limited to the near-wall viscous sublayer. The results of this work open an important research line aimed at reducing energy costs and the carbon footprint in pipe fluid distribution systems. [ABSTRACT FROM AUTHOR]

In the impinging region of opposing jets, strong mixing and significant energy dissipation are observed, but the mixing parameters invariably change with the opposed impinging strength (OIS). In this paper, the ratio of the turbulent kinetic energy (TKE) intensity at the theoretical impinging point to the nozzle exit is defined as the opposed impinging strength. To examine the mixing properties of opposed impinging jets (OIJs) in a limited field under various OIS, a renormalization group k-ε turbulence model is employed to calculate three-dimensional OIJs under various OIS. The nozzle exit diameter is set to 0.6 m, and the inlet velocity is between 0.08 and 8 m/s, so the simulations are performed at Re between 4.8 × 104 and 4.8 × 106. This work focuses on the radial and vertical jets produced after impinging as well as the distribution of the TKE, flow field, and vortices. A thorough investigation reveals that although the OIS of the jets is primarily determined by the degree of jet development, it increases with the Reynolds number. A low OIS results in less mixing in the surrounding water and relatively unconstrained jet generation; however, it also results in limited energy extraction from the fluid. Once the OIS is high, there is more mixing in the surrounding water, and more energy is also lost during impinging. The distribution of vortices in the vortex field is not only influenced by the OIS but is also very closely related to the scale of the limited mixing field. [ABSTRACT FROM AUTHOR]

In this paper, large-eddy simulation of the interaction between a shock wave and the hypersonic turbulent boundary layer in a compression corner with a fixed 34° deflection angle at Ma = 6 for different Reynolds number cases is conducted. For investigating the effects of the Reynolds number for hypersonic cases, three cases where the free-stream Reynolds numbers are 14000, 20000, and 30000/mm are selected. The averaged statistics, such as the mean velocity, the skin friction, the heat flux, and the wall pressure, are used in this paper. The flow structures in the compression ramp including the shock wave and interaction region are discussed. The decomposition of the mean skin-friction drag for the flat flow is extended to be used in the compression corner. In addition, the turbulent kinetic energy is studied through the decomposition of the mean skin-friction drag for the flat-plate region and the corner region. It is found that higher Reynolds numbers would increase the turbulent kinetic energy by turbulent dissipation at the interaction region, while higher Reynolds numbers would decrease the turbulent kinetic energy by turbulent dissipation after reattachment. In addition, it is also found that the turbulent kinetic energy is larger with a higher Reynolds number and higher turbulent kinetic energy inhibits the movement from the separation point to the inflection point (x = 0 mm), which deduces larger separation bubbles. [ABSTRACT FROM AUTHOR]

This paper presents a modal and non-modal stability analysis of the boundary layer developed on a hot plate. A liquid-type temperature-dependent viscosity model has been considered to account for the viscosity variation in the boundary layer region. The base flow is uniform and parallel to the surface at the leading edge. The base flow solution is obtained using an open-source finite volume source code. The Reynolds number (Re) is defined based on the displacement thickness (δ*) at the inlet of the computation domain. The spectral collocation method is used for spatial discretization of governing stability equations. The formulated generalized eigenvalue problem (EVP) is solved using Arnoldi's iterative algorithm with the shift and invert strategy. The global temporal eigenmodes are calculated for the sensitivity parameter β from 1 to 7, Re = 135, 270, and 405, and the span wise wave-number N from 0 to 1. The modal and non-modal stability analysis have been performed to study the least stable eigenmodes and the optimal initial conditions and perturbations (using mode superposition), respectively. The global temporal eigenmodes are found more stable for β > 0 at a given value of N. Thus, heating the boundary layer within the considered range of β (0 < β ≤ 7) leads to the stabilization of flow. The optimal energy growth increases with the β due to reducing the perturbation energy loss. Tilted elongated structures of the optimal perturbations are found near the outflow boundary. However, the length scale of the elongated cellular mode structure reduces with increase in β. The same qualitative structure of the optimal perturbations has been found at a given value of N. [ABSTRACT FROM AUTHOR]

Proper EMA-balance (balance of kinetic energy, linear momentum and angular momentum), pressure-robustness and Re-semi-robustness (Re: Reynolds number) are three important properties of Navier–Stokes simulations with exactly divergence-free elements. This EMA-balance makes a method conserve kinetic energy, linear momentum and angular momentum in an appropriate sense; pressure-robustness means that the velocity errors are independent of the pressure; Re-semi-robustness means that the constants appearing in the error bounds of kinetic and dissipation energies do not explicitly depend on inverse powers of the viscosity. In this paper, based on the pressure-robust reconstruction framework and certain suggested reconstruction operators in Linke and Merdon [Comput. Methods Appl. Mech. Eng.311 (2016) 304–326], we propose a reconstruction method for a class of non-divergence-free simplicial elements which admits almost all the above properties. The only exception is the energy balance, where kinetic energy should be replaced by a suitably redefined discrete energy. The lowest order case is the Bernardi–Raugel element on general shape-regular meshes. Some numerical comparisons with exactly divergence-free methods, the original pressure-robust reconstruction methods and the EMAC method are provided to confirm our theoretical results. [ABSTRACT FROM AUTHOR]

The object of this paper is to apply spectral geometry methods to predicting pressure losses in mildly curved pipes at high Reynolds numbers. The obtained formula for the pressure losses is theoretically justified and provides good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

This paper studies the interaction of two droplets splashing on a stationary film. A source term is included in the large-density-ratio pseudopotential lattice Boltzmann method to achieve tuneable surface tension. This model offers excellent numerical accuracy and stability for droplet impacts on liquid films. The influence of the Reynolds number, Weber number, film thickness, and horizontal/vertical distance between the droplets on the crown geometry evolution is investigated. The energy loss during the impact process and the velocity discontinuity in the liquid film are the two key factors affecting the stability and evolution process of the crown. A smaller Reynolds number or thicker liquid film enhances the energy loss and decreases the velocity discontinuity, leading to more stable side and central jets. An increase in the horizontal distance between the droplets reduces the velocity discontinuity, causing the central jet height to decrease. An increase in the Weber number does not affect the energy loss or velocity discontinuity, but the lower surface tension leads to a dramatic deformation in both the central and side jets. A vertical distance between the two droplets causes an asymmetrical evolution of the crown geometry, and postpones the breakup time of the central jet. [ABSTRACT FROM AUTHOR]

Hydrofoil is widely used in hydraulic machinery, the complex shear flow and wake vortex with it can cause energy dissipation of system, affecting the stable operation of the unit. This paper takes the guide vane of a Francis turbine as the research object, and adopts the SST k − ω turbulence model to simulate the complex unsteady flow in a three-dimensional hydrofoil-channel. Entropy production theory is introduced to evaluate the energy dissipation. The effects of attack angle, gap ratio and Reynolds number on the flow loss are studied. The results show that: (1) In the hydrofoil-channel, the dissipation caused by the velocity gradient always dominates the irregular flow of fluid field. So the loss caused by the turbulent dissipation term is the main source of the loss in mainstream zone, accounting for more than 95%, while the loss caused by the viscous dissipation term accounts for only about 2% of the total loss, which can be almost ignored; (2) Changing the attack angle have a significant effect on the flow separation of the suction surface and the shear effect in the wake vortex zone, causing the energy loss to fluctuate accordingly. And 2.5° is the optimal attack angle. On this working condition, the loss decreases to the minimum value of 2.2714W/K and 0.1432 m, a decrease of 57% compared to the initial value; (3) The bottom boundary of channel suppress the development of wing tip vortex, reducing gap ratio will reduce the loss caused by wake vortex to some extent. Among them, S = 0.3 is the optimal gap ratio. Under this operating condition, the loss is 2.2941W/K and 0.1447m, a decrease of 56% compared to the initial value; (4) Affected by flow separation and wake vortexes, more than 80% of energy loss occurs in the middle and downstream zones. The effects of attack angle, gap ratio and Reynolds number on the fluid field are different. Increasing attack angle and flow velocity significantly exacerbate the flow separation and shear flow, while reducing gap ratio would inhibit the adverse flow in downstream wing tip vortex zone. • Applying entropy production theory to energy dissipation in hydrofoil fluid fields. • Discovering the dominant role of velocity fluctuations in turbulence. • The effects of attack angle, gap ratio, Reynolds number on vortex structure and energy loss are discussed, and the optimal conditions are given. [ABSTRACT FROM AUTHOR]

In this paper a numerical investigation on shell-and-tube heat exchanger (STHE) with three configurations of novel segmental curved baffles (convex core baffles/convex peripheral baffles (CB1), convex peripheral baffles/convex core baffles (CB2), and convex peripheral baffles/concave core baffles (CB3)) for thermohydralic performance analysis and enhancement is presented. STHE with conventional straight baffles (SB) is also numerically studied for comparison. Each baffle configuration was tested under different Reynolds number in shell side (Re c) (17693–30331) based on the overall heat transfer coefficient (U), effectiveness (ε), and number of transfer unit (NTU). Also, pressure loss across the shell side is calculated to estimate the energy loss from the system due to the suggested design. In addition, the influence the inlet cold fluid temperature, baffle cutting ratio, and baffle spacing are tested, analyzed and discussed. The results showed that CB3 configuration results in a significant augmentation of the heat exchanger performance compared with the others two configurations for all investigated cases. The enhancements in U , ε, NTU by using CB3 configuration compared to SB are 48.98%−51.31% and 22.92%–23.62% and 48.98%–51.31% respectively, with decreasing in pressure loss by 12.40%–11.22%. [ABSTRACT FROM AUTHOR]