Laminar Region Research Articles

Large Eddy Simulation of Planar Free Jet Flow Using the Space-Time Conservation Element/Solution Element Scheme

In order to investigate the characteristics of planar free jet flows, the Monotonically Integrated Large Eddy Simulation (MILES) method with the high-resolution space-time conservation element/solution element (CE/SE) scheme was used to simulate the two-dimensional free jet flows. For free jet flows within the Reynolds number range 35300~2200, the flow structures, averaged and instant velocity, turbulence intensity, velocity profiles of different cross-sections were examined respectively. The simulation results agree well with the experiment data. A region of constant height exists right after the jet outlet, in which the average centerline velocity and turbulence intensity maintains same with the jet outlet, and the instant fluctuation amplitudes are also small. For turbulent flows, the length of the undisturbed region is equal to the length of region of constant height. While near the laminar region, the length of undisturbed region is slightly larger than the length of zone of constant height. The vortex structures start to generate after the undisturbed region, develop coherently in the region of flow establishment, and break down gradually in the region of fully established flow region. As the Reynolds number increases, the length of zone of constant height, the length of undisturbed region and the length of potential core region all decrease correspondingly.

Investigation of Inclined Turbulators for Heat Transfer Enhancement in a Solar Air Heater

ABSTRACTIn the present study, the effect of forward inclined turbulators on the heat transfer enhancement in a duct is investigated, for forced convection. Turbulator configurations with three different pitch ratios and three different inclination angles are investigated for seven Reynolds numbers within the range 500–50,000. Investigations are performed experimentally as well as computationally, within a computational fluid dynamics framework. A distinguishing feature of the latter has been the employment of a turbulence model, the transitional shear stress transport model that is applicable throughout the presently considered range of Reynolds numbers containing laminar, transitional, and turbulent regions. At the beginning of the study, measurements and predictions are validated against analytical and empirical expressions known for a plain duct. The results obtained for turbulators configurations indicate that Nusselt number increases with the inclination angle but decreases with the pitch ratio. The influence of the inclination angle on the Nusselt number and thermal enhancement factor is found to be stronger than that of the pitch ratio. For all Reynolds numbers and for all configurations, the thermohydraulic performance is observed to increase, leading to thermal enhancement factors within the range 2–5. In all cases, a quite good agreement of the predictions and experiments is observed, which increases the confidence in the accuracy of both approaches.

Assessment of self-adapting local projection-based solvers for laminar and turbulent industrial flows

In this work, we study the performance of some local projection-based solvers in the Large Eddy Simulation (LES) of laminar and turbulent flows governed by the incompressible Navier–Stokes Equations (NSE). On one side, we focus on a high-order term-by-term stabilization Finite Element (FE) method that has one level, in the sense that it is defined on a single mesh, and in which the projection-stabilized structure of standard Local Projection Stabilization (LPS) methods is replaced by an interpolation-stabilized structure. The interest of LPS methods is that they ensure a self-adapting high accuracy in laminar regions of turbulent flows, which turns to be of overall optimal high accuracy if the flow is fully laminar. On the other side, we propose a new Reduced Basis (RB) Variational Multi-Scale (VMS)-Smargorinsky turbulence model, based upon an empirical interpolation of the sub-grid eddy viscosity term. This method yields dramatical improvements of the computing time for benchmark flows. An overview about known results from the numerical analysis of the proposed methods is given, by highlighting the used mathematical tools. In the numerical study, we have considered two well known problems with applications in industry: the (3D) turbulent flow in a channel and the (2D/3D) recirculating flow in a lid-driven cavity.

Complexity of the laminar-turbulent boundary in pipe flow

Over the past decade, the edge of chaos has proven to be a fruitful starting point for investigations of shear flows when the laminar base flow is linearly stable. Numerous computational studies of shear flows demonstrated the existence of states that separate laminar and turbulent regions of the state space. In addition, some studies determined invariant solutions that reside on this edge. In this paper, we study the unstable manifold of one such solution with the aid of continuous symmetry-reduction, which we formulate here for the first time for the simultaneous quotiening of axial and azimuthal symmetries. Upon our investigation of the unstable manifold, we discover a previously unknown traveling wave solution on the laminar-turbulent boundary with a relatively complex structure. By means of low-dimensional projections, we visualize different dynamical paths that connect these solutions to the turbulence. Our numerical experiments demonstrate that the laminar-turbulent boundary exhibits qualitatively different regions whose properties are influenced by the nearby invariant solutions.

Direct Numerical Simulation of Laminar-Turbulent Transition in a Non-Axisymmetric Stenosis Model for Newtonian vs. Shear-Thinning Non-Newtonian Rheologies

Steady inflow through a non-axisymmetric stenotic model at Re = 500-1000 for Newtonian and shear-thinning non-Newtonian rheologies was studied numerically to investigate the experimental evidence of stabilizing effect of shear-thinning fluids. A minimally-dissipative and energy-preserving finite-element based code was used, and results were verified against a higher-order spectral element code. Below a critical Reynolds number (Recrit), both rheology models showed non-stationary and intermittent flow in time, with successive phases of laminar and turbulent regions that were quasi-periodic with long observation times. Using the conventional definition of Reynolds number based on high-shear viscosity, transition was delayed for the shear-thinning model, with Recrit of 760 vs. 700 for the Newtonian rheology, a delay broadly consistent with previous reports. However, using domain-averaged viscosity aposteriori, the Recrit for the shear-thinning model dropped to 710, closer to the Newtonian value. The transition process and the vortical structures for both rheologies were similar, albeit with some differences in the turbulent kinetic energy and evolution of non-stationary perturbations near the transition point. This suggests that previously-reported delays in transition to turbulence for blood vs. Newtonian fluids may be due to rheological factors other than shear-thinning, such as viscoelasticity. Our study also further highlights the challenges of defining non-Newtonian Reynolds numbers for flows in non-trivial geometries.

Experimental evidence of power law reinjection in chaotic intermittency

The main classical results on classical chaotic intermittency were based on a uniform reinjection on the laminar region. Recently, for a wide class of 1D maps, a generalized power law reinjection was found, so the classical theory of intermittency was generalized for 1D-maps. We propose experimental evidences that this generalization works also in an experiment based on an analog circuit. In the noisy case, we also found good agreement with a recently proposed theory for this scenario.

Formation of free round jets with long laminar regions at large Reynolds numbers

The paper describes a new, simple method for the formation of free round jets with long laminar regions by a jet-forming device of ∼1.5 jet diameters in size. Submerged jets of 0.12 m diameter at Reynolds numbers of 2000–12 560 are experimentally studied. It is shown that for the optimal regime, the laminar region length reaches 5.5 diameters for Reynolds number ∼10 000 which is not achievable for other methods of laminar jet formation. To explain the existence of the optimal regime, a steady flow calculation in the forming unit and a stability analysis of outcoming jet velocity profiles are conducted. The shortening of the laminar regions, compared with the optimal regime, is explained by the higher incoming turbulence level for lower velocities and by the increase of perturbation growth rates for larger velocities. The initial laminar regions of free jets can be used for organising air curtains for the protection of objects in medicine and technologies by creating the air field with desired properties not mixed with ambient air. Free jets with long laminar regions can also be used for detailed studies of perturbation growth and transition to turbulence in round jets.

Investigation of unsteady, hypersonic, laminar separated flows over a double cone geometry using a kinetic approach

Shock-dominated hypersonic laminar flows over a double cone are investigated using time accurate direct simulation Monte Carlo combined with the residuals algorithm for unit Reynolds numbers gradually increasing from 9.35 × 104 to 3.74 × 105 m−1 at a Mach number of about 16. The main flow features, such as the strong bow-shock, location of the separation shock, the triple point, and the entire laminar separated region, show a time-dependent behavior. Although the separation shock angle is found to be similar for all Re numbers, the effects of Reynolds number on the structure and extent of the separation region are profound. As the Reynolds number is increased, larger pressure values in the under-expanded jet region due to strong shock interactions form more prominent λ-shocklets in the supersonic region between two contact surfaces. Likewise, the surface parameters, especially on the second cone surface, show a strong dependence on the Reynolds number, with skin friction, pressure, and surface heating rates increasing and velocity slip and temperature jump values decreasing for increasing Re number. A Kelvin-Helmholtz instability arising at the shear layer results in an unsteady flow for the highest Reynolds number. These findings suggest that consideration of experimental measurement times is important when it comes to determining the steady state surface parameters even for a relatively simple double cone geometry at moderately large Reynolds numbers.

Computational study of heat transfer from the inner surface of a circular tube to force high temperature liquid metal flow in laminar and transition regions

Heat transfer through forced convection from the inner surface of a circular tube to force the flow of liquid sodium in the laminar and transition regions were numerically analysed for two types of tube geometries (concentric annular and circular tubes) and two types of equivalent diameters (hydraulic and thermal equivalent diameters). The unsteady laminar three-dimensional basic equations for forced convection heat transfer caused by a step heat flux were numerically solved until a steady state is attained. The code of the parabolic hyperbolic or elliptic numerical integration code series (PHOENICS) was used for calculations by considering relevant temperature dependent thermo-physical properties. The concentric annular tube has a test tube with inner and outer diameters of 7.6 and 14.3 mm, respectively, has a heated length of 52 mm, and an L/d of 6.84. The two circular tubes have inner diameters of 6.7 and 19.3 mm with L/d of 7.76 and 2.69, respectively, and a heated length of 52 mm. The inlet liquid temperature, inlet liquid velocity, and surface heat flux were equally set for each test tube as Tin≅573 to 585 K, uin = 0.0852 to 1 m/s, and q = 2×105 to 2.5×106 W/m2, respectively. The increase in temperature from the leading edge of the heated section to the outlet of the circular tubes (with a hydraulic diameter of dH = 6.7 mm and a thermal equivalent diameter dte = 19.3 mm) was approximately 2.70 and 1.21 times as large as the corresponding values of the concentric annular tube with an inner diameter of 7.6 mm and an outer diameter of 14.3 mm, respectively. A quantity in the laminar and transition regions was suggested as the dominant variable involved in the forced convection heat transfer in the circular tube. The values of the local and average Nusselt numbers, Nuz and Nuav, respectively, for a concentric annular tube with dH = 6.7 mm and for a circular tube with dH = 6.7 mm were calculated to examine the effects of q, Tin, and Pe on heat transfer. The general correlation in the laminar and turbulent regions for a high temperature liquid metal was identified for the circular tube with a hydraulic diameter.

A Combined Hybrid 3-D/2-D Model for Flow and Solidification Prediction during Slab Continuous Casting

A combined hybrid 3-D/2-D simulation model was developed to investigate the flow and solidification phenomena in turbulent flow and laminar flow regions during slab continuous casting (CC). The 3-D coupling model and 2-D slicing model were applied to the turbulent flow and laminar flow regions, respectively. In the simulation model, the uneven distribution of cooling water in the width direction of the strand was taken into account according to the nozzle collocation of secondary cooling zones. The results from the 3-D turbulent flow region show that the impact effect of the molten steel jet on the formation of a solidification shell is significant. The impact point is 457 mm below the meniscus, and the plug flow is formed 2442 mm below the meniscus. In the laminar flow region, grid independence tests indicate that the grids with a cell size of 10 × 10 mm2 are sufficient in simulations to attain the precise temperature distribution and solidification profile. The liquid core of the strand is not entirely uniform, and the solidification profile agrees well with the integrated distribution of cooling water in secondary cooling zones. The final solidification points are at a position of 400–500 mm in the width direction and are 17.66 m away from the meniscus.

An experimental investigation of the motion of long bubbles in high viscosity slug flow in horizontal pipes

The slug bubble velocity is one of the most important closure relations in multiphase flow models, and has been studied extensively for decades. Available models and correlations may provide quite reliable predictions for low viscosity liquids, but this is not always the case for high viscosities, mainly due to a lack of detailed data. An investigation of viscous slug flow was carried out, based on 241 experiments in a 15 m long horizontal pipe of 57 mm inner diameter, applying three different liquids with viscosities in the range of 240–730 cP. Measured parameters include slug bubble velocities, slug lengths, liquid holdup and pressure drop. A main objective has been to quantify the effect of viscosity on slug bubble velocities, in order to improve predictive models of slug flow in the laminar region. Slug front, tail and pattern velocities were obtained by cross-correlating holdup time series recorded at two locations at the end of the test section. The dependency of individual slug bubble velocities on slug lengths and bubble nose shape was investigated in detail for selected experiments. A new slug bubble velocity model has been developed, incorporating viscosity, which compares quite well with available high viscosity slug flow data.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Effect of the design parameters on mass transfer and energy consumption inside a lithium electrolysis cell

A 2D axisymmetric electrochemical model of a lithium experimental cell is solved using a finite element method. The model is considering the coupled effect of momentum, electric, kinetic and mass transfer phenomena. The dense diaphragm used in the setup, which is not hydraulically permeable, separates the cell into two regions: 1 (a) turbulent region, between the anode and the diaphragm, and 2 (a) laminar region between the diaphragm and the cathode. The k-epsilon model is used to solve the turbulent flow resulting from bubbles generation at the anode. A two-phase flow model is also developed to simulate the volume fractions of bubbles and electrolyte in the cell. The non-uniform bubbles distribution over the anode surface, derived from the non-uniform current distribution, has been added to the two-phase model. The effects of the diaphragm length, position and porosity on the electric and two-phase flow fields are simulated. In fact, the diaphragm position and length both influence the current distribution at the surface of electrodes and the velocity distribution in the cell, all of which influence ohmic and kinetic overpotentials. The results show that up to 40% of energy can be saved when running the lithium electrolysis cell with a shorter porous diaphragm located as far as possible from the anode. The maximum current density, found at the bottom corner of anode, is higher when the diaphragm is longer and when it is closer to the anode.

Modeling boundary-layer transition in direct and large-eddy simulations using parabolized stability equations.

We examine the potential of the nonlinear parabolized stability equations (PSE) to provide an accurate yet computationally efficient treatment of the growth of disturbances in H-type transition to turbulence. The PSE capture the nonlinear interactions that eventually induce breakdown to turbulence and can as such identify the onset of transition without relying on empirical correlations. Since the local PSE solution at the onset of transition is a close approximation of the Navier-Stokes equations, it provides a natural inflow condition for direct numerical simulations (DNS) and large-eddy simulations (LES) by avoiding nonphysical transients. We show that a combined PSE-DNS approach, where the pretransitional region is modeled by the PSE, can reproduce the skin-friction distribution and downstream turbulent statistics from a DNS of the full domain. When the PSE are used in conjunction with wall-resolved and wall-modeled LES, the computational cost in both the laminar and turbulent regions is reduced by several orders of magnitude compared to DNS.

Investigation of Submerged Jets with an Extended Initial Laminar Region

The method of producing laminar submerged jets using a device, whose length is comparable with the jet diameter, is described. A submerged air jet, 0.12 m in diameter, produced by means of this technique is experimentally investigated in the Reynolds number range from 2000 to 13 000. Hot-wire anemometer measurements of the flow parameters and laser visualization of the flow are performed. It is shown that the device developed makes it possible to produce submerged jets with the laminar regions as long as 5.5 jet diameters. The initial regions of such jets can be used to study the development of disturbances in submerged jets, as well as used in medicine and engineering in organizing various gasdynamic curtains which produce zones with given properties with respect to purity and composition inside another gas media.

LPS-stimulated Macrophage Activation Affects Endothelial Dysfunction

Intestinal microbiota is involved in the atherosclerotic process by development of an atheromatous core with foam cells in carotid arteries. It has reported that lipopolysaccharide (LPS) from Escherichia colilocalizes in human atherosclerotic plaque and causes inflammation via interaction with toll like receptor 4. However, there is no evidence that whether LPS-activated macrophages regulate endothelial cell (EC) function. We evaluated whether LPS-activated macrophage acts as one of the stimulants activating EC and its underlying signaling pathways. Using Western blotting and quantitative reverse transcription-polymerase chain reaction (qRT-PCR), we confirmed that intraperitoneal injection with LPS increases iNOS protein and inflammatory cytokine, TNF-α and IL-6 mRNA expressions. To determine whether LPS-mediated macrophage inflammatory condition affects EC activation and inflammation, human umbilical vein endothelial cells (HUVECs) were incubated with isolated peritoneal macrophages from LPS-injected mice. Interestingly, p90RSK Serine 380 phosphorylation and protein expression were significantly increased by macrophage treatment in EC. Messenger RNA levels of vascular cell adhesion molecule 1 and p90RSK was increased, but endothelial nitric oxide synthase was decreased. In addition, NF-κ B promoter activity, which plays an important role in the pathogenesis of inflammation, was strongly enhanced by the macrophage treatment in EC. We further evaluated the effects of LPS on EC function in the mouse aorta using en facestaining. In agreement with in vitroresult, p90RSK expression was strongly increased in the steady laminar flow region of the mouse aorta in mice injected with LPS. Together, our study demonstrates that p90RSK might be a one of the major therapeutic candidates for the prevention of vascular diseases mediated by LPS.

Natural laminar flow airfoil shape design at transonic regimes with multi-objective evolutionary algorithms

The natural laminar flow airfoil shape design at transonic regime is solved using multi-objective evolutionary algorithms in this paper. A shock wave control bump is used to reduce wave drag of natural laminar flow airfoil in transonic flow, and the eN transition prediction method based on the linear stability theory is used to predict the transition location. The multi-island parallel multi-objective evolutionary algorithms is implemented to optimize the airfoil shape equipped with shock wave control bump for obtaining a larger laminar flow region and a weaker wave drag simultaneously. Optimization experiment shows that it is easy to capture the Pareto front of wave drag minimization and laminar flow region maximization. Results demonstrate that both wave drag and friction drag performance of several chosen Pareto members are significantly improved via the optimal airfoil shape and shock wave control bump device compared to that of the baseline shape.

Experimental study of heat transfer to non-Newtonian fluids inside a scraped surface heat exchanger using a generalization method

The heat transfer process to a non-Newtonian pseudoplastic fluid inside a scraped surface heat exchanger has been experimentally analysed. The scraping device consists of a rod with semicircular pieces mounted on it, which are in contact with the inner surface of the pipe. The whole moves axially and thus, the pieces scrape the inner surface of the pipe, in order to avoid fouling formation and enhance heat transfer.Pressure drop, heat transfer and power consumption measurements, using non-Newtonian pseudoplastic fluids, have been carried out in static and dynamic conditions of the scraper. Four flow regions have been identified: attached laminar, detached laminar, transitional and turbulent flow regions. A generalization method for the fluid viscosity that includes the effects of the non-Newtonian behaviour has been used. Friction factor and Nusselt number have been successfully correlated by employing the generalized viscosity on pressure drop and heat transfer results, both in static and dynamic conditions, for three of the four identified flow regions (it was not possible to get correlations in the transition to turbulent flow region). The study shows that, despite the high power consumption of the scraping movement, the device is suitable for an industrial process using non-Newtonian fluids, since it prevents fouling, increases heat transfer, provides flexibility and enhances the final product quality. Furthermore, the obtained correlations are a valuable tool in the design of heat exchangers.

Experimental study of thermal conductivity, heat transfer and friction factor of Al2O3 based nanofluid

Presented research paper is an investigation of water, paraffin and ethylene glycol based Al2O3 nanofluid. Heat transfer coefficient is measured using shell and tube heat exchanger under the conditions of fully developed laminar and turbulent flow. Concentration of nanoparticles in the respective base fluids was varied from 0.01vol% to 0.08vol% in the step of 0.1%. Nanofluids were used as the working fluid and sent to tubular side. The effects of Reynold's number and concentration of nanoparticles in nanofluid on heat transfer characteristics were studied. A significant enhancement in heat transfer coefficient and thermal conductivity of nanofluid over base fluid is found in the study. As the concentration of nanoparticles was increased, heat transfer coefficient was also improved marginally. At greater turbulence, the improvement in the heat transfer coefficient of nanofluid over base fluid was found to be more than that at lower turbulence. This is due to homogeneous distribution of nanoparticles in a base fluid at higher temperatures was achieved. The various factors that were considered in the characterization of nanofluid heat transfer coefficients are: sonication time, temperature, pressure difference, base fluid (water, paraffin and EG). Pressure drop in case of nanofluid was higher than base fluid in the turbulent regime; however, no significant change was observed in the laminar region.

Random Number Generation From Intermittent Optical Chaos

We propose a method to generate physical random numbers based on intermittent optical chaos. Intermittent chaotic output is produced in a semiconductor laser subjected to optical feedback embedded in a photonic integrated circuit. This dynamics is characterized by a temporal waveform organized in a succession of laminar regions of low amplitude and bursts of high amplitude. The temporal randomness ruling the alternation of successive laminar regions and bursts is used as an entropy source to generate sequences of random bits. We compare the performances of the exclusive OR and reverse methods implemented in the bit generation process and evaluate the quality of the random bits with the Rabbit test of TestU01 for different bit sequences lengths.