Laminar State Research Articles

Analysis of turbulence and blocking effects on loading capacity for elementary texture cells of infinite width under water lubrication

PurposeThe loading mechanism of textures considering turbulence has not been fully covered. This paper aims to investigate the effect of turbulence on the textured loading capacity under water lubrication and to analyze the causes of the turbulence effect.Design/methodology/approachComputational fluid dynamic models with different textured shapes are established after validation. The transition shear stress transport (SST) model, which is suitable for predicting the transition process of fluid from laminar state to turbulent state, is adopted in the present study. To illustrate the effect of turbulence, the loading capacity of textures predicted by transition SST model and laminar model is compared.FindingsThe loading capacity is higher after considering turbulence because more lubricant enters into textures and the flow rate of lubricant to textured outlet increases. There exists an optimal textured depth ratio and density for loading capacity and the change of flow state would not affect the optimal values. The degree of fluid blockage at textured outlet has a dominant influence on loading capacity. As the textured shape changes to triangle or ellipse from rectangle, the vortices at the textured bottom move forward and the blockage at a textured outlet is enhanced, which makes loading capacity improved under the action of blocking effect.Originality/valueThe enhancement of the blocking effect is found to be crucial to the improvement of textured loading capacity after considering turbulence. Present research provides references to understand the loading mechanism of textures under turbulent conditions.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-04-2020-0149/

The Distributed-Suction Effect on the Development of Disturbances at the Nonlinear Stage of Their Evolution in the Boundary Layer

The results of experimental investigations into the effect of distributed suction through a finely perforated permeable section on the spatial development of disturbances at the nonlinear stage of their evolution in the boundary layer of a straight wing are presented. It is shown that distributed suction makes possible reducing the intensity of both natural pulsations of the boundary-layer velocity and artificial ones generated by an external acoustic field at the fundamental frequency and its harmonics by two orders of magnitude. Affecting significantly the average flow, suction rules out the turbulent state of the boundary layer below the impact area and shifts the flow separation keeping it in the laminar state.

Predicting Boundary-Layer Transition over Backward-Facing Steps via Linear Stability Analysis

Laminar–turbulent transition downstream of a backward-facing step (BFS) in a two-dimensional zero-pressure-gradient boundary layer is investigated computationally for step-height–to–local-displacement-thickness ratios of . The nominally sharp BFS in a previous wind tunnel experiment is approximated via a smooth shape with a specified maximum slope. The laminar basic state solutions with the BFS are computed by solving the Navier–Stokes equations over a curvilinear grid. The linear amplification of Tollmien–Schlichting instabilities from the effects of basic state modification due to the BFS is quantified in terms of the logarithmic amplification ratio (or factor) and computed with the harmonic linearized Navier–Stokes equations. The transition locations measured at nearly all test conditions with correlate well with a computed factor of , demonstrating the first successful stability-based transition criterion related to step excrescences. An explanation for this successful correlation is that the net destabilization due to the BFS is the dominant contributor to the resulting shift in the transition onset location. The effect of any additional receptivity near the BFS does not play an important role in determining the movement of the transition front.

Spiral laminar flow of yield-power-law fluids in partially blocked annulus

During drilling of directional and horizontal wells, cutting beds inevitably form in highly deviated and horizontal sections. Because of the centralizer, both concentric and eccentric annuli can exist in both highly deviated and horizontal sections. Existing studies on the helical laminar flow of a non-Newtonian fluid in partially blocked concentric and eccentric annuli are mainly based on numerical models, laboratory experiments, and computational fluid dynamic (CFD) simulations. Directly applying the research results to drilling engineering is difficult. To accurately and conveniently predict the pressure gradient of helical laminar flow with a fixed horizontal cutting bed, this work first presents a new model based on fluid mechanics to compute the pressure loss of a yield-power-law (YPL) flow in partially blocked concentric and eccentric annuli in a laminar helical-flow state. Second, the Fluent software is used to simulate the helical laminar flow of the YPL fluid with a fixed horizontal cutting bed. The existing experimental measurements, mathematical model calculation, and Fluent software simulation results are used to verify the proposed simplified model. The results show that the predicted results of the simplified model agree well with the existing numerical results, existing experimental data, and CFD simulation results; the error is only ±5%. The new simplified model can achieve accurate and convenient prediction of the pressure gradient of a spiral laminar flow in an annulus with a fixed horizontal cutting bed.

Mixing of non-Newtonian inelastic fluid in a turbulent patch of T-junction

In this paper, we present result from a direct numerical simulation (DNS) of turbulent flow in a converging T-junction for both Newtonian (water) and non-Newtonian inelastic fluid (dilute Xanthan Gum solution). Based on experimental data, the Bird-Carreau law is used to capture the inelastic shear thinning property of the solution. For the Xanthan solution, the viscosity at rest is about 100 times greater than the viscosity at high shear-rate. A passive scalar is introduced in the transverse branch to investigate the mixing in such configuration. The nominal Reynolds number at the exit varies from 4800 to 8000 for the Newtonian cases and for the same inflow rates, the non-Newtonian flow will be necessarily at lower nominal Reynolds number. Two regimes are explored as a function of the inlet velocity ratio r=Ub/Um : the ”deflecting” regime noted DR (r=1) and the ”impinging” regime noted IR (r=4). For the non-Newtonian cases, two viscous cores are observed before the junction. After the junction a laminar state is obtained for the lower flow rate conditions. Surprisingly, in spite of a large viscosity at rest, a self-sustained non-Newtonian turbulence is achieved except for one case. We describe existing vortex mechanisms which pilot the scalar mixing. In addition, we show that in the non-Newtonian cases, the existing peak of turbulence is only shifted in the DR case. The shift is probably due to the nature of the fluid and not to the dynamical regime. After an intense turbulent zone, we show that a re-laminarization zone appears in the non-Newtonian case which reduces the fluctuation as well as mixing. As a result, IR has a better mixing quality than DR.

Transition in the flow due to a rotating sphere: Simulating isolated roughness

We present results of a study on the roughness-induced receptivity of the boundary layer that exists in the flow due to a rotating sphere. Our approach exploits a computational methodology that allows us to capture the full range of fundamental fluid physics that arises in the flow from the early collision of boundary layers due to the sphere’s rotation to the development of a radial jet and subsequent instabilities in the spatially and temporally developing flow. We demonstrate that the flow can exhibit a transient convective instability in the form of a spiral vortex, but ultimately the boundary layer returns to a laminar state. Consideration is also given to the impact of this transient instability on the radial jet, which when advected into the radial jet at the sphere’s equator serves to render the jet turbulent.

Performance of high-speed hydrodynamic sliding bearings with lubricating oils combining laminar and turbulent flows

High-speed hydrodynamic sliding bearings use lubricating oil that can have laminar and turbulent flow states, yet turbulent states remain relatively unstudied. This study combines theoretical analysis, numerical modeling, and experiments to analyze lubrication fluids in such bearings. It considers Reynolds equations, energy equations, and temperature-viscosity relationships under laminar and turbulent flows. The governing equations are solved by the finite difference method. Two-dimensional distributions of Reynolds number, pressure, and temperature in the bearing film, as well as the lubrication characteristics like bearing capacity and frictional force under working conditions, are analyzed. Single and mixed flow states are compared, which demonstrates the coexistence states of laminar and turbulent flows in an oil film under specific working conditions. Oil film flow distributions differ significantly according to rotational speed and eccentric conditions. Flow changes under high eccentricity are complex. The characteristics of oil films in a single flow state deviate significantly from those in mixed flow. Changes in flow state and heat should not be ignored during analysis of the lubrication performance of high-speed bearings.

Recurrence Behavior Statistics of Blast Furnace Gas Sensor Data in Industrial Internet of Things

Blast furnace gas (BFG) produced from steel industries is generally one of the most important energy supplies in enterprises. Due to a great deal of output, fluctuation, and heterogeneity in data, it is very difficult to provide profound insights into its internal dynamic. In this article, a novel analysis framework is developed for the BFG data processing, considering the recurrence plot (RP) and the recurrence quantification analysis (RQA). The specific aim is to investigate the relationship between BFG output and its potential influencing factors. This framework can be deemed as a uniform and consistent system with functional components of qualitative visualization and quantitative analysis. Concretely, the BFG outputs related to five factors are separately projected to high-dimensional spaces, followed by that their internal dynamics can be embodied through a 2-D recurrence representation of states. Finally, five RQA parameters are used to quantify the influence of these factors on the BFG output. This is the first attempt revealing the relations among the BFG data from the qualitative and quantitative perspectives. The experimental results show that RP can discover the BFG output patterns of laminar state, chaos, and instability over given three states of influencing factors, and the ranked influential degree can be given by a two-stage standard deviation of all considered RQA measures. Besides, we also demonstrate that the temperature of the hot-blast stove is most relevant to the BFG output, while the influence of considered other factors directly depends on the selected series length.

Evaluation of thermal and fluid dynamic performance parameters in aluminum foam compact heat exchangers

This paper analyses the behavior of heat exchangers with metal foam. A numerical study in a laminar steady state two-dimensional model is provided to evaluate the thermal - fluid dynamic performance of the system. An aluminum foam block embedded with an array of five circular tubes and assigned uniform temperature, constitute the physical domain. The open-celled metal foam is considered as a porous medium in local thermal non equilibrium (LTNE) and the Darcy – Forchheimer – Brinkman model is assumed. The metal foam has a porosity equal to 0.9353 and a pore density equal to 20 pores per inch (PPI). The numerical simulations are carried out considering air as the cooling fluid with several mass flow rate for assigned temperature of the tube surface. The Reynolds number, Re, based on tube diameter, ranges from 56 to 1120, the ratios between the thickness of metal foam and tube diameter, Lmf/d, and between the tube pitch and tube diameter, 2H/d, are in the ranges from 1 to 20 and 1.5–25, respectively. The goal of this investigation is to find the compact heat exchanger size for an assigned metal foam that allows trade-off between the improvement of the heat transfer and the increment of pressure drop, and to study the fluid dynamics and thermal behavior of these configurations. The study focuses on thermal performances of a metal foam heat exchanger using several conventional parameters employed in the analysis of porous medium heat exchangers to find optimal geometrical configurations. Results are given in terms of Colburn factor, friction factor and area goodness factor as a function of Re, Lmf/d and 2H/d. It is shown that the compact heat exchanger, examined in the investigation, presents optimum Reynolds number values; where the ratio between the Colburn and the friction factor, AG, attains a maximum value for metal foam, with different thickness and distance between the tubes of the heat exchanger. Two correlations for AGmax are proposed for 56 ≤ Red ≤ 1120; one in terms of dimensionless thickness and another for the dimensionless pitch.

Heat transfer mechanism of concurrent flame spread over rigid polyurethane foam: Effect of ambient pressure and inclined angle

As a common building façade insulation material with numerous applications, the heat transfer mechanism and flame spread characteristics of rigid polyurethane (RPU) foam should have been concerned. This work aims to give a comprehensive understanding of the coupling effect of sample inclination and ambient pressure on the heat transfer process and flame spread behaviors over RPU foam. A series of contrastive bench-scale tests were performed, in which inclination (denoted θ, 10°–90°) and ambient pressure (P, 65.5 and 100.8 kPa) were varied, and the variation of flame spread rate, flame length and flame stand-off distance were thoroughly investigated. Under the competitive effect of gas-phase heat transfer and surface charring, the extinction phenomena of flame spread were found, and the critical extinction angle were: θcrit,max=85∘, P = 100.8 kPa; θcrit,max=60∘, θcrit,min=25∘, P = 65.5 kPa. Based on the laminar and turbulent flow states, a piece-wise correlation to evaluate flame spread rate was established, which matched the experimental results well. Moreover, spreading flames were classified as pool-like flame and wall-like flame, and a new modified dimensionless correlation and semi-empirical model were proposed to describe the variation of flame length. Average flame stand-off distance was found to linearly decrease with inclination, y‾~θ, and the slopes in the plain and on the plateau were −0.455 and −0.776, respectively. The results of this study can provide a better insight for the fire safety design of energy conservation system of high-rise buildings.

A complex evaluation of [C2mim][CH3SO3]– alumina nanoparticle enhanced ionic liquids internal laminar flow

This paper involves a numerical study of several water – ionic liquids mixtures and nanoparticle enhanced ionic liquids starting from a less studied ionic liquid: 1-Ethyl-3-methylimidazolium Methanesulfonate, [C2mim][CH3SO3]. The research start point was several previous published experimental results in regard to the water – ionic liquid mixtures and alumina based nanoparticle enhanced ionic liquids thermophysical properties. Consequently, all the considered fluids are completely described and can be successfully implemented in a numerical analysis using the single phase approach. The numerical analysis was performed in a cylindrical enclosure considering laminar steady state and results were collected in terms of Nusselt number, convective heat transfer coefficient, pressure drop and temperature on tube exit. Results were discussed in terms of heat transfer enhancement and pumping power requirement variation, as well as field synergy. Concluding, it can clearly say that mixing water with [C2mim][CH3SO3] ionic liquid it gets a decrease in laminar convective heat transfer coefficient, while adding nanoparticles the heat transfer is enhanced by maximum 50%, depending on the alumina mass concentration.

Characteristics of temperature distribution in atmospheric pulsed surface dielectric barrier discharge for ozone production

The temperature field distribution in air SDBD (surface dielectric barrier discharge) is investigated by measuring the temperatures of reactive plasma volume (Tp), electrode (Te) and gas (Tg) using the fiber optic thermometer. Results show that Tp is almost the same as Te, while Tg is lower than Tp. Both Tp and Tg follow a first-order model. The temperature in the area between the boundary of the plexiglass surfaces and the electrodes is higher than the temperature in the center. The gas flow rate has little effect on Tp due to that air flow is in steady laminar flow state.

Analytical model of the laminar traffic phase states and its use for calculating the level of service indicators for road network

The article is dedicated to the description and discussion of theoretical models that may be used for calculating the level of service indicators for road network. Authors developed the approach for calculating the level of service indicators for urban road network sections with signalized intersections. The core of the proposed models of assessing the level of service is based on the method, developed by American experts for main roads and adapted by the authors for the city roads. The approach to assessing the level of service is based on the advanced analytical model of the fundamental diagram, describing various phases of traffic flow. Herein authors introduce the concept of a laminar traffic flow in the sections of the road network, which is implemented for the level of service calculating.

Physiological and geometrical effects in the upper airways with and without mandibular advance device for sleep apnea treatment

Sleep apnea is a sleep disorder that occurs when the breathing of a person is interrupted during the sleep. This interruption occurs because of the patient has narrowed airways and the upper airways muscles relax, closes in and blocks the airway. Therefore, any forces or reaction originated by the air flow dynamics over the relaxed upper airways muscles could make to close the upper airways, and consequently the air could not flow into your lungs, provoking sleep apnea. Fully describing the dynamic behavior of the airflow in this area is a severe challenge for the physicians. In this paper we explore the dynamic behavior of airflow in the upper airways of 6 patients suffering obstructive sleep apnea with/without a mandibular advancement device using computational fluid dynamics. The development of flow unsteadiness from a laminar state at entry to the pharynx through to the turbulent character in the soft palate area is resolved using an accurate numerical model. Combining the airflow solution with a geometrical analysis of the upper airways reveals the positive effects of mandibular advance device in the air flow behavior (pressure drop). Improved modeling of airflow and positioning of mandibular advance device could be applied to improve diagnosis and treatment of obstructive sleep apnea.

Three-dimensional signatures of self-similarity in a high-energy-density plasma shear-driven mixing layer

A hydrodynamic shear mixing layer experiment at the National Ignition Facility had previously demonstrated Eulerian scaling of integrated, late-time quantities, including turbulent kinetic energy. In this manuscript, the experiment is repeated with new materials. Using the new dataset, we demonstrate that Euler-number scalings hold not just for late time, but dynamically throughout the experiment, for measurements in all three spatial dimensions. Incorporating the dynamic scaling leads to an enhanced calculation that the heavier of the two scaled experiments has approached three generations of mergers of its primary instability's structures and a consistent observation of such a merger in action in the lighter of the two scaled experiments. Furthermore, the improved scrutiny of the time evolution of instability structures leads to sharper estimates of turbulent kinetic energy, including a demonstration of different behaviors correlating with surface roughness (quantitatively consistent with transitions between laminar and turbulent initial states), as predicted by a Reynolds-averaged turbulent model, which evidently correctly handles the differing shock-roughness interactions to drive its internal state of the model into different regimes. Altogether, a picture arises of the analytical improvements in treating these variations (of times, densities, and roughnesses) as a unified whole and of multiple ways by which deviations from the scaling could indicate an onset of non-hydrodynamic behavior. Such deviations were not expected for these experiments (which models correctly indicated would remain hydrodynamic) but could be introduced by, for example, imposing external fields or increasing drive energy to test conditions relevant to inertial confinement fusion or other high-energy-density experiments.

Secondary flows in a longitudinally grooved channel and enhancement of diffusive transport

Flow in a longitudinally grooved channel is analysed with the primary objective of quantifying intensification of transport mechanism due to the onset of secondary flows resulting from hydrodynamic instability and amplification of unstable modes into the nonlinear regime. Considered geometry consists of a channel whose walls are fitted with sinusoidal corrugations, forming a system of longitudinal grooves parallel to the streamwise direction. Such configuration is energy efficient, because it reduces drag when compared to the smooth reference configuration and at the same time results in flow destabilization, due to travelling wave mode already at very low values of the Reynolds number ( < 102). The analysis is performed for a range of over-the-critical values of the Reynolds numbers and focuses on nonlinear flow solutions corresponding to the limit cycle to which flow transitions from a fixed-point laminar state through a supercritical Hopf bifurcation beyond which nonlinear solution is both three-dimensional and time-periodic allowing for development of complex advection patterns and consequently improved mixing. The main results of the current work are characterization of the nonlinear solutions, attained by the flow past bifurcation and for the first time illustration of changes in the diffusive transport mechanism due to nonstationary and three-dimensional form of the flow. Our analysis indicates that significant enhancement in advective transport may be attained and that it is not monotonically related to the Reynolds number, but rather, it is related to the ability of saturated flow to intensify spanwise motions. Implications of this study might help in development of small scale flow devices operating at low and moderate ranges of Reynolds number with the purpose of intensifying mixing, heat transfer or reaction of chemical or biological compounds.

Influence of agitation intensity on solution stability for rapidly grown KDP crystal through theoretical and experimental research

For rapid growth of KDP crystal, the influence of agitation intensity on solution stability was investigated by altering rotation speed in crystalline procedure. Based on the experiments, the results demonstrate that rotation speed has a great influence on growth rate, which increases with increase of rotation speed as is no more than 100 r/min. The corresponding metastable zone widths (MSZW) indicate that solution stability decreases with the agitation intensity increasing, while introducing a seed into the growing solution has little effect on MSZW. Combined with numerical simulation, the dependences of rotation speeds on turbulent dissipation and vorticity imply that growth solution with a laminar state has a better stability and a stronger ability to resist spontaneous crystallization at lower speeds. With increasing of rotation speeds ranging from 22 to 106 r/min, the growth solution is transformed from laminar to turbulent state causing the decrease of solution stability, while the turbulent dissipation rate increases gently with increasing of rotation speed. Furthermore, the vortex magnitude increases with rising of rotation speed, and an obvious variation of vorticity has a substantial impact on solution instability, especially at high-speed larger than 200 r/min. Thus, appropriately reducing speed is an effective way to maintain the stability of growing environment and beneficial to realize rapidly grown KDP crystal with high-quality.

Analysis of pressure characteristics under laminar and turbulent flow states inside the pilot stage of a deflection flapper servo-valve: Mathematical modeling with CFD study and experimental validation

Electro-hydraulic servo-valves are widely used components in the mechanical industry, aerospace and aerodynamic devices which precisely control the airplane or missile wings. Due to the small size and complex structure in the pilot stage of deflection flapper servo-valves, accurate mathematical models for the flow and pressure characteristics have always been very difficult to be built. In this paper, mathematical models for the pilot stage of deflection flapper servo-valve are investigated to overcome some gaps between the theoretical formulation and overall performance of the valve by considering different flow states. Here, a mathematical model of the velocity distribution at the flapper groove exit is established by using Schlichting velocity equations for in-compressible laminar fluid flow. Moreover, when the flow becomes turbulent, a mathematical model of pressure characteristics in the receiving ports is built on the basis of the assumption of the collision between the liquid and the jet as the impact of the jet on a moving block of fluid particles. To verify the analytical models for both laminar and turbulent flows, the pressure characteristics of the deflection flapper pilot stage are calculated and tested by using numerical simulation and experiment. Experimental verification of the theory is also presented. The computed numerical and analytical results show a good agreement with experimental data.

Experimental investigation on shell-side performance of a novel shell and tube oil cooler with twisted oval tubes

The lubrication oil cooler is widely used in many industries. The lubrication oil is usually operated in a laminar state with Reynolds number less than 1000 due to its high viscosity characteristic. It is hard to augment the heat transfer performance of high viscosity fluids without obvious pressure drop. A lot of researches have been conducted on the shell-side heat transfer and fluid flow characteristics of twisted oval tube heat exchangers. However, the previous study mainly focused on the high Reynolds number regime and the correlations obtained are not suitable for low Reynolds flow. In the present paper, a novel self-supported shell-and-tube oil cooler with twisted oval tube bundles (OCTT) is designed. Experiments are conducted to investigate the shell-side heat transfer and pressure drop of OCTT compared to the conventional shell-and-tube oil cooler with round tube bundles (OCRT) in low Reynolds number flow. The experimental results indicate that at identical oil volume flow rate, the shell-side pressure drop of OCTT is lower than that of OCRT. Furthermore the shell-side heat transfer coefficient per unit pressure drop of OCTT is larger than OCRT. The empirical equations for Nusselt number and friction coefficient of OCTT in low Reynolds number ranging of 50–600 are also deduced from the experimental data, which could provide a beneficial guidance for the design of OCTT.

Unified mechanisms for separation control around airfoil using plasma actuator with burst actuation over Reynolds number range of 103–106

We conduct large-eddy simulations of separated airfoil flows with control by a dielectric-barrier-discharge plasma actuator over a wide range of Reynolds numbers. The Reynolds numbers based on the chord length (Re) are set at Re = 5.0 × 103, 1.0 × 104, 6.3 × 104, 2.6 × 105, and 1.6 × 106. These Reynolds numbers cover most of the conditions used in the previous studies on separation control by a plasma actuator. The burst frequency nondimensionalized by the chord length and freestream velocity (F+) is used as the computational parameter, and the effective burst actuation and control mechanisms at each Reynolds number condition are investigated. With regard to cases without the control, the flows separate near the leading edge in the laminar state at the Reynolds number range of 103–105, and a substantial turbulent separation occurs at the Reynolds number of 1.6 × 106. Separation control with a high burst frequency [F+ ≃ O(10)] can cause early flow reattachment through the promotion of turbulent transition of a separation shear-layer for Re = 6.3 × 104 and 2.6 × 105. Flow reattachment is mainly caused by momentum entrainment into the boundary layer by fine-scale turbulent vortices. On the other hand, the large-scale spanwise vortices play an important role at F+ = 1 for Re = 1.0 × 104 and 1.6 × 106. In these cases, the dynamics of the spanwise vortices show similar behavior and the pairing of these vortices significantly contributes to the separation control by increasing the momentum entrainment. The optimum value of F+ changes with a Reynolds number. In contrast, when a nondimensional burst frequency based on the characteristics of the separation shear-layer (Fθs) is considered, a high lift-to-drag ratio is found at Fθs≃O(10−2) for all Reynolds numbers. This demonstrates that one of the effective burst frequencies is closely related to the scale of the separation shear-layer, especially for the spanwise vortex shed from the separation shear-layer.