Rough Plates Research Articles

Combined effect of viscosity variation on rough porous rectangular plates with magneto hydrodynamic effect

The squeezing action between rough porous parallel rectangular plates under the effect of magneto hydrodynamic force and viscosity variation effect is considered in this paper. The modified Reynolds equation is derived . The load carrying capacity of the plate thus derived is found to increase with increase in the magnetic, roughness and viscosity variation parameter.

Heat transport enhancement and scaling law transition in two-dimensional Rayleigh-Bénard convection with rectangular-type roughness

Direct numerical simulations (DNS) of Rayleigh-Bénard convection are performed in two-dimensional (2-D) square cavities with rough top and bottom plates. The roughness is introduced by setting heated/cooled rectangular elements called fins equidistantly onto lower/upper plates. The Rayleigh numbers (Ra) range from 107 to 1010. Different numbers of fins (S) are used with Prandtl number (Pr) fixed to 1. The Pr effect is also investigated for fixed S=8, with Pr varied from 0.71 to 34.1. It is found that in all roughness configurations, the heat transport is significantly enhanced when Ra exceeds a critical value. The power-law scalings of the Nusselt number (Nu) versus Ra keep in change in the studied Ra range for all Pr and S, with most of the exponents within [1/3,1/2], larger than those in smooth cases. In addition, we find Nu increases monotonically with Pr when Pr⩾1 and Ra is moderate for S=8. The dynamics of plumes, the flow structures between the fins and in the bulk, as well as the turbulence dissipation rates are found to be greatly influenced by the presence of fins, which are responsible for the heat transport enhancement, as well as the change in Nu∼Ra power-law scalings.

Molecular Dynamics of a Thin Liquid Argon Layer Squeezed between Diamond Surfaces with a Periodic Relief

The behavior of a liquid argon layer compressed between absolutely solid diamond surfaces with a periodic atomic relief has been studied by the method of molecular dynamics. The film consists of one or two layers of molecules for which the model of absolutely elastic spheres has been used. The equilibrium and dynamic properties of the film dependently on the load and shear force applied to surfaces and the number of argon layers have been investigated. The performed modeling allows us to compare the behavior of systems with smooth and rough plates. The results show that most of the properties of ultrathin argon films confined between surfaces of both types are similar.

Effects of surface roughness on a separating turbulent boundary layer

Separating turbulent boundary layers over smooth and rough flat plates are studied by large-eddy simulations. A suction–blowing velocity distribution imposed at the top boundary of the computation domain produces an adverse-to-favourable pressure gradient and creates a closed separation bubble. The Reynolds number based on the momentum thickness and the free-stream velocity before the pressure gradient begins is 2500. Virtual sand grain roughness in the fully rough regime is modelled by an immersed boundary method. Compared with a smooth-wall case, streamline detachment occurs earlier and the separation region is substantially larger for the rough-wall case, due to the momentum deficit caused by the roughness. The adverse pressure gradient decreases the form drag, so that the point where the wall stress vanishes does not coincide with the detachment of the flow from the surface. A thin reversed-flow region is formed below the roughness crest; the presence of recirculation regions behind each roughness element also affects the intermittency of the near-wall flow, so that upstream of the detachment point the flow can be reversed half of the time, but its average velocity can still be positive. The separated shear layer exhibits higher turbulent kinetic energy (TKE) in the rough-wall case, the growth of the TKE there begins earlier relative to the separation point, and the peak TKE occurs close to the separation point. The momentum deficit caused by the roughness, again, plays a critical role in these changes.

Elastic wave generated by granular impact on rough and erodible surfaces

The elastic waves generated by impactors hitting rough and erodible surfaces are studied. For this purpose, beads of variable materials, diameters, and velocities are dropped on (i) a smooth PMMA plate, (ii) stuck glass beads on the PMMA plate to create roughness, and (iii) the rough plate covered with layers of free particles to investigate erodible beds. The Hertz model validity to describe impacts on a smooth surface is confirmed. For rough and erodible surfaces, an empirical scaling law that relates the elastic energy to the radius Rb and normal velocity Vz of the impactor is deduced from experimental data. In addition, the radiated elastic energy is found to decrease exponentially with respect to the bed thickness. Lastly, we show that the variability of the elastic energy among shocks increases from some percents to 70% between smooth and erodible surfaces. This work is a first step to better quantify seismic emissions of rock impacts in natural environment, in particular on unconsolidated soils.

Jenkins model based ferrofluid lubrication of a squeeze film in rough curved circular plates considering slip velocity

An attempt has been made to study and analyse the combined effect of roughness and slip velocity on the performance of a Jenkins model based magnetic squeeze film in curved rough circular plates taking velocity slip into account. The curvatures of both the plates are described by exponential functions. The stochastic model of Christensen and Tonder has been used to evaluate the effect of surface roughness. The associated stochastically averaged Reynolds type equation has been solved to obtain the pressure distribution leading to the computation of load carrying capacity. The graphical representations of the solution reveal that although Jenkins model modifies the performance as compared to Neuringer-Rosensweig model, the magnetisation may not go too far in reducing the adverse effect of slip velocity and standard deviation associated with roughness because the material constant parameter brings down the load carrying capacity. However, the situation is fairly improved in the case of negatively skewed roughness particularly, when variance (-ve) is involved. This article also ensures that by keeping the slip parameter at minimum and choosing the curvature parameters the magnetic fluid lubrication may lead to an augmented performance.

Jenkins model based ferrofluid lubrication of a squeeze film in rough curved circular plates considering slip velocity

An attempt has been made to study and analyse the combined effect of roughness and slip velocity on the performance of a Jenkins model based magnetic squeeze film in curved rough circular plates taking velocity slip into account. The curvatures of both the plates are described by exponential functions. The stochastic model of Christensen and Tonder has been used to evaluate the effect of surface roughness. The associated stochastically averaged Reynolds type equation has been solved to obtain the pressure distribution leading to the computation of load carrying capacity. The graphical representations of the solution reveal that although Jenkins model modifies the performance as compared to Neuringer-Rosensweig model, the magnetisation may not go too far in reducing the adverse effect of slip velocity and standard deviation associated with roughness because the material constant parameter brings down the load carrying capacity. However, the situation is fairly improved in the case of negatively skewed roughness particularly, when variance (-ve) is involved. This article also ensures that by keeping the slip parameter at minimum and choosing the curvature parameters the magnetic fluid lubrication may lead to an augmented performance.

Experimental study of dynamic contact angles on rough hydrophobic surfaces

Rough hydrophobic surfaces have many applications in industry and technology. An experimental study was done on the spreading dynamics of different concentrations of polyethylene glycol (PEG) solutions on rough Teflon plates with different roughness. The experiments were conducted using Wilhelmy plate method. The advancing dynamic contact angle was found to be weakly dependent of capillary number. However, the receding dynamic contact angle decreases with increasing capillary number. The degree of roughness on rough Teflon surface has an important role on dynamic contact angle. The dynamics of receding motion was found to follow the molecular-kinetic theory. A power law relation between the receding dynamic contact angle and the capillary number was also obtained.

Intelligent modeling of thermohydraulic behavior in solar air heaters with artificial neural networks

The artificial neural network was applied to map the thermohydraulic performance criteria of the solar air heater with constant values of geometric parameters. Along with intelligent prediction of friction factor and Nusselt number, the main idea behind the research was modeling the performance of solar air heater through the temperature and velocity features instead of geometric parameters which cause opportunity for comparison and investigation of the roughnesses based on the Nusselt number and the friction factor and consequently the thermohydraulic performance. Two approaches were selected for ANN methodology: separate networks for each smooth and roughened plate and integrated model for both. Comparison between the predicted and experimental data in all designed networks indicated that the ANN predicts the Nusselt number better than the friction factor. Parameters of the roughened plate were inserted as the test data to the best-determined smooth plate networks in order to generalize the networks. The better result was observed for the Nusselt number of the roughened plate rather than the friction factor, in the procedure of generalizing the smooth plate networks. The least MSE for the Nusselt number and the friction factor of roughened duct in method 1 was 2.4999e−06 and 2.70199e−04, respectively. 1.7681e−06 and 4.0819e−04 were the least magnitudes of the MSE for the Nusselt number and the friction factor related to the smooth plate. The proposed model can be strongly recommended as a replacement for exhaustive and massive experimental procedure in the optimization of solar air heaters.

Roughness-Facilitated Local 1/2 Scaling Does Not Imply the Onset of the Ultimate Regime of Thermal Convection.

In thermal convection, roughness is often used as a means to enhance heat transport, expressed in Nusselt number. Yet there is no consensus on whether the Nusselt vs Rayleigh number scaling exponent (Nu∼Ra^{β}) increases or remains unchanged. Here we numerically investigate turbulent Rayleigh-Bénard convection over rough plates in two dimensions, up to Ra≈10^{12}. Varying the height and wavelength of the roughness elements with over 200 combinations, we reveal the existence of two universal regimes. In the first regime, the local effective scaling exponent can reach up to 1/2. However, this cannot be explained as the attainment of the so-called ultimate regime as suggested in previous studies, because a further increase in Ra leads to the second regime, in which the scaling saturates back to a value close to the smooth wall case. Counterintuitively, the transition from the first to the second regime corresponds to the competition between bulk and boundary layer flow: from the bulk-dominated regime back to the classical boundary-layer-controlled regime. Our study demonstrates that the local 1/2 scaling does not necessarily signal the onset of ultimate turbulence.

Heat transfer in rotating Rayleigh–Bénard convection with rough plates

This experimental study focuses on the effect of horizontal boundaries with pyramid-shaped roughness elements on the heat transfer in rotating Rayleigh–Bénard convection. It is shown that the Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in the case of smooth top and bottom surfaces, is unaffected by the roughness as long as the Ekman layer thickness $\unicode[STIX]{x1D6FF}_{E}$ is significantly larger than the roughness height $k$. As the rotation rate increases, and thus $\unicode[STIX]{x1D6FF}_{E}$ decreases, the roughness elements penetrate the radially inward flow in the interior of the Ekman boundary layer that feeds the columnar Ekman vortices. This perturbation generates additional thermal disturbances which are found to increase the heat transfer efficiency even further. However, when $\unicode[STIX]{x1D6FF}_{E}\approx k$, the Ekman boundary layer is strongly perturbed by the roughness elements and the Ekman pumping mechanism is suppressed. The results suggest that the Ekman pumping is re-established for $\unicode[STIX]{x1D6FF}_{E}\ll k$ as the faces of the pyramidal roughness elements then act locally as a sloping boundary on which an Ekman layer can be formed.

Investigation of thermal and hydrodynamic behaviour of Al2O3-water nanofluid through a rough parallel plate

The implementation of nanofluid within a preferred volume fraction in base fluid is one of the innovative and inexpensive methods of enhancing the thermal and hydrodynamic behaviour in terms of increased heat transfer rate and reduced pumping power. In the present paper, forced convection heat transfer and pumping power were studied for a rough parallel plate subjected to constant heat flux under turbulent flow condition. The investigation was performed for a wide range of Reynolds number 10,000 to 30,000 with Al2O3 nanoparticles volume fraction 1% to 5% dispersed in base fluid water and three different rough surfaces (relative roughness: 0.001, 0.002, and 0.003) were considered. Heat transfer performance was substantially improved more by employing roughness at the wall of the boundary than the smooth parallel plate and also for the increased more volume fraction of nanofluid than base fluid water. Augmentation was found significant at higher surface roughness and volume fraction by the virtue of superior thermo-physical properties of nanofluid up to 36.9% and 26.1% for the rough surface. Roughness did not increase the pumping power as its effect was mitigated by the nanofluid. 2% volume fraction of Al2O3-water nanofluid on 0.003 relative roughness that showed the superior behaviour for heat transfer enhancement with minimum pumping power requirement.

Describing flow behavior of polymer gel in fracture using rheology

In many successful conformance controls to reduce excessive water production, large volumes of gels were extruded through fractures during placement. The pressure gradient for gel extrusion depends strongly on fracture width and gel composition. Extrusion experiments directly measure gel properties in fractures, but they are both expensive and time-consuming. The rheology behavior of the gels showed a strong parallel to the results obtained from previous gel extrusion experiments. The previous researches showed two discrepancies and one similarity between the rheology estimation and extrusion experiment. The two discrepancies are: the pressure gradient required to extrude a gel through a fracture substantially greater than anticipated based on rheology measurements; different relationships of pressure gradient versus fracture width. The similarity is the relationships between pressure gradient versus flow rate and shear stress versus shear rate are fairly close. This paper examined these discrepancies and the similarity, in hopes of understand and ultimately substituting the rheology measurements for extrusion experiments.Extensive experiments established that wall slip and first normal stress difference were not responsible for the pressure gradient discrepancy. Steady shear and oscillatory shear data were collected with a rheometer using both smooth and rough parallel-plate geometries and employing various gap heights. Wall-slip effects were present with smooth plates but negligible with rough plates. To explain the discrepancy, we noted that the aperture for gel flow (for mobile gel wormholing through concentrated immobile gel within the fracture) was much narrower than the width of the fracture.Considering the shear-thinning properties of the gels, two models were developed combining power-law model and shell momentum balances. The first model explained the discrepancy of the relationships between pressure gradient versus fracture width. The second model correlated pressure gradient, shear stress, flow rate, and shear rate to bridge the gap between gel rheology in fractures versus in a rheometer and explained the similarity between the rheology measurement and extrusion experiment.

Turbulent thermal convection over rough plates with varying roughness geometries

We present a systematic investigation of the effects of roughness geometry on turbulent Rayleigh–Bénard convection (RBC) over rough plates with pyramid-shaped and periodically distributed roughness elements. Using a parameter $\unicode[STIX]{x1D706}$ defined as the height of a roughness element over its base width, the heat transport, the flow dynamics and the local temperatures are measured for the Rayleigh number range $7.50\times 10^{7}\leqslant Ra\leqslant 1.31\times 10^{11}$ and Prandtl numbers $Pr$ from 3.57 to 23.34 at four values of $\unicode[STIX]{x1D706}$ (0.5, 1.0, 1.9 and 4.0). It is found that the heat transport scaling, i.e. $Nu\sim Ra^{\unicode[STIX]{x1D6FC}}$ where $Nu$ is the Nusselt number, may be classified into three regimes in turbulent RBC over rough plates. In Regime I, the system is in a dynamically smooth state. The heat transport scaling is the same as that in a smooth cell. In Regimes II and III, the heat transport is enhanced. When $\unicode[STIX]{x1D706}$ is increased from 0.5 to 4.0, $\unicode[STIX]{x1D6FC}$ increases from 0.36 to 0.59 in Regime II and it increases from 0.30 to 0.50 in Regime III. The experiment thus clearly demonstrates that the heat transport scaling in turbulent RBC can be manipulated using $\unicode[STIX]{x1D706}$ in the heat transport enhanced regime. Previous studies suggest that the transition to heat transport enhanced regime, i.e. from Regime I to Regime II, occurs when the thermal boundary layer (BL) thickness becomes smaller than the roughness height. Direct measurements of the viscous BL in the present study suggest that the transition from Regime II to Regime III is likely a result of the viscous BL thickness becoming smaller than the roughness height. The scaling exponent of the Reynolds number $Re$ with respect to $Ra$ changes from 0.471 to 0.551 when $\unicode[STIX]{x1D706}$ is increased from 0.5 to 4.0, suggesting a change of the dynamics of the large-scale circulation. Interestingly, the transition from Regime II to Regime III in terms of the heat transport scaling is not reflected in the $Re$ scaling with $Ra$. It is also found that increasing $\unicode[STIX]{x1D706}$ increases the clustering of thermal plumes which effectively increases the plume lifetime. This leads to a great increase in the probability of observing large temperature fluctuations in the bulk flow, which corresponds to the formation of more coherent plumes or plume clusters that are ultimately responsible for the enhanced heat transport.

Antiplasmodial potential and quantification of aloin and aloe-emodin in Aloe vera collected from different climatic regions of India

BackgroundIn this study, Aloe vera samples were collected from different climatic regions of India. Quantitative HPTLC (high performance thin layer chromatography) analysis of important anthraquinones aloin and aloe-emodin and antiplasmodial activity of crude aqueous extracts was done to estimate the effects of these constituents on antiplasmodial potential of the plant.MethodsHPTLC system equipped with a sample applicator Linomat V with CAMAG sample syringe, twin rough plate development chamber (20 x 10 cm), TLC Scanner 3 and integration software WINCATS 1.4.8 was used for analysis of aloin and aloe-emodin amount. The antiplasmodial activity of plant extracts was assessed against a chloroquine (CQ) sensitive strain of P. falciparum (MRC-2). Minimum Inhibitory Concentration (MIC) of aqueous extracts of selected samples was determined according to the World Health Organization (WHO) recommended method that was based on assessing the inhibition of schizont maturation in a 96-well microtitre plate. EC (effective concentration) values of different samples were observed to predict antiplasmodial potential of the plant in terms of their climatic zones.ResultsA maximum quantity of aloin and aloe-emodin i.e. 0.45 and 0.27 mg/g respectively was observed from the 12 samples of Aloe vera. The inhibited parasite growth with EC50 values ranging from 0.289 to 1056 μg/ml. The antiplasmodial EC50 value of positive control Chloroquine was observed 0.034 μg/ml and EC50 values showed by aloin and aloe-emodin was 67 μg/ml and 22 μg/ml respectively. A positive correlation was reported between aloin and aloe-emodin. Antiplasmodial activity was increased with increase in the concentration of aloin and aloe-emodin. The quantity of aloin and aloe-emodin was decreased with rise in temperature hence it was negatively correlated with temperature.ConclusionsThe extracts of Aloe vera collected from colder climatic regions showed good antiplasmodial activity and also showed the presence of higher amount of aloin and aloe-emodin in comparison to collected from warmer climatic sites. Study showed significant correlation between quantities of both the anthraquinones used as marker compounds and EC50 values of the different Aloe vera extracts. Although, both the anthraquinones showed less antiplasmodial potential in comparison to crude extracts of different Aloe vera samples. Diverse climatic factors affect the quantity of tested compounds and antiplasmodial potential of the plant in different Aloe vera samples.

Experimental Investigation of Roughness Effect on Ship Resistance Using Flat Plate and Model Towing Tests

Towing tests on a thin flat plate of 3-mm thickness and on a ship model in smooth and rough condition were performed and extrapolation to ship scale was attempted. A newly designed experimental setup was constructed for the examination of the thin plate. The experiments on smooth flat plate included examination of a series of trip wires for flow stimulation, among which the optimum was 1.3 mm. In rough condition, the plate was covered with sandpapers of 40 and 80 grit. Both calculated roughness functions exhibited Nikuradse behavior, verifying the validity of the experiments. The equivalent sand roughness height was 1.7 times the average sandpaper roughness, as calculated by the Schlichting diagram for sand-roughened plates. Both roughness functions indicated transitionally rough regime, except for the last two data of the rougher sandpaper that lay on the fully rough regime. The results were extrapolated to ship scale using Granville method. Extrapolation of smooth model results in ship scale revealed that the traditional Froude method predicts higher resistance coefficient compared to the International Towing Tank Conference (ITTC) 78 method. Rough model results were extrapolated to ship scale by applying a newly proposed extrapolation method, using Schlichting resistance formula for rough plates as the friction correlation line, according to Froude method and for two length scales, namely the plate and ship length. The two versions of the proposed extrapolation method provided an upper and lower limit for the predicted rough hull total resistance coefficient.

Dynamic wall slip behavior of yield stress fluids under large amplitude oscillatory shear

Wall slip occurs under large amplitude oscillation shear (LAOS) for yield stress fluids. In this work, we investigated how the boundary conditions affect the nonlinear behavior under LAOS and proposed a simple methodology based on the geometric average of Lissajous curves to study the dynamic wall slip behavior under oscillatory shear. The results show that the stress-mean strain curve is a good candidate to define material's functions since it is almost not influenced by the wall slip effect. Meanwhile, the stress-mean strain rate curves from smooth plates and rough plates can be used to determine the wall slip velocity. It is found that the intercycle maximum slip strain rate follows the generalized Navier's law, while the intracycle slip behavior can be well described by a Maxwell-like dynamic slip model, which helps to determine the slip relaxation time. It is also found that the slip Deborah number is independent of the angular frequency and is a monotonically decreasing function of the reduced stress. Moreover, the slip Deborah number depends on the reduced stress through a power law, and there is an evident transition of the power law exponent at the yield stress.

Experimental Study on Cohesiveness Characteristics of Raw Cement Ash at Moderate Temperature

Raw cement ash deposition in SP boilers gives rise to serious operation problems. In the present, mechanism of raw cement ash deposition is still unclear because the ash components, gas atmosphere and gas temperature are quite different from existing studies. The characteristics of cohesiveness of raw cement ash in moderate temperature were investigated in a lab-scale electric furnace. Three typical raw cement ashes were selected as samples, three base plates, including rough steel plate, smooth steel plate, organic glass plate were studied. Ball-impacting method was adopted to evaluate the cohesiveness of ash. Experimental results show that, the ash cohesiveness is mainly dependent on the components of ash, and the material property of base plate, while it exhibits little relationship with the roughness of the plate surface. Low-melting substances appear during ash deposition on steel plate. The results reveal that ferrum possibly has catalytic to some low-melting substances forming.

Velocity fluctuations and boundary layer structure in a rough Rayleigh-Bénard cell filled with water

We report Particle Image Velocimetry of the Large Scale Circulation and the viscous boundary layer in turbulent thermal convection. We use two parallelepipedic Rayleigh-B{\'e}nard cells with a top smooth plate. The first one has a rough bottom plate and the second one has a smooth one so we compare the rough-smooth and the smooth-smooth configurations. The dimensions of the cell allow to consider a bi-dimensional mean flow. Lots of previous heat flux measurements have shown a Nusselt--Rayleigh regime transition corresponding to an increase of the heat flux in presence of roughness which is higher than the surface increase. Our velocity measurements show that if the mean velocity field is not clearly affected by the roughness, the velocity fluctuations rise dramatically. It is accompanied by a change of the longitudinal velocity structure functions scaling. Moreover, we show that the boundary layer becomes turbulent close to roughness, as it was observed recently in the air [Liot et al., JFM, vol. 786, pp. 275-293]. Finally we discuss the link between the change of the boundary layer structure and the ones observed on the Large Scale Circulation.

Local distribution of wall static pressure and heat transfer on a rough flat plate impinged by a slot air jet

Local distribution of wall static pressure and heat transfer on a rough flat plate impinged by a slot air jet