Skin Friction Reduction Research Articles

Unsteady MHD Stagnation Point Flow of Al2O3-Cu/H2O Hybrid Nanofluid Past a Convectively Heated Permeable Stretching/Shrinking Sheet with Suction/Injection

The numerical investigation of unsteady magnetohydrodynamic (MHD) stagnation point flow of Al2O3-Cu/H2O hybrid nanofluid past a convectively heated permeable stretching/shrinking sheet with suction/injection effect is underlined. The characteristic of MHD and boundary condition with suction/injection has received a lot of consideration due to its across-the-board application in mechanical and chemical engineering. The governing continuity, momentum and energy equations are transformed into a system of nonlinear ordinary differential equations using similarity transformation, which is then solved using the bvp4c routine. Numerical results are obtained for the skin friction coefficient, local Nusselt number as well as the velocity and temperature profiles for certain values of the governing parameters, namely suction/injection parameter, copper nanoparticle volume fraction parameter and MHD parameter. Results showed that both velocity profile and temperature increase as the suction/injection parameter increases for the first solution and decreases for the second solution. Similarly, when increase the value of MHD parameter M, the velocity and temperature profiles are decreases for both solutions. The magnitude of the reduced skin friction coefficient and the local Nusselt number are notably increased for the first solution with increasing values of the suction/injection and MHD parameters. Finding also revealed that the skin friction coefficient is intensified in conjunction with the local Nussel number by adding up the copper nanoparticle volume fraction. In general, dual solutions are found to exist to a particular extent of the stretching/shrinking sheet.

Heat Transfer under Magnetohydrodynamics Flow of Nanofluids Past an Inclined Plate with Non Uniform Boundary Conditions

This paper deals with natural thermal convection combined with the mass transfer of nanoparticles occurring in the boundary layers of a nanofluid subjected to magnetohydrodynamics. The wall consists of an inclined plate is considered according to a temperature as well as the volume fraction of the nanoparticles varying as the power of the axial coordinate. In addition, internal heat generation/absorption is taken into account in the mathematical formulation. The governing partial differential equations based on Buongiorno's approach are transformed into a set of ordinary differential equations. The two-level method of no-similarity equations is used to achieve higher accuracy. The whole calculation procedure is implemented using a limit value problem program written according to the Matlab computer language that applies the Lobbato IIIa finite difference method. The obtained results have revealed that small variations of the boundary conditions with the axial coordinate become very significant on the local Nusselt number and the local Sherwood number for nanoparticles. Moreover, a better heat transfer has been obtained with a larger S. However, a trade-off between desired heat transfer rate and level of reduced skin friction should be scheduled.

Hydrodynamic Boundary Layer Flow of Chemically Reactive Fluid over Exponentially Stretching Vertical Surface with Transverse Magnetic Field in Unsteady Porous Medium

This article addresses the hydrodynamic boundary layer flow of a chemically reactive fluid over an exponentially stretching vertical surface with transverse magnetic field in an unsteady porous medium. The flow problem is modelled as time depended dimensional partial differential equations which are transformed to dimensionless equations and solved by means of approximate analytic method. The results are illustrated graphically and numerically and compared with previously published results which shown a good agreement. Physically increasing Eckert number of a fluid amplifies the kinetic energy of the fluid, and as a novelty, the Eckert number under the influence of chemically reactive magnetic field is effective in controlling the kinematics of hydrodynamic boundary layer flow in porous medium. Interestingly, whilst the Eckert number amplifies the thermal boundary layer thickness and velocity as well as the concentration of the fluid, the presence of the magnetic field and the strength of the chemical reaction have a retarding effect on the flow. Also, the chemical reaction parameter and permeability of porous medium are effective in reducing skin friction in chemically reactive magnetic porous medium and are relevant in practice because reduced skin friction enhances the efficiency of a system. The results of the current study are useful in solar energy collector systems and materials processing.

Insight into significance of thermal stratification and radiation on dynamics of micropolar water based TiO2 nanoparticle via finite element simulation

The evolution of nanofluids is important for improving the thermal conductivity of base fluids. The influence of thermal radiation and thermal stratification on the magnetohydrodynamic micropolar nanofluid flow through a shrinking sheet with a prescribed heat flux on the surface has been examined. The most important parts of this study are the effects of magnetohydrodynamic microrotation, thermal radiation, the magnetic field, and the Cattaneo-Christov heat flux model. The efficiency of nanoparticles, heat, and mass transference rates are influenced by the magnetic field pattern, the characteristics of the source of heat, thermal radiation, and the dispersion of volume fraction. The partial differentials are transformed into the set of nonlinear differential equations through boundary layer estimations and similarity substitutions and then computed with the use of a variational finite element procedure. A MATLAB code has been developed to assess parametric simulations for reduced skin friction factor, micro-rotation, fluid velocity, heat transfer rate, and thermal properties for the Glariken formulation. The temperature field declined due to increasing values of the thermal stratification parameter and the heat transfer rate accelerated. There is a strong link between the two sets of results, which shows that the finite element method used here is accurate.

Experimental Study on the Effect of Pile-End Soil on the Pile Load Transfer Law

This paper compares and analyzes the difference in the skin friction between pile-end soilless compressive pile and conventional compressive pile at various stages during loading by the in situ test method. The influence of pile-end soil on the load transfer law of compressive piles in clay-dominated stratified foundations is further investigated. The results show that the overall load–displacement curves of the pile-end soilless compressive pile and the conventional compressive pile both present a slow decline followed by a steep drop. The length of the linear section on the load–displacement curve of the pile-end soilless compressive pile is less than that of the linear stage of the conventional compressive pile. Under the vertical load, the distribution laws and distribution forms of the skin friction ratio of the pile sections of the two piles are more consistent. The pile-end soil of the conventional compressive pile restricts the skin friction of the pile’s middle-lower and lower pile segments when compared to the pile-end soilless compressive pile. This restriction manifests itself as a reduction in pile skin friction, and the weakening effect decreases from bottom to top.

Modification of a turbulent boundary layer by circular cavities

It is shown how well-chosen perforations in a wall flow can locally reduce skin friction drag by modifying the generation of bursts in the boundary layer. For this purpose, a detailed hot wire boundary layer experimental investigation of the flow past a perforated plate, complemented with large eddy simulations, is carried out and compared to the smooth case. The perforated plate is obtained with an array of flush-mounted circular cavities. These cavities are disposed in a periodic staggered arrangement. For the three tested flow velocities, the momentum thickness-based Reynolds number varies from Reθ=1830 to 3380 and the cavity diameter and spacing in wall units, respectively, from d+=130 to 250 and L+=587 to 1075, the latter being identical in both spanwise and streamwise directions. The mean velocity profiles evidence a thickening of the viscous sublayer and a decrease in the friction velocity as compared to the smooth wall case. The application of the Variable Interval Time Averaging technique highlights an upward shift of the bursts from the wall and an attenuation of the average burst intensity and duration. Spanwise measurements evidence an overall bursts attenuation despite the lack of spanwise uniformity. The three-dimensional (3D) mean flow topology arising from the large eddy simulations provides evidence for the qualitative similarities between the current setup and the spanwise wall oscillations.

Effects of wall thermal state on the cooling and friction reduction characters for supersonic film using gaseous hydrocarbon fuel

• The region influenced by the wall thermal state is limited within 1 mm off the wall. • Increase of the wall cooling heat flux improves the hydrocarbon film cooling performance. • The cooling enhancement can be equivalent to an additional wall heat flux of −0.36 MW/m 2 . • Increase of the wall cooling heat flux is unfavorable for reducing skin friction. Film cooling using gaseous hydrocarbon fuel is an effective way to meet the thermal protection and friction reduction demands of hydrocarbon-fueled scramjet engines. The cooling and friction reduction characteristics of the hydrocarbon film can be influenced by the wall thermal state through the variation of endothermic and exothermic chemical reactions in the film. Numerical investigations based on the RANS method have been performed to evaluate the effects of the wall thermal state on the hydrocarbon slot film cooling performance. Four wall thermal states which represent different regenerative cooling levels are simulated. Results show that an increase of the wall cooling heat flux is beneficial to the film cooling performance while has unfavorable effects on the film friction reduction performance. Further analysis reveals that the net energy effects of the chemistry in the hydrocarbon film can be equivalent to an additional constant wall cooling heat flux of −0.36 MW/m 2 within a certain range. Besides, the proportion of this equivalent wall cooling heat flux in the total wall cooling heat flux decreases from 100% to 26% for different wall thermal states. Therefore, the cooling enhancement brought by the endothermic chemistry is comparable to the regenerative cooling. However, the wall shear stress is less sensitive to the wall thermal state. The maximum deviation of the total wall friction for different wall thermal states is less than 8% compared to the reference value.

A Homogenization Approach for Turbulent Channel Flows over Porous Substrates: Formulation and Implementation of Effective Boundary Conditions

The turbulent flow through a plane channel bounded by a single permeable wall is considered; this is a problem of interest since a carefully chosen distribution of grains and voids in the porous medium can result in skin friction reduction for the flow in the channel. In the homogenization approach followed here, the flow is not resolved in the porous layer, but an effective velocity boundary condition is developed (and later enforced) at a virtual interface between the porous bed and the channel flow. The condition is valid up to order two in terms of a small gauge factor, the ratio of microscopic to macroscopic length scales; it contains slip coefficients, plus surface and bulk permeability coefficients, which arise from the solution of microscale problems solved in a representative elementary volume. Using the effective boundary conditions, free of empirical parameters, direct numerical simulations are then performed in the channel, considering a few different porous substrates. The results, examined in terms of mean values and turbulence statistics, demonstrate the drag-reducing effects of porous substrates with streamwise-preferential alignment of the solid grains.

Free Convection Boundary Layer Flow from a Vertical Truncated Cone in a Hybrid Nanofluid

The present study investigates the mathematical model of free convection boundary layer flow from a vertical truncated cone immersed in Cu/water nanofluid and Al2O3-Cu/water hybrid nanofluid. The governing non-linear equations are first transformed to a more convenient set of partial differential equations before being solved numerically using the Keller-box method. The numerical values for the reduced Nusselt number and the reduced skin friction coefficient are obtained and illustrated graphically as well as temperature profiles and velocity profiles. Effects of the alumina Al2O3 and copper Cu nanoparticle volume fraction for hybrid nanofluid are analyzed and discussed. It is found that the high-density and highly thermal conductivity nanoparticles like copper contributed more in skin friction and convective heat transfer capabilities. The appropriate nanoparticles combination in hybrid nanofluid may reduce the friction between fluid and surface but yet still gave the heat transfer capabilities comparable to metal nanofluid.

Numerical investigation and optimization of flat plate transpiration-film combined cooling structure

To improve the limited overall cooling effectiveness of laminated structure with the increase of coolant under larger Bi-number, a new optimized structure named transpiration-film combined cooling structure is proposed for the thermal protection of aero-engine combustion chamber. By replacing the lower impingement plate of the laminated structure with a porous layer, a desirable outside film coverage are achieved and the internal heat transfer is enhanced owing to the blocking effect and the large specific surface area of the porous layer. Accurate mathematical modeling and fully-coupled numerical approach are adopted to investigate and compare the cooling effectiveness, cooling uniformity, and the effect of skin friction reduction of the combined cooling structure and the laminated cooling structure under practical combustor conditions with larger Bi-number. The results show that the combined structure brings about a 30% increment in overall cooling effectiveness and a considerable improvement in temperature uniformity of the whole structure, and moreover, the frictional resistance on the structure surface is greatly reduced. Furthermore, an automatic single-objective optimized method is adopted to obtain optimal schemes of the combined cooling structure under a practical combustion chamber working condition, powerfully promoting the cooling effectiveness by 8.5%–8.7%. This work aims to provide the designers of aero-engine with a valuable reference for searching the thermal management approach with high efficiency and low frictional resistance.

Entropy generation on unsteady stagnation‐point Casson nanofluid flow past a stretching sheet in a porous medium under the influence of an inclined magnetic field with homogeneous and heterogeneous reactions

AbstractThis study explores the entropy generation analysis on unsteady nonlinear radiative ethylene glycol‐based Casson nanofluid flow near stagnation point towards a stretching sheet through a porous medium. Analysis has been accomplished in the presence of an inclined magnetic field, heat generation, homogeneous–heterogeneous reactions, and viscous dissipation with velocity slip and convective boundary conditions. The nondimensional governing equations are solved by the shooting technique with the help of the RK45 method. We have experimented with copper and silver nanoparticles and a comparative analysis has been highlighted for both copper and silver nanofluids. Numerical outcomes are executed by the MATLAB built‐in bvp4c function. The consequences of the experiment for various pertinent flow parameters are portrayed by graphs and tables for both the Ag‐ and Cu‐Casson nanofluids. Results reveal that the enhancement of nanoparticles volume fraction accelerates temperature but it slows down concentration and velocity distributions. Higher values of the Eckert number boost velocity and temperature but reduce skin friction coefficient and Nusselt number. Enhancement of the Brinkman number boosts up entropy generation but it slows down Bejan's number. The results of the model can be applied in the movement of biological fluids, separation of biomolecules, glass manufacturing, paper production, food processing, crude oil purification, polymer drag reduction, and cooling atomic reactors.

Thermal stability of hybrid nanofluid with viscous dissipation and suction/injection applications: Dual branch framework

The thermal stability of nanomaterials is quite necessary for controlling the heat and cooling phenomenon. It is worthy observed that much research has been focused scientists towards the thermal significance of nanoparticles with multidisciplinary engineering and industrial applications. On this end, this report explores the improved thermal mechanism water base material with interaction of hybrid nanofluid stretching and shrinking surface. The cooling and heat phenomenon is observed in presence of viscous dissipation. The hybrid nanofluid characteristics are inspected with combination of copper (Cu) and aluminum oxide (Al2O3) nanoparticles with stable prospective. The consideration of such hybrid nanoparticles is due to impressive thermal characteristics and stable thermal performances. Although some studies are focused by researchers on hybrid nanofluid, however the measurement of thermal stability is not claimed yet. The stretching and shrinking configuration specify the porous medium features. The problem is compiled into dimensionless structure which is further preceded via bvp4c scheme. The resultant ODEs are successfully numerically solved using the bvp4c solver technique. Under restricting conditions, numerical findings are compared to previously published results. Non-dimensional profiles of velocity and temperature are shown graphically. Furthermore, graphs and tables show the effects of the physical parameters used on the reduced skin friction and heat transfer rate. Dual branches are found in specified domain of suction factor.

Heat Transfer Analysis of Nanostructured Material Flow over an Exponentially Stretching Surface: A Comparative Study

The objective of the present research is to obtain enhanced heat and reduce skin friction rates. Different nanofluids are employed over an exponentially stretching surface to analyze the heat transfer coefficients. The mathematical model for the problem has been derived with the help of the Rivilin–Erickson tensor and an appropriate boundary layer approximation theory. The current problem has been tackled with the help of the boundary value problem algorithm in Matlab. The convergence criterion, or tolerance for this particular problem, is set at 10−6. The outcomes are obtained to demonstrate the characteristics of different parameters, such as the temperature exponent, volume fraction, and stretching ratio parameter graphically. Silver-water nanofluid proved to have a high-temperature transfer rate when compared with zinc-water and copper-water nanofluid. Moreover, the outcomes of the study are validated by providing a comparison with already published work. The results of this study were found to be in complete agreement with those of Magyari and Keller and also with Lui for heat transfer. The novelty of this work is the comparative inspection of enhanced heat transfer rates and reduced drag and lift coefficients, particularly for three nanofluids, namely, zinc-water, copper-water, and silver-water, over an exponentially stretching. In general, this study suggests more frequent exploitation of all the examined nanofluids, especially -water nanofluid. Moreover, specifically under the obtained outcomes in this research, the examined nanofluid, Ag-water, has great potential to be used in flat plate solar collectors. Ag-water can also be tested in natural convective flat plate solar collector systems under real solar effects.

Two-Dimensional Mixed Convection and Radiative Al2O3-Cu/H2O Hybrid Nanofluid Flow over a Vertical Exponentially Shrinking Sheet with Partial Slip Conditions

Hybrid nanofluid is considered a modern and improvised form of nanofluid which usually used to enhance the performance of heat transfer in fluid flow systems. Previous studies found hybrid nanofluid offered a wide range of applications and this opened up numerous new opportunities to further explore the unknown behaviour of hybrid nanofluid under different body geometries and physical parameters. This paper numerically studied a two-dimensional mixed convection and radiative Al2O3-Cu/H2O hybrid nanofluid flow over a vertical exponentially shrinking sheet with partial slip conditions. The main objective is to investigate the effect of mixed convection and radiation on the velocity and temperature profiles, as well as the effect of suction on reduced skin friction and reduced heat transfer with respect to solid volume fraction of copper, velocity, and thermal slips. Exponential similarity variables transformed the governing system of partial differential equations into a system of ordinary differential equations which is solved via MATLAB’s bvp4c solver. Outcomes showed that the value of the reduced heat transfer upsurges in the first solution but declines in the second solution when the velocity slip rises. The reduced heat transfer decreases in both dual solutions when thermal slip is enhanced. As the intensity of thermal slip increases, the reduced skin friction rises in the first solution and decreases in the second. As the mixed convection parameter increases, no obvious variation is noticed in the temperature distribution within the first solution, but increasing trend is observed within the second solution. An increment in the temperature distribution also observed within the dual solutions as the thermal radiation parameter increases. In summary, findings from this study are particularly useful to understand various behaviour of Al2O3-Cu/H2O hybrid nanofluid under the influence of mixed convection, radiation, and partial slip conditions when it flows over a vertical exponential shrinking sheet.

Cattaneo\u2013Christov heat flow model for copper\u2013water nanofluid heat transfer under Marangoni convection and slip conditions

This report is devoted to the study of the flow of MHD nanofluids through a vertical porous plate with a temperature-dependent surface tension using the Cattaneo–Christov heat flow model. The energy equation was formulated using the Cattaneo–Christov heat flux model instead of Fourier’s law of heat conduction. The Tiwari–Das model was used to take into account the concentration of nanoparticles when constructing the momentum equation. The problem is described mathematically using the boundary layer approach as a PDE, which is then converted into an ODE with the help of the transformation process. The solution finding process was completed by running the bvp4c code in MATLAB. A quantitative analysis of the influence of some newly occurring parameters on physical quantities was carried out using graphics. The addition of nanoparticles to the base fluid leads to an increase in both skin friction and thermal conductivity. The increase in thermal conductivity is the advantage, while the increase in skin friction is the disadvantage of the nanoparticle concentration. Marangoni convection has proven to be one of the most cost-effective tools available that can reduce skin friction. Marangoni convection improves the heat transfer coefficient during suction but decreases the heat transfer coefficient during the injection.

Large-scale and small-scale contribution to the skin friction reduction in a modified turbulent boundary layer by a large-eddy break-up device

In this work, we assess the contributions of large-scale and small-scale Reynolds shear stress events to the skin friction reduction of a turbulent boundary layer modified by a large-eddy break-up device, based on the quadrant analysis of Reynolds shear stress, the Fourier mode decomposition, and an extension of a skin friction decomposition scheme.

Modification of energetic modes for transitional flow control

We introduce a mechanism of using collocated serpentine shaped plasma actuators for controlling the flow via an input electrical signal to either advance or delay transition by amplification or annihilation of energetic modes of turbulent structures. A wall resolved implicit large eddy simulation is conducted to examine the process of turbulent flow control due to this mechanism. Collocation allows for selective superposition of different energetic modes, which can either subtract or add energy to the baseline flow resulting in turbulent streak manipulation. Unlike most flow control methods that use either a localized large amplitude forcing or a low amplitude distributed forcing, this mechanism uses a localized low amplitude forcing to cause reduction in skin friction of more than 53% across the plate. This is achieved by manipulating strength as well as the spacing between the low-speed turbulent streaks, which are ubiquitous in a turbulent flow field. Reduction in skin friction drag can result in decreasing fuel consumption and in turn reducing pollution.

Reduction of skin friction and two-phase flow structure beneath wall in horizontal rectangular channel

From the viewpoint of the improvement of the air lubrication method for the ship's hull resistance reduction, an experimental study was conducted for two-phase flow in a horizontal rectangular channel with a length of 3.0 m, a gap of 0.05 m, and a width of 0.2 m, which simulates the external two-phase flow. The database on frictional drag and local two-phase flow parameters, including void fraction profile and gas chord length, was collected using a shear stress sensor and a double sensor probe. Measurement of two-phase flow structure was performed at three axial positions of z = 0.930, 1.43, and 1.93 m from the bubble injection port. The wall shear stress was obtained at z = 1.93 m. A total of 72 datasets are acquired a room temperature and at flow conditions of superficial liquid velocity <jf> ranging from 0.832 to 3.00 m/s and superficial gas velocity <jg> ranging from 0.0232 to 0.714 m/s. The effects of entry length, liquid and gas flow rates on the phase distribution characteristics beneath the wall were discussed. The bubble layer thickness was newly defined as the length scale. The bubble layer thickness was not so sensitive to the changes in the liquid flow rates and the entry length, and highly dependent on the gas flow rates in the present experimental conditions. The mean void fraction in the bubble layer was highly reliant on the liquid flow rate and decreased with the superficial liquid velocity. In contrast, the insignificant effect of gas flow rate on the mean void fraction was confirmed. A strong correlation between the drag reduction effect due to the air lubrication and the mean void fraction in the bubble layer was confirmed. The obtained data are expected to be used for the modeling of the interfacial area transport terms, development of the constitutive equations of the drift flux model, modeling of the drag reduction for the external two-phase flow and the benchmark tests of various CFD codes in the future study.

Soret and heat generation effects on unsteady MHD Casson fluid flow in porous medium

This article studies motion with quadratic speed on the unsteady free convective MHD Casson fluid flow in a porous medium. The fluid in electrically conducting and bounding plate has ramped the surface concentration. The characteristics of heat and mass transfer are analyzed with thermal diffusion, heat generation, chemical reaction, and thermal radiation. The governing dimensionless equations are solved using the Laplace transform technique, and analytical expressions for velocity, temperature, and concentration are obtained. For both boundary conditions, the expression for Skin friction, Nusselt number, and Sherwood number are obtained and represented in tabular form. Effects of several pertinent parameters on velocity, temperature, and concentration distribution are presented graphically. It is seen that the mass buoyancy force increases with Soret number which leads to accelerated fluid flow. Chemical reaction and porosity of the fluid tend to reduced Skin friction coefficient, while Casson fluid and magnetic field tend to reverse effect on it.

Lifestyle modifications and complementary treatments for hidradenitis suppurativa

AbstractBackgroundHidradenitis suppurativa (HS) is a chronic inflammatory dermatosis with recurrent painful nodules and abscesses affecting the intertriginous areas. Patients often experience a significant negative impact on quality of life from associated pain, discharge, odor, scarring, and social stigma. Even with complex medical regimens, patients may report inadequate disease control. Lifestyle changes and complementary and alternative medicines (CAM) are used by a majority of HS patients. Conventional medicines and lifestyle and CAM modalities strategically incorporated together can increase treatment satisfaction and success.ObjectiveWe aim to highlight lifestyle and CAM treatment modalities to use alongside conventional treatment plans for HS patients.FindingsLifestyle modifications for HS patients include weight loss, reducing skin friction, smoking cessation, and stress reduction. Dietary modifications for HS patients include limiting foods containing dairy, Brewer's yeast, and simple carbohydrates, appropriate caloric reduction, and consuming foods in the Mediterranean diet. The CAM modalities studied in HS are vitamin D, niacinamide, vitamin B12, zinc, and magnesium. Potential CAM modalities for HS include probiotics, cannabinoids, turmeric, bathing regimens, and acupuncture.ConclusionThis review is a guide for clinicians to provide evidence‐based care for HS patients seeking non‐pharmacologic treatment modalities such as weight loss, clothing choices, dietary changes, and use of supplements and complementary medicine. Lifestyle and CAM modalities may be used in parallel with conventional medicines to synergistically improve patient disease outcomes.