Boundary Layer Regime Research Articles

Investigation of the discharge coefficient in the laminar boundary layer regime of critical flow Venturi nozzles calibrated with different gases including hydrogen

This study represents the first step towards the introduction of critical flow Venturi nozzles into the traceability scheme for gaseous hydrogen and addresses the characterisation of the hydrogen discharge coefficient in the laminar boundary layer regime and the identification of a potential alternative gas for nozzle calibration. The experimental study presented was performed for two nozzles with a throat diameter of 0.175 mm and 0.436 mm. The tests were performed with six different gases, including hydrogen, covering a Reynolds number range from 1.5·103 to 6·104. The experimental results show that the discharge coefficient depends not only on the Reynolds number but also on the isentropic exponent of the gas. Based on the experimental data, theoretical models taking into account the isentropic exponent and the Prandtl number of the gas were evaluated. The study indicates possibilities to use inert gases instead of hydrogen in calibration processes.

Impulsive motion causes time-dependent nonlinear convective Williamson nanofluid flow across a rough conical surface: Semi-similar analysis

The time-dependent nonlinear convective Williamson nanofluid flow over a rough conical surface in the presence of numerous diffusions and Arrhenius activation energy effects are the main focus of the investigation in this paper. The unsteady flow is essentially due to the impulsive mainstream flow. Two diffusive species, namely, liquid Hydrogen and liquid Nitrogen, occur in the flow under investigation. A sinusoidal waveform mathematically models the rough conical surface with small amplitude and high frequency. Thus, the surface gradients, namely skin friction, exhibit wavy effects in the boundary layer regime. In the current flow problem, the governing equations for the fluid flow are highly coupled nonlinear PDEs that depend on the proper initial and boundary conditions. Mangler's transformations are used to convert them into non-dimensional forms, and implicit finite difference approximation and quasilinearization are employed to produce numerical semi-similar solutions. The numerical results representing the boundary layer effects are displayed graphically and analyzed for parametric values. The friction experienced by the rough surface is strongly fluctuating for higher values of Williamson parameter W. The heat transfer rate Rex-1/2Nu increases approximately by 61% and 63% at ξ=0.8 when Nt decreases from Nt=0.5 to Nt=0.1, for the cases of W = 0 (Newtonian fluid) and W = 1(Williamson fluid), respectively. Additionally, there is a strong correlation between the current findings and the related results previously published in the literature.

Magnetized dissipative Soret and Dufour effects on thermally radiative Casson fluid flow over a stretching cylinder with Cattaneo–Christov heat and mass flux models

The main aim and novelty of this numerical analysis is to investigate the thermodynamic features of magnetized Cattaneo–Christov heat and mass flux models on the radiative Casson fluid flow past a stretching cylinder manifested with the thermal and mass flow mechanism within the boundary layer regime qualitatively under the influence of Soret and Dufour effects. Pertaining to this numerical investigation, the primitive forms of produced nonlinear coupled partial differential equations are reduced to ordinary differential equations via similarity transformations and are solved by employing numerically efficient and stable Runge–Kutta fourth-order scheme. The graphical results for velocity, thermal and concentration fields, wall shear stress, Nusselt and Sherwood numbers are presented for the various values of parameters such as, , , , , , , , , and . In addition, the produced numerical solutions indicate that, the higher curvature number suppressed the velocity and thermal fields. Elevating thermal relaxation number diminished the thermal energy distribution over a cylinder. Magnifying Lorentz forces decayed the flow velocity and enhanced the thermal field. Finally, a reasonable agreement between the current analysis and previously published results is tabularized.

Heat and Mass Transfer Analysis of Single Walled Carbon Nanotubes-Water and Multi Wall Carbon Nanotubes-Water Based Maxwell Nanofluid Flow Over Stretchable Rotating Disks

Influence of thermal radiation and magnetic field on heat transfer and flow analysis of water-CNTs type nanofluid between two stretchable revolving disks with heat generation/absorption and convective boundary condition is numerically examined in this analysis. The most extensively validated Finite element technique is employed to solve the reduced non-linear ordinary differential equations together with boundary conditions. Velocity and temperature distributions are calculated and are displayed through graphs for various values of pertinent parameters entered into the problem. Furthermore, the values of rates of change of velocity and temperature are examined in detail and are portrayed in tabular form. The values of skin friction co-efficient at both upper and lower disks elevates in the boundary layer regime with rising values of Deborah number in both nanofluids and this augmentation is higher in MWCNTs-water than SWCNTs-water based Maxwell nanofluid. Temperature of the fluid in both nanofluids deteriorates as the values of nanoparticle volume fraction parameter upsurges and this deterioration in temperature distributions is higher in MWCNTs-water than the SWCNTs-water based Maxwell nanofluid.

Theoretical profiles of two-phase transcritical laminar boundary layer near the Widom line

Boundary layers can be defined as sub-, trans-, and supercritical regime according to whether their temperature profiles cross the Widom line. The transcritical boundary layers which contain liquid and supercritical phases are hard to solve. In this paper, adiabatic flat plate laminar boundary layers of supercritical carbon dioxide (SCO2) are preliminarily explored by the self-similar solution. We analyze the difference of boundary layer profiles at 8 MPa - inflow pressure by varying inflow temperature Te and Mach number Ma. The influence of Te and Ma on the velocity profiles exhibits complex diversity at three boundary layer regimes. Compared with subcritical and supercritical boundary layers, transcritical boundary layers with smaller skin friction coefficients are the most special, and they include the “sub-layer”. Notably, there is no transcritical boundary layer when Te increases to pseudo-critical temperature Tpc at small Ma. The wall temperature Tw of transcritical boundary layers may be greater than inflow total temperature T0 at larger Ma.

Bejan’s numerical heat and mass flow visualization in turbulent boundary layer regime

Low Reynolds number k-ϵ model is employed to demonstrate the Bejan's numerical heat and mass flow visualization in turbulent boundary layer regime. The governing turbulence kinetic energy and its dissipation rate equations are included along with conservation equations. The turbulent flow equations are highly nonlinear and coupled in nature, hence, an unconditionally stable Crank–Nicolson finite difference approach is deployed. The computer-generated heatlines, masslines, streamlines, and isotherms for different control parameters are analyzed in the turbulent flow regime. Average momentum, thermal, and concentration diffusion rates are increased upon the increment in in laminar regime and are decreasing with rising in turbulent regime. The stream, heat and masslines deviate more in turbulent regime when compared to laminar regime. Further, the kinetic energy and dissipation rate contours are deviated more in turbulent heat and mass flow regime for different values of . Mainly, in this research paper, the authors made an attempt to demonstrate the turbulent heat and mass flow visualization about a vertical plate using boundary layer approximations through LRN k-ϵ turbulence model.

Numerical Study of the Effects of a Hot Obstacle on Natural Convection Flow Regimes

A numerical study was conducted for natural turbulent and laminar convection induced by a circular hot obstacle in two different positions, the heat obstacle is introduced at the inlet of the cylinder and inside the cylinder. To determine the effects of a circular hot obstacle on the regime of the thermosiphon flow generated by a heated cylindrical channel, we suggest a theoretical and numerical model using the Navier–Stokes equations and the finite volume method. An experimental apparatus was used for the validation of the numerical model. A comparison of the numerical and experimental results showed a good agreement in the global flow behavior. The temperature and vertical velocity profiles demonstrated the presence of a boundary-layer regime along the heated channel's wall in the absence of a hot obstacle and the presence of the hot obstacle at different positions. The introduction of the hot obstacle generates a significant modification of the evolution of the flow above the hot obstacle. The numerical simulation was conducted for a specific radius of the hot obstacle that ensures the existence in the laminar flow regime, and different Rayleigh numbers (Ra=105; 107; 108; 109; and1010). The flow rate, velocity, temperature and Nusselt number profiles were presented and discussed. The effects of the hot obstacle are presented in our results by two maxima of the vertical component of the velocity profile. The temperature profile at the entrance of the vertical cylinder is presented by the appearance of a maximum above the hot obstacle. As the cylinder height increases, the air temperature decreases. The vertical displacement of the obstacle upward induces a modification in the behavior of the dynamical flow. A CFD code was used to study the different natural convection regimes presented by three different form factors, including the critical regime with a form factor of A=0.1.

Experimental Studies of Hypersonic Shock Impingement on a Transpiration-Cooled Flat Plate

Hypersonic vehicle design requires mitigation of the high heat fluxes present in regions of shock-wave/boundary-layer interactions. A candidate technology that may be applied locally to these regions is transpiration cooling. In this work, experiments were conducted in the University of Oxford’s high-density tunnel at Mach 6.1 in both laminar and turbulent undisturbed boundary-layer regimes where a 10 deg shock generator impinged a strong oblique shock wave onto a transpiration-cooled microporous injector. For the laminar boundary layer, due to the strength of the incident shock, a transitional shock-wave/boundary-layer interaction region was formed with peak heating over 50 times greater than the nominal laminar level. Both nitrogen and helium were used as coolants. Relatively low levels of helium injection of for the transitional and for the turbulent scenarios were sufficient to reduce the heat transfer downstream of shock interaction to approximately 50% of the value without cooling. In fact, helium is highly effective with a similar cooling performance achieved as eight times the equivalent mass flux of nitrogen. The experimental data are correlated, and both the turbulent and transitional shock-impingement scenarios display a similar trend of reduced surface heat transfer with higher blowing parameters. Empirical fits are proposed that may be used for initial systems design.

Thermo-solutal stratification and chemical reaction effects on radiative magnetized nanofluid flow along an exponentially stretching sensor plate: Computational analysis

Motivated by emerging technologies in nanofluid electromagnetic sensor systems, a mathematical model is developed for free convective chemically reacting magnetized Buongiorno nanofluid flow along a stretching exponential Riga plate with dual (thermal and solutal) stratification. Additionally, the effects of radiative heat flux and thermal sink/generation are included. The non-dimensional boundary layer conservation equations are solved with the associated boundary constraints using the Keller Box finite difference scheme, and authentication with earlier studies is conducted. With increasing magnetization parameter, velocity is elevated whereas temperature is suppressed. Increasing Grashof number enhances velocity strongly near the sensor surface region but reduces it further towards the free stream. The heat transfer is depleted throughout the boundary layer regime with greater Grashof numbers. The thermal distribution is substantially boosted with increment in radiative flux, heat source, thermophoresis and Brownian motion parameters, whereas it is strongly decreased with increment in Prandtl numbers and thermal stratification. The nanoparticle concentration is markedly reduced with rising nanoparticle solutal stratification, Brownian motion parameter, reacting species term and Schmidt number. However, there is a considerable increment in nanoparticle concentration with high thermophoresis values. An increase magnetization parameter also elevates the drag force and wall heat transfer rate whereas it reduces the species gradient at the wall. With increasing chemical reaction, a weak rise in the wall friction and temperature gradient is noticed, but a significant rise is computed in Sherwood number.

Spatial Variability of Nocturnal Stability Regimes in an Operational Weather Prediction Model

Forecast errors in near-surface temperatures are a persistent issue for numerical weather prediction models. A prominent example is warm biases during cloud-free, snow-covered nights. Many studies attribute these biases to parametrized processes such as turbulence or radiation. Here, we focus on the contribution of physical processes to the nocturnal temperature development. We compare model timestep output of individual tendencies from parametrized processes in the weather prediction model AROME-Arctic to measurements from Sodankylä, Finland. Thereby, we differentiate between the weakly stable boundary layer (wSBL) and the very stable boundary layer (vSBL) regimes. The wSBL is characterized by continuous turbulent exchange within the near-surface atmosphere, causing near-neutral temperature profiles. The vSBL is characterized by a decoupling of the lowermost model level, low turbulent exchange, and very stable temperature profiles. In our case study, both regimes occur simultaneously on small spatial scales of about 5 km. In addition, we demonstrate the model’s sensitivity towards an updated surface treatment, allowing for faster surface cooling. The updated surface parametrization has profound impacts on parametrized processes in both regimes. However, only modelled temperatures in the vSBL are impacted substantially, whereas more efficient surface cooling in the wSBL is compensated by an increased turbulent heat transport within the boundary layer. This study demonstrates the utility of individual tendencies for understanding process-related differences between model configurations and emphasizes the need for model studies to distinguish between the wSBL and vSBL for reliable model verification.

Lattice-Boltzmann calculations of rotor aeroacoustics in transitional boundary layer regime

The goal of this paper is to demonstrate the predictive capabilities of the Lattice-Boltzmann/Very-Large-Eddy-Simulation CFD solver SIMULIA PowerFLOW® when a transitional flow past a small drone rotor blade is simulated, without using any numerical or physical turbulence triggering system. A recently developed variant of PowerFLOW VLES model is validated against measurements performed at Delft University of Technology. A 2-bladed propeller of 0.3m diameter is considered, which is operated at 4000 and 5000 RPM and two different advance ratios, 0.0 and 0.6, for each angular velocity. In such conditions, corresponding to a Reynolds number based on the chord at 75% of the tip radius ranging between 7⋅104 and 9⋅104, the presence of a laminar separation bubble was experimentally observed and related to the occurrence of a high-frequency broadband noise hump in the far-field noise spectra. The numerical results are in a good agreement with the loading, noise and PIV experimental measurements, demonstrating the capability of the new PowerFLOW VLES model to predict boundary layer flow transitional phenomena such as laminar separation, reattachment and transition to turbulence, as well as the corresponding broadband trailing-edge noise radiation.

Estimation of the atmospheric boundary layer height by means of machine learning techniques using ground-level meteorological data

Aerosols affect air quality, weather and climate through many mechanisms and are dangerous to human health. They are mostly concentrated within the atmospheric boundary layer (ABL) its height is affected by the radiation emitted by the surface, causing turbulence and evolving along the day, influencing the vertical mixing of the air pollutants generated near the surface and therefore, their ground-level concentration from local sources. Lidars have demonstrated their capabilities to study the aerosol vertical distribution and their spatio-temporal evolution can provide very complete information on the ABL dynamics. In this work, machine learning techniques are employed to predict the ABL height. The meteorological variables measured at ground-level are used as features of the algorithm and the ABL height estimated by the STRATfinder algorithm using ceilometer profiles, a small lidar instrument with enhanced characteristics for unassisted continuous operation, are considered the truth in the supervised regression algorithm. The machine learning models allow considering combination of features in the regression algorithm and also allow characterizing the importance of each of the predictors to determine the final result. This property is used to study different boundary layer regimes. The ABL is difficult to study in certain parts of the day due to transitions between atmospheric regimes. In order to improve the performance of the model, each day was divided in four parts (nighttime, morning, daytime and evening). The Madrid ceilometer profile database has been studied for the year 2020, splitting the training datasets for the machine learning algorithm into season and part of the day, and the importance of predictors analyzed. Major influence of temperature and relative humidity is found in most of the situations, but also wind velocity in certain circumstances and pressure. The influence of radiation is small, contrary to expected. The main advantage of the proposed method is that MLHs and ABLHs can be retrieved directly from widely available ground-level meteorological data. Future work will focus on more relevant predictors, as latent heat or turbulence.

DYNAMICAL EFFECTS OF A HOT DISK ON THE DEVELOPMENT OF A NATURAL CONVECTION FLOW BETWEEN CYLINDRICAL WALLS

This study numerically investigates the problem of natural convection flow ina vertical cylinder. The importance of this issue mainly stems from thermal systems (e.g. chimneys, hot air generators, solar collectors, and many others.). Despite the numerous studies on the modeling of free convection flow between two perpendicular plates, there are very few studies that have investigated regarding the convective flow between two cylindrical walls. Therefore, the study is limited to vertical cylinders with different heating modes.The first configuration includes heated walls of thermosiphon flow in a cylindrical channel.The second configuration introduces a circular heat source at the channel entrance, and the third configuration includes an ended vertical cylinder with a heat source centered in the cylinder (zs = 0.04). The flow comparison from the three configurations demonstrates how the hot disk affects the flow from a dynamic point of view.Furthermore, details about the flow and dynamical fields can be obtained from the solution equation of the conservation of momentum and energy, considering the differences between the boundary conditions of the studied configurations. The study covers Rayleigh numbers and focuses on the effect of the cylinders geometry on the characteristic of the flow as well as dynamical and thermal field characteristics and variations in Nusselt number. The studied flow is natural convection, laminar, steady, two-dimensional, and incompressible flow. Solutions were obtained using a numerical model based on the finite volume method and the Navier–Stokes equations. The velocity–pressure coupling was resolved using the SIMPLER algorithm. The temperature and vertical velocity profiles of the flow showed the existence of a boundary layer regime along the heated channel wall without a heat source. The layer continued along the cylindrical wall with the introduction of the heat source at the channel inlet. However, introducing the heat source caused a considerable change in the flow structure at the channels central part.Calculations were performed for the channel aspect ratio R* = 0.2 and different Rayleigh numbers (Ra = 105, 107, 108, 109 and 1010). Numerical results included velocity, temperature, and Nusselt number profiles.

Heat and mass transfer analysis of radiative and chemical reactive effects on MHD nanofluid over an infinite moving vertical plate

A comparative study of nanofluid (Cu–H2O) and pure fluid (water) is investigated over a moving upright plate surrounded by a porous surface. The novelty of the study includes the unsteady laminar MHD natural transmission flow of an incompressible fluid, to get thermal conductivity of nanofluid is more than pure fluid. The chemical reaction of this nanofluid with respect to radiation absorption is observed by considering the nanoparticles to attain thermal equilibrium. The present work is validated with the previously published work. The upright plate travels with a constant velocity u0, and the temperature and concentration are considered to be period harmonically independent with a constant mean at the plate. The most excellent appropriate solution to the oscillatory pattern of boundary layer equations for the governing flow is computed utilizing the Perturbation Technique. The impacts of factors on velocity, temperature, and concentration are visually depicted and thoroughly elucidated. The fluid features in the boundary layer regime are explored visually and qualitatively. This enhancement is notably significant for copper nanoparticles.

Tribological study of iron infused carbon tubes additive in gearbox, engine, and vegetable-based lubricants

Tribo-film strength in the boundary layer regime depends predominantly on the type of additives present in lubricant. Here, we have synthesized iron-infused carbon tubes (Fe-CTs) that can be used as an additive to improve the tribo-film strength of the lubricants (gearbox, engine, and vegetable oil) under the boundary layer regime. Additive synthesis was done by adopting the thermal decomposition of carbon and iron precursors at 800 °C in inert atmosphere. Small amount of additive insertion into the lubricant are found to reduce the coefficient of friction of the lubricant by maximum ∼54% for gearbox oil and ∼22%, ∼15% for vegetable and engine oils respectively. Reduction in friction showed direct relation with the scar diameter and depth profiles. Tribo-surface analysis revealed, the Fe-CTs formulated lubricants provide strength to the tribo-film and protect the surface from severe damage by filling the asperities.

Unsteady absorption flow and dissipation heat transfer over a non-Newtonian fluid

The current research explores an unsteady Casson fluid flow through a stretching sheet with a thermal radiation effect. The influence of heat absorption/generation, viscous disputation and Joule heating is also incorporated. The governing partial differential equations of the flow problem are reduced into boundary layer regimes, which are then transformed using similarity variables into ordinary differential equations. The results are conducted graphically to present the feature of the different parameters in the problem. The physical quantities are tabulated and observed. It is examined that the velocity field is scaled back for thicker values of . Furthermore, higher values of increase the thermal layer thickness.

Stability Analysis of Boundary Layer Flow and Heat Transfer of Fe2O3 and Fe-Water Base Nanofluid οver a Stretching/Shrinking Sheet with Radiation Effect

In this paper, the radiation and slip effects are investigated on the boundary layer flow and heat transfer of Fe2O3 and Fe-water base nanofluids over a porous stretching/shrinking sheet. A similarity transformation is used to convert the system of governing partial differential equations into ordinary differential equations, which are then numerically solved in Maple software with the help of the shooting technique. At different ranges of the applied parameters, dual solutions are found. The effects of the different physical factors such as radiation, nanoparticle volumetric fractions, suction, and slip parameters are determined and discussed. The skin-friction coefficient and local Nusselt number are influenced significantly by the applied parameters. In the boundary layer regime, the increase in nanoparticle volume fractions and radiation parameters enhance the temperature and boundary-layer thicknesses, while increasing Prandtl number, suction, and thermal slip parameters decrease the temperature and reduce thermal boundary-layer thicknesses. The suspension of iron nanoparticles shows more enhancement in skin friction and Nusselt number than the iron oxide nanoparticles in base fluid water.

A Single-Column Comparison of Model-Error Representations for Ensemble Prediction

Various perturbation approaches have been proposed for representing model error in convection-permitting ensemble prediction. Their evaluation usually relies on time-averaged ensemble prediction statistics and on complex case studies. In this work, their detailed physical behaviour is studied in order to understand their differences, and to help their optimization. A process-level intercomparison framework is used to investigate the widely used SPPT (stochastic perturbations of physics tendencies), independent SPPT, and random-parameters model-perturbation approaches. Ensemble predictions with the single-column version of the Arome numerical-weather-prediction model are evaluated on three different boundary-layer regimes: cumulus convection, stratocumulus-topped boundary layer, and radiation fog. The independent SPPT approach is found to produce more dispersion than the SPPT approach, particularly when several physics parametrizations are in near equilibrium. It also appears to be more numerically stable near the surface. The random parameters approach perturbations are structurally very different from the other approaches, particularly regarding cloud structure. The independent SPPT and random parameters approaches have very different sensitivities to the atmospheric conditions, which suggests that intercomparisons of ensemble-model-error approaches should carefully account for situation dependency. Substantial forecast biases are produced by random parameters with respect to the unperturbed model. These results suggest that the independent SPPT approach can bring major improvements over the SPPT approach with minimal effort, that there is some complementarity between the independent SPPT and random parameters approaches, but that implementing random-parameters-type approaches in operational applications may require careful tuning to avoid creating forecast biases.

Spectral‐quasi‐linearization method and multiple regression analysis of reiner‐philippoff fluid flow

AbstractIn this paper, the flow and heat transmission of Reiner‐Philippoff fluid (RPF) past a stretchable sheet is investigated theoretically and analyzed by using the numerical and statistical tools. A set of similarity transformations is used to non‐dimensionalize the governing equations. These expressions are numerically evaluated by employing the Spectral‐Quasi‐Linearization Method (SQLM). The numerical values are validated with the published results, and an excellent agreement is noticed in limiting cases. The results are presented graphically for different cases of fluids such as dilatant, Newtonian and pseudo‐plastic fluids for various physical parametric values include Bingham number, thermal buoyancy parameter, inclination angle parameter. Further, the behavior of fluid velocity and its temperature profiles are discussed within the boundary layer regime. Multiple regression process is used to analyze the results of the heat transmission rate and skin‐friction coefficient against distinct physical parametric values. It is observed that the dilatant fluid and pseudo‐plastic fluid show a diverse behavior as Bingham's number increases. With the enhancement in the inclination angle parameter, the fluid temperature risen for dilatant, pseudoplastic, and Newtonian fluids. This type of flow model over the stretching sheet has gained eminent importance in fluid dynamics due to their enormous applications in engineering and environment such as drawing of plastic sheets, metal and polymer extrusion, liquid coatings on photographic films, and so forth.

Identifying meteorological influences on marine low-cloud mesoscale morphology using satellite classifications

Abstract. Marine low-cloud mesoscale morphology in the southeastern Pacific Ocean is analyzed using a large dataset of classifications spanning 3 years generated by machine learning methods. Meteorological variables and cloud properties are composited by the mesoscale cloud type of the classification, showing distinct meteorological regimes of marine low-cloud organization from the tropics to the midlatitudes. The presentation of mesoscale cellular convection, with respect to geographic distribution, boundary layer structure, and large-scale environmental conditions, agrees with prior knowledge. Two tropical and subtropical cumuliform boundary layer regimes, suppressed cumulus and clustered cumulus, are studied in detail. The patterns in precipitation, circulation, column water vapor, and cloudiness are consistent with the representation of marine shallow mesoscale convective self-aggregation by large eddy simulations of the boundary layer. Although they occur under similar large-scale conditions, the suppressed and clustered low-cloud types are found to be well separated by variables associated with low-level mesoscale circulation, with surface wind divergence being the clearest discriminator between them, regardless of whether reanalysis or satellite observations are used. Clustered regimes are associated with surface convergence, while suppressed regimes are associated with surface divergence.