Pressure Gradient Parameter Research Articles

Numerical investigation of heat and mass transfer of variable viscosity Casson nanofluid flow through a microchannel filled with a porous medium

Thermal behaviours and hydrodynamics of non-Newtonian nanofluids flow through permeable microchannels have large scale utilizations in industries, engineering and bio-medicals. Therefore, this paper presents the numerical investigation of heat and mass transfer of variable viscosity Casson nanofluid flow through a porous medium microchannel with the Cattaneo-Christov heat flux theory. The highly nonlinear PDEs corresponding to the continuity, momentum, energy and concentration equations are formulated and solved numerically via the second order implicit finite difference scheme known as the Keller-Box method. Accordingly, the numerical simulations reveal that variable viscosity parameter, thermal Grashof number, solutal Grashof number, thermophoresis parameter, Schmidt number and Casson fluid parameter show increasing effects on both velocity and temperature of the nanofluid. Furthermore, the temperature profile escalates with increasing values of the Eckert number and the thermal relaxation time parameter. Thus, the Cattaneo-Christov heat flux model is beneficial in warming the transport system of microfluidics when compared to that of the classical Fourier heat conduction law. The temperature profile however, indicates a retarding behavior with increasing values of the Brownian motion parameter, Prandtl number and porous medium parameters namely Forchheimer number and porous medium shape parameter and hence, the porous medium quite effectively controls the nanofluid temperature distribution which plays substantial roles in cooling the transport system of microfluidics. Moreover, the concentration profile shows an increasing pattern with escalating values of the Prandtl number, Schmidt number and thermophoresis parameter but it demonstrates a decreasing trend with the Casson fluid, variable viscosity, thermal relaxation time and solutal relaxation time parameters. It is also observed that coefficient of the skin friction increases with increasing values of the pressure gradient parameter, Eckert number, Forchheimer number and injection/suction Reynolds number. Besides, the heat transfer rate at both walls of the microchannel enhances with rising values of the Eckert number, variable viscosity, parameter and injection/suction Reynolds number. The Casson fluid and thermal relaxation time parameters reveal opposite scenarios on the heat transfer rate at the left and right walls of the microchannel. In addition, the mass transfer rate at both walls of the microchannel shows an increasing pattern as the Eckert number, variable viscosity parameter, Schmidt number and suction/injection Reynolds number increase.

Bubble dynamics in a pressure gradient with reentrant jet break through and energy loss

The dynamics of a bubble in a pressure gradient is investigated experimentally and numerically with particular emphasis on the behavior at reentrant jet impact and break through the opposite side of the bubble with corresponding energy loss and vorticity generation. High speed photography observations of a bubble generated by electric spark energy deposit in a low ambient pressure tank are coupled with wavelet based Optical Flow Velocimetry (wOFV) and Boundary Element Method (BEM) numerical analysis to examine the flow field resulting from the reentrant jet formation and break through. We study, as an illustration, the effects of the constant pressure gradient due to gravity on the bubble dynamics. Energy losses between the first and second cycle are measured for the bubbles generated under various conditions characterized by a non-dimensional pressure gradient parameter, and the corresponding measured energy loss is used in the numerical simulations. Good correspondence is seen between the image analysis, the wOFV computations, and the BEM results and insight is gained on the involved physics.

Unsteady MHD hybrid nanofluid flow over a convectively heated linear stretching cylinder with velocity slip: A comparative study

This paper provides a comparative numerical analysis of unsteady hybrid nanofluid ([Formula: see text]/H2O) and nanofluid (TiO2/H2O) flows through a permeable linear stretching cylinder placed in a porous medium has been presented in the presence of oblique Lorentz force. The flow regulating boundary layer equations has been governed due to the sway of the suction, viscous-Ohmic dissipation, heat source, thermal radiation and first-order velocity slip with imposing convective boundary conditions on the surface. The self-similar transformation has been employed to convert the governing coupled nonlinear PDEs into a set of ODEs and finally solved them numerically by fifth-order Runge–Kutta method with shooting technique. Finally, a comparative study has been made on the thickness of the momentum and thermal boundary layers due to the effect of governing parameters for simple and hybrid nanofluids. Further, a numerical observation on local skin friction and thermal-transport coefficients is presented with the help of tables. The result indicates that the progressiveness of the velocity profile reduces for the unsteadiness parameter, Hartree pressure gradient, velocity slip parameter, inclination parameter, porous medium parameter, magnetic parameter and suction parameter while improving for velocity ratio parameter and temperature profiles increases with Biot number, Eckert number, heat generation parameter and thermal radiation parameter, but decreases for Hartree pressure gradient and velocity ratio parameter. This work has found various applications in high-temperature and cooling processes, paints, conductive coatings, medicines, space technology, biosensors, conductive coatings and so on.

Computation of micropolar nanofluid from a wedge with heterogeneous carbon/metallic nanoparticles, viscous dissipation and heat sink/source: rheological nanocoating flow simulation

ABSTRACT Motivated by nanotechnological coating applications, a theoretical study is presented for the laminar, steady-state, incompressible nonlinear boundary layer flow of a non-Newtonian nanofluid external to a wedge-shaped configuration. The wedge surface is assumed to be isothermal. The Eringen micropolar model is deployed for rheological properties of the nanofluid. The dimensionless thermal perimeter layer equations are solved with the efficient MATLAB bvp4c numerical scheme. Validation with earlier studies is conducted. Aqueous-based nano-polymers are examined with either metallic/metallic oxide or carbon-based nanoparticles. The influence of Hartree pressure gradient parameter, Eringen vortex viscosity parameter, nanoparticle volume fraction, heat absorption parameter, Prandtl number and nanoparticle type on velocity, angular velocity, temperature, skin friction function and Nusselt number function are visualized graphically and in tables. Temperature is strongly elevated with increasing micropolar parameter and nanoparticle volume fraction. Velocity is boosted with increasing nanoparticle volume fraction. Temperatures are elevated with heat source but suppressed with heat sink. Temperatures are a maximum for silver and progressively lower for copper, diamond and with a minimum for titania. Skin friction is boosted with pressure gradient parameter whereas Nusselt number is depleted. Nusselt number is observed to be a maximum for diamond whereas it is a minimum for silver.

Heat and mass transport in an electrically conducting nanofluid flow over two-dimensional geometries

Engineering equipment in medicine, chemical and power engineering, electronics, and other human endeavours use nanofluids. The ability to improve mass and heat transport because of the low concentration of nanoparticles is the primary driver behind the vast array of nanofluid applications. Thus, the famous problems of viscous, incompressible, Newtonian, and 2-D laminar flow are revisited to investigate the mass and heat transmission rates for water-based carbon nanotubes (CNTs) with variable magnetic fields and external pressure gradients. Flow cases considered with varying pressure gradients are the flows upon a flat plate, flow in a planar diverging and converging channel, flow over a wedge, and plane stagnation flows, which are investigated. The impressions of thermophoresis and Brownian motion parameters are examined through the Buongiorno model. Using the Görtler transformation, the leading boundary layer (BL) equations are converted into dimensionless forms of ordinary differential equations (ODEs). Runge-Kutta Fehlberg Method (RKF45) is operated to tackle the ensuing ODEs to find the mass, heat, and skin friction rates. It has been found that the rates of shear stress, mass, and heat transport slow down with an escalating magnetic field. Although mass transport rates are decreased, shear stress and heat transport (HT) rates escalate due to the solid volume portion of carbon nanotubes. Furthermore, the pressure gradient parameter facilitates faster heat and shear stress transmission rates.

Simplified Stability-Based Transition Transport Modeling for Unstructured Computational Fluid Dynamics

Inspired by the model framework, a new -based model was developed and implemented into an unstructured Reynolds-averaged Navier–Stokes (RANS) code. The new model applies a modified version of the -transport equation of the model. This new implementation applies an alternative local formulation for the pressure-gradient parameter that allows the local assessment of the transition criterion within the boundary layer and, thereby, simplifies the model and increases the accuracy of the transition onset location. The resulting prediction approach is robust, user-friendly, and suitable for unstructured RANS solvers with high parallelization. To attain a wide application range of the prediction method, while keeping the simplicity of the approach, a physical path-independent transition criterion was adopted, which was calibrated based on linear stability theory results. The approach was successfully validated against experimental data for various relevant test cases. In addition, results were also compared with results obtained with the model and, for some cases, with the method, which showed a strong similarity in the predictions of the method and the new model.

Velocity spectra and scale decomposition of adverse pressure gradient turbulent boundary layers considering history effects

Experiments at relatively large Reynolds number were conducted to better understand the influence of an adverse pressure gradient on the turbulence in two-dimensional boundary layer flows. One focus is on documenting the scale contributions to the Reynolds normal and shear stress profiles under the influence of a positive streamwise pressure gradient. Toward these aims, velocity spectra at friction Reynolds numbers (7100≤Reτ≤8600) are presented, analyzed, and compared to a zero pressure gradient case from the same facility at nominally matching Reynolds number. Distance-from-the-wall scaling is central to numerous theoretical and modeling considerations of the canonical flat plate flow. Consistently, the present spectral measurements reveal clear evidence for its preservation on the inertial sublayer under modest adverse pressure gradients. Increases are seen in the viscous-scaled premultiplied spectra in the outer region as the pressure gradient increases — commensurate with the increases observed in the outer peaks of the streamwise and wall-normal velocity variances, as well as the Reynolds shear stress. Effects of varying Reτ on the streamwise and wall-normal velocity variances and the Reynolds shear stress are studied by comparing the present data at varying Clauser pressure gradient parameter, β, with previous experiments having similarly varying β at significantly lower Reτ (≈1900). This allows to study the effects of changing Reynolds number at matched β. Cases at similar β and Reynolds number, but with varying flow history are also examined, wherein the same β value is arrived either by starting from a smaller or larger initial β. The effects of β>0 on the large and small scale motions are investigated by segregating the signal contributions according to a cut-off wavelength established in previous zero pressure gradient flows. Under a normalization that uses a hybrid velocity scale this analysis reveals similarity between the outer regions of the spectrally-decomposed variances and Reynolds shear stress. These analyses also reveal a more dramatic reduction in the near-wall large scale contribution to the Reynolds shear stress gradient in the β>0 flow when compared to the β=0 flow.

Impact of viscous dissipation and entropy generation on cold liquid via channel with porous medium by analytical analysis

In the current investigation, it is examined numerically entropy generation (EG) on inherent irreversibility motion of couple stress cold liquid with porous medium via Horizontal channel in presence of viscous dissipation. Present work was studied heat generation on couple stress liquid with porous channel. This work considerable importance in many industrial applications like “Control Mechanism in Material Manufacturing”, “Manufactures of Electronic Chips”, “Crystal Formation”, “Scientific Treatment Problems of Irrigation”, “Soil Erosion and tile drainage” are the present focus of development of channel motion. The formulated physical liquid equations are subsequently calculated by shooting technique with R-K-F (“Runge-Kutta Fehlberg”) scheme. The velocity, temperature as predicted via graphically. we found the velocity dwindle, temperature high production by an escalating statistical value of K (“Couple Stress Parameter”), Ec (“Eckert number”) and Q (“Heat Generation parameter”) respectively. Gradient constraint in addition to improve by an enhancement into Bejan number for various values of “pressure gradient parameter”.

Experimental study of the effect of CO2 on rock seepage characteristics

The influence of different phase states of CO2 on the seepage characteristics of different rocks is one of the most important factors to enhance shale gas exploitation, such as CO2 fracturing and displacement, and achieve CO2 geological storage. Rock soaking experiments with different CO2 phase states were carried out to compare and analyze the influence of permeability, contact angle, starting pressure gradient and other seepage characteristic parameters of sandstone and shale before and after soaking. Research shows: After soaking in liquid and supercritical CO2, the permeability and contact angle of sandstone and shale increase in varying degrees, and the starting pressure gradient decreases in varying degrees. After soaking in liquid CO2, the permeability of sandstone and shale increased by 1.71% and 12.88% respectively, the contact angle increased by 3.23% and 7.47% respectively, and the starting pressure gradient decreased by 3.02% and 5.88% respectively. After supercritical CO2 immersion, the permeability of sandstone and shale increased by 37.37% and 48.82% respectively, the contact angle increased by 19.27% and 36.80% respectively, and the starting pressure gradient decreased by 35.39% and 39.34% respectively. Compared with sandstone, the seepage characteristics of shale after soaking in liquid and supercritical CO2 have greater changes; Compared with liquid CO2, the seepage characteristics of sandstone and shale change more after supercritical CO2 soaking. With the increase of supercritical CO2 soaking time, the contact angle of sandstone slightly increases, while that of shale obviously increases. Compared with liquid CO2, the change of seepage curves of sandstone and shale after supercritical CO2 soaking is more different.

Comparison of smooth- and rough-wall non-equilibrium boundary layers with favourable and adverse pressure gradients

Measurements were made in rough-wall boundary layers subject to favourable, zero and adverse pressure gradients. Profiles of mean velocity and turbulence quantities were acquired and velocity fields were measured in multiple planes to document flow structure. Comparisons were made to equivalent smooth-wall cases with the same free stream velocity distributions. Outer layer similarity was observed between the rough- and smooth-wall cases in all quantities in the favourable and zero pressure gradient regions, but large differences were observed with adverse pressure gradients. In both the smooth- and rough-wall cases, the favourable pressure gradient reduced the turbulence in the boundary layer, and increased the size of turbulence structures relative to the boundary layer thickness in both the streamwise and spanwise directions, while lowering their inclination angle with respect to the wall. When the boundary layer was returned to a zero pressure gradient following the favourable pressure gradient region, the turbulence level and the size and inclination of the structures returned to their canonical zero pressure gradient condition. The response of the boundary layer was somewhat faster in the rough-wall case, causing it to reach equilibrium in a shorter streamwise distance after the changes in pressure gradient than in the smooth-wall case. The adverse pressure gradient increased turbulence levels relative to the wall friction velocity, reduced the size of turbulence structures relative to the boundary layer thickness and increased their inclination angle. The changes with the adverse pressure gradient were significantly larger with the rough wall than the smooth. The results suggest that similarity might be achieved with adverse pressure gradients if smooth- and rough-wall cases with the same Clauser pressure gradient parameter history are compared.

Convection velocity in turbulent boundary layers under adverse pressure gradient

The convection velocity (UC) of turbulent structures has been studied in adverse-pressure-gradient (APG) turbulent boundary layers (TBLs) for a wide range of Reynolds numbers Reτ≈1400−4000. The study is based on estimation of the convection velocity using decomposed streamwise skewness factor introduced in (Dróżdż A., Elsner W., Int. J. of Heat and Fluid Flow. 63 (2017) 67-74) and verified by means of two-point correlation method. It was shown that in the overlapping region of APG flows, the convection velocity profiles (when scaled in viscous units) reassemble the universal logarithmic law characteristic for the ZPG flows up to Clauser-Rotta pressure gradient parameter β≲19 for the considered range of Reynolds number, what means that in the inner region of TBL the friction velocity in APG is not proportional to U (as in ZPG) but to UC instead. The physical mechanism that explains the impact of increased convection velocity on the mean flow is proposed. The difference between UC and U increases as a function of APG, which causes the stronger sweeping that enhances momentum transfer to the wall and compensates the weaker mean shear profile that is created by low vorticity near the wall. This effect is a result of an enhancement of amplitude modulation of the small scales by large-scale motion. The process becomes more pronounced as eddy density grows, so with increasing Re. The proposed model addresses a number of literature observations found in adverse pressure gradient flows which have been so far left without a well-founded explanation.

AN ALIGNED MAGNETIC FIELD IMPACT ON A MICROPOLAR FLUID BOUNDARY-LAYER FLOW ACROSS AWEDGE SUBJECTED TO SLIP EFFECTS: HOMOTOPY ANALYSIS METHOD

The heat transfer on laminar boundary-layer flow (BLF) of a micropolar non-Newtonian fluid (NNF) due to a wedge with an impact of an aligned magnetic field embedded in a porous stratum accompanied by an effect of velocity and thermal slips are scrutinized. The similarity transformations are utilized to modify the governing partial differential equations into nonlinear, coupled ordinary differential equations. Resultant equations were solved analytically by homotopy analysis method (HAM). The obtained results are shown graphically for angular momentum, velocity, and temperature distributions for emerging parameters like pressure gradient parameter, velocity, and thermal slip parameters, magnetic parameter, porosity parameter, material parameter, angle of inclination, and Prandtl number. The Nusselt number and skin-friction coefficient values are tabulated. It is noticed that extending values of the material variable suppressed the velocity. The impact of an angle of inclination and porosity parameter enhance the velocity profile. A velocity field was suppressed by larger values of velocity slip variable and temperature distribution declined at the surface and it begins to increase for a certain distance with rising values of thermal slip variable. The obtained results are compared with existing results in a limiting case which provides a good agreement.

Impact of an MHD Cattaneo-Christov model for a Williamson fluid flow across a wedge with a convective boundary condition: Homotopy Analysis Method

The present study aims to scrutinise the influence of the Cattaneo-Christov (CC) heat flux model on magnetohydrodynamic (MHD) laminar boundary-layer flow through a wedge in a non-Newtonian Williamson fluid along with convective boundary condition embedded in a porous medium. Significantly, the presence of the CC model explores the heat transfer characteristics which forecast the effect of thermal relaxation time on the boundary-layer phenomenon. The governing equations are formulated and modified into non-linear, coupled ODEs by utilising similarity transformations and then tackled by Homotopy Analysis Method (HAM). The obtained results are discussed and presented graphically for various values of emerging parameters on velocity and temperature distributions. The results reveal that the velocity is an enhancing function of the magnetic parameter, permeability parameter and Hartree pressure gradient parameter whereas decreasing with larger values of the Williamson fluid parameter. It is evident that the temperature upsurges due to the Biot number and decays with the relaxation parameter. The numerical values of skin-friction and Nusselt number are tabulated. Comparative reviews between the formerly published literature and the current data are made for specific cases, which are inspected to be in tremendous agreement. Abbreviations: BLF: Boundary layer flow; MHD Magnetohydrodynamic; CC model: Cattaneo-Christov model; NNF: non-Newtonian fluid; ODEs: Ordinary differential equations; PDEs: Partial differential equations

A computational numerical algorithm for thermal characterization of fractional unsteady free convection flow in an open cavity

A significant problem to study is how the fractional operators and physical structures may be interrelated and interconnected. The current model study reports the fractional time-dependent viscous, electric conductive fluid between two permeable infinitely large walls. The flow is driven by the mutual actions of imposed thermal buoyancy, pressure gradient, and transverse magnetic field of uniform strength. The suction and injection of the fluid take place at the right and left walls respectively. Transformations are used to convert the physical model into equivalent fractional partial differential equations (FPDEs). The finite difference computational code based on three difference fractional operators is developed to seek the behavior of modeled problem. The analysis of the model and code endorsement is provided through set of graphical plots and tabular form. It is noted that an increment into the Hartmann and Reynolds number causes a dropped pattern of the velocity while the drop is more substantial for the smaller choices of fractional parameters. Higher choices of heat source, thermal radiation, Ecker, pressure gradient and magnetic parameters cause an enhanced behavior of the thermal profile. The smaller values caused a slight increment on the thermal layer as higher choices of the fractional parameters. However, the patterns of the thermal and velocity layers are found clearer while using the ABC and CF fractional operators compared with the CC idea of fractional derivative.

Applications of the Bernoulli wavelet collocation method in the analysis of MHD boundary layer flow of a viscous fluid

This study focuses on the flow of viscous, electrically conducting incompressible fluid over a stretching plate. The Falkner–Skan equation is a nonlinear, third-order boundary value problem. No closed-form solutions are available for this two-point boundary value problem. Here, we developed a new functional matrix of integration using the Bernoulli wavelet and also generated a new technique called Bernoulli wavelet collocation method (BWCM) to solve the nonlinear differential equation that arises in the fluid flow over a stretching plate. The boundary layer model is transformed to a nonlinear ordinary differential equation called the Falkner-Skan type equation using suitable transformation. Using BWCM, we have solved the unbounded governing equations of different types that arise in the MHD boundary-layer flow of a viscous fluid over a stretching plate. Several aspects of this problem are justified using the Haar wavelet and the previously obtained theoretical results. It is observed that the boundary-layer thickness decreases as the pressure gradient and magnetic field parameters increase. The overshoots and undershoots are observed for some particular parameters using BWCM. Furthermore, our research yields dual solutions for some physical parameters, which are investigated for the first time in the literature using the Bernoulli wavelet approach. The nature of the flow problem is discussed through the graphs by varying the physical parameters.

Solidification of electrically conducting liquid flow driven by shear and pressure gradients subject to viscous and Joule heating

AbstractSolidification of a liquid in motion driven by shear and pressure gradients occurs in many natural settings and technological applications. When the liquid is electrically conducting, its solidification rates can potentially be modulated by an imposed magnetic field. The shearing motion results in viscous dissipation and the Lorentz force induced by the magnetic field causes Joule heating of the fluid, which can influence the structure of the flow, thermal fields, and thereby the solidification process. In this study, a mathematical model is developed to study the combined effects of shear and pressure gradients in the presence of a magnetic field on the solidification of a liquid between two parallel plates, with one of them being insulated and under constant motion, and the other being cooled convectively and at rest. Under the quasi‐steady assumption, closed‐form semianalytical solutions are obtained for the instantaneous location of the solid–liquid interface, Nusselt number, and dimensionless power density as a function of various characteristic parameters such as the Hartmann number, pressure gradient parameter, Brinkman number, and Biot number. Furthermore, an interesting remelt or steady‐state condition for the interfacial location is derived as arising from the competing effects of the solid side heat flux and viscous dissipation and Joule heating on the liquid side. The newly derived analytical results are shown to reduce to the various classical results in the limiting cases. A detailed systematic study is performed by the numerical solution of the semianalytical formulation, and the effects of different characteristic parameters on the solidification process are discussed.

Flow of an incompressible non-Newtonian fluid in a lipid concentrated cylindrical channel under the influence of magnetic field and metabolic heat

In this research, we used mathematical models in the investigation of incompressible non-Newtonian fluid flow through a lipid-concentrated cylindrical channel with the effect of metabolic heat and magnetic treatment. The nonlinear partial differential equations were scaled into linear partial differential equations and then solved using the Laplace method to obtain the exact solutions. The numerical simulation was carried out using Wolfram Mathematica, version 12, where graphical results were obtained showing the variation of various pertinent parameters and their effects on the velocity profile of the fluid. The graphs revealed the effects of the parameters such as the metabolic heat, Grashof parameter, magnetic field; Prandtl number, pressure gradient, and porosity parameter have on the flow profile. The significance of this study is in the therapeutic investigation of hyperthermia, especially on the regulation of blood flow and lipids in the blood.

An Empirical Wall Law for the Mean Velocity in an Adverse Pressure Gradient for RANS Turbulence Modelling

An empirical wall law for the mean velocity in an adverse pressure gradient is presented, with the ultimate goal of aiming at the improvement of RANS turbulence models and wall functions. For this purpose a large database of turbulent boundary-layer flows in adverse pressure gradients from wind tunnel experiments is considered, and the mean velocity in the inner layer is analysed. The log law in the mean velocity is found to be a robust feature. The extent of the log-law region is reduced in ratio to the boundary layer thickness with increasing strength of the pressure gradient. An extended wall law emerges above the log law, extending up to the outer edge of the inner layer. An empirical correlation to describe the reduction of the log-law region is proposed, depending on the pressure-gradient parameter and on the Reynolds number in inner viscous scaling, whose functional form is motivated by similarity and scaling arguments. Finally, there is a discussion of the conjecture of the existence of a wall law for the mean velocity, which is governed mainly by local parameters and whose leading order effects are the pressure gradient and the Reynolds number, but whose details can be perturbed by higher-order local and history effects.

Cross-flow linear instability in compressible boundary layers over a flat plate

The linear instability of compressible boundary layers over a flat plate in the presence of parameterized crossflow has been investigated by means of linear stability theory. A family of boundary layer with crossflow is obtained as the base flow from the compressible Falkner–Skan–Cooke (FSC) flow model. Two factors, that is, the local swept angle and the pressure-gradient parameter, are designed to create the crossflow with different directions and magnitudes, which further results in the variation of the cross-flow instability. Modal properties related to the cross-flow instability are emphasized. The association between the cross-flow mode and the Mack's mode is clarified by extending the base flow from two dimension to three dimension. The cross-flow instability is discovered to be integrated with the slow-mode instability, that is, the instability related to the Tollmien–Schlichting (T–S) mode or the first mode, and it can hardly be distinguished as an individual mode in most cases. The effects of Mach number, pressure gradient, local sweep, and wall temperature are studied. The behaviors of the cross-flow instability under such effects resemble those of the slow-mode instability in the two-dimensional boundary layers. It is found that the unstable modes in the three-dimensional boundary layers are mainly affected by the streamwise pressure gradient and the crossflow per se. Specifically, the first mode is much more sensitive to the crossflow than the second mode. As a result, more marked variations are commonly observed for the first mode in the boundary layers with crossflow.

Intense large-scale motions in zero and adverse pressure gradient turbulent boundary layers

Proper orthogonal decomposition (POD) is used to study coherent structures in wall-bounded turbulent flows. The present study uses POD in turbulent boundary layers to determine the contributions of the intense large-scale motions (LSMs) to the Reynolds stresses. This study uses the 2C-2D PIV measurements of zero pressure gradient turbulent boundary layers (ZPG-TBL) at Re_{δ2} = 7750, and adverse pressure gradient turbulent boundary layer (APG-TBL) at β = 2.27 and Re_{δ2}= 16240, where Re_{δ2} is the momentum thickness based Reynolds number and β is the Clauser’s pressure gradient parameter. The measurements were obtained in the Laboratoire de Mécanique des Fluides de Lille (LMFL) High-Reynolds-Number (HRN) Boundary Layer Wind Tunnel, Lille, France. The snapshots of the flow field are segregated into those dominated by the intense and mild LSMs based on the intensity of the temporal coefficients of the first POD mode. The intense LSMs are further decomposed into high-momentum (HM) and low-momentum (LM) motions. The relative contributions of the HM motions to the Reynolds stresses are larger near the wall as compared to the LM motions. At the wall-normal distance of the displacement thickness (δ1), HM and LM motions have similar contributions. Beyond δ1, the LM motions have larger contributions with their peaks located closer to the displacement thickness height. This shows that in the presence of an APG, the turbulence activity is shifted closer to the displacement thickness height.