The linear stability of nanofluid boundary-layer flow over a flat plate is investigated using a two-phase model that incorporates Brownian motion and thermophoresis, building upon the earlier work of Buongiorno (2006). Solutions to the steady boundary-layer equations reveal a thin nanoparticle concentration layer near the plate surface, with a characteristic thickness of $O({\Rey}^{-1/2}{Sc}^{-1/3})$, for a Reynolds number $\Rey$ and Schmidt number $Sc$. When Brownian motion and thermophoresis are neglected, this nanoparticle concentration layer disappears, resulting in a uniform concentration across the boundary layer. Neutral stability curves and critical conditions for the onset of the Tollmien--Schlichting wave are computed for a range of nanoparticle materials and volume concentrations. Results indicate that while the effects of Brownian motion and thermophoresis are negligible, the impact of nanoparticle density is significant. Denser nanoparticles, such as silver (Ag) and copper (Cu), destabilise the Tollmien--Schlichting wave, whereas lighter nanoparticles, like aluminium (Al) and silicon (Si), establish a small stabilising effect. Additionally, stability characteristics are influenced by the viscosity model. Finally, a high-Reynolds number asymptotic analysis is undertaken for the lower branch of the neutral stability curve.
In this paper, thermal conductivity measurement of liquids and nanofluids is numerically investigated by the steady state method. In which, it is required to omit the natural convection effects on the inaccuracy of measurement for heat lost quantification. One of the well-known steady-state apparatus is circular parallel-plate where the distance between circular plates is filled with liquids or nanofluids. So, it has to be designed accurately to neglect the effects of free convection. The numerical simulation is a convenient method which can omit the mentioned effects. Water, Ethylene and Ethy…
Even a small fraction of nanoparticles in fluids affects the splashing behavior of a droplet upon impact on a smooth surface. Nanofluid drop impact onto a smooth sapphire substrate is experimentally investigated over wide ranges of Reynolds ($10^2<\mathrm{Re}<10^4$) and Weber ($50<\mathrm{We}<500$) numbers for three nanofluid mass concentrations (0.01%, 0.1%, 1%) using high-speed photography. Nanofluids are prepared by diluting a commercial Al$_2$O$_3$-water nanofluid in aqueous glycerol solutions without dispersants. In total, 30 samples are prepared and 1799 data points are acquired. Every s…
Computational Exploration of Inclined Magnetic Fields and Variable Thermal Flux Effects on the Flow of Dusty Hybrid Nanofluid around Stretching/Shrinking Wedge
We investigate the retention force factor of sessile droplets of pure (ethanol) and binary (water-ethanol) fluids laden with alumina nanoparticles placed on a critically inclined substrate. It is observed that while the critical angle of an ethanol droplet increases with an increase in nanoparticles concentration, for water-ethanol binary droplets, it reaches to plateau and decreases slightly after 0.6 wt.\% nanoparticle loading. The effect of composition and concentration of nanoparticles on the retention force factor is studied, and correlations are proposed for the retention force factor an…
Ion transport in perm-selective media has been extensively investigated in recent years owing to its applications in advancements of fundamental understanding of nanoscale electrokinetics as well as innovative engineering applications. A key phenomenon in this context is ion concentration polarization (ICP) that occurs near perm-selective nanoporous membranes or nanochannels under dc bias. In classical settings, involving voltage-driven transport of ions through perm-selective membranes, concentration polarization is well understood as formation of steep concentration drop to reach near zero c…
The flow properties at the leading edge of a flat plate represent a singularity to the Blasius laminar boundary layer equations; by applying the Lagrangian approach the leading edge velocity profiles of the laminar boundary layer over a flat plate are studied. Experimental observations as well as the theoretical analysis show an exact Gaussian distribution curve as the original starting profile of the laminar flow. Comparisons between the Blasius solution and the Gaussian curve solution are carried out providing a new insight into the physics of the laminar flow.
The paper presents a theoretical study using the Karman-Pohlhausen method for describing the transient heat exchange between the boundary-layer free convection and a vertical flat plate embedded in a porous medium. The unsteady behavior is developed after the generation of an impulsive heat flux step at the right-hand side of the plate. Two cases are considered according to whether the plate has a finite thickness or no thickness. The time and space evolution of the interface temperature is evidenced.
We investigate the global frequency selection of two-dimensional vortex shedding in the flat plate wake. The analysis is based on the mean flow velocity profiles obtained from experimental measurements carried out for two values of Reynolds number, 1850 and 3350, which are based on the plate thickness and the free-stream velocity. Two different trailing edge geometries of the flat plate are considered in this study: blunt and circular. By performing local spatio-temporal analyses on the measured mean flow velocity profiles, we estimate the global shedding frequency of the flow. This is in exce…
The problem of a steady forced convection thermal boundary-layer past a flat plate with a prescribed surface heat flux is investigated both analytically and numerically. In view of the present formulation, the governing equations reduce to the well-know Blasius similarity equation and to the full boundary-layer energy equation with two parameters: Prandtl number and surface heat flux parameter. The range of existence of solutions is considered. The asymptotic solutions are derived and compared with the numerical solutions of the full boundary-layer equation. A very good agreement between these…
In this paper, a numerical analysis of boundary layer flow and heat transfer in Jeffrey fluid over a moving flat plate with Newtonian Heating have been presented. The governing partial differential equations were reduced to a transformed ordinary differential equation with the help of similarity transformation. Numerical solutions were obtained for these transformed ordinary differential equation by using the Runge-Kutta-Fehlberg method. The effect on the boundary layer flow and heat transfer behaviours of various parameters such as Deborah number λ2, relaxation time and retardation time ratio…
The turbulent boundary layer over a flat plate is computed by direct numerical simulation (DNS) of the incompressible Navier-Stokes equations as a test bed for a synthetic turbulence generator (STG) inflow boundary condition. The inlet momentum thickness Reynolds number is approximately 1,000. The study provides validation of the ability of the STG to develop accurate turbulence in 5 to 7 boundary layer thicknesses downstream of the boundary condition. Also tested was the effect of changes in the stabilization scheme on the development of the boundary layer. Moreover, the grid resolution requi…
We present a model appropriate to the initial motion (2-3 chords of travel) of a flat-plate airfoil accelerating in an inviscid fluid. The separated flow structures are represented as vortex sheets in the conventional manner and similarity expansions locally applicable to the leading and trailing edges of the plate are developed. The topological character of vortex sheets is maintained rather than resorting to point vortex discretizations. Beyond this, there are two theoretical novelties to our approach as compared to previous studies. First, an expansion is applied to the attached outer flow …
If a flat, horizontal, plate settles onto a flat surface, it is known that the gap $h$ decreases with time $t$ as a power-law: $h\sim t^{-1/2}$. We consider what happens if the plate is not initially horizontal, and/or the centre of mass is not symmetrically positioned: does one edge contact the surface in finite time, or does the plate approach the horizontal without making contact? The dynamics of this system is analysed and shown to be remarkably complex.
Global wake instabilities of low aspect-ratio flate-plates
本文研究了垂直于来流放置的平板后发展的三维定常尾流的线性不稳定性。这里考虑的是以低长宽比$L$为特征的板。通过在$1 \le L \le 6$范围内改变该纵横比，识别出三种不稳定情形。对于非常低的纵横比$1 \le L \le 2$，流动首先是不稳定的，当雷诺数增加时，由一个稳定的全球模式，打破了顶部/底部平面反射对称。对称定常流通过一个叉状分叉分叉，朝向一个非对称定常尾流分叉，类似于球体和圆盘后面的轴对称尾流的情况。对于大展弦比$2.5 \le L \le 6$，第一不稳定模态也破坏了顶底对称性，但是是不稳定的。对于二维圆柱后的尾迹，出现了Hopf分叉。最后，一个中间制度$2 \le L \le 2.5$被发现的流动变得第一不稳定的非定常模式，打破了左/右平面反射对称。
We determine the globally minimum time $T$ needed to translate a thin submerged flat plate a given distance parallel to its surface within a work budget. The Reynolds number for the flow is assumed to be large so that the drag on the plate arises from skin friction in a thin viscous boundary layer. The minimum is determined using a steepest descent, where an adjoint formulation is used to compute the gradients. Because the equations governing fluid mechanics for this problem are nonlinear, multiple local minima could exist. Exploiting the quadratic nature of the objective function and the cons…
The kinematics of a gliding flat-plate with spanwise oscillation has been optimized to enhance the power efficiency by using Bayesian optimization method, in which the portfolio allocation framework consists of a Gaussian process probabilistic surrogate and a hybrid acquisition strategy. We tune three types of acquisition function in the optimization framework and assign three different balance parameters to each acquisition function. The design variables are set as the dimensionless oscillating amplitude and reduced frequency of the spanwise oscillation. The object function is to maximize the…
In naval hydrodynamics most the reseach on the impact of the hull of a vesel to the water surface has focused primarily on mapping the forces on the hull and much less to the formation of the spray. The latter is the topic of the present work, where the spray generated by the impact of a flat plate on a quiescent water surface is studied by experiments and simulations at different impact Froude numbers. Overall two types of sprays are formed: Type I, which is a cloud of droplets and ligaments generated at the impact of the plate's leading edge with the water surface, and Type II, which is form…
In this study, a flat-plate channel configured with pyramidal protrusions are numerically analysed for the first time. Simulations of laminar single-phase fluid flow and heat transfer characteristics are developed using a finite-volume approach under steady-state condition. Pure water is selected as the coolant and its thermo-physical properties are modelled using a set of temperature-dependent functions. Different configurations of the channel, including a plain channel and a channel with nature-inspired protruded surfaces, are studied here for Reynolds numbers ranging from 135 to 1430. The e…
In an attempt to reduce jet noise, blended wing concept vehicle utilizes rectangular jet exhaust ports exiting from above the wing ahead of the trailing edge. In this study, we take another look at the rectangular exhaust port configuration with some notional modifications to the geometry of the trailing edge to determine if the emitted noise levels due to jet interactions can be reduced with respect to a baseline configuration. We consider various horizontal and vertical offsets of the jet exit with respect to a flat plate standing in for the aft wing surface. We then introduce a series of si…
A new composite heat transfer fluid consisting of tetralin and fullerene has been proposed for photovoltaic thermal hybrid solar harvesting. It features a unique absorption spectrum that is capable of sharply cutting off solar energy irradiated in the range of wavelength from 300 to 650 nm, making it a perfect candidate for simultaneous harvesting of both photovoltaic and thermal components of solar energy. The proposed composite revealed outstanding stability and facile synthesize root, which are the two main obstacles for applicability of nanofluids. It was shown experimentally that the addi…
In Computational Fluid Dynamics (CFD) studies composed of the coupling of different simulations, the uncertainty in one stage may be propagated to the following stage and affect the accuracy of the prediction. In this paper, a framework for uncertainty quantification is applied to the two-step simulation of the mechanical design of a swirling jet flow generated by a rotating pipe (\textit{Simulation 1}) impinging on a flat plate to provide convective heat transfer (\textit{Simulation 2}). The first approach is the Stochastic Collocation Method (SCM) with Clenshaw-Curtis sparse grids. The concl…
Three-dimensional simulations with fully resolved hydrodynamics are performed to study the dynamics of a single squirmer under gravity, in order to clarify its motion in the vicinity of a flat plate. Different dynamics emerge for different gravity strengths. In a moderate gravity regime, neutral squirmers and pullers eventually stop moving and reorient in a direction perpendicular to the plate; pushers, instead, exhibit continuous motion in a tilted direction. In the strong gravity regime, all types of squirmers sediment and reorient perpendicularly to the plate. In this study, the chirality i…
The effects of entropic-instabilities on the laminar-turbulent transition dynamics of a blunted flat plate at Mach~$4$ are numerically investigated through linear and nonlinear approaches. Linear wavepacket analysis reveals amplifying oblique first-modes as well as planar and oblique entropy-layer disturbances. The receptivity of entropic-instabilities is found to be largest for actuation seeded in the entropy-layer; the corresponding linear evolution is characterized by an intensification in the wall-normal plane, coupled with streamwise tilting. The transition process that arises from entrop…
Wall-resolved large-eddy simulations are performed to study the impact of spanwise traveling transversal surface waves in zero-pressure gradient turbulent boundary layer flow. Eighty variations of wavelength, period, and amplitude of the space- and time-dependent sinusoidal wall motion are considered for a boundary layer at a momentum thickness based Reynolds number of $Re_θ= 1000$. The results show a strong decrease of friction drag of up to $26\,\%$ and considerable net power saving of up to $10\,\%$. However, the highest net power saving does not occur at the maximum drag reduction. The dra…
In this paper, the shock dynamics due to the movement of a bump over a flat plate flying at supersonic speed are numerically investigated. The bump is located at the impingement position of the shock wave and is moved at different speeds. This study determines the suitable speed that achieves the minimum entropy change, which is the representation parameter of the transition period. The two-dimensional unsteady Navier-Stokes equations are solved using OpenFOAM to simulate the flow field variables, while the motion of the bump is tracked using the Arbitrary Lagrangian-Eulerian (ALE) technique. …
Flapping animal flight is often modeled as a combined pitching and heaving motion in order to investigate the unsteady flow structures and resulting forces that could augment the animal's lift and propulsive capabilities. This work isolates the heaving motion of flapping flight in order to numerically investigate the flow physics at a Reynolds number of 40,000, a regime typical for large birds and bats and challenging to simulate due to the added complexity of laminar to turbulent transition in which boundary layer separation and reattachment are traditionally more difficult to predict. Period…
The present study is a prelude to applying different flow control devices on pitching and plunging airfoils with the intention of controlling the growth of the leading edge vortex (LEV); hence, the lift under unsteady stall conditions. As a pre-requisite, the parameters influencing the development of the LEV topology must be fully understood, and this constitutes the main motivation of the present experimental investigation. The aims of this study are twofold. First, an approach is introduced to validate the comparability between flow fields and LEV characteristics of two different facilities …
The accurate description of the growth or dissolution dynamics of a soluble gas bubble in a super- or undersaturated solution requires taking into account a number of physical effects that contribute to the instantaneous mass transfer rate. One of these effects is the so-called history effect. It refers to the contribution of the local concentration boundary layer around the bubble that has developed from past mass transfer events between the bubble and liquid surroundings. In Part 1 of this work \citep{penas2016}, a theoretical treatment of this effect was given for a spherical, isolated bubb…
Linear stability analysis is performed using a combination of two-dimensional Direct Simulation Monte Carlo (DSMC) method for the computation of the basic state and solution of the pertinent eigenvalue problem, as applied to the canonical boundary layer on a semi-infinite flat plate. Three different gases are monitored, namely nitrogen, argon and air, the latter as a mixture of 79\% Nitrogen and 21\% Oxygen at a range of free-stream Mach numbers corresponding to flight at an altitude of 55km. A neural network has been utilised to predict and smooth the raw DSMC data; the steady laminar profile…
We provide an overview of the leading edge problem in this paper. We have used a self-similar function having a dependence on both the self-similar variable $η$ and Reynold's number R to covert the momentum and energy equations into a fourth-order, non-linear partial differential equation (PDE) and a second-order, non-linear PDE respectively. Attempts have been made to solve the energy equation in a variety of ways, which include solving the PDE approximating the terms of the order $\mathcal{O}(R^{2})$ and solving the PDE via the method of characteristics, but mostly being able to solve the en…
Numerical study of friction reduction and underlying mechanism of air film via parallel and tandem injections on a hypersonic flat plate at high altitude
The unsteady motion of a two-layer fluid induced by oscillatory motion of a flat plate along its length is mathematically analyzed. Two cases are considered: (i) the two-layer fluid is bounded only by the oscillating plate (Stokes' second problem), (ii) the two-layer fluid is confined between two parallel plates, one of which oscillates while the other is held stationary (oscillatory Couette flow). In each of the Stokes' and Couette cases, both cosine and sine oscillations of the plate are considered. It is assumed that the fluids are immiscible, and that the flat interface between the fluids …
Deep reinforcement learning-based active flow control of an elliptical cylinder: transitioning from an elliptical cylinder to a circular cylinder and a flat plate
Free surface, axially symmetric shallow flow is analysed in both the centrifugal and centripetal directions. Referring to the inviscid steady flow over a flat plate characterised by a unique value of specific energy, the analytical sub- and supercritical solutions are determined. Furthermore, the sub- and supercritical steady solutions for the flow with friction over a flat plate are determined, provided that inertial terms are important compared with frictional terms. As the inviscid case, the sub- and supercritical solutions over a bottom topography are determined, provided that the bottom u…
Comment on Magnetohydrodynamic non-Darcy mixed convection heat transfer from a vertical heated plate embedded in a porous medium with variable porosity, by Dulal Pal
Gas-surface scattering and collisional feedback in rarefied hypersonic flow past an inclined flat plate
Universitat Politècnica de Catalunya–BarcelonaTech (UPC)（加泰罗尼亚理工大学）；European Space Agency (ESA), European Space Research and Technology Centre (ESTEC)（欧洲空间局（ESA）欧洲空间研究与技术中心（ESTEC））
