Anthony A. Ruffa
An irrotational solution is derived for the steady-state Navier-Stokes equations that approximately satisfies the boundary conditions for flow over a finite flat plate. The nature of the flow differs substantially from boundary layer flow, with severe numerical difficulties in some regions.
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Do irrotational water waves remain irrotational in the limit of a vanishing viscosity?
研究发现，在无滑移边界条件下，无旋水流在粘性消失极限下会因涡旋对的脱落而产生扰动，表明该假设在特定条件下不成立。
A correction term for the asymptotic scaling of drag in flat-plate turbulent boundary layers
本文提出通过流体方向平均速度对数定律推导修正项，修正平板湍流边界层阻力渐近尺度的理论模型。
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.
In this fluid dynamics video, recent simulations of transition to turbulence in compressible (M = 0.2), zero-pressure-gradient flat-plate boundary layers triggered by fundamental (Klebanoff K-type) and subharmonic (Herbert H-type) secondary instabilities of Tollmien-Schlichting waves are highlighted.
A Similarity Solution of Rear Stagnation-point Flow over a Flat Plate in Two Dimensions
本文提出了一种二维平板后滞止点处不可压缩流体不稳流涡旋脱落发展的相似解方法。
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.
This paper presents a theoretical derivation of the empirical Blasius power law correlation for the friction factor. The coefficients in this correlation are shown to be dependent on the Reynolds number. Published experimental data is well correlated. Key words: Blasius, friction factor, turbulence, power law, log-law, wall layer
This paper investigates the nature of the development of two-dimensional laminar flow of an incompressible fluid at the reversed stagnation-point. ". In this study, we revisit the problem of reversed stagnation-point flow over a flat plate. Proudman and Johnson (1962) first studied the flow and obtained an asymptotic solution by neglecting the viscous terms. This is no true in neglecting the viscous terms within the total flow field. In particular it is pointed out that for a plate impulsively accelerated from rest to a constant velocity V0 that a similarity solution to the self-similar ODE is…
Effect of leading edge geometry on aerodynamic performance of a flat plate
本研究通过二维数值模拟，探究低雷诺数下平板前缘半球形曲率对涡脱、气动力等的影响，结果可为小型低雷诺数无人航行器的工程设计提供依据。
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…
Comment on Group method analysis of magneto-elastico-viscous flow along a semi-infinite flat plate with heat transfer,Helal and Abd-el-Malek, Journal of Computational and Applied Mathematics 173 (2005) 199-210. In the above paper the authors treat the boundary layer flow of a elasto- viscous liquid along an infinite plate in the presence of a transverse magnetic field. The plate temperature is higher than the ambient fluid temperature. The boundary layer equations are transformed into ordinary ones using the group theory and subsequently are solved numerically. Velocity, temperature, shear str…
The steady laminar supersonic flow past a flat plate having a stretch of an elastic membrane, the pressure on the other side of which is adjustable, is studied within the framework of the triple deck theory. The resulting lower deck problem is supplemented with a membrane equation relating the pressure difference across the membrane to its curvature. By pressurizing or depressurizing the membrane, it assumes the form of a hump or a dent that alters the flow characteristics. Numerical solutions obtained, in either case, give plausible account of the interaction between the membrane and the flow…
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…
Mixed Convection in a Differentially Heated Cavity with Local Flow Modulation via Rotating Flat Plates
本研究针对旋转平板局部调制的差热方腔混合对流，采用ALE有限元法求解动边界问题，模拟得到临界瑞利数等结果，明确了调制器数量与瑞利数对流动换热的影响规律。
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.
This paper investigates the linear destabilization of three-dimensional steady wakes developing behind flate plates placed normal to the incoming flow. Plates characterized by low length-to-width ratio $L$ are considered here. By varying this aspect ratio in the range $1 \le L \le 6$ three destabilization scenarios are identified. For very low aspect ratio $1 \le L \le 2$, the flow is first destabilized, when increasing the Reynolds number,by a steady global mode that breaks the top/bottom planar reflectional symmetry. The symmetric steady flow bifurcates, via a pitchfork bifurcation, towards …
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…
Transient growth in a flat plate boundary layer under a stream with uniform shear
本研究针对均匀剪切来流下半无限平板边界层的瞬态增长展开，发现初始最优扰动为流向涡，相比Blasius边界层能量更高、展向波长更大，且最大最优放大倍数随自由流剪切梯度指数增长。
Viscous flow past a finite flat plate moving in direction normal to itself is studied numerically.The plate moves with velocity $at^p$, where $p=0,0.5,1,2$. We present the evolution of vorticity profiles, streaklines and streamlines, and study the dependence on the acceleration parameter $p$. Four stages in the vortex evolution, as proposed by Luchini & Tognaccini (2002), are clearly identified. The initial stage, in which the vorticity consists solely of a Rayleigh boundary layer, is shown to last for a time-interval whose length shrinks to zero like $p^3$, as $p \to 0$. In the second stage, …
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…
This fluid dynamics video submitted to the Gallery of Fluid motion shows a turbulent boundary layer developing under a 5 metre-long flat plate towed through water. A stationary imaging system provides a unique view of the developing boundary layer as it would form over the hull of a ship or fuselage of an aircraft. The towed plate permits visualisation of the zero-pressure-gradient turbulent boundary layer as it develops from the trip to a high Reynolds number state ($Re_τ\approx 3000$). An evolving large-scale coherent structure will appear almost stationary in this frame of reference. The vi…
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…
How vortices enhance heat transfer from an oscillating plate
本研究通过数值模拟揭示平板振荡方向和速度是增强传热的关键因素，涡旋脱落影响局部传热，并探讨了系统输入功率与帕累托最优设计。
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…
Viscous flow past a finite plate which is impulsively started in direction normal to itself is studied numerically using a high order mixed finite difference and semi-Lagrangian scheme. The goal is to resolve details of the vorticity generation at early times, and to determine the effect of viscosity on flow quantities such as the core trajectory and vorticity, and the shed circulation. Vorticity contours, streaklines and streamlines are presented for a range of Reynolds numbers $Re \in [250, 2000]$ and a range of times $t \in[0. 0002, 5]$. At early times, most of the vorticity is attached to …
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…
Several previous experimental and theoretical studies have shown that a leading edge vortex (LEV) on an airfoil or wing can provide lift enhancement. In this paper, unsteady 2D potential flow theory is employed to model the flow field of a flapping flat plate wing. A multi-vortices model is developed to model both the leading edge and trailing edge vortices (TEVs), which offers improved accuracy compared with using only single vortex at each separation location. The lift is obtained by integrating the unsteady Blasius equation. It is found that the motion of vortices contributes significantly …
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…
Linear stability of nanofluid boundary-layer flow over a flat plate
研究了纳米流体平板边界层流动的线性稳定性，发现纳米粒子密度显著影响Tollmien-Schlichting波的稳定性，密度高者 destabilize，低者 stabilize。
Frequency Scaling Laws for Flat Plate Wing Active Separation Control
研究平板机翼主动分离控制的频率缩放规律，发现其与昆虫翼拍频率存在关联，通过斯特鲁哈尔数确定最佳控制参数。
Heat transfer in the flow of a cold, axisymmetric vertical liquid jet against a hot, horizontal plate
本文研究冷垂直液体射流撞击热水平平板的薄膜流动传热，采用修正Keller box方法获得大Reynolds数高精度数值解，并用先进多项式逼近验证结果。
Direct Numerical Simulations of K-type transition in a flat-plate boundary layer with supercritical fluids
本文通过直接数值模拟研究超临界流体平板边界层的K型转捩，发现亚临界和跨临界温度剖面下转捩均被延迟，且跨临界条件下壁面超伪沸腾温度会诱发额外发卡涡与近壁条纹结构。
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 Unsteady Lift Produced by a Flat-Plate Wing Translating Past Finite Obstacles
本研究通过拖曳水池实验测量平板翼平移经过通道、天花板和地面等有限障碍物时的非定常升力，发现障碍物间隙和起始距离通过改变前缘涡的时机与强度显著影响升力峰值。核心方法为实验测量，主要贡献在于揭示不同障碍物对升力峰值大小和出现时间的影响规律。
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…
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…
An augmented swirling and round jet impinging on a heated flat plate and its heat transfer characteristics
本文提出了一种增强旋流环形喷射的几何机制，通过不同分割比研究其传热特性，发现优化距离和分割比可显著提升传热效率。
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…
Dynamic interactions between an aerodynamic flow and a flexible flat plate
本研究通过高保真流固耦合模拟，探究柔性平板与涡脱落气动流的动态相互作用，发现流动诱发不稳定性引发结构运动，并依据模态动力学划分不同耦合状态。
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
本文通过数值模拟研究高空高超声速平板平行与串列气膜注入的减阻机理，揭示质量流率、孔间距和注入孔长宽比对低速层覆盖、流动损失及减阻效率的影响规律。
Analytical Green's function for the acoustic scattering by a flat plate with a serrated edge
本文提出一种解析格林函数，用于高频下锯齿边缘平板声散射的精确求解，并通过数值验证，适用于前缘和后缘散射，为高效三维后缘噪声建模奠定基础。
Libby-Fox perturbations and the semi-analytic adjoint solution for laminar viscous flow along a flat plate
研究基于利比 - 福克斯理论探讨平板层流粘性流二维伴随边界层方程解的性质，通过格林函数得到伴随解的显式表示，可推导相关约束、计算ATC项及评估流动敏感度，还讨论了非零压力梯度情况的扩展。
Reversal of a flat plate into its wake: a minimal model for wake capture
该研究以平板为对象，建立尾流捕获最小模型，揭示翼-尾流相互作用机制，发现反转前移动距离对阻力峰值及尾流作用力衰减的影响规律。
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
本研究将基于深度强化学习的主动流动控制技术应用于不同纵横比的椭圆圆柱与平板，利用PPO算法精确控制合成射流，在极低能耗下实现了显著的减阻、降升及涡旋脱落抑制。
This work establishes a closed-form analysis for two-dimensional transient heat transfer in an absorber plate based on the thermal wave model when the collector starts to work. The method of separation of variables is employed to determine the exact thermal response during the starting of a solar collector. The thermal field is represented by 2-D thermal contours and 3-D surface plots to provide an accurate prediction compared to the 1-D analysis using the time-lag theory. An influence of Fourier number on the heat flow in the absorber plate is studied for different thermal relaxation times. T…
In the sense of distributions, the derivative of the Heaviside unit step function $H(t)$ is a generalized Dirac-$δ$ distribution. If the velocity $V(t)$ of a flat plate is impulsive, as $V(t)=H(t)$ (i.e., it is suddenly set into motion with unit velocity at $t=0^+$), then its acceleration is $V'(t)=δ(t)$. The Dirac-$δ$ distribution has no point values. However, when the Dirac-$δ$ is the forcing term of an ODE (in $t$), it contributes to the solution. The recently published paper [Chaos Solitons Fractals 117 (2018) 68] incorrectly treats the Dirac-$δ$ function as being identically 0. This Comme…
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…
In the above paper the author treats the boundary layer flow along a vertical flat plate, immersed in a Darcy Brinkman Forchheimer porous medium. The porosity and the permeability of the porous medium are variable across the boundary layer. In addition a magnetic field with constant strength is applied normal to the plate. The fluid temperature at the plate is constant and different from that of the ambient fluid. This temperature difference creates a buoyancy force and the flow is characterized as mixed convection. The partial differential equations of the boundary layer flow are transformed …
Gas-surface scattering and collisional feedback in rarefied hypersonic flow past an inclined flat plate
本研究采用直接模拟蒙特卡罗方法，考察稀薄高超声速流过40°倾斜平板时，气体-表面散射定律通过分子碰撞对入射分子群体的反馈效应，评估Maxwell与CLL散射模型，发现群体反馈与力的投影决定散射响应在宏观量中的增强、掩盖或反转。
One of classical boundary problems of the kinetic theory (a problem about thermal sliding) of the rarefied gas along a flat firm surface is considered. Kinetic Boltzmann equation with model integral of collisions BGK (Bhatnagar, Gross, Krook) is used. As boundary conditions the boundary Maxwell conditions (mirror-diffuse) are used. The generalized method of a source is applied to the problem decision. Comparison with earlier received results is spent.
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