Frequency-domain unsteady lifting-line theory (ULLT) provides a means by which the aerodynamics of oscillating wings may be studied at low computational cost without neglecting the interacting effects of aspect ratio and oscillation frequency. Renewed interest in the method has drawn attention to several uncertainties however. Firstly, to what extent is ULLT practically useful for rectangular wings

This paper presents a method to evaluate the near-singular line integrals that solve elliptic boundary value problems in planar and axisymmetric geometries. The integrals are near-singular for target points not on, but near the boundary, and standard quadratures lose accuracy as the distance d to the boundary decreases. The method is based on Taylor series approximations of the integrands that capture

Streamwise vortices and the associated streaks evolve in boundary layers over flat or concave surfaces due to disturbances initiated upstream or triggered by the wall surface. Following the transient growth phase, the fully developed vortex structures become susceptible to inviscid secondary instabilities resulting in early transition to turbulence via ‘bursting’ processes. In high-speed boundary layers

Mack (1977) criticized methods referring to a single frequency perturbation for correlation of transition prediction because the external disturbance source (like free stream turbulence) should have a broadband spectrum. Delta-correlated perturbations are characterized by the mean square of physical amplitude, which is expressed as a double integral of the power spectral density in frequency and the

The effects of crossflow on the interaction between an impinging shock wave and a high-speed turbulent boundary layer are investigated using direct numerical simulations of statistically two-dimensional, three-component flow. The leading-order effect of crossflow is increased size and strength of the separation bubble, with upstream and downstream displacement of the separation and reattachment points

This work establishes a procedure to accurately compute heat transfer between an Eulerian fluid and Lagrangian point-particles. Recent work has focused on accurately computing momentum transfer between fluid and particles. The coupling term for momentum involves the undisturbed fluid velocity at the particle location which is not directly accessible in the simulation. Analogously, in the context of

In this paper, the mixing and combustion at low-heat release in a turbulent mixing layer are studied numerically using large eddy simulation. The primary aim of this paper is to successfully replicate the flow physics observed in experiments of low-heat release reacting mixing layers, where a duty cycle of hot structures and cool braid regions was observed. The nature of the imposed inflow condition

Using data from numerical simulations, we show that the lift experienced by both impulsively started and surging airfoils correlates well with the sum of the circulation of the leading-edge vortices truncated at the trailing edge. Therefore, we suggest that reasonable estimates of the lift can be obtained using only two vortex parameters, i.e., its circulation and its position. In addition to being

Hydroelastic responses of floating elastic surfaces to incident nonlinear waves of solitary and cnoidal type are studied. There are N number of the deformable surfaces, and these are represented by thin elastic plates of variable properties and different sizes and rigidity. The coupled motion of the elastic surfaces and the fluid are solved simultaneously within the framework of linear beam theory

A modified formulation of the interfacial boundary condition for the coupling of the Stokes and Darcy models describing the incompressible fluid flow in the free space and porous medium domains is proposed using the dimension analysis procedure. The case is considered for the porous media formed by circular or square cylinders located in the centers of rectangular cells. The vorticity is derived as

The Rossiter modes of an open cavity were studied using bi-global linear analysis, local instability analysis and nonlinear numerical simulations. Rossiter modes are normally seen only for short cavities; hence, in the study, the length over depth ratio was two. We focus on the critical region; hence, the Reynolds numbers based on cavity depth were close to 1000. We investigated the effect of the ratio

The results of the numerical study of oil recovery enhancement using nanosuspension are presented. The research was carried out using the volume of fluid method (VOF) for a 2D microporous core model. Experimentally measured values of interfacial tension (IFT) and the contact angle (CA) were used for numerical modeling. An aqueous suspension with silicon oxide nanoparticles (5 nm) is used. It was shown

The blood flow in a normal aorta is simulated directly based on patient-specific data for boundary conditions. This study provides insight into the implications of using two different outlet boundary conditions in accurate modeling of the blood flow based on using pressure or flow rate outlet boundary conditions. Both boundary conditions at the outlet provided similar blood flow characteristics through

The dynamics of a fully three-dimensional lid-driven cubical cavity (3D-LDC) flow at several post-critical conditions, i.e., beyond the first bifurcation, are elucidated using both linear and nonlinear analyses. When the Reynolds number is increased beyond the critical value, symmetry breaks down intermittently with subsequent gradual growth in spanwise inhomogeneity. This results in crossflow as well

Hill’s vortex is a three-dimensional vortex structure form-preserving solution of the Euler equations (Hill in Philos Trans R Soc Lond A 185:213–245, 1894). For small amplitude axisymmetric disturbances on the external surface, the linear stability analysis by Moffat and Moore (J Fluid Mech 87:749–760, 1978) predicted the formation of a tail. Successive linear and nonlinear investigations confirmed

We present direct numerical simulations for the pathophysiology of hypertrophic cardiomyopathy of the left ventricle of the human heart. This cardiovascular disorder manifests itself through systolic anterior motion (SAM), a drift of the mitral leaflets towards the aortic subvalvular region, sometimes causing ventricular obstruction during systole. This pathology is induced by a combination of factors

This paper reports the results from a direct numerical simulation of an initially turbulent boundary layer passing over a wall-mounted “speed bump” geometry. The speed bump, represented in the form of a Gaussian distribution profile, generates a favorable pressure gradient region over the upstream half of the geometry, followed by an adverse pressure gradient over the downstream half. The boundary

In this paper, experimental and numerical methods are presented to investigate the dam-break flow in a horizontal rectangular section flume. In the experimental part of the research, different configurations have been tested: dry flume and the presence of shallow ambient water downstream with varied depth. In addition, experiments with viscosity changes in the fluid have been conducted. Numerically

The capture mechanism of fibrous filters in a wet condition is studied by focusing on the single droplet impact to an initial droplet attached to the fiber. High-speed photography and numerical simulation are conducted to study the collision phenomenon. The eccentricity between the mass center of the impacting droplet and the initial droplet is varied to evaluate the threshold capture velocity of the

High-frequency ventilation is a type of mechanical ventilation therapy applied on patients with damaged or delicate lungs. However, the transport of oxygen down, and carbon dioxide up, the airway is governed by subtle transport processes which hitherto have been difficult to quantify. We investigate one of these mechanisms in detail, nonlinear mean streaming, and the impact of the onset of turbulence

In this work, the breakup of a droplet passing through an obstacle in an orthogonal cross section is numerically investigated. The relevant boundary data of the velocity field is numerically computed by solving the depth-averaged Brinkman equation via a self-consistent integral equation using the boundary element method. To study the dependence of the droplet breakup on the obstacle shape, two different

The present paper reports an investigation of the statistical properties of pressure fluctuations in the near field of subsonic compressible jets. The data-base analyzed has been obtained numerically by DNS and LES of two single-stream circular jets, having diameter-based Reynolds numbers of 3125 and 100,000 and Mach number 0.9, respectively, initially laminar and highly disturbed. Pressure fluctuations

A coupled level set and volume–of–fraction method is applied to investigate hollow oil droplet impacts on heated walls. Results show that given the increase in impact velocity, three evolutionary processes of spreading, transition, and central jet occur after the hollow oil droplet impact on a heated wall. The variation in the spreading length of hollow oil droplets is similar in different evolutionary

We present a comprehensive analysis of the cumulant lattice Boltzmann model with the three-dimensional Taylor–Green vortex benchmark at Reynolds number 1600. The cumulant model is investigated in several different variants, using regularization, fourth-order convergent diffusion and fourth-order convergent advection with and without limiters. In addition, a cumulant model combined with a WALE sub-grid

We present a constructive procedure for the calculation of 2-D potential flows in periodic domains with multiple boundaries per period window. The solution requires two steps: (i) a conformal mapping from a canonical circular domain to the physical target domain, and (ii) the construction of the complex potential inside the circular domain. All singly periodic domains may be classified into three distinct

The problem of the boundary condition setting is considered for creeping flows over cylindrical and spherical obstacles. The interaction of Newtonian and micropolar liquid with the solid surface is discussed in the context of the Stokes paradox and the cell model technique. Mathematical and mechanical aspects of various types of boundary conditions at the hypothetical liquid surface are considered

In this paper, the influence of the Mach number on the stability of two-dimensional compressible planar wakes is studied to gain physical insight into the turbine wakes. Two-dimensional instabilities of compressible wakes are studied using local spatiotemporal instability analyses. The absolute/convective boundary of a family of wake profiles is obtained at different Mach numbers. Then, local stability

We propose a simple model for the evolution of an inviscid vortex sheet in a potential flow in a channel with parallel walls. This model is obtained by augmenting the Birkhoff–Rott equation with a potential field representing the effect of the solid boundaries. Analysis of the stability of equilibria corresponding to flat sheets demonstrates that in this new model the growth rates of the unstable modes

Parametrically excited standing waves are observed on a cylindrical fluid filament. This is the cylindrical analog of the Faraday instability in a flat surface or spherical droplet. Using Floquet theory, a linear stability analysis is carried out on a viscous cylindrical fluid surface, which is subjected to a time-periodic radial acceleration. Viscosity of the fluid has a significant impact on the

The physical characteristics and evolution of a large-scale helium plume are examined through a series of numerical simulations with increasing physical resolution using adaptive mesh refinement (AMR). The five simulations each model a 1-m-diameter circular helium plume exiting into a $$(4~\hbox {m})^3$$ ( 4 m ) 3 domain and differ solely with respect to the smallest scales resolved using the AMR,

The mass transport in electrokinetically actuated microchannel flow is interesting when the wall reactions influence the wall potential, thereby affecting the hydrodynamics. This is the first work where the electro-osmotic flow is impacted by the chemical reactions. Since the wall potential is non-uniform, we have compared the results of the classical Poisson–Boltzmann equations with the generalized

Large scale industrial combustion devices, for example, internal combustion engines, gas turbine combustors, etc., operate under high-pressure conditions and utilize a variety of fuels. Unfortunately, the majority of the current numerical combustion modelling approaches are not fully validated for high-pressure and the non-unity Lewis number ( $$\hbox {Le} =$$ Le = thermal diffusivity/mass diffusivity)

The equations governing the dynamics of a periodically driven micro-spheroid in an unsteady viscous fluid at low Reynolds number are derived. Its oscillation properties in the presence/absence of memory forces are reported. The core part of the derivation is a perturbation analysis of motion of a sphere. The calculated solutions match with those available in the literature in the limiting case of a

Many aquatic organisms from copepods to harbor seals are able to detect and respond to flow disturbances. The physiological mechanisms underlying such behavior remain a challenge for current and future research. Here, we propose a simplified flow sensing scenario in which a mobile sensor reorients its heading in response to local flow stimuli, with the goal of tracing the wakes created by oscillating

We carry out a priori tests of linear and nonlinear eddy viscosity models using direct numerical simulation (DNS) data of square duct flow up to friction Reynolds number $${\text {Re}}_\tau =1055$$ . We focus on the ability of eddy viscosity models to reproduce the anisotropic Reynolds stress tensor components $$a_{ij}$$ responsible for turbulent secondary flows, namely the normal stress $$a_{22}$$

Oscillatory instability of buoyancy convection in a laterally heated cube with perfectly thermally conducting horizontal boundaries is studied. The effect of the spanwise boundaries on the oscillatory instability onset is studied. The problem is treated by Krylov-subspace-iteration based Newton and Arnoldi methods. The Krylov basis vectors are calculated by a novel approach, based on the SIMPLE iteration

In this study, we propose a novel computational model for simulating the coffee-ring phenomenon. The proposed method is based on a phase-field model and Monte Carlo simulation. We use the Allen–Cahn equation with a pinning boundary condition to model a drying droplet. The coffee particles inside the droplet move according to a random walk function with a truncated standard normal distribution under

Effect of finite ion size on the transport of a neutral solute across the porous wall of a nanotube is presented in this study. Modified Poisson–Boltzmann equation without the Debye–Huckel approximation is used to determine the potential distribution within the tube. Power law fluid is selected for the study, as its rheology resembles closely to the real-life physiological fluids. The flow within the

The paper describes the longitudinal dispersion of passive tracer materials released into an incompressible viscous fluid, flowing through a channel with walls having first-order reaction. Its model is based on a steady advection–diffusion equation with Dirichlet’s and mixed boundary conditions, and whose solution represents the concentration of the tracers in different downstream stations. For imposing

Identifying accurate and yet interpretable low-order models from data has gained a renewed interest over the past decade. In the present work, we illustrate how the combined use of dimensionality reduction and sparse system identification techniques allows us to obtain an accurate model of the chaotic thermal convection in a two-dimensional annular thermosyphon. Taking as guidelines the derivation

This study presents two different machine learning approaches for the modeling of hydrodynamic force on particles in a particle-laden multiphase flow. Results from particle-resolved direct numerical simulations (PR-DNS) of flow over a random array of stationary particles for eight combinations of particle Reynolds number ($Re$) and volume fraction ($\phi$) are used in the development of the models

This work deals with the characterization of the closed-loop control performance aiming at the delay of transition. We focus on convective wavepackets, typical of the initial stages of transition to turbulence, starting with the linearized Kuramoto–Sivashinsky equation as a model problem representative of the transitional 2D boundary layer; its simplified structure and reduced order provide a manageable

The mixing in three-dimensional enclosures is investigated numerically using flow in cubical cavity as a geometrically simple model of various natural and engineering flows. The mixing rate is evaluated for up to the value of Reynolds number $$\hbox {Re}=2000$$ Re = 2000 for several representative scenarios of moving cavity walls: perpendicular motion of the parallel cavity walls, motion of a wall

A major aim of the present study is to understand and thoroughly document the fluid dynamics in three-dimensional branching networks when an intermediate branch is partially or completely obstructed. Altogether, 26 different three-dimensional networks each comprising six generations of branches (involving 63 straight portions and 31 bifurcation modules) are constructed and appropriately meshed to conduct

Circulating tumor cells (CTCs) are regarded as important biomarkers for early cancer detection and treatment. Decades of research have made progress in CTC detection using deformability-based microfilters; however, developing a high-throughput CTC microfilter remains a challenging task due to the lack of the essential understanding of microscopic multiphase flow. To design and optimize a CTC microfilter

In recent years, machine learning has been used to create data-driven solutions to problems for which an algorithmic solution is intractable, as well as fine-tuning existing algorithms. This research applies machine learning to the development of an improved finite-volume method for simulating PDEs with discontinuous solutions. Shock-capturing methods make use of nonlinear switching functions that

Turbulence models facilitated by Kolmogorov’s theory play an important role for compressible flows. Typically the basis of these models is the power spectrum of the velocity $${\mathbf {u}}$$ u or of the density-weighted velocity $${\mathbf {w}}\equiv \rho ^{1/3}{\mathbf {u}}$$ w ≡ ρ 1 / 3 u . While for incompressible flow the quantity turbulent kinetic energy characterises turbulent motions, from

Previous research studies indicate that the proportion of Magnus force at the spinning projectile tail position is very high. Meanwhile, the large mutations of aerodynamic characteristics are found after adding boattail structures. In order to study the influences of boattail structures on aerodynamics of a spinning projectile, a 6.37-diameter long tangential-ogive-cylinder projectile is selected as

We present results of three-dimensional direct numerical simulations (DNS) and large eddy simulations (LES) of turbulent gravity currents with a discontinuous Galerkin finite elements method. In particular, we consider the lock-exchange test case as a benchmark for gravity currents. Since, to the best of our knowledge, non-Boussinesq three-dimensional reference DNS are not available in the literature

We propose a method to construct a reduced order model with machine learning for unsteady flows. The present machine-learned reduced order model (ML-ROM) is constructed by combining a convolutional neural network autoencoder (CNN-AE) and a long short-term memory (LSTM), which are trained in a sequential manner. First, the CNN-AE is trained using direct numerical simulation (DNS) data so as to map the

We develop an unsupervised machine learning algorithm for the automated discovery and identification of traveling waves in spatiotemporal systems governed by partial differential equations (PDEs). Our method uses sparse regression and subspace clustering to robustly identify translational invariances that can be leveraged to build improved reduced-order models (ROMs). Invariances, whether translational

Many previous studies have shown that the fidelity of three-dimensional cardiovascular flow simulations depends strongly on inflow and outflow boundary conditions that accurately describe the characteristics of the larger vascular network. These boundary conditions are generally based on lower-dimensional models that represent the upstream or downstream flow behavior in some aggregated fashion. However

This paper considers membranes of globular structure in the framework of the cell model technique. The flow of a micropolar fluid through a spherical cell consisting of a solid core, porous layer and liquid envelope is modeled using coupled micropolar and Brinkman-type equations. The solution is obtained in analytical form. Boundary value problems with different conditions on the hypothetical cell

We present a systematic approach for determining the optimal actuator location for separation control from input–output response data, gathered from numerical simulations or physical experiments. The Eigensystem realization algorithm is used to extract state-space descriptions from the response data associated with a candidate set of actuator locations. These system realizations are then used to determine

We introduce a new approach for adaptive mesh refinement in which adaptivity is driven by low rank decomposition and optimal sensing of the dynamically evolving flow field. This method seeks an ordered set of locations for mesh adaptation from the instantaneous data-driven basis of an online proper orthogonal decomposition of the velocity, which organizes features into sparse optimal orthogonal modes

An inviscid vortex shedding model for separated vortices from a solid body is studied. The model describes the separated vortices by vortex sheets and the attached flow via conformal mapping. We develop a computational model to simulate the vortex shedding of a moving body, with varying angle. An unsteady Kutta condition is imposed on the edges of the plate to determine the edge circulations and velocities

Molecular dynamics simulation is used to study flow rate behavior of monoatomic fluid through carbon nanotube (CNT) against the pore diameter. All armchair and zigzag CNTs with diameters below 1.5 nm were considered. Fluid flow rate versus diameter is investigated, and discrepancy was observed in the results. Its non-monotonic behavior is reported and attributed to diameter-dependent potential energy

The control of complex systems is of critical importance in many branches of science, engineering, and industry. Controlling an unsteady fluid flow is particularly important, as flow control is a key enabler for technologies in energy (e.g., wind, tidal, and combustion), transportation (e.g., planes, trains, and automobiles), security (e.g., tracking airborne contamination), and health (e.g., artificial

The problem of finding optimal forcing and response for unbounded base flows, exemplified by the Blasius boundary layer, is assessed by means of a locally parallel resolvent analysis. A new analysis of previous results in the literature, which stated that a maximum resolvent gain occurs for spanwise wavenumber $$k_z \approx 0.2$$ k z ≈ 0.2 , revealed that this result was not domain converged, and larger

We apply supervised machine learning techniques to a number of regression problems in fluid dynamics. Four machine learning architectures are examined in terms of their characteristics, accuracy, computational cost, and robustness for canonical flow problems. We consider the estimation of force coefficients and wakes from a limited number of sensors on the surface for flows over a cylinder and NACA0012