Wake Patterns Research Articles

Forced oscillations of a cylinder in the flow of viscoelastic fluids

We investigate the effects of fluid elasticity on the flow forces and the wake structure when a rigid cylinder is placed in a viscoelastic flow and is forced to oscillate sinusoidally in the transverse direction. We consider a two-dimensional, uniform, incompressible flow of viscoelastic fluid at $Re=100$ , and use the FENE-P model to represent the viscoelastic fluid. We study how the flow forces and the wake patterns change as the amplitude of oscillations, $A^*$ , the frequency of oscillations (inversely proportional to a reduced velocity, $U^*$ ), the Weissenberg number, $Wi$ , the square of maximum polymer extensibility, $L^2$ , and the viscosity ratio, $\beta$ , change individually. We calculate the lift coefficient in phase with cylinder velocity to determine the range of different system parameters where self-excited oscillations might occur if the cylinder is allowed to oscillate freely. We also study the effect of fluid elasticity on the added mass coefficient as these parameters change. The maximum elastic stress of the fluid occurs in between the vortices that are observed in the wake. We observe a new mode of shedding in the wake of the cylinder: in addition to the primary vortices that are also observed in the Newtonian flows, secondary vortices that are caused entirely by the viscoelasticity of the fluid are observed in between the primary vortices. We also show that, for a constant $Wi$ , the strength of the polymeric stresses increases with increasing reduced velocity or with decreasing amplitude of oscillations.

Numerical analyses of the flow past a short rotating cylinder

This work studies the three-dimensional flow dynamics around a rotating circular cylinder of finite length, whose axis is positioned perpendicular to the streamwise direction. Direct numerical simulations and global stability analyses are performed within a parameter range of Reynolds number $Re=DU_\infty /\nu <500$ (based on cylinder diameter $D$ , uniform incoming flow velocity $U_\infty$ ), length-to-diameter ratio ${\small \text{AR}}=L/D\leq 2$ and dimensionless rotation rate $\alpha =D\varOmega /2U_\infty \leq 2$ (where $\varOmega$ is rotation rate). By solving Navier–Stokes equations, we investigated the wake patterns and explored the phase diagrams of the lift and drag coefficients. For a cylinder with ${\small \text{AR}}=1$ , we found that when the rotation effect is weak ( $0\leq \alpha \lesssim 0.3$ ), the wake pattern is similar to the unsteady wake past the non-rotating finite-length cylinder, but with a new linear unstable mode competing to dominate the saturation state of the wake. The flow becomes stable for $0.3\lesssim \alpha \lesssim 0.9$ when $Re<360$ . When the rotation effect is strong ( $\alpha \gtrsim 0.9$ ), new low-frequency wake patterns with stronger oscillations emerge. Generally, the rotation effect first slightly decreases and then sharply increases the $Re$ threshold of the flow instability when $\alpha$ is relatively small, but significantly decreases the threshold at high $\alpha$ ( $0.9<\alpha \leq 2$ ). Furthermore, the stability analyses based on the time-averaged flows and on the steady solutions demonstrate the existence of multiple unstable modes undergoing Hopf bifurcation, greatly influenced by the rotation effect. The shapes of these global eigenmodes are presented and compared, as well as their structural sensitivity, visualising the flow region important for the disturbance development with rotation. This research contributes to our understanding of the complex bluff-body wake dynamics past this critical configuration.

A GPU-Accelerated Method for 3D Nonlinear Kelvin Ship Wake Patterns Simulation

The study of ship waves is important for ship detection, coastal erosion and wave drag. This paper proposed a highly paralleled numerical computation method for efficiently simulating three-dimensional nonlinear kelvin waves. First, a numerical model for nonlinear ship waves is established based on potential flow theory, the Jacobian-free Newton–Krylov (JFNK) method and the boundary integral method. To reduce the amount of data stored in the JFNK method and improve the computational efficiency, a banded preconditioner method is then developed by formulating the optimal bandwidth selection rule. After that, a Graphics Process Unit (GPU)-based parallel computing framework is designed, and we used the Compute Unified Device Architecture (CUDA) language to develop a GPU solution. Finally, numerical simulations of 3D nonlinear ship waves under multiple scales are performed by using the GPU and CPU solvers. Simulation results show that the proposed GPU solver is more efficient than the CPU solver with the same accuracy. More than 66% GPU memory can be saved, and the computational speed can be accelerated up to 20 times. Hence, the computation time for Kelvin ship waves simulation can be significantly reduced by applying the GPU parallel numerical scheme, which lays a solid foundation for practical ocean engineering.

Awake responses suggest inefficient dense coding in the mouse retina.

The structure and function of the vertebrate retina have been extensively studied across species with an isolated, ex vivo preparation. Retinal function in vivo, however, remains elusive, especially in awake animals. Here, we performed single-unit extracellular recordings in the optic tract of head-fixed mice to compare the output of awake, anesthetized, and ex vivo retinas. While the visual response properties were overall similar across conditions, we found that awake retinal output had in general (1) faster kinetics with less variability in the response latencies; (2) a larger dynamic range; and (3) higher firing activity, by ~20 Hz on average, for both baseline and visually evoked responses. Our modeling analyses further showed that such awake response patterns convey comparable total information but less efficiently, and allow for a linear population decoder to perform significantly better than the anesthetized or ex vivo responses. These results highlight distinct retinal behavior in awake states, in particular suggesting that the retina employs dense coding in vivo, rather than sparse efficient coding as has been often assumed from ex vivo studies.

Numerical investigation of flow across three co-rotating cylinders in side-by-side arrangement

Flow across three side-by-side co-rotating cylinders is investigated at a Reynolds number of 100 and non-dimensional rotation rates varied from 0 to 8, for spacing ratios of L/D=1.5, 2, and 4 through two-dimensional numerical simulations, where D and L are cylinder diameter and the center-to-center spacing between the cylinders, respectively. For L/D=1.5 and 2, the wakes are classified into regime FF (flip-flopping) at smaller rotation rates and regime SB (single-body) at higher rotation rates. Each regime can be further divided into sub-regimes based on the wake patterns. Regime FF is a regime where the flow switches between two patterns intermittently. The three sub-regimes of SB at L/D=1.5: vortex shedding (SB-VS), steady state (SB-SS), and secondary instability (SB-SI) are the same as those of a single rotating cylinder as the flow through the gap is too weak to have effect on global wake. A new sub-regime single-body quasi-steady (SB-QS) is found for L/D=2, where the two shear layers in the wake of the three cylinders interact weakly with each other but do not form strong vortices. For L/D=4, two new regimes are found: regime 3V-to-3S (transition from three vortex shedding wake to three steady wake), where the vortex shedding from the three cylinders are suppressed consecutively one by one with the increase in the rotation rate, and regime TB (two-body) where two of the three cylinders behave as a single body. Regime TB for L/D=4 has two sub-regimes: steady state (TB-SS) and secondary instability (TB-SI). The effects of the flow regimes on the force coefficients are quantified. For all the spacing ratios L/D=1.5, 2, and 4, the standard derivation drag and lift coefficients are significantly greater than that of a single cylinder when vortex shedding occurs.

Mean wake evolution behind low aspect-ratio wall-mounted finite prisms

The evolution in the mean wake of wall-mounted prisms with varying depth-ratio (length-to-width) between 0.016 and 4 is examined numerically over a range of Reynolds numbers (Re = 50 - 500). The aspect-ratio (height-to-width) of the prism is limited to 1. This study aims to ascertain the mechanism of the evolution of mean wake topology due to changing depth-ratio. The mean wake topology is classified into Dipole, Multipole, and Quadrupole types as functions of Reynolds number and depth-ratio. The threshold depth-ratio for the wake evolution changes with Reynolds number. The Multipole-type wake appears as an evolutionary or intermediate wake pattern between Dipole and Quadrupole-type wakes. Increasing the depth-ratio leads to the enhancement of the downwash flow, resulting in the mean wake evolving from Quadrupole to Dipole wake. This suggests that the downwash flow dominates in dictating the wake topology downstream of the prism. Enhancing the downwash flow (with increase in depth-ratio) results in the reattachment of the flow on the prism surfaces, which further results in the suppression of velocity gradients and streamwise momentum transfer downstream of the prism trailing edge. This suppression of gradients and momentum leads to the deterioration of the base vortex, which evolves the wake from Quadrupole (at a small depth-ratio) to Dipole-type (at a large depth-ratio).

Vortex wake patterns in superfluid 4He

Excitations in the form of quantized vortex rings are known to exist in superfluid 4He at energies and momenta exceeding those of the Landau phonon–roton spectrum. They form a vortex branch of elementary excitations spectrum which is disconnected from the Landau spectrum. Interference of vortex ring excitations determines wake patterns due to uniformly traveling sources in bulk superfluid at low speeds and pressures. The dispersion law of these excitations resembles that of gravity waves on deep water with infrared wave number cutoff. As a result, vortex wake patterns featuring elements of the Kelvin ship wake are predicted. Specifically, at lowest speeds the pattern with fully developed transverse and diverging wavefronts is present. At intermediate speeds transverse wavefronts are absent within a cone whose opening angle increases with the source velocity. At largest speeds only diverging wavefronts confined within a cone whose opening angle decreases with the source velocity are found. When experimentally observed, these changes in appearance of wake patterns serve as indicators of the beginning part of the vortex branch of elementary excitations.

Numerical study on flow-induced vibration of an oscillating cylinder with an unattached downstream square plate at a low Reynolds number

Based on the Lattice Boltzmann Method, we numerically studied the flow around a transversely oscillating cylinder with a downstream stationary square plate at a Reynolds number Re = 150 and a mass ratio m* = 2.0. This paper initially considered a zero-damping ratio and a wide range of reduced velocities (Ur = 3–30) to investigate the effects of the gap ratio (L/D = 0.3–3.0) and the dimensionless width of the square plate (W/D = 0.5–2.0) on the response characteristics, force coefficients, wake patterns, and pressure distribution. The energy conversion performance is further tested by applying different non-zero damping ratios (ζ = 0.01–0.3). The main conclusions are as follows: (1) Three vibration behaviors are identified: vortex-induced vibration (VIV), wake interference galloping, and a newly reported “bifurcation” phenomenon of oscillation. For L/D = 0.3, at high reduced velocities (Ur ≥ 15), the cylinder experiences the “bifurcation” phenomenon, where it oscillates on one side of the square plate with a small vibration amplitude. Similar behavior is achieved for the system with L/D = 0.5 and 0.7 but requires a wider square plate than one diameter and higher reduced velocities of Ur ≥ 17.5. (2) Considering both the response amplitude and drag coefficient exerted on the square plate, the cylinder-square plate system with L/D = 0.7 and W/D = 1.0 exhibits a wide wake interference galloping response and an expected vibration amplitude with a lower drag coefficient. (3) For L/D = 0.7 and W/D = 1.0, as Ur increases from 10 to 15, the inclusion of the square plate leads to an increased difference in pressure coefficient between the front of the square plate and the rear of the cylinder. The amplified pressure difference enhances the response of the cylinder, potentially explaining the transition from VIV to wake interference-induced galloping. (4) With a non-damping ratio (ζ = 0.01–0.3), the cylinder-square plate system with L/D = 1.0 and W/D = 1.0 achieves the highest maximum energy harvesting efficiency ηM = 12.01% at the optimum damping ratio ζO = 0.2.

Features and mechanisms of asymmetric wake evolution downstream of two parallel circular cylinders

Uniform flow past side-by-side circular cylinders is a classical fluid dynamic model that triggers rich phenomena, from which asymmetric wakes usually emerge. Such asymmetry arising from a completely symmetric geometric setting is of theoretical interest when exploring the system bifurcations. Using direct numerical simulation, a detailed parametric map of the wakes behind two side-by-side circular cylinders is presented with several asymmetric wake patterns. These include asymmetric anti-phase (AAP), typical and special deflected (DF), and in-phase (IP) flows, for which AAP and special DF flows are discovered for the first time. Additionally, the IP flow is simulated by both two- and three-dimensional grids to explore the effect brought by three-dimensional vortical structures. The evolution of these asymmetric wakes is analyzed in different phases, with the aid of the wavelet transform, Hilbert–Huang transform, and dynamic mode decomposition, to reveal their temporal variations of developing features. Interestingly, although revealing with distinct fully developed flow fields, there are several common dynamics identified among these wake patterns: AP and IP vortex shedding, wake transition, and gap flow oscillation. The vicissitudes of dynamic flow evolution allow us to further differentiate several wake patterns and ultimately contribute to a deeper understanding of asymmetric flows.

Flow-induced reconfiguration and cross-flow vibrations of an elastic plate and implications to energy harvesting

We numerically study flow-induced vibrations (FIV) of an elastic cantilevered plate in a cross-flow configuration. The plate is kept upright in an open domain with a fixed bottom end. Such a configuration eliminates energy losses in the wall boundary. Due to allowed vortex shedding from its bottom end, the resulting novel dynamics of the fluid-structure interaction (FSI) system is explored. An in-house sharp-interface immersed boundary method-based flow solver, coupled with a Saint-Venant Kirchhoff material model-based open-source finite element solver, is employed. Two sets of simulations were carried out in two-dimensional Cartesian coordinates at Reynolds number of 100 for a wide range of reduced velocity (UR), by performing a parametric sweep of dimensionless Young’s modulus (E) and mass ratio (M). In general, FIV characteristics of the plate are similar to the transverse vortex-induced vibration of an elastically-mounted cylinder. The plate deforms to a curved mean position depending on E and M and exhibits angular oscillations. In particular, the following FIV regimes are obtained with increasing UR: initial excitation, lock-in with the first mode, desynchronization after the first lock-in, lock-in with the second mode, and desynchronization after the second lock-in. During lock-in, the plate oscillation and vortex shedding frequency are closer to the first or second mode natural frequency of plate. A larger angular amplitude and increased energy exchange between the structure and fluid characterize the lock-in. We quantify the modal contributions of Euler-Bernoulli modes in the plate’s vibration response with a curved mean position. The spatiotemporal variation of energy exchange between the fluid and plate is analyzed to understand the observed plate dynamics. The flow physics of the wake is discussed using wake patterns, vortex formation length, and vortex strength. Drag over the plate reduces due to flow-induced reconfiguration and strongly correlates with these parameters. Lastly, based on the spatial distribution of strain energy density over the plate, we present a potential design for an ambient fluid energy harvester to harness the plate’s vibration energy with optimized piezoelectric patching. The optimum location and height of a piezoelectric patch on the plate for different cases are reported.

Propulsive performance and vortex dynamics of jellyfish-like propulsion with burst-and-coast strategy

The propulsive performance and vortex dynamics of a two-dimensional model for the jellyfish-like propulsion with burst-and-coast strategy are investigated using a penalty-immersed boundary method. The simplified model comprises a pair of pitching flexible plates with their leading edges connected. The effects of two key parameters are considered, i.e., the duty cycle (DC, the ratio of the closing phase to the whole period) and the bending stiffness (K). Three different wake patterns, i.e., periodic symmetric, periodic asymmetric, and chaotic wakes, are identified in the DC–K plane. Numerical results indicate that a significant fast-close-slow-open motion is more likely to achieve higher speed, efficiency, and stability than a slow-close-fast-open motion, and proper higher bending stiffness is conducive to improving efficiency. A force decomposition based on the weighted integral of the second invariant of the velocity gradient tensor is performed to gain physics insight into the self-propulsive mechanism. It is found that the repulsive force induced by the strain-rate field between the body and the previous vortex pair is the main driving force of the jellyfish-like motion and that capturing the previous vortex pair during the closing phase can significantly enhance the strain rate as well as the thrust. This clarifies why the jellyfish can achieve thrust by pushing back vortex pairs. This study provides inspiration for the design and control of flexible jet propulsion devices.

Understanding the GABAA Receptor: Implications for Anesthesia and Beyond

AbstractGamma-aminobutyric acid (GABA), a nonpeptide amino acid transmitter, is a major component of modern neuropharmacology and one of the most crucial target sites for general anesthetics and therapeutic drugs. GABA type A receptors (GABAARs) are the most abundant inhibitory neurotransmitter receptors in the central nervous system. They are part of the rapid-acting, ligand-gated ion channel (LGIC) receptor category, a pentameric Cys-loop superfamily member that mediates inhibitory neurotransmission in the mature brain. GABAARs mainly consist of two α subunits, two β subunits, and one additional subunit from either γ or δ arranged around a central chloride (Cl-) selective channel. Multiple GABAAR subunit subtypes and splice variants have been identified. Each variant of GABAAR exhibits distinct biophysical and pharmacologic properties. Several compounds allosterically modulate the GABAAR positively or negatively. The widely used positive GABAAR modulators include benzodiazepines (anxiolytic and anticonvulsant), general anesthetics (volatile agents like isoflurane, and intravenous agents like barbiturates, etomidate, and propofol), long-chain alcohols, some anticonvulsants, and neuroactive steroids. The binding sites for each drug are distinctly different. The anesthetic drugs enhance receptor-mediated synaptic transmission and thus interrupt the thalamocortical transmission, which controls the sleep–wake patterns. Abnormality in the GABAAR function has been implicated in several neurological conditions, such as sleep disorders, seizures, depression, cognitive function, neurological recovery after injury, and neuroplasticity. Understanding the GABAAR lays the foundation for the development of highly specific drugs in the treatment of neurological disorders and general anesthesia.

Laminar flow over a rectangular cylinder experiencing torsional flutter: Dynamic response, forces and coherence modes

This study investigates the flutter response of a rectangular cylinder model with an aspect ratio of 5 at the Reynolds number Re = 100 via direct numerical simulation. The effects of two key parameters, i.e., the moment of inertia and reduced flow velocity, on the aerodynamic performance and dynamic responses of the cylinder in the state of torsional flutter are investigated. To reveal the flutter mechanism, the high-order dynamic mode decomposition (HODMD) analysis is conducted to decompose the flow field. The results show that both an increase in the moment of inertia and a higher reduced flow velocity lead to a larger torsional amplitude and a corresponding decrease in torque. At the same time, the primary frequency decreases and the size of the shedding vortex gradually enlarges. The vortices shed from the leading edge and the trailing edge of the model form a 2P wake pattern. The leading-edge vortex is significantly larger than the trailing-edge vortex in terms of strength and size. The leading edge plays a dominant role and only contributes to the odd-order HODMD modes while the even-order modes are deemed inconsequential. As the moment of inertia increases, the total energy of the higher-order modes increases, which has the same results as the power spectral density of torque, reflecting increased nonlinearity and complexity of the system. Similarly, increasing the reduced flow velocity at the same moment of inertia has similar excitation effects.

Normal variants and artifacts: importance in EEG interpretation.

Overinterpretation of EEG is an important contributor to the misdiagnosis of epilepsy. For the EEG to have a high diagnostic value and high specificity, it is critical to recognize waveforms that can be mistaken for abnormal patterns. This article describes artifacts, normal rhythms, and normal patterns that are prone to being misinterpreted as abnormal. Artifacts are potentials generated outside the brain. They are divided into physiologic and extraphysiologic. Physiologic artifacts arise from the body, and include EMG, eyes, various movements, EKG, pulse, and sweat. Some physiologic artifacts can be useful for interpretation such as EMG and eye movements. Extraphysiologic artifacts arise from outside the body, and in turn can be divided into the environments (electrodes, equipment, cellphones) and devices within the body (pacemakers, neurostimulators) Normal rhythms can be divided into awake patterns (alpha rhythm and its variants, mu rhythm, lambda waves, posterior slow waves of youth, HV-induced slowing, photic driving, photomyogenic response) and sleep patterns (POSTS, vertex waves, spindles, K complexes, sleep-related hypersynchrony, frontal arousal rhythm). Breach can affect both awake and sleep rhythms. Normal variants or variants of uncertain clinical significance include variants that may have been considered abnormal in the early days of EEG but are now considered normal. These include wicket spikes and wicket rhythms (the most common normal pattern over-read as epileptiform), small sharp spikes (aka benign epileptiform transients of sleep), rhythmic midtemporal theta of drowsiness (aka psychomotor variant), Cigánek rhythm (aka midline theta), 6 Hz phantom spike-wave, 14&6 Hz positive spikes, subclinical rhythmic epileptiform discharges of adults (SREDA), slow fused transients, occipital spikes of blindness, and temporal slowing of the elderly. Correctly identifying artifacts and normal patterns can help avoid overinterpretation and misdiagnosis. This is an educational review paper addressing a learning objective of the International League Against Epilepsy (ILAE) curriculum.

Numerical investigations of two vibrating cylinders in uniform flow using overset mesh

Abstract In this research, flow around two vibrating cylinders in a tandem arrangement is simulated at Reynolds number Re = 200 using the dynamic overset mesh technique in finite volume-based commercial software. This investigation aims to study the combined influences of the spacing between the two identical circular cylinders and their excitation frequency in the flow. The cylinders are excited by a transverse forced vibration in a uniform cross-flow by applying a simple harmonic motion. The gap distance between the vibrating cylinders is chosen to be L/D = 1.5 and 4, and the vibration amplitude is kept constant at A/D = 0.25. The study focuses on three frequency ratios of the cylinders’ excitation frequency to Strouhal shedding frequency of the single stationary cylinder f e/f s = 0.8, 1.0, and 1.2. Simulation results showed that the flow characteristics over the two vibrating circular cylinders differ from that of a single vibrating cylinder. Also, it is observed that the lock-in state (resonance) for the two vibrating cylinders and the vortex wake patterns are highly affected by the gap distance between cylinders and the excitation frequency.

Nonlinear Kelvin wakes and exponential asymptotics

A notable feature of Kelvin surface-wave patterns due to submerged sources at small Froude numbers (F≪1) is that the wave amplitudes are exponentially small with respect to F. Moreover, as F→0 the assumption of linearization is not appropriate. Thus, the role of nonlinear effects in Kelvin wake patterns at small F hinges on exponential (‘beyond-all-orders’) asymptotics. This outstanding theoretical issue is addressed here in the context of a simple partial differential equation with a quadratic nonlinear term, controlled by a parameter ɛ, and a locally confined forcing term in two spatial dimensions. The linear far-field wave response of this model equation is akin to a Kelvin wake whose wave amplitudes are exponentially small with respect to a parameter μ, analogous to F in water waves. To understand how nonlinearity affects this Kelvin-like pattern for ɛ, μ≪1, we develop an exponential asymptotics technique motivated by our earlier analysis in the wavenumber domain of the fKdV equation. Results are presented for three forcing terms that mimic a submerged source, a submerged doublet and a surface pressure. While the linear wave-crest geometry remains intact, the wave amplitudes in the Kelvin wake are significantly enhanced/suppressed by nonlinearity, depending on the type, strength and sign (polarity) of the forcing term. The asymptotic predictions generally are supported by direct numerical computations for various ɛ and μ.

Particle dispersion over side-by-side square cylinders: Proximity interference effects

This paper investigates the dispersion of tiny inertial particles in the two-dimensional laminar wake from a pair of square cylinders placed side-by-side to each other. The flow Reynolds number (defined based on the cylinder size h and uniform inflow velocity Uc) in our study is fixed at 75. The wake pattern resulting from the interaction between vortices shed from individual cylinders depends on the cross-stream spacing between the cylinders, i.e. spacing ratio s/h. We examine the effect of varying the cross-stream spacing between the cylinders on the body impaction and dispersion of particles in the wake flow over a range of their responses. Three different spacing ratios, s/h = 0.3, 2, and 4 are considered. We find that the impaction efficiency of Stokes number St = 0.1 particles is not sensitive to the spacing ratio. However, the impaction efficiency of St = 1 and 10 particles is dependent on the spacing ratio and increases with the latter. We illustrate how the distribution and clustering of representative inertial particles (i.e., with Stokes number unity) evolve over a vortex shedding cycle uniquely due to the wake flow in each spacing ratio configuration and explain the physical mechanism of particle clustering using backward tracking. The non-uniform clustering of particles with different responses to the wake flow in each spacing ratio configuration is analyzed using the Voronoï diagrams. The spacing ratio effects on the dispersion of particles are quantified in terms of mean statistical quantities such as mean local concentration, clustering intensity, and velocity statistics. The local concentration of St = 1 and 10 particles at streamwise positions away from cylinders is significantly influenced by the wake flow behavior governed by the choice of the spacing ratio. Further, the combined effect of gravity and hydrodynamic drag forces on the impaction and clustering behavior of particles is reported, where the Froude number Fr=0.32. In all the cases, particles exhibit ballistic behavior, showing a significantly reduced tendency to form clusters and a drastic increase in the impaction efficiency.

Mapping of the flow structure and hydrodynamic properties of a round-ended cylinder

This paper reports the numerical results of flow past a round-ended cylinder with various incidence angles in a low Reynolds number range of Re = 60–160. Mapping of the flow structure and hydrodynamic properties is examined in the incidence angle range of α = 0°–90° with increment of 15°. Three wake patterns are identified, including the steady and symmetric wake without vortex shedding (Pattern I), the Karman vortex street (Pattern II), and the Karman vortex street with the occurrence of subordinate vortex (Pattern III). The reattachment of boundary layers results in the occurrence of a subordinate vortex and hence the non-single-frequency fluctuation of hydrodynamic coefficients (CL and CD, which are lift and drag coefficients, respectively). Elliptical and figure-eight CL–CD curves are observed, depending on the frequency ratios of the two coefficients and their weights. Non-zero time-averaged CL occurs when 0° < α < 90°, due to the asymmetric boundary layer separation. The backward migration of boundary layer separation point contributes to the reduction of frictional drag and the vortex formation length. The shortening of the vortex formation length results in the enhanced fluctuations of CL and CD.

Effect of Sleep Restriction during Pregnancy on Fetal Brain Programming and Neurocognitive Development of Offspring: A Review

We spend one-third of our lives in sleep, yet the core function of it still remains an enigma due to underlying complex neural processing in this altered state of consciousness. Sleep requirement varies with phase of development. Neonates spent about 85% of their time in sleep, which is polyphasic in nature. Gradually, this pattern takes the shape of a monophasic sleep in adolescents and adults, with changing micro- and macroarchitecture in every phase. Deprivation of sleep in adults impairs learning and memory, and reduces theta coherence among hippocampus and amygdale during sleep. However, sleep loss during pregnancy can affect the ontogenetic development of networks for sleep–wakefulness and the cognitive development of offspring. Even in normal pregnancy, poor sleep quality, reduced rapid eye movement (REM) sleep, and sleep fragmentation are common observation during the last trimester of pregnancy. Delta power, a marker for the homeostatic drive for sleep, in the NREM sleep during the last trimester of pregnancy and postpartum is increased. However, further sleep loss during late pregnancy is a growing concern. Neonates that are born to the total sleep-restricted dams present significant alterations in their emotional development (symptoms of hyperactivity, increased risk-taking behavior during periadolescence) and immature sleep–wakefulness patterns. The REM sleep restriction during late pregnancy elicits depressionlike traits in neonates, which persist until middle age. For a healthy development of brain and body, thorough understanding of the dynamic nature of sleep in relation to age and state (pregnancy) is instrumental in preventing the above-mentioned conditions of prenatal origin. Although sleep is essential for an active brain (for work during day), it remains an underestimated phenomenon. This review highlights the importance of sleep during pregnancy for a healthy brain network programming in offspring.

Aspect ratio effect on flow past an elliptical cylinder near a moving wall

In this paper, flow over an elliptical cylinder located near a moving wall is investigated numerically to ascertain the effect of aspect ratio (AR) on flow characteristics in terms of wake pattern, separation and stagnation points, and hydrodynamic forces. The two-dimensional simulations span both steady and unsteady flow regimes, where the effect of AR is analyzed in conjunction with other important parameters, such as gap ratio (GR∈ [0.6, 1.2]), Reynolds number (Re ∈ [5, 150]), and angle of attack (AOA∈ [−45°, 45°]). At a low AR = 0.1, significant differences in the wake pattern and critical Re for steady to unsteady wake transition are observed in GR-Re space in comparison with previous results for high AR cylinders. This results from an influence of AR on vorticity convection into the wake. Additionally, the effect of variation in AR from a flat plate (AR = 0) to a circular cylinder (AR = 1) on the wake pattern is discussed and presented in AR-Re space at a low GR. The locations of separation and stagnation points are shown to result from an interplay of AR and wall suppression depicting a low sensitivity to Re with low AR or high wall suppression effect. Subsequently, the effect of AR in conjunction with AOA is analyzed where counterclockwise inclination is observed to reduce wall suppression, while a competing effect arising from the contrasting movements of cylinder bottom and rear side governs the wake pattern for clockwise inclination that is explicit at a certain AR. Eventually, the variations in drag and lift forces with AR for different AOA are characterized.