Unsteady Dynamics Research Articles

Numerical investigation on aerodynamic and inertial couplings of flexible spinning missile with large slenderness ratio

There are aerodynamic and inertial couplings in rigid-body motion and structural vibration of a flexible spinning missile in flight, and when slenderness ratio is larger, the couplings are more obvious. As for the flexible spinning missile with large slenderness ratio, aerodynamic and inertial couplings should be considered. Based on rigid-motion mesh and radial-basis-function (RBF) mesh deformation techniques, unsteady Reynolds-averaged Navier-Stokes (URANS) Equations and flight dynamics equations of flexible spinning missile derived from Lagrange equation are coupled simultaneously to simulate the dynamic response of flexible spinning missile with large slenderness ratio. The URANS equations are solved by Computational Fluid Dynamics (CFD) technique with the in-house code. Not only the aerodynamic coupling between rigid-body six degree of freedom (DOF) motion and structural vibration are included in the flight dynamics equations of flexible spinning missile, but also the variation of inertia tensor and extra moment terms caused by structural vibration are included, too. More importantly, the extra force term caused by angular acceleration of rigid-body motion, Coriolis and centrifugal loading terms caused by angular velocity of rigid-body motion are included. The rigid-motion mesh and RBF mesh deformation techniques are both based on unstructured mesh. The rigid-motion mesh is adopted to treat the large rigid-body motion due to flight dynamics, while the RBF mesh deformation is employed for flexible structural deformation caused by aeroelasticity. Numerical results of free flight case and aeroelastic case are well agreed with the experimental results, which validates the numerical method. A missile model with X-X configuration is constructed to quantitatively investigate the aerodynamic and inertial couplings between rigid-body motion and structural elastic vibration, and aerodynamic force, extra generalized force terms caused by rigid-body motion and extra moment terms caused by structural vibration are all investigated. Final results show that for the critical stable case studied in this paper, the aerodynamic force plays a major role in coupling effects between rigid-body motion and structural vibration, and the inertial coupling terms in rigid-body motion caused by structural vibration are negligible compared with the aerodynamic force. However, the inertial coupling terms in structural vibration due to rigid-body motion are not negligible compared with the aerodynamic force.

Visualizing three-dimensional vortex shedding through evolution surface clusters

Turbulent vortex shedding in the wake of a bluff body often contains cycle-to-cycle variations in the shape, trajectory, and intensity of vortices. Existing flow visualization techniques cannot effectively present these variations and, consequently, their influence on the aerodynamics to the user. This paper explores a new flow visualization approach to represent quasi-periodic vortex shedding over multiple shedding cycles concurrently. This approach uses a reduced-dimension representation of spatiotemporal vortex progression (called evolution surfaces) and ensemble visualization techniques (clustering). The resulting visualization can be used to identify topological changes in the behavior and strengths of coherent structures (i.e., vortices) in unsteady flows. This approach is applied in two case studies of bluff body wakes with Reynolds Numbers Re = 1200 (Hemmati et al. in Progress in Turbulence VII, Bertinoro, Italy, pp 227–232, 2017. https://doi.org/10.1007/978-3-319-57934-4_32 ) and Re = 300 (Morton et al. in J Heat Fluid Flow 72:109–122, 2018. https://doi.org/10.1016/J.IJHEATFLUIDFLOW.2018.05.014 ). In prior work, classification of these wakes’ dynamics was based on energy fluctuation and shedding topology. However, these techniques are not well suited for representing characteristic changes between shedding regimes. In the present work, it has been shown that evolution surface clusters help to identify topological changes characterizing cycle-to-cycle variations in vortex behavior, while reducing visual clutter. The results indicate that evolution surface clusters are a promising visualization tool for comparative analysis of unsteady vortex dynamics in turbulent wakes.

Hydraulic resistance due to unsteady ow in river channels: numerical simulation results

The hydraulic resistance to ow in a river-type channel is due to the action of a large number of factors. We created numerical models of unsteady water dynamics in an inhomogeneous channel by varying its geometric and hydraulic characteristics. We focused on the study of ows at small values of the Manning roughness coefficient, when the water movement is unsteady. The transitional regime from a pulsating ow to a laminar ow is characterized by a complex system of large vortices covering the entire channel. The nonstationarity factor enhances the hydrological resistance to ow, and thus provides an additional contribution to the effective Manning coefficient.

On the highly unsteady dynamics of multiple thermal buoyant jets in cross flows

Thermal plumes of small scale generated by spatially separated heat sources can form, like atoms in a chemical compound, complex structures of different kinds and with distinct behaviors. The situation becomes even more complex if plumes can interact with imposed vertical shear (a horizontal wind). In this analysis, a “minimal framework” based on the application of a filtering process to the governing balance equations for mass, momentum, and energy (falling under the general heading of “Large Eddy Simulation” approach) is used together with direct numerical simulation to inquiry about the relative importance of buoyancy and vertical shear effects in determining the patterning scenario when highly unsteady dynamics are established (turbulent flow). Emerging patterns range from the flow dominated by a static rising jet produced by the aggregation of plumes that are pushed by horizontal leftward and rightward winds toward the center of the physical domain to convective systems with disconnected thermal pillars of smaller scale, which travel in the same direction of the prevailing wind. The classical sheltering effect, which for flows that are steady “in mean” simply consists of an increased deflection of the leading buoyant jet with respect to the trailing ones, is taken over by a variety of new phenomena, including (but not limited to) fast plume removal-rebirth mechanisms (with local increase in the velocity frequency and shrinkage in the related amplitude), “bubble” formation-rupture, and local departure of the frequency spectrum from the Kolmogorov similarity law.

Computational simulation and free flight validation of body vibration of flapping-wing MAV in forward flight

FWMAVs (Flapping Wing Micro Air Vehicles) turn to be a flexible multi-body dynamic (FMBD) system when the inertia and flexibility of flapping wing are considered. The vibration caused by interaction of unsteady aerodynamics, structural dynamics and multi-body dynamics makes the analysis of aerodynamic and acquisition of equilibrium become full of challenge. In this study, a method coupled three-dimensional CFD (Computational Fluid Dynamics) and CSD (Computational Structural Dynamics) considering FMBD (flexible multi-body dynamics) between flexible wings and body is developed. And a trim algorithm is developed to satisfy three aspects of conditions including convergence of structural deformation and periodic condition of body's vibration in longitudinal direction and conditions of equilibrium. The computational results show that the trim conditions when body's vibration is taken into account are different from that when body's vibration is ignored. The vibration of body mainly increases the aerodynamic drag while having a little effect on lift. When the flight is trimmed, the corresponding flapping frequency increases and angle of attack decreases. In order to validate the method of simulation, the free flight data including the time course of oscillated plunging acceleration and pitch angle is collected based on the steady flight ability of “DOVE” FWMAV. Through comparison, though there is slight discrepancy between computational results and free flight data, it proves that CFD/FMBD method provides a sufficiently good accuracy for the simulation of body vibration of FWMAVs. Based on the proposed and validated model, the influence of various take-off mass on the vibration of body is studied and it indicates that a proper mass ratio between wing to body can suppress the maximum acceleration of vibration to protect the precise sensors and equipment.

Dynamic Thermal Behavior of a GDI Spray Impacting on a Heated Thin Foil by Phase-Averaged Infrared Thermography

<div class="section abstract"><div class="htmlview paragraph">The regulations about pollutant emissions imposed by Community’s laws encourage the investigation on the combustion optimization in modern engines and in particular in those adopting the gasoline direct injection (GDI) or direct injection spark-ignited (DISI) configuration. It is known that the piston head and cylinder surface temperatures, coupled with the fuel injection pressure, strongly influence the interaction between droplets of injected fluid and the impinged wall. In the present study, the Infrared (IR) thermography is applied to investigate the thermal footprint of an iso-octane spray generated by a multi-hole GDI injector impinging on a heated thin foil. The experimental apparatus includes an Invar foil (50 μm in thickness) heated by Joule effect, clamped within a rigid frame, and the GDI injector located 11 mm above the surface. Thermal images of the impinging spray are acquired from the dry side of the foil at several time delays after the impact instant at two different injection pressures (10 and 20 MPa). The temperature difference distributions are employed to describe the unsteady dynamics of the impingement.</div></div>

A fully Eulerian multiphase model of windblown sand coupled with morphodynamic evolution: Erosion, transport, deposition, and avalanching

Modeling unsteady windblown sand dynamics requires not only treatment of the sand present in the air as a suspended constituent of a mixture but also consideration of erosion and sedimentation phenomena and consequently of the morphodynamic evolution of the sand-bed surface, including avalanching, especially in the presence of natural or human-built obstacles, artifacts, and infrastructures. With this aim in mind, we present a comprehensive multiphase model capable of accurately simulating all the physical phenomena mentioned above, producing satisfactory results, with reasonable computational effort. As test cases, two- and three-dimensional simulations of dune evolution are reported, as is windblown sand transport over a straight vertical wall. Examples of sand transport around other obstacles are given to show the flexibility of the model and its usefulness for such engineering applications.

Flame dynamics in a stepped micro-combustor for non-adiabatic wall conditions

Flame stabilization in micro scale combustors has generated significant research interest to develop combustion-based power generators for microelectromechanical (MEMS) systems. Flame stabilization in such systems is significantly impacted by increased susceptibility to quenching due to increased heat-loss to walls and flame stability is governed by thermal-wall coupling. The present work is carried out to understand the role of thermal-wall coupling on flame stabilization, and unsteady flame dynamics in a backward step micro-combustor. The effect of various operating parameters such as mixture equivalence ratio and flow velocities on flame dynamics is reported for a range of non-adiabatic conditions by changing the geometrical parameters such as the diameter and length of the heat recirculating cup around the micro-combustor. The role of these parameters on flame stabilization and flame dynamics is delineated through flame regime diagrams. It is observed that flame stability limits are significantly enhanced due to a change (increase) in the dimensions of the heat recirculation cup, which enhances the heat recirculation through combustor walls. Steady and unsteady flame propagation modes are observed for the range of conditions investigated. The role of various parameters leading to the formation of stable and unstable flame propagation modes is experimentally investigated. The flame stabilization is observed to depend on heat recirculation through combustor walls and heat recirculation cup. Various interesting aspects of rotating and pulsating flame propagation modes are presented. The unstable flame modes lead to a significant decrease in combustion and thereby reducing combustion efficiency.

Thermodynamic analysis of unsteady cavitation dynamics in liquid hydrogen

This paper presents a numerical study regarding the evolution of cavitation dynamics in liquid hydrogen cavitating flows with special emphasis on the shedding process and thermodynamic characteristics. The liquid hydrogen cavitating flows around an ogive body in different thermal cavitation modes (the quasi-isothermal mode, the transition mode, and the thermo-sensitive mode) were numerically investigated. Both 2-D and 3-D computations were compared and validated. The results show that the thermal transition of cavitation dynamics is mainly affected by the combined effects of thermodynamic effects and Reynolds number. Lagrangian methods were utilized to investigate the shedding process. When cavitation dynamics changes from the quasi-isothermal mode to the thermo-sensitive mode, the distribution of Lagrangian Coherent Structure (LCS) is closer to the ogive surface. Several vortexes exist between the LCS and the ogive surface. The free-stream liquid flows into the attached cavity because of the smaller vapor volume fraction, and the impact of the re-entrant jet on attached cavity becomes slight and finally disappears. The thermodynamic characteristics of liquid hydrogen cavitating flows were further investigated by the entropy transport equation. The analysis indicates that the distribution of entropy production rate caused by direct viscous dissipation Spro.D becomes mushy and gets closer to ogive surface while the distribution of the entropy production rate caused by heat transfer Spro.C is always along the liquid-vapor interphase with the increasing temperature.

Memory-Augmented Temporal Dynamic Learning for Action Recognition

Human actions captured in video sequences contain two crucial factors for action recognition, i.e., visual appearance and motion dynamics. To model these two aspects, Convolutional and Recurrent Neural Networks (CNNs and RNNs) are adopted in most existing successful methods for recognizing actions. However, CNN based methods are limited in modeling long-term motion dynamics. RNNs are able to learn temporal motion dynamics but lack effective ways to tackle unsteady dynamics in long-duration motion. In this work, we propose a memory-augmented temporal dynamic learning network, which learns to write the most evident information into an external memory module and ignore irrelevant ones. In particular, we present a differential memory controller to make a discrete decision on whether the external memory module should be updated with current feature. The discrete memory controller takes in the memory history, context embedding and current feature as inputs and controls information flow into the external memory module. Additionally, we train this discrete memory controller using straight-through estimator. We evaluate this end-to-end system on benchmark datasets (UCF101 and HMDB51) of human action recognition. The experimental results show consistent improvements on both datasets over prior works and our baselines.

Predicting octane number from microscale flame dynamics

The standard method for measuring the octane number of fuels requires large sample volumes ( ∼ 1L) and access to a Cooperative Fuel Research (CFR) engine. This method reliably quantifies the knock resistance of fuels in spark ignition engines, however the large sample volume requirement prevents testing of new experimental fuels (often produced in quantities of just ∼ 1 mL), and the large equipment size impedes mobile, decentralized testing of remote fuel supplies. When direct measurements of octane number are impractical, other methods are needed. Micro flow reactors have shown promise in measuring ignition characteristics that are sensitive to octane number, and they are compact and operate on small volumes ( ∼ 1 mL). This study uses simulations to demonstrate that measurements of the unsteady flame dynamics in a micro flow reactor can provide valuable data for accurate octane number predictions. Simulations of the flow reactor are used to obtain ignition characteristics for over 200 ethanol-toluene primary reference fuels (ETPRF) and 21 biofuel blends. A feed forward neural network is trained using the micro flow reactor ignition characteristics, fuel properties, and known research octane number (RON) and motor octane number (MON) for the ETPRF fuels. The neural network is able to predict the RON and MON of the biofuel blends to within 2 octane number on average. Prediction results are compared to other methods available in the literature. Additional neural network models are trained that show improved prediction accuracy as additional fuel training data becomes available.

Assessment of the transforming impact of global value chains on international trade

The article discusses the problems of international fragmentation of production and global value chains and how they relate to modern aspects, trends and challenges of international trade development. Methodologically the research rests upon the key principles of classical and alternative theories of international trade, the theory of value chains and the basic theories of foreign investment. In the course of the study, the authors apply both general theoretical (formalization, synthesis, analysis) and particular (measurement, comparison, observation) research methods. The main results of the research are as follows: a conceptual rethink of the nature of today’s international trade in terms of influencing factors of global production decomposition; theoretical justification and criticism of contemporary approaches to the organization of value chains between developed and developing countries; the formulation of measures to modify trade and economic policy in the current system-related and structural challenges of value chains’ transformation. The authors conclude that instability of modern global value chains in the world production and international trade is due to the unsteady dynamics of the global economy, industrial differentiation of global value chains, changes in international strategies of multinational companies, innovation and digitalization of global production networks.

Canonical description of wing kinematics and dynamics for a straight flying insectivorous bat (Hipposideros pratti).

Bats, with highly articulated wings, are some of the most agile flyers in nature. A novel three-dimensional geometric decomposition framework is developed to reduce the complex kinematics of a bat wing into physical movements used to describe flapping flight: namely flapping, stroke plane deviation and pitching, together with cambering and flexion. The decomposition is combined with aerodynamic simulations to investigate the cumulative effect of each motion on force production, and their primary contribution to the unsteady vortex dynamics. For the nearly straight and level flight of Hipposideros pratti, results show that the flapping motion by itself induced a moderate drag and lift. Stroke plane deviation increased lift, and nullified the drag. With the inclusion of the pitching motion into the kinematics, lift production further increased by a factor of more than 2.5, and exhibited a positive net thrust by virtue of the favorable wing orientation during the upstroke. The primary contribution of cambering, which included a maximum chord line displacement of ≈40% standard mean chord, was the stabilization of the leading edge vortex during the downstroke. This increased mean lift by about 35% at the expense of net thrust. Flexion was perhaps the most complex motion with maximum displacements of 75% standard mean chord. This was instrumental in reducing the negative lift during the upstroke by preventing the formation of strong leading edge vortices. The aerodynamic effective angle of attack emerged as a heuristic parameter to describe lift and net thrust production across movements.

Influence of pitch rate on freely translating perching airfoils

We numerically investigated the unsteady dynamics of a two-dimensional airfoil undergoing a continuous, prescribed pitch-up motion and freely translating as a response to aerodynamic forces and the gravity field. The pitch-up motion was applied about an axis located $1/6$ chord away from the leading edge and was parameterized using the shape change number, with a Reynolds number set to 2000. It was shown that the minimum kinetic energy reached by the airfoil depends stochastically and asymptotically on shape change numbers for values below and above 1, respectively. Very low kinetic energy levels (close to zero) can be reached in both stochastic and asymptotic regions but high shape change numbers are accompanied by significant gain in altitude which may be undesirable from a practical perspective. Rather, shape change numbers in the range [0.1–0.3] allow us to reach relatively low levels of kinetic energy for close perching locations. We showed that highly nonlinear fluid–structure interactions induced by massive flow separations and strong vortices are conducive to low kinetic energy, but responsible for the stochastic dependence of kinetic energy to shape change number, which can make perching manoeuvres hardly controllable for flying vehicles.

Unsteady evolution of the two-phase flow in sparkling wine tasting and the subsequent role of glass shape

The purpose of this paper is to demonstrate the link between the unsteady dynamics of the liquid phase, closely dependent on the glass shape, and the release of CO2 by a diffusion-convection mechanism, which is the superposition of two mechanisms, the molecular diffusion (Fick’s law) and the bubbles induced mass-transfer (convective transfer). Four glasses of different shapes were investigated. For each one, we followed the time evolution of the liquid (i.e. the wine) and gaseous (i.e. CO2) phases of the wine. The monitoring of the gaseous phase was carried out by measuring the time evolution of the diffusion velocity of gas-phase CO2 released from the air/wine interface during a tasting. In complement, particle image velocimetry (PIV) has been used to monitor the dynamic behaviour of the liquid phase. The comparison of the results for both phases revealed that the velocity at which CO2 molecules in the gas phase returns to the atmosphere is closely related to the vorticity and velocity of the liquid medium which are highly dependent on the shape of the glass. These results suggest that in a glass with a significant wine swirling intensity such as glass 1, CO2 will be released early, which will have a definite impact on the way wine is perceived.

Fluid dynamics of oscillatory flow in three-dimensional branching networks

The present study is aimed at understanding and thoroughly documenting the complex unsteady fluid dynamics in six generations of a model human bronchial tree, comprising 63 straight sections and 31 bifurcation modules, during a complete breathing cycle. The computational task is challenging since the complexity of an elaborate network is augmented with adopted stringent criteria for spatial and temporal accuracy and convergence at each time step (10−8 for each scaled residual). The physical understanding of the fluid dynamics of steady expiratory flow is taken to a similar level of fine details that have been previously established for steady inspiratory flow in earlier publications of the authors. The effects of three-dimensional arrangement of the same branches on the oscillatory flow structure are determined. It is found that the quasisteady assumption is approximately valid in the neighborhood of the peak flow rate, both during inspiration and expiration. Unsteady effects are at their maximum during the changeover from expiration to inspiration and inspiration to expiration. At these time instants, regions of bidirectional flow are observed in all branches with significant secondary motion at various cross sections (none of these features can be predicted by steady state simulations). It is described how the symmetry of the solution with respect to both space and time—found in the oscillating, fully developed flow in a pipe—are destroyed in the unsteady effects that occur in the oscillating flow in a branching network. As the Womersley number is increased, the unsteady effects at all branches increase, and bidirectional flow exists over a greater portion of a cycle. The flow division at a bifurcation module during inspiratory flow generates large asymmetry in the flow field with nonuniform mass flow distribution among the branches of a generation (even in a geometrically symmetric network), whereas flow combination at the same bifurcation module during expiratory flow tends to produce more symmetry in the flow field, displaying essential irreversibility of fluid dynamics.

Impact of Unsteady Flow Events on Bedload Transport: A Review of Laboratory Experiments

Recent advances in understanding bedload transport under unsteady flow conditions are presented, with a particular emphasis on laboratory experiments. The contribution of laboratory studies to the explanation of key processes of sediment transport observed in alluvial rivers, ephemeral streams, and river reaches below a dam is demonstrated, primarily focusing on bedload transport in gravel-bed streams. The state of current knowledge on the impact of flow properties (unsteady flow hydrograph shape and duration, flood cycles) and sediment attributes (bed structure, sediment availability, bed composition) on bedload are discussed, along with unsteady flow dynamics of the water-sediment system. Experiments published in recent years are summarized, the main findings are presented, and future directions of research are suggested.

Numerical and experimental study of the flow around a 4:1 rectangular cylinder at moderate Reynolds number

This paper presents the results of investigations into the flow around a rectangular cylinder with a chord-to-depth ratio equal to 4. The studies are performed through wind tunnel dynamic pressure measurements along a cross section combined with two-dimensional Unsteady Reynolds-Averaged Navier-Stokes (urans) and three-dimensional Delayed-Detached Eddy Simulation (ddes). These experimental and numerical studies are complementary and combining them allows a better understanding of the unsteady dynamics of the flow. These studies aim mainly at determining the effects of the rectangle incidence and freestream velocity on the variation of the flow topology and the aerodynamic loads, and at assessing the capability of the industrially affordable urans and ddes approaches to provide a sufficiently accurate estimation of the flow for different incidences. The comparison of experimental and numerical data is performed using statistics and Dynamic Mode Decomposition. It is shown that the rectangular cylinder involves complex separation-reattachment phenomena that are highly sensitive to the Reynolds number. In particular, the time-averaged lift slope c¯lα increases rapidly with the Reynolds number in the range 7.8×103≤Re≤1.9×104 due to the modification of the time-averaged vortex strength, thickness and distance from the surface. Additionally, it is shown that both urans and ddes simulations fail to accurately predict the flow at all the different incidence angles considered. The urans approach is able to qualitatively estimate the spatio-temporal variations of vortices for incidences below the stall angle α=4∘. Nonetheless, urans does not predict stall, while ddes correctly identifies the stall angle observed experimentally.

Mixed Convective Transport Around Staggered Rows of Square Cylinders

The unsteady mixed convective transport around multiple bluff objects placed in a staggered configuration with respect to a uniform free stream flow is analyzed through two-dimensional numerical computation. The bluff objects are identical in shape and size with square cross-section and arranged in two different rows within an unconfined domain. A small temperature difference between the objects and the free stream results in the free convection in addition to the forced flow. Simulation is carried out using a finite volume based method considering a uniform cross flow of air (Prandtl number = 0.71) at a moderate Reynolds number (= 100). The transverse spacing between the cylinders may anticipated to influence significantly the wake dynamics, which in turn affects the thermal transport. Simultaneously, the mixed convective strength also influences the wake dynamics and vortex structure formation. An interplay between these two effects aptly dictates the resulting flow dynamics and associated thermal transport. Accordingly, the dimensionless transverse spacing is varied (= 1, 3 and 5) along with the mixed convective strength (Richardson number = 0-2). It is observed that the flow and thermal fields show chaotic nature at smaller transverse spacing. However, at larger spacing, the usual unsteady vortex dynamics persists. Very interestingly it is observed that the chaotic flow at smaller transverse spacing reduces its instability to become unsteady periodic at larger strength of the thermal buoyancy. The average heat transfer from the cylinders is found more at smaller transverse spacings and it increases with increasing mixed convective strength.

The gap size effect on leakage flow and cavitation in the clearance of a 2D hydrofoil: high-speed visualization and velocity measurements

In the study, tip-clearance cavitation inception and development was studied in the gap between an end face of a NACA0022-34 hydrofoil and a transparent sidewall of a test channel. The experiments were carried out for the attack angles of 3° and 9° and the gap width of 0.4 to 0.8 mm under various flow conditions on the cavitation number. In order to observe the tip-clearance cavitation inception and its dynamics, a high-speed visualization was applied. The leakage flow velocity was measured inside the clearance by a modified PTV technique. It was shown that unsteady dynamics of the primary cavity on the foil suction side affects the leakage flow direction and the gap cavitation evolution. An increase of the gap width causes the gap cavitation to be initiated at higher cavitation numbers.