Unsteady Dynamics Research Articles

Large eddy simulation of turbulent supersonic hydrogen flames with OpenFOAM

A high-fidelity numerical solver, RYrhoCentralFoam, is developed based on OpenFOAM® to simulate turbulent compressible reactive flows. It is designed for accurately simulating combustion with detailed chemistry, turbulence, shock wave and their interactions. The features that we develop in this work include: (1) multi-species transport, (2) detailed fuel chemistry, and (3) turbulent combustion models in Large Eddy Simulations (LES). Two hydrogen flames with detailed measurements are studied, including turbulent auto-igniting flame in hot co-flowing jet and supersonic combustion in a supersonic burner. For the first flame, the lift-off height, overall flow and flame behaviors, as well as the statistics of the velocity and reactive scalar are computed accurately. For the second flame, the RYrhoCentralFoam is also shown to have the ability for modelling supersonic combustion in model combustors, in terms of the velocity and temperature fields as well as unsteady flame lift-off dynamics in a recirculating zone. The accuracies of LES with RYrhoCentralFOAM in both flames are comparable to those with other well-validated compressible flow solvers.

Boundary rod elements for modeling the railway at the bridge crossing under the action of the speed railway load

New rod boundary elements on an elastic foundation (finite elements of large length, when approximating the displacements by a set of linear and trigonometric functions) are proposed for calculating the path oscillations outside the bridge and on the bridge, combined with a double-track beam bridge connected through a layer with a high-speed rolling stock movement. The selection of interlayer material is related to the dynamics of the bridge. To build the methodology, the articles published earlier by the author are used: a step-by-step procedure for solving the problems of unsteady dynamics of structures and the method of “nodal accelerations” to take into account the action on the structures of a moving inertial load.

Global eigenmodes of thin liquid sheets by means of Volume-of-Fluid simulations

The unsteady dynamics of planar liquid sheet flows, interacting with unconfined gaseous environments located on both sides of the liquid phase, is numerically investigated by means of the Volume-of-Fluid (VOF) technique for supercritical regimes. The global behavior of the non-parallel flow is analyzed by perturbing the initial steady configuration by means of a Gaussian bump in the transverse velocity component of relatively small amplitude, thereby exciting sinuous modes. To gain more physical insights into the fluid system, a theoretical linear one-dimensional model is also developed. A physical interpretation of this model relates the sheet dynamics to transverse vibrations of tensional string forced by terms containing the lateral velocity and subjected to a total damping coefficient, which can assume negative values. The VOF simulation satisfactorily confirms that the velocity impulse perturbation splits into two wave fronts traveling downstream with the theoretical wave velocities. A good agreement is found in comparing the crossing times over the entire domain length of such waves with the almost constant spacing between the frequencies of the eigenvalue spectrum. Surface tension plays a stabilizing role, and for relatively high values of density ratio rρ of gaseous-to-liquid phases, the sheet becomes unstable. It is argued that the distribution of transverse velocity component of the gaseous phase represents the forcing term, which leads the system toward the instability when, for relatively high rρ, the total damping becomes negative. An analogy seems to exist between the global unstable behavior exhibited by the liquid sheet as rρ increases and the shear-induced global instability found by Tammisola et al. [Surface tension-induced global instability of planar jets and wakes,” J. Fluid Mech. 713, 632–658 (2012)] in the presence of surface tension. However, for the gravitational sheet, the surface tension is stabilizing.

Variable thrust and high efficiency propulsion with oscillating foils at high Reynolds numbers

Bio-inspired oscillatory foil propulsion has the ability to traverse various propulsive modes by dynamically changing the foil’s heave and pitch kinematics. This research characterizes the propulsion properties and wake dynamics of a symmetric oscillating foil, specifically targeting the high Reynolds number operation of small to medium surface vessels whose propulsive specifications have a broad range of loads and speeds. An unsteady Reynolds-averaged Navier–Stokes (URANS) solver with a k-ω SST turbulence model is used to sweep through pitch amplitude and frequency at two heave amplitudes of h0∕c=1 and h0∕c=2 at Re=106. At h0∕c=2, the maximum thrust coefficient is CT=8.2 due to the large intercepted flow area of the foil, whereas at a decreased Strouhal number the thrust coefficient decreases and the maximum propulsive efficiency reaches 75%. Results illustrate the kinematics required to transition between the high-efficiency and high-thrust regimes at high Reynolds number and the resulting changes to the vortex wake structure. The unsteady vortex dynamics throughout the heave–pitch cycle strongly influence the characterization of thrust and propulsive efficiency, and are classified into flow regimes based on performance and vortex structure.

Route to chaos on a dragonfly wing cross section in gliding flight

The route from linear towards nonlinear and chaotic aerodynamic regimes of a fixed dragonfly wing cross section in gliding flight is investigated numerically using direct Navier-Stokes simulations (DNSs). The dragonfly wing consists of two corrugations combined with a rear arc, which is known to provide overall good aerodynamic mean performance at low Reynolds numbers. First, the three regimes (linear, nonlinear, and chaotic) are characterized, and validated using two different fluid solvers. In particular, a peculiar transition to chaos when changing the angle of attack is observed for both solvers: The system undergoes a sudden transition to chaos in less than 0.1^{∘}. Second, a physical insight is given on the flow interaction between the corrugations and the rear arc, which is shown as the key phenomenon controlling the unsteady vortex dynamics and the sudden transition to chaos. Additionally, aerodynamic performances in the three regimes are given, showing that optimal performances are closely connected to the transition to chaos.

Computational investigation of the cavitation vortex dynamics in flow over a three-dimensional hydrofoil by a new transport-based model

Cavitation is of significant practical interest due to its unsteady features which could induce destructive effects such as drastic drop in efficiency, noise, vibration, and corrosion for propulsion systems, rudders and other hydraulic machinery. A thorough understanding of the hydrodynamics in the cavitating flow past a three-dimensional hydrofoil makes indicators for an improved control performance of these hydraulic systems. Hence, a computational investigation of the cavitating flow over the Delft Twist-11 hydrofoil was performed with special emphasis on the cavitation vortex dynamics. A new transport-based cavitation model was proposed and compared with the conventional Schnerr–Sauer model through the predictions of cavitation characteristics, for which available experimental data were also provided. The results show that, as compared with the Schnerr–Sauer model, the proposed model can predict closer engineering parameters, including time-mean lift coefficient and vapor shedding frequency with the experiments. In addition, more reasonable structure and dynamics of the three-dimensional unsteady sheet/cloud cavitation patterns, including the cavity growth, break-off, and collapse downstream are captured by the proposed model. With the help of the vorticity transport equation in a variable density flow, further analysis of the flow field predicted by the proposed model reveals that cavitation promotes the production of vortex as well as the flow instabilities. Vorticity production in the cavitating flow is mainly induced by the terms of vortex stretching and vortex dilatation, while the baroclinic torque only contributes in the region of shedding and collapse of the cloud cavity and the contribution of the viscous diffusion term is negligible as compared with the other three terms. The main significance of this study is that it demonstrates the potential of a robust transport-based cavitation model to investigate the unsteady dynamics in the cavitating flow past a three-dimensional twisted hydrofoil and expected to make sense for other hydraulic machinery.

Coarse-Grained Modeling of Pore Dynamics on the Red Blood Cell Membrane under Large Deformations

Transient pore formation on the membrane of red blood cells (RBCs) under high mechanical tensions is of great importance in many biomedical applications, such as RBC damage (hemolysis) and mechanoporation-based drug delivery. The dynamic process of pore formation, growth, and resealing is hard to visualize in experiments. We developed a mesoscale coarse-grained model to study the characteristics of transient pores on a patch of the lipid bilayer that is strengthened by an elastic meshwork representing the cytoskeleton. Unsteady molecular dynamics was used to study the pore formation and reseal at high strain rates close to the physiological ranges. The critical strain for pore formation, pore characteristics, and cytoskeleton effects were studied. Results show that the presence of the cytoskeleton increases the critical strain of pore formation and confines the pore growth. Moreover, the pore recovery process under negative strain rates (compression) is analyzed. Simulations show that pores can remain open for a long time during the high-speed tank-treading induced stretching and compression process that a patch of the RBC membrane usually experiences under high shear flow. Furthermore, complex loading conditions can affect the pore characteristics and result in denser pores. Finally, the effects of strain rate on pore formation are analyzed. Higher rate stretching of membrane patch can result in a significant increase in the critical areal strain and density of pores. Such a model reveals the dynamic molecular process of RBC damage in biomedical devices and mechanoporation that, to our knowledge, has not been reported before.

The 2D lumped wake vortex method applied to unsteady flapping

PurposeAdvancements in aerospace technologies, which rely on unsteady fluid dynamics, are being hindered by a lack of easy to use, computationally efficient unsteady computational fluid dynamics (CFD) software. Existing CFD platforms are capable of handling unsteady flapping, but the time, money and expertise required to run even a basic flapping simulation make design iteration and optimization prohibitively expensive for the average researcher.Design/methodology/approachIn the present paper, a remedy to model the effects of viscosity is introduced to the original vortex method, in which the pitching moment amplitude grew over time for simulations involving multiple flapping cycles. The new approach described herein lumps far-field wake vortices to mimic the vortex decay, which is shown to improve the accuracy of the solution while keeping the pitching moment amplitude under control, especially for simulations involving many flapping cycles.FindingsIn addition to improving the accuracy of the solution, the new method greatly reduces the computation time for simulations involving many flapping cycles. The solution of the original vortex method and the new method are compared to published Navier–Stokes solver data and show very good agreement.Originality/valueBy utilizing a novel unsteady vortex method, which has been designed specifically to handle the highly unsteady flapping wing problems, it has been shown that the time to compute a solution is reduced by several orders of magnitude (Denda et al., 2016). Despite the success of the vortex method, especially for a small number of flapping cycles, the solution deteriorates as the number of flapping cycles increases due to the inherent lack of viscosity in the vortex method.

Interplay of unsteady aerodynamics and flight dynamics of transport aircraft in icing conditions

Airframe icing causes significant degradation of aerodynamic characteristics and influences the flight safety. Wind tunnel study of longitudinal steady and unsteady aerodynamic characteristics of a transport aircraft in icing conditions is carried out in order to develop mathematical model of aerodynamics in the extended flight envelope. The wind tunnel results are validated through flight tests conducted for the real aircraft. Large, glaze-horn ice shapes, corresponding to holding flight phase, are considered. Influence of an ice protection system as well as its failure is examined. Effect of icing on the unsteady aerodynamics characteristics is studied not only through wind tunnel tests but also via analysis of subsequent influence on the flight dynamics of the aircraft. The conducted study shows that the ice shapes of the holding phase lead to reduced stall angle of attack (AoA), maximum lift, and longitudinal damping. Flight dynamics analysis demonstrates that dangerous aircraft behaviour in the form of high AoA departure and limit cycle oscillations (LCO) can be observed at smaller elevator deflections for the iced aircraft. The account for icing influence on the unsteady aerodynamics in the flight dynamics simulations revealed degradation of the dynamic response and deterioration of phase portraits of the system. Even for small AoA and elevator deflection, the aircraft might be trapped into the basin of attraction of high-AoA LCO. In addition, incorporating icing effects in unsteady aerodynamics manifest larger amplitude of LCO.

SHORT PULSE ELECTRON LINAC WITH FULL BEAM LOADING COMPENSATION

Acceleration of intense short-pulsed electron beams in transient mode with energy spread about a few tenths of a percent is the actual problem for RF linacs, in an instance, for an electron injector of a storage ring. In this work we study both analytically and numerically unsteady self-consistent beam dynamics in the one section RF linac fed from SLED. For compensation of the transient beam loading, resulting in beam energy spread, a method of a beam delay with respect to RF pulse is consider. In order to increase efficiency of this method we choose a homogenous dick-loaded waveguide with π/2 phase advance as an accelerating structure that characterized by the minimal frequency dispersion of group velocity. The preliminary results show that the full beam loading compensation is possible for the 1.2 A beam at its acceleration up to 60 MeV during 50 ns.

Investigation of unsteady liquid nitrogen cavitating flows with special emphasis on the vortex structures using mode decomposition methods

• Liquid nitrogen cavitating flows in a C-D nozzle were simulated by LES. • Thermal effects on the unsteady cavitation dynamics were investigated. • Vorticity transport equation was utilized to analyze the vortex structures and shedding dynamics. • Coherent structures and corresponding frequencies were investigated by mode decomposition methods. This paper presents the large-eddy simulations with significant thermal effects to investigate the evolution of cavitation dynamics in liquid nitrogen cavitating flows with special emphasis on the vortex dynamics and coherent structures. The liquid nitrogen cavitating flows in the converging-diverging nozzle under different flow conditions were numerically investigated and verified by the experimental data. The thermal effects on cavity structures and shedding dynamics were investigated by comparing the thermal and isothermal calculations. The cavitation dynamics of two typical thermal cavitation modes (the inertial mode and the thermal mode) were analyzed based on the temporal-spatial behaviors, the Fast Fourier Transformation (FFT), the Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) methods. The results show that: (1) The thermal effects could enhance the magnitude of the vorticity gradient inside the cavity. The detachment of the small-scale cavity is affected by the great vorticity gradient under the thermal condition. (2) For the inertial mode, the cloud cavity detaches due to the re-entrant jet and soon collapses, while for the thermal mode, the small-scale cloud cavity collapses slowly and the co-existing cloud cavities are observed. Three kinds of characteristic frequency for the inertial mode and thermal mode are obtained based on the temporal-spatial distribution of vapor volume fraction and the FFT analysis of the cavity area. The FFT power of the characteristic frequency for the single attached cavity process is not significant. This is because the cavity area processed by FFT cannot distinguish the attached cavity and the shedding cloud cavity. The characteristic frequencies in the thermal mode are much larger than those in the inertial mode. (3) The coherent structures of POD and DMD for the inertial mode and the thermal mode consist of the counter-rotating structures. The first four POD modes contribute more than 70% of the total energy. The first three modes with high energy obtained by the POD method have low frequencies. The locations of POD Mode 1 with the most energetic structure for two thermal cavitation modes are significantly different. The DMD algorithm can consistently extract the characteristic frequency for the single attached cavity process. The DMD modes with high frequency are characterized by the fragmented counter-rotating structures due to the co-existing shedding of cloud cavities, especially for the thermal mode. The obtained results between the inertial mode and the thermal mode including cavitation dynamics, frequency characteristics and corresponding coherent structures provide a reference for better understanding the cavitating flows in practical engineering applications.

Control of unsteady partial cavitation and cloud cavitation in marine engineering and hydraulic systems

Cavitation is a process of liquid evaporation, bubble or vapor sheet formation, and further collapse of vapor structures, which plays a destructive role in many industrial applications. In marine transport and hydraulic machinery, cavitation usually occurs nearby the surface of a ship propeller and rudder, impeller blades in a pump, and distributor vanes and runner blades in a hydroturbine and causes various undesirable effects such as vibrations of frameworks and/or moving parts, material erosion, and noise enhancement. Based on an extensive literature review, this research is aimed at an experimental investigation of a passive approach to control cavitation on a benchmark hydrofoil using a wedge-type vortex generator in different flow regimes with a high Reynolds number. In this study, we employed a high-speed imaging method to explore the spatial patterns and time evolutions of cavitation structures and utilized a hydroacoustic pressure transducer to record and analyze local pressure pulsations due to the collapse of the cavities in the hydrofoil wake region. The results show that the examined control technique is quite effective and capable of hindering the formation of cloud cavities and reducing the amplitude of pressure pulsations associated with unsteady cavitation dynamics. This study provides important experimental information, which can be useful for improving industrial technologies and for promoting new developments in this particular research field.

Spatio-temporal dynamics of flow separation induced by a forward-facing step submerged in a thick turbulent boundary layer

The effects of large-scale motion (LSM) on the spatio-temporal dynamics of separated shear layers induced by a forward-facing step submerged in a thick turbulent boundary layer (TBL) are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the free-stream velocity and step height was 13 200. The oncoming TBL was developed over a cube-roughened surface and the thickness was 6.5 times the step height. The step height was chosen to coincide with the elevation where the dominant frequency of streamwise fluctuating velocity in the TBL occurred. At this elevation, the local turbulence intensity was 14.5 %. Distinct regions of elevated Reynolds stresses were observed upstream and downstream of the leading edge of the step. The unsteady dynamics of the separation bubbles upstream and downstream of the step was investigated using the reverse flow area. Both separation bubbles exhibit low-frequency flapping motion, and the dominant frequency of the downstream separation bubble is identical to the dominant frequency of the streamwise fluctuating velocity in the oncoming TBL at the step height. As the low-velocity region of LSM passes over the step, the downstream separation bubble is enlarged and subsequently undergoes a high-frequency oscillation. Turbulence motions were partitioned into low-, medium- and high-frequency regimes based on spectral analysis of the Reynolds stresses. The contributions from these partitioned turbulence motions are used to elucidate the effects of LSM on the elevated Reynolds stresses in the shear layers upstream and downstream of the step.

Scaling of cylinder-generated shock-wave/turbulent boundary-layer interactions

Scaling parameters of shock-wave/turbulent boundary-layer interactions generated by a semi-infinite standing cylinder were explored in a combined numerical and experimental effort, consisting of Reynolds-averaged Navier–Stokes simulations and high-speed schlieren imaging. The primary interaction variable, the cylinder diameter (d), and a secondary interaction variable, the boundary-layer thickness ($$\delta $$), were varied to study the effects of the parameter $$d/\delta $$. This was found to be an appropriate scaling parameter for mean features. The characteristic interaction variables (the maximum separation distance, S, and the triple-point height, $$h_\mathrm{tp}$$) followed a linear trend when normalized as $$S/\delta $$ and $$h_\mathrm{tp}/\delta $$; however, when the boundary layer became larger than the cylinder diameter, a power law trend became more representative. The parameter $$d/\delta $$ also determined the role that viscous effects have on the strength of the interaction, where a lower $$d/\delta $$ was characterized by a greater interaction scale and lower surface pressure peaks. Moreover, the high surface pressure on the cylinder leading edge due to the Edney interaction was found to be reduced for $$d/\delta \le 0.45$$, as the boundary layer encompassed the lambda-shock structure. Trends in the shapes or peaks of the auto-spectral density function were not observed based on $$d/\delta $$, but appeared to be dominated by broadband low-frequency ($$f < 1~\hbox {kHz}$$) content. While the position and structure of the interaction may change as a result of varying $$d/\delta $$, the effects on the unsteady dynamics were minimal.

Active hydrodynamic imaging of a rigid spherical particle

A body with mechanical sensors may remotely detect particles suspended in the surrounding fluid via controlled agitation. Here we propose a sensory mode that relies on generating unsteady flow and sensing particle-induced distortions in the flow field. We demonstrate the basic physical principle in a simple analytical model, which consists of a small spherical particle at some distance from a plate undergoing impulsive or oscillatory motion. The model shows that changes in pressure or shear on the plate can be used to infer the location and size of the sphere. The key ingredient is to produce strong shear or strain around the sphere, which requires careful tuning of the viscous boundary layer on the moving plate. This elucidates how some organisms and devices may control their unsteady dynamics to enhance their range of perception.

Dynamic bridging modeling for coarse grained simulations of shock driven turbulent mixing

We focus on simulating the consequences of material interpenetration and mixing arising from perturbations at shocked material interfaces, as vorticity is introduced by the impulsive loading of shock waves, e.g., as in Inertial Confinement Fusion (ICF) capsule implosions. The flow physics is driven by flow instabilities such as Richtmyer-Meshkov, Kelvin-Helmholtz, Rayleigh-Taylor, and vortex stretching; it is capturable with both, classical large-eddy simulation (LES) and implicit LES (ILES) – where small-scale flow dynamics is presumed enslaved to the dynamics of the largest scales. Beyond the complex multiscale resolution issues of shocks and variable density turbulence, we must address the difficult problem of predicting flow transitions promoted by energy deposited at the material interfacial layers during the shock interface interactions. Transition involves unsteady large-scale coherent-structure dynamics resolvable by the coarse grained simulation but not by Reynolds-Averaged Navier-Stokes (RANS) modeling based on equilibrium turbulence assumptions and single-point-closures. We describe a dynamic blended hybrid RANS/LES bridging strategy for applications involving variable-density turbulent mixing applications. We report progress testing implementation of our proposed computational paradigm for relevant canonical problems, in the context of LANL’s xRAGE Eulerian hydrodynamics and BHR unsteady RANS code. Proof-of-concept cases include the Taylor-Green vortex – prototyping transition to turbulence, and a shock tube experiment – prototyping shock-driven turbulent mixing.

Experimental study on combustion stability characteristics in liquid-fueled gas turbine model combustor: Fuel sensitivities and flame/flow dynamics

In this paper, combustion stability characteristics of a laboratory-scaled lean premixed prevaporized gas turbine model combustor operated with different liquid fuels were experimentally examined. Test fuels involved one linear alkane (n-dodecane), one branched alkane (iso-octane), one cyclic alkane (methylcyclohexane (MCH)) and practical multi-component RP-3 jet fuel. Tests conducted under a wide range of equivalence ratios (∅) and inlet air velocities suggested that n-dodecane flame featured the smallest (∅=0.34) lean blow out (LBO) limit, while iso-octane flame was characterized by the highest (∅=0.39). On the other hand, n-dodecane flame shifted from stable to unstable combustion at the highest equivalence ratio (∅=0.52), which is ~18% higher than the lowest case RP-3 (∅=0.44). Additional flame and flow dynamics were characterized by simultaneous high-speed OH* chemiluminescence (CL) imaging and planar particle image velocimetry (PIV) measurements. Results suggested that the instability frequency and amplitude were dependent on the fuel type and could be well related to their fundamental combustion properties. Moreover, both phase-averaged and time-resolved OH* CL imaging and PIV results demonstrated that the global unsteady flame and flow dynamics were similar for different fuels. However, a time-lag behavior was observed when iso-octane and MCH flames were subject to similar acoustic pressure oscillations in the combustor. This was speculated to be dominantly caused by their distinct ignition delay times (IDTs), which resulted in different phase lags between heat release rate and acoustic pressure oscillations. Such findings were further supported by proper orthogonal decomposition (POD) analysis performed on the simultaneously recorded OH* CL and PIV measurements.

Large-eddy simulation study of unsteady wake dynamics and geometric sensitivity on a skewed bump

Large-eddy simulations of a skewed bump are performed with the bump at angles of and with respect to the free stream. The wake has a large separation bubble and unsteady vortex structures that are highly sensitive to the bump angle. The bump has secondary separation with forward-facing flow in the main separation bubble. The Reynolds stress suggests large-scale motions in the wake, but are insufficient to understand the shedding dynamics. The quasi-periodic shedding cycle is reconstructed using spectral proper orthogonal decomposition. For the case, opposite-sign streamwise vortices are alternately advected downstream in the wake, but only a single vortex is distinguishable in the case. The wake streamwise velocity pulses twice each cycle in the bump, but once per cycle in the bump. The separation bubble and wake oscillate in the spanwise direction. Conditional averaging shows that the vortices are associated with low streamwise velocity and high turbulence. Surface pressures on the bump have negatively skewed distributions in regions on the upstream face and in the separation bubble. In the separation bubble, fluctuations associated with the shedding cycle contribute to the negative skewness.

Features of heat and mechanical characteristics of pulsating flows in gas-air paths of piston engines with turbocharging

This article provides a comparative analysis of unsteady gas dynamics and instantaneous local heat transfer of pulsating flows in the intake and exhaust systems of reciprocating internal combustion engines in the case of a turbo-compressor installed without it and based on the results of experimental studies. Experimental studies were carried out on full-scale laboratory stands under the conditions of gas-dynamic nonstationarity. The article provides an original method for determining the instantaneous values of the local heat transfer coefficient in pipes, and describes the procedure for conducting experiments. It has been established that the presence of a turbo compressor in the gas-air system of a piston engine leads to significant differences in the patterns of changes in the gas-dynamic and heat exchange characteristics of pulsating flows. The obtained new data can be used to improve engineering methods for calculating the quality indicators of gas exchange processes, to refine the working processes of the engine when installing a turbocharger, as well as to develop advanced gas-air ICE systems with turbocharging.

Publisher’s Note: “On the highly unsteady dynamics of multiple thermal buoyant jets in cross flows” [Phys. Fluids 31, 115105 (2019)

Publisher’s Note: “On the highly unsteady dynamics of multiple thermal buoyant jets in cross flows” [Phys. Fluids 31, 115105 (2019)