Unsteady Modes Research Articles

Modelling finger propagation in elasto-rigid channels

Abstract

Heat transfer and fluid flow of helium coolant in a model of the core zone of a pebble-bed nuclear reactor

Using the RNG k-ε model of turbulence in the approximation for porous media, a numerical study of fluid flow and heat transfer in a model of the core zone of a pebble-bed nuclear reactor was performed. Validations of the results of simulations against experimental data (for the friction factor) and analytical solutions (for the velocity profiles) confirmed applicability of the RNG k-ε model of turbulence for the stated problem. An unsteady mode of operation with a decrease in the coolant mass flowrate at the inlet to the reactor core zone model, as well as a steady-state mode were studied. The effect of the pebble bed permeability on the dynamics of flow temperature variation was also investigated.

Studies on unsteady mode transition of a turbine based combined cycle (TBCC) inlet with multiple movable panels

Using the overset grid method, this paper numerically studies the unsteady mode transition from the turbojet mode to the ramjet mode of a 2D TBCC inlet with multiple movable panels, for the first time. The flow field during mode transition is highly unsteady so that the steady simulation is not appropriate. Under the baseline mode transition strategy, the instability of shock waves near the throat causes the oscillation of inlet exit performance parameters. In addition, mode transitions under different turbojet back pressure are investigated, and the oscillation occurs in most cases. In order to suppress the oscillation and increase the total pressure recovery, the Unsteady Panel to Panel Optimization Method (UPPOM) is proposed to optimize the final rotation angles of movable panels. In UPPOM, the movable panels operate in a designed motion pattern, where the mode transition process is divided into two steps, namely geometry adaption and flowpath switching. After geometry adaption, the oscillation is successfully removed, and the total pressure recovery at the turbojet flowpath exit is increased by 4.24%. In the second step of optimization, a new optimization index of Weighted Total Pressure Recovery (WTPR) is employed to increase the overall performance of TBCC inlet. In the sense of the WTPR, the performance of the optimization result is increased by 5.92%.

Two-Dimensional Flood Inundation Modeling in the Godavari River Basin, India—Insights on Model Output Uncertainty

Most flood inundation models do not come with an uncertainty analysis component chiefly because of the complexity associated with model calibration. Additionally, the fact that the models are both data- and compute-intensive, and since uncertainty results from multiple sources, adds another layer of complexity for model use. In the present study, flood inundation modeling was performed in the Godavari River Basin using the Hydrologic Engineering Center—River Analysis System 2D (HEC-RAS 2D) model. The model simulations were generated for six different scenarios that resulted from combinations of different geometric, hydraulic and hydrologic conditions. Thus, the resulted simulations account for multiple sources of uncertainty. The SRTM-30 m and MERIT-90 m Digital elevation Model (DEM), two sets of Manning’s roughness coefficient (Manning’s n) and observed and estimated boundary conditions, were used to reflect geometric, hydraulic and hydrologic uncertainties, respectively. The HEC-RAS 2D model ran in an unsteady state mode for the abovementioned six scenarios for the selected three flood events that were observed in three different years, i.e., 1986, 2005 and 2015. The water surface elevation (H) was compared in all scenarios as well as with the observed values at selected locations. In addition, ‘H’ values were analyzed for two different structures of the computational model. The average correlation coefficient (r) between the observed and simulated H values is greater than 0.85, and the highest r, i.e., 0.95, was observed for the combination of MERIT-90 m DEM and optimized (obtained via trial and error) Manning’s n. The analysis shows uncertainty in the river geometry information, and the results highlight the varying role of geometric, hydraulic and hydrologic conditions in the water surface elevation estimates. In addition to the role of the abovementioned, the study recommends a systematic model calibration and river junction modeling to understand the hydrodynamics upstream and downstream of the junction.

Flow Structures on a Planar Food and Drug Administration (FDA) Nozzle at Low and Intermediate Reynolds Number

In this paper, we present a general description of the flow structures inside a two-dimensional Food and Drug Administration (FDA) nozzle. To this aim, we have performed numerical simulations using the numerical code Nek5000. The topology patters of the solution obtained, identify four different flow regimes when the flow is steady, where the symmetry of the flow breaks down. An additional case has been studied at higher Reynolds number, when the flow is unsteady, finding a vortex street distributed along the expansion pipe of the geometry. Linear stability analysis identifies the evolution of two steady and two unsteady modes. The results obtained have been connected with the changes in the topology of the flow. Finally, higher-order dynamic mode decomposition has been applied to identify the main flow structures in the unsteady flow inside the FDA nozzle. The highest-amplitude dynamic mode decomposition (DMD) modes identified by the method model the vortex street in the expansion of the geometry.

Wall pressure beneath a transitional hypersonic boundary layer over an inclined straight circular cone

Properties of wall pressure beneath a transitional hypersonic boundary layer over a 7∘ half-angle blunt cone at angle of attack 6∘ are studied by Direct Numerical Simulation. The wall pressure has two distinct frequency peaks. The low-frequency peak with f≈10−50 kHz is very likely the unsteady crossflow mode based on its convection direction, i.e. along the axial direction and towards the windward symmetry ray. High-frequency peaks are roughly proportional to the local boundary layer thickness. Along the trajectories of stationary crossflow vortices, the location of intense high-frequency wall pressure moves from the bottom of trough where the boundary layer is thin to the bottom of shoulder where the boundary layer is thick. By comparing the pressure field with that inside a high-speed transitional swept-wing boundary layer dominated by the z-type secondary crossflow mode, we found that the high-frequency signal originates from the Mack mode and evolves into the secondary crossflow instability.

Combustion of Lean Methane–air Flames in Mesoscale Reactor with Opposite Gas Flows

ABSTRACT In this work, we study a countercurrent premixed rich-lean coaxial reactor to realize an overall lean combustion of hydrocarbon fuels. Flame topology and stability of the axially symmetric reaction fronts of methane-air mixtures are investigated both numerically and experimentally. These studies show that depending on the equivalence ratio of the mixture fed into the inner tube, playing the role of supporting a pilot burner, there can occur two or three-layered stable flame structures. It is demonstrated that flames can be stabilized over a wide range of mixture compositions and flow rates. In particular, it is possible to stabilize methane-air flames with the overall equivalence ratio below the lean flammability limit for this mixture due to the existence of the weak supporting flame and combined effect of the flame stretch and heat recirculation realized in the examined configuration. The diverse unsteady combustion modes, including flame oscillations and flame periodic extinction and ignition, are also found. The prospects of further investigations are discussed.

Unsteady combustion mode with a super-high frequency induced by a curved shock

Previous studies of a high-speed blunt projectile in a combustible mixture found two oscillating unsteady combustion modes induced by the curved shock, referred to as high- and low-frequency modes. A new unsteady combustion mode is observed in the present study. The frequency reaches approximately twice the high frequency and is referred to as the super-high frequency to maintain consistency with the terminology used in previous works. The super-high-frequency mode appears in cases of a small sphere diameter, and with a proper diameter, an intermediate mode arises with the co-existence of both high and the super-high frequencies. An analysis of pressure and temperature gradients along the stagnation streamline attributes the oscillation of combustion to the interaction of compression and entropy waves between the shock and flame front. If the compression/entropy waves affect the flame front of the next cycle, the high-frequency mode arises; this is consistent with the results of previous works. However, weakened compression/entropy waves in cases of a small sphere diameter only affect the flame front of every other cycle, leading to the super-high-frequency mode.

Causes of cracks on the edges of the combustion chamber of the diesel engine piston

Internal combustion engines operate in non-stationary operating modes, when the crankshaft rotational speed changes over wide ranges, not only the torque but also the angular speed changes. While transients occurring in the cylinder of an internal combustion engine during operating modes, especially for tractor diesels, are the most unfavorable for parts of the cylinder-piston group. Long cycle changes during which pressure, temperature, piston speed and heat transfer surface are constantly changing creates very difficult loading conditions. Moreover, the peak values of thermal powers through the heat exchange surfaces of the piston bottom in unsteady modes are 2 ... 2.5 times higher than the values at steady modes. High values of thermal and mechanical loads on the piston lead to cracks on the edge of the combustion chamber and premature destruction. Failure in pistons can be caused by mechanical and thermal loading, but thermal loads make the greatest contribution to the value of equivalent stresses. Therefore, cracks gradually develop, which often leads to a deterioration of the working process, and then to the destruction of the piston.

Time-dependent solution for natural convection in a porous enclosure using the Darcy–Lapwood–Brinkman model

Natural convection (NC) in high permeable porous media is usually investigated using the Darcy–Lapwood–Brinkman model (DLB). The problem of the porous squared cavity is widely used as a common benchmark case for NC in porous media. The solutions to this problem with the DLB model are limited to steady-state conditions. In this paper, we developed a time-dependent high accurate solution based on the Fourier–Galerkin method (FG). The solution is derived considering two configurations dealing with unsteady and transient modes. The governing equations are reformulated using the stream function. The Temperature and the stream functions are expended as unknowns in space using Fourier series which are appropriately substituted in the equations. The equations are then projected to the spectral space using Fourier trigonometric trial functions. The obtained developed equations form a nonlinear differential algebraic system of equations. An appropriate technique is used to integrate the spectral system in time and to ensure high accuracy. The results of the FG method are compared to a finite element solution for different Rayleigh and Darcy numbers values. The transient and unsteady solutions are obtained with a feasible and low computational cost. The paper provides high accurate time-dependent solutions useful for benchmarking numerical models dealing with NC in porous media. The results of the developed solutions are efficient to gain physical insight into the time-dependent NC processes.

Novel flame dynamics in rich mixture of premixed propane–air in a planar microcombustor

This paper reports the first experimental observations of various novel unsteady flame propagation modes in a two-dimensional, high-aspect ratio rectangular quartz channel with a positive wall temperature gradient for rich premixed propane–air mixtures. Various flame propagation modes are observed on progressively increasing the mixture velocity, while keeping the equivalence ratio fixed: FREI (Flame with Repetitive Extinction and Ignition) mode, oscillating FREI mode, oscillating flame mode, and wavy flame mode. The FREI mode resembles the classical FREI flame propagation reported earlier in the literature. In the oscillating FREI mode, the flame front oscillates in the transverse direction between the upper and lower walls of the channel, while propagating upstream as in the classical FREI mode. A sudden peak in flame intensity is observed in this mode before its extinction at an upstream location. In the oscillating flame propagation mode, the flame front anchors itself at an axial location and exhibits periodic oscillations in the transverse direction without extinction. In the wavy flame mode the flame anchoring happens at a location close to the downstream end of the channel. The flame front exhibits visibly irregular fluctuations, while anchored at this axial location. A Fast Fourier transform analysis of the flame intensity data shows that FREI and oscillating FREI modes consist of a single dominant frequency of ∼100 Hz, whereas multiple dominant frequencies are present for oscillating and wavy flame modes. The appearance of these multiple oscillating and propagating flame modes is attributed to flame bifurcation behavior due to thermal-wall coupling in the channel.

Emission Modes in Electrospray Thrusters Operating with High Conductivity Ionic Liquids

Electrospray thruster life and mission performance are strongly influenced by grid impingement, the extent of which can be correlated with emission modes that occur at steady-state extraction voltages, and thruster command transients. Most notably, we experimentally observed skewed cone-jet emission during steady-state electrospray thruster operation, which leads to the definition of an additional grid impingement mechanism that we termed “tilted emission”. Long distance microscopy was used in conjunction with high speed videography to observe the emission site of an electrospray thruster operating with an ionic liquid propellant (EMI-Im). During steady-state thruster operation, no unsteady electrohydrodynamic emission modes were observed, though the conical meniscus exhibited steady off-axis tilt of up to 15°. Cone tilt angle was independent over a wide range of flow rates but proved strongly dependent on extraction voltage. For the geometry and propellant used, the optimal extraction voltage was near 1.6 kV. A second experiment characterized transient emission behavior by observing startup and shutdown of the thruster via flow or voltage. Three of the four possible startup and shutdown procedures transition to quiescence within ∼475 μs, with no observed unsteady modes. However, during voltage-induced thruster startup, unsteady electrohydrodynamic modes were observed.

On the Cutting of Metals: A Mechanics Viewpoint

AbstractThe mechanics of large-strain deformation in cutting of metals is discussed, primarily from viewpoint of recent developments in in situ analysis of plastic flow and microstructure characterization. It is shown that a broad range of deformation parameters can be accessed in chip formation—strains of 1–10, strain rates of 10–105/s, and temperatures up to 0.7Tm—and controlled. This range is far wider than achievable by any other single-stage, severe plastic deformation (SPD) process. The resulting extreme deformation conditions produce a rich variety of microstructures in the chip. Four principal types of chip formation—continuous, shear-localized, segmented, and mushroom-type—as elucidated first by Nakayama (1974, “The Formation of ‘Saw-Toothed Chip’ in Metal Cutting,” Proceedings of International Conference on Production Engineering, Tokyo, pp. 572–577) are utilized to emphasize the diverse plastic flow phenomena, especially unsteady deformation modes that prevail in cutting. These chip types are intimately connected with the underlying flow, each arising from a distinct mode and triggered by an instability phenomenon. The role of plastic flow instabilities such as shear banding, buckling, and fracture in mediating unsteady flow modes is expounded, along with consequences of the flow modes and chip types for the cutting. Sinuous flow is shown to be the reason why gummy (highly strain-hardening) metals, although relatively soft, are so difficult to cut. Synthesizing the various observations, a hypothesis is put forth that it is the stability of flow modes that determines the mechanics of cutting. This leads to a flow-stability phase diagram that could provide a framework for predicting chip types and process attributes.

Heat Transfer and Fluid Flow During Instantaneous Unsteady Condensation

The present study outlines a methodology for modeling instantaneous unsteady film condensation of vapor on a vertical flat wall. Both analytical and numerical solutions of the problem were obtained and compared with each other. Time variation of the thickness of the condensate film, velocity, and temperature profiles in the film as well as the friction and heat transfer coefficients were analyzed. At unsteady film condensation of vapor, a decrease in the heat transfer coefficient and simultaneous increase in the friction coefficient with time were observed. As a result, the Reynolds analogy is violated in the unsteady mode. The value of the parameter characterizing the time necessary for the unsteady film condensation of vapor to reach the steady state was obtained.

A global approach for the time-dependent solution of natural convection in a tilted porous cavity

Natural convection in an inclined cavity filled with saturated porous media has been investigated. A time-dependent solution using a spectral weighted residual method is determined. Preparing the governing equations by the substitution of the stream functions, we obtained a system of nonlinear ordinary differential equations. Integrating in time using the Runge-Kutta method of variable order the final system is determined. The study consisted of the investigation of the problem in the transient and unsteady modes. For each configuration, the solution is developed for different values of Rayleigh numbers and different inclination levels of the cavity. The high accuracy of the results with the reduced computational cost is well proved by the comparison with results determined using a finite element method that has shown an excellent agreement with our developed solution. This work is very efficient to understand from a physical insight the effect of the inclination parameters on the unsteady and transient mode of changes in the mechanisms of natural convection. The results provide a set of high-accurate data that can be used as a benchmark case for the time-dependent natural convection problems. As a result, the inclination effect of the cavity has clearly influenced the heat transfer process and the fluid flow. Therefore, the inclination can be considered as a control parameter for heat and fluid flow effectively.

An Experimental Study on Dynamics Features of Three Side-by-Side Flexible Risers Undergoing Vortex-Induced Vibrations in A Uniform Flow

A vortex-induced vibration (VIV) experiment on three side-by-side risers subjected to a uniform flow was carried out in a combined wave-current flume. The dynamic features of interference effect on three side-by-side risers were investigated by varying fluid velocity and inter-riser spacing. The distributions of dimensionless displacement, dominant frequency, and displacement trajectory of the model risers were measured using mode decomposition and wavelet transform techniques. The coupled interference of inter-riser fluid to adjacent risers at different spacings was disclosed by introducing the “interference ratio” concept. The results show that at spacings smaller than 6.0D, the three model risers display appreciable deviations in their displacement responses in cross-flow or in-line direction, attributable to the strong proximity disturbance and wake interference between the risers. When the spacing is increased to 8.0D, wake interference still makes great difference to the dynamic response of the risers in both directions. As reduced velocity increases, the three risers show higher agreement with an isolated riser in overall dominant vibration frequency in CF direction than that in IL direction at all spacings and the side risers, although symmetrically placed, do not vibrate symmetrically, as a result of the steady deflection of clearance flow within the riser group. Interference effect results in a remarkable unsteady mode competition within the risers; quantitation of the interference levels for the three risers at different spacings with interference ratio revealed that under low flow velocities and large spacing ratios, clearance flow constitutes a non-neglectable interferer for three side-by-side risers.

A single unsteady DBD plasma actuator excited by applying two high voltages simultaneously for flow control

Dielectric barrier discharge plasma actuators have attracted a lot of attention to use as new technologies for active flow control. In this paper, an experimental investigation of a single unsteady plasma actuator driven by two simultaneous sinusoidal high voltages is carried out. The aerodynamic performance of this plasma actuator is investigated for flow control of a wind turbine blade. Leading edge separation control at Reynolds number of 0.26×106 and in a wide range of angles of attack including linear, stall, and post-stall regions is considered as the test case. The momentum imparted by the plasma actuator to flow is investigated via measuring induced electric wind velocity, which represents that the induced velocities in the steady mode, with vmax≅5 m/s, are higher than that of the unsteady mode. The important aspect of exciting the unsteady dielectric barrier discharge plasma actuator in this new approach is improving its efficiency through increasing the authority of momentum addition to flow and reducing the minimum input power for discharge ignition relative to its typical grounded structure. This can be very important practically in flow control applications where the amount of consumed energy is a substantial factor in determining the actuator's efficiency. The obtained aerodynamic results reveal that the unsteady plasma actuator has the best operation in post-stall angles of attack, which is of great importance for operation characteristics of wind turbine blades. The power spectral density of pressure time series illustrates that the unsteady plasma actuator affects the flow through instabilities of the separation layer and vortex shedding structure.

Discrepancies in Flood Modelling Approaches in Transboundary River Systems: Legacy of the Past or Well-grounded Choices?

Flood modelling in transnational rivers requires efficient cross-border collaboration among the riparian countries. Currently, each country/region usually uses a different hydraulic modelling approach, which may hinder the modelling of the entire river. For the sake of accurate and consistent river modelling there is a necessity for the establishment of a framework that fosters international collaborations. This study investigates the current hydraulic modelling approach across the whole length of the River Meuse, the main course of which crosses three north-western European countries. The numerical models used by French, Belgian, and Dutch agencies and authorities were interconnected by exchanging boundary conditions at the borders. At the central part of the river, the Belgian hydraulic model assumed steady flow conditions, while the rest of the river was modelled in unsteady mode. Results for various flood scenarios revealed a distinctive pattern of water depths at the Belgian-Dutch border. To clarify whether this is a bias induced by the change in modelling approach at the border (steady vs. unsteady), we remodelled a stretch of the river across the Belgian-Dutch border using a consistent unsteady modelling approach. The steady and unsteady approaches led to similar patterns across the border, hence discarding the hypothesis of a bias resulting from a change in the employed model. Instead, the pattern in water depths was attributed to a change in the topography of the Meuse Valley, where there is a transition from a narrow steep corridor with limited water storing capacity in Ardennes massif to wide floodplains in the Dutch lowlands. The associated flood damping for the 100-year discharge is less than 1% in the Ardennes and exceeds 15% in the Dutch lowlands. It can be inferred that the current differences in regional hydraulic modelling approaches for the River Meuse are generally well-grounded and not just a legacy of the past.

STUDY OF DYNAMICS OF THE TWISTING PROCESS IN PNEUMOMECHANICAL SPINNING IN THE PRESENCE OF DOUBLE FALSE SPINNING

For the modern textile industry, one of the characteristic trends remains the desire to increase the productivity of technological machines by increasing speed, which sets the task of reducing the cost of electricity in pneumatic mechanical spinning, the successful solution of which requires systematizing the accumulated information on new methods of shaping yarn and structures of shaping and twisting devices to clarify a number of problems in the development of the yarn forming method. This paper work presents the results of a study of the dynamics of the process of torsion of a yarn of pneumomechanical spinning (rotor spinning) in the presence of double false torsion, there were constructed dynamic mathematical models of the process of yarn twisting of pneumomechanical spinning in the presence of double false twisting in unsteady start modes and stopping of the forming-twisting device.

Cutting of tantalum: Why it is so difficult and what can be done about it

Tantalum has long drawn the ire of machinist, being particularly difficult to cut. Often referred to as being ‘gummy,’ cutting of tantalum is characterized by very thick chips, large cutting forces, and a poor surface finish on the machined surface. These unfavorable attributes of the cutting have usually been attributed to bcc tantalum's high strain-hardening capacity, relative softness, and low thermal conductivity; and a small shear plane angle. Here, we show using in situ high-speed imaging at low speeds, and ex situ chip morphology observations at higher commercial cutting speeds (625 mm/s), that the gummy nature of Ta in cutting, including the large forces and thick chips, is actually due to the prevalence of a highly unsteady plastic flow – sinuous flow – characterized by large-amplitude folding and extensive redundant deformation. The sinuous flow and the associated folding are much more amplified and extreme in tantalum (chip thickness ratio, 30–50), and with different morphology, than that observed in fcc Cu and Al (chip thickness ratio, 10–20), wherein this flow mode was first uncovered. The in situ observations are reinforced by force measurements, and chip morphology and cut-surface characterization. The observations also suggest that the sinuous flow, in the same genre of unsteady mesoscale flow modes such as shear banding and segmented flow, is quite prevalent in cutting of highly strain-hardening metal alloys.By application of a surface-adsorbing (SA) medium, e.g., permanent marker ink, to the initial workpiece surface, we show that sinuous flow can be disrupted and replaced by a more energetically favorable flow mode – segmented flow – with thin chips and >70% reduction in the cutting force. This flow disruption is mediated by a local ductile-to-brittle transition in the deformation zone, due to the action of the SA medium – a mechanochemical (MC) effect in large-strain deformation of metals. Equally importantly, the MC effect and underlying segmented flow are beneficial also for machined surface quality – producing nearly an order of magnitude improvement in the surface finish, creating a surface with minimal residual plastic strain, and reducing level of material pull-out. Thus, by use of the SA medium and triggering the MC effect, a promising new opportunity is demonstrated for improving the machinability of Ta by ameliorating its gumminess. The results could enhance the viability of Ta for applications such as gun barrel liners – applications for which it has been hitherto considered but discarded due to its poor machining characteristics.