Unsteady Transition Research Articles

STUDY ON THE INFLUENCE OF RATING CHARGE ON TEMPERATURE AND PHASE BEHAVIOR OF STABILIZED MATRIX WAX-BASED THERMAL STORAGE

The rating charge characteristic is crucial for the heat energy storage, particularly for low-temperature system which uses paraffin wax. The influence of different rating charge causes severe temperature fluctuation, which is undesirable for the heat system. It is affected by unstable phase transformation, reducing the heat distribution and makes the average rating charge insufficient. In this work, the fluctuation can be minimized by adding polyethylene plastic as phase stabilizer for the wax. The polyethylene is added by ratio of weight of 15% from the total mass of 134 grams. Each material is mixed in the liquid state, stirred and molded into the apparatus. The phase transition evaluation is performed, showing the mixture indicates an additional transition peak after the principal phase transformation, ranging between 15.26 °C – 19.34 °C (melting) and 11.98 °C – 9.45 °C (freezing). The steady operation makes the lowest rating variation is only 0.3 °C/min, which is obtained by the mixture of wax and polyethylene. In the same rating charge, the wax has rating variation of 2.9 °C/min, signifying the effect of unsteady melting transition. It clearly demonstrates the addition of polyethylene for the mixture promotes a better phase transformation under various rating charge, maintaining the effective temperature distribution for the heat storage.

Transition of Edney shock–shock interactions due to the whipping phenomenon of liquid jet in supersonic crossflow

In this paper, we experimentally study the unsteady dynamics of shock–shock interaction between the bow shock generated by a liquid jet in supersonic crossflow (LJISC) and an oblique shock. Images of shock–shock interactions were captured using high-speed focusing schlieren. Due to the whipping nature of the liquid jet, a coupling happens between the instantaneous bow shock shape and violent oscillations of the liquid jet. Proper orthogonal decomposition reveals that the dominant coherent structures of LJISC are convective and flapping modes, and these modes are responsible for unsteady variation in the local bow shock angle. An oblique shock emanating from a wedge is made to interact with the oscillating bow shock of a liquid jet near the sonic line. At this shock interaction location and for a constant momentum flux ratio between liquid jet and crossflow, unsteady transitions between the types of Edney shock–shock interactions were observed. The types of Edney shock–shock interactions that can occur depend on the local average bow shock angle and the momentum flux ratio. Support vector machine (SVM) model was used to classify three types of Edney shock–shock interactions based on ten features related to the nearest knee point, shock interaction point, and maximum penetration height. Using the SVM model, three dominant features that affect the type of shock–shock interaction were identified. Experimental results, when compared with shock polar, reveal some short-duration abnormal presence of overall regular interaction instead of overall Mach interaction regime.

Improvements in the Prediction of Steady and Unsteady Transition and Mixing in Low-Pressure Turbines by Means of Machine-Learnt Closures

Abstract In this article, novel machine-learnt transition and turbulence models are applied to the prediction of boundary-layer transition and wake mixing in a low-pressure turbine (LPT) cascade, including unsteady inflow cases with incoming wakes. A laminar kinetic energy (LKE) transport approach is employed as baseline transition framework. The application of the machine-learnt explicit algebraic Reynolds stress models (EARSMs) takes advantage of a zonal strategy based on a newly developed sensing function that allows automated wake demarcation. This work compares the performance of several approaches that are based on the application of improved transition models without machine learnt EARSMs, baseline transition model with EARSMs trained for improved wake mixing, and new transition and turbulence closures that are simultaneously developed in a fully coupled way and aimed at improving both transition and wake mixing predictions in LPTs. The investigation is carried out on a cascade of industrial footprint, representative of modern LPT bladings. First, the various modeling frameworks are evaluated, without bar wakes (steady conditions), over a wide range of Reynolds number values. Unsteady Reynolds-averaged Navier–Stokes (URANS) analyses have also been carried out to investigate the predictive capabilities of machine-learnt closures in the presence of incoming bar wakes. Reynolds-averaged Navier–Stokes (RANS)/URANS results are scrutinized against large eddy simulation (LES) calculations and experimental data. It will be demonstrated that machine-learnt EARSMs are capable of producing realistic wake mixing when simply applied on top of the baseline transition models. However, the most accurate predictions will be shown to be provided by fully integrated machine-learnt transition/turbulence closures.

Weakly nonlinear bifurcation behaviour at the onset of instability in horizontal convection using an advective buoyancy approximation

Horizontal free convection is studied under an extended buoyancy approximation constructed to capture centrifugal effects in strongly rotating regions within buoyancy driven flows. Under this approximation, the Gay-Lussac parameter (Ga) arises in the momentum equation to characterise non-Boussinesq behaviour. The scaling of average Nusselt numbers (Nuavg) are determined, as is its dependence on Ga. Increasing Ga is found to decrease Nuavg when convection dominates the flow, while no influence is detected in the low Rayleigh number regime dominated by conduction. The influence of Ga on the onset of the time-dependent regime is investigated and the critical Rayleigh number corresponding to different Ga are determined. Results demonstrate that increasing Ga stabilises the flow, delaying transition to the time-dependent regime. An Orr–Sommerfeld type stability analysis is performed to determine the local stability at different horizontal stations. A zone of convective instability is first detected, growing from the hot end of the enclosure, at Rayleigh numbers three orders of magnitude lower than the predicted global stability threshold at the maximum investigated Ga value. Ga is found to play a key role in determining the preferred orientation of instability roll structures, with increasing Ga being accompanied by a bias toward transverse-roll instabilities over longitudinal rolls. The Stuart—Landau model is used to reveal that the non-linear characteristics of this unsteady transition are consistent with a supercritical Hopf bifurcation.

Direct Numerical Simulation of a High-Pressure Turbine Stage: Unsteady Boundary Layer Transition and the Resulting Flow Structures

Abstract In the present study, we investigate the unsteady boundary layer transition based on the direct numerical simulation database of a high-pressure turbine (HPT) stage (Zhao and Sandberg, 2021, High-Fidelity Simulations of a High-Pressure Turbine Stage: Effects of Reynolds Number and Inlet Turbulence, ASME Turbo Expo 2021, Paper No. GT2021-58995), focusing on the transition mechanisms on the rotor blade, affected by the incoming periodic wakes and the background freestream turbulence (FST). On the basis of the fully resolved flow fields, we provide detailed analysis of the flow structures responsible for the transition, and two distinctive transition paths have been identified. The first path is the typical bypass transition via the instability of Klebanoff streaks, which happens when the transition region is not directly affected by the wake. The suction-side boundary layer is disturbed at the leading edge, resulting in the formation of streamwise streaks. These streaky structures endure varicose instability in the region with adverse pressure gradient (APG), then quickly break down into turbulent spots, which then evolve into fully turbulent flow. The other transition path is a consequence of the direct interaction between the wake structures and the blade boundary layer, when the wake apex starts to affect the transitional region. To be specific, the wake structures directly interact with the separation bubble in the APG region, causing sudden breakdown into turbulence. A calmed region is found to follow the wake-induced turbulent boundary layer. It is observed that the recovery to a calmed region can be impacted by the FST, as the calmed region in case with no FST is much longer compared to cases with stronger FST.

An investigation on the laminar flame propagation and auto-ignition characteristics of a coal-derived rocket kerosene-Part I: Experimental study

The energy structure of coal-rich and oil-poor in China, makes the conversion of coal to liquid fuels an attractive option to reduce petroleum dependence. In this study, the combustion characteristics of a newly developed coal-derived rocket kerosene were investigated to provide fundamental parameters for evaluating the feasibility and compatibility of the new fuel in potential applications. The laminar flame propagation properties of the fuel were studied at pressures of 1, 2 and 5 bar over a temperature range of 423–483 K for equivalence ratio of 0.7–1.8 in a constant volume combustion bomb, and the auto-ignition characteristics of the target fuel were then experimentally studied in a heated shock tube. Flame propagation process was studied based on the recorded flame morphology firstly. The unsteady transition from spark assisted ignition kernel propagation to normal flame was analyzed and was found to become less conspicuous in flames with stronger reaction intensity (higher initial pressure or temperature) and smaller Lewis number (higher equivalence ratio). The Markstein length was extracted by extrapolating the flame trajectories and used to investigate the flame instability. The results shows that the increase of pressure, equivalence ratio enhance the flame instability, but the impact of initial temperature shows no prominent pattern. The laminar burning velocity and ignition delay times were reported over a wide range to strengthen the understanding of combustion characteristics of the new fuel. A kinetic model was proposed to simulate the combustion chemistry of the fuel and kinetic analysis was performed to identify the key reactions driving the flame propagation and auto-ignition. The present study is hoped to provide insights into the evaluation of fuel application in the future and database support for further combustion reaction kinetic mechanism development.

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.

Effect of size and spacing on the wake characteristics of two spheres placed in tandem

In the present study, the flow past two spheres placed in a tandem arrangement is investigated numerically using open source field operation and manipulation (OpenFOAM) at a fixed Reynolds number (Re) of 300, where the Reynolds number is defined based on the diameter (D) of the downstream sphere (DS) and the freestream velocity at the inlet. Keeping size of the DS constant, the diameter of the upstream sphere (US) has been varied, so that the diameter ratio, DR (ratio of the diameters upstream and downstream spheres), takes the values of 0.4, 0.6, 0.8, 1.0, and 1.5. The spacing between the spheres (S) is also varied from 1D to 5D. Iso-Q surfaces show that both upstream and downstream wakes undergo various transitions with changes in the values of DR and S. For US, transition from steady symmetric to planar symmetric occurs at DR = 0.6 and S = 1, which corresponds to a local Re of 180. For DR = 0.8, steady to unsteady transition occurs at S = 2, whereas for all other values of S, the wake remains steady. For DR > 0.8 and S > 1, both upstream and downstream wakes are found to be unsteady. Hilbert analysis revealed that unsteady wakes are periodic for DR values of 0.4, 0.8, and 1.0 and are aperiodic for DR = 1.5. In addition, the extent of wake nonlinearity has also been quantified in terms of degree of stationarity. The drag force on both the spheres increases with an increase in spacing between the spheres.

A new hypothesis of steady-state flank wear progression of cutting tools with similar geometry and its verification

A new hypothesis is proposed and verified in this study to clarify the relationship between the progression and the mechanism of the flank wear of cutting tools in the steady-state region, assuming that the flank wear part expands while maintaining a geometrically similar profile. Based on Preston's law, the proposed hypothesis analytically leads to the conclusions that in the steady-state wear region where the cutting temperature is low and the abrasive wear is dominant, the flank wear width increases linearly, and the normal stress on the flank wear part does not change. Furthermore, this hypothesis suggests that the nonlinear and faster wear progression in the initial wear region is caused by an unsteady transition in the flank wear profile. Cutting experiments were conducted using tungsten carbide tools and carbon steel workpieces. The results demonstrated that the flank wear part expands with a certain similarity profile in the steady-state wear region. The normal stress distribution on the flank wear part was estimated from the similarity profile based on the proposed hypothesis, and the result was in rough agreement with that calculated by the finite element method in the literature.

Large eddy simulations of periodic wake effects on boundary-layer transition of low-pressure turbine cascades

The periodic wake effect is one of the most important sources of unsteady disturbance in turbines. Its influence on the boundary layer transition process of the downstream blade suction surface is an important factor determining the turbine loss and aerodynamic performance, and also an effective potential approach of turbine loss control. In this paper, the high-load low-pressure turbine (LPT) cascade is taken as the research object, and the large eddy simulation based on the inhouse coed Multiblock Parallel Large-eddy Simulation is used to study the periodic influence of upstream wake. The unsteady transition process of the boundary layer on the suction surface of the turbine cascade and the spatial–temporal evolution of the vortex are discussed in detail. It is shown that there are three modes of boundary layer transition on the suction surface of the LPT cascade under the effect of wake, occurring alternately during the wake passing period. Each mode of transition has different characteristics in vortex structures, as well as boundary-layer separation and reattachment, thereby makes different losses. Although the transition mechanism and evolution process of the three modes are different, the calming regions exist in all three modes, which is important for the control of the boundary layer. This study gives an important reference for reducing the flow loss in high-load turbines by means of periodic wakes.

Numerical Investigations on Unsteady Features of Rounded Trailing Edge Airfoils

As a practical rounded trailing edge airfoil for coaxial rotors, DBLN-526 is a fore and aft symmetrical airfoil with two steps on its lower side. This airfoil has been used at the inboard section of the coaxial rotor system. As there are always two eddies behind the airfoil because of its rounded trailing edge, the interaction between the separation and transition should be considered. Thus, the unsteady Reynolds-averaged Navier-Stokes- (RANS-) based γ − Re ¯ θ t model was used to analyze the unsteady transition and separation features. Three different rounded trailing-edge airfoils were compared with DBLN-526. Power spectrum density analysis and Δ Cl calculations demonstrated that the lift coefficient fluctuation of DBLN-526 was smaller than that of other rounded airfoils with different angles. Further investigation indicated that the locations of transition for DBLN-526 can be fixed at a wide range of angles by the unique design on its lower side. Because of this settled transition location, the size of separation is decreased, and the position of separation is settled as well, which leads to a lower lift coefficient fluctuation. The turbulent kinetic energy after the transition was higher, which injected a lot of energy into the boundary layer, and the separation zone near the transition position was relatively smaller. This study provides an indication for controlling separation and reducing unsteady fluctuations for rounded trailing edge airfoils.

Unsteady Flow Transition Effects on a Pitching Transonic Laminar Airfoil

This paper presents a computational study of unsteady transonic effects on a pitching supercritical laminar airfoil using high-order implicit large-eddy simulation. A CAST10-2 airfoil operating at a chord-based Reynolds number of , a transonic freestream Mach number of , and a small nominal angle of attack of deg was subjected to forced pitching oscillations about the quarter-chord with a small amplitude of deg and reduced frequencies of , 0.23, and 0.285. Complex interactions between flow separation, the motion of multiple shocks, and excursions of transitional flow were elucidated and compared against inviscid simulations. The boundary layer was found to be highly sensitive to pitch frequency, due in large part to the effects of frequency on shock motion. was found to be indirectly affected by the impact of shock motion on the boundary-layer state. Aerodynamic damping diminished with increasing oscillation frequency until becoming negative at , indicating potential for aeroelastic instability.

Stability criteria of Samara-like decelerator in unsteady transitions

AbstractThe physics behind the transitions of natural Samaras, or the bio-inspired counterparts, to steady autorotation has been unclear. Theoretical and experimental investigations explore the inertial and aerodynamic characteristics required to guarantee stable transitions of an artificial Samara-like decelerator from chaotic tumbling motions to azimuthal autorotation. A non-dimensional inertial criterion is proposed, which is in accord with experiments.

The unsteady wake transition behind a wall-mounted large-depth-ratio prism

The wake of a long rectangular wall-mounted prism is investigated at Reynolds numbers of $Re=250\unicode{x2013}1200$ by directly solving the Navier–Stokes equations. The aim of this study was to examine the unsteady transition mechanism in the wake of a large-depth-ratio (streamwise length to width) prism as well as to characterize the unsteady wake evolution at low Reynolds numbers. The results highlighted that increasing Reynolds number significantly altered the dominance of upwash and downwash flows in the time-averaged flow and changed the characteristics of coherent structures, including their size, dominant frequency and interaction with other structures in the flow. The wake is, therefore, categorized into three regimes within the transition process: steady regime at $Re \leq 625$ , regular unsteady regime at $625 < Re < 750$ and irregular unsteady regime at $Re \geq 750$ . Particularly, the wake started to exhibit unsteady features at $Re=625\unicode{x2013}650$ , which transitioned to an early irregular unsteady wake topology at $Re=750$ . At $Re \geq 675$ , horseshoe vortices transformed to vortex loops. There were hairpin-like structures formed on the upper and side faces of the long prism. The results further indicated that the transition to unsteadiness is attributed to separated leading-edge shear-layer instabilities and interactions of the shear layer with the horseshoe structures. The wake was more complex due to the interactions of multiple coherent structures in the flow, which resulted in a multiple-periodicity wake system. A skeleton model is proposed for a large-depth-ratio prism, to incorporate details of the unsteady flow features and specify various flow coherent structures at low Reynolds numbers.

Unsteady Flow Transition Effects on a Transonic Laminar Airfoil

This paper presents a computational study of unsteady transonic effects on a supercritical laminar airfoil using high-order implicit large-eddy simulation. The unsteady transitional flow behavior on a static CAST10-2 airfoil operating at a chord-based Reynolds number of , a transonic freestream Mach number of , and several small angles of attack (0 deg ) was examined alongside corresponding inviscid simulations. Shock- and boundary-layer behaviors were found to be rather sensitive, producing significant change in shock structure, flow transition location, and laminar separation bubbles over this small range of . One intermediate case exhibited a buffeting phenomenon in which flow transition alternated between two positions, causing pronounced oscillations in the pitching moment.

Drag, lift, and torque coefficients for various geometrical configurations of elliptic cylinder under Stokes to laminar flow regimes

In this study, the drag, lift, and torque coefficients are derived as a function of the axis ratio (AR) and angle of attack (AOA) for elliptic cylinders with simple and straightforward correlations in flow regimes ranging from Stokes to laminar flow. An immersed boundary method, based on an implicit direct forcing scheme to impose a more accurate no-slip condition for viscous flow, is utilized to compute the hydrodynamic forces on the elliptic cylinder. Numerical simulations are performed for two-dimensional flow around an elliptic cylinder. The ARs and AOAs of the elliptic cylinder are within 0.3–1.0 and 0°–90°, respectively. A critical Reynolds number (Recrt) map is obtained, indicating a transition between steady and unsteady flows as a function of the AR and AOA. Based on Recrt for the circular cylinder, it is observed that a more prolonged elliptic cylinder delays Recrt to a higher Re value at low AOAs but causes an unsteady transition at a lower Re value when the AOA exceeds ∼30°. The correlations are selected as simple functions, such as power laws and trigonometric functions, based on the hydrodynamic force behaviors according to variations in the AR, AOA, and Re. The prediction accuracy of the proposed correlations assessed in terms of mean relative errors is ∼1.5%, 8.9%, and 11.2% for drag, lift, and torque, respectively. This comparison demonstrates that the proposed correlations are suitable for accurately predicting hydrodynamic forces in Stokes to laminar flow regimes, even when using simple basic forms.

Wake transitions and steady -instability of an Ahmed body in varying flow conditions

The paper investigates experimentally the flow past a flat-back, taller than wide Ahmed body having rectangular base aspect ratio $H/W=1.11$ in the context of ground vehicle aerodynamics. Parametric studies at Reynolds number $2.1\times 10^5$ explore the sensitivity of the aerodynamic force and body pressure distribution with respect to varying flow conditions defined from variable ground clearance $C$ (taken at mid-distance from the front and rear axles of the body), pitch angle $\alpha$ , and yaw angle $\beta$ (equivalent to a crosswind). Two-dimensional parametric maps in the parametric spaces $(\beta,C)$ and $(\beta,\alpha )$ are obtained for parameter ranges covering real road vehicle driving conditions. The study of the base pressure gradient reveals non-trivial and sharp transitions identified as significant changes of near wake orientation in both parametric spaces. All unsteady transitions correspond to fluctuation crises of the vertical gradient component only. These transitions are summarized in phase diagram representations. Both phase diagrams in $(\beta,C)$ and $(\beta,\alpha )$ parametric spaces can be unified at large yaw in the $(\beta,C_r)$ space, where the rear clearance $C_r$ separating the lower edge of the base from the ground is a simple function of the pitch $\alpha$ and the clearance $C$ . The impacts of the wake transitions are clearly identified in the base drag, drag, lift and side force coefficients. The contribution of the steady near wake $z$ -instability (Grandemange et al., Phys. Fluids, vol. 25, 2013a, pp. 95–103) is assessed by repeating the sensitivity analysis with a rear cavity. As reported previously, the rear cavity suppresses the steady instability by symmetrizing the wake. A domain for the existence of the instability is finally proposed in the attitudes map $(\beta,\alpha )$ defined from regions where the mean lateral force coefficients are significantly decreased by the presence of the rear cavity. In addition, it is found that the steady instability forces the wake to be less unsteady for all attitudes that do not correspond to unsteady transitions.

Surface Roughness Effects on Unsteady Transition Property over a Pitching Airfoil

Surface Roughness Effects on Unsteady Transition Property over a Pitching Airfoil

A Cobalt Enrichment Strategy for Suppressing the 4.2 V Adverse Phase Transition in Ni-Rich Layered Materials

In this study, a Co-rich Ni-rich layered material with a core–shell structure is designed, in which LiNi0.82Co0.12Mn0.06O2 (NCM-Ni82) is used as the core wrapped in the shell by doping Al into LiNi0.735Co0.15Mn0.1Al0.015O2 to form the hybrid particle LiNi0.795Co0.13Mn0.07Al0.005O2 (NCM-HA). NCM-HA is divided modularly into the core part NCM-Ni82 and the single hybrid part without doped Al (NCM-HS), and then all modules were compared with the pristine LiNi0.8Co0.1Mn0.1O2 via various characterization methods to reveal the superiority of the design. The core–shell structure, which prevents the diffusion of microcracks caused by the lattice shrinkage of a high content of cobalt, is used to improve the morphological strength of the material so that the cathode material is capable of fully playing the excellent stable cycling performance brought by the remarkable cationic order degree of Co-rich treatment. The excellent cathode material NCM-HA still has a capacity retention rate of 83.35% after 200 cycles, while the pristine material has a rate of 55.42%. Moreover, NCM-HA successfully inhibits the unsteady phase transition of layered materials at 4.2 V and reduces the degree of polarization during the cycling process. This study provides a new strategy for the modification of Cobalt-enriched Ni-rich layered materials.

A complete analytical solution of unsteady coagulation equations and transition between the intermediate and concluding stages of a phase transformation

A complete analytical solution of unsteady coagulation equations and transition between the intermediate and concluding stages of a phase transformation