Evolution Of Flow Structures Research Articles

Spatial-Temporal evolution characteristics of pre-Stage-stall in a centrifugal compressor with a vaneless diffuser and a volute

The continuous throttling process is of great interest for compressor designers. Previous experimental studies mainly consider the temporal evolution characteristics of the pre-stage stall at the impeller inlet or outlet regions. In this study, we explore the spatial-temporal evolution characteristics of pre-stage-stall using a pressure array consisting of 15 high-response sensors. The dynamic evolution characteristics of the pressure signal on the time and frequency domains and the underlying unsteady flow structure evolution during the transition process were analyzed. We find that the evolution characteristics of pressure irregularity varied with the streamwise positions. The evolution process from the pre-stage stall point to the stage stall point can be divided into three stages based on the disturbance scale and pressure fluctuations, namely (I) the coexistence of the impeller RI, the mild impeller stall, and the mild diffuser stall, (II) the interaction of stall cell and RI disturbance, (III) a single large stall cell. In stage I, the streamwise influential regions of the stall cells are mainly confined in the impeller passage. In stage II, the stall cell expands upstream and gradually interacts with the RI disturbance. The wavelength of RI-related disturbance increases while the related pressure fluctuation decreases. In stage III, several stall cells merge into one large cell.

Evolution of unsteady secondary flow structures near the onset of stall in a tip-critical transonic axial flow compressor stage

The aerodynamic stability of an axial compressor stage depends on the rotor and stator. However, due to the specific design requirements, the evolving flow field and the resulting secondary flow structures are different in both components. This study investigates the evolution of dominant secondary flow structures occurring in the rotor and stator of a tip-critical transonic compressor stage at the near-stall condition using unsteady numerical analysis and validates performance characteristics with the experimental data. The investigation reveals that the presence of rotor tip shock creates a large difference in the pressure gradient across the pressure surface and the suction surface, intensifying tip leakage flow and shock-induced boundary layer separation. The higher incidence angle near the hub leading edge creates a local separation and reattachment zone. The radial pressure gradient causes the low momentum flow from this local separation zone to migrate radially upward. This migrated flow interacts with the tip leakage flow and separated blade boundary layer, eventually creating a colossal recirculation zone and subsequent rotor blockage of around 46%. The increasing streamwise adverse pressure gradient pushes the tip shock upstream, and at the onset of the stall, the flow directly separates from the rotor leading edge avoiding the shock formation. The stator flow field is dominated by the asymmetric hub corner separation induced by the streamwise adverse pressure gradient and the tip corner separation caused by the vortex structures convected downstream from the rotor tip region leading to stator blockage of around 48%. Along with the blade passing frequency, four other dominating frequencies (0.06×BPF, 0.12×BPF, 0.44×BPF, and 0.84×BPF) related to the aerodynamic instabilities are observed at the inception of stall.

The unsteadiness of tip leakage vortex breakdown and its role in rotating instability

The unsteadiness due to tip leakage vortex (TLV) breakdown was studied using a special experimental test campaign in parallel with numerical simulations. The back flow vortex (BFV), an isolated vortex caused by TLV spiral-type breakdown, was found to play a key role in rotating instability (RI). High-speed pressure transducers were used to measure the unsteady pressure field at the casing end wall of the blade in an isolated subsonic compressor rotor, which identified a low-frequency fluctuation at the near stall condition. A single-passage unsteady Reynolds-averaged Navier–Stokes simulation was used to study the evolution of unsteady flow structures, validated by the experimental measurements. Two distinct kinds of periodically unsteady flow were revealed by the simulations. A high-frequency fluctuation corresponding to 1.0 blade pass frequency (BPF) was caused by the spiral-type breakdown of the TLV. The other low-frequency fluctuation corresponding to 0.5BPF was caused by the feedback interaction between the BFV and the blade loading. The BFV was generated by the TLV breakdown, which was separated from the twisted vortex core of the TLV, and it moved downstream along the pressure side of the adjacent blade. A larger sized BFV reduced the local loading of the adjacent blade. The TLV was weakened as a consequence of the reduced loading, resulting in a smaller sized BFV. The blade tip loading was relatively less affected by the small sized BFV rather than the larger sized BFV. Therefore, the blade loading recovered and the size of the BFV increased, repeating the cycle. This feedback mechanism produced a pressure fluctuation with a frequency equal to 0.5BPF, which was closely related to RI.

Effect of additional periodic motion on gas-liquid two-phase secondary flow behavior in inclined pipes

Secondary flow significantly changes the flow structure and increases flow resistance. The pipeline motion further contributes to the evolution of secondary flow. This study aims to elucidate the characteristics of two-phase secondary flow in inclined pipelines under additional periodic motion conditions. Based on the CFD technique, three-dimensional oscillating two-phase flow fields were simulated using the VOF model and dynamic mesh method. The focus was on four common flow patterns observed in inclined pipes, namely stratified flow, slug flow, turbulence flow, and transitional flow. The impact mechanisms of swaying, heaving, and coupling motion on the structural evolution, velocity profile, and relative intensity of secondary flow were analyzed. The results indicate that additional periodic motion exhibits a significant dominant behavior on secondary flow when the amplitude exceeds 1.0D. Heave motion has a more pronounced effect on the secondary flow structure. Additional periodic motion induces periodic evolution of secondary flow structure in stratified flow, with the evolution period aligning with the period of the additional motion. Under additional periodic motion, there is no one-to-one correspondence between secondary flow velocity and axial velocity. Additional motion significantly suppresses the axial velocity of slug flow. Furthermore, additional periodic motion significantly increases the relative intensity of secondary flow, exceeding 10%. When the amplitude is less than 1.5D, additional motion suppresses the relative intensity of secondary flow in turbulence flow and transitional flow.

Qualitative and quantitative analysis of interaction between cavitation patterns and vortices of a pitching hydrofoil from Lagrangian viewpoint

The evolution of transient flow structures and mass transport in cavitating flow around a pitching hydrofoil is investigated qualitatively and quantitatively, and the interaction between cavitation patterns and vortices is elucidated from Lagrangian viewpoint. First, turbulence effects are estimated by the density-corrected k–ω model to account for the local compressibility of the multiphase flow at Reynolds number Re=6.4×105. Then, the formation and evolution of vorticity structures during the whole pitching cycle are analyzed using Lagrangian averaged vorticity deviation method. By comparing the flow structures and hydrodynamic properties at varying angles of attack, the cavitating flow is divided into two distinct stages, namely multi-scale cloud cavitation phase from α+=10° to α−=8°, and traveling sheet cavitation phase from α−=8° to α+=10°. Specifically in cloud cavitation, the formation of the cavitation pattern is closely related to the development of the main vortex. Furthermore, the quantitative analysis method based on Lagrangian flow network is developed to deeply analyze the transport and mixing processes. Importantly, the coherence ratio and the mixing parameter are proposed as transport indicators to precisely quantify the spatial connectivity behavior. Finally, the correlations between vapor fraction, codelength, global coherence ratio and global mixing parameter are evaluated. As the conclusion, it is shown that Lagrangian methods are powerful tool for both qualitative and quantitative analysis, and the results obtained could provide a key and important understanding of the flow structure and changing mechanism between cavitation and vortices in marine hydro and propulsion systems.

Numerical Simulation and Characterization of Swirl Fluid Motion through Cylindrical Chambers

Axisymmetric solution of radial, axial and tangential components of velocity of flow through a cylindrical chamber with an inflow containing swirl and outflowis investigated numerically using Finite Volume Method. An in-house developed computer code for solving the three components of velocity and pressure is developed and validated using experimental data available from literature on the problem of axial vortex breakdown of confined flow in a cylinder due to rotation of one of the end walls. The code uses a staggered approach and fractional step projection method for decoupling the velocity and pressure. The convection and diffusion terms are approximated using a second-order accurate central difference scheme. This paper discusses the characteristics features of flow such as mixing streams, streams of fluid flowing close to the solid walls (cooling streams), swirl motion and development of flow structures.

Reducing the aerodynamic drag of high-speed trains by air blowing from the nose part: Effect of blowing speed

To reduce the aerodynamic drag of high-speed trains, this work proposes an air blowing configuration on the head and tail cars of high-speed trains. The variation in the aerodynamic drag and slipstream velocity is analyzed under different blowing velocities, and the flow mechanism for train aerodynamic performance alteration is explained. The results show that under the blowing speeds of Ub = 0.05Ut, 0.10Ut, 0.15Ut, and 0.20Ut, where Ut is the train speed, the total drag coefficient (Cd) decreases by 5.81%, 10.78%, 13.70%, and 15.43% compared to the without-blowing case, respectively. However, with the increase in the blowing speed, the reduction trend of Cd tends to be smoother; namely, the decrement ratio compared to the previous blowing speed for the head car is 9.08%, 0.11%, 0.60%, and 1.14% for Ub = 0.05Ut, 0.10Ut, 0.15Ut, and 0.20Ut, respectively. The blowing measure generates an air gap between the coming flow and train surface, consequently causing a reduction in the viscous and pressure drag. In addition, the structure size and strength of the wake flow under different blowing cases show a decreasing trend from Ub = 0.00Ut to 0.10Ut and then an increasing trend from Ub = 0.10Ut to 0.20Ut. Thus, considering the blowing cost, efficiency, and flow structure evolution comprehensively, the case of Ub = 0.10Ut is recommended. Under this blowing speed, the reduction ratio of the aerodynamic drag is 9.18%, 12.77%, 10.90%, and 10.78% for the head, middle, tail car, and total train, respectively.

Pore-scale study of calcite dissolution during CO2-saturated brine injection for sequestration in carbonate aquifers

The calcite reaction with carbonate acid is a prerequisite to release cations providing the reactant for mineral trapping, however, the complicated multiple physicochemical processes during calcite dissolution are hardly precisely captured in carbonate aquifers, impeding the CO2 sequestration technology. In this work, a pore-scale numerical study based on the lattice Boltzmann method coupled fluid flow, mass transport, heterogeneous reactions, and calcite structure evolution is implemented to investigate the calcite dissolution process. Firstly, the impact of pressure difference on this reaction is analyzed to identify two characteristic dissolution patterns. Selecting two typical pressure differences, namely 0.036 and 0.36 Pa, the dissolution dynamics under different temperatures from 25 to 85 °C is then investigated by species concentration and solid distributions elucidating that the effect of pressure difference on calcite dissolution will drop with the increment of temperature. Moreover, the regime diagram of calcite dissolution is plotted based on the relation between temperature and pressure. Finally, to upscale the simulation results to the representative element volume scale, the temporal evolution of normalized permeability, the normalized permeability-porosity relationship and the optimal operations are explored to find that the maximum discrepancy of the normalized permeability with the time among all cases is approximately seven times and the best operating conditions promoting the mineral trapping are a high-temperature formation with a high injection rate.

Impact of Operating Conditions and Axial Casing Grooves on the Evolution of Flow Structure Across Blade Rows in an Axial Compressor

Abstract Stereo particle image velocimetry (SPIV) measurements in a series of axial planes investigate the impact of operating conditions and semicircular axial casing grooves (ACGs) on the evolution of flow structure across multiple blade rows in an axial compressor. The field of view extends radially from the hub to the tip and circumferentially over entire blade passages. Previous studies in this machine have shown that the ACGs improve the stall margin significantly but reduce the peak efficiency. At pre-stall flowrate and without ACGs, intermittent reverse axial flow near the casing is induced by backflow vortices, tip leakage vortex (TLV), and the leakage flow extend upstream of the rotor leading edge. Inside the rotor, the tip region blockage, characterized by low axial and high circumferential momentum, expands radially inward as the flow evolves axially. This extreme non-uniformity diminishes rapidly within the stator. In addition to previously shown ACGs effects, the current data reveal that the flow jetting out from the groove upstream of the rotor generates axially aligned vortices on both sides of each jet. These vortices substantially reduce the flow non-uniformity over the entire passage by entraining the faster mid-span flow into the tip region. Near the best efficiency point, the jets become weaker, the blockage is confined to the tip region, and differences between the global flow structure with and without ACGs become subtle. However, interactions of the TLV with secondary flows entrained from the grooves into the passage expand the TLV signature, which has adverse effects on the compressor performance.

Flow Structure and Short-Term Riverbed Evolution in Curved Flumes

River bending is the major effect responsible for bed topography and bank changes. In this study, fluid velocity (measured by a three-dimensional Doppler advanced point current meter) and bed topographical data have been collected in 40 sections of an experimental model. The whole flume was composed of an organic glass bend, upstream and downstream water tanks, two transition straight sections, a circulation pump, and a connection pipeline. Each section has been found to be characterized by a primary circulation and a small reverse circulation, with some sections even presenting three more or more circulation structures. The minimum circulation intensity has been detected in proximity to the top of the curved channel, while a region with small longitudinal velocity has been observed near the concave bank of each bend, corresponding to the flat bed formed after a short period of scouring. The maximum sediment deposition and scour depth in the presence of a uniform distribution of living flexible vegetation within 10 cm of the flume wall have been found to be smaller than those observed in the tests conducted without vegetation.

Experimental investigation of effects of dual-cavity configuration on ignition and flame stabilization in a kerosene-fueled supersonic combustor

The effects of two kinds of dual-cavity configuration on ignition and flame stabilization in a high Mach number (Ma) kerosene-fueled scramjet were experimentally investigated in this study. To better understand the combustion characteristics, flow structure and flame development were studied by schlieren photography, flame self-illumination photography, and wall-pressure measurement. Results were obtained with an isolator inlet Ma 3.0, total pressure 2.9 MPa, and total temperature 1900 K, which correspond to Maf 8.0 condition. The symmetrical dual-cavity configuration did not achieve kerosene auto-ignition, while the staggered dual-cavity configuration did achieve kerosene auto-ignition because shock waves were much more complex. In the staggered dual-cavity scramjet, flame stabilization was achieved after successful ignition by a lower equivalence ratio (ER) of kerosene, but when the ER of kerosene was increased to 0.8, flame stabilization was not achieved and the flow structure oscillated quasi-periodically.

Identification and dynamics prediction of a plane vortex structure based on a mathematical model of a point vortices system

The aim of the article is developing and analyse an algorithmic method for solution finding of one inverse problem of 2d vortex fluid dynamics. It is identification and prediction of the flow structure evolution of the based on the data on fluid velocity vectors in a set of reference points. Theoretical analysis of convergence and adequacy of the method is difficult due to the ill-posedness typical of inverse problems, these issues studied experimentally. Methods. The proposed method uses a mathematical model of a point vortex dynamics system for identification and prediction flow structures. The parameters of the model system are found by minimising the functional that evaluates the closeness of the original and model vectors fields at the reference points. The prediction of the vortex structure dynamics is based on the solution of the Cauchy problem for a system of ordinary differential equations with the parameters found in the first stage. Results. As a result of the calculations, we found it out: the algorithm converges to the desired minimum from a wide range of initial approximations; the algorithm converges in all cases when the identified structure consists of sufficiently distant vortices; the forecast of the development of the current gives good results with a steady flow; if the above conditions are violated, the part of successful calculations decreases, false identification and an erroneous forecast may occur; with the convergence of the method, the coordinates and circulation of the eddies of the model system are close to the characteristics of the eddies of the test configurations; the structures of the streamlines of the flows are topologically equivalent; convergence depends more on location than on the number of vectors used for identification. Conclusion. An algorithm for solving the problem of identifying and the evolution forecast of a 2d vortex flow structure is proposed when the fluid velocity vectors in a finite set of reference points are known. The method showed its high efficiency when using from 40 to 200 reference points. The results of the study make it possible to recommend the proposed algorithm for identifying flat vortex structures, which consist of vortices separated from each other.

The low frequency pressure pulsation and control of the open-jet wind tunnel

An open jet wind tunnel has low-frequency pressure pulsation in common wind speed range due to its unique structural form, which seriously damages the quality of flow field in the test section. The low-frequency pressure fluctuation performance and control mechanism of Jilin University open jet and return flow wind tunnel are investigated by experiments and numerical simulation. The results show that the low-frequency pressure fluctuation is a narrow pulse phenomenon that only occurs in certain intervals, and several velocity intervals may be found in the same wind tunnel. The reliability of the numerical simulation is verified by comparing the peak frequency and amplitude of pressure fluctuation in numerical simulation and wind tunnel tests. A simplified model similar to and amplifying the phenomenon is established. The flow structure and vortex evolution are analyzed via detached eddy simulation. In the test section, large-scale shedding vortices are formed at the nozzle exit, introducing periodic pulsating instantaneous velocity and acting with the collector to form an edge-feedback. This acoustic feedback forms resonance with the pipeline circuit, resulting in poor flow field quality. In accordance with the mechanism of nozzle jet, two methods of controlling pulsation are proposed: spoiler and flow-follow device. The study shows that the effects of two methods are abrupt, and the frequency of pressure pulsation is changed. The spoiler destroys the complete structure of vortex ring in free jet and develops into a complementary double vortex ring structure, which is highly sensitive to size factors. The flow-follow device supplements the velocity loss of the free jet at the nozzle and develops into a double vortex ring with master–slave structure in the middle of the test section. Its vibration reduction effect is greatly affected by the flow velocity. It takes effect in an appropriate range where the flow velocity is higher than the nozzle velocity. If the follow velocity is extremely low, the flow-follow device cannot change the original jet structure. If the follow velocity is extremely high, the momentum of the fan will be greatly reduced, the flow field will be unstable, and another order of pulsation may be induced. This work lays a solid foundation for further understanding the aerodynamic characteristics and optimization mechanism of open jet wind tunnel.

Turbulence suppression by streamwise-varying wall rotation in pipe flow

Direct numerical simulations of turbulent pipe flow subjected to streamwise-varying wall rotation are performed. This control method is able to achieve drag reduction and even relaminarize the flow under certain control parameters at friction Reynolds number $Re_\tau =180$ . Two control parameters, which are velocity amplitude and wavelength, are considered. It is found that increasing the wavelength rather than increasing the amplitude seems to be a better choice to improve the control efficiency. An annular boundary layer, called the spatial Stokes layer (SSL), is formed by the wall rotation. Based on the thickness of the SSL, two types of drag-reduction scenarios can be identified roughly. When the thickness is low, the SSL acts as a spacer layer, inhibiting the formation of streamwise vortices and thereby reducing the shear stress. The flow structures outside the SSL are stretched in the streamwise direction due to the increased velocity gradient. Within the SSL, the turbulence intensity diminishes dramatically. When the thickness is large, a streamwise wavy pattern of near-wall streaks is formed. The streak orientation is dominated by the mean shear-strain vector outside the viscous sublayer, and there is a phase difference between the streak orientation and local mean velocity vector. The streamwise scales of near-wall flow structures are reduced significantly, resulting in the disruption of downstream development of flow structures and hence leading to the drag reduction. Furthermore, it is found that it requires both large enough thickness of the SSL and velocity amplitude to relaminarize the turbulence. The relaminarization mechanism is that the annular SSL can absorb energy continuously from wall-normal stress due to the rotational effect, thereby the turbulence self-sustaining process cannot be maintained. For the relaminarization cases, the laminar state is stable to even extremely large perturbations, which possibly makes the laminar state the only fixed point for the whole system.

Investigation of hemocompatibility and vortical structures for a centrifugal blood pump based on large-eddy simulation

The evolution of complex flow structures has a large impact on the hemocompatibility of the centrifugal blood pump. In this study, the hemodynamic performance and the hemocompatibility of a centrifugal blood pump are investigated based on large-eddy simulation (LES). Comparisons are conducted between the LES results and the results predicted by the renormalization group (RNG) k−ε model and delayed detached eddy simulation (DDES) methods. The local trace criterion is utilized to analyze the vortical structures within the blood pump. Results show that the tip leakage vortex, the Taylor–Couette flow, and the flow separation are the most important flow structures in the blood pump. These structures have a significant influence on the hemodynamic performance and hemocompatibility. Quantitative comparison between the hemodynamic performance and the hemocompatibility is conducted between DDES, RNG k−ε, and LES results. Little difference is shown between DDES and LES results, while the RNG k−ε model tends to underestimate the pressure and hemolysis due to adopting the steady-state approach, and the assumption of isotropy and equilibrium turbulence transport. In detail, the accuracy of RANS in predicting the strength of the main vortical structures is insufficient, which tends to underestimate the leakage vortex strength and overestimate the Taylor vortex strength. Furthermore, an analysis of the relationship between hemocompatibility and vortical structures indicates that the interaction between the boundary layer and the vortical structures, such as leakage vortex and Taylor vortex, induces more blood damage, while the blood damage caused by vortical structures in the mainstream is limited.

Evolution of flow structure from a coaxial synthetic jet

• Influence of phase differences (∅) on novel coaxial synthetic jet (CSJ) is studied. • Azimuthal instability widens the jet for ∅ = 45°, and 90°. • Vortex pairing results in a strong jet for ∅ = 180°. • Entrainment ability of CSJ is high compared to single cavity synthetic jets. • Modulation of CSJ can be achieved with ∅ without investing any extra power. In this work, we propose and demonstrate a novel coaxial synthetic jet (CSJ) in which two cavities are arranged coaxially with 0° orientation angle with inner and annular orifices of equal hydraulic diameter. The flow behavior of the CSJ is studied at a low Reynolds number; Re = 135. The Navier-Stokes equations are solved (Direct Numerical Simulation) with the finite volume solver of openFoam, an open-source platform for numerical simulations. During the evolution of CSJ, two vortex rings IVR (inner vortex ring), and AVR (annular vortex ring), which are single toroid and double toroid in nature, are emanated from inner and annular orifices, respectively. Both the inner and annular jets are operated at equal mass fluxes. The evolution and interaction of the IVR, and AVR are studied by providing five-phase differences (∅ = 0°, 45°, 90°, 135°, and 180°) between the inner and outer diaphragms, and also its influence on the nature of the CSJ is highlighted. The phase-averaged and time-averaged flow fields are analyzed, and the CSJ is compared with the single cavity synthetic jet (SJ) at Re = 135, which is obtained by operating the inner cavity and annular cavity alone. The CSJ with ∅ = 0° reveals a wide and strong jet as compared to the single cavity SJ. The pairing of IVR and AVR results in a narrow and strong jet for ∅ = 180° as compared to ∅ = 0°. Merging and pulling mechanisms in case of ∅ = 45° and 90° enhances the azimuthal instability resulting in a wide and strong jet as compared to ∅ = 0°. A significant increase in the integrated jet area is observed for CSJ, which evidences the increase in entrainment ability of the CSJ. This novel device may find wide applicability for various engineering applications.

Evolution and composition of flow structures generated by nanosecond pulsed surface arc discharge in a magnetic field

The complete flow structure and the composition of the flow structure generated by Pulsed Surface Arc Discharge in a Magnetic Field (M-PSAD) are the keys to explaining the plasma application mechanism. The experimental results show that the M-PSAD can produce plasma clusters and detonation waves. The thermal motion of charged particles in the plasma cluster is the main reason for the thermal effect of arc discharge. The detonation wave consists of the shock wave, Taylor expansion wave, and two different thickness zones of charged particles. The experimental results provide a profound physical mechanism for applying plasma.

Analysis of tip leakage flow unsteadiness in a transonic turbine cascade using data-driven modal decomposition methods

The evolution of tip leakage flow and subsequent vortical structures is accompanied by inherent unsteadiness. This paper presents a novel characterization for the unsteady dynamics of turbine tip flow using data-driven, i.e., equation-free modal decomposition methods, which are applied to the hybrid Reynolds-averaged Navier–Stokes/large eddy simulation data at a transonic condition. By combining these techniques, the identified eigen-tuples (eigenvalues, eigenvectors, and time evolution) are well determined, and the differences between the obtained patterns (modes) are also pronounced. The snapshot proper orthogonal decomposition (POD) analysis can isolate the large-scale fluctuating structures that populate the rear part of the suction-side, which is mainly attributed to the shock-induced vortex instability. Similar to the turbulence cascade, macro-scale coherent structures that correspond to the tip leakage vortex shedding phenomena and the subsequently transitional and noisy parts closely related to the dissipation can be well derived by a quadruple reconstruction. Three dynamic mode decomposition (DMD) variants including the amplitude selecting-DMD method, the DMD with criterion method, and the sparsity promoting (SP)-DMD method are also compared in extracting dominant modes from the periodic tip flow, and the SP-DMD method which can distill modes of broadband frequencies and low dissipation is proved to be more conducive to representing and reconstructing the complex tip flow. Additional spectral-kernel-based POD (SPOD) analysis that can identify the similar primary unsteadiness frequencies as the DMD method is also encompassed in this study. Specifically, although it manifests that a physical resemblance of the pattern of pressure fluctuations to tip eddy unsteadiness can be captured by all these approaches, the behavior of small-scale vortical interaction downstream of the trailing edge can be clearly isolated with the intrinsic Karman-type vortex layer shedding process via DMD and SPOD approaches, which also demonstrates that these two techniques are more favorable to decomposing the complex tip flows into uncoupled single-frequency coherent structures compared to the conventional POD method. On this basis, resulting modes of velocity components have been accounted for verifying their contributions to the turbulent kinetic energy fields. The ensuing observations can offer a glimpse of the complex dynamics in the tip region, which also sheds light on features previously masked by conventional analysis approaches.

Pressure and velocity fluctuations characteristics of the tip clearance flow in an axial compressor stage at the near-stall condition

Pressure and velocity fluctuations characteristics of tip clearance flow in the rotor and stator of the axial compressor are studied by large eddy simulation (LES) at the near stall condition. In this paper, our work mainly emphasizes that the spatiotemporal evolution of complex flow structure, the pressure fluctuation, the velocity fluctuation of tip clearance flows, probability density function (PDF) of velocity and pressure fluctuations in the upstream and downstream of the rotor. Pressure fluctuations of the downstream rotor blade gradually decrease from the tip leading edge to the trailing edge. The motion law of leakage vortex may be related to the passage of the blade channel and the leakage vortexes mainly affect the region from the leading edge to the middle of the blade tip, but has little effect near the trailing edge. The frequency amplitude of suction surface is significantly higher than that of the pressure surface, and the frequency amplitude decreases along the flow direction. The passage shock has a major impact on the space-time correlation for pressure fluctuation. The intensity of space-time correlation of the downstream clearance flow of the rotor tip is similar to that of the upstream scraping vortex flow of the rotor tip. The strong space-time correlation for pressure fluctuation mainly concentrates in the separated flow on both sides of the rotor. Downstream flow has a better local temporal correlation than that of the upstream flow. The space-time correlation decreases more rapidly in velocity fluctuation. The space-time correlations of pressure and velocity fluctuations are vital for understanding the behaviors of unsteady flow characteristics of tip clearances at the near-stall conditions.

Quantitative analysis of fluid transport in dynamic stall of a pitching airfoil using variational Lagrangian coherent structures and lobe dynamics

The evolution of flow structures during dynamic stall of a two-dimensional pitching National Advisory Committee for Aeronautics 0012 airfoil is studied using the variational Lagrangian coherent structures (LCSs), and the mass transport and vorticity transport are precisely analyzed using LCSs and lobe dynamics for further understanding the nature of flow phenomena in dynamic stall. First, the variational LCS algorithm is improved to be efficiently used in the accurate extraction of flow structures. Then, both the hyperbolic LCSs and elliptic LCSs are computed numerically in the whole process of dynamic stall to analyze the evolution of flow structures in detail. Further, a high-accuracy LCS-advection method is used in the advection of LCSs to quantitatively analyze the mass transport and vorticity transport in the evolution of LCSs utilizing lobe dynamics based on nonlinear dynamics. Finally, the evolution and motion of primary leading edge vortex (LEV) and trailing edge vortex (TEV) identified by elliptic LCSs are analyzed in depth. The results obtained can provide a deeper insight into the nature of flow phenomena in dynamic stall from the viewpoint of nonlinear dynamics. Specifically, the nature of evolution of primary LEV and the TEV and the reasons for the changes of lift coefficients are clarified from the viewpoint of fluid transport. To explain it briefly, the variational LCSs and lobe dynamics are powerful tools to quantitatively analyze the evolution of flow structures and fluid transport.