This paper presents a series of systematic multi-passage unsteady RANS simulations on a transonic axial flow compressor rotor (Rotor 35). The objective is to have a better understanding of the ...underlying flow mechanism which connects the phenomena of the tip leakage vortex (TLV)'s breakdown to the appearance of flow unsteadiness. It has been revealed that both bubble-type and spiral-type breakdown of the TLV can result in a self-sustained flow unsteadiness at high-loading flow conditions. The origin of such unsteadiness lies in that the vorticity region redistributed by the vortex breakdown is capable of affecting the pressure distribution on the pressure side of a passage. Once this threshold event is met, the swirl intensity of the TLV and the strength of shock wave, which are key factors controlling the vortex breakdown, varies accordingly, thus leading to an instantaneous rather than a stationary vortex breakdown occurring in the confined rotor–passage system. As compared to the bubble-type breakdown of TLV, the spiral-type breakdown of TLV exerts more severe impact on the pressure distribution on the PS of the passage. As a result, a significant change in the scale of the breakdown region occurs. This gives rise to not only an appearance of a new vortex structure but also a blockage transfer across the passage against the rotor turning direction. The new vortex structure, termed as tip secondary vortex (TSV), is essentially a vortex segment arising from the spiral-type breakdown of TLV. The necessary condition for the inception of the rotating wave like RI is the blockage transfer induced by the spiral-type breakdown of TLV and its resultant interaction with the tip leakage in the adjacent passage.
Reversible pump turbine (RPT)’s runner inter-blade flow unsteadiness onset and development especially under off-design conditions is still one of unresolved issues, yet continues to cause different ...complications on a daily basis within pumped storage power plants, detrimental pressure pulsations and resulting structural vibrations among others. Though different methods have been used to solve the issue at hand, an adequate solution suitable for all designs has not yet been found; a fact that calls for a more deep understanding in terms of RPT runner flow dynamics and possible influencing parameters. Therefore, the present study seeks to investigate the RPT flow dynamics and associated runner inlet pressure pulsations under off-design operating conditions, as well as the effect of runner blade number on the same. Three RPT models with different runner blade numbers were numerically investigated. The results showed that runner flow unsteadiness has a close relationship with the machine flow conditions, where flow vortices are mostly located at the runner shroud and vicinities. The runner flow unsteadiness decreased with the increasing blade number. The runner blade number considerably influenced runner inlet pressure pulsations where pulsation levels of models with 9 and 10 blades were the highest and lowest respectively.
•RPT runner flow was simulated for three different runner blade numbers.•Fast Fourier Transform-based pressure pulsations were extracted from different flow zones.•Pressure and flow field characteristics were analyzed for different runner blade numbers.
The development of supersonic and hypersonic aeronautical projects has led to a renewal of interest and research in supersonic mixing processes and methods to control them. Due to the compressibility ...effects, the mixing process in supersonic condition is significantly inhibited. Limited flow residence time (at the order of microseconds) for mixing of supersonic airstream and fuel in a scramjet combustor calls for the development of techniques for mixing enhancement. In the present paper, the applicable techniques for mixing enhancement of supersonic mixing layer flows are reviewed comprehensively. Following the brief introduction of fundamentals of supersonic mixing layer, this paper discusses the mixing enhancement methods in the categories of: passive flow control techniques, active flow control techniques and shock wave induced mixing enhancement strategy. Based on the review of former progress, the gaps in current knowledge and areas where more research is needed are pointed out. Meanwhile, the mechanisms of different enhanced mixing methods are summarized, which can be a reference and guidance for the future techniques design for mixing enhancement of supersonic flows.
•Enhanced mixing methods for supersonic mixing layers are reviewed.•Overview of the fundamentals of supersonic mixing layers is presented.•The mechanisms of different methods for mixing enhancement are revealed.•The present review can be a reference and guidance for future strategy design.
This paper reveals that air injection at tip region of an axial compressor rotor blade row is an effective technique for enhancing its aerodynamic performance. Method of investigation is based on ...solution of the Navier–Stokes equations exploiting shear stress transport (SST) k–ω turbulence model. This model attempts to predict the turbulence by two partial differential equations for two variables of turbulence kinetic energy (k) and specific rate of dissipation (ω). Initially, computational performance curves of the proposed rotor blade row for no-injection case are compared with available experimental data, which show reasonable agreement. Then, effects of air tip injection on flow field and general aerodynamic performance of the rotor blade row are studied in detail. Results are presented in terms of the blade row span-wise distributions of total pressure rise coefficient and diffusion factor for no-injection and injection cases. Flow patterns at the blade row tip region are also carefully demonstrated. Six pseudo static pressure taps were mounted along the blade chord line within the blade tip gap region for detection of the flow fluctuations. So, it was possible to extract frequency spectrum of the fluctuating flow in this region. Numerical simulations confirmed that injection of air flow at the blade row tip region boosts the flow momentum and reduces blockages due to the flow separation from the blades surfaces. Results showed that air injection as small as of only 0.5% of the whole annulus mass flow rate causes the stall margin of the rotor blade row to increase by 15.5%.
Accurate and reliable experimental data of a sloshing-induced, rapidly-evolving spilling breaker, are used to understand the specific physics of this phenomenon and to partially evaluate a simplified ...analytical model by Brocchini and co-workers (Brocchini , 1996; Misra et al., 2004, 2006). Such model is based on a three-layer structure: an underlying potential flow, a thin, turbulent single-phase layer in the middle and a turbulent two-phase layer (air–water) on the upper part. The experiments were carried out by using a 3 m long, 0.6 m deep and 0.10 m wide tank, built in Plexiglas and forced through an hexapode system, this allowing for an high accuracy of the motion. Mean and turbulent kinematic quantities were measured using the Particle Image Velocimetry (PIV) technique. To ensure repeatability of the phenomenon, a suitable breaker event was generated to occur during the first two oscillation cycles of the tank. The tank motion was suitably designed using a potential (HPC) and a Navier–Stokes solver. The latter, was useful to understand the dimension of the area of interest for the measurements. The evolution of the breaker is described in terms of both global and local properties. Wave height and steepness show that after an initial growth, the height immediately decays after peaking, while the wave steepness remains constant around 0.25. The evolution of the local properties, like vorticity and turbulence, vortical and turbulent flows displays the most interesting dynamics. Two main stages characterize such evolution. In stage (1), regarded as a “build-up” stage, vorticity and TKE rapidly reach their maximum intensity and longitudinal extension. During such stage the thickness of the single-phase turbulent region remains almost constant. Stage (2), is regarded as a “relaxation” stage, characterized by some significant flow pulsation till the wave attains a quasi-steady shape. In support to the analytical, three-layer model of Brocchini and co-workers it is demonstrated that the cross-flow profile of the mean streamwise velocity U inside the single-phase turbulent layer is well represented by a cubic polynomial. However, differently from available steady-state models the coefficient of the leading-order term is function of time: A=A(s,t). During stage (1) a fairly streamwise-uniform distribution of U is characterized by A(s,t)≈1, while during stage (2) U is less uniform and A varies over a much larger range.
Three-dimensional numerical simulations are conducted to investigate the origin of flow unsteadiness and its associated unsteady flow phenomena in a transonic compressor rotor. The predicted results ...are compared with the available experimental data and a good agreement is achieved. The numerical monitoring results and further analyses of the flow field indicate that flow unsteadiness is detected in the passage with the operating condition approaching the stability limit, and the highest oscillating region is at the leading edge of the blade pressure surface; the tip leakage vortex breakdown is not a decisive factor for the flow unsteadiness, and the shock oscillation is a unsteady flow phenomenon resulted from the vibration of the recirculation region; a U-type vortex emerges in the tip leakage vortex breakdown region, and its periodic impingement on the pressure surface of the adjacent blade is treated as a trigger that leads to the flow unsteadiness.
An experimental investigation was conducted in a cold flow test facility to identify the origin of various flow conditions that lead to side-load generation in a truncated ideal contour nozzle (of ...area-ratio 20.66) especially at moderate nozzle pressure ratio range of 20 to 42 during the start-up and shutdown sequences. The major contributors seem to be the transition in flow conditions namely, the change in the circumferential shape of re-circulation region inside the nozzle from a cylindrically dominated regime to a conical one and the end-effect regime that initiate highly unsteady flow conditions in the separation region preceding these transitions. Other flow transitions such as those initiated by the onset of test gas condensation and vice-versa result in a downstream or an upstream jump in separation, respectively, that causes the overall side-load signal to increase. During this flow regime, an increase in the length of the upstream influence region accompanied by a rise in the peak standard deviation value is also observed.
In this study, we propose a novel dynamic mode decomposition (DMD) energy sorting criterion that works in conjunction with the conventional DMD amplitude-frequency sorting criterion on the ...high-dimensional schlieren dataset of the unsteady flow of a spiked-blunt body at Ma = 2.2. The study commences by conducting a comparative analysis of the eigenvalues, temporal coefficients, and spatial structures derived from the three sorting criteria. Then, the proper orthogonal decomposition (POD) and dynamic pressure signals are utilised as supplementary resources to explore their effectiveness in capturing spectral characteristics and spatial structures. The study concludes by summarising the characteristics and potential applications of DMD associated with each sorting criterion, as well as revealing the predominant flow features of the unsteady flow field around the spiked-blunt body at supersonic speeds. Results indicate that DMD using the energy sorting criterion outperforms the amplitude and frequency sorting criteria in identifying the primary structures of unsteady pulsations in the flow field, which proves its superiority in handling an experimental dataset of unsteady flow fields. Moreover, the unsteady pulsations in the flow field around the spiked-blunt body under supersonic inflow conditions are observed to exhibit multi-frequency coupling, with the primary frequency of 3.3 kHz originating from the periodic motion of the aftershock.
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