Effects of temperature on drag reduction in a subsonic turbulent boundary layer via micro-blowing array

A comparative study of two micro-blowing temperature cases has been performed to investigate the characteristics of drag reduction in a subsonic flat-plate flow (where the freestream Mach number is 0.7) by means of Direct Numerical Simulation (DNS). With minute amount of blowing gas injected from a 32 × 32 array of micro-holes arranged in a staggered pattern, the porosity of micro-holes is 23% and the blowing coefficient is 0.125%. The simulation results show that a drag reduction is achieved by micro-blowing, and a lower wall-friction drag can be obtained at a higher blowing temperature. The role of micro-blowing is to redistribute the total kinetic energy in the boundary layer, and the proportion of stream-wise kinetic energy decreases, resulting in the thickened boundary layer. Increasing micro-blowing temperature can accelerate this process and obtain an enhanced drag reduction. Moreover, an explanation of drag reduction by micro-blowing related to the micro-jet vortex clusters is proposed that these micro-jet vortex clusters firmly attached to the wall constitute a stable barrier, which is to prevent the direct contact between the stream-wise vortex and the wall. By Dynamic Mode Decomposition (DMD) from temporal/spatial aspects, it is revealed that small structures in the near-wall region play vital role in the change of turbulent scales. The high-frequency patterns are clearly strengthened, and the low-frequency patterns just maintain but are lifted up.     

Hierarchical model updating strategy of complex assembled structures with uncorrelated dynamic modes

In structural simulation and design, an accurate computational model directly determines the effectiveness of performance evaluation. To establish a high-fidelity dynamic model of a complex assembled structure, a Hierarchical Model Updating Strategy(HMUS) is developed for Finite Element(FE) model updating with regard to uncorrelated modes. The principle of HMUS is first elaborated by integrating hierarchical modeling concept, model updating technology with proper uncorrelated mode treatment, and...     

Influence of sub boundary layer vortex generator height and attack angle on cross-flows in the hub region of compressors

It has been recently shown that Sub Boundary layer Vortex Generator (SBVG, abbreviated as VG hereafter) can suppress the Cross-Flow (CF), and therefore, can eliminate corner separation and increase aerodynamic loading when installed on the end wall inside middle-load compressor passages. However, when VGs are applied in high-load compressors, it is difficult to achieve ideal results. This is because the definition of the VG attack angle in the presence of CF in existing research is confusing, and the stronger CF in high-load compressors worsens the problem and results in an improper design and optimization range of VG attack angle. Therefore, this paper clarifies the definition of the VG attack angle in the presence of CF and reveals the CF controlling mechanism of VG on a flat plate. The differences in the flow phenomena around a VG both with and without CF are also studied. The numerical results show that a larger height or attack angle of the VG generates a greater CF suppression effect. However, the cross velocity increases when surmounting the primary vortex induced by the VG, except that this enhanced CF is less conspicuous for larger VG heights. Compared to the cases without CF, the VG suffers an additional loss because of the stronger separation and primary vortex loss caused by the CF.     

Large eddy simulations of a turbulent mixing layer periodically excited with fundamental and third harmonic frequency

A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications. Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency. There is a lack of detailed studies on applying low and higher frequency excitation. In this study, we have performed large-eddy simulations of periodically excited turbulent mixing layers. The excitation consists of a fundamental frequency and its third harmonic. We have used phase-averaging to identify the vortex structure and strength in the mixing layer, and we have studied the vortex dynamics. Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies. The influence of these two mechanisms on the mixing of a passive scalar is also studied. It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process; however, it improves the mixing further downstream of the splitter plate with the excitation using a phase shift of $\mathrm{\Delta }\varphi =\mathrm{\pi }$ showing the best mixing performance. The present works shed lights on the fundamental vortex dynamics, and has great potential for aeronautical, automotive and combustion engineering applications.     

Coupled evolution of piston asperity and cylinder bore contour of piston/cylinder pair in axial piston pump

The wear condition of the piston/cylinder pair is crucial to the performance and reliability of the axial piston pump. The hard piston surface, the soft cylinder bore surface, and the interface oil film affects each other during the wear process. Specifically, in the mixed lubrication region, the geometry of the hard piston surface asperity directly affects the wear of soft cylinder bore surface, while the asperities may deform or even degrade when penetrating and sliding against the cylinder bore. So far, there is no suitable method to simulate their coupled evolution. This paper proposed a wear process simulation model considering the real-time interaction between the elasto-plastic deformation of the piston surface asperity, the wear contour of the cylinder bore, and the lubrication condition of the interface. An offline library of the elasto-plastic constitutive behavior of the asperity based on the finite element method (FEM) is established as a part of the simulation model to precisely analyze the deformation and degradation of the asperity and quickly invoke them in the numerical wear process simulation. The simulation and experimental results show that the piston asperity and the cylinder bore contour converge to a steady state after running-in for about 0.5 h. The distribution of the simulated asperity degradation and wear depth is also verified by the experiment.     

Effects of swirl and hot streak on thermal performances of a high-pressure turbine

Advanced civil aero-engines tend to adopt lean burn combustors to meet emission requirements. The exit of a lean burn combustor experiences highly non-uniformities in both temperature (Hot Streak, HS) and flow (swirl). This paper presents a numerical investigation on the behaviors of a High-Pressure (HP) turbine under a combined effect of swirl and hot streak. The investigation was conducted on a GE-E3 HP turbine with unsteady numerical simulations, which considered the realistic clocking position of the HP Nozzle Guide Vane (NGV) relative to the combustor. The influences of swirl orientations on the HS migration and thermal performances on the blade surface were examined. Results indicate that, inside the NGV passage, the swirl’s induced incidence angle effect dominates the HS radial migration. The transversal movement of HS follows the cross flow and thus makes itself approach the Suction Side (SS) and keep away from the Pressure Side (PS) as passing through the NGV, so that HS near the SS is more influenced by the incidence angle effect than that near the PS. As for the heat transfer, swirl affects the Heat Transfer Coefficient (HTC) on the NGV’s PS and SS mainly through the incidence angle effect. Different from the NGV, the inlet swirl and HS have limited effect on the HTC on the rotor blade’s PS, while on the rotor blade’s SS, the original vortex system dominates; therefore, the inlet non-uniformities merely enhance the HTC on the SS rather than alter its distribution characteristics.