جستجوی مقالات مرتبط با کلیدواژه « boundary-layer separation » در نشریات گروه « مکانیک »

This study utilizes numerical simulations and dimensional analysis to investigate the impact of the two-phase outlet on flow field characteristics and separation efficiency of the separator. The study revealed a boundary layer separation at the water outlet, which was subsequently addressed to reduce energy losses in the separator. Dimensional analysis considered the influences of operational, structural, and physical parameters on the separator's performance. With other structural parameters held constant, separation efficiency is directly proportional to the ratio of inlet and oil-outlet diameter. Additionally, the separation efficiency is also associated with Re and the ratio of the inlet to the water-outlet diameter. When the diameter of the water outlet is constant, the axial vortex separator achieves optimal separation when the ratio of inlet and water-outlet diameter is 0.563, with a maximum separation efficiency of 97.00%. The optimal separation efficiency is reached at Re=22,908 under various operational conditions. Separation efficiency increases with water content, peaking at an inlet water content of 0.9 across different structural parameters. Separation efficiency shows an increase followed by a decrease with the rise in inlet flow rate(vi), achieving the best performance at vi=3m/s for the different separator structures studied.

The transonic turbulent two-dimensional airflow over a symmetric flat-sided double wedge is studied numerically. Solutions of the Reynolds-averaged Navier-Stokes equations are obtained with ANSYS-18.2 CFX finite-volume solver of second order accuracy on a fine mesh. The solutions demonstrate an extreme sensitivity of the flow field and lift coefficient to variation of the angle of attack α or free-stream Mach number M∞. Non-unique flow regimes and hysteresis in certain bands of  α  and  M∞ are identified. Interaction of shock waves and local supersonic regions is discussed. The study confirms a concept of shock wave instability due to a coalescence/rupture of supersonic regions. In addition to the instability of shock wave locations, the numerical simulation shows a buffet onset, i.e., self-exciting oscillations due to instability of a boundary layer separation at the rear of wedge. Curious flow regimes with positive lift at negative angles α and, vice versa, with negative lift at positive angles α, are pointed out. A piecewise continuous dependence of the lift coefficient on two free-stream parameters, α and M∞, is discussed.

Unstart/restart phenomena induced by backpressure in a general inlet with a freestream of M = 2.7 are investigated in an in-draft supersonic quiet wind tunnel. The boundary layers are turbulent on the forebody while are laminar on the lip wall, which could mimick real flight conditions. The high-speed Schlieren imaging system and the nanoparticle-based planar laser scattering (NPLS) method are used to visualize the inlet flowfield. The inlet wall pressure is measured by high-frequency pressure transducers. The backpressure is reproduced by downstream transverse jets other than mechanical throttlers, which is more suitable to mimic backpressure caused by combustion. The high spatio-temporal resolution full-view images of inlet flow features during the complete unstart/restart process are captured, which are seldom seen before. The formation and disappearance process of massive boundary layer separation at the entrance of the unstarted inlet is observed. The backpressure transmits upstream through the shock wave/boundary layer interaction (SWBLI) regions. The shock structures change the angles and merge upstream to balance the pressure rise. The Mach shock reflection configuration is observed in both unstart/restart process, accompanied by the boundary layer separation extending to the leading edge. The experiment also revealed notable hysteresis in the unstart/restart process.

Unsteady magnetohydrodynamic mixed convection flow of an incompressible hybrid nanofluid (Cu-Al2O3/water) past an isothermal cylinder with thermal radiation effect has been studied. Appropriate non-dimensional variables are initiated to reduce the governing equations into a convenient form. By utilizing the procedure of finite difference, reduced equations are then solved for all time. Besides, series solutions are obtained using perturbation technique for short time and asymptotic method for long time which agree with the acquired numerical solution up to a good accuracy. When the mixed convection parameter Ri, radiation conduction parameter Rd, magnetic field parameter M and the volume fractions of nanoparticles ϕ1 and ϕ2 are increased, the local skin friction coefficient and the local Nusselt number are found to increase. Results revealed that the hybrid nanofluid (Cu-Al2O3/water) enhances the heat transfer about 28.28% in comparison to the Al2O3-water nanofluid and about 51.15% than the pure fluid. Contrary to this, the heat transfer of hybrid nanofluid is augmented about 41.76% than the Cu-water nanofluid and 71.41% than the base fluid. The streamlines and isotherms reveal that higher values of Ri, M and Rd delay the boundary layer separation and accordingly shrink the vortices. Moreover, the thermal boundary layer is thickened for the increment of aforesaid quantities. The surface temperature parameter augments the local skin friction coefficient, however, the reverse characteristic is observed for the local Nusselt number.

Suitable slot structure of the compressor blade can generate high-momentum jet flow through pressure difference between the pressure and suction surface, it has been proved that the slot jet flow can reenergize the local low-momentum fluid to effectively suppress the flow separation on the suction surface. In order to explore a slotted method for better comprehensive suppressing effects on the boundary layer separation near blade midspan and the three-dimensional corner separation, a diffusion stator cascade with large camber angle is selected as the research object. Firstly, the Slotted_1 and Slotted_2 whole-span slotted schemes are set up, then the Slotted_3 scheme with whole-span slot and blade-end slots is proposed, finally the performance of original cascade and slotted cascades is computed under a wide range of incidence angles at the Mach number of 0.7. The results show that: in the full range of incidence angles, compared with the whole-span slotted cascades, the development of the endwall secondary flow on the suction surface of Slotted_3 cascade is effectively suppressed, the degree of the mutual interference between the secondary flow and the main flow is reduced. Besides, on the suction surface of Slotted_3 cascade, the boundary layer separation near blade midspan and the corner separation are basically eliminated. As a result, compared with those of original cascade, the total pressure losses of Slotted_3 cascade are reduced in the full range of incidence angles, and its operating range of incidence angles is broadened. Moreover, compared with the whole-span slotted schemes, Slotted_3 scheme has a better adaptability to wide range of incidence angles.

Both experiments and computations are performed and analyzed to investigate the effectiveness and mechanisms of different slotted aspiration schemes in controlling the separated flows in a highly-loaded axial compressor cascade. It is found that the boundary layer aspiration on the blade suction surface can improve the incidence characteristics of the airfoil within most of the incidence range except of the extremely high incidence and the profile loss coefficient is reduced remarkably as the aspirated massflow increases. The combined aspiration is the most effective scheme to control both the separated flow on the blade suction surface and the three-dimentional hub corner separation, and an improper design of aspiration would lead to a deterioration of the flow field. Different aspiration schemes have different effectiveness in controlling the flow separation, which leads to various influences on the blade loading and the diffusion abilities. The cascade incidence characteristics of different aspiration schemes show that the part-span aspiration scheme (SS1) located on the blade suction surface can only improve the overall flow field in very high incidences, while the other schemes can reduce the overall loss coefficient within almost the whole incidence range, especially for the combined aspiration scheme. There always exists a closed separation in the cascade when the boundary layer separation is not removed completely on the blade suction surface and in the hub corner. In addition, the type of critical point is affected by the spanwise static pressure gradient, which has significant effects on the cascade performance.