Volume 28, Issue 1

The present analysis undertakes the complex research program aimed at understanding the aerodynamic characteristics of two square surface mounted bluff bodies immersed in a boundary layer. The main attention has been put on the role of interference of the wake flow connected with a process of vortex shedding as a factor stimulating the aerodynamics loads. The analysis has been performed for the three-dimensional case of two in-line surface-mounted bluff bodies, arranged in tandem. The program of the study consists of parallel wind-tunnel experiments and numerical simulation performed with the use of phase-averaged form of k - ε turbulence model in RNG version which have shown overall changes in flow pictures observed for different values of objects “immersion” in boundary layer H / δ . The flow structure around surface-mounted obstacles depends on various factors: the oncoming boundary layer thickness δ related to the height of obstacles H , the oncoming flow turbulence, the non-dimensional distance ratio ( S / D ) and the height ratio of the two consecutive bodies. Conjugation of two research methods improved the efficiency of the conducted investigations. The most important finding of present research has confirmed that the interference of the flow structure around the system of rectangular objects and aerodynamic characteristic can be modified in promising ways through a change of their geometrical parameters. This article is the second part of a twopart series. The first part, “Aerodynamic Characteristics of Two-Dimensional Sharp-Edged Objects in Tandem Arrangement”, appeared in the Archives of Mechanics ([Gnatowska, 60: 475–490, 2008]).

The helicopter to be operated in a severe flight environmental condition must have a very reliable propulsion system. On-line condition monitoring and fault detection of the engine can promote reliability and availability of the helicopter propulsion system. A hybrid health monitoring program using Fuzzy Logic and Neural Network Algorithms can be proposed. In this hybrid method, the Fuzzy Logic identifies easily the faulted components from engine measuring parameter changes, and the Neural Networks can quantify accurately its identified faults. In order to use effectively the fault diagnostic system, a GUI (Graphical User Interface) type program is newly proposed. This program is composed of the real time monitoring part, the engine condition monitoring part and the fault diagnostic part. The real time monitoring part can display measuring parameters of the study turboshaft engine such as power turbine inlet temperature, exhaust gas temperature, fuel flow, torque and gas generator speed. The engine condition monitoring part can evaluate the engine condition through comparison between monitoring performance parameters the base performance parameters analyzed by the base performance analysis program using look-up tables. The fault diagnostic part can identify and quantify the single faults the multiple faults from the monitoring parameters using hybrid method.

This paper is concerned with the numerical study of the effect of injection momentum on the flow field, fuel-air distribution and temperature distribution within the cavity of an axisymmetric trapped vortex combustor (TVC) under reacting flow condition. Shear Stress Transport (SST) k - ω model was used for turbulence modeling while combustion was simulated using the Eddy Dissipation Model (EDM). The predicted velocity field and temperature profile inside the cavity matched reasonably well with the experimental results available in literature. Reacting flow studies revealed that when the cavity momentum flux ratio, MFR c ≤ 1, the TVC exhibited similar flow features, irrespective of the relative momentum between the cavity and the mainstream flow. For these cases, a single vortex, rotating in the stream-wise was established within the cavity. The temperature distribution was also found to be similar within the cavity for these cases. However, nonuniform fuel air distribution was observed for these cases. In contrast, MFR c > 1 cases exhibited entirely different flow pattern. For these cases, multiple vortices were observed within the cavity, with a counter streamwise vortex oriented at the bottom wall. Within this counter streamwise vortex, the temperature distribution was observed to be uniform. Hence, MFR c is the dictating factor in determining the flow and temperature field within the TVC cavity. Beside this, by maintaining same aft- and fore-wall dimensions, a counter rotating vortex characterized by better fuel-air mixing within the entire cavity domain can be established for MFR C > 1 cases, which needs further investigation in future.

In this paper, the optimization of a Francis turbine is carried out by the flow diagnosis method in the turbine runner based on vorticity parameters. Firstly, a new RNG k - ω turbulence model is proposed based on the RNG k - ε model, which brings the nonlinear term of the mean fluid flow transition to the ω equation in the original k - ω model. Secondly, the Reynolds-averaged Navier–Stokes equations are solved numerically by this RNG k - ω model. And, the prediction of the energy performances for Francis turbine is completed. Finally, the detailed flow behavior is revealed by vorticity parameters. In the paper, the simulation results for original and optimized Francis turbines have been compared as well. The model test results indicate that the efficiency of the optimized turbine will be improved from 93.6% to 94.5% in the case of optimum operating condition.

This paper presents the results of an experimental investigation carried out to control the base pressure in a suddenly expanded axi-symmetric passage. Active control in the form of micro jets was employed to control the base pressure. Air injection at four locations at the base, symmetric to the nozzle axis was used as the active control. The jet Mach numbers at the entry to the suddenly expanded duct studied were 1.87, 2.2 and 2.58. The area ratio of the present study was 2.56. The length-to-diameter ratio of the suddenly expanded duct varied from 10 to 1. Nozzles generating the above jet Mach numbers were operated with nozzle pressure ratio (NPR) in the range 3 to 11. In addition to base pressure, wall pressure field along the duct was also studied. It is found that the active control in the form of blowing through small orifices (micro jets) are effective in controlling the base pressure field. As high as 65 percent increase in base pressure was achieved for certain combination of parameters of the present study. Micro jets do not disturb the flow field in the enlarged duct.

Cavitation causes serious damages to Francis turbine, e.g., noise and vibration. Its mechanism is complex and may be affected by many factors. The present paper compares cavitation behavior of flow in a Francis turbine with different content of non-condensable gas (NCG) concluded from experiment and numerical simulations. The experimental results show small difference in characteristics of cavitation with different content of non-condensable gas, while numerical simulation shows larger difference. It thus can be concluded that present simulation over-predict this difference.

In this paper, the deflected circumferential distance of wake related to some aerodynamic and geometrical parameters was studied to determine the optimum clocking position in axial-flow turbomachinery. A predicting method on how to calculate the deflected circumferential distance of the wake was also put forward. To verify this method, a 2-D time-accurate, viscous flow solver had been used to analyze unsteady flow associated with different clocking positions. A 1.5 stage turbine was employed to observe the effects of clocking positions on the machine performance and tend to find out the optimum clocking position. It shows that the error of the optimum clocking position between the simulation result from CFD and the result from the predicting method is only 0.361%. In fact, there are three assumptions implied in the predicting method, and their rationality has been understood and explained one by one through analyzing machine efficiency, instantaneous entropy contours and viscous stress on the surface of the stator suction side. In order to get further confirmation, the method was also used in the case of a 1.5 stage compressor and the error between the predicted position and the maximum efficiency position obtained from flow field simulation is as much as 1.2%. Furthermore, based on numerical simulation of a 2.5 stage turbine, it is found that clocking positions of the first stator row have little effect on the incoming wake and free stream turbulence of the third stator. This conclusion should be helpful and beneficial to the designs of the multistage turbomachinery, in which clocking effect has been fully utilized.

The effect of flow angularity on an S-duct inlet with icing is computationally investigated. Flow angularity is simulated through angle-of-attack, and sideslip in addition to asymmetrical ice accretion on the inlet lip. A commercial CFD code, STAR-CCM+ is used for the steadystate computations with the shear-stress transport (SST) k - ω turbulence model. Symmetrical and asymmetrical glaze ice shapes are computationally simulated on the inlet lip. The symmetrical glaze ice uniformly covers the entire cowl lip; whereas the asymmetrical glaze ice is simulated on a 1/4 sector of the inlet lip and is positioned on top, bottom or side of the inlet lip. The results indicate that flow angularity, whether in angle-of-attack or sideslip, aggravates the low performance of inlets with icing. The total pressure recovery suffers an additional ~2% loss and the inlet mass flow rate drops by ~7% when the inlet is at +20° angle of attack, as compared to zero angle, for flight Mach number of 0.34. The extent of loss in total pressure and a drop in mass flow rate depends on the asymmetrical icing location as well as the inlet angle-of-attack and sideslip. In addition, the ice-induced flow blockage is identified as a critical inlet performance parameter, since the symmetrical (360°) glaze ice with its wider flow blockage creates a lower total pressure recovery than the asymmetrical (90°) glaze ice at all angles of attack or sideslip.

An internally mixed twin fluid atomizer has been characterized in terms of its spray uniformity and liquid distribution by using an optical patternator. The atomizer was operated at an injection pressure of 819 kPa and images were acquired at three different planes normal to the injector axis for three gas-to-liquid ratio (GLR) cases. These planar images were processed and divided into ‘ N ’ rectangular sectors. Subsequently, the patternation indices were calculated based on the two newly defined indices, namely (i) mean-to-maximum ratio (MR) and (ii) global mean-to-maximum ratio (GMR). The MR and GMR attained a maximum value for 10% GLR as compared to other GLR cases, which indicated better distribution for 10% GLR case. The results from the mechanical patternator showed that the maximum normalized value of flow rate decreased slightly as the GLR was increased from 5 to 15% where the spray becomes finer. However, due to better spatial resolution, this trend was more perceptible in the optical patternation results even for 5 and 10% GLR. The trend of the qualitative information of the spray uniformity was similar to that of the results from the mechanical patternator. Thus, the present optical patternation technique could be employed effectively to predict spray uniformity for different operating conditions of an internally mixed atomizer.