In order to determine the types of rain producing weather in Tehran, the rainy days of the station (days with more than or equal to 1 millimeter) were identified during the 1990-95 period. The 12 GMT pressure and height data of the area 200-47.5° and 35°-67.5°E were -extracted from the NCEP Reanalysis file for the grid points 2.50 apart. The synoptic systems and flow directions were determined according to the objective method of computing vorticity. The results first showed 24 pressure patterns for the three groups of light, moderate, and heavy precipitation days. Then these patterns were re-grouped according to their similarities into 10 weather types. These types consist of two cyclonic and anticyclonic, and 8 directional types corresponding to 8 geographical directions. The most frequent type was the Easterly and the most important rain producing type was the cyclonic type. The anticyclonic type was the least rain producing type. Cyclonictypes were more dominant and developed mostly in winter. The anticycloictypes showed lower frequency and concentrated in spring.This study showed that the vorticity computation method is very effective and accurate and capable of quantifying the power of the weather types. The 500 hpa level had more significant effect on the precipitation than the surface pressure changes. The Syrian trough was the most important upper level system which controls the precipitation of Tehran.

In the present study, the performance of the vorticity confinement method has been improved by combining it with the vortex feature detection methods. In the conventional vorticity confinement method, the only parameter to apply or not to apply vorticity confinement is the non-zero value of vorticity. On the other hand, the presence of vorticity in some cases, like the boundary layer and the shear layer flows, does not imply the presence of vortices. Applying the vorticity confinement at these points can lead to errors, in addition to loss of solution time. In order to solve this problem, using the combination of vorticity confinement method and four methods of vortex feature detection (non-dimensional 􀜳 , non-dimensional 􀟣 􀬶 , non-dimensional modified Δ , and the 􀜵 − Ω correlation) the vorticity confinement term is applied only in vortex regions. In order to investigate the effects of this combination, the compressible Euler equation has been investigated for the problem of two-dimensional stationary single vortex at Mach number 0. 5. The results indicate significant positive effects in reducing the solving time, decreasing the sensitivity of the solution to the amount of confinement parameter and significant elimination of the oscillation.

In this paper, variable confinement parameters were successfully developed for compressible vorticity confinement. Three variable confinement parameters, that have velocity dimension, were defined, based on three artificial dissipation schemes. The resulting confinement parameters are functions of the spectral radii of the Jacobian matrices and the Jacobian matrices themselves.Therefore, the confinement parameter implicitly contains the grid size and other local fluid properties. Preliminary results for moving vortices showed that the new confinement parameters allow the capture of vortical layers that, effectively, do not decay in time, like Hu et al.'s confinement. Calculations of the supersonic base flow and supersonic shear layer showed good agreement with experimental and analytical data, especially for the variable CUSP confinement parameter. When variable confinement parameters are used, the tuning constant is equal to, or larger than, the equivalent value of the constant confinement (Hu et al.). This means that the tuning constant varies in a range smaller than that of the Hu et al. constant confinement, especially for the CUSP confinement parameter.

This paper describes a design methodology for ducted propellers operating in a non-uniform wake field. An improved lifting line model is used to represent propeller blades where more realistic slipstream deformation behind the propeller is considered, while the duct and hub are modeled by ring vortex distributions. Duct and hub geometries are assumed to be known and their effects on the propeller are calculated during propeller design stage. At the same time, mutual interaction between duct/hub geometry and propeller is taken into consideration based on an iterative procedure. For numerical application of the present method, a design example is carried out and the effect of slipstream deformation is investigated in detail. Another application based on experimental work is also taken into account to present the accuracy of the calculated results by present method and Computational Fluid Dynamics (CFD) code. Finally, propulsive characteristics of ducted propellers obtained from present method are compared to those of CFD code and experimental results.

The apparent mass effect is enhanced significantly when the motion of a body changes quickly, such as a flapping wing or an impulsively started plate. Previous method for calculating the apparent mass of a given body needs to adopt the assumption of ideal flow and know the potential of velocity field arising in the fluid due to the motion of the body. However, the assumption of ideal flow is contrary to real fluid field and it is hard to obtain the potential of velocity field in most cases. In this paper, a new method based on the vorticity moment theorem for calculating the apparent mass of the body of revolution in the axial direction due to axial acceleration is developed. This method has no assumption of ideal flow and establishes the relationship between the apparent mass and the vorticity loops adjacent to the surface of the body. Using this method, the value of the apparent mass can be easily figured out and the physical mechanism of the apparent mass can be revealed from the view of the vorticity loop. The comparisons between different bodies have shown the influences of the fineness ratio (the ratio of the length to the maximum diameter) and the trailing edge type on the apparent mass.