John C. Chien

July 22, 1999 03:04 
Re: [Q] Boundary conditions in Turbomachinery
(1). Look like that you are working on a centrifugal fan instead of compressor. Well, that is not important. (2). You really have to be very careful here. (3). First of all, the appearance of the governing equations depends on the coordinate system used, whether it is stationary (fixed, nonmoving) Cartesian, stationary cylindrical, or moving (rotating) cylindrical coordinates. You do not add or substract terms from the equations! They are there because of the coordinate system you selected in the first place. (4). Once you have the coordinate system selected, the boundary conditions must be specified on that coordinate system as well. (5). If the wall was stationary in the stationary coordinate system originally, then it will become a moving wall (rotating wall) in the rotating coordiante system. If the rotating coordinate system is moving clockwise, then the wall will rotate counterclockwise. And the inlet condition in the new rotating coordinate system will likewise pick up this counterrotating velocity component. That is all you need. (6). For steadystate stage calculation, the situation is quite different. Here, you are trying to joint the two separate problems ( one is formulated on rotating coordiante system and the other is formulated on stationalry coordinate system) into one combined problem through the use of "mixing plane boundary conditions". In principle, they are still two separate problems, but now they exist in one program and the solution is obtained through this mixing plene boundary condition iteratively. (7). For the real transient flow problems, the formulation is still on one single coordinate system , say stationary coordiante system, but the boundary conditions on the balde will become moving boundary. In this case, the steadystate mixing plane boundary between two computational zones is no longer used. (8). So, make sure that you know what kind of cases you are dealing with before setting the boundary conditions. The guideline is When one coordiante system is used, there is only one probem. If you have to use both stationary and moving coordinate systems, then there are two separate problems. These two problems are separated by the mixing plane boundary conditions. (9). It can be very confusing if you are not careful.
