The flow structure in cavities is a key issue for trapped vortex combustors. Numerical method is verified by experimental results. According to the numerical simulation in this paper, series of streamwise vortexes which exist in the form of counter-rotating vortex pairs would be induced by the addition of specific rectangular stacks in the jet slots in the front body of cavities. Three cases in which the stack blockage ratio BR (tab area to jet slot area ratio) are 0，0.2，0.4 respectively are investigated. In cases BR=0.2,0.4, the results show that: (1) the streamwise vorticities are significantly increased by nearly 100% compared with that in the case BR=0, and the streamwise vorticities in the case BR=0.2 are slightly stronger than the streamwise vortexes in the cases BR=0.4; (2) in the streamwise vortex mixing layer, the mean streamwise vorticity increases first and then decreases along the axis direction; (3) the total pressure loss of the combustor is about 1% bigger than that in the case BR=0.