In this paper, we address the issue of integrating packet level simulation with fluid model based simulation for IEEE 802.11-operated wireless LANs (WLANs), so as to combine the performance gains of the latter with the accuracy and packet level details afforded by the former. In such mixed-mode simulation, foreground flows operate in the packet mode, while the other background flows are approximated into a collection of fluid chunks and simulated in the fluid mode. As these two types of flows influence each other at the point of interaction, e.g. the wireless channel in a WLAN, we derive the model of interaction at the wireless medium. We then implement mixed mode simulation in ns-2, and conduct a comprehensive simulation study to evaluate mixed mode simulation with respect to its capability to keep track of network dynamics (in terms of the TCP behavior), accuracy (in terms of error discrepancy in throughput), and efficiency (in terms of speed-up in carrying out simulation). Simulation results indicate that for IEEE 802.11-operated WLANs, it is feasible to blend packet level simulation into fluid model based simulation, and the performance improvement is quite significant while the accuracy and the packet level details are not compromised. Specifically, the mixed mode simulation accurately retain packet level details for the TCP flows of interest, incurs only approximately 2 % of the error discrepancy, and maximally reduces the execution time by two orders of magnitude.