1. INTRODUTION

Considering the safety at an accidental explosion of the rocket, the launch site must be located far from the populated area. When the explosion occurs, a hemisphere-shaped shock wave is generated and propagates in the radial direction as shown in Fig. 1 . The damage to the surrounding area is mainly caused by the rapid rise in the pressure due to the shock wave. In order to secure the safety in the populated area, the off-limits zone is set around the launch site, since the shock wave is weakened with the increase in the distance from the center of explosion. To determine the radius of the off-limits zone, which is often called as the "safety distance", the accurate prediction of the distance at which the shock wave is weakened to the safety level is necessary. However, the characteristics of the shoek wave propagation are affeeted by the surface topography and the presence of obstacle around the launch site. In Japan, it is quite difficult to find a flat, clean and huge area for the launch site due to the limitation of the land. Hills and forest are commonly seen. As for the effects of the surface topography on the shock wave propagation, the precise computational studies have been made by Shimizu et al ^{1 )}. They clarified that the blast wave strength is significantly influenced by the ground surfaee geometry. However, the effects of the forest has not been clarified yet. To consider the characteristics of the shock wave propagation in the forest, we must know the drag force of the forest rather than the detailed flow pattern around each tree. In the present study, our interests are mainly focused on such overall features of the flow in an array of obstacles.

Lots of studies have been done on the shock wave propagation through a single obstacle both experimentally and computationally. For the shock wave through an array of obstacles, however, a little amount of studies have been made. Rogg et al ²⁾. experimentally investigated the shock-induced flow in an array of cylinders with staggered arrangement by using the shock tube. They compared the experimental results on the drag force of the cylinder array with the empirical relations. The detailed description of the unsteady flow field induced by the shock wave propagation in a cylinder array was obtained by Takayama et al^3,4), who used the interferograms to visualize both the experimental and computational flow field.

In the present study, the propagation of the shock wave through a cylinder array is experimentally investigated by using the shock tube. The flow in the forest is essentially three-dimensional and quite complicated. From a viewpoint of the overall features of the flow, however, the forest is regarded as a kind of porous media which produces the drag force to the incident flow. The shape of each obstacle is not important. Hence, we consider an array of cylinders to represent the forest in the experiment. In the case of an actual explosion, the shock wave propagation is three-dimensional. However, we consider the propagation of the normal shock wave in a channel having an array of cylinders as shown in Fig. 1, since such situation is easily obtained by using the shock tube. After these simplification of the problem, the essential properties of the phenomena are expected to be still retained. The propagation of the normal shock wave in the channel is described by the one-dimensional flow model. The drag force of a cylinder array is evaluated by considering the pressure difference between the upstream and downstream regions of the cylinder array. In the experiments, the unsteady pressure measurements are made at various locations in the channel. To consider the wide variety of the shape, diameter and the arrangement of trees in a forest, various cylinder arrangement and diameter are tested. The flow visualization by the schlieren method is made to reveal what happens at the interaction of the shock wave with a cylinder array.

The major objectives in the present study are as follows:
(1) To clarify the phenomena which occur at the propagation of the normal shock wave through a cylinder array with the emphasis on the attenuation and augmentation of the shock wave strength,
(2) To present a simple numerical model for description of such flow and evaluation of the drag force of a cylinder array,
(3) To investigate the effects of the cylinder arrangement and diameter.