This is called a "funnelflow" pattern, in which some material moves while the rest remains stationary during discharge.
这叫做“通道流动”模式。其中，一些材料输运的同时，另一些材料在卸料时静止。
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Other causes to look for are insufficient drying or funnelflow inside the drying hopper-when material at the center flows quickly while material near the wall stagnates.