The term cluster or streamer is commonly applied to a collection of particles held together by hydrodynamic forces. Inside a circulating fluidized bed a single cluster has a significant effect on the gas that flows around it. Recent experiments have suggested that large scale gas fluctuations occur as the air flows around the cluster body. This paper examines the effect a single cluster has on the surrounding gas flow. Experimental work focuses on a small riser with the ability to release a single cluster. A specially designed probe measures the gas velocity as the cluster travels by. Numerical work simulates the effect of airflow around a porous body. The fluctuation levels observed are comparable to the experiments. Both results show that large scale velocity fluctuations occur in the gas in the vicinity of a cluster. This has a large impact on gas and particle modeling and mixing inside a circulating fluidized bed. (C) 2003 Elsevier Science Ltd. All rights reserved.