Gas-Solid Clustering: Not Your Ordinary Instability

In high velocity, gas-solid flows, the presence of the clustering instability – collections of highly-concentrated particles that continuously form, rearrange and breakup – are the norm rather than the exception. For decades, such instabilities have been predicted qualitatively by dozens of researchers using a kinetic-theory analogy for the continuum description. The ability to quantitatively predict clusters, however, has received relatively scant attention. Achieving such accuracy is non-trivial given the complexity of the problem – transient, three-dimensional flows with a flow instability that evolves over several length scales – as well as the seemingly inappropriateness of a “small-gradient” starting assumption. In this talk, the mechanisms for clustering will be discussed along with the competing influences that impact stability/instability, as well as recent progress on assessing the quantitative accuracy of a recent theory.