Mass addition to flowing tenuous plasmas by the ablation of embedded clumps alters the flows and the observational characteristics of both flows and clumps. The boundary layers between the clumps and the flows are the sites of enhanced radiative losses. Flows which in smooth media would be driven by thermal pressure, are instead driven by momentum. There are many possible types of flows and we explore some of them in the context of Wolf-Rayet and planetary nebulae. Flows in which transsonic tenuous plasmas exit from mass-loading cores into smooth haloes are relevant for planetary nebulae. On intermediate scale lengths, the flow-clump interactions produce extended ‘tails’. We give a general discussion of this and describe applications to the cometary tails behind globules in the planetary nebula NGC 7293 and to the tail of the galactic centre red supergiant IRS7. We finally briefly discuss diagnostics of the boundary layers themselves.
Practically all diffuse media of astrophysical significance are clumpy media which are responding to energy sources. The most important distinction between flows initiated in clumpy as opposed to homogeneous media, is that in the former, there is mass, momentum and energy interchange at clumptenuous plasma boundaries, i.e. in boundary layers. The consequences are major (Hartquist & Dyson 1993). This interchange reacts back on the dynamical, physical and even chemical state of the global tenuous plasma flow; conversely, the state of the global flow influences the interchange process.