The collapse of dust particle clouds directly to \si { \kilo \meter } -sized planetesimals is a promising way to explain the formation of planetesimals , asteroids and comets .
In the past , this collapse has been studied in stratified shearing box simulations with super-solar dust-to-gas ratio \epsilon , allowing for streaming instability ( SI ) and gravitational collapse .
This paper studies the non-stratified SI under dust-to-gas ratios from \epsilon = 0.1 up to \epsilon = 1000 without self-gravity .
The study covers domain sizes of L = \SI { 0.1 } { \scaleheight } , \SI { 0.01 } { \scaleheight } and \SI { 0.001 } { \scaleheight } , in terms of gas disk scale height \si { \scaleheight } , using the PencilCode .
They are performed in radial-azimuthal ( 2-d ) and radial-vertical ( 2.5-d ) extent .
The used particles of \mathrm { St } = 0.01 and 0.1 mark the upper end of the expected dust growth .
SI-activity is found up to very high dust-to-gas ratios , providing fluctuations in the local dust-to-gas ratios and turbulent particle diffusion \delta .
We find an SI-like instability that operates in r - \varphi even when vertical modes are suppressed .
This new azimuthal streaming instability ( aSI ) shows similar properties and appearance as the SI .
Both , SI and aSI , show diffusivity at \epsilon = 100 only to be two orders of magnitude lower than at \epsilon = 1 , suggesting a \delta \sim \epsilon ^ { -1. } relation that is shallow around \epsilon \approx 1 .
The ( a ) SI ability to concentrate particles is found to be uncorrelated with its strength in particle turbulence .
Finally , we performed a resolution study to test our findings of the aSI .
This paper stresses out the importance of properly resolving the ( a ) SI at high dust-to-gas ratios and planetesimal collapse simulations , leading else wise to potentially incomplete results .