Thermal Instability in Shearing and Periodic Turbulence

The thermal instability with a piecewise power law cooling function isinvestigated using one- and three-dimensional simulations with periodic andshearing-periodic boundary conditions in the presence of constant thermaldiffusion and kinematic viscosity coefficients. Consistent with earlierfindings, the flow behavior depends on the average density, . When$\bra{\rho}$ is in the range 1-5 x 10^{-24} g cm^{-3} the system is unstableand segregates into cool and warm phases with temperatures of roughly 100 and10^4 K, respectively. However, in all cases the resulting average pressure

is independent of and just a little above the minimum value. For aconstant heating rate of 0.015 erg g^{-1} s^{-1}, the mean pressure is around24 x 10^{-14} dyn (corresponding to p/k_B ~ K cm^{-3}). Cool patches tend tocoalesce into bigger ones. In all cases investigated there is no sustainedturbulence, which is in agreement with earlier results. Simulations in whichturbulence is driven by a body force show that when rms velocities of between10 and 30 km/s are obtained, the resulting dissipation rates rates arecomparable to the thermal energy input rate. The resulting mean pressures arethen about 30 x 10^{-14} dyn, corresponding to p/k_B ~ 2170 K cm^{-3}. This iscomparable to the value expected for the Galaxy. Differential rotation tends tomake the flow two-dimensional, that is, uniform in the streamwise direction,but this does not lead to instability.Comment: 10 pages, 13 figure