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The maximum sustainable heat flux in stably stratified channel flows

In analogy to the nocturnal atmospheric boundary layer, a flux‐driven, cooled channel flow is studied using direct numerical simulations. In agreement with earlier studies, turbulence collapses when the surface cooling exceeds a critical value. In that case, laminarization occurs. Here, the so‐called maximum sustainable heat flux (MSHF) hypothesis is tested. It explains why laminarization will occur at strong cooling rates. It states that in stratified flows the downward heat flux is limited to a maximum, which, in turn, is determined by the momentum of the flow. If the heat extraction at the surface exceeds this maximum, near‐surface stability will increase rapidly, which hampers efficient vertical heat transport further. This positive feedback eventually causes turbulence to be suppressed fully by the intensive density stratification. This framework is used to predict the collapse of turbulence and a good agreement between theory and simulations is found. Therefore, it is concluded that the maximum sustainable heat flux mechanism explains the collapse of turbulence in this kind of flow. In future work, there is the need for an extension to more realistic configurations, allowing for Coriolis effects and more realistic surface boundary conditions.