Internal wave excitation by turbulent convection

Internal Wave Excitation by Turbulent Convection D. Lecoanet 1 , M. Le Bars 2,3 , K. J. Burns 4 , E. Quataert 1 , G. M. Vasil 5 , B. P. Brown 6 & J. S. Oishi 7 University of California, Berkeley, USA IRPHE, Marseille, France University of California, Los Angeles, USA MIT, Cambridge, USA University of Sydney, Sydney, Australia University of Colorado, Boulder, USA Farmingdale State College, Farmingdale, USA Abstract Convection near a stably stratified region can excite internal gravity waves. This occurs in a wide range of geophysical settings, including the Earth’s atmosphere and core, as well as in astrophysical systems like some gas giants like Saturn, and most stars including the sun. In each of these circumstances, the wave excitation mechanism and subsequent wave energy flux spectrum is of great interest. In this work, we study both the excitation mechanism and the wave energy flux spectrum by a combination of experiments and numerical simulations. We find the waves are excited in the bulk of the convective region. Preliminary comparisons to analytic predictions of the wave energy flux are presented. Introduction In nature, it is not unusual to find stably stratified fluid adjacent to convectively unstable fluid. This can occur in the Earth’s atmosphere, where the troposphere is convective and the stratosphere is stably stratified; in lakes, where surface solar heating can drive convection above stably stratified fresh water; in the oceans, where geothermal heating can drive convection near the ocean floor, but the water above is stably stratified due to salinity gradients; possible in the Earth’s liquid core (Hirose et al., 2013), where gradients in thermal conductivity and composition diffusivities maybe lead to different layers of stable or unstable liquid metal; and, in stars, as most stars contain at least one convective and at least one radiative (stably stratified) zone. Internal waves can play a dynamically important role via nonlocal transport. Momen- tum transport by convectively excited internal waves is thought to generate the quasi- biennial oscillation of zonal wind in the equatorial stratosphere (Baldwin et al., 2001), an important physical phenomenon used to calibrate global climate models. Angular momentum transport by convectively excited internal waves may play a crucial role in setting the initial rotation rates of neutron stars (Fuller et al., 2015). In the last year of life of a massive star, convectively excited internal waves may transport even energy to the surface layers to unbind them, launching a wind (Quataert and Shiode, 2012). In each of these cases, internal waves are able to transport some quantity—momentum, an- gular momentum, energy—across large, stable buoyancy gradients. Thus, internal waves represent an important, if unusual, transport mechanism. The nonlocal transport by internal waves depends on the amplitude of the excited waves, as well as what type of waves are excited. For instance, in some astrophysical contexts, only the highest frequency waves can propagate far enough to lead to interesting transport (Fuller et al., 2014). The literature also includes various proposed mechanisms VIII th Int. Symp. on Stratified Flows, San Diego, USA, Aug. 29 - Sept. 1, 2016