Part 2 of a Computational Study of a Drop-Laden Mixing Layer

This second of three reports on a computational study of a mixing layer laden with evaporating liquid drops presents the evaluation of Large Eddy Simulation (LES)models.The LES models were evaluated on an existing database that had been generated using Direct Numerical Simulation (DNS).The DNS method and the data- tively coated thin electroactive-polymer (EAP)films be developed for use in spaceborne microwave and optical systems.In these mirrors,the EAP films would serve as both structures and ac- tuators.EAPs that are potentially suit- able for such use include piezoelectric, electrostrictive,ferroelectric,and di- electric polymers.These materials ex-THE POWER MANAGEMENT LEADERFlexible Shields for Protecting Spacecraft Against Debris A report presents the concept of Flexshield —a class of versatile,light- weight,flexible shields for protecting spacecraft against impacts by small me- teors and orbiting debris.The Flexshield concept incorporates ele- ments of,but goes beyond,prior space- craft-shielding concepts,including those of Whipple shields and,more recently, multi-shock shields and multi-shock blankets.A shield of the Flexshield type includes multiple outer layers (called “bumpers ”in the art)made,variously,of advanced ceramic and/or polymeric fibers spaced apart from each other by a lightweight foam.As in prior such shields,the bumpers serve to shock an impinging hypervelocity particle,caus- ing it to disintegrate,vaporize,and spread out over a larger area so that it can be stopped by an innermost layer (back sheet).The flexibility of the fabric layers and compressibility of the foam make it possible to compress and fold the shield for transport,then deploy the shield for use.The shield can be at- tached to a spacecraft by use of snaps, hook-and-pile patches,or other devices. The shield can also contain multilayer insulation material,so that it provides some thermal protection in addition to mechanical protection. This work was done by Eric L.Christiansen and Jeanne Lee Crews of Johnson Space Center . derived by apply- ing a spatial filter to the DNS set,gov- ern the evolution of the larger scales of the flow and can therefore be solved on a coarser grid.Consistent with the reduction in grid points,the DNS drops would be represented by fewer drops,called “computational drops ”in the LES context.The LES equations contain terms that cannot be directly computed on the coarser grid and that must instead be modeled.Two types of models are necessary:(1)those for the filtered source terms representing the effects of drops on the filtered flow field and (2)those for the sub-grid scale (SGS)fluxes arising from filter- ing the convective terms in the DNS equations.All of the filtered-source- term models that were developed were found to overestimate the filtered source terms.For modeling the SGS fluxes,constant-coefficient Smagorin- sky,gradient,and scale-similarity mod- els were assessed and calibrated on the DNS database.The Smagorinsky model correlated poorly with the SGS fluxes,whereas the gradient and scale- similarity models were well correlated with the SGS quantities that they represented.