| Turbulence exits in most surface parts of vehicles like aircrafts or ships.Reducing drags caused by turbulence can save a large mount of energy and increasevehicles'loading ability, enlarge their voyage or improve missile's attacking range inthe same situation. So design low-drag surface and make deeper research aboutmechanics of how to decrease drag have important practical meanings. Models withbionic knaggy unsmooth surface based on cuticles of organisms such as humpbackand dung beetles, get good results in drag-reducing in wind tunnel test. The purposeof this article was to analyze characteristics of flow and probe into drag reducingmechanism of unsmooth surface by flow visualization and numerical simulation.Three models which showed better drag reducing effects in ex-experiments wereused in force measuring experiments in wind tunnel .(These three models were No.1model having convex dome unsmooth structure in tail, No. 2 model having dimpleconcave in head and No. 3 model having dimple concave in tail.) In the experiments,Models had flow velocity ranging from 30m/s to 44m/s, having 00 attack angles in288k environmental temperature. The results showed that all these three models havesmaller drag than smooth model in experimental flow velocity. Especially the modelNo. 3, it had the best drag reducing effects in all experimental flow velocity andchanged softly among those flow velocity. Moreover a 12.65% reducing abilityappeared in 30m/s of flow velocity. The most remarkable factor what affectedinfluenced drag was flow velocity: drag increased as flow velocity was becominglarger. The way of layout of unsmooth structure also laid in notable factors, whichmean unsmooth structure in tail had better effects than that in head.Smooth and three unsmooth models were tested with oil flow visualization. Thistechnique was one of convenient way to show separated flows in complicated air flow.Through analyses of oil flow spectrum, characteristics of flow flied around rotatingmodels can be obtained: flows had same characters in all sections in flow direction,i.e. flows were symmetry. Unsmooth structures affected flows around themdramatically. Marked particles stimulated in each dimple concave unit, where hadsmall cutting stresses. In addition, adjacent parts were influenced by these structuresand striping zones with same dimension of dimple diameter showed stress stimulatedin these places. However convex dome units had same flow appearance withball-shape objects. Separated flows exited behind every single dome. Distinct oilstimulation parts appeared in models'head, shoulder part and tail which had strongcurvature changing. But compared with the smooth model, oil stimulation zones inthree unsmooth models had changed a lot both in width and position, whichabsolutely resulted from these little unsmooth structures. These changed showed thatflows separated much later in unsmooth models than in smooth, especially in modelsNo.1 and No. 3, having unsmooth structure in tail, which had the smallest oilstimulation zones showing the effects of postponed separated point. So these modelshad smaller pressure drag. Because some disadvantages in wind tunnel experiments like long experimentalcircle, large investigation funds and difficulties in analyzing some variables of a flowparticle in amount, numerical simulation for wind tunnel test with FLUENT was anadditional method, in order to reinforce experimental results and verify with eachother. Moreover simulation results also could supply data for mechanism analyzing.Through comparison with drag and oil flow pictures from experiments, the results ofsimulation showed their own reliability and efficiency. Numerical simulations were made for the models which had unsmooth units ofconvex domes and dimple concaves with diameter 1mm, depth 0.5mm,rectangle-shape distributing in tails. Drag reducing mechanism of these two types ofunsmooth structure was issued without being influenced by any other factor. Theresults showed that pressure drag of models shared a portion more than 70% in totaldrag. And for the unsmooth models, friction drags of them were increased slightlythan the smooth model but pressure drags were decreased about 16% whichcounteracted friction drags'increases and at last made total pressure drags decreasedabout 10%. Comparing with flow field structure and variables in flows of both smoothand unsmooth structures, drag reducing mechanism of unsmooth structure could besummarized as follow. Unsmooth structures made flows slow down at the places likedimple concaves inside and space between each two convexes. When air flowsthrough unsmooth surfaces, higher speed flow contact with some lower speed flowsthat exist in unsmooth structures. In this condition, speed gradient is much smaller... |
PDF Full Text Request