简介：Thecontrolofturbulencebydimples/pimpleshasdrawnmoreandmoreattention.Theobjectiveofthispaperistoinvestigatetheeffectivenessoftheactivedimples/pimplesforthedragreductionintheincompressibleturbulentflow.Firstly,thedragreductionbytheoppositioncontrolbasedonactivedimples/pimplesatthelowerwallisstudiedviathedirectnumericalsimulationoftheturbulentchannelflow.Itisfoundthatlargeactivedimples/pimplescannotsuppressthestreamwisevorticessignificantlyandthusalmostnodragreductionisachieved.Smallactivedimplesandpimpleswiththediameterofonefourthofthestreakwidthcanbothreducethefrictiondrag,butpimpleswillinducealargerpressuredragthandimples.Thenthesuboptimalcontrolschemeisexaminedbasedonsmallactivedimplesusingthespanwisewallshearinformationonly.Itisshownthatthefrictiondragdecreasesbyabout4.5%butthetotaldragisonlyreducedbyabout2.7%abatedbythepressuredrag.Comparedwiththeactuationoftheall-pointblowing/suctionortheall-pointwallmovement,theeffectivenessoftheturbulentdragreductionbasedonactiveshallowdimplesismuchsmaller.

简介：Inphysicalmodeltestsforhighlyreflectivestructures,oneoftenencountersaproblemofmultiplereflectionsbetweenthereflectivestructuresandthewavemaker.Absorbingwavemakerscancancelthere-reflectivewavesbyadjustingthepaddlemotion.Inthispaper,weproposeamethodtodesignthecontrollerofthe2-Dabsorbingwavemakersysteminthewaveflume.Basedonthefirst-orderwavemakertheory,afrequencydomainabsorptiontransferfunctionisderived.Itstimerealizationcanbeobtainedbydesigninganinfiniteimpulseresponse(IIR)digitalfilter,whichisexpectedtoapproximatetheabsorptiontransferfunctionintheleastsquaressense.AcommonlyusedapproachtodeterminetheparametersoftheIIRfilterisapplyingtheTaylorexpansiontolinearizethefilterformulationandsolvingthelinearleast-squaresproblem.However,theresultisnotoptimalbecausethelinearizationchangestheoriginalobjectivefunction.Toimprovetheapproximationperformance,weproposeaniterativereweightedleast-squares(IRLS)algorithmanddemonstratethatwiththefiltersdesignedbythisalgorithm,theapproximationerrorscanbereduced.Physicalexperimentsarecarriedoutwiththedesignedcontroller.Theresultsshowthatthesystemperformswellforbothregularandirregularwaves.