car-aerodynamics - 10 Vehicle body aerodyna(ics The...

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Unformatted text preview: 10 Vehicle body aerodyna(ics The constant need for better fuel economy, g reater v ehi cle p erformance, reducti on i n w i nd noi se lev el and i mp rov ed road holdi ng and stabi li ty for a v ehi cle on the mov e, has p romp ted v ehi cle manufacturers to i nv esti g ate the nature of ai r resi st- ance or drag for di fferent body shap es under v ari ous op erati ng condi ti ons. ) erodynami cs i s the study of a soli d body mov i ng throug h the atmosp here and the i nteracti on w hi ch tak es p lace betw een the body surfaces and the surroundi ng ai r w i th v aryi ng relati v e sp eeds and w i nd di recti on. ) erodynami c drag i s usually i nsi g ni fi cant at low v ehi cle sp eed but the mag ni tude of ai r resi stance becomes consi der- able w i th ri si ng sp eed. Thi s can be seen i n F i g . 6 7 . 6 w hi ch comp ares the aerodynami c drag forces of a p oorly streamli ned, and a v ery hi g hly streamli ned medi um si 9 ed car ag ai nst i ts constant rolli ng resi st- ance ov er a typ i cal sp eed rang e. ) v ehi cle w i th a hi g h drag resi stance tends only marg i nally to hi nder i ts accelerati on but i t does i nhi bi t i ts ma1 i - mum sp eed and i ncreases the fuel consump ti on w i th i ncreasi ng sp eed. , ody styli ng has to accommodate p asseng ers and lug g ag e sp ace, the functi onal p ow er trai n, steeri ng , susp ensi on and w heels etc. thus v ehi cle desi g n w i ll confli ct w i th mi ni mi 9 i ng the body surface drag so that the body shap e fi nally accep ted i s nearly alw ays a comp romi se. ) n ap p reci ati on of the fundamentals of aero- dynami cs and the methods used to counteract hi g h ai r resi stance for both cars and commerci al v ehi cles w i ll now be e1 p lai ned. 10. 1 Viscou s air f low f u nda(ent als 14.1.1 Boundary layer : F i g . 6 7 . 2 ; ) i r has v i scosi ty, that i s, there i s i nternal fri cti on betw een adB acent layers of ai r, w henev er there i s relati v e ai r mov ement, conseD uently w hen there i s sli di ng betw een adB acent layers of ai r, energ y i s di ssi p ated. " hen ai r flow s ov er a soli d surface a thi n boundary layer i s formed betw een the mai n ai rstream and the surface. ) ny relati v e mov ement betw een the mai n ai rstream flow and the surface of 1600 12 00 8 00 4 00 4 0 8 0 12 0 160 ro llin g re s is ta n c e V e h ic le s p e e d (k m /h ) lo w a ir r e s is ta n c e ( C = .2 5 ) D h i g h a i r r e s i s t a n c e ( C = . 5 ) D R e s i s t i n g f o r c e s o p p o s i n g m o t i o n ( N ) Fig. 14.1 Comparison of low and high aerodynamic drag forces with rolling resistance 4 3 7 the body then takes place within this boundary layer via the process of shearing of adBacent layers of air. "hen air flows over any surface, air particles in intimate contact with the surface loosely attach themselves so that relative air velocity at the sur- face becomes 9ero, see Fig. 67.2. The relative speed of the air layers adBacent to the arrested air surface film will be very slow@ however, the ne1t adBacent...
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car-aerodynamics - 10 Vehicle body aerodyna(ics The...

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