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viskoznost - Hrvatska enciklopedija
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Viskoznost uzrokuju međumolekulske kohezijske sile u fluidu i adhezijske sile između fluida i krutoga tijela kroz koje se ili oko kojeg se strujanje odvija. Slojevi fluida uz stijenke posude (cijevi, korita rijeke itd.) usporavaju brže slojeve.</p><p>Viskoznost koja djeluje na površinu ploštine <span style="color: #000000"><i>S</i></span> između dvaju slojeva fluida razmjerna je <a href="https://www.enciklopedija.hr/clanak/gradijent">gradijentu</a> relativne brzine <span style="color: #000000"><i><b>v</b></i></span>, tj. brzini kojom se relativna brzina gibanja mijenja od sloja do sloja (d<span style="color: #000000"><i>y</i></span>):</p><p><span style="color: #000000"><i>F</i></span> = <span style="color: #000000"><i>ηS</i></span> (d<span style="color: #000000"><i><b>v</b></i></span>/d<span style="color: #000000"><i>y</i></span>),</p><p>gdje je koeficijent <span style="color: #000000"><i>η</i></span> dinamička viskoznost.</p><p><span style="color: #000000"><i>Otpor gibanju čvrstih tijela kroz fluid</i></span> ovisi o dimenzijama i obliku tijela, brzini gibanja, dinamičkoj viskoznosti i gustoći fluida. Za glatku kuglu polumjera <span style="color: #000000"><i>r</i></span> koja se malom brzinom <span style="color: #000000"><i>v</i></span> giba u fluidu dinamičke viskoznosti <span style="color: #000000"><i>η</i></span>, ili taj fluid laminarno struji oko nje, otpor je: <span style="color: #000000;"><i>F</i></span> = 6π<span style="color: #000000;"><i>ηvr</i></span> (Stokesov zakon). (→ <span style="font-variant: small-caps; color: #000000;"><a href="https://www.enciklopedija.hr/clanak/stokes-george-gabriel">stokes, george gabriel</a></span>)</p><p><span style="color: #000000"><i>Dinamička viskoznost</i></span> (znak <span style="color: #000000"><i>η</i></span>) je fizikalna veličina kojom se opisuje svojstvo fluida da se opire različitim brzinama strujanja, tj. unutarnje trenje među slojevima fluida. Mjerna jedinica dinamičke viskoznosti je paskalsekunda (Pas). Pokretljiviji fluidi imaju manju dinamičku viskoznost: dinamička viskoznost glicerina na sobnoj temperaturi iznosi 1,4 Pas, vode 0,001 Pas, a zraka 17 × 10<sup>–6</sup> Pas. Dinamička viskoznost različitih stakala, smole i sl. veoma je velika, u pravilu veća od 10<sup>9</sup> Pas. Dinamička viskoznost tekućina smanjuje se povećanjem temperature, jer termičko gibanje smanjuje privlačne međumolekulske sile, a dinamička viskoznost plinova povećava se s povećanjem temperature jer temperaturom povećana difuzija uzrokuje izjednačavanje brzina susjednih slojeva, što je ekvivalentno sili trenja među slojevima. Dinamička viskoznost motornih ulja vrlo malo ovisi o temperaturi.</p><p><span style="color: #000000"><i>Kinematička viskoznost</i></span> (znak <span style="color: #000000"><i>ν</i></span>) je fizikalna veličina koja je omjer dinamičke viskoznosti <span style="color: #000000"><i>η</i></span> i gustoće fluida <span style="color: #000000"><i>ρ</i></span>, dakle: <span style="color: #000000"><i>v</i></span> = <span style="color: #000000"><i>η</i></span>/<span style="color: #000000"><i>ρ</i></span>. Mjerna jedinica kinematičke viskoznosti je kvadratni metar po sekundi (m²/s).</p> </div> </div> </div> <div class="container citiranje"> Citiranje: <p> viskoznost. <i>Hrvatska enciklopedija</i>, <i>mrežno izdanje.</i> Leksikografski zavod Miroslav Krleža, 2013. – 2025. 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