Lecture_29_notes - ChE 374~Lecture 29—External...

Info iconThis preview shows pages 1–10. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Background image of page 10
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: ChE 374~Lecture 29—External Flows—Drag External flows are flows over objects: cars, wings, buildings, etc. Objective: Understand and compute drag forces on objects. — Concepts: Flow separation, Lift, Drag, Drag Coefficient, Laminar/ Turbulent. Drag is the force a fluid exerts on an object IN THE FLOW DIRECTION. — Lift is the fierce a fluid exerts on an object perpendicular to the flow direction. — Drag is due to pressure forces and viscous (friction) forces. — Consider a flat plate (1) aligned with the flow; (2) angled with the flow; (3) perpendicular to the flow. We have friction only; pressure and friction; and pressure only, respectively. —- Pressure drag also called form drag, and is due to flow separation (which is caused by friction). Flow separation. Streamlines detatch from objects. — As flow around oject velocity: low then high, then low; pressure: high then low then high. Bernoulli equation relates pressure and velocity. * Friction causes a loss in pressure so P cannot recover fully and the flow separates, leaving a low pressure wake behind the object. — This is counteracted by streamlining, to avoid separation. * Reduces pressure (form) drag, but increases friction drag. * At high velocities, pressure drag dominates over friction drag. — SEE FIGURES ON PPT SLIDES Drag Coefficients — Recall: f = 75217—2. — NOW: CD = Tifm. * A is the frontal projected area (or sometimes the plan-view projection, e.g., wings). * Usually combine effects of pressure and friction. e Spheres: * Low Re (up to 3) Stokes law: CD = 24/Re. * Higher Re requires correlations: see Table 11-2, Figs, 11—34, 11—36. ~ Other shapes in Table 11—1, 11—2. — Flat Plates: =o< Critical Re between laminar and turbulent is 5 X 105. * Laminar: CD = 1.33/Rel/2. * Turbulent: CD = 0.074/Re1/5. * Laminar and Turbulent on one plate (in the turbulent region): CD = 0.074/R81/5 — 1742/Re. * See correlation for rough plates. 0 Terminal Velocity Example — Balance weight, buoyancy, and drag force. ~ Drag force depends on velocity: FD = %W2ACD. — Iteration required since CD = OD(Re). LEL-‘FW 1"[' ~— ExhonaJ Flag; “flamwmmwuga "‘1' 1'“ Tex-1'. HQ I Ham, Ln 77/73 I I v "(mum-«4!, ax 'TD-ktwwaw) UPDJM7 z/éy‘fifi I (sparkle, F’a} wt)“ ’ A #0 QW+A¥~ awvv wu {v Deimw‘n-r neéJ/u! "-“PW/‘ulf { bra], Hm Lav/H. bfggma {x-Li'r’x‘u/ //¢.o; A») T331 «2 Formal) I I ,W_~.. ._. M ' Exk/Muj Plan-7% m '71-“ 91’795')’ 3’ :n4fifina” ’ 1M4“) 7’ (Mr-4'7 ,4,» in t has J “and; H's» MW ,7 " F’9U} (fl-1M M53? ’5 MW»! M "L Q "‘ ‘5"; 00 V‘) 'YH‘ QM [Ir straw up,“ ’51"/. , Exampiao g . Dfa; an Gay-Gr 57 .. 'T’rcoa éu,‘/v'»'w;fi/ M}, Punk‘t/go . A‘fithyv‘é mic; Lrw Kfi/ Dbj(£4_:ffmim U»c’ms-}v-J «J 1" 05!’ 1"” Ora? do?!“ 0. adj-Eric Cord-(Pt); ‘_ 4 L4,- ‘ L‘M"““"7 /{-7Lu/J W . 1354'!) r 014de glwpm. D(;<U (id'ép . F712“) (2“ PM‘I};W. M 71’" 6J7]! a. (V6373; cm a” 4494/ J, £9 TL»: flflg ’}>;mc./)m‘ D08 Me ‘3)“: {a CD "PM/MW ion,“ ..__‘7 waall {a L”; @ V§$com fingm| k1, Fahd] ~62 Snafu “‘4; :79 a. of n. J 6(¢_r) __,_ ,, c, A m“! T 7mm)- 791W W, 1.5, ‘D W ,LD éaPeHQJur— QFM a." 1‘ 0"”— Flak) v.5; sygow‘...“ ‘rMJ ¢—..a PM“ émet-"'M '47 Q’t‘w’f'“ Walk/t4 11w 714 nip)“ng Ecol! "’9 Efofif’l’ ' CM wave) w/W’ '— W A W M W W m 37. 261% _./'\-— __._, v 5 “Raw 5 ‘ Fla-«3 61F. ficlauJ/f f“ ’4‘?!“ m“? Java, 7 {NJ/'69 [op all [MA—Q;- awake! ) Dam ‘p’h’tm—n WV"; ’ flfrflL-A 5"“ QM" léssw poo $9? ;s 73%; to /fi¢}£o--~. ..E;:”;ZI:,T”"7 9W” A9 51/0“) W m?“ sp‘w Shcnwlrm 7?}, (last {oi-PM} \M/oa'y’y Incl/(.444 - "rt—UM: % 4% 4"" Chi-wt”. + P \l?_ % f’vivé " [35}? “FHA. £7~1 ,u—v, w say-mm s7 ammzw- ‘ Red...“ T!» (MQa’H‘M. “4% . RECIth £07“ bf“? (?l(mr baa-7 3 _ W fink-Jim wk.) 7 " Imam. . é’l’lfldn‘vq‘a QMB 6rcfl*” g{ ww ~Do-fAutgi é», [Ag/cw D“; a; In“) g?“ch LIL“: )ovJ) ‘9, 4%. D3 éemocwt A. “I... E”*“‘”” 5/“) | CD ' C» (12!, PEP“ “’1. Fr 1 f D) \J) fl/ 6 é PM’M 3 Dim/5 '3 éfwfi’i MP, 951i HEM W"? FD, f, L, v, fl, .5 D “’7 $0” m7, 4,7 w:th .p' 4? 3 *1" 0? VD. A? 1> 0‘ “ ' M‘ AP VJa-MMT r“, 7. fut/L L C v Z a7 4, : FD - ’ 4 .TZWUA I“? C, ; (1,747., 1,5110] 121 7 10 Chemical Engineering 374 Fluid Mechanics Fall 201 1 External Flows gBYU' IIKHKID [ Hum ‘\r “Ill Hm! rmfln’urul I-' .1 mi" :1 Ill) ‘.1 0| w r mum mm um [L] 1.0 in mi) moo lnmuIIX),(l1)0|l,llm,0(H) l'unn-lv le-inulxls munlx‘r hfll'p “=1” For mitigation of vortex—shedding induced vibration : Eliminates cross-wind vibration, but increases drag coefficient and along—wind vibration t j ‘ Helical strakes _. 1/ ...
View Full Document

This note was uploaded on 03/11/2012 for the course CHE 374 taught by Professor Davidlignell during the Fall '12 term at Brigham Young University, Hawaii.

Page1 / 10

Lecture_29_notes - ChE 374~Lecture 29—External...

This preview shows document pages 1 - 10. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online