This chapter is a study of the shear stress as a function of the shear rate for Newtonian and non-Newtonian biological materials.PowerPoint PresentationSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Viscosity and Flow of Liquids and Semisolids…Chapter 601/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity1This chapter is a study of the shear stress as a function of the shear rate for Newtonian and non-Newtonian biological materials.Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity2•“When a fluid or semisolid is subjected to a constant shearing force it flows, ie., it deforms continuously at a velocity that increases as the applied shearing force increases.”•Viscosity: quantifies the resistance of the fluid to flowLecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity3•Liquids and semisolids are usually pumped during processing•Viscosity plays a huge part in pump and conveyance system design•Viscosity may be dependent on moisture content, concentration, composition and prior treatments.Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity4•Newtonian Fluids (Newton 1687)–Simplest model–Covers most, but not all, ag products–Velocity behaves linearly w/ distance–Shear stress is linear function of the shear rate–Dynamic viscosity: proportionality constant for this relationshipLecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity5•The viscosity can be measured where the fluid of interest is sheared between two flat plates which are parallel to one another •Known as planar Couette flow. The shear stress is the ratio of the tangential force F needed to maintain the moving plate at a constant velocity V to the plate area A. •Couette flow:Low-speed, steady motion of a viscous fluid between two infinite plates moving parallel to each other. Lecture 10 – Viscosity and Flow (Ch. 6)•http://www.answers.com/topic/viscosity?cat=biz-fin01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity6•Dynamic viscosity (Figure 6.1)21; where =shear stress (N/m = Pa), =shear rate (s ), proportionality constant t m tm-=-=zyzzdvdydvdy aka dynamic viscosity (Pa-s)Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity7•Kinematic viscosity: dynamic viscosity/density(no force involved)2, m s stokesmur= =Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity8•Non-Newtonian Fluids•Relationship between shear stress and shear rate is NOT linear•Some also have a yield stress which must be obtained before flow begins.Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity9•Most common: pseudoplastic…convex curve towards the shear stress axis (Fig. 6.1b)–Apparent viscosity will decrease as shear rate increases•Dilatant fluids: concave toward shear stress axis (corn flour, wet beach sand: stiffens when walked on..select pumps carefully!)–Apparent viscosity increases as shear rate increases•Plastic: linear but intercept is at the yield stress (toothpaste: must stay on brush but must be exudable)•Casson-type plastic: has a yield stress but is not linear (chocolate)Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity10•Apparent viscosity = shear rate ratio at any given shear rate•Pseudoplastic and Dilatant materials, eqtn. 6.2, Table 6.2)•Newtonian: n=1, k=dynamic viscosity; : =shear stress k=consistency coefficient n=flow behavior indexndvk wheredyt t� �=� �� �Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity11•Plastic and Casson-type plastic behavior (more general case…Herschel-Bulkley model, eqtn. 6.3 Table 6.3)•Chocolate and other Casson materials follows this where N = ½ and the yield stress is taken to the ½ power0 0; : =shear stress, yield stress k=consistency coefficient n=flow behavior indexndvk wheredyt t t t� �= + =� �� �Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity12•Temperature Dependency:–Viscosity decreases with an increase in Temp.–Typically 2% per degree C–For some materials (fruit juices) the T effect follows an Arrhenius relationship (Eqtn. 6.5 page 193)exp , viscosity, Pa-sm m m�� �= =� �� �EaRTLecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity13•Time dependent Viscosity (figure 6.2 page 196)Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity14•Time dependent Viscosity–Thixotropic examples (viscosity decreases with time)»Gelatin, shortening, cream, paints–Rheopectic examples (viscosity increases with time)»Highly concentrated starch solutions…gravyLecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity15•Flow in a pipe:•Darcy-Weisbach –Newtonian–-Non-newtonian22L vH fd gD =4 8nK L VPd d� �D =� �� �Lecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity16•Examples of viscometersLecture 10 – Viscosity and Flow (Ch. 6)01/13/19 BAE2023 Physical Properties of Biological Materials Lecture 10 Viscosity17Viscosity and Flow of Liquids and Semisolids…Chapter
View Full Document
Unlocking...