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Lecture #3 Mechanics of Fluids

Lecture #3 Mechanics of Fluids. Plan of the lecture. I . Basic concepts and formulae of hydrodynamics 1. Ideal liquid definition 2. The Bernoulli equation 3. The equation of continuous flow 4. Real liquid. Laminar and turbulent flow

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Lecture #3 Mechanics of Fluids

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  1. Lecture #3 Mechanics of Fluids

  2. Plan of the lecture I. Basic concepts and formulae of hydrodynamics 1. Ideal liquid definition 2. The Bernoulli equation 3. The equation of continuous flow 4. Real liquid. Laminar and turbulent flow 5. Newton’s formula for the viscous friction force 6. Reynolds’s number 7. Poiseuille formula 8. Hydraulic resistance II. Foundations of Biorheology 1. Basic definitions 2.Flow curves for Newtonian and non-Newtonian liquids 3. Rheological properties of blood III. Haemodynamics 1. Basic characteristics of blood dynamics in the blood circulation system 2. Work and power of heart 3. Pulse wave

  3. Basic Concepts and Formulae of Hydrodynamics Ideal liquid is a completely incompressible and non-viscous liquid Incompressiblemeans that at compression the distance between molecules and atoms in liquid is not changed, i.e. the interdependence of density and pressure is neglected Non-viscous means that there is no internal friction between molecules and atoms in liquid

  4. Bernoulli equation(for the ideal liquid) Pis pressure in some point of the liquid (Pa),  is liquid density (kg/m3), h is height of the point with the reference of some level (m), v is velocity of liquid at this point (m/s), gis free fall acceleration (9.8 m/s2) P is static pressure (Pa) gh is hydrostatic pressure (Pa) ρv2/2 is hydrodynamic pressure (Pa)

  5. Bernoulli equation is a corollary to the law of mechanical energy conservation.

  6. Equation of continuous flow (for real and ideal liquids) Q = S1v1= S2v2= const Q [ m3·s-1] is the volume velocity of liquid flow (the volume of liquid passing through the cross-section of a pipe per unit time) S [m2] is the area of the pipe cross-section v [m·s-1] is the average velocity of the liquidfor the section This equation is the corollary to the law of mass conservation for non-compressible liquid.

  7. Real liquidsare compressible and viscous liquids. The real fluid flows by layers. Liquid layer is a part of liquid where velocity of any point is the same. There is a friction between the layers. Forces of internal, or viscous friction F arise from the relative movement of layers.

  8. Newton’s formula (1687) (for the viscous friction force) F is the force of viscous friction between the liquid layers (N)  is coefficient ofviscosity (or dynamic viscosity) (Pa·s) V is the velocity of the liquid layer movement (m/s) z is the coordinate perpendicular to the layer contact boundary (m) is a velocity gradient absolute value S is the area of the liquid layers contact surface

  9. Coefficient of viscosity, or viscosity () in metric system SI is measured in Pascal-seconds (Pas) For example, viscosity of water is 1.00510-3 Pas ≈ 1 mPa·s at temperature 20С; blood viscosity is (4 - 5) ·10-3 Pa ·s ≈ 4 - 5 mPa·s at temperature 37°C.

  10. Real liquids are subdivided into Newtonian and non-Newtonian liquids with respect to their viscosity properties. Non-Newtonian liquids do not comply with the Newton’s formula(like complex liquids inhomogeneous in the composition of particles forming the liquid). Newtonian non-Newtonian Coefficient of viscosity alsodepends on the values, characterizing the conditions of the liquid flow. For example, on the speed gradient. Coefficient of viscosity () depends only on the kind of liquid and its temperature As the temperature increases, the coefficient of viscosity decreases!

  11. For Newtonian liquids speed of the liquid layers increases as the distance from the wall increases. The layer touching the walls is considered to be at rest. Liquid layer speed (v) in a portion of a cylindrical pipe with the length L and radius R on the distance (r) from the center of the pipe is described by the formula: P1is the pressure at the beginning of the pipe section; P2 is the pressure at the end of the pipe section

  12. The particles moving along the pipe axis ( r = 0)have the maximum velocities (vmax) : The mean value of speed (vmean) :

  13. POISEUILLE FORMULA: Qis the volume of real liquid passing through the cross-section of a pipe per unit timeat stationary laminar flow (m3/s)

  14. Analogy between the processes of the liquid flow in the pipe and electric current flow in a conductor Liquid flow in the pipe Electric current flow in a conductor Q is volume velocity P1 - P2 is pressures difference X is hydraulic resistance I is current strength 1- 2 is potentials difference R is electrical resistance .

  15. Total hydraulic resistance(Xtotal )(rules of calculation) Pipes with hydraulic resistance X1, X2, ... , Xn are connected in parallel : in series : ,

  16. Liquid flow types : Laminar flow Turbulent flow Liquid layers slide relative to each other, without mixing (vortex flow) Liquid layers are mixing. Vortices are created due to the high friction forces.

  17. Reynolds’ number (Re) characterizes conditions under which the liquid can change type of flow - from laminar to turbulent : The value of the Reynolds’ number, at which transition from laminar to turbulent flow takes place, is calledcritical Reynolds’ number (Recr). For smooth cylindrical pipesRecr ≈2300. Re < Recr – flow is laminar Re > Recr – flow is turbulent

  18. Kinematic viscosity ( ) The unit is [ m2·s-1] Reynolds’ number:

  19. Viscosimetry Viscosity measurement is called viscosimetry, the instruments for measuring of viscosity are calledviscometers. Ostvald’s viscometer Hess’ viscometer (used in medicine for measurement of blood viscosity)

  20. Foundations of Biorheology The formulae of hydrodynamics are mainly used to describe Newtonian liquids only. That is why to describe the flow of blood (a non-Newtonian liquid) through blood vessels, notions and formulae of rheology are used. Rheologyis a field of physics, studying deformations and fluidity of real liquids and media. In biorheology the blood-flow is regarded as a shear deformation. The basic quantitative parameters of biorheological processes are: . • shear stress () • shear angle ( ) • shear velocity ( )

  21. Rheological model of liquids flow • Applied force (F) causes elastic shear deformation and the parallelepiped undergoes deformation Shear angle characterizes the value of deformation • The horizontal layers of parallelepiped are shifted relatively to each other in the direction of the OX axis Under such conditions an elastic force F is created in the layers contact surfaces • Elastic force F causes the shear stress between thelayers contact surfaces, which is calculated by formula

  22. In the process of layer shifting the shear angle  increases. The time derivative of  is called shear velocityand isdesignated as Shear velocityin hydrodynamics called as velocity gradient absolute value. . Newton’s formula for the force of viscous friction in terms of rheology:

  23. Flow curves The graph showing the dependence of on is called flow curve 1 – Newtonian liquid 2 – Pseudo-plastic liquid 3 – Dilatant liquid 4 and 5 – Viscous-plastic liquids (blood) 0is the limit shear stress

  24. Models describing the behavior of viscous-plastic liquids at different shear rates Shvedov-Bingham’s model Caisson’s model at low values of shear rate at high values of shear rate k is a coefficient equidimensional with viscosity k is Kesson’s coefficient

  25. Rheological Properties of Blood Blood and blood plasma are non-Newtonian liquids. Blood is one of the viscous-plastic liquids. Blood serum is a Newtonian liquid. Non-Newtonian properties of blood are linked first of all with the blood cells in blood plasma.

  26. Erythrocytes are most of all responsible for the non-Newtonian properties of blood. This can be accounted for by a number of factors: • a large amount of erythrocytes in blood • (of the order of 5 mln per 1 mm3);

  27. the capacity of undergoing deformation when passing through small-diameter vessels; • the capacity of erythrocytes to form aggregate arrangements, the size of which depends on the conditions of the blood flow.

  28. Haematocrit value (Н) is equal to the relative volume of blood cells in blood, in percentage. Normally, this index in humans is about 46 - 48 %. The haematocrit value increases, the blood viscosity increases. Blood viscosity, mPa ·s Haematocrit

  29. HAEMODYNAMICS Basic Characteristics of Blood Dynamics in the Blood Circulation System

  30. The distribution curve of the mean flow velocity of blood and meanblood pressure in different parts of blood circulation system V, P

  31. Indirect bloodless way of blood pressure measuring (according to Korotkov) The physical idea of this method is based on an approximate equality of pressures in the cuff and in the soft tissues, and on the possibility of registering the audio signals accompanying the vibration of the vessel walls in turbulent flow of blood through the vessels.

  32. The manometer readings at the first pulse beat show the maximum, or systolic pressure (normally 16 kPa, or 120 mmHg). The manometer readings at the moment of a sharp decrease of tones correspond to the minimum, or diastolic pressure(11 kPa, or 80 mmHg)

  33. Work and Power of Heart When contracting, the heart does work against the force of the blood pressure, thus pushing the blood into the artery, and gives it kinetic energy. Work which done by the ventricle to push the blood per one contraction can be calculated using the formula A = P·ΔV, where P is the mean pressure in the ventricle at contraction, ΔV is a variation in the volume of the ventricle. The variation in the volume of the heart chamber per one contraction is referred to as heartbeat volume (or stroke volume), that is, ΔV=Vs . The kinetic energy of the blood pushed onto the artery is where m is the mass of the pushed blood, ρ is the blood density, v is the mean velocity of the blood in the artery.

  34. Work and Power of Heart(continuation) Thus, work done by the left ventricle (Alv): Alv = Vs(P+ρv2/2) Work done by atrium being neglected, and assuming that work done by the right ventricle approximately equals 20% of that of the left one, we get A = 1,2·Vs(P+ρv2/2) If we represent this in numerical expression, we will see that work done by the heart per one heart contraction approximately equals to 1 J. The time of one heart contraction being approximately 0.3 s, the power of heart approximately equals to 3.3 W.

  35. Pulse wave • Pulse wave is a high-pressure wave that spreads through aorta and arteries. It is caused by blood being pushed from the left ventricle during the systole. • Pulse wave speedis 5-10 m·s-1 . The pulse wave corresponds to the blood stream pulsation speed in big arteries. • Blood speed (0,5 m·s-1) is considerably lower than the speed of the pulse wave. • Contracting during the systole, the heart pushes blood to aorta and branching arteries. If the walls of these vessels were rigid, the pressure developed at the heart output would be transmitted to the peripherals with the speed of the sound. • The walls of the vessels being elastic, the blood pushed by the heart during the systole expands the aorta, arteries and arterioles. The blood inflow to the big vessels during the systole exceeds its outflow to the peripherals.

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