What are Essentials Topics of Fluid Mechanics for CFD Modeling
by
Dr. Sharad N. Pachpute
Introduction of Fluid Flows
 Fluid mechanics deals with flow physics for liquids, gases, or any flowing medium by considering external forces and their effect on the deformation of fluid flows
 It is an essential subject for many engineering and applied sciences
 Mechanical, chemical, civil engineering, and environmental science
 Geophysics, Meteorology, Oceanography
 Astrophysics, and Biology
 Computational Fluid Dynamics (CFD): It is a subject based on fluid mechanics and numerical methods
 Convective Heat or Mass Transfer: Fluid mechanics plays a role in the bulk motions of fluid with heat transfer. The modeling of convective heat transfer is mentioned on the webpage of CFD flow engineering.
 Due to the importance of fluid in real life, nature, and industrial applications, this subject has been important for many decades for scientists, engineers, and astrophysicists
Major Applications of fluid Flows
 Any application relevant to fluid flow
 Automobile, chemical, and processing industries
 Energy sector
 Transportation Sector
 Environmental Fluid Flows
 Weather or meteorology
 Biological Applications
 CFD modeling applications
Outline of Fluid Mechanics
 Fluid mechanics is divided into statics and dynamics
 In fluid static, the result force and moments are zero
 The fluid dynamics which deals with flows in motions, resultant forces or moments can not be zero
 Fluid flows deal with gas and liquids
The Concept of Field
 Any physical quantity that generates or carries the force in the fluid is called a field
 Some basic properties of the fluid (like density, viscosity, surface tension etc.) with surroundings conditions (walls, moving surfaces etc.) are responsible for the occurrence of the flow field.
 The fields can be scalar and vector or both in nature
 Scalar fields are quite easier to understand with than vector fields
 Most of the resources, energy, and work are in the form of scalars. These scalars can be used to create vector fields like velocity, force and stress
 First step is to analyze fluid flows using bas properties of scalar and vector fields
 During flowing process, the properties of working medium changes with a timedependent. Such flows are unsteady
 In fluid flows, physical quantities are functions of time (t) and space (x)
 Vector: velocity, stresses acting on the fluid
 Scalar: Thermodynamic properties of fluid such as pressure, temperature, density, enthalpy etc.
Basic Physical Quantities in Fluid Mechanics
Scalar (Zerothorder tensor)
 Scalar has a definite magnitude but not a definite direction
 Example: Density, Pressure, Temperature, Concentration
 If N is the order of tensor Total component in 3 directions (X, Y, and Z directions): 3^{N}
 Total component for scalar: 3^{0 }= 1
Vector (Firstorder tensor)
 A firstorder tensor consists of definite magnitude, with three components and a definite different three directions
 Example: velocity, force, heat flux, etc.
 Heat flux vector
 The spread of Fire in a Room is an example of a heat flux vector, a firing rate is not the same in all direction
 ▼ is Differential Operator, then the heat flux vector
Here, del (▼)is a differential operator
Secondorder Tensor
 A secondorder tensor has nine definite components and nine definite directions
 Example: Velocity gradient, Stress, Strain, thermal conductivity
 Velocity Gradient:
A velocity gradient consists of deformation and rotation tensor.
 Stress Tensor:
 Thermal conductivity:
 For isotropic materials has only one component and it is a scalar quantity. The thermal conductivity is same in all direction.
 The thermal conductivity is different in all direction which is called an anisotropic material. It has 9 different components.
Hydro statics
 Field variables of pressure and gravity and their effect were presented by Pascal
 To start with simple Fluid Mechanics traditionally with hydrostatics, only pressure and external force (e.g. gravity) are essential
 You can refer to the post on experimental fluid pressure measurement methods in the post.
Where p is the pressure field, Φ is the external force field potential and ρ is density.
 The static pressure is independent on the shapes of water storage
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 Due to high pressure at the bottom, the velocity of the jet is higher at the bottom and lower for the top jet
 However, analytical pressure drop for fluid flow through the pipe is calculated using loss coefficient and average velocity.
Physical Properties of Tensor
Scalar is a mother of Vector Field
 Start from the path integral Work which is equal to the dot product of force and displacement vectors
 Vector is a gradient of Scalar
 The gradient in scalar creates a vector field
System conversions for Gradient for the different coordinate system
 Cartesian coordinate
 Cylindrical coordinate
 Spherical coordinate
Gauss’s Divergence Theorem
 Divergence (Gauss’s) theorem
 This theorem is useful for the expansion or contraction (divergence or convergence) of material inside a volume which is equal to what entering and leaving across the boundary of the material
 The divergence theorem is mainly used
 to convert a surface integral into a volume integral.
 to convert a volume integral to a surface integral.
The Divergence Effect in A Fluid Flow
 A velocity field is the major vector field essential to describe a flowing fluid
 The divergence is used to measure the expansion and contraction in fluid flows
 A divergence of vector field can have a positive or negative value
 Diverging Flows: a vector field whose divergence is positive
 Converging Flows: a vector field whose divergence is negative
 Solenoidal Flows: a vector field whose divergence is zero
 Application of convergence and divergence for atmospheric flow over a ground
Types of NonSolenoid Flows
 Incompressible flow is vectordominated as variation in scalar (density) is negligible
 Example: flow over a car
 For compressible Flow, both vector and scalar are essential for flow analysis
 Example: flow over a fighter jet or space shuttle
Mass Conservation or Continuity Equation
Continuity equation in differential form
One Dimensional Form Continuity equation
 For onedimensional compressible flow, the mass flow rate is constant
Mass flow rate at any crosssection = ρAV
 For incompressible flow: flow rate at any crosssection, Area*average Velocity =constant
The curl of a vector field
 The Curl of a vector field is a measure of flow Circulation which is defined as
 Circulation is the amount of force that pushes along a closed boundary or path
 Curl measure the amount of circulation per unit area, circulation density, or rate of rotation (twisting action at a single point)
 Curl has a magnitude equal to the maximum “circulation” at each point
 Its direction is oriented perpendicularly to this plane of circulation for each point
The Curl of Velocity Field
 A vorticity vector is defined as the curl of a velocity vector
 Example: Atmospheric flow is highly rotational
Rotational vs Irrotational flow
 Flows with vorticity are called rotational flows
 Flows without vorticity (zero value of curl of velocity) are said to be irrotational flow
 Cyclonic Flows:
 AntiCyclonic Flows:
Vorticity Transport Equation
 Vortex Stretching is in turbulent flow presented by the Vorticity Transport Equation
 In this equation, there are two terms o the RHS: viscous torque and vortex stretching term
 Vortex lines possess vorticity which is defined as the curl of the local velocity vector and its magnitude is indicative of the angular rotation of fluid elements about the local ‘axis’.
 Largeeddy has Reynolds number in terms of length scale hence it is almost inviscid and Largeeddy angular momentum is conserved as it does not have any viscous torque acting on it
 The Energy Cascade is observed. Large eddies have most of the turbulence kinetic energy (TKE) which is transferred to smaller eddy. Hence, the angular velocity for the large eddy is decreasing.
 To maintain their angular momentum conservation, the moment of inertia (MOI) of the large eddies increases and large eddies are stretched.
 Vortex lines become distorted from the induced velocities of the bigger eddies.
 This phenomenon can be explained using the vorticity transport equation
Forces Acting on Fluid Flows
Types of forces acting on Fluid Flows
1) Body (Volumetric) Force:

 Gravity Force
 Electrical Force:
 Dielectric force
 Coulomb force
 Electrostriction force
 Magnetic: Lorentz force
2) Normal Surface Force: Pressure, surface tension,viscous forces
3) Combined Normal and Tangential Surface Forces

 Force due to thermodynamic Effects: Buoyancy forces or surface
 PhysioChemical or concentrationbased forces: Environmental or Biofluid mechanics
Stress Acting on Fluid Flows is the mother of Surface Force
 Force acting on the surface of a fluid element is the divergence of the Stress tensor field
 The stress tensor is usually divided into its normal and shear stress parts
Momentum Acting on Fluid Flows
 The momentum equation is integrated over a system to get drag, lift forces forces due to pressure and viscosity
 Forces due to viscous forces, and gravitational forces act on fluid
 Depending on the application, Other forces can affect fluid flows such as electrodynamic, electrostatic, and magnetic forces
 Newton’s second law of motion
Flows Driven by Gravitational Force
 Water fall under gravity is an example of flow driven by a gravitational force
 The momentum equation is given below
 The gravitational force acting on a fluid system is a volume integral of the product of mass and acceleration due to gravity
Flows Driven by Electromagnetic Force
 In such cases, the forces are identified as electrodynamic and electrostatic
 Supplying high voltage between the Collecting Electrode and Discharge Electrode generates a Corona Discharge that produces minus ions
MagnetoHydrodynamics Flows
 Magnetofluid dynamics: Fluid Dynamics of electricallyconducting fluids and their interactions with magnetic fields.
 There is an interaction of three fields: Magnetic field, Electric field and Plasma flow
 Magnetic field flow (B) can produce an Electric field (J).
 The interaction of magnetic and electrical fields can produce Lorentz Force to accelerate the flow
 The principle of Magnetohydrodynamics dynamics is shown below
 Using electric and magnetic fields, the flow can be accelerated with an increase in flow
Differential Form of Momentum Conservation Equations for Fluid Flows
 Newton’s Second Law to a system of fluid flow
 Newton’s Second law to a control volume of fluid element
 Stress acting on fluid is a function for strain rate and rotation tensor
Viscosity of Fluid
Newtonian (linear) Viscous Fluid
 Newton’s Law of Viscosity: for straight, parallel and uniform flow, the shear stress , between layers is proportional to the velocity gradient in the direction perpendicular to the layers
 Shear stress = Viscosity * Velocity Gradient
 Newton’s Law of viscosity provides a more economical solution to complex flows
 Example: Air, water
Nonlinear viscosity
 Most of fluid does not show a linear variation of shear stress with the rate of strain
 For such nonlinear viscous fluid is called NonNewtonian fluid
 Nonlinear (nonNewtonian) fluids do not have a constant viscosity. They can exhibit different viscosity behaviors based on the shear rate or stress applied. There are several types of nonNewtonian behavior given below
 Shearthinning (Pseudoplastic):

 The viscosity of the fluid decreases as the shear rate or stress increases.
 Common examples include ketchup and certain types of paints. These fluids become less viscous and flow more easily when you apply force.

 Shearthickening (Dilatant):
 The viscosity of the fluid increases as the shear rate or stress increases.
 A popular example is a mixture of cornstarch and water.
 When force is applied, the mixture becomes thicker and more viscous.
 Thixotropic:
 Thixotropic fluids become less viscous over time when they are subjected to continuous shear stress.
 When the stress is removed, they gradually return to their original, higher viscosity state.
 Many gels and some types of clay exhibit thixotropic behavior.
 Rheopectic:
 Rheopectic fluids, on the other hand, become more viscous over time under shear stress and return to their original lower viscosity state when the stress is removed.
 This behavior is less common than thixotropy.
 Viscosity changes due to changes in force, pressure gradient, temperature or concentration of liquid or solid particles
 The following figure shows the effect of nonlinear viscosity on velocity profiles
 Example of NonNewtonian Fluids: Honey, toothpaste, Ketch up, slurry flow, salt solution, corn starch, blood, melted butter, shampoo, starch suspension
Euler’s Equation
 For the memoir of 1755, Euler obtained the momentum equations of fluid flow
 He equated the product of mass and acceleration aof fluid element to the resultant of the pressures and external body forces acting the fluid element
Bernoulli’s Equation
 Bernoulli integrated the Euler equation for a steady, incompressible, and inviscid flow
 Bernoulli equation shows that the total energy of fluid for a particular section is constant
Viscous BoundaryLayer
 When fluid flows over a surface, a very thin region is formed adjacent to the nonslip wall. This results in the formation of gradient in velocity normal to the wall
 When fluid flows over a sudden down portion. The fluid separates due to positive pressure gradient
Flow Over A curved Surface
 Two flow regions are formed over a curved surface
 In a favorable pressure gradient, the fluid flow accelerates
 In an adverse pressure gradient, the fluid flow separates due to wake region
Laminar vs Turbulent Flow
 The relative magnitude of inertial and viscous terms is called as Reynolds number
 Increasing velocity of fluid flow (Reynolds number), the nonlinearity inertia term of NavierStokes(NS) equations that causes disturbances in fluid flows
For a more detailed understanding of turbulent flow, Read Turbulent Flow Physics and Methods of Analysis
Conclusion
 Fluid mechanics is an important subject for many engineering applications. It helps to understand of flow physics of liquids and gases, external forces, and their effect on the deformation of fluid flow
 The governing equations of fluid flows are Mass and momentum (Navier Stokes equations). These equations are solved numerically in CFD simulation.
 Volumetric, surface forces have different applications in fluid flows
 Viscous forces are important in the boundary layer and inertia forces are significant out of the boundary layer
 Using Flow features we can define laminar and turbulent (chaotic) flows
The basics have been explained nicely.
Thanks Dr.Alankrita
Strong understanding of engineering fundamentals, particularly fluid mechanics, heat transfer, and solid mechanics are essentials