Ansys Fluent Day 1, Intro

Overview: CFD workflow:

+ CFD (in general):                    + Ansys:

–  Pre-processor                              –  CAD Prep, Meshing, and Setup

–  Solver                                            –  Solve/Converge

–  Post processor                            –  Review Results

(Re)Introduction to Fluids:

  • A Fluid is: a substance with no fixed shape that deforms easily under external forces.
  • Examples: Liquids and Gases
  • Two most common fluids: Air & Water

(Re)Introduction to Fluids:

  • Fluid Properties:
  • Density, rho = Mass/Volume
  • Viscosity, mu = Resistance to deformation (as a result of shear stress). Commonly called the “thickness” of a fluid.

(Re)Introduction to Fluids:

  • Most fluids have a constant Viscosity, and are thus called a “Newtonian Fluid”.
  • Special case #1 = Non-Newtonian Fluid: Variable Viscosity, usually as a function of the shear rate (speed with which the fluid is being moved)
    • “Shear Thinning”
      • Blood
      • Ketchup
      • “Non Drip Paint”
  • “Shear Thickening”
    • Oobleck

(Re)Introduction to Fluids:

  • Flow Characteristics:
  • Velocity, V = Distance/Time
  • Flow Regimes:
    • Subsonic: < 650 mph
    • Transonic: 650 – 767 mph
    • Supersonic: 767 – 4,000 mph
    • Hypersonic: >4,000 mph

(Re)Introduction to Fluids:

  • Speed of Sound = ~767 mph (M=1.0)
  • When M>1.0, the air is rapidly compressed (increase in pressure, temperature and density)
  • Special case #2 = Compressible Flow: Where fluids (mostly gasses) which exhibit significant changes in density.

(Re)Introduction to Fluids:

  • Special case #3 = Inviscid: The viscous forces within the flow are negligible. This can be true for fluid of non-zero viscosity! (e.g. High speed, high atmospheric flow away from a wall).

(Re)Introduction to Fluids:

  • Fluid-Flow Characteristics:
  • Reynold’s Number, Re: The ratio of Inertial Forces to Viscous Forces
  • Low Re – Laminar
  • High Re – Turbulent

(Re)Introduction to Fluids:

  • Boundary Layer
    • Region of fluid flow near a wall, where velocity transitions from 0 to V
    • Region where viscous forces dominate the flow!
  • Critical Reynolds Number, Rex: Where Laminar flow transitions to Turbulent
  • Internal Flow: For Flow through a Pipe, Rex = 2300
  • External Flow: For Flow over a Plate, Rex = 5x10E5

Pre-CFD Checklist:

  • What are the fluid properties?
    • Density, Viscosity
  • What are the flow characteristics?
    • Velocity, Reynold’s Number at entrance, where are there Boundary Layers/Walls?
  • Do any special cases apply?
    • Non-Newtonian?
    • Compressible?
    • Inviscid?

Start:

  • Open Workbench
  • Drag/Drop a Geometry Component
  • Drag/Drop Fluent (with Fluent Meshing)
  • Save the Workbench File

Start:

  • Launch SpaceClaim from Workbench
  • Import the Nozzle/Diffuser file here: nozzle_diff.x_t

Prepare:

  • Use SpaceClaim to prepare the geometry for meshing
    • Cut into a "quarter symmetry" model
    • Establish Names Selections
      • Inlet (Larger Diameter)
      • Outlet (Smaller Diameter)
      • Symmetry Planes

Mesh:

  • Import into the Watertight Workflow
  • Create a volume mesh using the default mesh sizes

Best Practice:

  • Check units in Workbench beforehand

Setup:

  • After familiarizing yourself with the interface, audit the model for accuracy
    • Physics Models & Materials
    • Boundary Conditions
      • Set Inlet to 12.5 m/s

Best Practice:

  • Check mesh upon entry

Solve the Analysis:

  • Set to Laminar & Solve 50 Iterations
  • Observe Convergence
  • Post Process
    • Create a Velocity Contour
    • Create Mesh Display
    • Create Scene with Velocity & Mesh

Post Process:

  • Use various Post Processing Tools:
    • Vectors
    • Pathlines
    • Particle Tracks
    • Iso Surfaces
  • Create your Favorite Scene

Double Check Your Work:

  • How do we know we have the right answer?
    • We compare it to “The Math”
    • What velocity should we have at the narrow section? The exit?
  • Create Reports
    • Inlet Flow Rate & Outlet Flow Rate
  • Create Plot along pipe Centerline

File Management:

  • Don't forget to save often!
  • Discuss the different files types
    • Mesh
    • Case
    • Data

Introduction to CFD Mathematics

  • Infinitesimal Fluid Element (Control Volume)
  • This volume has both Fluid Properties and Flow Characteristics at a certain point in time
    • Density
    • Viscosity
    • Pressure
    • Velocity
    • Temperature

Introduction to CFD Mathematics

  • Assume the fluid is represented by a “Control Volume”
  • Bernoulli says: P + ½*rho*V^2 + rho*g*h = constant
    • Pressure, density, Velocity, Gravity, Height
  • Additionally:
    • A1V1 = A2V2 & P1 – P2 = ½*rho (V2^2 – V1^2)
    • So, as Area goes down, Velocity goes up (linearly)
    • And as Velocity goes up, Pressure goes down (quadratically)

Modeling Turbulent Flow

  • Should we include turbulence?
  • Calculate Inlet Re (recall: Turb>2,300+)
  • Let’s add turbulence, and solve 100 more iterations
  • Why are we able to just continue at the last iteration, even though the entire physics have changed?
  • Why did the analysis stop early?

More CFD Mathematics:

  • Governing Equations conserve Mass, Momentum, (sometimes) Energy
  • These equations must be solved...
    • at every iteration
    • for every element

More CFD Mathematics:

  • Velocity, Mass, Force, and Temperature, are all COUPLED. Thus, the need for an iterative approach.
  • We sum this all up to a set of equations called the “Navier Stokes equations (shown here, broken down to the 3 directions (u,v,w), which are velocities in (x, y, z):

Residuals:

  • Residuals are the leftover values from each iteration (in the real world, they are zero)
    • This is not computationally feasible
  • Rule of Thumb:
    • Default = .001 (1E-03) – This is for a quick answer.
    • Tighter = .0001 (1E-04) – This is for a more accurate answer.
    • Fine = .00001 (1E-05) – This is for a solution that you can start to trust.
    • Extra Fine = .000001 (1E-06) – This is for an answer that you plan to publish and/or utilize academically.

Globe Valve:

  • Import, Extract Water Volume, Cut in Half
  • Suppress Structure for Physics
  • Named Selections: Inlet, Outlet, Symmetry

Download here: globe_valve7_x_t

Mesh:

  • Create a Poly-Hex Core Mesh using the Fluent Watertight Workflow

Setup:

  • Fluid = Water
  • Inlet = 0.002 meter/sec
  • Is it turbulent?
  • Set Residuals to 1e-4, and solve 100 iterations

Assess for Accuracy & Quality:

  • How can we trust the results?
  • Check the velocity at the exit, or at any other location – Compare to Bernoulli's equation
  • Check for mesh independence!
  • Add a mesh control, and re-solve
    • Surface Control
    • Gap Elements

Tell the Right Story, and Optimize the Design:

  • Create a scene showing the pressure drop and velocity streamlines on the symmetry plane
  • Make a design change to minimize the pressure drop in the flow

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