Ansys Fluent Day 2, Turbulence & Meshing

Turbulence Validation: NACA0012 Airfoil

Setup:

  • Audit the model
  • Note the Mesh density!
  • Inlet = 106 m/s
  • Initialize and solve 150 iterations

Post-Process:

  • Review the velocity, note the distance between the airfoil and the model limits
  • Check for Mesh Independence
  • Create a Pressure Coefficient (Cp) Contour

Post Process:

  • Discuss the definition of Cp
  • Plot the Cp along the top edge of the airfoil
  • Correlate the Cp ratio to the NASA Data
    • Report: Vertex Max, Pressure, Cp
    • Report: Vertex Min, Pressure, Cp
    • Report: Expression = Max Cp/Min Cp
  • The answer should be 0.41!

Investigate:

  • Let's use Fluent to determine the stall Angle of Attack of the NACA0012 airfoil.
  • During this investigation:
    • Post-process the turbulence values
    • Observe the convergence for signs of oscillation
  •  Download the pre-modeled mesh files here: NACA0012_aoa_sweeps.zip

Turbulence:

  • “Unpredictable” changes in the flow characteristics: velocity and pressure
  • Characterized by vortices (eddies) within the flow
    • Vortices can be large or small
    • Vortices can be fast or slow
    • Vortices can live for a long time for a short time
    • This all happens at the same time
  • Turbulence is constantly changing in Length AND Time

Turbulence:

  • Very computationally expensive, so its "modeled, not meshed"
  • CFD codes are put in categories based on how they handle Turbulence:
    • RANS – Reynold’s Averaged Navier Stokes (Time-Averaged Turbulence)
    • DES – Detached Eddy Simulation (Combo of RANS & LES)
    • LES – Large Eddy Simulation (Ignores small vortices)
    • DNS – Direct Numerical Simulation (Resolving flow on time and length scales!) (EXAMPLE)

Reynolds Averaged Navier-Stokes (RANS):

  • In the boundary layer, the mesh must be accurately sized (not too big, and not too small!)
  • The Law of the Wall:
    • “The average velocity of turbulent flow is proportional to the log of the distance from the wall”
  • We use the “y+” value to determine the proper wall layer height:
    • y+ = y * Friction Velocity/Kinematic Viscosity
    • y+ = distance from the wall * flow characteristics/fluid properties

Reynolds Averaged Navier Stokes (RANS):

  • k-epsilon: Standard turb model used in many applications (30<y+<300)
  • SST k-omega: Recommended for external Aero, detached flows (1<y+<10)
  • SAS: Vortex shedding, variable wake
  • DES: Separation/High Reynold’s Number
  • RNG: Reattachment (Flow over a backward facing step)
  • Low Re k-e: 1,500<Re<5,000, and “jets”
  • Mixing Length: Designed for Internal Natural Convection (gases)
  • Eddy Viscosity: For low speed turbulent flow if the k-epsilon won’t work…
  • GEKO – Generalized K-Omega
  • More Information in Ansys Help

Steady State Subsonic External Flow - The Vehicle in Ground Effect

  • Performance characteristics are defined by the forces acting on the vehicle
    • Drag, Thrust, Lift, Gravity
  • Lift forces come from: Fluid being redirected and the Bernoulli Effect
  • Drag Forces come from:
    • Form: Creation of turbulence and low pressure zone in wake
    • Skin Friction Drag: Viscous Shear forces in Boundary Layer
    • Lift-Induce Drag: Creation of turbulent vortices in the wake

Steady State Subsonic External Flow - The Vehicle in Ground Effect

  • Fdrag = Cd*(1/2*rho*V^2)*A
  • Flift = Cl*(1/2*rho*V^2)*A
  • Typically, Cd & Cl are obtained experimentally
    • Cd is relatively easy to research for passenger vehicles ~0.30 (wiki)
    • Cl not so much… (so let’s use CFD!)
  • Simple Convertible Calcs:
    • A = 17.7 ft2
    • Cd = 0.38 (Top down)
    • V = 100mph

Prepare:

  • Use SpaceClaim to prepare the geometry for meshing
  • Fix "dirty" CAD
    • Extra Edges; Inexact Edges
  • Create Named Selections
  • Download model here: miata_air.x_t

Mesh:

Setup:

  • Inlet - 100 mph
  • Start with K-Epsilon Turbulence model
    • 30<y+<300
  • Start with 1e-3 residuals
  • Start with 200 iterations
  • Solve & Review

Refine:

  • The turbulent wake needs to be resolved finer
  • Utilize a Body of Interest (BOI) in SpaceClaim & Fluent Meshing
  • What surfaces need a surface refinement?

 

Assess for Accuracy in Turbulence Modeling:

  • Review the y+, does it correspond to our Turbulence Model?
  • Calculate the drag force, does it correlate to the published Cd value?
  • Would a K-Omega turbulence model be more accurate?

 

Optimize:

  • Can we use the results of this analysis to make the design better?
  • How might we reduce the drag?
  • Pick an idea and try it!

 

Brief Overview on CFD Validation:

  • In a Wind Tunnel
    • Quantitatively: Measuring pressure with a Pitot Tube
    • Qualitatively: Using smoke trails
  • In the real world
    • Quantitatively: Pitot tubes, Coast Down test
    • Qualitatively: Flow viz paint

 

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