Warming up a Viscous Fluid#

This example introduces how to solve another physics along with the CFD solver.

Features#

  • Solver: lethe-fluid

  • Transient problem

  • Multiphysics

  • Displays the use of heat transfer physics

  • Analytical solution

Files Used in This Example#

  • Parameter file: examples/multiphysics/warming-up-viscous-fluid.prm

Description of the Case#

A viscous fluid lays between two parallel plates: one fixed and insulated, and the other moving and heated. The velocity profile and the temperature evolution are simulated. The parameter file used is warming-up-viscous-fluid.prm.

The following schematic describes the simulation.

Simulation schematic

  • bc = 0 : no slip and thermal insulation boundary condition

  • bc = 1 : flow in the y-direction (\(v=2\)) and heating at Tw

Important

The whole simulation is carried out in the frame of one-way coupling: the fluid velocity influences the heat generated through viscous dissipation, but the heat transfer does not influence the fluid velocity. Moreover, fluid state changes are not considered.

Parameter File#

Simulation Control#

Time integration is defined by a 2nd order backward differentiation (bdf2), for a 7.0 seconds simulation (time end) with a 0.05 second time step, as stated in the subsection simulation control:

subsection simulation control
  set method           = bdf2
  set time step        = 0.05
  set time end         = 7.0
  set output name      = warming-up
  set output frequency = 1
end

Note

Heat transfer phenomenon occur at a much larger characteristic time than fluid transport phenomenon. To reach a stable state for the system, the end time much be quite big, but the time step can also be increased (in the limit of numerical stability).

FEM#

The order of resolution for the velocity, pressure and temperature are given in the subsection FEM:

subsection FEM
  set velocity order    = 1
  set pressure order    = 1
  set temperature order = 2
end

Physical Properties#

The fluid’s physical properties are defined in the following subsection, according to the properties of oil.

subsection physical properties
  subsection fluid 0
    set density              = 0.9
    set kinematic viscosity  = 0.5
    set thermal conductivity = 0.12
  end
  # water = 1 density, 0.01 kinematic viscosity, 0.59 conductivity
  # oil = 0.9 density, 0.5 kinematic viscosity, 0.12 conductivity
end

Warning

If no physical properties are defined, default values (of 1.0) are taken for the simulation.

Mesh#

The mesh considered is a very basic rectangle, using the dealii grid type hyper_rectangle, to represent the fluid volume considered between the two plates. Here, the width between the two plates is set to 0.5.

subsection mesh
  set type               = dealii
  set grid type          = hyper_rectangle
  set grid arguments     = 0, 0 : 0.5, 1 : true
  set initial refinement = 4
end

Note

As the fluid velocity is not influenced by heat transfer (one-way coupling), the fluid velocity will remain constant for the whole simulation across the domain, and as heat transfer occurs at a larger scale, the mesh can be coarse.

Multiphysics#

The multiphysics subsection enable to turn on (true) and off (false) the physics of interest. Here heat transfer and viscous dissipation must be set (see Bonuses for results without viscous dissipation).

subsection multiphysics
  set heat transfer       = true
  set viscous dissipation = true
end

Analytical Solution#

The analytical solution is defined, according to the fluid and simulation properties:

\[T(x) = T_\omega + \frac{\rho \nu v^2}{2K}\left ( 1 - \left ( \frac{x}{B} \right )^2 \right )\]

with \(x\) the axis perpendicular to the plates, \(\rho\) the density, \(\nu\) the kinematic viscosity, \(K\) the thermal conductivity, \(T_\omega\) the heating temperature and \(v\) the velocity of the right plate (bc 1), and \(B\) is the width between the two plates.

subsection analytical solution
  set enable    = true
  set verbosity = verbose
  subsection uvwp
    set Function expression = 0 ; 0 ; 0
  end
  subsection temperature
    set Function constants  = rho=0.9, nu=0.5, K=0.12, Tw=80, v=2, B=0.5
    set Function expression = Tw+(((rho*nu)*v*v)/(2*K))*(1-(x/B)*(x/B))
  end
end

Boundary Conditions#

The boundary conditions are set for:

  • the fluid dynamic in subsection boundary conditions, with noslip at the left wall (bc 0) and a velocity of 2 in the y-direction at the right wall (bc 1),

  • the heat transfer in subsection boundary conditions heat transfer, with a convection imposed at the left wall (bc 0) with a heat transfer coefficient h = 0 to represent an insulation condition, and an imposed temperature of 80 at the right wall.

subsection boundary conditions
  set number = 2
  subsection bc 0
    set id   = 0
    set type = noslip
  end
  subsection bc 1
    set id   = 1
    set type = function
    subsection u
      set Function expression = 0
    end
    subsection v
      set Function expression = 2
    end
  end
end

subsection boundary conditions heat transfer
  set number = 2
  subsection bc 0
    set id   = 0
    set type = convection-radiation-flux
    subsection h
      set Function expression = 0
    end
    subsection Tinf
      set Function expression = 0
    end
  end
  subsection bc 1
    set id    = 1
    set type  = temperature
    subsection value
      set Function expression = 80
    end
  end
end

Running the Simulation#

The simulation is launched in the same folder as the .prm file, using the lethe-fluid solver. It takes only about 5 seconds with one cpu. Assuming that the lethe-fluid executable is within your path, the simulation can be launched by typing:

lethe-fluid warming-up-viscous-fluid.prm

Results#

Visualizations#

Convergence with regards to the analytical solution on the temperature:

Convergence with regards to the analytical solution on the temperature

Domain with temperature:

Domain with temperature (t = 0) Domain with temperature (t = 1) Domain with temperature (t = 2) Domain with temperature (t = 3)

Temperature evolution over time:

Temperature evolution over time

Physical Interpretation#

From \(t=0s\) to \(t=2s\), the right plate (\(T=80^\circ\)) heats up the fluid (initially at \(T=0^\circ\)). At \(t=2s\), the temperature is quasi-homogeneous in the fluid, with \(T=80^\circ\). As the fluid continues to be forced to flow at the right wall, viscous dissipation generates more heat, so that the wall with a fixed temperature of \(T=80^\circ\) now cools down the fluid. A steady state between viscous dissipation heating and the fixed temperature cooling is reached at about \(t=4.5s\).

Bonuses#

Results for Water#

For water, physical properties are:

subsection physical properties
  subsection fluid 0
   set density              = 1
   set kinematic viscosity  = 0.01
   set thermal conductivity = 0.59
  end
# water = 1 density, 0.01 kinematic viscosity, 0.59 conductivity
# oil = 0.9 density, 0.5 kinematic viscosity, 0.12 conductivity
end

As water has a higher thermal conductivity than oil, the temperature becomes quasi-homogeneous sooner (around \(t=1s\)). And as it is far less viscous, the heat generated by viscous dissipation is not visible on the temperature-over-time plot. However it still exists, as seen when the temperature scale is adapted.

Temperature evolution over time Domain with temperature (t = 0.5) Domain with temperature (t = 7) Rescaled domain with temperature (t = 7)

Results without Viscous Dissipation#

The viscous dissipation can be disabled physically, if the two plates remain fixed (v=0 for bc 1), or numerically with set viscous dissipation = false. Both cases give the same results shown below. The fluid considered is still water.

After the fluid has been heated up by the right plate, the temperature is really homogeneous throughout the domain, and both minimum and maximum temperatures stay at \(T_\omega=80^\circ\). Adapting the temperature scale shows that there is no viscous dissipation at all.

Temperature evolution over time Domain with temperature (t = 0.5) Domain with temperature (t = 7) Rescaled domain with temperature (t = 7)

Horizontal Domain#

Several adjustments have to be made in the .prm to turn the domain clockwise, so that it becomes horizontal, with the upper wall being the no slip and thermal insulation boundary condition, and the lower wall with the flow in the y-direction (\(v=2\)) and heating at Tw:

  • in subsection mesh: set grid arguments = 0, 0 : 1, 0.5 : true

  • in subsection analytical solution, subsection temperature:

    set Function expression = Tw+(((rho*nu)*v*v)/(2*K))*(1-(y/B)*(y/B))

  • and most importantly, the id of boundary conditions should be adapted to use the bottom and top wall (see the deal.II documentation on hyper_rectangle grid generator for further details):

subsection boundary conditions
  set number = 2
  subsection bc 0
    set id   = 2
    set type = noslip
  end
  subsection bc 1
    set id   = 3
    set type = function
    subsection u
      set Function expression = 2
    end
    subsection v
      set Function expression = 0
    end
  end
end

subsection boundary conditions heat transfer
  set number = 4
  subsection bc 2
    set id   = 2
    set type = convection-radiation-flux
    subsection h
      set Function expression = 0
    end
    subsection Tinf
      set Function expression = 0
    end
  end
  subsection bc 3
    set id    = 3
    set type  = temperature
    subsection value
      set Function expression = 80
    end
  end
end

Important

For the fluid boundary conditions, we use set number = 2, whereas for boundary conditions heat transfer we use set number = 4. These two notations are perfectly equivalent, as the boundary conditions are none by default (or noflux in the case of heat transfer, see Boundary Conditions - Multiphysics). However, it is important to make sure that:

  • the index in subsection bc .. is coherent with the number set (if number = 2, bc 0 and bc 1 are created but bc 2 does not exist),

  • the index in set id = .. is coherent with the id of the boundary in the mesh (here, the deal.II generated mesh).

Possibilities for Extension#

  • Study the sensitivity to the time step, namely to assess how large the time step can be before stating any difference in the heat transfer solution.

  • Test a different time integration scheme and see if there is any difference in the computational cost and/or the precision with regards to the analytical solution.

  • See how the resolution order (velocity order, pressure order and temperature order) affects the precision with regards to the analytical solution.