2-D Kelvin-Helmholtz test

This test consists of a dense central layer moving with respect to a lower density layer above and below the dense layer. Using periodiuc boundaries on all sides, a steady state can be established. Noise is added to the dense layer velocity field at the 1% level to seed the Kelvin-Helmholtz instability (KH instability).

There is not an analysitcal solution, but the growth of the Y component of the kinetic energy grows in essentially the same fashon as the test performed by the ATHENA code: see the details here. There is a brief settling period, followed by exponential growth and finally saturation. The three resolutions excite increasing maximum wavenumber modes, and the higher wavenumebr modes grow more quickly than the lower wavnumber modes, as expected.

Initial conditions

  • Adiabatic index: Gamma = 1.4
  • Three layers
    • Upper, 0.25 < y < 0.50, Density, d = 1.0, Pressure, P = 2.5, Velocity, v_x0 = -0.5
    • Middle, -0.25 < y < 0.25, Density, d = 2.0, Pressure, P = 2.5, Velocity, v_x0 = +0.5
    • Lower, -0.50 < y < -0.25, Density, d = 1.0, Pressure, P = 2.5, Velocity, v_x0 = -0.5, same as upper
  • Pressure is set to P = 2.5 everywhere
  • Middle layer, velocity components v_x and v_y are modified with white noise, amplitude 0.01 peak to peak, ie +/- 0.005
    • v_x = v_x0 + 0.01*(ran(1.0)-0.5), where ran(1.0) returns uniform random number 0.0 <= n < 1.0
    • v_y = 0.01*(ran(1.0)-0.5)

Grid

  • 128 x 128, 256x256, 512x512 cells.
  • Domain: -0.5 < x < 0.5, -0.5 < y < 0.5, (0.0, 0.0 in the centre of a unit square)
  • All boundaries are periodic.

Algorithm settings

  • CFL number: 0.8
  • Striping correction on, artificial viscosity parameter: alpha = 0.001
  • DivV threshold: 1.0, Scale: 1.0
  • Flattening: minimum 0.0, maximum 1.0

Ending condition

  • Time, t = 5.0

Results

Y-axis kinetic energy 0.5*d*v_y^2

2-D Kelvin-Helmholtz layer

QuickTime movies

t = 0.0 - 5.0, 100 frames, one each for 512x512, 256x256 and 128x128 cell grids

128 x 128 cells

128x128 cells - initial density

Initial density

128x128 cells - density, t=5.0

t = 5

128x128 cells - density, t=2.5

t = 2.5

128x128 cells - density, t=10

t = 10

 

min 0.8  2.2 max

256 x 256 cells

256x256 cells - initial density

Initial density

256x256 cells, density, t=5.0

t = 5

256x256 cells, density, t=2.5

t = 2.5

256x256 cells, density, t=10

t = 10

 

min 0.8  2.2 max

512 x 512 cells

512x512 cells, initial density

Initial density

512x512 cells, density, t=5.0

t = 5

512x512 cells, density, t=2.5

t = 2.5

512x512 cells, density, t=10

t = 10

 

min 0.8  2.2 max

Updated:  21 October 2017/Responsible Officer:  RSAA Director/Page Contact:  Webmaster