This numerical simulation shows a von Karman vortex street in the wake of a bluff body. As flow moves over the object, vortices are periodically shed off the object’s upper and lower surfaces at a steady frequency related to the velocity of the flow. The simulation takes place in a channel; note how the thickness of the boundary layers on the walls increases with downstream distance, forcing a slight constriction on the vortex street in the freestream.
Tag: numerical simulation
2D Convection
This simulation shows 2D Rayleigh-Benard convection in which a fluid of uniform initial temperature is heated from below and cooled from above. This is roughly analogous to the situation of placing a pot of water on a hot stovetop. (In the case of the water on the stove, the upper boundary is the water-air interface, while, in the simulation, the upper boundary is modeled as a no-slip (i.e. solid) interface.) The simulation shows contours of temperature (black = cool, white = hot). In general, the hot fluid rises and the cold fluid sinks due to differences in density, but, as the simulation shows, the actual mixing that occurs is far more complex than that simple axiom indicates.
Underwater Cloaking
Researchers have suggested that it may be possible to cloak submerged objects as they move through a fluid using layers of mesh and micro-pumps. By redirecting the fluid so that it enters and leaves the mesh surrounding the object in the same speed and direction that it entered, it is theoretically possible to have zero drag and no wake. So far researchers have only simulated this set-up computationally using a sphere with 10 layers of mesh. It’s also unfortunately limited in size and speed: a vehicle 1 cm across could only remain wake-free at speeds below 1 cm/s. (Photo credit: Michael J Rinaldi) #
Computational Shock Compression
[original media no longer available]
Computational modeling can help verify and visualize experimental results, as in this video of supersonic flow. Oak Ridge National Laboratory produced the work as part of a project using shock compression and turbines to capture carbon dioxide gas. Shock waves and velocity profiles are shown throughout the computational field, and velocity isosurfaces paint a telling portrait of the complicated flow pattern. Wired Science features other award-winning simulation videos, many of which also feature fluid dynamics. #
Starting Vortices
Whenever a wing stops or starts in a fluid, it produces a vortex. This 2D numerical simulation shows an airfoil repeatedly starting and stopping, shedding a vortex each time. Note how the line of vortices drifts downward in the wake; this is an indication of downwash. (submitted by jessecaps)
High-Res Rayleigh-Taylor Instability
When a heavy fluid sits atop a lighter fluid, the interface between the two breaks down through the Rayleigh-Taylor instability. This computation of a 2D interface shows the near fractal behavior of this instability as whorls and eddies of all different scales form and mix the fluids. (submitted by @markjstock)
Vortex Shedding from a Hot Cylinder
This numerical simulation shows vortex shedding behind a hot cylinder. The behavior is very similar to what one sees behind an unheated cylinder, until the coefficient of thermal expansion increases and the von Karman vortex street is completely distorted. Describing the particulars of the computation, jessecaps writes (links added):
I wrote an incompressible flow solver to simulate flow past a heated cylinder. The Navier-Stokes equations are discretized on a Cartesian grid and solved explicitly in time. The pressure-Poisson equation is solved implicitly using a bi-conjugate gradient method. The Boussinesq approximation was used (density is constant everywhere except for the gravity term) to account for buoyancy. Reynolds number = 250, Froude number = 1 (gravity is pointing down). The two simulations show the effect of the coefficient of thermal expansion. Each video shows a plot of velocity and temperature.
(submitted by jessecaps)
Rayleigh-Taylor Art
The Rayleigh-Taylor instability occurs when a denser fluid lies atop a lighter fluid (relative to the gravitational field). The interface between the fluids deforms and the two fluids form finger-like protrusions that turn into mushroom caps and mix the dissimilar fluids together. This video, though based on a 2D Rayleigh-Taylor instability numerical simulation, was actually part of an art exhibit. (submitted by Mark S)
Personally, I recommend putting together a playlist of your favorite late 60s/early 70s rock (Pink Floyd, late Beatles, Jimi Hendrix, etc.) and sticking it on in the background while you watch the video in HD. It’s totally worth the 15 minutes. Especially in the later stages of each segment, the mixing between fluid layers really brings to mind cloud patterns on Jupiter or Saturn.
Supersonic Bullet
[original media no longer available]
This video shows a CFD simulation of a bullet passing through a parallel channel at Mach 2. The simulation captures 3 milliseconds of real-time and shows the Mach number in the top view and the temperature in the bottom view. Note how the bow shock near the front of the bullet and the trailing shock behind it reflect off the walls of the channel and interact. Even though the calculation is inviscid, the shock waves cause intense heating (white) in front of and behind the bullet.
Leapfrogging Vortices
This numerical simulation shows two pairs of vortices interacting in a leap-frogging motion. Another version shows the same situation but with a small perturbation in the rotational alignment that causes even more interesting interactions. Both simulations are of potential flow–an idealized flow without viscosity where velocity can be described as the gradient of a scalar function. The mathematics governing potential flow are notably easier than the full Navier-Stokes equations that govern fluid mechanics. (submitted by jessecaps)
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