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: vortex shedding
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)
Island Vortex Street
The von Karman vortex street is a series of vortices shed periodically in the wake of a bluff body. Although they are commonly observed in the lab behind cylinders, they also occur in nature, as seen here in the wake of Juan Fernandez Islands near Chile. The strong equatorward wind creates steady flow over the mountainous island, creating a pattern in the clouds that stretches 10,000 times longer than vortex streets created in a laboratory. (via freshphotons)
Wake of a Rising Sphere
This flow visualization shows the wake left by a freely rising sphere. Observations of rising and falling spheres date at least back to Newton, who observed that the inflated hog bladders he used “did not always fall straight down, but sometimes flew about and oscillated to and fro while falling”. That vibration is caused by the vortices seen here in the wake. There are actually four vortices shed per oscillation cycle–two primary vortices (marked P) and two secondary vortices (marked S). #
Vortex Street
A flow visualization behind a cylinder shows the formation of a von Karman vortex street. The frequency of vortex shedding in the wake is directly related to the speed of the airflow–the higher the velocity, the faster vortices will shed from the cylinder. This relationship is expressed in the Strouhal number, which remains constant for any cylinder. (via freshphotons)
Island Vortices
The von Karman vortex street isn’t just found in the wake of cylinders in a lab. Wind moving over islands frequently creates the effect, as in this MODIS Aqua image of the coast of Baja California, Mexico. #
Vortex Shedding
Whenever a bluff (i.e. non-aerodynamic) body is placed in a flow of sufficient Reynolds number, it will shed periodic vortices, creating a pattern known as a von Karman vortex street. The animation above shows the phenomenon in the wake of a cylinder, but vortex streets form behind many other bodies as well, including islands. Each vortex shed causes forces on the body and alternating vortices can cause the body to vibrate. This is what causes suspended power lines to “sing” in the wind. #
