Co-Flowing Jet Experiments

About the Experiment :

Coherent Motion of a Compound Jet

   The closed surface water tunnel in the Aerospace and Mechanical Engineering Hydrodynamics Laboratory is currently being used to study the large scale dynamics of a coflowing jet. The coflowing jet mechanics are relevant to many real-world applications including environmental mixing and lifted flames as well as to the general study of free shear flows. The effect of the additional coflow tends to stabilize the jet for a given Reynolds number when compared to the free jet. This added stability makes the coflowing jet a good choice for studying large scale motions without the use of outside forcing. The ongoing experimental work is being pursued in an attempt to develop relationships between the mean and time-dependent behavior of the flow field, with emphasis on the existence of coherent motions.

Laser Doppler Velocimetry and the Mean Flow

   The diagram depicts the basic mean flow field of the compound jet. A jet issues from the end of the tube within which the flow is laminar. The initially parabolic exit velocity profile rapidly takes on a Gaussian-like shape typical of the free jet. Unlike the free jet which is self-similar, this flow has a region of jet like self similarity just beyond the formation region followed by a second region which is wake-like in nature. This can be seen on the plot below which depicts the decay of the centerline velocity, acuired via LDV measurements, as a function of dimensionless distance from the jet exit.

Laser Induced Fluorescence and Scalar Structure

   The image was obtained from multiple LIF images of the cross section of the compound jet, which are acquried at the standard frame rate (30 Hz) with a CCD camera over the period of 3.5 seconds. An iso-concentration surface is obtained from the collection of LIF images which are stacked in time forming and x-y-t concentration volume. The diagram shown is rendered by viewing the x-t face of this surface. Note that topological surface structure is present in the form of ridges which are made visible on the plot as ridges highlighted by light and shadow. Convection speeds of these structures are computed from the images and used for other measurements.

Particle Image Velocimetry and Coherent Motion

   Particle Image Velocimetry (PIV) is used to compute velocity vector fields for the compound jet using a unique frame of reference. The data is acquired with the imaging equipment moving in a frame of reference with the convective speed determined from the scalar measurements. This technique allows us to isolate the vortical motions and effectively track them as they develop downstream. The vorticity, computed from finite differencing the velocity output, is displayed in the Quicktime format video displayed below. Click on the image and watch as the vorticity contours are displayed. Blue contours represent regions of negative vorticity while red regions represent positive vorticity. Notice how the initially flat shear layer grows and breaks up into distinct regions of coherent motion.

Future Work

   We are hoping to tie together the information gained from each of the three experimental techniques to develop a simple, consistent description of the compound jet flow field.