An experimental and computational study was made of a single circular jet
impinging onto a flat ground board. A 1/2" nozzle running at a fixed nozzle pressure ratio
of 1.05 was used in the experimental phase (giving an nozzle exit Reynolds number of
90xlO'), the nozzle to ground plane separation being varied between 2 and 10 nozzle
diameters. Measurements were performed in the free and wall jets using single and
cross-wire hot-wire anemometry techniques and pitot pressure probes in order to
detemine mean velocity and normal and shear stress distributions. Some analysis is also
presentedo f earlier measurementso n high pressurer atio impinging jets.
Nozzle height was found to effect the initial thickness of the wall jet leaving the
impingement region, increasing nozzle to ground plane separation increasing the wall jet
thickness, although this separation distance did not seem to affect the rate at which the
wall jet grew. Nozzle height was also found to have a large effect on the peak level of
turbulence found in the wall jet up to a radial distan ce from the jet axial centre line of
4.5 nozzle diameters, after which the profiles become self-similar. Lowering the nozzle
tended to increase the peak level measured in all the turbulent stresses within this
development region. The production of turbulent kinetic energy in the wall jet, which is
an indication of the amount of work done against the mean flow by the turbulent flow
was found to increase dramatically with decreasing nozzle height. This was attributed to
greater shearing of the flow at lower nozzle heights due to a thinner wall jet leaving the
impingement region. A moving impingement surface was found to cause separation of
the wall jet inner boundary layer on the 'approach' side leading to very rapid decay of
peak velocity. The point of separation was found to occur at radial positions in the
region of 7.0 to 8.0 nozzle diameters, this reducing slightly for lower nozzle heights.
A parametric investigation was performed using the k-e turbulence model and the
PHOENICS CFD code. It was found that due to inadequacies in the model, it failed to
predict accurately the growth of the wall jet, both in terms of its initial thickness and the
rate of growth. It did, however, predict an increase in wall jet thickness with both increasing nozzle height and exit turbulence intensity and decreasing nozzle pressure
ratio. Modifications were made to the constants in the model to try and improve the
predictions,w ith a limited degreeo f successT. he low Reynoldsn umber k-F-t urbulence
model was shown to give a slightly improved non-dimensional wall jet profile, although
this did not improve the predicted rate of growth of the wall jet.
| Identifer | oai:union.ndltd.org:CRANFIELD1/oai:dspace.lib.cranfield.ac.uk:1826/3888 |
| Date | 27 October 2009 |
| Creators | Myszko, M |
| Contributors | Knowles, K |
| Publisher | Department of Aerospace and Guidance Systems |
| Source Sets | CRANFIELD1 |
| Language | English |
| Detected Language | English |
| Type | Thesis or dissertation, Doctoral, PhD |
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