Research Articles

The density and drag of the accretion wake of a massive body moving through a uniform stellar distribution

Winston L. Sweatman & Douglas C. Heggie

Download: Bodies in Space.pdf (205 KB)



We calculate the change in density within a uniform distribution of field stars (point masses) caused by a single massive body passing through with a constant velocity. Starting with the simplest case in which the field stars are initially stationary this leads to an infinite density wake behind the body.

Introducing a small thermalisation within the field stars removes this infinity whilst leading to similar results off the path of the massive body. Results are in good agreement with those previously derived. An approximation can be made for the density in the thermalised case and this can be used to deduce the force exerted on the massive body due to the drag caused by the accretion wake.

A model for crop spray adhesion, bounce and shatter at a leaf surface

Geoffry N. Mercer, Winston L. Sweatman and W. Alison Forster

Download: Spray Bounce.pdf (877 KB)


Improvements in crop spray methods result in environmental and cost benefits. By developing a greater understanding of the physical processes involved, it should be possible to tailor spray formulations to maximise retention by plant foliage. This enables the reduction of the chemical active required to achieve agrochemical efficacy. In the present poster one important aspect of the retention process is considered: a droplet-leaf impaction model is presented allowing for bounce, shatter or adhesion of the droplets by the leaf surface.

Cold point determination in heat-treated steel coils

New Zealand Steel, A/Prof Mark McGuiness (Victoria University of Wellington), Dr Winston L. Sweatman (Massey University, Auckland

Download: NZSteel _Heat Treated Steel_ MISG 2007.pdf (176 KB)


During steel manufacture, the process of cold rolling introduces stresses into the crystalline structure of the metal. The cold rolled steel, in the form of coiled sheets, requires further heat treatment (annealing) to release the metal stresses and reform the crystalline structure in order to reintroduce desirable mechanical properties. To achieve this aim all parts of the sheet must reach the required annealing temperature and then be held at this temperature for a period of time.

Transport of Droplets in Sprays

Sharleen Harper. Supervised by: Prof. Robert McKibbin & Prof. Graeme Wake

Download: Spray Drift.pdf (1,412 KB)


I aim to produce a model for droplet motion in the atmosphere that will help to minimize spray drift.

The model should include the effect of release height, direction, and initial speed of droplets, as well as air movement, temperature and humidity. Studies suggest that drift is a problem for droplets in the diameter range 10µm to 150µm, as smaller droplets evaporate and larger droplets settle on the ground or are captured by foliage. Spraying usually takes place in wind speeds of less than 5m/s. So this is the range I have focused on - the other parameters vary with different spray applications, so the model should be able to handle this.

Initially I have assumed that the droplets are pure water, this will be adjusted as my research progresses.

Critical initial conditions for spontaneous thermal ignition

R. O. Weber, E. Balakrishnan and G. C. Wake

Download: Spontaneous Ignition (Wake).pdf (125 KB)


Critical initial conditions for thermal ignition are calculated numerically for planar, cylindrical and spherical geometries. In each case a comparison is made on the bifurcation diagram with the intermediate steady state(s). It is clearly seen that the actual critical initial condition which depends upon the initial temperature profile, is partly below the intermediate steady state(s). The variation with ambient temperature, exothermicity and Biot number are all explicitly shown. The results are important for calculations relevant to the critical hot product assembly problem where a hot manufactured organic product is packed into containers or stockpiled at a subcritical ambient storage temperature. An example calculation for an interesting case study (milk powder) is given to illustrate the utility of our results.

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