Wind forces and their influence on gas loss from grain storage structures

1978

A simple mathematical model is proposed, which combines the effects of mean wind speed, plant spacing and the drag coefficients of individual plants to calculate the fluctuating airflow within a waving crop canopy. The model is non-linear and is only amenable to analytical treatment when linearized; however, the full non-linear version can be solved on an analogue computer.The linear and non-linear results show unexpectedly good agreement, and the success of the linearization allows clear conclusions to be drawn about the relationship between the elasticity, geometry and mean wind speed through a stand of plants and the fluctuating components of airflow and stalk motion.Finally, the analytical results are in qualitative agreement with wind-tunnel results from two model canopies with different characteristics.

Section: Comparison With Wind-tunnel Resultsmentioning

confidence: 99%

A simple mathematical model of airflow in waving plant canopies

1978

“…The working section is 12 m long, 0.61 m high, and 1.83 m wide. A full description can be found in Mulhearn et al (1976). A thick turbulent boundary layer was developed in the tunnel by placing 1.2 m of smooth surface followed by 4.28 m of a very rough surface upstream of the model canopy.…”

Section: Experimental Apparatus and Proceduresmentioning

confidence: 99%

Modelling waving crops in a wind tunnel

1978

Analysis of movie films of a field of barley, combined with observations of the motions of individual plants, show that single stalks oscillate at a well-defined natural frequency even when stimulated by turbulent winds. Treating single stalks as resonant cantilevers allows the use of standard engineering methods to determine their elastic properties. Armed with these values, the application of similarity analysis to the equation of motion of a single stalk leads to criteria for aeroelastic modelling of wheat plants in the wind tunnel. A representative value for the spacing of stalks in a small section of model wheat field was calculated by referring to published data on momentum absorption in a variety of real and model canopies. Preliminary measurements of first and second moments of velocity in the model appear to confirm the importance of including elastic properties in wind-tunnel simulations of airflow in flexible crops.

“…WIND-TUNNELSETUP Experiments were performed in an open-return, blower wind-tunnel with working section 1.83 m wide, 0.61 m high, and 11 m long. More details on this tunnel are in Wooding (1968) and Mulhearn et al (1976). To generate a deep turbulent flow, a 49 mm high fence was placed at the start of the working-section.…”

Section: Introductionmentioning

confidence: 99%

Turbulent flow over a very rough, random surface

1978

A knowledge of the nature of turbulent flow over very rough surfaces is important for an understanding of the environment of crops, forests, and cities. For this reason, a wind-tunnel investigation was carried out on the variations in mean velocity, Reynolds shear-stress, and other turbulence quantities in a deep turbulent flow over a rough surface having a fair degree of randomness in the shapes, sizes, and positions of its elements.There was a layer close to the surface with considerable variations in both mean velocity and shear-stress, and the horizontal scale over which the mean velocity varied was much larger than the average distance between roughness elements. Above this layer, whose depth was of the order of the spacing between roughness elements, shear stress was constant with height, and the velocity profile had a logarithmic form. The usefulness of both mean profile and eddy-correlation methods for estimating fluxes above very rough terrain is discussed in the light of these findings.