FINE SCALE MOMENTUM ADJUSTMENT ACROSS A FOREST EDGE: A WIND TUNNEL EXPERIMENT by Stuart Nulty A thesis submitted in partial fulfilment of the requirements for the degree of Bachelor of Applied Science (Honours) at the University of Canberra Faculty of Applied Science August 2012 i
The research described in this thesis is my own. I conducted all of the experimental work myself and I am solely responsible for the validity of the data. I did most of the data analysis myself. I was fortunate to work alongside a very good engineer at CSIRO who maintained the technical working order of the tunnel. I certify that I am the sole author of the thesis submitted today entitled Fine Scale Momentum Adjustment Across a Forest Edge: A Wind Tunnel Experiment in terms of the Statement of Requirements for a thesis issued by the University Research Degrees Committee. August 2012 ii
Acknowledgements I would like to acknowledge the time, effort and enthusiasm my supervisor Margi Böhm gave to me and my project and the countless hours of time she spent educating me on fluid mechanics and wind tunnel experimentation. Without her support and energy I would not have been able to complete an honours project of this size within 9 short months. To my parents and my brother for supporting me through thin and thin, financially, emotionally and spiritually. Their care and support was integral to completing this research. I acknowledge the help and guidance I was given from John Finnigan and Ian Harman who steered me in the right direction when I got lost. To Dale Hughes for all his help keeping the wind tunnel in top condition, even when I broke things thank you. Finally to the University of Canberra and the Faculty of Applied Science who gave me the opportunity to undertake this project, and the means to present some of its results overseas. iii
Abstract This body of research investigates the adjustment of momentum across and downwind of a model plant canopy edge and was conducted at the Pye Laboratory Wind Tunnel, CSIRO, Canberra, Australia. Two hundred and forty vertical profiles were taken across the canopy edge. These data were compared with 80 profiles taken well downwind of the edge from a previous experiment with which I was involved. Measurements were taken using Laser Doppler Velocimetry to resolve the 3D wind field adjusting across the model forest edge. Some key results emerge: Experimental evidence for the existence of the Equilibrium Layer at the top of the canopy. Streamwise wind velocity decelerates exponentially within the canopy and the rate of deceleration increases with canopy depth. Adjustment of different wind parameters varies with location above and within a canopy. Streamwise wind velocity adjusts much faster within the canopy than above it. iv
Table of Contents Chapter 1: Introduction 1 1.1 Background and Motivation 1 1.2 Theoretical considerations and notation 9 1.2.1 Mean velocity wind equation 9 1.2.2 The momentum equation 10 1.2.3 Spatial averaging 11 1.2.4 Second order moments 12 1.2.5 Adjustment equations 13 Chapter 2: Methods 15 2.1 Experimental framework 15 2.2 The Pye laboratory wind tunnel 15 2.3 Black Tombstones model 16 2.4 Experimental methodologies 20 2.4.1 Laser Doppler Velocimetry 20 2.4.2 Sampling configuration 24 2.4.3 Coordinate rotations 25 Chapter3: Results 29 3.1 Adjustment of mean streamwise windspeed 29 3.2 Adjustment of streamwise standard deviation 31 3.3 Adjustment of vertical standard deviation 35 3.4 Adjustment of Reynolds stress 37 Chapter 4: Discussion 43 4.1 Adjustment of individual profiles 43 4.2 Lateral and block average adjustment 46 4.2.1 U adjustment 46 v
4.2.2 Sd (u) adjustment 47 4.2.3 Sd (w) adjustment 48 4.2.4 u w adjustment 49 Chapter 5: Conclusion 51 References 52 vi
List of Figures 1.1Atmospheric layers 2 1.2 Mixing layer profile 4 1.3 Mixing layer vortex 5 1.4 Schematic of edge flow region 7 2.1 Pye laboratory wind tunnel 16 2.2 The Black Tombstones experiment 17 2.3 Surface configuration 18 2.4 LDV sampling volume 23 2.5 Probe schematic 24 2.6 Experimental plan 27 2.7 Phi coordinate rotation 28 2.8 Theta coordinate rotation 28 3.1 Profile strip adjustment 30 3.2 Streamwise P3 profiles 32 3.3 Streamwise P6 profiles 32 3.4 Standard deviation P3 34 3.5 U Standard deviation P6 34 3.6 W standard deviation P3 36 3.7 W standard deviation P6 36 3.8 Reynolds stress P3 38 3.9 Reynolds stress P6 38 3.10 Adjustment ratios U 40 3.11 Adjustment ratios Sd (u) 40 3.12 Adjustment ratios Sd (w) 41 3.13 Adjustment ration u w 41 3.14 Mean profile development 42 vii