The CSIRO Vertical Wind Tunnel This document describes the design, construction and subsequent modification of a vertical wind tunnel located at CSIRO Forestry and Forest Products, Yarralumla, ACT. This facility enabled the first study of the aerodynamic and combustion properties of eucalyptus bark as firebrands.
A vertical wind tunnel for the investigation of the aerodynamic and combustion behaviour of firebrands at their terminal velocity CSIRO Forestry and Forest Products Technical Report No 133 I.K Knight, P.F.M Ellis and A.L Sullivan CSIRO Forestry and Forest Products Bushfire Behaviour and Management The CSIRO Vertical Wind Tunnel CSIRO Forestry and Forest Products Technical Report No 133 I.K Knight, P.F.M Ellis, and A.L Sullivan ISSN : 1329-2218 ISBN: 643 06526 © 2001 CSIRO Australia CSIRO Forestry and Forest Products PO Box E4008 Kingston ACT 2604 Australia Ph: (02) 6281 8211 Int: +61 6281 8211 Fax: (02) 6281 8312 Int: +61 6281 8312 Email: Enquiries@ffp.csiro.au Web: http://www.ffp.csiro.au Design & Layout: Andrew Sullivan Date of Publication: September 2001 Note: The use of trade, firm or corporation names in this report is for the information and convenience of the reader only Such use does not constitute any official endorsement or approval by CSIRO Forestry and Forest Products of any product or service to the exclusion of others that may be suitable Summary T his document describes the design, construction and subsequent modification of a vertical wind tunnel located at CSIRO Forestry and Forest Products, Yarralumla, ACT This facility enabled the first study of the aerodynamic and combustion properties of eucalyptus bark as firebrands The 20 m long wind tunnel had a variable-speed fan, horizontal divergence section, and a turning section below a contractor and a vertical tapered working section The working section was 4250 mm long and 770 mm H 770 mm in cross-section at its base, diverging to 1000 mm H 1000 mm at the exit The air velocities within the working section could vary between and 20 m s-1, and had a variation in the uniform section of the air flow of approximately ±1% (P = 0.05) of the mean Two features in wind tunnel design facilitated the study of firebrands The vertical working section had a divergent taper that allowed a firebrand to find its terminal velocity within a velocity gradient The second feature was the modification of the air flow in the working section that reduced impact of non-tethered firebrands with the walls of the working section A series of screens at the base of the working section resulted in relatively high air velocity adjacent to the walls, and relatively low air velocity in the centre zone These features, and the fan speed control, allowed observation of burning firebrands throughout their viable lifetime The centre zone was square in cross-section and was 300 mm H 300 mm at the base of the working section The velocity in this zone could reach up to 10 m s-1 and had a variation of about ±4% (P = 0.05) of the mean air velocity across the zone The air flow in the centre zone was calibrated so that the air velocity at any location could be derived as a function of contractor differential pressure (CDP) and the height of the location above the base of the working section It was assumed that when there was no apparent vertical movement of a bark sample, its terminal velocity equalled the air velocity At such times, the CDP and height of the sample were recorded iii iv Contents Summary iii Table of Contents v List of Figures vi List of Plates vii General Design 1.1 Principles of wind tunnel design 1.2 Previous firebrand studies using wind tunnels 1.3 CSIRO design requirements and constraints 2 Design and construction of components 2.1 General 2.2 The fan 2.3 The diffuser .6 2.4 The screens .7 2.5 The turning section 2.6 Straightening section 2.7 Contractor section 10 2.8 Adaptor 15 2.9 Working section 16 2.10 The wind tunnel housing 19 Instrumentation 21 3.1 Air speed control 21 3.2 Contractor differential pressure 21 3.3 Working section pitot tube 22 3.4 Pressure transducers and control board 23 The unmodified air flow in the working section 24 4.1 The variation in air flow measured for a central transect 24 4.2 Comparing the air flow along a central transect with that of the surrounds 27 4.3 Conclusion 29 Modification of the air flow .30 Sources of variation in the air flow 33 6.1 The variation in contractor differential pressure 33 6.2 The variation in pitot differential pressure of the unmodified air flow 33 6.3 The variation in pitot differential pressure (PDP) of the modified air flow .34 Calibration of the modified air flow 35 Discussion and conclusions .38 Acknowledgements 40 10 References 41 Appendix 41 v vi General Design 1.1 Principles of wind tunnel design A ll modern wind tunnels that produce a highly uniform flow of air operate on the same fundamental design principles Highly turbulent air from the fan is decelerated in a widening duct called the diffuser This slowed air is then passed through screens which moderate the velocity extremes produced by the fan The air is then passed through a straightening section which straightens the flow by removing large diameter eddies This air, now moving slowly with a degree of uniformity in the widest cross-section of the tunnel, then enters the contractor The contractor constricts the air flow, forcing the air to accelerate Energy is added equally to each parcel of air Variations in energy (mean or turbulent) of the in-going air stream remain but are an insignificant fraction of the accelerated flow at the exit The degree of curvature of the contractor walls is critical; the flow must be accelerated smoothly because any separation of the flow from the contractor walls will produce undesirable eddies in the final flow The contractor is the final step of air flow preparation, reducing variation in the flow to values as low as 0.5% of mean air velocity The uniform accelerated air flow then enters the working section where the aerodynamic experiments are performed Most wind tunnels have the critical components of the tunnel aligned on one axis because bends introduce turbulence and vorticity When a bend in the tunnel is needed, the tunnel is normally designed so that the bend is well upstream of the working section This allows maximum time for unwanted vorticity to dissipate To reduce the height of the CSIRO vertical wind tunnel (and the cost of the building required to house it), a turning section was installed just upstream of the straightening section 1.2 Previous firebrand studies using wind tunnels Both vertical and horizontal wind tunnels have been used previously for firebrand studies These studies used tethered and non-tethered samples, mostly of solid wood shapes (Tarifa 1965; 1967, Muraszew et al 1975; 1976; Clements 1977) Tarifa et al (1967) tethered firebrand samples on balance devices in both vertical and horizontal wind tunnels and at constant or variable air velocities Samples tended to break off the General Design tether when small, but it was assumed that this problem was unimportant, as the ignition potential of very small firebrands was also small Tarifa et al (1967) also found that a conical vertical wind tunnel was useful for determining the burn-out times of non-tethered samples, although measurement of terminal velocities was difficult due to the tumbling motion of the firebrands Figure 1.1 The construction of the CSIRO vertical wind tunnel was supported by Mayne Nickless Ltd The facility was opened by Dr Malcom McIntosh, Chief Executive of CSIRO, on January 17 1997 Clements (1977) used a vertical wind tunnel to observe burning characteristics of non-tethered pine cones He noted the problem of collision of the samples with the walls and that this method was unsuitable for leaf and moss samples because they clung to the walls 1.3 CSIRO design requirements and constraints The CSIRO vertical wind tunnel was designed to allow the study of bark firebrand samples Eucalypt bark is notorious in Australia as a spotting agent but there have been no studies of its firebrand characteristics The bark characteristics of some eucalypt species give them a spotting potential which is not equalled by any other type, both in spotting distance and in spotfire concentration (Cheney and Bary 1969) Muraszew et al (1976) stated that “the lower rate of flat plate burning coupled with the low density of bark allows burning bark to fly large distances and makes it one of the most hazardous firebrand materials” It was supposed that, ideally, the samples should be non-tethered, both because of the fragility of bark and because this method permits the study of the latter part of flight when samples become very small An additional advantage is that during non-tethered flight, samples assume orientations which are likely to occur under natural conditions It is possible that the restriction of natural movement of bark samples (viz because they are tethered) could change their combustion behaviour, although Tarifa et al (1967) showed that this effect was not significant for wood samples Thus, the CSIRO vertical wind tunnel needed to be large enough to allow for a horizontal component of movement of non-tethered samples The CSIRO vertical wind tunnel=s modular design and construction aided on-site assembly and allowed for future reconfiguration to various experimental needs A working section with a cross-section of 750 mm H 750 mm and that was 4250 mm tall was considered a suitable size that allowed free movement of samples A range of - 20 m s-1 in air velocity was also considered appropriate as this covered the range of up-draught velocities of plumes of low to very high-intensity line fires (Raupach 1990) A second stage contractor and smaller working section could be constructed if firebrand samples with terminal velocities > 20 m s-1 were to be studied The minimum path-length from the fan to the exit working section for such a tunnel exceeded 20 metres A straight vertical tunnel of this height would be impractical and costly to con2 The CSIRO Vertical Wind Tunnel Housing Screens Adaptor Section Straightening Section Garage Contractor Diffuser Fan Turning Section 4.8 m 2.65 m Figure 1.2 The general layout and components of the CSIRO vertical working section wind tunnel The tunnel is housed in a tower constructed of clad scaffolding General Design Modification of the working section air flow B efore experiments were conducted, it was necessary to install a primary screen at the base of the working section directly on top of the adaptor section The purpose of this screen was to prevent burning samples falling into the contractor and straightening sections The screen material was standard aluminum insect mesh with square apertures of about 1.8 mm H 1.8 mm It was anticipated that the turbulence introduced by this screen would dissipate within • 200 mm above the screen (Bradshaw and Pankhurst 1962) The firebrand study required the bark samples to remain untethered and free to move during combustion at their terminal velocity However, when the tunnel was initially tested the combusting samples of bark of Eucalyptus obliqua had frequent collisions with the walls As anticipated (see sections 1.2 & 1.3), the bark tended to become trapped in the boundary layer and stick against or slide down the walls The cause of the wall collisions appeared to be the horizontal component of velocity which is often developed by both regular- and irregular-shaped free-falling objects The air flow in the working section was modified to minimise the horizontal movement of the non-tethered sample This modification was accomplished by the addition of square concentric layers of mesh placed at the base of the working section Three additional screens of standard insect mesh with sides of 750, 500 and 300 mm were fastened to the primary screen at the bottom of the working section (Figure 5.1 (a)) These additional screens were laid so that the orientation of the mesh wire on successive screens differed by 45E This was to minimise the occurrence of irregular areas where successive layers of mesh were >in phase=, which caused large variations in air velocity across the centre zone The modified air flow profile had higher velocities near the walls and a central zone of lower velocity (Figure 5.1 (b)) This centre zone (Z) in plan view was a 300 mm square at the base of the working section and decreased in width by approximately 20 mm per metre of height above the screens The modified air flow profile was investigated by measuring the maximum and minimum air velocities at 10 mm intervals along a transect 1000 mm above the screens at contractor differential pressures of Pa and Pa The profile of mean air velocities from the wall to the middle of the centre zone within the modified working section is shown in Figure 5.2 The profile had low velocity air flow in the centre, high velocity near the wall, and, in between, a series of steep velocity gradients associated with the changes from screen to screen The high velocity zone acted as a barrier to a sample traversing to the wall so that the fire30 The CSIRO Vertical Wind Tunnel to pitot tube (not shown) (b) (2) Velocity profile of modified airflow Boundary layer READOUT UNIT (3) Centre Zone is (c) calibrated so that sample terminal velocities can be measured PRESSURE TRANSDUCER LP (d) (4)Combusting (a) (1) Additional mesh 157 sample screens which modify the airflow HP CONTROL BOARD Original mesh screen to contractor Figure 5.1 Schematic diagram of the modified working section and the instrumentation used to determine air velocity in the working section during calibration The diagram shows the arrangement of the screens used to modify the air flow (a) The velocity profile (b) and the location of the centre zone (c) of this air flow are also shown A combusting firebrand sample (d) is shown within this zone brand was pushed back towards the centre (Figure 5.1) The modified air flow did not completely eliminate collisions with the walls of the working section for all samples all the time However, the amount of impact which occurred with regular-shaped messmate stringybark (E obliqua) samples was very much reduced The differences in air velocity between zones decreased as mean air velocity in the working section decreased The areas of turbulent air flow were characterised by large fluctuations in PDP at a given position but the magnitude of these fluctuations were not investigated due to the time constraints of the study Combusting samples typically moved in three dimensions throughout the air flow profile, but tended to spend at least part of their flight within the centre calibrated zone (Z) When in this Modified air flow 31 zone, samples tended to adopt a flat spin which could be fast or slow Some samples showed both clockwise and anti-clockwise spin during their combusting flight While samples were moving in the centre zone there was little variation in vertical movement Although the variation of air flow in this centre zone was not measured we considered that if this zone had a wide range of air velocities the vertical movement of samples would have been erratic Therefore, we considered the air flow within this section was sufficiently uniform for these experiments CDP Pa CDP Pa Air velocity (m s-1 ) 2.5 1.5 0.5 Modifying screens Wall Centre Distance from North wall (mm) Figure 5.2 The profile of air velocity within the modified working section at a height of 1000 mm for two contractor differential pressures(CDP) 32 The CSIRO Vertical Wind Tunnel 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 80 60 40 20 0 Sources of variation in the air flow 6.1 The variation in contractor differential pressure T he air flow in the working section was described in terms of the pitot differential pressure (PDP) measured at points along transects or within designated areas of the air flow During these measurements the contractor differential pressure (CDP) was maintained at a predetermined value which was monitored and adjusted as required At a constant fan speed setting, the measurement of CDP (displayed on the readout device) fluctuated by one or two units This fluctuation was attributed to changes in ambient wind velocity outside the structure, which affected the air flow inside the structure The measurement of CDP appeared to be very sensitive to small changes in external ambient air movement as the readout device only displayed whole units on a - 250 unit scale One unit corresponded to Pa when using the - 250 Pa Furness pressure transducer, and 0.08 Pa when using the 0-20 Pa pressure transducer The problem of ambient air movement is discussed more fully below Similarly, the reading of PDP could also fluctuate The causes of this variation in PDP are also discussed below 6.2 The variation in pitot differential pressure of the unmodified air flow The variation in the PDP of the unmodified air flow (the air flow in the working section without any modifications to improve the air flow for firebrand observation) appeared to result from a combination of fluctuations in ambient wind, velocity fluctuation due to turbulence in the boundary layer, and low frequency vibration or resonance of the walls of the diffuser and turning section 6.2.1 Variation in air flow velocity due to ambient wind The variations due to ambient wind are a problem only during calibration Consequently calibration was performed on still days During relatively still days the fluctuation of the readings of PDP or CDP due to ambient wind appeared to be < 1% of their mean values Sources of variation 33 6.2.2 Variation in air flow velocity due to turbulence A relatively large difference between the maximum and minimum differential pressure at one point was assumed to indicate turbulence Variations between 25 and 50% of measured PDP may occur in the boundary layer, which occurs near the walls (see Figure 4.1) 6.2.3 Variation in air flow velocity due to vibration The timber walls had a natural harmonic frequency that was examined by attaching a cathode ray oscilloscope to the output voltage of the pressure transducer The pressure oscillation had a consistent frequency of approximately Hz for fan speeds up to the maximum speed used during experiments The amplitude of wall movement was small It was not measured but was perceptible to touch This acoustic vibration was considered to have an insignificant effect on the accuracy of air velocity measurements for the purpose of determining firebrand terminal velocities However, it is anticipated that these vibrations could be reduced by externally bracing the larger panels of the tunnel walls 6.3 The variation in pitot differential pressure (PDP) of the modified air flow In order to control the flight of untethered firebrands, modifications were made to the air flow within the working section by adding wire screens across the entrance to the working section (see Figure 5.1 and Section for more discussion) When the air flow was modified in this way large fluctuations in PDP occurred in the wall boundary layer Large fluctuations were also found in the horizontal distance intervals •100 - 170 mm and •230 - 260 mm from the walls of the working section (see Figure 5.2) These areas, which are assumed to be turbulent, are located above the edges of the additional screens However, the variation was small within the centre zone, where firebrand sample was intended to burn and the terminal velocities were measured Observations of the motion of the samples within the centre zone also indicated that levels of turbulence were low 34 The CSIRO Vertical Wind Tunnel Calibration of the modified air flow T he aim of calibration was to express the PDP in the lower 1250 mm of the centre zone (Zone Differential Pressure or ZDP) as a function of CDP and height (h) above the screens The air velocity at that height could then be calculated from the derived ZDP The measurements of ZDP for calibration were obtained along a single North-South transect of the centre zone at different heights and for different CDPs It was necessary to test if the measurements along this N-S transect were representative of the measurements throughout this centre zone The results of the test were: mean pressure (S-N centre transect) = 2.6 Pa and mean pressure (combined transects) = 2.44 Pa (P=0.25) Although this test was conducted under less than ideal conditions because of fluctuating ambient wind speeds, there was no significant difference between the mean differential pressures of the whole centre zone and the single central transect The pressure measurements for calibration (ZDP) were made by taking measurements at eight locations along the North-South transect through the centre of the zone at five different heights and for six contractor differential pressures These locations were approximately 10, 30, 70 and 120 mm from each of the south and north boundaries of the centre zone (Figure 7.1) Centre zone boundary positions on a central transect South Positions are 10mm, 30mm, 70mm and 120mm from S and N boundaries Figure 7.1 Positions within the centre zone used to take the measurements for calibration The heights of the transects were 250, 500, 750, 1000 and 1250 mm above the screens The six contractor differential pressures were approximately 2, 4, 8, 16, 24 and 32 Pa The coefficients of variation were calculated for the measurements North at each of the 30 CDP/height categories This population of 30 coefficients had a mean of 4.6% and a standard deviation of 1.77 This error included an unmeasured effect due to ambient wind movement (see section 6.1) Thus, the 95% confidence interval for the error in ZDP is approximately " 8% This is Calibration of air flow 35 equivalent to an error in derived air velocity of about " 4% or " 0.2 m s-1 and " 0.04 m s-1 at air velocities of and at m s-1 respectively Constrained linear regression was performed for the 48 measurements at each of the five heights to derive expressions of the form: ZDP = k × CDP (7.1) Figure 7.2 shows the mean value for centre zone differential pressure (ZDP) versus each value for contractor differential pressure (CDP) for each category of height 18 16 h = 250 mm h = 500 mm h = 750 mm h = 1000 mm h = 1250 mm 14 ZDP (Pa) 12 10 0 10 15 20 25 30 35 CDP (Pa) Figure 7.2 The mean value for centre zone differential pressure (ZDP) versus contractor differential pressure (CDP) for five heights within the working section The values obtained for k using constrained regression are shown in Table 7.1: Table 7.1 Values of coefficient k and its standard error (s.e.) derived by regression for Equation 12 for each height category Height h (m) k (mean) s.e P r2 0.25 0.5 0.0028 < 0.001 0.99 0.5 0.46 0.0044 < 0.001 0.99 0.75 0.41 0.0027 < 0.001 0.99 0.36 0.0029 < 0.001 0.99 1.25 0.32 0.0027 < 0.001 0.99 36 The CSIRO Vertical Wind Tunnel Linear regression of the mean values for h and k, tabled above gave: r = 0.99 (se = 0.0037) (se = 0.0031) k = - 0.18 h + 0.55 (7.2) The air velocity during an experiment at a given height in the centre zone was derived from the CDP, which was displayed on the readout device, was determined in the manner: 1) the slope k for the given height was derived using Equation 7.2 2) the ZDP was derived using Equation 7.1 3) the air velocity was then derived by substituting this value for ZDP into Equation 7.3, which is specific to the pitot tube: v = 28 ZDP − 013 T − − P 288 d (7.3) where v is air velocity (m s-1), ZDP is centre zone differential pressure (Pa), P is station barometric pressure during experiment (Bar), T is ambient temperature during the experiment (oK) and d is relative air density (1) The visual estimate of the height above screens of the firebrand sample (h), was a further source of error in the estimate of air velocity Height was estimated to the nearest 100 mm The consequence of estimating a height of 0.5 m (500 mm) instead of 0.6 m (600) mm was an error of •4% of ZDP and hence •2% of air velocity Calibration of air flow 37 Discussion and conclusions T he CSIRO vertical wind tunnel was constructed economically and within a limited budget The diffuser, straightening section, contractor, and tapered working section proved sound in principle Uniform air flow was maintained for the full length of the working section without boundary layer separation The divergent taper working section, at 3.4E total angle, was sufficient for practical observation of burning embers The tunnel exhibited low frequency vibrations which were attributed to resonance of panels in the diffuser, the turning section and working section, and the length of the tunnel All these panels were constructed from 12 mm thick plywood Sections with dimensions 1.2 m H m are currently unbraced External bracing could easily be attached and should remove most of this vibration Figure 8.1 A piece of Eucalyptus obliqua bark burning at its terminal velocity in the working section of the CSIRO vertical wind tunnel Contractor differential pressure can be seen on the digital display in the background Boundary layer growth in the working section was greater than predicted but was still considered acceptable for the study of nontethered firebrands Mismatches of a few millimeters in the joins of the contractor to the working section were most likely responsible for accelerated boundary layer growth on one wall This boundary layer growth could be reduced by fairing the joints between the modules and improving the quality of the joints Because it is difficult to work timber and plywood to millimetre accuracy, we suggest fabricating sections in pairs, using the previous completed component as a jig for the following stage Turning sections will inevitably introduce turbulence of the scale of the vane spacing (140 mm) These sections are normally placed well upstream from critical working sections to allow this turbulence to dissipate However, the placing of the straightener and contractor a short distance from the turning section did not have a great adverse affect on the quality of air flow 38 The CSIRO Vertical Wind Tunnel Construction difficulties increase exponentially with size Calculations following the kinetic energy increases of individual air parcels showed that a 7.1:1 contractor will reduce a 20% variation in input flow to 0.5 % at the contractor exit If the widest cross-section was reduced from metres to 1.5 metres, a 4:1 contraction could be used A 4:1 contraction would reduce a 20% variation to 1.1 % which is most probably adequate for firebrand studies Screens of similar porosity to those presently used would be needed in the diffuser sections, but note that a smaller cross-section tunnel will generate higher back pressures However, fans of the type selected have a generous reserve of pressure capability to permit wind tunnel builders to increase the number of screens to improve the flow quality into the contractor and so compensate for a lower contraction ratio The above suggestion maintains the working section with a 750 mm entrance into the adapter This is a practical size We would not recommend making it smaller Boundary layers will build up at the same rate regardless of tunnel width The boundary layer will eventually encroach on a small tunnel, producing an unworkably small centre zone a shorter distance into the working section A pitot tube was mounted so that it could be traversed remotely to measure differential pressure of the air flow in the working section Measurements made directly under the pitot support beam u n d e r- estimated the air velocity in the surrounding area by •1% The air velocities in the uniform zone at the bottom and top of the working section varied by between " 0.5% and " 1.5% (P = 0.05) respectively, of the mean air velocity This air flow was adequate for the study of firebrands Figure 8.2 A flaming firebrand being launched through the access door into the lower half of the working section of the CSIRO vertical wind tunnel The firebrand was lit with a gas flame at the work bench in the background Additional screens were placed at the base of the working section to modify the air flow and reduce the rate of collision of untethered samples with the walls The screens resulted in a central zone of lower air velocity than the surrounding areas and tended to restrict the horizontal movement of samples, reduce collisions with the walls of the working section so that most samples spent at least a part of their flight in the calibrated centre zone This method was successful for experiments burning samples of the bark of E obliqua It is possible that more sophisticated methods exist to achieve this, but time and budget constraints prevented further investigation Discussion and Conclusions 39 The calibration method allowed the terminal velocity of a firebrand sample within the modified working section to be determined as a function of contractor differential pressure (CDP) and the height of the sample above the base of the working section The CDP was monitored continuously during experiments using a readout device mounted in the north wall of the working section The variation in air velocity throughout the centre zone, at a given height and for a constant CDP, was approximately " 4% (P = 0.05) of the mean air velocity Fluctuation in air velocity in the working section due to ambient air movement was a problem during calibration Consequently calibration was only performed on calm days Large fluctuations in ambient wind did not affect the accuracy of measurements during experiments but made it difficult to take measurements and did result in the loss of a few samples from the working section While the tapered working section allowed a burning ember to fluctuate over a significant range of terminal velocities, the ember had to be kept within a practical viewing >window= of the working section This was achieved by manually adjusting the overall air flow velocity by altering the speed of the fan While the speed of the fan responded quickly to the speed controller, human reaction times and hand/eye coordination while observing the ember=s flight were the limiting factors in the operational use of the wind tunnel It was found that a good percentage of the 4.25 metre tapered working section length was needed to steady an ember and keep it captive The wind tunnel was successfully used as part of a doctoral study into the aerodynamics and burning characteristics of E obliqua (Ellis 2000) Accurate measurements of the terminal velocity of burning bark samples enabled a model of maximum spotting distance to be constructed 40 The CSIRO Vertical Wind Tunnel Acknowledgements T he construction of this wind tunnel was funded by Mayne Nickless Ltd The tunnel was prefabricated in the CSIRO Division of Forestry and Forest Products= carpentry workshop at Yarralumla, ACT, by Ricky Jordan Members of the Bushfire Behavior and Management Team assisted with the construction of the housing for the wind tunnel and with its final installation and assembly: Phil Cheney, Jim Gould, Peter Hutchings, Sean Cheney and John Coleman 41 10 References Bertin, J.J 1984 Engineering Fluid Mechanics Prentic-Hall, Inc., Englewood Cliffs, New Jersey 461 pp Borger G.G 1976 The optimisation of wind tunnel contractions for the subsonic range NASA Technical Translation: NASA TT F-16899 Translation of "Optimierung von Windkanalduesen fur den Unterschallbereich, Ruhr-Universitat, Fakultat fur Maschinebau und Konstructiven Ingenieurbau, Doctoral Dissertation, 1973 151 pp Bradshaw, P and R.C Pankhurst 1962 The design of low speed wind tunnels National Physical Laboratory, Teddington, UK Reprinted from NPL Aero Report 1039 64 pp Clements H.B 1977 Lift-off of forest firebrands USDA Forest Service Research Paper SE-159 11 pp Ellis, P.F 2000 The aerodynamic and combustion characteristics of eucalypt bark A firebrand study PhD Thesis, Australian National University, Canberra 187 pp Mentah R.D 1994 Aspects of the design and performance of blower tunnel components Thesis (Ph D) London University Photocopy Boston Spa, England: British library Muraszew, A., J.B Fedele and W.C Kuby 1975 Firebrand Investigation Aerospace report ATR-75(7470)-1 The Aerospace Corp., El Segundo, Calif 103 pp Muraszew, A., J.B Fedele and W.C Kuby 1976 Investigation of fire whirls and firebrands Aerospace report ATR-76(7509)-1 The Aerospace Corp., El Segundo, Calif 155 pp Rae, W.H and Pope, A 1984 Low-speed wind tunnel testing Wiley, New York Raupach, M.R 1990 Similarity analysis of the interaction of bushfire plumes with ambient winds Mathl Comput Modelling, 13(12):113-121 Tarifa,C.S., del Notario, P.P., and F.G Moreno 1965 On the flight paths and lifetimes of burning particles of wood Tenth symposium (International) on Combustion, pp 1021 - 1037 Tarifa, C.S., del Notario, P.P., F.G Moreno, and A.R Villa 1967 Transport and combustion of fire brands Final report of grants FG-SP-114 and FG- SPEED-146, Aeronautical Institute of Madrid for USDA 90 pp 42 The CSIRO Vertical Wind Tunnel References Appendix Pressure measurement and wind tunnel air velocity determination D efinition of static, dynamic and total pressure help in the description of wind tunnel performance These pressures are not measured in absolute terms but as a differential pressure between two points The air flow in the vertical wind tunnel was initially calibrated using pitot tube anemometers to take measurements of point velocity throughout the working section The total air flow of the working section air flow was determined as a function of Contractor Differential Pressure (CDP) In subsequent experiments, where an unobstructed working section was required, air flow was determined with a differential pressure reading between two static pressure taps, one at the inlet of the contractor and one at the outlet of the contractor The difference between these two provided the CDP Static pressure was obtained by tapping into the tunnel wall The inside opening was flush with the inside wall such that the air flow suffered little disturbance The pressure reading was that of the free stream pressure within the tunnel at that location The air flow within the contractor was essentially non-turbulent and Bernoulli's principal was valid, i.e the static pressure (P) plus the kinetic energy of the air stream was a constant For the contractor the pressure and velocity at the inlet (v i) is related to the pressure and velocity at the outlet (v o) by Bernoulli's equation : Pi + ρv i2 = Po + ρv o2 (A1) or ∆Poi = ρv o2 - ρv i2 (A2) where P i and P o are the pressures at the inlet and outlet respectively (Pa), ∆P oi is the difference between the pressures at the inlet and outlet (Pa), and ρ is the density of the air (kg/m3) Knowing that the ratio of exit velocity to the inlet velocity was fixed by the contraction ratio : vo/vi = C (A3) meant that the directly measured differential pressure ∆P oi could be converted to velocity 43 The other means of determining velocity is with a pitot tube anemometer This is also a differential pressure device, one that measures both static and total pressure The difference between static and total pressure is dynamic pressure Dynamic pressure provides a measure of velocity Dynamic pressure is defined as the pressure exerted on a stationary object by the motion of a fluid For a fluid to exert dynamic pressure on an object, the fluid must be undergoing acceleration, e.g deflection by the object In the case of a pitot tube anemometer, the pressure measured at the tip inlet of the tube (which faces into the fluid stream) is the total pressure of the fluid stream This total pressure is a the sum of the static pressure and the work done on the fluid in order to bring it to rest in the inlet This work is equal to the fluid=s initial kinetic energy (2ρv 2) The ports further down the shaft of the pitot tube parallel to the fluid stream, where the fluid is moving close to the free stream velocity, reads the static pressure A differential pressure transducer connected between the static and total pressure inlet ports gives a measure of the dynamic pressure and provides a measure of the kinetic energy of the airstream The kinetic energy of the air, 2ρv 2, is often loosely referred to as the dynamic pressure Bernoulli=s principle is then that the sum of dynamic plus static pressure (i.e the total pressure) is constant However, this is only true for a frictionless or drag-free system In practice, wind tunnels have resistance from screens, honeycombs, walls, etc, which are described as losses in total pressure 44 The CSIRO Vertical Wind Tunnel Appendix ... of the final design of the CSIRO vertical wind tunnel is shown in Figure 1.2 The components and modules of the wind tunnel are described in detail in Section Figure 1.3 The external view of the. .. external view of the completed CSIRO vertical wind tunnel The CSIRO Vertical Wind Tunnel Design and construction of component 2.1 General T he components of the wind tunnel were square in cross-section... axes to produce the overall shape of the contractor (-xw, -yw)exit to (xw, yw)inlet 2.7.4 The parameters for the CSIRO contractor In the case of the CSIRO vertical wind tunnel, the contractor