|Review of “Using computational fluid dynamics on field experiments to improve vehicle-based wind measurements for environmental monitoring, authored by T. Hanlon and D. Risk.|
Overall, this manuscript presents an interesting analysis of the biases that occur when trying to measure winds on a moving vehicle. As this is common practice for a number of land-based research and operational activities (e.g., severe storms research, chemical plume monitoring), the analysis and discussions presented in the manuscript will contribute valuable information to future users of vehicle-based wind sensors.
My primary concerns with the manuscript are that in many places the writing is unclear or written in an overly-complicated (sometimes too condensed) manner, making the results hard to follow. The manuscript reads as a technical report for a very specialized set of domain area (e.g., CFD) experts who already understand all the terminology. This is not appropriate for a journal article and more needs to be done to explain/clarify the methods and results. Also, there were some missed opportunities to do some direct comparisons between the stationary and mobile wind data that would have strengthened the manuscript.
1. Abstract line 14-15 – The authors note the difference between the stationary and mobile anemometers increased with yaw angle, but nowhere in the paper are these results shown. I would expect a table or graphic showing a comparison between the actual observations at least using the observations from the stationary and mobile anemometers that are coincident in time (as provided in your supplemental and published data set). I am not certain this statement is supported by the presented results. Certainly, the CFD shows changes with increased yaw angle, but there is a disconnect (something missing in the results/discussion) when it comes to comparing the stationary and mobile anemometer field data.
2. Section 2.1 – There are a number of terms presented (inlet velocity, yaw angle), coordinate systems (for the vehicle, anemometer, Earth), and instrument placement/height information that it would be nice to see presented in a schematic. A simple top view of the truck, noting the location of the anemometer and identifying the coordinate systems that come into play would be very enlightening. This would also allow the authors to add lines/reference marks that would show the locations of the cross sections used from the CFD results for Figures 3 and 4. As a reader I was very confused as to whether the cross section in Figure 3, for example, ran down the centerline of the truck or was offset to the driver’s side where the anemometer was located.
3. When you developed the string model of the truck in figure 1, did you also include the roof rack and instrument mast in that model? As you note later, the mast can influence the observations and the FLUENT model should be able to assess that impact.
4. Page 3, line 21 – Why did you choose exactly 25.4 m for the extent of the virtual wind tunnel? This seems a rather precise measurement. Why not 26, or 30 m? Did it relate to the analysis grid used by the CFD model?
5. Page 4, line 6 – Can you comment on the impact/role of friction in your field experiment? How did the conditions differ from the smooth/no slip conditions of the CFD?
6. Page 4, Line 8 – which “manufacturer’s specifications” were used? I assume this was ANSYS, but please specifically state that here as you have mentioned Toyata, Nissan, and other manufacturers earlier in the paper.
7. Page 5, ~line 8 – See my comment #2. Again, I think a schematic would make the paper easier to understand.
8. CFD – general comment – It would be interesting to see the results if you turned the vehicle fully broadside (90˚) to the wind (this is one of the worst angle for flow distortion on ships, even with bow mounted anemometers) and rotated to 180˚ to provide the tail wind example (this would likely be the worst case for tail wind flow, but may be enlightening). The lack of any CFD from the tail wind configuration gave you no standard for comparison for your tail wind field data.
9. Figure 2 – Please add a north reference to the map and note how much difference there was between true north on the Earth (via your GPS data) and the roads on your grid. I assume they are not running exactly N/S or E/W.
10. Page 6 – I would like to see a bit more regarding your experimental design here
a. You used two different Sonic anemometers (RM Young on the truck, Decagon on the stationary towers). What is the difference in their specifications? If placed side by side, what is the bias you would expect between the two sensors.
b. What is the orientation of the anemometer on the truck and how was this confirmed? I assume the 0˚ line on the sonic was pointed towards the front of the truck, but this is not stated (nor is it essential as long as you account for an offset angle in the calculation of true winds – see Smith et al 1999). Again, this could be on a schematic of the truck and the relevant coordinate systems.
c. Are the measurements from the Decagon averaged or instantaneous at the minute the values are reported?
d. Were anemometers at the two heights on the truck run simultaneously, or were the positions swapped between test?
e. When you ran the test at a lower height, did you also lower the height of the stationary anemometers?
11. Page 7, line 18-19 – This seems an expected result, and you should state this if it was expected. Trucks are designed to be aerodynamic going into the wind as you drive down the road. Most manufacturers probably do not pay much attention to the aerodynamics in cross winds as this plays little role in fuel economy (though they may look at this some for safety specifications).
12. Figure 3 – In the caption (or the suggested schematic) please clarify the plane on which these CFD results are presented. I assume they are on a plane running the length of the truck that intersects the mast on the driver’s side (not a plane down the center of the truck).
13. Figure 4 – Same comment as Figure 3 (although here you do mention “along the roof racks, but the exact location of the rack vs the anemometer could be shown in a schematic – or even a photograph taken in the field if you have one).
14. Equation 2 – you note wind direction units are degrees, but you do not mention the coordinate system. Are these wind directions from the anemometer in the truck’s coordinate system or do they need to be corrected to the Earth’s coordinate system. Or does it not matter at all?
15. Figure 6 – There is no explanation in the manuscript of how the synoptic conditions varied between your field tests. Were the days all very windy, calm, stormy, benign? Also does figure 6 show the comparison for just a single trial around your box for high and low anemometers positions, or is this an average/compilation of multiple circuits around your box?
16. Figure 7 – These are unacceptable for publication. They are too small to read and it is impossible to see the changes you try to address in the text.
17. Figure 8 – The contents of the tables need to be described in the figure caption. Also are the wind directions represented in the wind roses direction “from which” the wind is blowing versus the vectors from the stationary anemometer being an arrow pointing “to which” the wind blows? Further clarification is needed in the caption. For example, looking at the arrows from the bottom leg of the box, it is not clear these are a tail wind without some additional explanation.
18. Page 15, lines 12-19. This is really the big conclusion of the work and I think it could be stated more strongly. Based on your results and these previous studies, the conclusion is that an anemometer mounted on a truck should be placed “as far forward and a high as possible”, with a location on a bumper mounted forward mast being the best location. This is exactly the guidance given to ship operators regarding wind measurement – forward and high is best. As you noted, if you mount on top of the truck anywhere behind the cab, you would need to correct the winds for flow distortion, but since the flow would vary with each type of vehicle used and CFD modeling is expensive, a recommendation to go with a forward mounted (bumper) mast seems the best choice.
19. Page 15, starting line 20 – This entire paragraph seems out of place and the paper would not be adversely affected if it was removed.
20. Page 17, data availability – I downloaded the file with the anemometer measurements but found it to be lacking in documentation. There is some mention of units, etc. on the source web page, but I suggest taking the time to add another header line with the units for all the columns. For example, what are the time units (local or UTC)? And what is the EPOCH – a time stamp of some sort.
21. I would also encourage the authors to provide their CFD model results in some way. You could provide the resultant flow grids, animations, etc. that clearly show the differences between yaw angles, etc. I have seen these presented for ship flow modeling and they are very enlightening.
22. Supplemental material
a. This material is not really long enough or sufficiently outside the scope of the main paper to be “supplemental”. This text and figures should be added to the main text in the appropriate location.
b. S1 – although I found this method of presenting the vector translation to be overly complex and likely confusing to many readers, it is an elegant approach. That said, the sentence near the end starting “To average measurements” is not clear – Is this referring to an average of each component (as I expect is the case) or what is written: the average of two vector components?