Experimental Study on the Performance of an Innovative Tide-Induced Device for Artificial Downwelling
Round 1
Reviewer 1 Report
The manuscript describes a device that can cause artificial downwelling driven by tidal currents. The hydrodynamic theory for the device is described. In an experiment it is shown that the downwelling works as expected. The theoretical pumping capacity is given for a given current speed and vertical density difference and the vertical cross-sectional area of the tube. The outlet velocity at the bottom of the tube is less than the tidal speed because of the density difference between the end of the tube and frictional losses.
Only two sets of quantitative observations are shown. Fig. 9 shows the effect of stratification on the downwelling intensity and Fig. 10 shows how the energy loss in the device depends on the Reynolds number. Fig. 8b shows that there also is downwelling outside the tube, but this is not explained in the manuscript. Is it just a curiosity or is the flow important in the experiment? Please explain how it is forced in the experiment.
The authors have only shown that if the tidal speed is large enough and the density difference between the ends of the pipe is small enough, the device may create artificial downwelling of oxygen rich surface water into hypoxic deepwater. However, they have not shown that the surface water really is mixed into the deepwater and thereby delivers its content of oxygen to the deepwater. If the outlet speed from the pipe is too small, there will be only little mixing between the down-welled surface water and the deepwater. Due to its buoyancy the deepwater will rise. If it again reaches the surface layer without substantial mixing with the deepwater the efficiency of the process is very low.
On line 94 – 96 it is said that “The static mixer is necessary in order to increase surface-bottom water mixing and avoid the released warm, light, oxygen-rich surface water near the bottom just rise the again to the upper thermocline and the mixed layer.” However, the efficiency of the mixing by the static mixer is not investigated in the experiment. Since the mixing is very important for the transfer of oxygen to the deepwater the mixing problem should be discussed in the manuscript. There should be a description of the functioning of the static mixer in the manuscript. If the mixing fails, the developed method is of less value. It would be mentioned that in the method practiced by Stigebrandt et al. (2015) in the By Fjord the initial mixing at the outlet of the pipe was considered very important and it was performed by horizontal jets with the speed 2 m/s.
Minor points
Line 44 & Table 1: The downwelling in the By Fjord experiment used power from the grid because it would have been more expensive to use wind power at that time.
Line 130-131: Please explain how the average power available from tidal currents is estimated.
Line 219: 1300 m3 s-1 must be wrong!
Line 440 – 441: Richardson
The English language is bad and must be improved before publication.
Author Response
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Reviewer 2 Report
This manuscript entitled “Experimental study on the performance of an innovative tide-induced device for artificial downwelling” addressed the importance to relieve hypoxia/anoxia for aquaculture by pumping O2-rich surface water to the bottom layer. To achieve this goal, the authors introduced a tide-driven device for artificial downwelling. Additionally, the authors ran a series of experiments to identify the relationship between downwelling flow rate, horizontal tidal current velocity, relative density difference, diameter of the tube, and bend radius of the tube. I like the solid results from the experiments. This idea is interesting from the point of view of hypoxia/anoxia, while I show my concerns about the feasibility of this idea by the limitation in the field practices. To address this issue, I would like to see how the authors connect their lab experiments with field physical and chemical properties.
Some major comments are listed below:
The authors need to address the novelty of this manuscript. As in the authors’ previous work, “Xiao, C.; Fan, W.; Qiang, Y.; Xu, Z.; Pan, Y.; Chen, Y., A tidal pump for artificial downwelling: Theory 526 and experiment. Ocean Engineering 2018, 151, 93-104.”, the authors have addressed this device. I have noticed that there are minor differences between them, including tube shape, size and so on. However, the authors addressed it’s an innovative device in this manuscript. I am thinking that it’s best for the authors to demonstrate the improvements of this device in both engineering design and theory. The authors listed an important study site called Muping marine ranch and monitored the changes in DO and temperature in the upper, middle, and lower layers in August. Additionally, the authors depicted the profiles of temperature, salinity, and density on Aug. 21, during which O2 was free in bottom waters. These values are important and can be used as threshold to double check the feasibility of the tide-induced device. However, in the latter sections, the authors didn’t mention this important information again to show the advances of their device. Instead, the authors just described their experimental results only. I am thinking that the authors should put their experimental results with field data together in the discussion. Otherwise, study site and lab experiments are totally separated. What is the interannual variation in the thickness of the summer pycnocline? As we only need to deploy this device in summer due to the formation of hypoxia and anoxia. Variation in the depth of pycnocline matters. Otherwise, the top of the tube might be located below the pycnocline. How large is the area of summer hypoxia and anoxia? The authors should give us an estimate of how many tubes are needed to relieve hypoxia and save aquaculture. If we need a lot of tubes, it’s not economic and must hinder boat sailing and trawling. In Fig.8. The authors mentioned the effectiveness of downwelling induced by the deflector. I question about this experiment, as we know the upper and lower tanks are separated with a stainless plate. Moreover, there is a gap between pvc tube and edge of the plate as shown in Fig. 7. Therefore, the downwelling outside the tubing might be caused only by water exchange rather than the influence of deflector.
There are many specific comments as follows:
Line 37, define “DCG” Numbers and units should have a space, such as L38-“12000m3/day”, L50-15.36m… correct them over the whole manuscript. L42, should delete “which” in “project which using”. L46, has been should be have been. L49, “Xiao et al.” is a wrong citation method. L51, “consisting” is “consisting of”. Table 1, in applicable sea condition part, need to add detailed information about “Tide” because tide influenced most of the area. L58, “threaten” should be “threatens”. L59, grammar error for “hardly have engineering”. And “the status quo” is Latin phrase, the authors can just use “the current situation”. L66, “concept” is “conceptual”. L81, when discussing the flow cross section between round and square pvc tube, does size matter? L91, “in the case of” is “in the case that”. L99, “trawers” is a wrong word, should be “trawling nets” or “trawler”? L103, Fig.1, “Sendiment” is “Sediment“ 1, H2S, NH3, SO4, numbers should be subscript. Define “MWL”, is it mixed water layer? L105, “where is “ should be “which is”. L108, “that are” should be “there is”. L118, if the authors calculate density, salinity changes play a more important role in density change instead of temperature. L122, “condition” should be “conditions”. L124, I am thinking “hypoxia” should be “anoxia” because there was no O2. 4, y-axis should have labels. Don’t use numerical values. Use separate y-axis to demonstrate DO and Temp with units. “analysis” is “analyze”, “regard” is “regarding”. L226, “made” is “are made”. L257, “dyeing” is “dying”. L378-379, “become zero when the bottom of the tube reaches the seabed”? I am guessing “reaches the pycnocline”? Correct me if I am wrong. L431, “functions” is “a function of”.Author Response
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Reviewer 3 Report
The paper presents an interesting device to generate downwelling in anoxic shallow water. It is a current and important problem around the world and worth investigating it. However, despite the presented device is interesting the adopted research method is not clearly described and it would nice for the Authors to revisit it in order to produce a clearer explanation of their technique and assumption. So, I suggest major revision for the manuscript, confident that the improvement will make a nice document to be published in this Journal.
Introduction:
Mention the WEBAP device, the only one of this category tested in the real field, Margheritin et al 2011.
The numerical modelling perform by Antonini et al. 2016 )Applied Ocean Researh 61, pp 16-31), it is worth mentioning in the introduction as it was the first numerical model of this type.
Pag.1 line 37: define DCG before,
Table 1: the OXYFLUX device was mainly designed to work in very low wave energy climate. Indeed it can generate downward flux even with breeze waves.
Pag. 3 line 84: Even if the device is below the water surface the access to the are form the ships should be forbidden.
Pag. 3 lines 91-93: rephrase not clear.
Figure 1: the buoy under the wave action might strongly affect the device, please describe countermeasure.
Pag. 4 line 131: 704MW and 3-9kW/m2, please provide reference or calculation.
Figure 4: describe better what happened after 27 Aug., upper, middle and lower temperature are roughly the same in the Figure.
Pag. 6 lines 156-157, rephrase, it is the required head to overcome the difference in density.
Pag. 6 line 159, where is "o" used except as subscription of z?
Eq. 5 provide the reference
Eq.6 no needed
Eq. 8 which is the density you assume? in the equation is only written rho_i
Eq. 9, please provide the reference.
Pag. 7 lines 196-198: it is not clear, please rephrase.
Pag. 7 line 204: 0.0003 please provide the unit of measurments
Pag.7 lnes 205-206: please explain mor ein detail
Pag. 8 lines 221-224: please explain better is not clear, What doe it do?
Figure 6: use dimensions instead of lengths
Figure 6: what is the weighing machine?
Flow measurements:
Better describe the overall set-up, provide quantitative information, provide analysis to the camera parameters, What is and how does it work the portable flow velocity measurement, please be more technical in your description. Eq. 12 please describe better Pag. 10 line 267: "several" it is not enough for a paper, please provide quantitative information in order to evaluate the statistical relevance.Eq. 14, and generally about Richardson number: have you used the Richardson number as scale law? Can you neglect Froude? Gravity and inertial forces might be quite relevant for your test.
Pag. 14 lines 347-348: what about the Reynolds number?
Pag. 14 lines 348-349: How does Figure 10 quantify the scale effects? I do not think you can quantify them.
Pag. 15 generally about critical velocity: it is not clear the concept. What do you mean for critical velocity? Become zero when to tube reach the bottom? can you explain this concept is not clear at all, for me, it reaches 0 when the tube length is 0.
Pag. 15 line 389: and internal losses.
Please provide a better description of the symmetrical guide plates.
In the conclusion you mention the critical Richard number, but this concept was never mentioned in the manuscript. please introduce it before.
Author Response
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Round 2
Reviewer 2 Report
I am glad to see that the authors have revised the manuscript carefully. By the way, Point 3: What is the interannual variation in the thickness of the summer pycnocline? should be addressed to make sure that the fixed-length tube will have its inlet in the surface mixed layer .
Point 1: The authors need to address the novelty of this manuscript. As in the authors’ previous work, “Xiao, C.; Fan, W.; Qiang, Y.; Xu, Z.; Pan, Y.; Chen, Y., A tidal pump for artificial downwelling: Theory 526 and experiment. Ocean Engineering 2018, 151, 93-104.”,
Should be addressed again in the introduction to let us know it's based on a previous effort and what's new. Showing it only in the method section will conceal other people's contribution.
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Reviewer 3 Report
I think two main references must be considered in the paper in order to properly provide a full description of the benchmark papers about the artificial downwelling devices:
Antonini, A. and Lamberti, A. and Archetti, R., 2015. OXYFLUX, an innovative wave-driven device for the oxygenation of deep layers in coastal areas: A physical investigation. Coastal Engineering, 104, 54-68.
Antonini, A. and Lamberti, A. and Archetti, R. and Miquel, A.M., 2016. CFD investigations of OXYFLUX device, an innovative wave pump technology for artificial downwelling of surface water, Applied Ocean Research, 61, 16-31.
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