Flume Test Simulation and Study of Salt and Fresh Water Mixing Influenced by Tidal Reciprocating Flow
Abstract
:1. Introduction
2. Materials and Methods
2.1. Flume Layout and Brine Treatment
2.2. Data Acquisition
2.2.1. Flow Velocity Data Acquisition
2.2.2. Salinity Data Acquisition
2.2.3. Other Data Acquisition
2.3. Test Repeatability
- Velocity repeatability test: 12 min was taken as a tidal cycle and the velocity repeatability under the conditions of several cycles on the same day, the same input conditions on different dates and different flow measurement equipment at the same measuring points were verified.
- Buoy displacement test: A buoy was placed on the water surface at a certain interval to record the limit position that the buoy reached in each tidal cycle. The results showed that the drift range of the buoy tended to be stable.
- Salty boundary repeatability test: The position of the salt boundary was recorded at set intervals. The results showed that the change of the salt boundary at the quasi-steady state tended to be within a certain range.
- Wedge thickness repeatability: Near the upstream, the brine intruded in a wedge shape. The video records showed that the wedge thickness and water depth at any time were almost constant after a tidal cycle.
- Salinity repeatability: The salinity of the same section in each layer was measured for two consecutive cycles. The repeatability of both the salinity of each layer and the depth averaged salinity must meet the test requirements.
2.4. Test Conditions
3. Results
3.1. Influence of Tidal Range on the Mixed Type of Salt-Fresh Water
3.2. Influence of Runoff on the Mixed Type of Salt-Fresh Water
3.3. Change of Salt-Fresh Water Interface in the Condition of Tidal Reciprocating Flow
4. Discussion
4.1. Improvement of Flume Test Technology
4.2. Characteristics of Gravitational Circulation and Residual Circulation
4.2.1. Circulation Characteristics under Tidal Range Change
4.2.2. Circulation Characteristics under Runoff Change
4.3. Analysis of Salt-fresh Water Interface Stability
5. Conclusions
- The salt-fresh water mixing under the reciprocating tidal flow was successfully reproduced through the tide and runoff control systems at both ends of the flume, which realized the simulation of the partially mixed type and highly stratified type of salt-fresh water in estuaries. The research scope of the current flume test was expanded by improving the length of the straight segment of the flume. The laboratory pollution problem was effectively solved by developing the preparation, recycling and recovery system of experimental water. The simulation technique was improved by developing the synchronous, multi-section, multi-layer sampling system.
- The flume experiment was carried out by using the control variable method. Under the combined action of runoff and tidal current, the characteristics of salt-fresh water mixing showed obvious differences in different segments. It is roughly divided into three segments upstream from the entrance. The salt-fresh water mixing in the first segment reflected the obvious characteristics of fully mixed type. The second segment presented the remarkable characteristics of partial mixed type. In addition, the salt-fresh water was significantly stratified in the third segment where the saltwater wedge intruded. The distribution range of these three segments varied greatly with the flood and ebb tide, but was consistent in terms of the order.
- Based on the dynamic-state and steady-state, the influence of tidal range and runoff volume on the velocity and salinity distribution in the area of salt-fresh water mixing has been analyzed dialectically. And the contradictory effect of tide and runoff on changing mixing type has been considered respectively.The contradictory effect of tide: tidal stirring and tidal straining. The longitudinal dispersion of salinity caused by tidal straining will lead to the uneven vertical distribution of salinity, thus inhibiting the mixing, which is the opposite effect caused by tidal stirring. During the period of flood tide, the tidal stirring effect is more dominant than the tidal straining effect, while they lead to the fully mixed trend of salt and fresh water jointly. During the period of ebb tide, the tidal straining effect plays a more important role and the tidal stirring effect is inhibited, while the salt-fresh water shows the trend of stratification.The contradictory effect of runoff: stabilizing the stratification and breaking the stratification. The increase of runoff volume can not only stabilize the stratification by strengthening the gravitational circulation and maintaining the vertical density gradient of salt-fresh water interface, but also break the stratification by enhancing the ebb current and weakening the flood current in the upper layer to increase the velocity shear near the interface until it exceeds the critical value. The balance of these two contradictory effects is not only the balance of vertical density gradient and velocity shear, but also the balance of longitudinal convection transport and vertical diffusion.
- The stability of salt-fresh water interface under the condition of tidal reciprocating flow has been studied experimentally, and the periodic transition between stability and instability of the interface has been investigated. The interface stability is closely related to the vertical gradient of the density and velocity near the interface. The Keulegan number (Θ) has been employed in the quantitative analysis on the stability of interface. The experimental results show that, under the condition of turbulent flow, when Θ ≥ 0.152, the interface is stable; when 0.112 ≤ Θ < 0.152, the interface is instable and shows the K-H instability; When Θ < 0.112, the interface is broken. As a result, the critical value Θc of interface stabilization is 0.152.
6. Patents
Author Contributions
Acknowledgments
Conflicts of Interest
References
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Case | Tidal Range (cm) | Runoff (m3/h) | Average Depth (cm) | Initial Salinity | Water Temperature (°C) |
---|---|---|---|---|---|
1 | 4 | 3 | 35 | 10.1 | 9.9 |
2 | 2 | 3 | 35 | 9.5 | 9.7 |
Case | Tidal Range (cm) | Runoff (m3/h) | Average Depth (cm) | Initial Salinity | Water Temperature (°C) |
---|---|---|---|---|---|
3 | 4 | 0 | 28 | 10.1 | 10.4 |
4 | 4 | 3 | 28 | 9.4 | 9.6 |
5 | 4 | 11 | 28 | 18.8 | 10.8 |
Time Sequence (s) | Depth (cm) | Thickness of Upper Layer (cm) | Interface State | Quantity of Wave Peaks within 50 cm Length | Wave Speed (cm/s) | Riw | Re | Frw | Θ |
---|---|---|---|---|---|---|---|---|---|
0 | 29.4 | 6.1 | Steady | \ | \ | 9.47 | 666 | 0.32 | 0.243 |
57 | 29.7 | 5.5 | Steady | \ | \ | 12.31 | 691 | 0.29 | 0.262 |
90 | 30.0 | 6.8 | K-H wave | 4.5 | 8.80 | 2.92 | 1056 | 0.59 | 0.140 |
134 | 29.0 | 7.0 | K-H wave | 5 | 11.63 | 2.55 | 1352 | 0.63 | 0.124 |
181 | 28.8 | 8.3 | K-H wave | 5 | 14.08 | 4.50 | 1826 | 0.47 | 0.135 |
193 | 28.8 | 8.3 | Broken | A large number | 14.97 | 2.82 | 2307 | 0.60 | 0.107 |
242 | 27.5 | 9.5 | Broken | A large number | 10.94 | 3.20 | 2492 | 0.56 | 0.109 |
447 | 25.5 | 13.0 | K-H wave | 3 | 4.11 | 6.56 | 2530 | 0.39 | 0.138 |
550 | 26.5 | 12.0 | K-H wave | 6 | 8.33 | 5.11 | 2438 | 0.44 | 0.128 |
577 | 27.0 | 12.0 | K-H wave | 5 | 9.67 | 5.36 | 2533 | 0.43 | 0.129 |
687 | 28.8 | 6.8 | Steady | \ | \ | 10.30 | 948 | 0.31 | 0.222 |
691 | 28.8 | 6.6 | Steady | \ | \ | 4.98 | 1305 | 0.45 | 0.157 |
Time Sequence (s) | Depth (cm) | Thickness of Upper Layer (cm) | Interface State | Quantity of Wave Peaks within 50 cm Length | Wave Speed (cm/s) | Riw | Re | Frw | Θ |
---|---|---|---|---|---|---|---|---|---|
40 | 30.5 | 9.5 | Steady | \ | \ | 6.78 | 1813 | 0.38 | 0.155 |
62 | 30.5 | 8.5 | Steady | \ | \ | 9.56 | 1336 | 0.32 | 0.193 |
95 | 29.9 | 7.9 | Steady | \ | \ | 5.66 | 1635 | 0.42 | 0.152 |
114 | 29.5 | 8.0 | K-H wave | 4 | 9.16 | 2.75 | 1812 | 0.60 | 0.115 |
135 | 29.5 | 8.5 | K-H wave | 4 | 10.75 | 3.06 | 2006 | 0.57 | 0.115 |
173 | 29.3 | 9.3 | K-H wave | 4 | 10.94 | 3.27 | 2345 | 0.55 | 0.112 |
185 | 29.1 | 10.1 | Broken | A large number | 10.71 | 1.58 | 3695 | 0.79 | 0.076 |
216 | 28.6 | 16.6 | Broken | A large number | 9.65 | 3.89 | 3941 | 0.51 | 0.100 |
263 | 27.5 | 15.5 | Broken | A large number | 8.91 | 4.12 | 3165 | 0.49 | 0.109 |
353 | 26.2 | 14.2 | Broken | A large number | 7.60 | 3.55 | 3814 | 0.53 | 0.098 |
377 | 26.0 | 14.0 | K-H wave | 7 | 7.40 | 7.22 | 2597 | 0.37 | 0.141 |
412 | 25.9 | 13.9 | K-H wave | 6 | 5.86 | 5.92 | 2654 | 0.41 | 0.131 |
466 | 26.0 | 14.0 | Standing wave | A large number | \ | 1.74 | 4205 | 0.76 | 0.075 |
485 | 26.0 | 14.0 | Broken | A large number | 4.67 | 1.72 | 4023 | 0.76 | 0.075 |
515 | 26.3 | 14.3 | K-H wave | 7.5 | 8.28 | 4.60 | 2522 | 0.47 | 0.122 |
552 | 27.0 | 15.0 | Broken | A large number | 12.79 | 1.58 | 4792 | 0.80 | 0.069 |
591 | 27.3 | 15.3 | Broken | A large number | 14.25 | 3.21 | 3990 | 0.56 | 0.093 |
618 | 28.0 | 16.0 | Broken | A large number | 10.62 | 4.05 | 3877 | 0.50 | 0.102 |
704 | 29.6 | 14.6 | Steady | \ | \ | 9.27 | 1867 | 0.33 | 0.171 |
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Xia, W.; Zhao, X.; Zhao, R.; Zhang, X. Flume Test Simulation and Study of Salt and Fresh Water Mixing Influenced by Tidal Reciprocating Flow. Water 2019, 11, 584. https://doi.org/10.3390/w11030584
Xia W, Zhao X, Zhao R, Zhang X. Flume Test Simulation and Study of Salt and Fresh Water Mixing Influenced by Tidal Reciprocating Flow. Water. 2019; 11(3):584. https://doi.org/10.3390/w11030584
Chicago/Turabian StyleXia, Weiyi, Xiaodong Zhao, Riming Zhao, and Xinzhou Zhang. 2019. "Flume Test Simulation and Study of Salt and Fresh Water Mixing Influenced by Tidal Reciprocating Flow" Water 11, no. 3: 584. https://doi.org/10.3390/w11030584
APA StyleXia, W., Zhao, X., Zhao, R., & Zhang, X. (2019). Flume Test Simulation and Study of Salt and Fresh Water Mixing Influenced by Tidal Reciprocating Flow. Water, 11(3), 584. https://doi.org/10.3390/w11030584