Master thesisInner flow structures of turbidity currents based on applied ultrasonic techniques​

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Master thesis

Inner flow structures of turbidity currents based on applied ultrasonic techniques


Laboratory for Flow Control,

Division of Energy and Environmental Systems, Graduate School of Engineering,

Hokkaido University.

1st February 2019


The flow behavior of experimental turbidity currents is evaluated quantitatively in order to reveal their long-distance propagation mechanism. Turbidity currents occurring in nature have complicated and large-scale of flow structures, so it is difficult to obtain sufficient knowledge about their flow behaviors from currents with a limited occurrence observed by a field research. Our research group, therefore, thinks that it is important to obtain basic knowledge that can be extended to actual phenomena from experimental turbidity currents. The test section is an inclined rectangle flume with 4,548 mm in length, 210 mm in height and 143 mm in width. The gate, which separates a heavier fluid and an ambient fluid, is positioned at roughly half length of the flume. It is revealed that the gravity currents generated by the lock-exchange method in such a high-volume release propagate with almost constant values of front velocity Uf. As the heavier fluids, three kinds of fluids are examined; quartz-suspended fluid, opalin-suspended fluid, and saline-dissolved fluids. The center diameters d50 of quartz particles and opalin particles are 12.2 and 18.9 m, respectively. Such small particles have been not used for UVP measurement basically, because the intensity of reflected waves from the particle is too small to be detected by an ultrasonic transducer. In this study, however, a supplemental experiment reveals that the volume fraction () range of quartz particles 0.1 ≤  ≤ 5% shows reasonable velocity distributions obtained by UVP with 4 MHz transducer. Total twenty-five cases of experiments are carried out changing the kind and initial density of the heavier fluids H. In the seven cases of them, two-layer turbidity currents are generated, and the experimental results are discussed. The details about the experimental facility, sediment materials, and initial conditions are summarized in chapter 2.

In chapter 3, analysis method is mentioned. To evaluate the flow behaviors of the gravity

currents quantitatively, it is essential to extend the use range of ultrasonic velocity profiler (UVP), which can be applied to opaque flows. In this study, the measurement by a pair of UVPs makes it possible to obtain horizontal and vertical velocity components (and v) along the whole measurement line of the ultrasonic beam using time correction method based on Uf. Additionally, the particle concentration is estimated by the calculation using the spatio-temporal echo distribution. In case of Rayleigh scattering, the echo values vary with the particle concentration existing in a unit volume. In this study, the boundary condition at the bottom is set to solve the non-linear integral equation between the echo amplitude and the particle concentration. Besides, the scattering intensity is varying depending on the diameter of particles. Appling this characteristic to echo intensity distributions, it makes possible to detect interfaces in the cases of two-layer turbidity currents.

In chapter 4.1., the flow structures between the quartz-suspended currents and the saline density currents are compared. The initial density of all heavier fluids mentioned in this chapter is

H = 1008 kg/m3. Appling pattern matching method to sequential experimental images, it is revealed

that the Uf values of the turbidity currents are greater on average 19.0% than the density currents. The

wall shear stress W is calculated by the least square fitting method for the time-averaged u distributions. The W of the turbidity current is larger than the density current. Therefore, it is revealed that the turbidity current propagates with larger values of Uf than the density current, although the turbidity currents have larger friction coefficient Cf about wall shear stress on the bed. In the body region of the two types of currents, the maximum velocity of horizontal component (umax) is also larger in the turbidity current than the density current. On the other hand, both values of the height hm that

takes the maximum velocity umax and the thickness of the flows are smaller in case of the turbidity current. From these results, the existence of the suspended particles with heavier density than water suppresses the influence of the diffusion, which is predominant factor in the density currents, and helps the continuous supply of driving force due to the density difference.

In chapter 4.2., the turbulent flow structures of the quartz-suspended turbidity current as solid–liquid two–phase flow is discussed. The momentum conservation equation based on two-fluid model is applied to the distributions of the body region. The computed results indicate that the viscous stress and Reynolds shear stress among the five shear stress components are dominant like single– phase channel flows, but the distribution in the height direction of the shear stresses shows a different shape from that. The values of the viscous stress and Reynolds shear stress get canceled out in the outer region (hm < ht, where the reaches zero), so it is revealed that the pressure gradient is working just in the inner region (0 < hm) of the current. Such a phenomenon shows a negative momentum transfer against the mean velocity gradient, which is working to keep a stable stratification. In chapter 4.3., the flow behaviors of several kinds of currents are discussed. These experimental cases show that the turbidity currents containing quartz particles propagates 38.5% faster than turbidity currents containing opalin particles. The analysis results about two-layer turbidity currents show that the kind of the particles suspended in the lower layer determines the front velocity Uf, when the bulk density H in initial condition is equal. In the two-layer turbidity currents composed of different kinds of particle-suspended fluid, it is confirmed that the lower layer is rolled up in the head region and the upper layer is covered with the fluid forming the lower layer. According to such a flow structure of the lower layer, not only the friction with the bed but also the interaction with the ambient fluid in the vicinity of the head region are dominated by the kind of lower layer, which means that the value of Uf can be almost estimated by the friction at the bottom and the interaction at the head region regardless the influence of the shear flow at the upper boundary. In addition to the comparison about Uf, the friction velocity u* is estimated from the distribution of the horizontal velocity component in the body region. The relationship between the friction velocity u* and the maximum velocity umax in the body region is confirmed as u*/umax ≈ 0.114 in every experimental case. It is also

confirmed that the friction coefficient Cf =2(u*/umax)2 ≈ 0.0260 is obtained using the relationship.

Table of contents

  1. Introduction 6
    1. Turbidity currents 6
    2. Ultrasonic measurement techniques and multi-phase flows 9
    3. Final goal and objective of this study 12
    4. Nomenclature 14
  2. Experimental method 15
    1. Experimental facility 15
    2. Sediment particles 17
    3. Experimental cases 19
  3. Development of analysis method 21
    1. Velocity measurement 21
      Supplemental experiments by stirring flow 21
      Velocity measurements for gravity currents 25
    2. Concentration profiles 27
    3. Pattern matching method and front velocity 31
  4. Results and discussions 39
    1. Comparison of flow structures between quartz-suspended currents and saline density currents 39
      Inner velocity structures of the currents 39
      Time variation of the height and velocity of the currents 40
      Flow structures in the body region 42
      Comparison of fluctuation components in the body region 45
      Summary of the difference of flow structures between the turbidity currents and the density currents 46
    2. Inner flow structures of turbidity currents as solid–liquid two–phase flow 49
      Flow field of the current 50
      Concentration profile 52
      Momentum equation in the body region 55
    3. Comparison of flow structures between quartz-suspended currents, and opalin-suspended currents, and two–layer turbidity currents 63
      Experimental snapshots of the currents 64
      Inner flow structures 67
      Interfaces in two–layer turbidity currents 76
      Summary of flow behaviors in the some kinds of turbidity currents 84
  5. Conclusion 86
  6. Bibliography 89

Acknowledgements Appendix

  1. Introduction
    1. Turbidity currentsGravity currents can be observed in many places, for instance in the seas, rivers, and so on. Turbidity currents, a kind of gravity currents driven by density difference between particle-suspended fluid and ambient fluid, are strongly related to the transportation or the deposition of fine particles. Consequently, they play important roles not only for a general understanding of global sediment transport process, but also for estimating the potential environmental hazards which they cause (reservoir sedimentation, trigger of Tsunami, cutting of submarine pipeline systems and cables, effluent dispersal and volcanic hazard (Middleton,1993)). Chamoun et al. (2016) suggested the problem about reservoir sedimentation and a solution for the efficient discharge by the venting. In order to settle the reservoir sedimentation problem, Oehy et al. (2010) evaluated the effect of inclined jet screen on turbidity currents. The result of their study indicates that in certain configurations turbidity currents can be partially stopped by the jet screen and the deposits downstream of the screen may be reduced up to a factor of two as compared with deposits of a free-flowing turbidity current. In addition, there is also the theory that the specific deposition formed by turbidity currents is related to the production or the melting process of fossil fuels such as methane hydrate. The reason why turbidity currents have been payed attention as an important subject is not limited in the field of civil engineering, and the interest for the long-distance propagation mechanism is attracting much attention with a view toward multi- phase fluid dynamics or Earth science.Actually, turbidity currents have been reported to propagate several thousand or hundred kilometers in the seas or rivers by field researches. For example, Azpiroz-Zabala et al. (2017) reported the measurement results by acoustic Doppler current profiler (ADCP) of actual turbidity currents occurring in Congo Canyon (see Table 1-1). In that research, turbidity currents are reported to have occurred six times in the period of seven months, and their thicknesses are from 48 to 77 m and the duration time of the currents is from 5.2 to 10.1 days. Additionally, Symons et al. (2017) also reported the field measurement results in Monterey Bay obtained by ADCP. In the study, the evolution of flow structure and composition are discussed as shown in Fig. 1-1.Contrary to such field researches, many laboratory experiments and numerical simulations have been conducted to collect detailed knowledge of the velocity and turbulent flow structures in the turbidity currents. Paker et al. (1986) suggested the self-acceleration mechanism of turbidity currents from numerical simulation. In the study, it is reported that the sediment entrainment from the bed resupplies potential energy, and it makes possible the acceleration and the long-distance propagation of turbidity currents. Cesare et al. (2001) established a novel numerical model for computational fluid dynamics of the turbidity current, which takes into account the interaction between the current and the deposited sediment. That three-dimensional (3D) numerical model can simulate the balance between deposition and erosion, and the currents provides good agreement with the turbidity currents in alaboratory flume as well as field measurements at the Luzzone Reservoir in swiss Alps. In the study by Gladstone et al. (2004), the multi-layer saline density currents were examined in the lock-exchange flume. It was revealed that the flow behaviors of those currents depend on a dimensionless densityratio between the layers * and dimensionless difference in the driving buoyancy B* (see Fig. 1-2);   g'*    U      W    U , ( 1-1 )   g'andL W LB h g ' MB*          U                  U    U                    U      , ( 1-2 )U WU U L LU LB  B h g '  h g '  M  Mwhere U, L, and W denote the density of the upper layer, lower layer, and water, respectively. In the study by Longo et al. (2016), the saline density currents produced in a circular cross-section channel were focused on, and then they established a theoretical model which coincides well with the experimental results.Table 1-1 Summary of flow properties of the turbidity currents observed in Congo Canyons (Azpiroz-Zabala et al., 2017)
      Fig. 1-1 Schematic of evolution of flow structure and composition of the turbidity current observed in Monterey Bay (Symons et al., 2017)
      Fig. 1-2 Experimental images of the developing two-layer saline density current in case of * = 0.55 and B* = 0.36 (Gladstone et al., 2004)
    2. Ultrasonic measurement techniques and multi-phase flowsIn some laboratory experiments and industries, acoustic measurement techniques have been widely used, because they have strong advantages of (i) utility to apply to opaque fluids and inside the opaque materials (e.g. metal pipeline), (ii) non-invasive measurement, and (iii) wide measurable velocity range. Ultrasonic velocity profiler (UVP) (Takeda, 2012) is one of the most used devices in the field of fluid measurement, and UVP made it possible to measure the spatio-temporal velocity distributions along the ultrasonic beam, using frequency veering based on Doppler effects of ultrasonic waves. Along with the advance in ultrasonic measurement technique, the needs for ultrasonic measurement have also diversified and become complicated. In order to obtain such complicated flow fields, some advanced techniques have been established. One example is ultrasonic imaging velocimetry (UIV) or echo-PIV using array transducers (e.g. Poelma, 2017). In this method, two-dimensional echo image is generated by converting echo amplitude values received by each transducer element to gray scale values. Then, the cross-correlation method used in particle imaging velocimetry (PIV) is applied for two consecutive echo images, and vector fields can be obtained. In the study by Zheng et al. (2006), the results measured by UIV provided good agreement with the vector fields measured by optical-PIV (see Fig. 1-3).
      Fig. 1-3 Validation of echo-PIV using a vertical flow: (a) B-mode particle image of the flow; (b) velocity field measured by echo-PIV; and (c) echo-PIV and optical PIV velocities along one radial line within the flow field (Zheng et al., 2006)Another one is phased-array transducer system (e.g. Kikura et al., 2016; Kang et al., 2016). In this method, adding a phase difference to the emitted ultrasonic waves from each element, spatial two- or three-dimensional echo images as well as measurement line can be generated. Murakawa et al. (2008) established unique ultrasonic measurement device using a dual-frequency Doppler transducer for bubbly flows, which makes it possible to measure both velocity distributions of tracer particles (liquid phase) and bubbles (gas phase) (see Fig. 1-4).
      Fig. 1-4 Schematic image of measurement principle using ultrasonic multi-wave method (Murakawa et al., 2008)In addition to the contraption for the arrangement of ultrasonic transducers, another approach has been conducted, which is utilizing echo intensities information termed echo intensity method. Echo intensity, which is the strength of reflected ultrasonic waves scattered on interfaces between two media having different acoustic impedance, gives us beneficial information. In past studies, echo intensity obtained from UVP could be used to detect moving interfaces of air-water bubbly channel flow by Murai et al. (2010). Hitomi et al. (2017) used echo intensity of pulse repetition method to detect and monitor the air-oil-water three-layer pipe flows (see Fig. 1-5). Su et al. (2017) revealed the relationship between the attenuation coefficient and the phase fraction in oil-water two- phase pipe flows. Shi et al. (2019) established the method to characterize oil-gas-water three-phase flow using time-frequency decomposition. Dong et al. (2015) measured velocity distributions of oil- water two-phase flows using continuous ultrasonic waves. Additionally, echo intensity method was applied to obtain the profiles of suspended sediment concentration mentioned below and to measure the profiles of void fraction in bubbly flows (Murai et al., 2009).
      Fig. 1-5 Samples of interface detection: (a) optical visualization, (b) echo intensity distribution,(c) phase distribution, and (d) Doppler velocity distribution (Hitomi et al., 2017)
    3. Final goal and objective of this studyAs mentioned above, the final goal of this study is to elucidate the long-distance propagation mechanism of turbidity currents occurring in nature under rivers, seas, and so on. In order to achieve this purpose, it is required to reveal the inner flow structures and the condition determining the flow behaviors of turbidity currents. The turbidity currents in nature, however, have huge flow structures, so it seems difficult to obtain sufficient and quantitative detailed data from field researches. Additionally, these have complicated flow structures due to turbulence and complex interaction caused between each particle. The turbidity currents show local mixture density fluctuation accompanying clusters and clouds of particles, which is hardly investigated by optical approaches and numerical simulations. Our research group, therefore, has ascribe experimental studies as a suitable option to take not only basic but also advanced knowledge to estimate the behavior of turbidity currents.To evaluate the flow behaviors of such experimental turbidity currents quantitatively, the measurement techniques, which can be applied to opaque flows with high resolutions, is required. Therefore, it is necessary to expand the utility of ultrasonic measurement techniques for solid–liquid two–phase flows. Based on the above discussions, the objective of this study is written below.
      • To elucidate the long-distance propagation mechanism of turbidity currents fromexperimental results by the lock-exchange technique
      • To expand the utility of ultrasonic measurement techniques, which can be applied to solid–liquid two–phase flows
      Experiments, analysis and discussions along the above objectives are described from the next chapter.
    4. Nomenclature
  2. Experimental method

    1. Experimental facilitySeveral types of gravity currents were generated in a flume by means of the lock-exchange technique, which has been widely used to investigate unsteady currents in laboratory experiments (e.g. Theiler and Franca, 2016). Schematic diagram of the experimental facility is shown in Fig. 2-1 and the picture of the facility is shown in Fig. 2-2. The test section is an inclined rectangle flume with 4,548 mm in length, 210 mm in height and 143 mm in width. Surface on side and bottom walls of the flume walls is lubricated, and top boundary of the fluid layer is free surface. One of the walls is transparent whereas the other is coated with a non-reflective black film, producing a dark uniform and contrasting background for flow visualization. The region surrounded by a dotted square in Fig. 2-1 indicates the area for optical visualizations. A lock gate 1 made of plastic plate is positioned at roughly half length of the flume. As an initial condition, the left-side region of the flume separated by the gate 1 is occupied by a heavier fluid with bulk density H while the right-side region is filled with tap water with densityW as the ambient fluid. As experimental conditions of ambient fluid summarized in Table 2-1,temperature and density of tap water were almost constant, 8.0°C and 999.9 kg/m3, respectively. When the gate 1 is released, the heavier fluid invades under the ambient fluid with front velocity Uf according to the density difference between both fluids. The horizontal displacement from the gate in the right region is defined as axis and axis denotes the height from the bed. For one-layer turbidity currents, which are examined in laboratory experiments normally, one separation gate (gate 1 only) is enough as shown in Fig. 2-1(a). In this study, in addition to the one-layer currents, two-layer currents were examined using two separation gates (gate 1 and gate 2) as shown in Fig. 2-1(b) to observe the difference of flow behaviors depending on the initial conditions or variation of sediment particles. In cases of two-layer currents, the gate 2 was firstly released to generate density difference between heavier particle-suspended fluid in the tank A and lighter particle-suspended fluid in the tank B. Then, the gate 1 was barely released before the head of heavier particle-suspended fluid reached the gate 1, and two-layer currents propagated in the right-side region.