Effect of boundary layer on simulation of benthic microfossils in coastal seabed

doi:10.13745/j.esf.sf.2023.5.32

Abstract

Abstract:

Computational fluid dynamics (CFD) has been used extensively in palaeontology in recent years. This method is important for the study of the relationships among individual morphology of fossils, function of organs, and their living environments. Currently, CFD methods in paleontology are mostly applied to simulate centimeter-sized organisms, while there are relatively few simulations on millimeter-sized microscopic organisms. In contrast to centimeter-sized macrofauna, microbenthos lived closer to the surface of the seabed and are more significantly influenced by the low velocity zone of the near-bottom viscous boundary layer, and therefore the reduction of the boundary layer basin environment needs to be taken into account in the simulation of these fossils. In this paper, a new model of an uneven coastal seabed surface is constructed using a random surface generation method, and the flow state of the bottom boundary layer is simulated as influenced by the topography of the seabed surface. Based on the velocity data obtained from this simulation, we carried out another simulation to analyze the drag forces of sedentary Cambrian cnidaria fossil Quadrapyrgite at four varied locations on the undulating seabed surface environment. The results illustrate that the flow near the seabed surface can form a distinct low-flow region, and the thickness of the low-flow region becomes thinner with the increase of the flow velocity. The bottom boundary layer is thinner in the upslope region of the waterward side where the flow velocity varies rapidly, and thicker, by contrast, in the downslope and gully regions of the backwater side where the flow velocity is slower and more prone to vortex formation. The influence of the difference in flow velocity is also shown in the amounts of drag forces of Quadrapyrgite at different locations, which can vary by a factor of 1-10 to several tens. This variation in drag forces can influence the distribution and feeding behavior of different microbenthos at the micrometer-centimeter scale. The work in this paper provides more in-depth ideas and methods for simulating the survival environment of benthic microfossils.

Key words: benthic microfossils, computational fluid dynamics, bottom boundary layer, Fourier transform, random surface generation

CLC Number:

ZHANG Yiheng, ZHANG Tao, YONG Yuanyuan, YU Chiyang, XIAO Juyue, HE Kaiyue, WANG Deng, WANG Xing, WANG Bin, YANG Xiaoguang, HAN Jian. Effect of boundary layer on simulation of benthic microfossils in coastal seabed[J]. Earth Science Frontiers, 2024, 31(2): 410-422.

Figures/Tables 9

Fig.1 Bottom boundary layer basin in the modern ocean. A and B adapted after [10], D adapted after [11].

Fig.2 Two-dimensional random surfaces generated with different parameters A—A=0.1,N=1;B—A=0.02,N=5;C—A=0.008,N=10;D—A=0.004,N=20。

Fig.3 3D random surfaces generated with different parameters

Fig.4 Construction of the simulation model

Fig.5 The process of constructing the model with Quadrapyrgites fixed on the bottom

Fig.6 Top view of simulated seabed surface height and flow velocity distribution at different inlet velocities. The color legend on the right represents the velocity magnitude of the arrows in the picture and the color legend below represents the height of the simulated seabed surface.

Fig.7 Flow patterns of simulated seabed surface sections at different inlet velocities. The left column shows the macroscopic flow at the simulated seafloor surface, the right row shows the microscopic flow shown in the red boxed area. The arrows represent the flow direction. The height difference between the arrows in each row is 0.005 m.

Fig.8 Visualizations of the simulation with Quadrapyrgites fixed

Table 1 Drag forces of Quadrapyrgites at Q1-Q4

位置	阻力/N	绝对值与最大值(Q1)之比/%
Q1	5.872 8×10^-8	100
Q2	-5.280 5×10^-9	8.991
Q3	-4.085 8×10^-9	6.957
Q4	-1.341 7×10^-9	2.285

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