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PhD Final Defense for Pallav Ranjan

Event Type
Civil and Environmental Engineering
2015 Hydrosystems Bldg.
May 15, 2023   11:00 am  

Bridging the gaps between numerical and physical modeling of aquatic vegetation: Experimental bias, near-bed hydrodynamics, and sediment-induced turbulence modulation

Advisor: Prof. Rafael Tinoco


This study presents the results of high-resolution Large Eddy Simulations (LES) and flume-based experiments of flow through rigid emergent vegetation. The aim of the study is to bridge the gap between numerical and physical modeling of vegetated flows by identifying the strengths and challenges of each approach, building a methodology to leverage their advantages to provide a better understanding of hydrodynamics and transport in aquatic vegetation. Direct measurement of precise flow fields within large vegetation canopies is challenging and thus near-bed processes have remained elusive. Wall resolving LES provides a complete description of the flow field in the near-bed region, allowing LES results within the canopy to reveal wake interactions within the cylinders. Results show that the near-bed flow structure changes drastically with increasing 𝑅𝑒, thus affecting the near-bed turbulence statistics. The turbulence budget near the bottom boundary layer is also found to be out of balance across all 𝑅𝑒, which has fundamental implications for near-bed processes such as sediment transport.

In addition, LES and complementary laboratory experiments using Particle Image Velocimetry (PIV) were performed to provide a detailed quantitative assessment of flow response to gaps in cylinder arrays. The base canopy consists of a dense array of emergent rigid cylinders placed in a regular staggered pattern. The gaps varied in length from Δ𝑔/𝑑= 4 to 24 in intervals of 4𝑑, where 𝑑 is the diameter of the cylinders. The analysis was performed under subcritical conditions with Froude numbers 𝐹𝑟 𝜖 [0.08,0.2] and bulk Reynolds numbers 𝑅𝑒 𝜖 [0.8,2]× 10^4. Results show that the gaps affect the flow statistics at the upstream and downstream proximity of the canopy. The affected zone was Δ 𝑥/𝑑 ≈ 5 for the mean flow and Δ 𝑥/𝑑≈ 3 for the second-order statistics. Dimensionless time-averaged streamwise velocity within the gap exhibited minor variability with gap spacing; however, in-plane turbulent kinetic energy, 𝑘, showed a consistent decay rate when normalized with that at 𝑥/𝑑≥1 from the beginning of the gap. The emergent canopy acts as a passive turbulence generator for the gap flow for practical purposes. The streamwise dependence of 𝑘 follows an exponential trend within 1≤ 𝑥/𝑑 ≲ 2.5 and transitions to a power-law at 𝑥/𝑑≥ 4. The substantially lower maximum values of 𝑘 within the gap compared to 𝑘 within the canopy evidences a limitation of gap measurements representative of canopy flow statistics. A simplified framework is presented for estimating representative in-canopy statistics from measurements in the gap.

Aquatic vegetation has the potential to increase suspended sediment capture while also increasing sediment resuspension and bedload transport. Suspended sediment can induce density stratification, which modulates the turbulence in the water column. We derive a Rouse-based formulation for suspended sediment concentration (SSC) including the effect of sediment-induced density stratification. We perform high-resolution Large Eddy Simulations (LES) of vegetated and non-vegetated channels to explicitly highlight the effect of stratification on SSC profiles. We found that the impact of stratification is limited to the near-bed region within the effective boundary layer, affecting both sediment resuspension and bedload transport. Analysis of turbulence metrics revealed that stratification reduces the likelihood of dominant sweep and ejection events near the bed and makes either event equally likely. Modifications to existing models of sediment pickup and bedload transport are suggested to account for the effects of sediment-induced stratification on vegetated and non-vegetated channels.

Finally, detailed qualitative and quantitative analysis of Reynolds stress generation is presented for vegetated canopy flows. It is found that near-bed mechanisms governing the Reynolds stress generation are fundamentally different from the mechanisms far from the bed. Quadrant analysis reveals that increasing 𝑅𝑒 also increases the contribution of each quadrant towards generation of Reynolds stresses in the canopy. Further, quadrant analysis of velocity fluctuations in the gap reveals that measurements in the gap don't provide a representative picture of the flow structure within the canopy. This has implications for the development of process-based models based on statistics measured in the gap. A new model for eddy viscosity is thus proposed based on the accurate quantification of near-bed flow field from LES. The model can be used for practical engineering applications such as stream restoration, measurement of sediment and gas fluxes from the bed, transport of microplastics with sediments, and other related applications.

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