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1. Performance Studies of an Axial Flow Waterjet Pump Using an Unsteady Reynolds-Averaged Navier-Stokes Model

   https://doi.org/10.22191/nejcs/vol6/iss1/5  

   Monroe, Stephen E; Wang, Junfeng; Liang, Chunlei (September 2024, Northeast Journal of Complex Systems)

   In this study, an Unsteady Reynolds-Averaged Navier-Stokes (URANS) model is demonstrated its suitability for studying the flow and performance of open marine propellers and waterjet pumps. First, the accuracy of the URANS model is validated by studying turbulent flow past counter-rotating propellers (CRPs). Specifically, experimental data from Miller (1976) is employed for comparison against the URANS results. Subsequently, URANS is used to study the flow and performance of an Office of Naval Research (ONR) axial flow waterjet pump (AxWJ-2). Due to the large number of degrees of freedom for both simulations, parallel computations over 80 cores are performed. For the CRP study, torque and thrust coefficients are assessed against a range of advance ratios, ensuring a Reynolds number of less than 600,000. For the waterjet, torque and head coefficients are computed for a range of flow rates at a Reynolds number of 1.25 million. For both studies, two levels of mesh resolution are utilized. The finer meshes of both studies contained roughly four times the total number of cells employed in their respective coarser counterparts. These refinements lead to minor improvements, suggesting good grid resolutions with the coarser grids. Across all advance ratios for the CRP set, the URANS torque and thrust coefficients show good agreement with experimental results, remaining within 10% difference. The torque and head coefficients for the waterjet displayed even better agreement, with the greatest error across all flow conditions remaining under 3%. Moreover, URANS studies revealed that the stator is responsible for 20% of the waterjet’s power production. 

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2. On computational simulations of dynamic stall and its three-dimensional nature

   https://doi.org/10.1063/5.0170251  

   Khalifa, Nabil M; Rezaei, Amirsaman; Taha, Haithem E (October 2023, Physics of Fluids)

   In this paper, we investigate the three-dimensional nature of dynamic stall. Conducting the investigation, the flow around a harmonically pitching National Advisory Committee for Aeronautics (NACA) 0012 airfoil is numerically simulated using Unsteady-Reynolds-Averaged Navier–Stokes (URANS) and multiple detached eddy simulation (DES) solvers: the Delayed-DES (DDES) and the Improved-DDES (IDDES). Two- and three-dimensional simulations are performed for each solver, and the results are compared against experimental measurements in the literature. The results showed that three-dimensional simulations surpass two-dimensional ones in capturing the stages of dynamic stall and predicting the lift coefficient values, with a distinguished performance of the DES solvers over the URANS ones. For instance, the IDDES simulations, as an inherently three-dimensional solver, predicted the necessary cascaded amalgamation process of vortices to form the adequate strength of the dynamic stall vortex. This vortex size and timing provided accurate and sufficient suction that resulted in identical matching of the numerical and experimental lift coefficients at the peak value. Hence, the hypothesis that dynamic stall has a three-dimensional nature is supported by the superiority of the three-dimensional simulation in all aspects. In conclusion, it is found that dynamic stall is intrinsically a three-dimensional phenomenon. 

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3. On the universality of local dissipation scales in turbulent channel flow

   https://doi.org/10.1017/jfm.2015.664  

   Bailey, S. C.; Witte, B. M. (January 2016, Journal of Fluid Mechanics)

   Well-resolved measurements of the small-scale dissipation statistics within turbulent channel flow are reported for a range of Reynolds numbers from $$Re_{{\it\tau}}\approx 500$$ to 4000. In this flow, the local large-scale Reynolds number based on the longitudinal integral length scale is found to poorly describe the Reynolds number dependence of the small-scale statistics. When a length scale based on Townsend’s attached-eddy hypothesis is used to define the local large-scale Reynolds number, the Reynolds number scaling behaviour was found to be more consistent with that observed in homogeneous, isotropic turbulence. The Reynolds number scaling of the dissipation moments up to the sixth moment was examined and the results were found to be in good agreement with predicted scaling behaviour (Schumacher et al. , Proc. Natl Acad. Sci. USA , vol. 111, 2014, pp. 10961–10965). The probability density functions of the local dissipation scales (Yakhot, Physica D, vol. 215 (2), 2006, pp. 166–174) were also determined and, when the revised local large-scale Reynolds number is used for normalization, provide support for the existence of a universal distribution which scales differently for inner and outer regions. 

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4. Flow instability and transitions in Taylor–Couette flow of a semidilute non-colloidal suspension

   https://doi.org/10.1017/jfm.2021.75  

   Kang, Changwoo; Mirbod, Parisa (June 2021, Journal of Fluid Mechanics)

   null (Ed.)

   Flow of a semidilute neutrally buoyant and non-colloidal suspension is numerically studied in the Taylor–Couette geometry where the inner cylinder is rotating and the outer one is stationary. We consider a suspension with bulk particle volume fraction $${\phi _b} = 0.1$$ , the radius ratio $$(\eta = {r_i}/{r_o} = 0.877)$$ and two particle size ratios $$\mathrm{\epsilon }\,( = \; d\textrm{/}a) = 60,\;200$$ , where d is the gap width ( $$= {r_o} - {r_i}$$ ) between cylinders, a is the suspended particles’ radius and $$r_i$$ and $$r_o$$ are the inner and outer radii of the cylinder, respectively. Numerical simulations are conducted using the suspension balance model (SBM) and rheological constitutive laws. We predict the critical Reynolds number in which counter-rotating vortices arise in the annulus. It turns out that the primary instability appears through a supercritical bifurcation. For the suspension of $$\mathrm{\epsilon } = 200$$ , the circular Couette flow (CCF) transitions via Taylor vortex flow (TVF) to wavy vortex flow (WVF). Additional flow states of non-axisymmetric vortices, namely spiral vortex flow (SVF) and wavy spiral vortex flow (WSVF) are observed between CCF and WVF for the suspension of $$\mathrm{\epsilon } = 60$$ ; thus, the transitions occur following the sequence of CCF → SVF → WSVF → WVF. Furthermore, we estimate the friction and torque coefficients of the suspension. Suspended particles substantially enhance the torque on the inner cylinder, and the axial travelling wave of spiral vortices reduces the friction and torque coefficients. However, the coefficients are practically the same in the WVF regime where particles are almost uniformly distributed in the annulus by the axial oscillating flow. 

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5. Turbulence and blade deformations of flexible emergent canopies in water flows

   https://doi.org/10.1063/5.0285010  

   Suresh, Dhanush Bhamitipadi; Deng, Qianxi; Palagummi, Saketh; Shores, Jack; Jin, Yaqing (August 2025, Physics of Fluids)

   Turbulence statistics and blade deformations of flexible emergent canopies impinged by water flows were experimentally investigated across a range of Reynolds numbers Reb=Ubb/ν (where Ub is the bulk incoming flow velocity, b is the blade width, and ν is the water kinematic viscosity) and blade aspect ratios AR=h/b (h is the blade length). Time-resolved particle image velocimetry was used to characterize both the deformation of flexible blades and the surrounding flow fields. Results showed that the blade deformation increased with the growth of both Reb and AR, with higher blade bending causing stronger variations in vertical profiles of streamwise velocities and Reynolds stresses. The drag produced by the presence of flexible canopies was identified as the dominant fluid loading balancing the pressure gradient. This term exhibited distinctive reduction near the water surface region with high blade deformation due to the large local blade inclination angle. Interestingly, in contrast to fully submerged flexible blades where the flow-induced drag increases monotonously with flow speed, a critical Reynolds number Reb,cri was observed, beyond which drag decreased with increasing flow speed until the blade became fully submerged. This phenomenon was explained with theoretical interpretations, which exhibited reasonable agreement with experimental results. Further analysis of unsteady flow dynamics revealed that Reynolds stress within the canopy was dominated by ejection events due to the absence of shear layer at the top of emergent canopy. Additionally, streamwise velocity spectra indicated that flow fluctuations inside the canopy were governed by periodic vortex shedding from blade. 

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