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1. Parallel Unsteady Reynolds-Averaged Navier-Stokes (URANS) Studies of the Performance of ONR Waterjet AxWJ-2

   Stephen E. Monroe (March 2023, AIAA Region I Conference, March 2023)

   The accuracy of SimericsMP+ Unsteady Reynolds-Averaged Navier-Stokes (URANS) model is validated by studying turbulent flow past counter-rotating propellers (CRPs). Subsequently, URANS is used to study the axial flow in an Office of Naval Research (ONR) waterjet and its performance. Specifically, experimental data from Miller (1976) is employed for comparison against the URANS results. Due to the large number of degrees of freedom for both simulations, parallel computing over 80 cores is involved. For the CRP study, torque and thrust coefficients are plotted against a range of advance ratios, ensuring a Reynolds number of less than 500,000. For the waterjet, torque and head coefficients are plotted for a range of flow rates at a Reynolds number of 1.25 × 106. For both studies, two different mesh resolutions are utilized. The finer meshes of both studies contained roughly four times the total number of cells found in their respective coarse meshes. These refinements lead to minor improvements, showing good convergence. The URANS torque and thrust coefficients are found to be within 10% of that from experimental data across all advance ratios for the CRP set, showing good agreement. The torque and head coefficients for the waterjet displayed even better agreement, with the greatest error across all flow conditions remaining under 3%. It is concluded that the stator is responsible for 20% of the waterjets power production. 

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2. 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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3. Experimental Investigation of Rotor Blade Structural Response In Hovering and Advance Flight

   https://doi.org/10.4050/F-0080-2024-1228  

   Croke, Alexander; Green, Richard; Wild, Oliver; Jones, Anya (May 2024, The Vertical Flight Society)

   A towing tank investigation of a single rotor blade operating at hovering and high advance ratio conditions is presented. A custom blade was manufactured and instrumented with fully bridged axial strain gauges to monitor the flap bending strain at three radial locations. Measurements of rotor thrust and torque were obtained to characterise the rotor aerodynamic environment for advance ratios ranging from 0.4 to 1.00 and to identify the presence of stalled and reverse flow. Strain measurements obtained at three locations across the blade span show minima and maxima at approximately the same azimuthal location as the load data. Moreover, the strain distribution shows a growth in strain magnitude with increasing advance ratio. Spectra of strain shows a dominant 1/rev signal and for the ∅ = 25° collective, non-harmonic frequencies are observed due to aperiodic vortex shedding from the presence of stalled flow. 

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4. 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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5. 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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