scholarly journals Two-scale dynamics of flow past a partial cross-stream array of tidal turbines

2013 ◽  
Vol 730 ◽  
pp. 220-244 ◽  
Author(s):  
Takafumi Nishino ◽  
Richard H. J. Willden
Keyword(s):  
Analytical Model ◽  
Three Dimensional ◽  
Navier Stokes ◽  
Wake Region ◽  
Near Wake ◽  
New Model ◽  
Tidal Turbines ◽  
Type Design ◽  
Flow Past ◽  
Far Wake

AbstractThe characteristics of flow past a partial cross-stream array of (idealized) tidal turbines are investigated both analytically and computationally to understand the mechanisms that determine the limiting performance of partial tidal fences. A two-scale analytical partial tidal fence model reported earlier is further extended by better accounting for the effect of array-scale flow expansion on device-scale dynamics, so that the new model is applicable to short fences (consisting of a small number of devices) as well as to long fences. The new model explains theoretically general trends of the limiting performance of partial tidal fences. The new model is then compared to three-dimensional Reynolds-averaged Navier–Stokes (RANS) computations of flow past an array of various numbers (up to 40) of actuator disks. On the whole, the analytical model agrees well with the RANS computations, suggesting that the two-scale dynamics described in the analytical model predominantly determines the fence performance in the RANS computations as well. The comparison also suggests that the limiting performance of short partial fences depends on how much of device far-wake mixing takes place within the array near-wake region. This factor, however, depends on the structures of the wake and therefore on the type/design of devices to be arrayed.

scholarly journals Hyperbolicity, shadowing directions and sensitivity analysis of a turbulent three-dimensional flow

2019 ◽  
Vol 863 ◽  
pp. 644-669 ◽  
Author(s):  
Angxiu Ni
Keyword(s):  
Reynolds Number ◽  
Large Fraction ◽  
Three Dimensional ◽  
Flow Simulation ◽  
Wake Region ◽  
Near Wake ◽  
Three Dimensional Flow ◽  
Long Time ◽  
Cylinder Flow ◽  
Far Wake

This paper uses compressible flow simulation to analyse the hyperbolicity, shadowing directions and sensitivities of a weakly turbulent three-dimensional cylinder flow at Reynolds number 525 and Mach number 0.1. By computing the first 40 covariant Lyapunov vectors (CLVs), we find that unstable CLVs are active in the near-wake region, whereas stable CLVs are active in the far-wake region. This phenomenon is related to hyperbolicity since it shows that CLVs point to different directions; it also suggests that for open flows there is a large fraction of CLVs that are stable. However, due to the extra neutral CLV and the occasional tangencies between CLVs, our system is not uniform hyperbolic. By the non-intrusive least-squares shadowing (NILSS) algorithm, we compute shadowing directions and sensitivities of long-time-averaged objectives. Our results suggest that shadowing methods may be valid for general chaotic fluid problems.

The Efficiency of Tidal Fences: A Brief Review and Further Discussion on the Effect of Wake Mixing

2013 ◽  
Author(s):  
Takafumi Nishino ◽  
Richard H. J. Willden
Keyword(s):  
Energy Balance ◽  
Theoretical Investigation ◽  
Three Dimensional ◽  
Physical Factors ◽  
Balance Equations ◽  
Order Model ◽  
Near Wake ◽  
New Model ◽  
Actuator Disk ◽  
Limiting Efficiency

Recent discoveries on the limiting efficiency of tidal fences are reviewed, followed by a new theoretical investigation into the effect of wake mixing on the efficiency of ‘full’ tidal fences (i.e. turbines arrayed regularly across an entire channel span). The new model is based on the momentum and energy balance equations but includes several unclosed terms, which depend on the actual (three-dimensional) characteristics of turbine near-wake mixing and therefore need to be modelled empirically. The new model agrees well with three-dimensional actuator disk simulations when those unclosed terms are assessed based on the simulations themselves, suggesting that this low-order model could serve as a basis to analyse how various physical factors (such as the design of turbines) affect the limiting efficiency of tidal fences via changes in those terms describing the characteristics of turbine near-wake mixing. Also discussed is the effect of wake mixing on the efficiency of ‘partial’ tidal fences.

Flow Unsteadiness and Stability Characteristics of Low-Re Flow Past an Inclined Triangular Cylinder

2017 ◽  
Vol 139 (12) ◽  
Author(s):  
Wei Zhang ◽  
Hui Yang ◽  
Hua-Shu Dou ◽  
Zuchao Zhu
Keyword(s):  
Growth Rate ◽  
Sensitivity Analysis ◽  
Unsteady Flow ◽  
Maximum Velocity ◽  
Flow Patterns ◽  
Wake Flow ◽  
Near Wake ◽  
Flow Unsteadiness ◽  
Flow Past ◽  
Far Wake

The present study investigates the two-dimensional flow past an inclined triangular cylinder at Re = 100. Numerical simulation is performed to explore the effect of cylinder inclination on the aerodynamic quantities, unsteady flow patterns, time-averaged flow characteristics, and flow unsteadiness. We also provide the first global linear stability analysis and sensitivity analysis on the targeted physical problem for the potential application of flow control. The objective of this work is to quantitatively identify the effect of cylinder inclination on the characteristic quantities and unsteady flow patterns, with emphasis on the flow unsteadiness and instability. Numerical results reveal that the flow unsteadiness is generally more pronounced for the base-facing-like cylinders (α → 60 deg) where separation occurs at the front corners. The inclined cylinder reduces the velocity deficiency in the near-wake, and the reduction in far-wake is the most notable for the α = 30 deg cylinder. The transverse distributions of several quantities are shifted toward the negative y-direction, such as the maximum velocity deficiency and maximum/minimum velocity fluctuation. Finally, the global stability and sensitivity analysis show that the spatial structures of perturbed velocities are quite similar for α ≤ 30 deg and the temporal growth rate of perturbation is sensitive to the near-wake flow, while for α ≥ 40 deg there are remarkable transverse expansion and streamwise elongation of the perturbed velocities, and the growth rate is sensitive to the far-wake flow.

Transient growth in the near wake region of the flow past a finite span wing

2019 ◽  
Vol 866 ◽  
pp. 399-430 ◽  
Author(s):  
Navrose ◽  
V. Brion ◽  
L. Jacquin
Keyword(s):  
Periodic Structures ◽  
Time Integration ◽  
Tip Vortex ◽  
Wake Region ◽  
Wind Tunnel Experiments ◽  
Near Wake ◽  
Optimal Perturbation ◽  
Tip Vortices ◽  
Naca0012 Airfoil ◽  
Flow Past

We investigate optimal perturbation in the flow past a finite aspect ratio ($AR$) wing. The optimization is carried out in the regime where the fully developed flow is steady. Parametric study over time horizon ($T$), Reynolds number ($Re$), $AR$, angle of attack and geometry of the wing cross-section (flat plate and NACA0012 airfoil) shows that the general shape of linear optimal perturbation remains the same over the explored parameter space. Optimal perturbation is located near the surface of the wing in the form of chord-wise periodic structures whose strength decreases from the root towards the tip. Direct time integration of the disturbance equations, with and without nonlinear terms, is carried out with linear optimal perturbation as initial condition. In both cases, the optimal perturbation evolves as a downstream travelling wavepacket whose speed is nearly the same as that of the free stream. The energy of the wavepacket increases in the near wake region, and is found to remain nearly constant beyond the vortex roll-up distance in nonlinear simulations. The nonlinear wavepacket results in displacement of the tip vortex. In this situation, the motion of the tip vortex resembles that observed during vortex meandering/wandering in wind tunnel experiments. Results from computation carried out at higher $Re$ suggest that, even beyond the steady flow regime, a perturbation wavepacket originating near the wing might cause meandering of tip vortices.

scholarly journals Numerical simulations of flow past an autonomous underwater vehicle at various drift angles

2012 ◽  
Vol 9 (2) ◽  
pp. 135-152 ◽  
Author(s):  
Sreekar Gomatam ◽  
S Vengadesan ◽  
S K Bhattacharyya
Keyword(s):  
Autonomous Underwater Vehicle ◽  
Three Dimensional ◽  
Symmetry Plane ◽  
Underwater Vehicle ◽  
Navier Stokes ◽  
Structure Variation ◽  
Flow Past ◽  
Computational Fluid Dynamics Cfd ◽  
Marine Engineering ◽  
Flow Variables

Three dimensional (3D) flow past an Autonomous Underwater Vehicle (AUV) is simulated using a Computational Fluid Dynamics (CFD) approach at a Reynolds (Re) number of 2.09x106. A non-linear k-? (NLKE) turbulence model is used for solving the Reynolds Averaged Navier-Stokes (RANS) equations. The effect of control surfaces over the flow, the flow interaction between the hull and the appendages at various Angles of Attack (AoA) and the effect of the symmetry plane is studied. Flow structure, variation of flow variables and force distribution for various AoA are presented and discussed in detail.DOI: http://dx.doi.org/10.3329/jname.v9i2.12567 Journal of Naval Architecture and Marine Engineering 9(2012) 135-152

The effect of base bleed on the steady separated flow past bluff objects

1969 ◽  
Vol 39 (4) ◽  
pp. 735-752 ◽  
Author(s):  
L. G. Leal ◽  
A. Acrivos
Keyword(s):  
Bluff Body ◽  
Separated Flow ◽  
Pressure Profile ◽  
Reynolds Numbers ◽  
Two Dimensional ◽  
Wake Region ◽  
Near Wake ◽  
Flow Past ◽  
Modifying Effect ◽  
Base Bleed

The modifying effect of base bleed on the steady separated flow past a two-dimensional bluff body is considered. Detailed experimental results are presented for Reynolds numbers R between 50 and 250 and for bleed coefficients b in the range 0 to 0·15. The streamline pattern near the object is found to be strongly affected by small changes in the rate of bleed, with the recirculating closed wake disappearing altogether for b > 0·15. Nevertheless, the qualitative dependence on R of the physical dimensions of the near-wake region and the associated streamwise pressure profile appear to be unaffected by base bleed.

scholarly journals Computational Algorithms for Solving Spectral/$hp$ Stabilized Incompressible Flow Problems

2016 ◽  
Vol 8 (4) ◽  
pp. 21 ◽  
Author(s):  
Rakesh Ranjan ◽  
Anthony Theodore Chronopoulos ◽  
Yusheng Feng
Keyword(s):  
Stokes Equations ◽  
Three Dimensional ◽  
Higher Order ◽  
Navier Stokes ◽  
Krylov Methods ◽  
Higher Order Methods ◽  
The Past ◽  
Flow Past ◽  
Flow Past A Cylinder ◽  
Computationally Intensive

In this paper we implement the element-by-element preconditioner and inexact Newton-Krylov methods (developed in the past) for solving stabilized computational fluid dynamics (CFD) problems with spectral methods. Two different approaches are implemented for speeding up the process of solving both steady and unsteady incompressible Navier-Stokes equations. The first approach concerns the application of a scalable preconditioner namely the element by element LU preconditioner, while the second concerns the application of Newton-Krylov (NK) methods for solving non-linear problems. We obtain good agreement with benchmark results on standard CFD problems for various Reynolds numbers. We solve the Kovasznay flow and flow past a cylinder at Re-$100$ with this approach. We also utilize the Newton-Krylov algorithm to solve (in parallel) important model problems such as flow past a circular obstacle in a Newtonian flow field, three dimensional driven cavity, flow past a three dimensional cylinder with different immersion lengths. We explore the scalability and robustness of the formulations for both approaches and obtain very good speedup. Effective implementations of these procedures demonstrate for relatively coarse macro-meshes<br />the power of higher order methods in obtaining highly accurate results in CFD. While the procedures adopted in the paper have been explored in the past the novelty lies with applications with higher order methods which have been known to be computationally intensive.

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