55 research outputs found
A New Point of View on Skin-Friction Contributions in Adverse-Pressure-gradient Turbulent Boundary Layers
Skin-friction decompositions such as the so-called FIK identity (Fukagata et al., 2002) are useful tools in identifying relevant contributions to the friction, but may also lead to results difficult to interpret when the total friction is recovered from cancellation of multiple terms with large values. We propose a new formulation of the FIK contributions related to streamwise inhomogeneity, which is derived from the convective form of the momentum equation and using the concept of dynamic pressure. We examine turbulent boundary layers subjected to various pressure-gradient conditions, including cases with drag-reducing control. The new formulation distinguishes more precisely the roles of the free-stream pressure distribution, wall-normal convection, and turbulent fluctuations. Our results allow to identify different regimes in adverse-pressure-gradient turbulent boundary layers, corresponding to different proportions of the various contributions, and suggest a possible direction towards studying the onset of mean separation
Applying Bayesian Optimization with Gaussian Process Regression to Computational Fluid Dynamics Problems
Bayesian optimization (BO) based on Gaussian process regression (GPR) is
applied to different CFD (computational fluid dynamics) problems which can be
of practical relevance. The problems are i) shape optimization in a lid-driven
cavity to minimize or maximize the energy dissipation, ii) shape optimization
of the wall of a channel flow in order to obtain a desired pressure-gradient
distribution along the edge of the turbulent boundary layer formed on the other
wall, and finally, iii) optimization of the controlling parameters of a
spoiler-ice model to attain the aerodynamic characteristics of the airfoil with
an actual surface ice. The diversity of the optimization problems, independence
of the optimization approach from any adjoint information, the ease of
employing different CFD solvers in the optimization loop, and more importantly,
the relatively small number of the required flow simulations reveal the
flexibility, efficiency, and versatility of the BO-GPR approach in CFD
applications. It is shown that to ensure finding the global optimum of the
design parameters of the size up to 8, less than 90 executions of the CFD
solvers are needed. Furthermore, it is observed that the number of flow
simulations does not significantly increase with the number of design
parameters. The associated computational cost of these simulations can be
affordable for many optimization cases with practical relevance
Boundary induced non linearities at small Reynolds Numbers
We investigate the influence of boundary slip velocity in Newtonian fluids at
finite Reynolds numbers. Numerical simulations with Lattice Boltzmann method
(LBM) and Finite Differences method (FDM) are performed to quantify the effect
of heterogeneous boundary conditions on the integral and local properties of
the flow. Non linear effects are induced by the non homogeneity of the boundary
condition and change the symmetry properties of the flow inducing an overall
mean flow reduction. To explain the observed drag modification, reciprocal
relations for stationary ensembles are used, predicting a reduction of the mean
flow rate from the creeping flow to be proportional to the fourth power of the
friction Reynolds number. Both numerical schemes are then validated within the
theoretical predictions and reveal a pronounced numerical efficiency of the LBM
with respect to FDM.Comment: 29 pages, 10 figure
DNS study of a pipe flow following a step increase in flow rate
Direct numerical simulation (DNS) is conducted to study the transient flow in a pipe following a near-step increase of flow rate from an initial turbulent flow. The results are compared with those of the transient flow in a channel reported in He and Seddighi (2013). It is shown that the flow again exhibits a laminar–turbulent transition, similar to that in a channel. The behaviours of the flow in a pipe and a channel are the same in the near-wall region, but there are significant differences in the centre of the flow. The correlation between the critical Reynolds number and free stream turbulence previously established for a channel flow has been shown to be applicable to the pipe flow. The responses of turbulent viscosity, vorticity Reynolds number, and budget terms are analysed. Some significant differences have been found to exist between the developments of the vorticity Reynolds number in the pipe and channel flows
Destabilizing turbulence in pipe flow
Turbulence is the major cause of friction losses in transport processes and it is responsible for a drastic drag increase in flows over bounding surfaces. While much effort is invested into developing ways to control and reduce turbulence intensities, so far no methods exist to altogether eliminate turbulence if velocities are sufficiently large. We demonstrate for pipe flow that appropriate distortions to the velocity profile lead to a complete collapse of turbulence and subsequently friction losses are reduced by as much as 90%. Counterintuitively, the return to laminar motion is accomplished by initially increasing turbulence intensities or by transiently amplifying wall shear. Since neither the Reynolds number (Re) nor the shear stresses decrease (the latter often increase), these measures are not indicative of turbulence collapse. Instead an amplification mechanism, measuring the interaction between eddies and the mean shear is found to set a threshold below which turbulence is suppressed beyond recovery
Direct Numerical Simulation of Drag Reduction with Uniform Blowing over a Rough Wall
Direct numerical simulation of turbulent channel flow over a rough wall with uniform blowing (UB) for drag reduction has been performed to have our understanding of UB toward practical applications. It turns out that UB is applicable to rough materials as well as, or possibly better than, ideal smooth ones. The drag reduction rates achievable with UB are smaller in the rough case but the amount is larger. In addition to the conventional friction drag reduction mechanisms by UB, i.e., moving the turbulent structures outward, pressure drag, which does not appear on the planar wall, is suppresed as well due to the supression of stagnation
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