Numerical Simulation Of Turbulent Flow Research Articles

Direct numerical simulation of turbulent flows through concentric annulus with circumferential oscillation of inner wall

Periodic wall oscillations in the spanwise or circumferential direction can greatly reduce the friction drag in turbulent channel and pipe flows. In a concentric annulus, the constant rotation of the inner cylinder can intensify turbulence fluctuations and enhance skin friction due to centrifugal instabilities. In the present study, the effects of the periodic oscillation of the inner wall on turbulent flows through concentric annulus are investigated by the direct numerical simulation (DNS). The radius ratio of the inner to the outer cylinders is 0.1, and the Reynolds number is 2 225 based on the bulk mean velocity Um and the half annulus gap H. The influence of oscillation period is considered. It is found that for short-period oscillations, the Stokes layer formed by the circumferential wall movement can effectively inhibit the near-wall coherent motions and lead to skin friction reduction, while for long-period oscillations, the centrifugal instability has enough time to develop and generate new vortices, resulting in the enhancement of turbulence intensity and skin friction.

Numerical Study of Suction and Blowing Approaches to Control Flow over a Compressor Cascade in Turbulent Flow Regime

In the present study, a numerical simulation of turbulent flow over the NACA 65-100 compressor cascade was conducted to control the boundary layer using both suction and blowing separately. In order to study the effects of suction and blowing over the blade, 10 slots were considered on the upper surface of the blade. The slots were considered at intervals of 5% to 95% of the blade chord length. The results showed that applying both suction and blowing caused to delay the flow separation point and move it downstream which led to increase the lift coefficient and decrease the drag coefficient. Also, it was concluded that the effect of applying suction especially in the location where is near the trailing edge was significantly more than the effect of applying blowing. So that the lift coefficient had increased about 45% at 95% of chord length for suction compared to 5% of chord length and it had increased about 10% at 95% of chord length for blowing compared to 5% of chord length. Moreover, the best location to apply suction and blowing in a compressor cascade was near the flow separation point where is near the trailing edge.

Numerical simulation of turbulent flow forced convection in a twisted elliptical tube

The numerical simulations by turbulent water forced convection in a three-dimensional twisted elliptical tube with constant wall temperature are investigated in this study. Flow resistance and heat transfer characteristics of water in the twisted elliptical tube are studied with the parameters, including Reynolds number and the twist pitch. Effects of the above-mentioned parameters on the performance of the twisted elliptical tubes are analyzed and the overall thermal-hydraulic performance is evaluated. The thermal resistance of the heat exchanger is defined based on the entransy dissipation theory. The results show that in the twisted elliptical tubes, rotational motions are produced in the flowing fluid that improves the synergy between velocity vectors and the gradient of temperature and enhances the heat transfer performance compared with an oval tube. The twist elliptical tubes bring on the pressure drop because of the twisted wall. The results are then compared with the results of the oval tube, the pressure drop of the twisted elliptical tube with d = 96 mm increases 58%–60%. The averaged Nusselt number with d = 96 mm increases 16%–19% compared with the oval tube. In addition, the average Nusselt number and the pressure drop both increase with increasing Reynolds number, while both decrease with the increasing of the twist pitch. The twisted elliptical tube can reduce the entransy dissipation based thermal resistance that provides the great benefit in heat transfer. The twist pitch has an influence on the overall thermal-hydraulic performances. The results show that the twisted elliptical tube with d = 128 mm has the best overall thermal-hydraulic performance.

Time-parallel simulation of the decay of homogeneous turbulence using Parareal with spatial coarsening

Direct Numerical Simulation of turbulent flows is a computationally demanding problem that requires efficient parallel algorithms. We investigate the applicability of the time-parallel Parareal algorithm to an instructional case study related to the simulation of the decay of homogeneous isotropic turbulence in three dimensions. We combine a Parareal variant based on explicit time integrators and spatial coarsening with the space-parallel Hybrid Navier–Stokes solver. We analyse the performance of this space–time parallel solver with respect to speedup and quality of the solution. The results are compared with reference data obtained with a classical explicit integration, using an error analysis which relies on the energetic content of the solution. We show that a single Parareal iteration is able to reproduce with high fidelity the main statistical quantities characterizing the turbulent flow field.

Correction to: Numerical Simulation of Turbulent Flow and Heat Transfer in a Three-Dimensional Channel Coupled with Flow Through Porous Structures

Correction to: Numerical Simulation of Turbulent Flow and Heat Transfer in a Three-Dimensional Channel Coupled with Flow Through Porous Structures

Janus surface concept for three-dimensional turbulent flows

Three-dimensional numerical simulations of turbulent flow over a partial super-hydrophobic cylinder, hereinafter referred to as the “Janus cylinder”, are performed using large-eddy simulation (LES) with OpenFOAM solver. The slip-wall velocity at the super-hydrophobic surface is evaluated from a third-type (Robin) boundary condition whose coefficient is adjusted based on the available experimental data. Compared to no-slip smooth cylinder, the Janus cylinder has a lower drag coefficient and a lower root-mean-square (rms) lift coefficient, whereas its dominant vortex shedding frequency is higher. The power density distribution of the oscillations, obtained from the Fourier transform of the time-history of the lift coefficient, of the Janus cylinder has a less-sharp and broader distribution compared to the one of the smooth cylinder. No significant difference is observed between the wall shear stress distributions of a Janus cylinder, where super-hydrophobic surface covers only the frontal half of the cylinder, and a totally super-hydrophobic cylinder. This helps the potential manufacturers to reduce the manufacturing costs by partially processing the surface while achieving the highest possible drag reduction.

Direct numerical simulation of turbulent flow and heat transfer in a spatially developing turbulent boundary layer laden with particles

Direct numerical simulations of particle-laden flows in a spatially developing turbulent thermal boundary layer over an isothermally heated wall have been performed with realistic fully developed turbulent inflow boundary conditions. To the authors’ best knowledge, this is the first time the effects of inertial solid particles on turbulent flow and heat transfer in a flat-plate turbulent boundary layer have been investigated, using a two-way coupled Eulerian–Lagrangian method. Results indicate that the presence of particles increases the mean streamwise velocity and temperature gradients of the fluid in the near-wall region. As a result, the skin-friction drag and heat transfer are significantly enhanced in the particle-laden flows with respect to the single-phase flow. The near-wall sweep and ejection motions are suppressed by the particles and hence the Reynolds shear stress and wall-normal turbulent heat flux are attenuated, which leads to reductions in the production of the turbulent kinetic energy and temperature fluctuations. In addition, the coherence and spacing of the near-wall velocity and temperature streaky structures are distinctly increased, while the turbulent vortical structures appear to be disorganized under the effect of the particles. Moreover, the intensity of the streamwise vortices decreases monotonically with increasing particle inertia.

Numerical simulations of turbulent flows within an infinite array of randomly placed cylinders

We address the task of modelling numerically turbulent flows within random arrays of circular cylinders, relevant for several industrial and environmental applications. Numerical simulations are employed to model infinite domains of randomly placed emergent and rigid cylinders validated by a laboratory database acquired by a PIV system. The main goals are: (i) to discuss the effect of the numerical domain size and the grid resolution on the first and second order moments; and (ii) to characterise the spatial distribution of mean flow and turbulence variables in the drag-wake controlled stratum. Five domains of different sizes (9–44 cylinders) and four grid resolutions were independently tested. The results show that the time-averaged velocity field and the Reynolds stress tensor were not significantly affected by the size of the tested numerical domains. The analysis of the grid resolution influence shows how the results improve with mesh refinement, while none of the tested meshes produces un-physical results. The present work provides guidance on the acceptable compromises, in terms of mesh resolution and domain size, when predicting, with eddy resolving computational fluid mechanics tools, first and second-order moments of turbulent flows within infinite arrays or randomly placed cylinders.

Effect of texture randomization on the slip and interfacial robustness in turbulent flows over superhydrophobic surfaces

Superhydrophobic surfaces demonstrate promising potential for skin friction reduction in naval and hydrodynamic applications. Recent developments of superhydrophobic surfaces aiming for scalable applications use random distribution of roughness, such as spray coating and etched process. However, most of previous analyses of the interaction between flows and superhydrophobic surfaces studied periodic geometries that are economically feasible only in lab-scale experiments. We conduct direct numerical simulations of turbulent flows over randomly patterned interfaces considering a range of texture widths $w^+\approx 4-26$, and solid fractions $\phi_s=11\%$ to $25\%$. Slip and no-slip boundary conditions are implemented in a pattern, modeling the presence of gas-liquid interfaces and solid elements. Our results indicate that slip of randomly distributed textures under turbulent flows are about $30\%$ less than those of surfaces with aligned features of the same size. In the small texture size limit $w^+\approx 4$, the slip length of the randomly distributed textures in turbulent flows is well described by a previously introduced Stokes flow solution of randomly distributed shear-free holes. By comparing DNS results for patterned slip and no-slip boundary against the corresponding homogenized slip length boundary conditions, we show that turbulent flows over randomly distributed posts can be represented by an isotropic slip length in streamwise and spanwise direction. The average pressure fluctuation on gas pocket is similar to that of the aligned features with the same texture size and gas fraction, but the maximum interface deformation at the leading edge of the roughness element is about twice larger when the textures are randomly distributed.

Causal analysis of self-sustaining processes in the logarithmic layer of wall-bounded turbulence

Despite the large amount of information provided by direct numerical simulations of turbulent flows, their underlying dynamics remain elusive even in the most simple and canonical configurations. Most common approaches to investigate the turbulence phenomena do not provide a clear causal inference between events, which is essential to determine the dynamics of self-sustaining processes. In the present work, we examine the causal interactions between streaks, rolls and mean shear in the logarithmic layer of a minimal turbulent channel flow. Causality between structures is assessed in a non-intrusive manner by transfer entropy, i.e., how much the uncertainty of one structure is reduced by knowing the past states of the others. We choose to represent streaks by the first Fourier modes of the streamwise velocity, while rolls are defined by the wall-normal and spanwise velocity modes. The results show that the process is mainly unidirectional rather than cyclic, and that the log-layer motions are sustained by extracting energy from the mean shear which controls the dynamics and time-scales. The well-known lift-up effect is also identified, but shown to be of secondary importance in the causal network between shear, streaks and rolls.

Direct numerical simulation of high aspect ratio spanwise-aligned bars

We conduct minimal-channel direct numerical simulations of turbulent flow over two-dimensional rectangular bars aligned in the spanwise direction. This roughness has often been described as $d$-type, as the roughness function $\unicode[STIX]{x0394}U^{+}$ is thought to depend only on the outer-layer length scale (pipe diameter, channel half-height or boundary layer thickness). This is in contrast to conventional engineering rough surfaces, named $k$-type, for which $\unicode[STIX]{x0394}U^{+}$ depends on the roughness height, $k$. The minimal-span rough-wall channel is used to circumvent the high cost of simulating high Reynolds number flows, enabling a range of bars with varying aspect ratios to be investigated. The present results show that increasing the trough-to-crest height, $k$, of the roughness while keeping the width between roughness bars, ${\mathcal{W}}$, fixed in viscous units, results in non-$k$-type behaviour although this does not necessarily indicate $d$-type behaviour. Instead, for deep surfaces with $k/{\mathcal{W}}\gtrsim 3$, the roughness function appears to depend only on ${\mathcal{W}}$ in viscous units. In these situations, the flow no longer has any information about how deep the roughness is and instead can only ‘see’ the width of the fluid gap between the bars.

Numerical simulation of turbulent flow around the dtmb4119 propeller in open water conditions

In this study, ANSYS-FLUENT packages are employed to simulate the turbulent flow around DTMB4119 propeller in open water conditions. In order to form a mesh, the multiple reference frame (MRF) methodology is used. The results are compared with the experimental results and a good conformity is obtained, which endorses numerical simulation. Furthermore, the turbulence model is used, which is superior to other turbulence models in modeling marine propellers. The investigation focuses on aspects related to the influence of the pressure coefficient and the advance coefficient on hydrodynamic performance and cavitation of the propeller. The results reveal that the pressure coefficient at first decreases and then augments as it moves from the leading edge to trailing edge in the suction surface. Moreover, by increasing the blade radius and its speed, the minimum pressure increases in a way that pressure coefficient reaches its minimum value. Furthermore, volume fraction of the vapor over the blades decreases as the advance coefficient increases. As a result, the possibility of cavitation decreases.

Numerical simulation of turbulent flow and heat transfer though sinusoidal ducts

Turbulent forced convection heat transfer in corrugated plate surfaces was studied by means of CFD. Flow through corrugated plates, which are sets of sinusoidal ducts, was analyzed for different inlet flow angles (0° to 50°), aspect ratios (0.1 to 10), Reynolds numbers (2000 to 40,000) and Prantdel numbers (0.7 to 5). Heat transfer is affected significantly by variation of aspect ratio. A maximum heat transfer coefficient is observed at a particular aspect ratio although the aspect ratio has a minor effect on friction factor. Enlarging inlet flow angle also leads to a higher heat transfer coefficient and pressure loss in aspect ratios close to unity. Dependency of Nusselt and friction factor on the angle and aspect ratio was interpreted by means of appearance of secondary motions and coexistence of laminar and turbulent flow in a cross section. Comparing the results with experimental data shows a maximum 12.8% difference. By evaluating the results, some correlations were proposed to calculate Nusselt number and friction factor for entrance and fully developed regions. A corrugated plate with an aspect ratio equal to 1.125 and an inlet flow angle equal to 50° gives the best heat transfer and pressure drop characteristics.

The sensitivity of ship resistance to wall-adjacent grids and near-wall treatments

Numerical simulations of turbulent flows around KCS have been performed to study the sensitivity of ship resistance to wall-adjacent grids and disclose the influence of near-wall treatment on the sensitivity of ship resistance. The resistance coefficients of viscous and pressure forces were compared when using realizable k-ε and SST k-ω turbulence models in structured and unstructured grids, respectively. The calculation of friction velocity was found to be mainly responsible for the reduction of viscous and total resistances when the height of wall-adjacent cells increased. Since the assumption of equilibrium state between turbulent production and dissipation was not met in a bulbous bow, it was more reasonable to iteratively calculate the friction velocity from empirical laws of the wall for near-wall treatment rather than explicitly estimate it from the turbulent kinetic energy.

Numerical Simulation of Turbulent Flow and Heat Transfer in a Three-Dimensional Channel Coupled with Flow Through Porous Structures

This study investigates numerically the turbulent flow and heat transfer characteristics of a T-junction mixing, where a porous media flow is vertically discharged in a 3D fully developed channel flow. The fluid equations for the porous medium are solved in a pore structure level using an Speziale, Sarkar and Gatski turbulence model and validated with open literature data. Overall, two types of porous structures, consisted of square pores, are investigated over a wide range of Reynolds numbers: an in-line and a staggered pore structure arrangement. The flow patterns, including the reattachment length in the channel, the velocity field inside the porous medium as well as the fluctuation velocity at the interface, are found to be strongly affected by the velocity ratio between the transversely interacting flow streams. In addition, the heat transfer examination of the flow domain reveals that the temperature distribution in the porous structure is more uniform for the staggered array. The local heat transfer distributions inside the porous structure are also studied, and the general heat transfer rates are correlated in terms of area-averaged Nusselt number accounting for the effects of Reynolds number, velocity ratio as well as the geometrical arrangement of the porous structures.

On the Stability and Accuracy of the BGK, MRT and RLB Boltzmann Schemes for the Simulation of Turbulent Flows

This paper presents an analysis of the stability and accuracy of different Lattice Boltzmann schemes when employed for direct numerical simulations of turbulent flows. The Single-Relaxation-Time scheme of Bhatnagar, Gross and Krook (BGK), the Multi-Relaxation-Time scheme (MRT) and the Regularized Lattice Boltzmann scheme (RLB) are considered. The stability and accuracy properties of these schemes are investigated by computing three-dimensional Taylor-Green vortices representing homogeneous isotropic turbulent flows. Varying Reynolds numbers and grid resolutions were considered. As expected, the BGK scheme requires sufficiently high grid resolutions for stable and accurate simulations. Surprisingly, the MRT scheme when used without any turbulencemodel fails to obtain mesh convergence for the type of flow considered here. The RLB scheme allows for stable simulations but exhibits a strong dissipative behavior. A similar behaviorwas foundwhen employing thementioned LBMschemes for numerical simulations of turbulent channel flows at varying Reynolds numbers and resolutions. The obtained insights on accuracy and stability of the considered Lattice Boltzmann methods can become useful especially for the design of effective turbulence models to be used for high Reynolds number flows.

Vortex-Resistance Hypothesis: Large Eddy Simulation of Turbulent Flow in Isolated Pool- Riffle Units

By numerical simulations of turbulent flow in isolated poolriffle units with various riffle heights, four different types of vortices were found and named as follows: surface rollers (SR), corner rollers (CR), ramp rollers (CR), and axial tails (AT). Surface rollers are shaped on the flow surface due to submerged hydraulic jump or any obstacles in the forced poolriffle units. Corner rollers are shaped close to the corners near the walls at the pool head. Ramp rollers are formed at the bed of the channel on the ramp into the head of the pool. All kinds of vortices stretch in the streamwise direction as they travel to the downstream, which they are called axial tails. The simulations showed that all four types of vortices interact with each other, combine, amplify or cancel out each other as they travel downstream. The strength of vortices and how they interact result into different types of flow patterns. The surface rollers combine with corner rollers to make a jet like plunging flow near the pool bed. In other cases with lower riffle heights, ramp rollers tend to push the flow up, which in turn leads to higher turbulence near the bed and higher velocity near the flow surface (skimming flow). Moreover, if both surface rollers and ramp rollers have the similar strength (e.g., vorticity) and scale, the streamwise velocity profile has a peak around the middle of the flow, and minimum velocities near the bed and free surface. This flow pattern was named as “rifting flow.” Based on these findings, a new hypothesis is proposed called ‘vortex-resistance,’ which states that the turbulent structures, by increasing the eddy viscosity and changing the pressure domain, act as an obstacle that steers the flow. Plunging and skimming flow can thus be understood as the products of different types of turbulent structures. These findings provide new clarifications to long-standing questions related to the hydraulics of pools and riffles.

Large-Scale Streamwise Vortices in Turbulent Channel Flow Induced by Active Wall Actuations

Direct numerical simulations of turbulent flow in a plane channel using spanwise alternatively distributed strips (SADS) are performed to investigate the characteristics of large-scale streamwise vortices (LSSVs) induced by small-scale active wall actuations, and their role in suppressing flow separation. SADS control is obtained by alternatively applying out-of-phase control (OPC) and in-phase control (IPC) to the wall-normal velocity component of the lower channel wall, in the spanwise direction. Besides the non-controlled channel flow simulated as a reference, four controlled cases with 1, 2, 3 and 4 pairs of OPC/IPC strips are studied at M = 0.2 and R e = 6,000, based on the bulk velocity and the channel half height. The case with 2 pairs of strips, whose width is Δz + = 264 based on the friction velocity of the non-controlled case, is the most effective in terms of generating large-scale motions. It is also found that the OPC (resp. IPC) strips suppress (resp. enhance) the coherent structures and that leads to the creation of a vertical shear layer, which is responsible for the LSSVs presence. They are in a statistically steady state and their cores are located between two neighbouring OPC and IPC strips. These motions contribute significantly to the momentum transport in the wall-normal and spanwise directions showing potential for flow separation suppression.

Numerical simulation of turbulent flow and heat transfer in tube under injection of gas through permeable walls

Using a differential turbulence model, a numerical simulation of turbulent flow in a tube is carried out and the dependences of the flow and heat transfer characteristics on the intensity of the coolant injection through permeable walls are obtained. It is shown that the coefficients of friction and heat transfer decrease with increasing of injection rate. The comparison of calculation results reflecting the main features of rather complicated processes of flow in tube under gas injection with well-known experimental data over a wide range of the Reynolds number and the intensity of injection show their satisfactory agreement.

Distribution characteristics of inertial sediment particles in the turbulent boundary layer of an open channel flow determined using Voronoï analysis

This paper presents the numerical investigation of the distribution of inertial sediment particles in the turbulent boundary layer of an open channel flow with the particle Stokes number ranging from 0.6 to 20.4. The methodology is a combination of three numerical approaches, i.e. direct numerical simulation of turbulent flow, the point-particle immersed boundary method, and the discrete particle method. By applying the Voronoï analysis, the preferential concentration characteristics of sediment particles were investigated quantitatively. It was found that the normalized area of the Voronoï cells follows a log-normal particle distribution. The inertial sediment particles distributed unevenly in the turbulent boundary layer and the unevenness, governed by the particle Stokes number, was more significant as the particle Stokes number approaches unity. The inertial sediment particles in the turbulent boundary layer accumulated preferentially in streamwise-aligned streaky structures and this pattern was less significant with increasing particle Stokes number.