Turbulent Inlet Conditions Research Articles

Tailoring anisotropic synthetic inflow turbulence generator for wind turbine wake simulations

In computational fluid dynamics, defining precise boundary conditions, especially at inlets, is of great importance. Inlet flows typically exhibit natural turbulence, which is managed in various ways in scale-resolving simulations. Methods to establish turbulent inlet conditions are commonly created using natural transition, uncorrelated oscillations, periodic boundary conditions from auxiliary simulations, or synthetic turbulent fields. In this study, we explore a technique aimed at generating a divergence-free synthetic inflow turbulence with arbitrary anisotropy. The methodology is based on the conventional portrayal of turbulence as consisting of several coherent structures. While our approach adeptly emulates predefined statistical characteristics across different scales, its primary focus is on generating input parameters that impact the airflow within the wake of individual wind turbines and the atmospheric boundary layer within a wind farm. The results are compared with high-resolution velocity experimental measurements, large eddy simulations, and the digital filter-based inlet boundary condition already available in OpenFOAM. The findings demonstrate that the applied inflow generator outperforms the default OpenFOAM filter, particularly in the context of a single wind turbine.

Experimental investigation of the variation of the local gas velocities in a cold flow pebble bed reactor (PBR) using a hot wire anemometry technique

Obtaining accurate results and new benchmark data for the local velocity of the gas flowing within the pebble bed reactor (PBR) is a key step for understanding and benchmarking simulations of the thermal hydraulics in the PBR core as it significantly affects the design and safety operation of these reactors. Therefore, this work focused on studying the local gas velocities inside a pebble bed using a sophisticated hot wire anemometry (HWA) technique, which was supported with a novel probe-protector case that protected the probe, allowing the measurements to be conducted at various locations in a pebble bed with pebble diameter of 5cm and an aspect ratio of 6. The measurements were conducted at various superficial inlet gas velocities (0.3≤Ug≤2.4m/s), covering both transitional and turbulent flow conditions (993.78≤Re≤7950.24) at three axial levels and four radial locations (r/R=0, 0.33, 0.67, 0.9). The results highlighted the effect of the wall on the variation of the local gas velocities inside the pebble bed, as higher gas velocities were recorded at the near-wall region, where the void fractions are higher, providing a path of the least resistance to the flow of the gas, compared to the center of the bed. Furthermore, a second order polynomial correlation with an R2=93.96% and an AARE equal to 1.47% was developed for the prediction of the local gas velocity inside the bed, within the experimental range of the study. The accurate gas velocity measurements obtained in this study can serve as benchmark data for the validation of CFD simulations coupled with heat transfer calculations.

Numerical investigation of local scour around tandem piers using proposed self-sustaining inlet turbulent boundary conditions

Turbulent characteristics significantly affect the numerical results of local scour in tandem piers. The novel self-sustaining model was used to quickly develop homogeneous isotropic turbulence to maintain the consistency of the numerical and physical flow fields. The Reynolds averaged N-S equations, which are close to the standard k−ε turbulence model, were employed to solve the complex flow field around the bridge piers. The sediment transport model, combined with dynamic mesh updating methods, simulated the evolution process of the local scour for tandem piers. The results indicated that the maximum scour depth occurred at the upstream pier at different distances between the centres of the two piers (L/D). The scour depth reached its largest value when L/D = 4, and the maximum scour depth reached its minimum value when L/D = 2. When L/D > 5, the shielding effect can be neglected for downstream piers. In addition, based on the numerical simulation results, a shielding coefficient was established, providing a method for quickly predicting the local scour depth based on the scour depth of the isolated pier and a reference for the scour resistance design of bridges, offshore fans, and other structural foundations.

The Influence of Turbulence and Reynolds Number on Endwall Heat Transfer in a Vane Cascade

Abstract Endwall heat transfer measurements have been acquired in a vane cascade over a range of turbulence conditions and Reynolds numbers using an array of small commercial infrared (IR) cameras. The linear cascade was tested over five inlet turbulence conditions ranging from low turbulence (0.7%) to high turbulence (17.4%) and three exit chord Reynolds numbers ranging from 500,000 to 2,000,000. The small commercial IR cameras made by Therm-App have a resolution of 384 by 288 pixels and were connected to individual smartphones to record the images. The cascade was modified with small zinc selenide windows to provide IR access for the cameras. The five cameras were calibrated against a constant temperature test plate and the output images were adjusted for the fisheye effect and thermal droop at the edges. The large-scale low-speed cascade, used in the endwall heat transfer study, was configured in a four-vane three full passage arrangement. The vane design includes a large leading and aft loading. This same cascade has been used in the acquisition of vane surface heat transfer distributions, vane suction surface heat transfer visualizations, and vane surface film cooling distributions. This paper includes comparisons with two large eddy simulation calculations, which were conducted prior to the acquisition of the heat transfer data. The influence of the secondary flows on the endwall heat transfer distributions, including the leading edge horseshoe vortex system, is particularly visible at lower turbulence levels and lower Reynolds numbers. However, at higher turbulence levels, the influence of secondary flows is less visible but the influence of Reynolds number and turbulence on transition can be discerned.

Influence of inlet turbulent condition on the formation mechanism of local scalar concentrations

AbstractIn the existing ventilation design, the ventilation rate is defined by the time‐averaged flow rate, and the fluctuating (turbulence) component is not typically considered. However, inlet turbulent conditions are also assumed to have some influence on the formation of contaminant distributions. Therefore, the influence of turbulent kinetic energy at the inlet boundary on scalar transportation in an indoor environment needs to be elucidated when discussing the ventilation rate setpoint via the supply inlet in terms of local contaminant concentration control. This study discusses the impact of turbulent kinetic energy in the ventilation design on scalar transfer and its distribution within an enclosed space. To understand the influence of various inlet turbulent boundary conditions on scalar transfer, a computational fluid dynamics analysis was conducted using two different room models: a simple room and a room with a ventilation system that creates a large velocity gradient. The results indicate that scalar transfer within the room is not solely dominated by the averaged velocity input at the inlet boundary but is also strongly affected by the turbulence conditions at the inlet boundary. The numerical results indicate the possibility of a new ventilation design strategy that simultaneously considers the transfer of turbulent components and contaminants.

PIV measurement of turbulent flow characteristics inside an open-cell metal foam replica

Metal foam structures have been of interest because their geometrical characteristics are theoretically desirable to many applications such as heat exchanger, chemical reactor, filter, etc. Better understanding of flow characteristics inside metal foam is essential to utilize metal foam in thermo-fluid systems. A test section with a 3-D printed 8 times scaled-up transparent metal foam replica has been prepared. Time-resolved 2-D particle image velocimetry was used to measure velocity fields in the center planes at upstream, inside, and downstream of the replica model. The investigation was conducted for 3 different Reynolds numbers, 1000, 5,000 and 10,000 based on the channel hydraulic diameter. The turbulent flow characteristics inside an open-cell metal foam was compared between laminar and turbulent inlet flow conditions. Upstream turbulence intensity was 10% in case of turbulent inlet condition. For the ensemble averaged fields, clear similarities are found in mean velocity distributions and turbulent stresses inside the metal foam regardless of Reynolds number. The velocity magnitude inside metal foam increased up to 1.6 times of the channel bulk velocity for all Reynolds number. The temporal fluctuation of streamwise velocity component is increased up to 1.4 times to channel bulk velocity inside the metal foam replica. This increase was similar to that of laminar inlet condition even there is no fluctuations in the upstream. Normalized vorticity and turbulent kinetic energy also showed the similarity in three different Reynolds numbers, but the magnitude is higher at lower Reynolds number. Temporal analysis was also conducted at several points inside metal foam. The integral length scale is similar to 0.2 size of ligament in the case of Re = 10,000. Energy spectrum shows that the spectral slope is -5/3 in inertial subrange which confirm that flow inside metal foam is turbulent even the inlet flow is laminar.

Unsteady Simulation of a Transonic Turbine Stage with Focus on Turbulence Prediction

The flow in a transonic turbine stage still poses a high challenge for the correct prediction of turbulence using an eddy viscosity model. Therefore, an unsteady RANS simulation with the k-ω SST model, based on a preceding study of turbulence inlet conditions, was performed to see if this can improve the quality of the flow and turbulence prediction of an experimentally investigated turbine flow. Unsteady Q3D results showed that none of the different turbulence boundary conditions could predict the free-stream turbulence level and the maximum values correctly. Luckily, the influence of the boundary conditions on the velocity field proved to be small. The qualitative prediction of the complex secondary flows is good, but there is lacking agreement in the prediction of turbulence generation and destruction.

PIV measurement of complex flow characteristics in open-cell metal foam replica

Time-resolved 2-D particle image velocimetry was used to study on turbulent flow characteristics inside an open-cell metal foam under the laminar and turbulent inlet conditions. A study on the effect of Reynolds number was conducted with different three channel Reynolds numbers, 1000, 5000 and 10000. Uniform upstream flow is divided by the pore network of metal foam and it is found that there are flow disturbances induced by metal foam structure even at a laminar inlet condition. It is confirmed that there is a similarity of the preferred flow path flows take regardless of Reynolds number.

Large Eddy Simulation of the By-pass Transition Process under Different Inlet Turbulence Conditions

The transition process of the boundary layer developing over a flat plate with elevated inlet Free Stream Turbulence Intensity (FSTI) has been studied by means of Large Eddy Simulation (LES). To this purpose, four cases with different inflow disturbances have been tested varying the magnitude and the length scale of turbulence. LES has been performed by using the finite-volume ANSYS Fluent code. The computational domain, which was constituted by a rectangular domain with a zero thickness plate, was based on an ERCOFTAC test case in order to provide a validation with a well-known set of data by comparing the boundary layer integral parameters and mean and fluctuating streamwise velocity profiles.The four cases were discussed within the paper by looking at classical statistical properties as well as advanced post-processing tools. It was shown that the decrease in the free stream turbulence level postpones the transition location, whereas the variation of the integral length scale has a very low influence on the distribution of the time-mean flow properties. Proper Orthogonal Decomposition (POD) has been applied to the instantaneous LES flow fields in order to provide a statistical representation of the structures responsible for transition and their response to free-stream turbulence intensity and length scale. The presence of vortical filaments parallel to the wall, typically referred as boundary layer streaks, is clearly identified; their characteristic dimensions and how they change as a function of FSTI properties were analyzed within the paper.

Numerical simulation of local scour around bridge piers using novel inlet turbulent boundary conditions

The numerical results of local scour for bridge piers are significantly affected by the inlet turbulent boundary conditions. A novel self-sustaining model (SSM) and corresponding the new inlet turbulent boundary conditions are proposed to quickly obtain a stable flow field of horizontal homogeneity. The Reynolds averaged Navier-Stokes (RANS) equations, and standard k-ԑ turbulence model are employed to solve the complex flow around the bridge piers. Based on the sediment transport model considering the effect of the arbitrary slope of riverbed and the collapse of sediment, the dynamic evolution process of local scouring around bridge piers is simulated. Then, the applicability of the numerical approach for local scour around bridge pier is validated by the flume test comparing with the flow pattern, the maximum scour depth and the morphology of scour pit. Finally, the influence factors of local scour for bridge piers including common pier shapes and different angle of attack are studied. The numerical results indicate that the proposed SSM can develop homogeneous isotropic turbulence to fine simulate local scour with prescribed inlet turbulent boundary conditions. The established sediment collapse model can reasonably reflect the collapse process in scouring. Importantly, the proposed numerical methods can provide a reasonable prediction of maximum scour depth. In addition, the performances of the local scouring are greatly sensitive to the pier shape and the angle of attack. The inflow parallelling to piers or choosing appropriate pier shape helpful to ease the scour.

Effect of Inlet Turbulent Boundary Conditions on Scour Predictions of Coupled LES and Morphodynamics in a Field-Scale River: Bankfull Flow Conditions

AbstractThis paper presents a systematic numerical investigation to study the effects of inlet turbulent boundary conditions on the coupled hydrodynamics and morphodynamics computations of a natura...

Effect of turbulent inlet conditions on the prediction of flow field and hemolysis in the FDA ideal medical device

Hemolysis is one of the most important issues of blood contacting artificial organs. Computational fluid dynamics, in conjunction with hemolysis models, has been widely used to predict the flow field and hemolysis during the development phase of these devices. It is widely accepted that hemolysis is related to flow field parameters, such as stress and energy dissipation. It is known that inlet boundary conditions such as turbulent intensity have important effects on flow fields. Nonetheless, the influence of inlet turbulent intensity on hemolysis is not yet clear. This study investigates the influence of turbulent intensity on the prediction of flow field and hemolysis in the FDA benchmark nozzle model. Three configurations are investigated: first is the original nozzle model (with a gradual contraction, throat, and sudden expansion); the second is composed only of the throat and sudden expansion; the third one is similar with the second, but with a shorter throat. Four turbulent intensities are considered, ranging from 1% to 20%. For the first configuration, the influence of turbulent intensity on both flow field and hemolysis is very small and limited prior to the gradual contraction, while for the other two configurations, the influence is considerable. The jet breaks down sooner with increased turbulent intensity. Both turbulence dissipation rate and Reynolds stress increase in the throat with increased turbulent intensity, but decrease after the jet breaks down. The influence of turbulent intensity is more considerable for the configuration with a shorter inlet. The influence of turbulent intensity on the prediction of hemolysis is up to 38.5%. This study shows that turbulent intensity can have considerable influence on the prediction of flow field and hemolysis in blood-contacting devices, thus should be taken in account when conducting blood compatibility design.

Investigation of flow and heat transfer instabilities and oscillation inhibition of n-decane at supercritical pressure in vertical pipe

The characteristics of the flow and heat transfer instabilities of n-decane at supercritical pressures were evaluated under different inlet temperature, pressure, mass flow, and flow direction conditions. The experiment was carried out in a vertical heating tube with an inner diameter of 2 mm, at a maximum heat flux density of 200 kW/m2, under both laminar and turbulent inlet conditions. The results revealed that an increase in the inlet temperature, pressure, or mass flow rate can weaken the flow and heat transfer oscillation. Two oscillation stages of were observed, one is the transition oscillation that occurs when the fluid flows from laminar to turbulent, the other is the Helmholtz oscillation due to the uneven distribution of the radial density and the density distribution in the flow direction. Furthermore, a novel oscillation inhibition method was proposed, which involves the insertion of a twisted stainless-steel wire from certain parts of the heating tube. The influence of the insertion methods on the heat transfer enhancement and oscillation inhibition was evaluated using a comprehensive heat transfer coefficient. The results revealed that the insertion of a twisted wire from the tube inlet was more effective for the inhibition of the oscillation than that from the tube outlet. The insertion of a twisted wire into the entire length of the tube resulted in the best comprehensive performance of heat transfer and flow resistance when compared with those of other insertion methods.

A study of two-phase annular flow using unsteady numerical computations

Flow behavior of thin liquid films in an annular flow regime is an important element of the thermal hydraulics of a BWR – it controls the heat transfer from the fuel rods to the coolant. Despite its relevance, the flow behavior of dynamic liquid films is not well understood even in adiabatic conditions. To investigate it, unsteady numerical simulations with an interface tracking model were performed in a double subchannel geometry with a P/D ratio of 1.325. Different turbulence models, namely the large eddy simulation (LES) and linear/nonlinear eddy viscosity unsteady-RANS (URANS) models, were tested. A novel approach for generating turbulent inlet conditions in a periodic flow domain was developed to reduce computational efforts. Validation against experimental data revealed shortcomings of the linear eddy-viscosity RANS model in predicting key flow parameters. By capturing the effects of the secondary flow structures in a subchannel geometry, improved predictions were obtained with a non-linear SST k–ω (QCR) turbulence model. Time-averaged liquid film thickness (LFT) and tracer distribution obtained with LES were found to have the best agreement with experimental data. Instantaneous and time-averaged velocity profiles were analyzed to understand the influence of the gas-liquid interface. Secondary flow structures in the subchannel gap region were found to enhance the turbulent mixing of the passive scalar in the liquid film. This finding is relevant towards the prediction of thermal-hydraulic parameters in a multichannel flow assembly by accounting for inter-channel mixing phenomena.

CFD modelling of pump-around jet mixing tanks: a discrepancy in concentration profiles

ABSTRACTJet mixing tanks are important in chemical processes. Over the past two decades or so, computational fluid dynamics (CFD) has been employed to study jet mixers. The shortfalls of the previous CFD models were the discrepancy in concentration profiles between simulation and experiment and the absence of exact inlet turbulence conditions. So, in our present work, the CFD model was developed to investigate the proper conditions for jet mixing tank simulation and improve the accuracy of concentration profile prediction by using an appropriate grid arrangement, a realizable k-epsilon model, and a second-order upwind discretization scheme. The results revealed that the CFD model with proper inlet conditions predicted the overall mixing time well and somewhat improved the predicted concentration profiles. Further, the reasons for discrepancies in concentration profiles were inappropriate inlet turbulence conditions and overprediction in total momentum available for mixing due to the flat top liquid surface assumption. In addition, this discrepancy may be caused by the dynamic response of concentration measuring device.

Large eddy simulation of the influence of synthetic inlet turbulence on a practical aeroengine combustor with counter-rotating swirler

To study the influence of inlet turbulence on the prediction of flow structure in practical aeroengine combustor, large eddy simulation with dynamic Smagorinsky subgrid model is used to explore the complex unsteady flow field in a single burner of a typical aeroengine combustor with two-stage counter-rotating swirler. The complex geometric configuration including all film cooling holes is fully simulated without any conventional simplification in order to reduce the modeling errors. First, unsteady process that flow developing from static to statistically stationary state is fully simulated under laminar inlet condition to obtain a fundamental understanding of flow characteristics in the combustor. Afterwards, synthetic eddy method is utilized to generate a turbulent inlet condition so that a perturbation with about 5% turbulence intensity is superimposed to the inlet plane. Simulation result shows that for the laminar inflow case, flow separation occurs in the near-wall region of the diffusion section, inducing a boundary layer transition and consequently introducing turbulence with nonuniformity in space before the swirler. In contrast, synthesized inflow generated under turbulent inlet condition by synthetic eddy method is more spatially homogeneous. Time-averaged flow field inside the swirler cup reveals that turbulent inflow ultimately causes the swirling flow with higher rotating speed in central region and more uniform distribution along the circumferential direction. It also enhances the transverse jet flow from primary holes and reverse flow in the central recirculation zone, and makes streamlines corresponding to the recirculation vortices more symmetrical on central profile. Maximum recirculating velocity predicted in central recirculation zone is −27.65 m/s and −17.86 m/s in turbulent and laminar case respectively, and corresponding total pressure recovery coefficient is 96.03% and 96.81%.

Numerical study of oscillatory flow instability in upward flow of supercritical water in two heated parallel channels

A 3-D numerical study of turbulent flow of supercritical water flowing upward in two heated parallel channels with constant applied wall heat flux was developed using a RANS model in the Computational Fluid Dynamics (CFD) code ANSYS CFX. Oscillatory flow instabilities were investigated using the standard k-ε turbulence model with scalable wall functions. The effects of changes to the grid sizes and the time step size on flow instabilities were studied first. Then the instability thresholds of experimental cases were obtained with the CFX code. For comparison purposes, Chatoorgoon’s 1-D non-linear SPORTS code was also used to determine the instability boundary. These two new numerical results were compared with the experimental data and previous numerical results by other investigators. Additionally, the effects on the instability thresholds of changing the outlet plenum volume, the turbulent Prandtl number, the turbulence inlet conditions, the channel outlet K factor, the maximum iterations per time step in the transient analysis, and the order of the transient scheme were examined.

Numerical simulations of wake characteristics of a horizontal axis tidal stream turbine using actuator line model

The wake of a laboratory scale tidal stream turbine in a shallow water channel with a turbulent inflow is simulated using the hybrid LES/ALM technique, which combines large eddy simulation with the actuator line method. The turbulent inlet conditions are generated using the mapping method to avoid a precursor running and large space for saving data. The numerical results demonstrated the usefulness of the mapping technique as well as some shortcomings that still remain to be addressed. Good agreement between numerical predictions and experimental data is achieved for both the mean and turbulent characteristics of the flow behind the turbine. The examination of changes in turbulence intensity and turbulent kinetic energy in the streamwise direction confirms the existence of a peak and transition to a highly turbulent flow about three diameters downstream of the turbine, which means that the distinct characteristics of the streamwise changes of turbulence intensity or turbulent kinetic energy may serve as an effective indicator for the flow regime transition and wake behaviour.

On the importance of inlet boundary conditions for aerothermal predictions of turbine stages with large eddy simulation

Abstract The analysis of a combustion chamber effects on the aerodynamics and thermal loads applied on a turbine stage is proposed. To do so, an integrated wall-modeled Large-Eddy Simulation of a combustion chamber simulator along with its high pressure turbine stage is performed and compared to a standalone turbine stage computation operated under the same mean conditions. For the standalone stage simulations, a parametric study of the turbulence injected at the turbine stage inlet is also discussed. For this specific configuration and with the mesh resolution used, results illustrate that the aerodynamic expansion of the turbine stage is almost insensitive to the inlet turbulent conditions. However, the temperature distribution in the turbine passages as well as on the stator and rotor walls are highly impacted by these inlet conditions underlying the importance of inlet conditions in turbine stage computations and the potential of integrated combustion chamber/turbine simulations in such a context.

RETRACTED ARTICLE: Evaluation of the Filtered Noise Turbulent Inflow Generation Method

Flows around buildings and in urban areas have the ability to exchange mass and momentum through mixing layers. The complex dynamical phenomena arising in mixing layers can be studied using Large Eddy Simulation (LES). As mixing layers depend on the turbulence conditions upstream of the buildings or urban areas, appropriate turbulent inlet conditions have to be provided to a simulation. Due to the high efficiency and level of control, the filtered noise inflow method was selected. The control over the Reynolds stresses as well as nine length scales make this method suitable to replicate conditions measured in experiments. In this paper, a formal method to obtain the filter coefficients is presented. This is achieved by relating the spatial filtering to a Finite Impulse Response (FIR) filter and the temporal filtering to an Autoregressive (AR) model. Three closed-form solutions for the spatial filter coefficients are presented having a Gaussian, double-exponential and exponential correlation function. By means of an LES simulation of a turbulent wall-bounded flow, the input-output behaviour is investigated. It was found that a combination of a Gaussian filter with length scales that increase with increasing wall distance result in the fastest downstream development of the artificial turbulence and the smallest loss of turbulent kinetic energy.