Numerical analysis of a 3-D printed porous trailing edge for broadband noise reduction

Lattice Boltzmann simulations were carried out to investigate the noise mitigation mechanisms of a 3-D printed porous trailing-edge insert, elucidating the link between noise reduction and material permeability. The porous insert is based on a unit cell resembling a lattice of diamond atoms. It replaces the last 20 % chord of a NACA 0018 at zero angle-of-attack. A partially blocked insert is considered by adding a solid partition between 84 % and 96 % of the aerofoil chord. The regular porous insert achieves a substantial noise reduction at low frequencies, although a slight noise increase is found at high frequencies. The partially blocked porous insert exhibits a lower noise reduction level, but the noise emission at mid-to-high frequency is slightly affected. The segment of the porous insert near the tip plays a dominant role in promoting noise mitigation, whereas the solid-porous junction contributes, in addition to the rough surface, towards the high-frequency excess noise. The current study demonstrates the existence of an entrance length associated with the porous material geometry, which is linked to the pressure release process that is responsible for promoting noise mitigation. This process is characterised by the aerodynamic interaction between pressure fluctuations across the porous medium, which is found at locations where the porous insert thickness is less than twice the entrance length. Present results also suggest that the noise attenuation level is related to both the chordwise extent of the porous insert and the streamwise turbulent length scale. The porous inserts also cause a slight drag increase compared to their solid counterpart.

Evaluating turbulent length scales from local MOST extension with different stability functions in first order closures for stably stratified boundary layer

<p>According to the Monin-Obukhov similarity theory (MOST), in the stratified surface layer of the atmosphere, the mean vertical velocity and scalars gradients are related to the turbulent fluxes of these quantities and to the distance z from the surface in a universal manner. The stability parameter ζ=z/L, where L is the Obukhov turbulent length scale, is the only dimensionless parameter that determines the flux-gradient relationships. This imposes a dependency of the dimensionless velocity and buoyancy gradients on ζ in form of universal nondimensional stability functions for  the surface layer. Over the decades a number of them were proposed and derived mostly from extensive field campaigns of measurements in the ABL. The stability functions differ from each other by both open coefficients and functional dependence on  ζ.  They have a limited range of applicability, which is often extended by incorporating the assumption about their asymptotic behavior.</p><p>           A generalization of MOST by considering the dependence of the dimensionless gradients on the local stability parameter z/Λ  in the framework of first order closures allows the extension of  the universal stability functions from the surface layer to most of the ABL. However, because of applicability constraints, differences in the asymptotic behavior and in other implied assumptions, it is not immediately obvious, which set of stability functions will perform best. In this study we analyze a set of stability functions which are implemented in a uniform manner into a one-dimensional first-order closure.  The latter applies a turbulent mixing length with generalized local MOST scaling which fits to a surface schemes employing corresponding functions for consistency. We use two numerical experiment setups accompanied with LES data for validation which correspond to the weakly stable GABLES1 case and to LES simulations of the very stable ABL based on measurements at the Antarctic station DOME-C (van der Linden et al. 2019). We also focus on the sensitivity of the 1D model results to coarser grids with respect to both the used  surface flux schemes and  the ABL turbulence closures since their are meant to be used in climate models because of numerical efficiency.</p><p>Authors want to aknowledge partial funding by Russian Foundation for Basic Research (RFBR project N 20-05-00776), sensitivity analysis and closure development were performed with support  of Russian Science Foundation (RSF No 20-17-00190). Steven van der Linden for providing LES data of DOME-C based experiments.</p><p>References:</p><p>van der Linden S.J. et al. Large-Eddy Simulations of the Steady Wintertime Antarctic Boundary Layer // Boundary Layer Meteorology 173.2 (2019): 165-192.</p>

The Hydrodynamics and Mixing Performance in a Moving Baffle Oscillatory Baffled Reactor through Computational Fluid Dynamics (CFD)

Oscillatory baffled reactors (OBRs) have attracted much attention from researchers and industries alike due to their proven advantages in mixing, scale-up, and cost-effectiveness over conventional stirred tank reactors (STRs). This study quantitatively investigated how different mixing indices describe the mixing performance of a moving baffle OBR using computational fluid dynamics (CFD). In addition, the hydrodynamic behavior of the reactor was studied, considering parameters such as the Q-criterion, shear strain rate, and velocity vector. A modification of the Q-criterion showed advantages over the original Q-criterion in determination of the vortices’ locations. The dynamic mesh tool was utilized to simulate the moving baffles through ANSYS/Fluent. The mixing indices studied were the velocity ratio, turbulent length scale, turbulent time scale, mixing time, and axial dispersion coefficient. We found that the oscillation amplitude had the most significant impact on these indices. In contrast, the oscillatory Reynolds number did not necessarily describe the mixing intensity of a system. Of the tested indices, the axial dispersion coefficient showed advantages over the other indices for quantifying the mixing performance of a moving baffle OBR.

The Impact of Combustor Turbulence on Turbine Loss Mechanisms

A blade row that is located downstream of a combustor has an extremely high turbulence intensity at the inlet, typically above 10%. The peak turbulent length scale is also high, at around 20% of the chord of the downstream blade row. In a combustor, the turbulence is created by impinging jets in crossflow. This may result in the turbulence being anisotropic in nature. The aim of this paper is to investigate the effect of combustor turbulence on the loss mechanisms which occur in a turbine blade row. The paper has a number of important findings. The combustor turbulence is characterized and is shown to be isotropic in nature. It shows that, when no pressure gradient is present, combustor turbulence increases the loss of a turbulent boundary layer by 22%. The mechanism responsible for this change is shown to be a deep penetration of the turbulence into the boundary layer. It shows that the presence of combustor turbulence increases the profile loss and endwall loss in the turbine cascade studied by 37% and 47%, respectively. The presence of combustor turbulence also introduces a freestream loss resulting in the total loss of the turbine cascade rising by 47%. When these loss mechanisms were applied to the vane alone, of an engine representative high-pressure turbine, it was found to result in a 1.3% reduction in stage efficiency.

A one-dimensional model of turbulent flow through “urban” canopies (MLUCM v2.0): updates based on large-eddy simulation

In mesoscale climate models, urban canopy flow is typically parameterized in terms of the horizontally averaged (1-D) flow and scalar transport, and these parameterizations can be informed by computational fluid dynamics (CFD) simulations of the urban climate at the microscale. Reynolds averaged Navier–Stokes simulation (RANS) models have previously been employed to derive vertical profiles of turbulent length scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as large-eddy simulations (LES), we observed that vertical profiles of turbulent kinetic energy and associated turbulent length scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of turbulent flow through urban areas and updated the parameterization of turbulent length scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density (λp) is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). In order to ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and direct numerical simulation (DNS) studies. After implementing the updated drag coefficients and turbulent length scales in the 1-D model of urban canopy flow, we evaluated the results by (a) testing the 1-D model against the original LES results and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and (b) testing the 1-D model against LES results for a test case with realistic geometries. Results suggest a more accurate prediction of vertical turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoscale.

A One-Dimensional Model of Turbulent Flow Through ‘Urban’ Canopies: Updates Based on Large-Eddy Simulation

In mesoscale climate models, urban canopy flow is typically parameterized in terms of the horizontally-averaged (1-D) flow and scalar transport, and these parameterizations can be informed by Computational Fluid Dynamics (CFD) simulations of the urban climate at the microscale. Reynolds Averaged Navier-Stokes Simulation (RANS) models have been previously employed to derive vertical profiles of turbulent length scale and drag coefficient for such parameterization. However, there is substantial evidence that RANS models fall short in accurately representing turbulent flow fields in the urban roughness sublayer. When compared with more accurate flow modeling such as Large Eddy Simulations (LES), we observed that vertical profiles of turbulent kinetic energy and associated turbulent length scales obtained from RANS models are substantially smaller specifically in the urban canopy. Accordingly, using LES results, we revisited the urban canopy parameterizations employed in the one-dimensional model of turbulent flow through urban areas, and updated the parameterization of turbulent length scale and drag coefficient. Additionally, we included the parameterization of the dispersive stress, previously neglected in the 1-D column model. For this objective, the PArallelized Large-Eddy Simulation Model (PALM) is used and a series of simulations in an idealized urban configuration with aligned and staggered arrays are considered. The plan area density is varied from 0.0625 to 0.44 to span a wide range of urban density (from sparsely developed to compact midrise neighborhoods, respectively). To ensure the accuracy of the simulation results, we rigorously evaluated the PALM results by comparing the vertical profiles of turbulent kinetic energy and Reynolds stresses with wind tunnel measurements, as well as other available LES and DNS studies. After implementing the updated drag coefficients and turbulent length scales in the 1-D model of urban canopy flow, we evaluated the results by a) testing the 1-D model against the original LES results, and demonstrating the differences in predictions between new (derived from LES) and old (derived from RANS) versions of the 1-D model, and b) testing the 1-D model against LES results for a test-case with realistic geometries. Results suggest a more accurate prediction of vertical turbulent exchange in urban canopies, which can consequently lead to an improved prediction of urban heat and pollutant dispersion at the mesoscale.

Cross-Correlation of POD Spatial Modes for the Separation of Stochastic Turbulence and Coherent Structures

This article describes a proper-orthogonal-decomposition (POD) based methodology proposed for the identification and separation of coherent and turbulent velocity fluctuations. Typically, POD filtering requires assumptions to be made on the cumulative energy content of coherent modes and can therefore exclude smaller, but important contributions from lower energy modes. This work introduces a suggested new metric to consider in the selection of POD modes to be included in a reconstruction of coherent and turbulent features. Cross-correlation of POD spatial modes derived from independent samples is used to identify modes descriptive of either coherent (high-correlation) or incoherent (low-correlation) features. The technique is demonstrated through application to a cylinder in cross-flow allowing appropriate analysis to be carried out on the coherent and turbulent velocity fields separately. This approach allows identification of coherent motions associated with cross-flow transport and vortex shedding, such as integral length scales. Turbulent flow characteristics may be analysed independently from the coherent motions, allowing for the extraction of properties such as turbulent length scale.

The Impact of Combustor Turbulence on Turbine Loss Mechanisms

A blade row which is located downstream of a combustor has an extremely high turbulence intensity at inlet, typically above 10%. The peak turbulent length scale is also high, at around 20% of the chord of the downstream blade row. In a combustor, the turbulence is created by impinging jets in cross flow. This may result in the turbulence being anisotropic in nature. The aim of this paper is to investigate the effect of combustor turbulence on the loss mechanisms which occur in a turbine blade row. The paper has a number of important findings. The combustor turbulence is characterized and is shown to be isotropic in nature. It shows that, when no pressure gradient is present, combustor turbulence increases the loss of a turbulent boundary layer by 22%. The mechanism responsible for this change is shown to be a deep penetration of the turbulence into the boundary layer. It shows that the presence of combustor turbulence increases the profile loss and endwall loss in the turbine cascade studied by 37% and 47%, respectively. The presence of combustor turbulence also introduces a freestream loss resulting in the total loss of the turbine cascade rising by 47%. When these loss mechanisms were applied to the vane alone, of an engine representative high pressure turbine, it was found to result in a 1.3% reduction in stage efficiency.