Partially-Covered Fractal Induced Turbulence On Fins Thermal Dissipation

The impacts of partially-covered fractal grids induced turbulence on the forced convective heat transfer across plate-fin heat sink at Reynolds number ReDh=22.0×103 were numerically and experimentally investigated. Results showed that partially covered grids rendered a higher thermal dissipation performance, with partially covered square fractal grid (PCSFG) registering an outstanding increase of 43% in Nusselt number relative to the no grid configuration. The analyzation via an in-house developed single particle tracking velocimetry (SPTV) system displayed the findings of unique “Turbulence Annulus” formation, which provided a small degree of predictivity in the periodic annulus oscillations. Further assessments on PCSFG revealed the preferred inter-fin flow dynamics of (i) high flow velocity, (ii) strong turbulence intensity, (iii) vigorous flow fluctuations, (iv) small turbulence length scale, and (v) heightened decelerated flow events. Furthermore, power spectra density unveiled the powerful vortex shedding effect, with PCSFG achieving fluctuation frequency f=18.5Hz close to an optimal magnitude. Such intricate flow structures pave the way for superior thermal transfer capabilities, benefiting the community in developing for higher efficiency heat transfer systems.

A Framework for Simulating the Tropical-Cyclone Boundary Layer Using Large-Eddy Simulation and Its Use in Evaluating PBL Parameterizations

Boundary layer turbulent processes affect tropical cyclone (TC) structure and intensity change. However, uncertainties in the parameterization of the planetary boundary layer (PBL) under high-wind conditions remain challenging, mostly due to limited observations. This study presents and evaluates a framework of numerical simulation that can be used for a small-domain [O(5 km)] large-eddy simulation (LES) and single-column modeling (SCM) to study the TC boundary layer. The framework builds upon a previous study that uses a few input parameters to represent the TC vortex and adds a simple nudging term for temperature and moisture to account for the complex thermodynamic processes in TCs. The reference thermodynamic profiles at different wind speeds are retrieved from a composite analysis of dropsonde observations of mature hurricanes. Results from LES show that most of the turbulence kinetic energy and vertical momentum flux is associated with resolved processes when horizontal grid spacing is O(10 m). Comparison to observations of turbulence variables such as momentum flux, effective eddy viscosity, and turbulence length scale show that LES produces reasonable results but highlight areas where further observations are necessary. LES results also demonstrate that compared to a classic Ekman-type boundary layer, the TC boundary layer is shallower, develops steady conditions much quicker, and exhibits stronger wind speed near the surface. The utility of this framework is further highlighted by evaluating a first-order PBL parameterization, suggesting that an asymptotic turbulence length scale of 40 m produces a good match to LES results.

Effect of Different Subgrid-Scale Models and Inflow Turbulence Conditions on the Boundary Layer Transition in a Transonic Linear Turbine Cascade

The aim of this work is to study the influence of different subgrid-scale (SGS) closure models and inflow turbulence conditions on the boundary layer transition on the suction side of a highly loaded transonic turbine cascade in the presence of high free-stream turbulence using large eddy simulations (LES) of the MUR237 test case. For the numerical simulations, the MUR237 flow case was considered and the incoming free-stream turbulence was reproduced using the synthetic eddy method (SEM). The boundary layer transition on the blade suction side was found to be significantly influenced by the choice of the SGS closure model and the SEM parameters. These two aspects were carefully evaluated in this work. Initially, the influence of three different closure models (Smagorinsky, WALE, and subgrid-scale kinetic energy model) was evaluated. Among them, the WALE SGS closure model performed best compared to the Smagorinsky and KEM models and, for this reason, was used in the following analysis. Finally, different values of the turbulence length scale, eddies density, and inlet turbulence for the SEM were evaluated. As shown by the results, among the different parameters, the choice of the turbulence length scale plays a major role in the transition onset on the blade suction side.

A Budget-Based Turbulence Length Scale Diagnostic

<p>The most frequently used boundary-layer turbulence parameterization in numerical weather prediction (NWP) and general circulation (GC) models are turbulence kinetic energy (TKE) based schemes. However, these parameterizations suffer from a potential weakness, namely the strong dependence on an ad-hoc quantity, the so-called turbulence length scale. The turbulence length scale is used to parameterize the molecular dissipation of TKE and is required to calculate the turbulence exchange coefficients. Traditional turbulence length scale formulations are designed for scales that are located above the energy production range of the turbulence spectra, hence the transfer of TKE across scales is not considered. However, as computational power increase, there is an increase in the potential for simulating turbulence at resolutions that are within the energy production range of turbulence. This is a gray zone problem. In order to represent turbulence processes accurately at these resolutions, the transfer of TKE across scales needs to be accounted for. For this purpose, a new turbulence length scale diagnostic, that can be used in the development of new turbulence length scale formulations, has been developed.  The new diagnostic uses the budget of TKE and the budgets of scalar variances to estimate the effective dissipation rate, which encapsulate the sum of the molecular dissipation and the cross-scale TKE transfer. The effective dissipation rate is then associated with the new scale-dependent turbulence length scale. Several idealized LES cases, simulated with the MicroHH model, are used to diagnose the turbulence length scale. It has been found that in the gray zone of turbulence the new turbulence length scale strongly depends on the horizontal grid spacing, and that this scale-dependence is also height-dependent. The new diagnostic is used for the evaluation of existing turbulence length scale formulations.</p>

The two-energies turbulence scheme coupled to the assumed PDF method

<p>An update of the two-energy turbulence scheme is presented. The two-energy scheme is an extension of a Turbulence Kinetic Energy (TKE) scheme following the ideas of Zilitinkevich et al. (2013), but valid for the whole stability range and including the influence of moisture. The additional turbulence prognostic energy is used for the calculation of the stability parameter. The stability parameter is thus not anymore strictly local and has a prognostic character. These characteristics enable the two-energy scheme to model both turbulence and clouds in the atmospheric boundary layer. The original implementation of the two-energy scheme is able to successfully model shallow convection without the need of an additional parameterization for non-local fluxes. However, the performance of the two-energy scheme is worse in stratocumulus cases, where it tends to overestimate the erosion of the stable layers due to over-mixing. We have identified the causes of the over-mixing in the stable layers.  First, the non-local stability parameter does not consider local stratification, which leads to its underestimation and subsequent over-mixing.  Second, the scheme lacks an internal parameter that could distinguish between a shallow convection regime and a stratocumulus regime, thus the scheme can not be calibrated in this respect.  And third, the turbulence length scale formulation is not flexible enough to adjust to all possible regimes in the ABL. To alleviate this problem, we propose several modifications: an update of the stability parameter, a modified computation of the turbulence length scale, and introduction of the influence of entropy potential temperature into the scheme. In addition, the two-energy scheme is coupled to a simplified assumed PDF method in order to achieve a more universal representation of the cloudy regimes. The updated turbulence scheme is evaluated for selected idealized and real cases in the ICON modeling framework.</p>

A Budget-Based Turbulence Length Scale Diagnostic

<p>The most frequently used boundary-layer turbulence parameterization in numerical weather prediction (NWP) models are turbulence kinetic energy (TKE) based schemes. However, these parameterizations suffer from a potential weakness, namely the strong dependence on an ad-hoc quantity, the so-called turbulence length scale. The physical interpretation of the turbulence length scale is difficult and hence it cannot be directly related to measurements or large eddy simulation (LES) data. Consequently, formulations for the turbulence length scale in basically all TKE schemes are based on simplified assumptions and are model-dependent. A good reference for the independent evaluation of the turbulence length scale expression for NWP modeling is missing. We propose a new turbulence length scale diagnostic which can be used in the gray zone of turbulence without modifying the  underlying TKE turbulence scheme. The new diagnostic is based on the TKE budget: The core idea is to encapsulate the sum of the molecular dissipation  and the cross-scale TKE transfer into an effective dissipation, and associate it with the new turbulence length scale. This effective dissipation can then be calculated as a residuum in the TKE budget equation (for horizontal sub-domains of different sizes) using LES data. Estimation of the scale dependence of the diagnosed turbulence length scale using this novel method is presented for several idealized cases.</p>

Numerical and experimental analysis of turbulent boundary layer control with piezo-ceramic actuator

The effects of smart-material-based active surface perturbation (i.e. piezo-ceramic actuators) on wall shear stress and noise metric have been investigated by simulations and wind tunnel experiments. A periodic vibration through the application of piezo-ceramic actuators is imposed on the surface of a plate, and the vibration position is located on the upper part of the leading edge of the plate. Both the control results from simulations and experiments are close to each other, when the control parameters are the same. The simulations and wind tunnel experiments show that downstream skin-friction drag and noise metric can be reduced with the active control, and the reductions strongly depend on control parameters. Comparing with the near wall flow structures, the turbulent kinetic energy and characteristic turbulence length scale in the turbulent boundary layer can be controlled with the piezo-ceramic actuator.

Effects of Freestream Turbulence Intensity, Turbulence Length Scale, and Exit Reynolds Number on Vane Endwall Secondary Flow and Heat Transfer in a Transonic Turbine Cascade

In gas turbine engines, the first-stage vanes usually suffer harsh incoming flow conditions from the combustor with high pressure, high temperature and high turbulence. The combustor-generated high freestream turbulence and strong secondary flows in a gas turbine vane passage have been reported to augment the endwall thermal load significantly. This paper presents a detailed numerical study on the effects of high freestream turbulence intensity, turbulence length scale, and exit Reynolds number on the endwall secondary flow pattern and heat transfer distribution of a transonic linear turbine vane passage at realistic engine Mach numbers, with a flat endwall no cooling. Numerical simulations were conducted at a range of different operation conditions: six freestream turbulence intensities (Tu = 1%, 5%, 10%, 13%, 16% and 20%), six turbulence length scales (normalized by the vane pitch of Λ/P = 0.01, 0.04, 0.07, 0.12, 0.24, 0.36), and three exit isentropic Mach number (Maex = 0.6, 0.85 and 1.02 corresponding exit Reynolds number Reex = 1.1 × 106, 1.7 × 106 and 2.2 × 106, respectively, based on the vane chord). Detailed comparisons were presented for endwall heat transfer coefficient distribution, endwall secondary flow field at different operation conditions, while paying special attention to the link between endwall thermal load patterns and the secondary flow structures. Results show that the freestream turbulence intensity and length scale have a significant influence on the endwall secondary flow field, but the influence of the exit Reynolds number is very weak. The Nusselt number patterns for the higher turbulence intensities (Tu = 16%, 20%) appear to be less affected by the endwall secondary flows than the lower turbulence cases. The thermal load distribution in the arc region around the vane leading edge and the banded region along the vane pressure side are influenced most strongly by the freestream turbulence intensity. In general, the higher freestream turbulence intensities make the vane endwall thermal load more uniform. The Nusselt number distribution is only weakly affected by the turbulence length scale when Λ/P is larger than 0.04. The heat transfer level appears to have a significant uniform augmentation over the whole endwall region with the increasing Maex. The endwall thermal load distribution is classified into four typical regions, and the effects of freestream turbulence, exit Reynolds number in each region were discussed in detail.

A Budget-Based Turbulence Length Scale Diagnostic

The most frequently used boundary-layer turbulence parameterization in numerical weather prediction (NWP) models are turbulence kinetic energy (TKE) based-based schemes. However, these parameterizations suffer from a potential weakness, namely the strong dependence on an ad-hoc quantity, the so-called turbulence length scale. The physical interpretation of the turbulence length scale is difficult and hence it cannot be directly related to measurements or large eddy simulation (LES) data. Consequently, formulations for the turbulence length scale in basically all TKE schemes are based on simplified assumptions and are model-dependent. A good reference for the independent evaluation of the turbulence length scale expression for NWP modeling is missing. Here we propose a new turbulence length scale diagnostic which can be used in the gray zone of turbulence without modifying the underlying TKE turbulence scheme. The new diagnostic is based on the TKE budget: The core idea is to encapsulate the sum of the molecular dissipation and the cross-scale TKE transfer into an effective dissipation, and associate it with the new turbulence length scale. This effective dissipation can then be calculated as a residuum in the TKE budget equation (for horizontal sub-domains of different sizes) using LES data. Estimation of the scale dependence of the diagnosed turbulence length scale using this novel method is presented for several idealized cases.