Session R25: LES: Applications and Modeling

Wavelet-based LES modeling of bluff-body flow with variable thresholding

The ability to represent coherent structures have made wavelet-based methods very useful for developing multi-resolution variable fidelity approaches to the computational modeling of turbulence. Following the wavelet-based adaptive LES approach, the turbulent velocity field is decomposed into two different parts: a coherent more energetic velocity field, which is computed, and a residual less energetic coherent/incoherent one, whose effect is approximated through SGS modeling. A new space-time varying thresholding procedure that consists in tracking the wavelet thresholding parameter within a Lagrangian frame, by directly solving the corresponding evolution equation and exploiting a path-line diffusive averaging approach, is used. The method is applied to the simulation of the turbulent flow past a square cylinder, where the geometry is enforced through Brinkman volume-penalization. Wavelet-based adaptive LES supplied with the one-equation localized dynamic kinetic-energy-based model is successfully performed at moderately high Reynolds number. The present method of physics-based Lagrangian varying thresholding fully exploits the intermittency of turbulence, overcoming the major limitation for wavelet multi-resolution techniques that make use of constant and uniform thresholding.   

Adaptive Wavelet-based Large Eddy Simulations of Compressible Turbulent Flows

Adaptive wavelet simulation exploits intermittency in turbulent flows by resolving locally on the most coherent structures, offering improved computational efficiency and a priori fidelity control. Adaptive LES utilizing a wavelet grid filter, hereto developed for incompressible flows, is extended to the compressible regime with a kinetic energy equation-based approach. Nonlinear filtered terms are scaled by the SGS kinetic energy and model coefficients are locally determined through a dynamic procedure. The influence of the modeled terms relative to the resolved physics can be used as a fidelity-based feedback control for the adaptive grid. Several benchmark cases, including turbulent mixing layers, are considered for the validation of this approach across multiple Mach numbers. Of particular interest is capturing compressibility and variable density effects within turbulent flows, notably the reduced growth rate of turbulent shear layer thickness and accurate modeling of the SGS heat flux. These simulations have been performed solving the filtered compressible Navier-Stokes equations with the adaptive wavelet collocation method.   

Inhomogeneous differential filters for explicit LES

Variable width filters in space and time are essential for the large eddy simulation of complex turbulent flows, when inhomogeneous unstructured grids are applied. Unfortunately inhomogeneous filters produce the so called commutation error difficult to calculate and to model. Here we will examine the commutation error associated to inhomogeneous differential filters both as regards the space and the time derivatives. This class of explicit filters is particularly appealing for explicit LES and is used in many explicit filtering procedures: they have been applied as test filters in the dynamic approach and as a method to rigorously derive the constitutive equations for the large scale field and to separate the filtering and the discretized operators. The commutation errors are in this case explicitly derived, and the particular case of the inhomogeneous parabolic differential filter is examined in detail.   

Large-eddy simulation of separation-reattachment of a flat-plate turbulent boundary layer

We describe large-eddy simulations (LES) of turbulent boundary-layer flow over a flat plate at high Reynolds number in the presence of three-dimensional flow separation. The stretched-vortex subgrid-scale model is used in the bulk of the flow domain combined with a wall-model that is a two-dimensional extension of that described by Chung and Pullin [J. Fluid Mech., 631, 281-309, 2009]. Wall-normal averaging of the wall-parallel, stream-wise momentum equations combined with local inner scaling for the resolved-scale velocity gives an ordinary differential equation describing the wall shear-stress vector at each wall point. Together with a specification of a slip velocity at a raised, wall-parallel plane, this provides a boundary condition for the outer LES that allows local backflow. The present LES is motivated by experiments on flows exhibiting separation induced by the response of a turbulent boundary layer to an adverse-favorable pressure-gradient profile. Detailed discussion of detachment and reattachment of the separation bubble will be presented.   

LES of propeller crashback

Crashback is an operating condition to quickly stop a propelled vehicle, where the propeller is rotated in the reverse direction to yield a negative thrust. In crashback, the freestream interacts with the strong reverse flow from the propeller leading to massive flow separation and highly unsteady loads. We have used Large-Eddy Simulation (LES) in recent years to accurately simulate the flowfield in crashback around a stand-alone open propeller, hull-attached (posterior alone) open propeller and a ducted propeller with stator blades. This talk will discuss our work towards LES of crashback inclusive of the entire hull. The results will be compared to available experimental data, and the flow physics will be discussed.   

Numerical Simulations of Separated Flows Using Wall-Modeled LES

Calculations using an unstructured, wall-modeled large eddy simulation (WMLES) solver are performed for several high Reynolds number test cases of interest. While the equilibrium formulation of this wall-model (Bodart, Larsson {\&} Moin, AIAA 2013-2724) has proven to be accurate for steady, attached boundary layers, its application to non-equilibrium or highly three-dimensional problems has yet to be fully explored. A series of turbulent flows that exhibit boundary layer separation due to the geometries involved in each test case are considered. First, spanwise-periodic simulations for the flow over periodic hills are performed at multiple Reynolds numbers. Next, calculations involving separation caused by three-dimensional bodies are used to generate more complex flow fields and to evaluate the accuracy of the WMLES in the separated wake region downstream. The performance of the WMLES is quantified through comparisons with existing numerical and experimental data sets. The effects of grid resolution and variations in several wall-model parameters are also investigated to determine their influence on the overall calculation.   

Characteristics of tip-leakage flow in an axial fan

An axial fan with a shroud generates complicated vortical structures by the interaction of the axial flow with the fan blades and shroud near the blade tips. Large eddy simulation (LES) is performed for flow through a forward-swept axial fan, operating at the design condition of \textit{Re} = 547,000 based on the radius of blade tip and the tip velocity. A dynamic global model (Lee \textit{et al}. 2010) is used for a subgrid-scale model, and an immersed boundary method in a non-inertial reference frame (Kim \& Choi 2006) is adopted for the present simulation. It is found that two vortical structures are formed near the blade tip: the main tip leakage vortex (TLV) and the auxiliary TLV. The main TLV is initiated near the leading edge, develops downstream, and impinges on the pressure surface of the next blade, where the pressure fluctuations and turbulence intensity become high. On the other hand, the auxiliary TLV is initiated at the aft part of the blade but is relatively weak such that it merges with the main TLV.   

Large-eddy simulations of a mixed-flow pump at off-design conditions

Reduced flow-rates in turbopumps produce significant unsteady phenomena, characterized by separation and back-flow. In this study an LES approach coupled with an immersed-boundary methodology is utilized to investigate the changes in the flow physics, when compared to nominal flow-rates. The present methodology has been already validated for the design case through comparison with PIV experiments in the literature. It will be shown that for a reduced flow rate (40\% of the design one) separation phenomena are generated on the suction side of the rotor blades and on the pressure side of the stator ones. Significant spanwise non-uniformity is produced in the diffuser channels, with a displacement of the flow towards the hub side and back-flow on the shroud side. The values of turbulent kinetic energy are increased by an order of magnitude at off-design conditions and the main source of turbulence is not anymore the flow from the suction side and the trailing edge of the rotor blades: most turbulence is generated now at the leading edge of the diffuser blades. The increased interaction between rotating and stationary parts implies also a stronger dependence of the flow features on the relative position between impeller and diffuser blades.   

Modelling boundary layer flow over barnacle-fouled surfaces

Macro-biofouling is a critical concern for the marine industry. However, there is little data on flow and drag over such surfaces. Accurate modelling of such multi-scale flows remains a big challenge. Such simulations are vital in providing insights into the fundamental flow physics, and they can be used to estimate the timing, need and effectiveness of measures used to counteract bio-fouling. This talk focuses on the use of a sharp-interface immersed boundary method coupled with a wall model and large-eddy simulations to carry out accurate simulations of a turbulent boundary layer flow over macro-fouled surfaces. For the current study, high resolution scans of barnacles were used to create simple geometrical representations. Simulations were then carried out to test how well these simpler geometric models mimic the flow over actual barnacles. Simulations of array of modeled barnacles, with different barnacle densities have also been carried out and we present results on the effect distribution density on the flow physics and drag on the surfaces. This work is funded by ONR grant N00014-12-1-0582.   

Large eddy simulation of a wing-body junction flow

We present numerical simulations of the wing-body junction flow experimentally investigated by Devenport $\&$ Simpson (1990). Wall-junction flows are common in engineering applications but relevant flow physics close to the corner region is not well understood. Moreover, performance of turbulence models for the body-junction case is not well characterized. Motivated by the insufficient investigations, we have numerically investigated the case with Reynolds-averaged Naiver-Stokes equation (RANS) and Large Eddy Simulation (LES) approaches. The Vreman model applied for the LES and SST k-$\omega$ model for the RANS simulation are validated focusing on the ability to predict turbulence statistics near the junction region. Moreover, a sensitivity study of the form of the Vreman model will also be presented.   

Effects of cylinder Reynolds number on the turbulent horseshoe vortex system and near wake of a surface-mounted circular cylinder

The turbulent horseshoe vortex (HV) system and the near-wake flow past a circular cylinder mounted on a flat bed in an open channel are investigated based on results of eddy-resolving simulations and supporting flow visualizations. Of particular interest are the changes in the mean flow and turbulence statistics within the HV region as the necklace vortices wrap around the cylinder's base and the variation of the mean flow and turbulence statistics in the near wake, in between the channel bed and the free surface. While it is well known that the drag crisis induces important changes in the flow past infinitely-long circular cylinders, the changes are less understood and more complex for the case of flow past a surface-mounted cylinder. A detailed discussion of the changes in the flow physics between cylinder Reynolds numbers at which the flow in the upstream part of the separated shear layers (SSLs) is laminar (Re$=$16,000, subcritical flow regime) and Reynolds numbers at which transition occurs inside the attached boundary layers away from the bed and the flow within the SSLs is turbulent (Re$=$500,000, supercritical flow regime). The changes between the two regimes in the dynamics and level of coherence of the large-scale coherent structures (necklace vortices, vortex tubes shed in the SSLs and roller vortices shed in the wake) and their capacity to induce high-magnitude bed friction velocities in the mean and instantaneous flow fields and to amplify the near-bed turbulence are analyzed.   

Large eddy simulations of swirl-promoted turbulent flow

Swirlers are used in applications such as furnaces and boilers, due to their ability to enhance fluid mixing in confined spaces. However, the origins and nature of swirler-induced instabilities of recirculating flow and vortex breakdown are still not well understood. Furthermore, the effects of swirl-promoted turbulent flow on sprays still need to be studied to be able to design proper swirler-spray systems. Recent studies have revealed that the large eddy simulation (LES) technique is capable of capturing the unstable features of swirling flows. In this study, the flow dynamics behavior on the downstream side of a swirler is explored using LES by considering two swirl numbers. The effects of swirl number (SN) on biofuel blend sprays and atomization are also discussed. The simulations have been validated using experimental data in terms of mean flow velocity profiles. This work presents insight into several features of swirling flows including recirculating flow generation and vortex breakdown. Results show that high SN at lower flowrate leads to greater turbulence kinetic energy and more compact of recirculation zone than at higher flowrate and lower SN. Moreover, the results also show that the shape of spray is affected by Swirl Number significantly.