# Application of hybrid RANS/VMS modeling to rotating machines

Florian Miralles, Bastien Sauvage, Stephen Wornom, Bruno Koobus, Alain Dervieux

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Florian Miralles, Bastien Sauvage, Stephen Wornom, Bruno Koobus, Alain Dervieux. Application of hybrid RANS/VMS modeling to rotating machines. CMF 2022 - The 18th International Conference on Modelling Fluid Flow, Aug 2022, Budapest, Hungary. hal-03922768

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# APPLICATION OF HYBRID RANS/VMS MODELING TO ROTATING MACHINES

Florian MIRALLES<fnref n="1" />, Bastien SAUVAGE<fnref n="2" />, Stephen WORNOM<fnref n="3" />, Bruno KOOBUS<fnref n="4" />, Alain DERVIEUX<fnref n="5" />,

## ABSTRACT

The proposed communication deals with hybrid RANS-LES modeling. The target application is the study of flows around rotating machines like helicopters and drones. In fine, the simulations should provide accurate estimates concerning the noise emission. Each of these flows can involve mean and high Reynolds turbulent regions with detached eddies and with thin laminar and turbulent boundary layers. A hybrid model, like DDES, is then mandatory, with possibly an improved resolution of LES regions, which are mainly turbulent wakes. It is then interesting to apply there a more sophisticated LES model than the LES part of DDES. In our study, we use there the Dynamic variational multiscale model (DVMS). In the other regions, a DDES or simply a RANS modeling is applied. In both cases a two-equation closure is chosen. After a discussion of the modeling ingredients, we shall present a comparison of the RANS, LES, and hybrid models for two series of flows. Although computed by many researchers, flows around cylinders remain difficult to predict. The comparison will continue with a flow around a cross shaped mixing device rotating inside a cylinder.

**Keywords:** hybrid turbulence models, variational multiscale model for LES, chimera method, cylinder flow, rotating machine, unstructured grids

## NOMENCLATURE

$\underline{W}$ [-] flow variables

$\rho$ [kg/m<fnref n="3" />] density

$\underline{u}$ [m/s] velocity vector

$\underline{E}$ [J/m<fnref n="3" />] total energy per unit volume

$k$ [$m^2/s^2$] turbulence kinetic energy

$\varepsilon$ [$m^2/s^3$] dissipation rate of k

$\underline{W}_h$ [-] discrete flow variables

$\underline{W}'_h$ [-] small resolved scales of $\underline{W}_h$

$\langle \rangle$ [-] Reynolds average

$\underline{F}$ [-] convective and diffusive fluxes

$\underline{\tau}$<sup>RANS</sup> [-] RANS closure term

$\underline{\tau}$<sup>LES</sup> [-] LES closure term

$\underline{\tau}$<sup>DDES</sup> [-] DDES closure term

$\nu$ [$m^2/s$] kinematic viscosity of the fluid

$\nu_t$ [$m^2/s$] turbulent kinematic viscosity

$\underline{C}_d$ [-] mean drag coefficient

$\underline{C}'_l$ [-] r.m.s. of the lift coefficient

$\underline{C}'_{p_b}$ [-] mean base pressure coefficient

$\theta$ [deg.] mean separation angle

$\Delta$ [m] LES filter width

$l_{RANS}$ [m] RANS turbulence length scale

$\mu_{SGS}$ [kg/m/s] LES eddy viscosity

$\mu_{RANS}$ [kg/m/s] RANS eddy viscosity

## 1. INTRODUCTION

In this paper, hybrid models are evaluated for the simulation of massively separated flows around fixed and moving geometries, with the objective of applying them to aeroacoustic problems involving complex industrial flows at high Reynolds numbers. For this purpose, the turbulence models need to behave preferentially like large eddy simulation models (LES) in order to introduce little dissipation and thus better capture the small scales and the fluctuations of the unsteady flows considered. However, these flows involve very thin boundary layers, which current computers and softwares cannot compute in LES mode, only in RANS mode. To accommodate both needs, LES and RANS, the hybrid turbulence approach, combining LES and RANS in a somewhat zonal manner, is considered by many research teams.

<footnote n="1">¹Corresponding Author. IMAG, Univ. Montpellier, CNRS, Place Eugène Bataillon, 34090 Montpellier, France. E-mail: florian.miralles@umontpellier.fr</footnote>

<footnote n="2">²Univ. Côte d'Azur/INRIA Projet Ecuador, Sophia-Antipolis, France, E-mail: Bastien.Sauvage@inria.fr</footnote>

<footnote n="3">³IMAG, Univ. Montpellier, CNRS, Montpellier, France. E-mail: stephen.wornom@inria.fr</footnote>

<footnote n="4">⁴IMAG, Univ. Montpellier, CNRS, Montpellier, France. E-mail: bruno.koobus@umontpellier.fr</footnote>

<footnote n="5">⁵Lemma, Biot, France, and Univ. Côte d'Azur/INRIA Projet Ecuador, Sophia-Antipolis, France, E-mail: alain.dervieux@inria.fr</footnote>
In this work, beside the classical Delayed Detached Eddy Simulation model (DDES, [1]), we study an approach which combines zonally this last model and the dynamic variational multiscale model (DVMS, [2]). In this hybrid DDES/DVMS approach [3], the DVMS model is preferentially activated in the wake regions where this latter model introduces less dissipation than the LES component of DDES. A hybrid RANS/DVMS model [4], where the RANS component is Goldberg's $k-\epsilon$ model [5], is also applied in this study. A smooth blending function, which is based on the value of a blending parameter, is employed in these hybrid strategies, for switching from RANS to DVMS or DDES to DVMS. In [4], these models are applied to flows around a cylinder at moderate Reynolds number, starting from Reynolds number 20000.

In many industrial cases, and particularly cases with rotating geometries like propellers, the Reynolds number is much higher.

In the present paper, these hybrid models are applied to the flow around a circular cylinder in the supercritical regime, namely at Reynolds numbers 1 million and 2 million. At Reynolds number 1 million, the supercritical regime shows turbulent flow separation on both the upper and lower surface of the cylinder with a laminar-turbulent transition in the boundary layers located between the stagnation point and the separation one. At Reynolds number 2 million, the high transition regime shows a fully turbulent boundary layer on one side of the cylinder, and a partially laminar, partially turbulent boundary layer on the other side. This benchmark, which contains many characteristics encountered in industrial flows, is challenging due to the complex physics of the flow and the considered high Reynolds numbers, and remains a difficult calculation to perform. The second application concerns the flow at Reynolds number 1.8 million around a cross shaped mixing device rotating inside a cylinder.

This work is part of a cooperation program with Keldysh Institute of Applied Mathematics of Moscow focussing on the “Efficient simulation of noise of rotating machines”, see e.g. [6, 7, 8].

The remainder of this paper is organized as follows. Section 2 presents the hybrid turbulence models used in this work. Section 3 describes the numerical scheme. In Section 4, applications are presented. The obtained results are analyzed and compared with those obtained in other numerical studies and with experimental data. Finally, concluding remarks are drawn in Section 4.

# 2. TURBULENCE MODELING

## 2.1. RANS

First, we want to specify that RANS stands for unsteady RANS throughout the document.

In this work, and as far as the closure of the RANS equations is concerned, we use the low Reyn-

olds $k-\epsilon$ model proposed in Goldberg et al. [5]. This model was designed to improve the predictions of the standard $k-\epsilon$ one for adverse pressure gradient flows, including separated flows. For the sake of brevity, the equations of this model are not recalled in this document.

## 2.2. LES-like model: DVMS

In this section, we present the DVMS model which is preferred to the classical LES approach in our hybrid strategies because of some specific properties, as will be specified hereafter, that allow this model to have a better behavior in some regions of the flow, such as shear layers and wakes.

The Variational Multiscale (VMS) model for the large eddy simulation of turbulent flows has been introduced in [9] in combination with spectral methods. In [2], an extension to unstructured finite volumes is defined. This method is adapted in the present work. Let us explain this VMS approach in a simplified context. Assume the mesh is made of two embedded meshes. On the fine mesh we have a $P^1$-continuous finite-element approximation space $V_h$ with the usual basis functions $\Phi_i$ vanishing on all vertices but vertex $i$. Let $V_{2h}$ represents its embedded coarse subspace. Let $V'_h$ be the complementary space: $V_h = V_{2h} \oplus V'_h$. The space of small scales $V'_h$ is spanned by only the fine basis functions $\Phi'_i$ related to vertices which are not vertices of $V_{2h}$. Let us denote the compressible Navier-Stokes equations by:

$$ \frac{\partial \underline{W}}{\partial t} + \nabla \cdot \underline{F}(\underline{W}) = 0 \quad \text{where} \quad \underline{W} = (\rho, \rho u, E). $$

The VMS discretization writes for $\underline{W}_h = \sum \underline{W}_i \Phi_i$:

$$ \left( \frac{\partial \underline{W}_h}{\partial t}, \Phi_i \right) + \left( \nabla \cdot \underline{F}(\underline{W}_h), \Phi_i \right) = - \left( \underline{\tau}^{\text{LES}}(\underline{W}_h'), \Phi_i' \right) \quad (1) $$

For a test function related to a vertex of $V_{2h}$, the RHS vanishes, which limits the action of the LES term to small scales. In practice, embedding two unstructured meshes $V_h$ and $V_{2h}$ is a constraint that we want to avoid. The coarse level is then built from the agglomeration of vertices/cells as sketched in Fig. 1. It remains to define the model term $\underline{\tau}^{\text{LES}}(\underline{W}_h')$. This

<figure><img src="image_3.png" /><figcaption>Figure 1. Building the VMS coarse level</figcaption></figure>

term represents the subgrid-scale (SGS) stress term, acting only on small scales $W'_h$, and computed from the small scale component of the flow field by applying either a Smagorinsky [10] or a WALE SGS model [11], the constants of these models being evaluated by the Germano-Lilly dynamic procedure [12, 13]. The resulting model is denoted DVMS in this paper. It has been checked [14] that combining VMS and dynamic procedure effectively brings improved predictions.
A key property of the VMS formulation is that the modeling of the dissipative effects of the unresolved structures is only applied on the small resolved scales, as sketched in Fig. 2. This property is not satisfied by LES models which damp also the other resolved scales. Important consequences are that a VMS model introduces less dissipation than its LES counterpart (based on the same SGS model) and that the backscatter transfer of energy from smallest scales to large scales is not damped by the model. These VMS models then generally allow a better behavior near walls, in shear layers and in the presence of large coherent structures.

Moreover, in this work, the dynamic procedure, which provides a tuning of the SGS dissipation in space and time, is combined with the VMS approach, which limits its effects to the smallest resolved scales, so that the resulting DVMS model enjoys synergistic effects.

<figure><img src="image_4.png" /><figcaption>Figure 2. VMS principle.</figcaption></figure>

## 2.3. DDES model

The DDES approach used in this work is based on the Goldberg's $k-\epsilon$ model [5]. In other words, this RANS model is introduced in the DDES formulation [1] by replacing the $D_k^{RANS} = \rho\epsilon$ dissipation term in the $k$ equation by $D_k^{DDES} = \rho \frac{k^{3/2}}{l_{DDES}}$ where

$$l_{DDES} = \frac{k^{3/2}}{\epsilon} - f_d \max(0, \frac{k^{3/2}}{\epsilon} - C_{DDES}\Delta)$$

with $f_d = 1 - \tanh((8r_d)^3)$

and $r_d = \frac{\nu_t + \nu}{\max(\sqrt{u_{i,j}u_{i,j}}, 10^{-10})} K^2 d_w^2$.

$K$ denotes the von Karman constant ($K = 0.41$), $d_w$ the wall-normal distance, $u_{i,j}$ the $x_j$-derivative of the $i$-th-component of the velocity $u$, and the model constant $C_{DDES}$ is set to the standard value 0.65.

## 2.4. Hybrid RANS/DVMS

The central idea of the hybrid RANS/DVMS model [3] applied in this study is to combine the mean flow field obtained by the RANS component with the application of the DVMS model wherever the grid resolution is adequate. In this hybrid model, the $k-\epsilon$ model of Goldberg (subsection 2.1) is used as the RANS component. First, let us write the semi-discretization of the RANS equations :

$$\left(\frac{\partial \langle W_h \rangle}{\partial t}, \Phi_i\right) + \left(\nabla \cdot F(\langle W_h \rangle), \Phi_i\right) = -\left(\tau^{\text{RANS}}(\langle W_h \rangle), \Phi_i\right).$$

A natural hybridation writes:

$$\left(\frac{\partial W_h}{\partial t}, \Phi_i\right) + \left(\nabla \cdot F(\langle W_h \rangle), \Phi_i\right) = -\theta\left(\tau^{\text{RANS}}(\langle W_h \rangle), \Phi_i\right) - (1 - \theta)\left(\tau^{\text{LES}}(\langle W'_h \rangle), \Phi'_i\right)$$

where $\langle W_h \rangle$ denotes now the hybrid variables and $\theta$ is the blending function which varies between 0 and 1 and is defined by:

$$\theta = 1 - f_d(1 - \tanh(\xi^2))$$

$$\text{with } \xi = \frac{\Delta}{l_{\text{RANS}}} \quad \text{or} \quad \xi = \frac{\mu_{\text{SGS}}}{\mu_{\text{RANS}}}$$

the shielding function $f_d$ being defined as in Subsection 2.3.

The blending function $\theta$ allows for a progressive switch from RANS to LES (DVMS in our case) where the grid resolution becomes fine enough to resolve a significant part of the local turbulence scales or fluctuations, i.e. computational regions suitable for LES-like simulations. Additionally, this blending function, thanks to the shielding function $f_d$, prevents the activation of the LES mode in the boundary layer. By way of example, the isocontours of the blending function $\theta$ for the flow past a circular cylinder are shown in Fig. 3. We can see in particular that in the boundary layer the RANS model is activated, while in the wake the LES approach is recovered.

<figure><img src="image_5.png" /><figcaption>Figure 3. Isocontours of the blending function for the circular cylinder benchmark (Re= $10^6$): for $\theta = 1$ (in red) the RANS model is activated, wherever $0 < \theta < 1$ additional resolved fluctuations are computed, and in the limit $\theta \to 0$ (in blue) the LES approach (DVMS in this work) is recovered.</figcaption></figure>

## 2.5. Hybrid DDES/DVMS

The key idea of the proposed hybrid DDES/DVMS model [4] is to use the DVMS approach instead of the LES component of DDES in locations where this component is expected to be activated, especially in wake regions where the DVMS approach allows more accurate prediction.

Assuming that the semi-discretization of the DDES equations writes:

$$\left(\frac{\partial W_h}{\partial t}, \Phi_i\right) + \left(\nabla \cdot F(\langle W_h \rangle), \Phi_i\right) = -\left(\tau^{\text{DDES}}(\langle W_h \rangle), \Phi_i\right),$$
the hybrid equations are then defined by:

$$ \left( \frac{\partial W_h}{\partial t}, \Phi_i \right) + \left( \nabla \cdot F(\underline{W}_h), \Phi_i \right) = -\theta \left( \underline{\tau}^{\text{DDES}}(\underline{W}_h), \Phi_i \right) - (1 - \theta) \left( \underline{\tau}^{\text{LES}}(\underline{W}_h'), \Phi_i' \right) $$

where $\underline{W}_h$ denotes the hybrid variables and $\theta$ is the blending function defined as in the previous subsection.

# 3. NUMERICAL DISCRETIZATION

## 3.1. Numerical scheme

The governing equations are discretized in space using a mixed finite-volume/finite-element method applied to unstructured tetrahedral grids. The adopted scheme is vertex-centered, i.e. all degrees of freedom are located at the vertices. The diffusive terms are discretized using P1 Galerkin finite-elements on the tetrahedra, whereas finite-volumes are used for the convective terms. The numerical approximation of the convective fluxes at the interface of neighboring cells is based on the Roe scheme [15] with low-Mach preconditioning [16]. To obtain second-order accuracy in space, the Monotone Upwind Scheme for Conservation Laws reconstruction method (MUSCL) [17] is used, in which the Roe flux is expressed as a function of reconstructed values of W at each side of the interface between two cells. A particular attention has been paid to the dissipative properties of the resulting scheme, since this is a key point for its successful use in LES, and therefore in simulations performed with a hybrid turbulence model. The numerical dissipation provided by the scheme used in the present work is made of sixth-order space derivatives [18] and thus is concentrated on a narrow-band of the highest resolved frequencies. This is expected to limit undesirable damping of the large scales by numerical dissipation. Moreover, a parameter $\gamma$ directly controls the amount of introduced numerical viscosity and can be explicitly tuned in order to reduce it to the minimal amount needed to stabilize the simulation. Time advancing is carried out through an implicit linearized method, based on a second-order accurate backward difference scheme and on a first-order approximation of the Jacobian matrix [19]. The resulting numerical discretization is second-order accurate both in time and space. It should be noted that the spatial discretization used in this work leads to a superconvergent approximation, i.e. the accuracy can be well above second-order for some Cartesian meshes. One can also add that the objective is not a higher-order convergence but a strong reduction of dissipation and a certain reduction of the dispersion in the general case of a non-Cartesian (but not too irregular) mesh.

## 3.2. Mesh adaptation for rotating machines

Our numerical model has been extended to rotor-stator simulation with a Chimera technique.

The Chimera method aims at solving partial differential equations by decomposition into subdomains with overlap in order to avoid having to use a global mesh. This method allows the communication between the computational subdomains thanks to the overlapping of the subdomains. In our case we consider a decomposition in two domains as shown in Fig. 4, a fixed domain in red and a rotating domain in blue. For computations we start by locating the boundary nodes of domain 1 in domain 2, and reciprocally we locate the boundary nodes of domain 2 in domain 1, then the aerodynamic values of each boundary node are determined by interpolation and each domain performs its calculation with the new interpolated values.

<figure><img src="image_6.png" /><figcaption>Figure 4. Definition of rotor and stator from an initial mesh : in red, the stator, in blue the rotor, and in gray, the overlap region.</figcaption></figure>

A mesh adaptation loop based on a metric depending of the Mach number has been combined with the Chimera algorithm. The Transient Fixed Point mesh adaptation algorithm [20, 21, 22] is applied to each domain from a metric field evaluated on the whole domain.

# 4. APPLICATIONS

## 4.1. Flow past a cylinder

The predictions of the flow around a circular cylinder are presented. Two Reynolds numbers, $10^6$ and $2 \times 10^6$, based on the cylinder diameter, D, and on the freestream velocity, are considered. Only a few numerical investigations have been performed for Reynolds numbers higher than $10^6$. This interval is inside the supercritical regime which appears at Reynolds number higher than $2 \times 10^5$ and for which the separation becomes turbulent [23].

The computational domain is such that $-15 \le (x, y)/D \le 15$, and $-1 \le z/D \le 1$ where x, y and z denote the streamwise, transverse and spanwise directions respectively, the cylinder axis being located at $x = y = z = 0$. The mesh contains 4.8 millions nodes.

The Steger-Warming conditions [24] are imposed
at the inflow and outflow. The flow is assumed to be periodic in the spanwise direction. The inflow Mach number is set to 0.1 so that the compressibility effects can be considered as negligible. The free-stream turbulence intensity is below 1%. In order to control the computational costs, a wall law is applied in the close vicinity of the wall. For accuracy purpose, the Reichardt analytical law [25], which gives a smooth matching between linear, buffer and logarithmic regions, is chosen. As the y⁺ normalized distance is generally subject to large variations in complex flows, the wall law is combined with low Reynolds modeling which locally damps the fully turbulent model in regions in which the wall law does not cover the buffer zone.

### Reynolds number 10⁶

The behavior of the hybrid models presented in Section 2 are first investigated in terms of flow bulk coefficients. From Table 1, it can be noted that the predictions of these coefficients are globally in good agreement with the experimental data and the numerical results in the literature. The lift time fluctuations are nevertheless better predicted by the RANS/DVMS model compared to the other two hybrid models. This good behavior is confirmed by the correct prediction of the distribution of the mean pressure coefficient, see Fig. 5. On the other hand, it can be observed from Table 1 and Fig. 5 that Smagorinsky and WALE SGS models give globally comparable results. It is also worth noting that the RANS model predicts rather well the bulk coefficients.

Fig. 6 shows instantaneous isocontours of the vorticity magnitude for each of the turbulence models used for this benchmark. As one might expect, the unsteady RANS model is more dissipative and captures less flow detail than the hybrid models, with in particular a much more damped and regular wake.

<figure><img src="image_7.png" /><figcaption>Figure 5. Distribution of the mean pressure coefficient over the cylinder surface. Comparison between experimental data and numerical results at Reynolds number 10⁶.</figcaption></figure>

<table><caption>Table 1. Bulk coefficients of the flow around a circular cylinder at Reynolds number 10⁶. The subscripts *S* and *W* holds respectively for Smagorinsky and WALE SGS models.</caption><thead><tr><th></th><th>C&#773;<sub>d</sub></th><th>C'<sub>l</sub></th><th>-C&#773;<sub>pb</sub></th><th>&theta;&#773;</th></tr></thead><tbody><tr><td><strong>Present simul.</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>RANS</td><td>0.20</td><td>0.02</td><td>0.20</td><td>130</td></tr><tr><td>DDES</td><td>0.20</td><td>0.04</td><td>0.22</td><td>138</td></tr><tr><td>DDES/DVMS<sub>S</sub></td><td>0.20</td><td>0.02</td><td>0.22</td><td>135</td></tr><tr><td>DDES/DVMS<sub>W</sub></td><td>0.20</td><td>0.02</td><td>0.26</td><td>132</td></tr><tr><td>RANS/DVMS<sub>S</sub></td><td>0.25</td><td>0.09</td><td>0.25</td><td>132</td></tr><tr><td>RANS/DVMS<sub>W</sub></td><td>0.26</td><td>0.11</td><td>0.22</td><td>134</td></tr><tr><td><strong>Other simul.</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>RANS [26]</td><td>0.40</td><td>-</td><td>0.41</td><td>-</td></tr><tr><td>LES [26]</td><td>0.31</td><td>-</td><td>0.32</td><td>-</td></tr><tr><td>LES [27]</td><td>0.27</td><td>0.12</td><td>0.28</td><td>108</td></tr><tr><td><strong>Measurements</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Exp. [28]</td><td>-</td><td>0.24</td><td>0.33</td><td>-</td></tr><tr><td>Exp. [29]</td><td>0.22</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Exp. [30]</td><td>0.25</td><td>0.32</td><td>-</td><td>-</td></tr><tr><td>Exp. [31]</td><td>-</td><td>-</td><td>-</td><td>130</td></tr><tr><td>Exp. [32]</td><td>0.2-0.4</td><td>0.1-0.15</td><td>0.2-0.34</td><td>-</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Figure 6. Circular cylinder, Re= 10⁶: instantaneous isocontours of the vorticity magnitude. From top to bottom: RANS, DDES, DDES/DVMS and RANS/DVMS.</figcaption></figure>
$$ \text{Reynolds number } 2 \times 10^6 $$

The main outputs obtained by the RANS, DDES, DDES/DVMS and RANS/DVMS models are summed up in Table 2. Regarding the two hybrid models using the DVMS approach, only the results obtained with the Smagorinsky SGS model are shown, the WALE SGS model giving very similar results. It can be noticed that all models overpredict the separation angle, and that the fluctuations of the lift coefficient are overpredicted by the RANS and DDES/DVMS model. The RANS/DVMS approach gives on the whole the most satisfactory results. In Fig. 7, which shows the distribution of the mean pressure coefficient, the numerical results obtained with both hybrid models RANS/DVMS and DDES/DVMS are in very good agreement with the experimental results, in particular RANS/DVMS.

<table><caption>Table 2. Bulk coefficients of the flow around a circular cylinder at Reynolds number $2 \times 10^6$. The subscript S holds for Smagorinsky SGS model.</caption><thead><tr><th></th><th>C&#773;<sub>d</sub></th><th>C'<sub>l</sub></th><th>-C&#773;<sub>pb</sub></th><th>&theta;&#773;</th></tr></thead><tbody><tr><td><strong>Present simul.</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>RANS</td><td>0.26</td><td>0.066</td><td>0.30</td><td>128</td></tr><tr><td>DDES</td><td>0.28</td><td>0.038</td><td>0.27</td><td>132</td></tr><tr><td>DDES/DVMS<sub>s</sub></td><td>0.26</td><td>0.070</td><td>0.35</td><td>130</td></tr><tr><td>RANS/DVMS<sub>s</sub></td><td>0.24</td><td>0.030</td><td>0.30</td><td>132</td></tr><tr><td><strong>Other simul.</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>LES/TBLE [33]</td><td>0.24</td><td>0.029</td><td>0.36</td><td>105</td></tr><tr><td><strong>Measurements</strong></td><td></td><td></td><td></td><td></td></tr><tr><td>Exp. [28]</td><td>0.26</td><td>0.033</td><td>0.40</td><td>105</td></tr><tr><td>Exp. [29]</td><td>0.32</td><td>0.029</td><td>-</td><td>-</td></tr></tbody></table>

<figure><img src="image_9.png" /><figcaption>Figure 7. Distribution of the mean pressure coefficient over the cylinder surface. Comparison between experimental data and numerical results at Reynolds number $2 \times 10^6$.</figcaption></figure>

## 4.2. Flow around a rotating cross

We present a preliminary computation obtained with the combination of the mesh adaptation, the Chimera approximation and the DDES model. They

are used for simulating the mixing process obtained with the rotation of a cross in a cylindrical box. The Reynolds number is 1.8 million, based on the thickness of the blades. The number of vertices of the initial mesh is 10.466. After two turns, and five mesh adaptations, the number of vertices of the adapted mesh is 1.05 million.

<figure><img src="image_10.png" /><figcaption>Figure 8. DDES computation of a rotating mixing device with a mesh adaptive Chimera approach (horizontal cut). View of the mesh and of Q-factor (colored with velocity magnitude) after the vertical upper blade.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 9. DDES computation of a rotating mixing device with a mesh adaptive Chimera approach (horizontal cut). Partial view of the mesh and velocity magnitude.</figcaption></figure>

In Fig. 8a, we present a view of this final adapted mesh. Mesh size normal to the cylinder is yet not small enough to allow the creation of strong non-2D features. This is verified from the examination of the
Q-factor, Fig. 8b. Fig. 9 shows details of mesh and velocity magnitude. It seems from this first calculation that despite the high Reynolds number, the flow separates at blade angle is a rather stable mode.

## 5. CONCLUSION

Several hybrid strategies, based on the DDES, RANS and DVMS models, are first evaluated for the simulation of the supercritical flow around a circular cylinder. This benchmark is characterized by turbulent boundary-layer separation. The cylinder flow is first computed at Reynolds $10^6$. The predictions of the main flow parameters are in overall good agreement with experimental data and the results of simulations in the literature, especially the RANS/DVMS model. A second series of computations of the flow past a circular cylinder is carried out at Reynolds $2 \times 10^6$. The predictions obtained by the hybrid models used to perform this benchmark are a little less accurate compared to the previous Reynolds number, more specifically for the mean separation angle which is overestimated. Again, the RANS/DVMS model is the hybrid model among those applied which gives the best results overall. It is also to be noted that the pressure distribution is well predicted by the hybrid models for both Reynolds numbers, especially by the RANS/DVMS model. Then, the DDES formulation is applied to the flow around a rotating cross inside a cylinder with a Reynolds number of $1.8 \times 10^6$. Preliminary results using mesh adaptation with meshes of 1 million vertices show that separations are more stable than in the flows around the cylinder. RANS/DVMS computations are planned. The new tool is also being applied to other rotating devices like propellers and will produce mesh adapted results for these geometries in a near future.

## ACKNOWLEDGEMENTS

This work was supported by the ANR NORMA project, grant ANR-19-CE40-0020-01 of the French National Research Agency. The authors gratefully acknowledge GENCI for granted access to HPC resources through CINES (grants 2020-A0092A05067 and 2021-A0102A06386) and IDRIS (grant 2021-A0112A05067).

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