Sanjin Saric

A Periodically Perturbed Backward-Facing Step Flow by Means of LES, DES and T-RANS: An Example of Flow Separation Control

Turbulent flow over a backward-facing step, perturbed periodically by alternative blowing/suction through a thin slit (0.05 H width) situated at the step edge, was studied computationally using (LES) large eddy simulation, (DES) detached eddy simulation, and (T-RANS) transient Reynolds-averaged Navier-Stokes techniques. The flow configuration considered (Re H =U c H/v=3700) has been investigated experimentally by Yoshioka et al. [1,2]. The periodic blowing/suction with zero net mass flux is governed by a sinusoidal law: v e =0.3U c sin(2πf e t), U c being the centerline velocity in the inlet channel. Perturbation frequencies f e corresponding to the Strouhal numbers St=0.08, 0.19, and 0.30 were investigated (St=f e H/U c )

LES, Zonal and Seamless Hybrid LES/RANS: Rationale and Application to Free and Wall-Bounded Flows Involving Separation and Swirl

An overview is given of the activities in the framework of the German-French Research Group on ”LES of Complex Flows” (DFG-CNRS FOR 507) with respect to the development of zonal and seamless hybrid LES/RANS computational methods based on a near-wall Eddy-Viscosity Model (EVM) and a near-wall Second-Moment Closure (SMC) respectively. The zonal scheme represents a two layer model with a two-equation EVM-RANS model covering the near-wall layer and the true LES employing the zero-equation subgrid-scale (SGS) model of Smagorinsky resolving the core flow. Due attention was payed to the exchange of the variables between the ensemble-averaged RANS field and the spatially-filtered LES field across the discrete interface separating the two sub-regions. A procedure for controlling the interface position in the flow domain was also in focus of the present investigations. After considering a few introductory test cases (fully-developed channel flow, flows separating from sharp-edged surfaces) the feasibility of the method was validated against the available experiments in a single tubo-annular, swirl combustor configuration (Exp.: Palm et al., [39]) and in the separated flows in a 3-D diffuser (Exp. Cherry et al., [10]) and over a 2-D hump including the case with the separation control by steady suction (Exp.Greenblatt et al., [23]). The seamless LES/RANS method employs the so-called Elliptic-Blending Reynolds-Stress Model (EB-RSM, Manceau, [33]; Manceau and Hanjalic, [34]) being active in the entire flow field. This RANS-based SGS model represents a near-wall Second-Moment Closure model relying on the elliptic relaxation method. The model coefficient multiplying the destruction term in the transport equation for the scale-supplying variable e (dissipation rate of the turbulence kinetic energy) was made filter-width (corresponding to the grid spacing) dependent, i.e. dependent on the location of the spectral cutoff, by applying a multiscale modelling procedure originating from spectral splitting of filtered turbulence in line with the Partially Integrated Transport Model (PITM) proposed by Dejoan and Schiestel, [48] and Chaouat and Schiestel, [8]. Herewith, the dissipation rate level was obtained, which suppresses the turbulence intensity towards the subgrid (i.e. subscale) level in the regions where large coherent structures dominate the flow. The resulting model was validated by computing some free flows (a temporal mixing layer) and wall-bounded flows (a fully-developed channel flow). Finally, the PITM method applied to the high-Reynolds number RSM model due to Speziale et al., [53] was used to compute the flow separated from a 2-D hill (with reference LES by Frohlich et al., [19] and Breuer, [6]). In addition, all relevant cases were computed by the conventional LES method aiming at mutual comparison of the predictive capabilities of the afore-mentioned methods with respect to the quality of results and space-time resolution issues.

Merging Near-Wall RANS Models With LES for Separating and Reattaching Flows

A method of coupling a low-Reynolds-number k–e RANS (Reynolds-Averaged Navier-Stokes) model with Large-Eddy Simulation (LES) in a two-layer Hybrid LES/RANS (HLR) scheme is proposed in the present work. The RANS model covers the near-wall region and the LES model the remainder of the flow domain. Two different subgrid-scale (SGS) models in LES were considered, the Smagorinsky model and the one-equation model for the residual kinetic energy (Yoshizawa and Horiuti, 1985), combined with two versions of the RANS e equation, one governing the “isotropic” (e; Chien, 1982) and the other the “homogeneous” dissipation rate (eh ; Jakirlic and Hanjalic, 2002). Both fixed and self-adjusting interface locations were considered. The exchange of the variables across the interface was adjusted by smoothing the turbulence viscosity either by adjusting the RANS model parameters, such as Cμ (Temmerman et al., 2005), or by applying an additional forcing at the interface using a method of digital-filter-based generation of inflow data for spatially developing DNS and LES due to Klein et al. (2003). The feasibility of the method was illustrated against the available DNS, fine- and coarse grid LES, DES (Detached Eddy Simulation) and experiments in turbulent flow over a backward-facing step at a low (Yoshioka et al., 2001) and a high Re number (Vogel and Eaton, 1985), periodic flow over a series of 2-D hills (Frohlich et al., 2005) and in a high-Re flow over a 2-D, wall-mounted hump (Greenblat et al, 2004). Prior to these computations, the method was validated in a fully-developed channel flow at a moderate Reynolds number Rem ≈ 24000 (Abe et al., 2004).Copyright

Comparative Assessment of Hybrid LES/RANS Models in Turbulent Flows Separating from Smooth Surfaces

Several hybrid LES/RANS (LES - Large-Eddy Simulation; RANS - Reynolds- Averaged Navier-Stokes) models have been assessed in computations of separated flows over smooth-contoured, wall-mounted hills, including flow control. The models considered include DES (Detached Eddy Simulation; Spalart et al. 1997, Travin et al. 2002), DDES (Delayed DES; Spalart et al. 2006), a zonal hybrid LES/RANS scheme (HLR; Jakirlic et al. 2006) and an Instability-Sensitized (IS) k-e model. We report on models performance in the two configurations: periodic flow over a symmetric 2-D hill at moderate Reynolds number (Re b =10595; LES: Frohlich et al.,2005; Breuer et al., 2005) and flow over a 2-D hump at high Re number( Re c ≈ 106,Exp.: Greenblatt et al., 2004). In the latter case the separation was controlled by steady suction through a narrow opening at the natural separation line in addition to the baseline flow. The computational results obtained confirm a crucial role of the LES/RANS interface treatment.

Computational Study of Mean Flow and Turbulence Structure in Inflow System of a Swirl Combustor

Flow structure in the annular section of the inlet system of a tuboannular swirl combustor with respect to the swirl intensity influence was investigated computationally complementary to the recent experimental study by Palm et al. [1]. In addition to the non-swirling case, two different swirling configurations corresponding to the swirl numbers S = 0.6 and 1.0 were considered. The simulations were performed by using Large Eddy Simulation (LES) method and a two-layer model scheme hybridizing a near-wall k —e RANS (Reynolds-averaged Navier Stokes) model covering the wall layer and LES method in the outer layer employing Smagorinsky model. Special attention was devoted to the position of the interface. An in-depth analysis of the mean velocity and turbulence fields reveals an increasingly asymmetric axial velocity profile in the annular pipe and an appropriately shaped profile of the Reynolds stress components corresponding to the enhanced turbulence production in the outer part of the concentric annulus. The present study also aimes at generation of reliable swirling inflow data for future LES of the flow in the combustor flue.

LES Based POD Analysis of Jet in Cross Flow

The paper presents results of a POD investigation of the LES based numerical simulation of the jet-in-crossflow (JICF) flowfield. LES results are firstly compared to the pointwise LDA measurements. 2D POD analysis is then used as a comparison basis for PIV measurements and LES, and finally 3D POD analysis is conducted on the LES datasets, giving some clear depictions of interaction processes between dominant flow structures pertinent to the JICF flowfield.

Turbulent Flow Separation Control by Boundary-Layer Forcing: A Computational Study

have demonstrated that active flow control (AFC) has a potential to enable significant advances in many engineering applications. Though demonstrated experimentally, unsteady separation flow control remains a challenge for Computational Fluid Dynamics (CFD). The main goal of this work was a computational study of the effects of boundary-layer forcing on the mean flow and turbulence using various methods for turbulent flow computations: Large-eddy simulation (LES), Reynolds-averaged Navier-Stokes (RANS) and Detached-eddy Simulation (DES), aiming also at mutual comparison of their features and performance in complex flow situations. Predictive capability of various CFD methods were evaluated for the three representative complex separated flow configurations without flow control. A potential of the methods for unsteady flow computations: LES, DES and URANS was investigated by predicting the flow and turbulence field for the two experimentally investigated AFC configurations. They involve the two recent experimental works pertinent to AFC: periodically perturbed backward-facing step (BFS) flow at a low Reynolds number (Yoshioka et al., 2001) and high Reynolds number flow over a wall-mounted hump (Greenblatt et al., 2004). In general, both the LES and DES computations have reproduced all important effects observed in the BFS experiments. The imposed perturbation frequency corresponding to St=0.19 was found to be the optimum one, leading to the maximum reduction of the reattachment length. URANS underpredicts substantially the intensity of the reduction, exhibiting a very weak sensitivity to the perturbations. Beside a close agreement with the experiment concerning time-mean behaviour of the flow for all perturbation frequencies, the extracted phase-averaged LES results for the case with the optimum frequency (St=0.19) compare well with the reference experimental data. The LES and DES predictions of the main characteristics of separated flow over a wall-mounted hump, obtained on relatively coarse grids with respect to the flow Reynolds number considered (Re_c=9.36x10^5), are encouraging, outperforming significantly the examined RANS models. The numerous simulations of the flow configurations pertinent to active flow control (AFC) have been carried out providing a picture of the current status of CFD in AFC applications.

Experimental characterization and modelling of inflow conditions for a gas turbine swirl combustor

One of the most important processes in a gas turbine combustor, influencing to a large extent the efficiency of the entire combustion process, is the mixing between a swirling annular jet (primary air) and the non-swirling inner jet (fuel). To study this fundamental flow geometry an experimental facility has been built which allows independent flow rate adjustment of the central (mean stream) and coaxial jet flow Vc/Vm and furthermore, good optical access for laser-based flow measurement techniques. Further important flow parameters include the Reynolds number (Rem), the swirl intensity (S) and the combustor confinement, expressed in terms of an expansion ratio (ER). The work presented focuses on the features of the swirling flow in the concentric annuli being the part of the inlet system. The laser Doppler technique has been used to measure the velocity profile and the gradient of the velocity in the annular cross section. The circumferential velocity profile follows the so-called free-vortex flow type, being characterized by an increase in the mean angular momentum by radius of curvature. The outcome of the experiment is that the axial velocity profile becomes increasingly asymmetric with increased swirl intensity. The velocity increases from the inside to the outside of the annular flow (with a decreasing gradient) corresponding to an intensified radial movement towards the outer wall due to imposed swirl. The numerical investigations, especially those accounting for the complete swirl generator system and using a second-moment closure reproduced all important mean flow and turbulent features in good agreement with available experimental data. Both the modelling and the Large Eddy Simulations of an equilibrium, fully developed swirling flow, performed with the method for the swirling inflow data generation due to Pierce and Moin (1998), revealed some interesting departures with respect to the opposite sign of the axial velocity gradient.