Suad Jakirlić

Dynamic contact angle of spreading droplets: Experiments and simulations

This paper presents results of an experimental investigation of a single drop impact onto a dry, partially wettable substrate and its numerical simulation. Particularly, the drop spreading diameter and the dynamic contact angle are measured at different time instants after impact. Two surfaces, wax (low wettability) and glass (high wettability), are used to study the effect of surface wettability (static contact angle) on the impact dynamics. It is shown that existing empirical models for the dynamic contact angle (e.g., Hoffman–Voinov–Tanner law) do not predict well the change of the dynamic contact angle, especially at high capillary numbers. In addition to the experimental investigations, the drop impact was studied numerically, focusing primarily on the contact angle treatment. The singularity in the neighborhood of the moving contact line is removed from the computational domain and replaced by a local force with some dependence on the instantaneous advancing/receding contact-line velocity. The predi...

Modeling Rotating and Swirling Turbulent Flows: A Perpetual Challenge

Severaltypesofrotatingandswirlinge owsfora rangeofReynoldsnumbersandrotationratesorswirlintensities have been studied computationally, aimed at identifying specie c features that require special consideration in turbulence modeling. The e ows considered include turbulent channel e ows subjected to streamwise and spanwise rotation,withstationaryandmovingboundaries;developingandfullydevelopede owsinaxiallyrotatingpipes;and swirling e owsin combustorgeometriesand long pipes.Computationsperformed with threeversionsof thesecondmoment closure and two eddy-viscosity models show that the second-moment models are superior, especially when the equations are integrated up to the wall. Such models reproduced the main e ow parameters for all e ows considered in acceptable agreement with the available experimental data and direct numerical simulations. However,challengesstillremaininpredictingaccuratelysomespecie ce owfeatures,suchascapturingthetransition from a freevortex to solid-body rotation in a long straight pipewith a weak swirl, or reproducing the normal stress components in the core region. Also, the so-called uw anomaly in fully developed e ows with streamwise rotation remains questionable. For rotating e ows, the low-Reynolds-number models yield a somewhat premature e ow relaminarization at high rotation speeds.

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 )

On unified boundary conditions for improved predictions of near-wall turbulence

A novel formulation of the wall boundary conditions relying on the asymptotic behaviour of the Taylor microscale λ and its relationship to the homogeneous part of the viscous dissipation rate of the kinetic energy of turbulence e h =5ν q 2 /λ 2 , applicable to near-wall turbulence, is examined. The linear dependence of λ on the wall distance in close proximity to the solid surface enables the wall-closest grid node to be positioned immediately below the edge of the viscous sublayer, leading to a substantial coarsening of the grid resolution. This approach provides bridging of a major portion of the viscous sublayer, higher grid flexibility and weaker sensitivity against the grid non-uniformities in the near-wall region. The performance of the proposed formulation was checked against available direct numerical simulation databases of complex wall-bounded flows featured by swirl and separation.

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

On “Steady” RANS Modeling for improved Prediction of Wall-bounded Separation

It is well-known that the separation process is inherently a highly unsteady phenomenon. To capture it correctly LES-relevant models – conventional LES and hybrid LES/RANS models (DES schemes, PITM, PANS) have to be applied. Because of their high spatial and temporal requirements the application of these methods is not straightforwardly affordable for the flow configurations of industrial relevance. On the other hand, apart of the backward-facing step flow geometry characterized by the sharpe-edge separation of a flat plate boundary layer which can be reasonably well solved by an advanced steady RANS model, the flows involving separation are in general beyond the reach of the conventional RANS method independent of the modeling level. Typical outcome is a low level of turbulence activity in the separated shear layer and a correspondingly long recirculation zone. The latter issues motivated the present work demonstrating the possibility to appropriately improve the computational results pertinent to the flow configurations featured by wall-bounded separation in the “Steady RANS” framework. An appropriately designed term modeled in terms of the von Karman length scale (adopted from the SAS modeling strategy for ”unsteady” flow computations, Menter and Egorov, 2010) was introduced into the scale-supplying equation governing the homogeneous part of the inverse time scale ( = ⁄ ). This term (denoted by ) being active only in the narrow area of the separation region acts towards an appropriate enhancement of the (fully-modeled) turbulence in the separated shear layer resulting in a correct mean velocity development and proper size of the recirculation zone. Predictive performances of the proposed model equation solved in conjunction with the Jakirlic and Hanjalic’s Reynolds stress model equation (2002) were illustrated by computing several configurations featured by boundary layer separation including the flow over a periodical arrangement of smoothly contoured 2D hills in a range of Reynolds numbers, flow over a wall-mounted fence and in a 3D diffuser.

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.

Rear-End Shape Influence on the Aerodynamic Properties of a Realistic Car Model: A RANS and Hybrid LES/RANS Study

The present work is concerned with the computational investigation of aerodynamic properties of the so-called ‘DrivAer’ car model representing a ‘generic realistic car configuration’ created by ‘merging’ the original geometries of two medium sized cars from the Audi A4 and the BMW 3 series, Heft et al. [7]. Three down-scaled (1:2.5) configurations differing in the rear-end shape—fastback, notchback and estate back geometries—investigated experimentally at the Institute of Aerodynamics and Fluid Mechanics Technical University in Munich are presently considered. The present numerical study focuses on the application of the ERM-capable (Elliptic-Relaxation Method) eddy-viscosity-based \(\zeta -f\) RANS model [6] describing turbulence within the Unsteady RANS (Reynolds-Averaged Navier-Stokes) and the so-called PANS (Partially-Averaged Navier-Stokes) computational frameworks. The latter approach representing a variable-resolution Hybrid RANS/LES (Large-Eddy Simulation) method is formulated and implemented into the CFD software package AVL-FIRE by Basara et al. [3]. The main objective of the present work is to check the models’ feasibility in computing the unsteady flow past the ‘DrivAer’ car configuration. The focus is on the varying structural properties of the flow arising from differently-shaped rear-ends and their impact on the surface pressure distribution and subsequently on the drag and lift force coefficients.

Predictive Capability Assessment of the PANS-\zeta -f Model of Turbulence. Part II: Application to Swirling and Tumble/Mean-Compression Flows

The present work is concerned with the application of the PANS-\(\zeta \)-f (Partially-Averaged Navier-Stokes) variable resolution model by Basara et al. (AIAA J 49:2627–2636, 2011), formulated in conjunction with the universal wall treatment, to the process of generation and destruction of a tumbling vortex in a square-piston compression machine (investigated experimentally by Boree et al. Phys Fluids 14:2543–2556, 2002) and the swirling flow in a tube generated by two tangential inlets with the outlet geometry resembling an orifice with an eccentrically positioned opening (reference experiment is by Grundmann et al. Int J Heat Fluid Flow 37:51–63, 2012). In addition, the complementary RANS computations (by using the \(\zeta -\textit{f}\) model of Hanjalic et al. Int. J. Heat Fluid Flow 25:1047–1051, 2004), representing also the background RANS formulation in the present PANS model) and LES (in conjunction with the Standard Smagorinsky subgrid-scale model) of both configurations are performed. The PANS-\(\zeta \)-f model description and its preliminary validation by simulating a fully-developed channel flow and a separating flow over a series of axisymmetric hill-shaped constrictions are given in a companion article by Chang et al. (5th International Symposium on Hybrid RANS-LES Methods, 2014)