Showing papers by "Nikolaus A. Adams published in 2015"

Large-eddy simulation of cavitating nozzle flow and primary jet break-up

Abstract: We employ a barotropic two-phase/two-fluid model to study the primary break-up of cavitating liquid jets emanating from a rectangular nozzle, which resembles a high aspect-ratio slot flow. All components (i.e., gas, liquid, and vapor) are represented by a homogeneous mixture approach. The cavitating fluid model is based on a thermodynamic-equilibrium assumption. Compressibility of all phases enables full resolution of collapse-induced pressure wave dynamics. The thermodynamic model is embedded into an implicit large-eddy simulation (LES) environment. The considered configuration follows the general setup of a reference experiment and is a generic reproduction of a scaled-up fuel injector or control valve as found in an automotive engine. Due to the experimental conditions, it operates, however, at significantly lower pressures. LES results are compared to the experimental reference for validation. Three different operating points are studied, which differ in terms of the development of cavitation regions and the jet break-up characteristics. Observed differences between experimental and numerical data in some of the investigated cases can be caused by uncertainties in meeting nominal parameters by the experiment. The investigation reveals that three main mechanisms promote primary jet break-up: collapse-induced turbulent fluctuations near the outlet, entrainment of free gas into the nozzle, and collapse events inside the jet near the liquid-gas interface.

Cavitation erosion prediction based on analysis of flow dynamics and impact load spectra

Abstract: Cavitation erosion is the consequence of repeated collapse-induced high pressure-loads on a material surface. The present paper assesses the prediction of impact load spectra of cavitating flows, i.e., the rate and intensity distribution of collapse events based on a detailed analysis of flow dynamics. Data are obtained from a numerical simulation which employs a density-based finite volume method, taking into account the compressibility of both phases, and resolves collapse-induced pressure waves. To determine the spectrum of collapse events in the fluid domain, we detect and quantify the collapse of isolated vapor structures. As reference configuration we consider the expansion of a liquid into a radially divergent gap which exhibits unsteady sheet and cloud cavitation. Analysis of simulation data shows that global cavitation dynamics and dominant flow events are well resolved, even though the spatial resolution is too coarse to resolve individual vapor bubbles. The inviscid flow model recovers increasingly fine-scale vapor structures and collapses with increasing resolution. We demonstrate that frequency and intensity of these collapse events scale with grid resolution. Scaling laws based on two reference lengths are introduced for this purpose. We show that upon applying these laws impact load spectra recorded on experimental and numerical pressure sensors agree with each other. Furthermore, correlation between experimental pitting rates and collapse-event rates is found. Locations of high maximum wall pressures and high densities of collapse events near walls obtained numerically agree well with areas of erosion damage in the experiment. The investigation shows that impact load spectra of cavitating flows can be inferred from flow data that captures the main vapor structures and wave dynamics without the need for resolving all flow scales.

Coupled simulation of shock-wave / turbulent boundary-layer interaction over a flexible panel

Abstract: We investigate the interaction of an oblique shock generated by a pitching wedge with a turbulent boundary-layer at a free-stream Mach number of Ma = 3:0 and a Reynolds number based on the incoming boundary-layer thickness of Re_delta0,I = 205 x 10^3. Large-eddy simulations (LES) are performed for two configurations, that differ in the treatment of the wind-tunnel wall and shock generator movement within the experiment: A fixed panel with a static shock generator that deflects the flow by Theta = 20°, and the transient interaction of a pitching shock generator with an elastic panel. Besides mean and instantaneous flow quantities, we investigate unsteady aspects of the interaction region by means of wall-pressure spectra and provide comparison with experimental data whenever possible.

LES of cavitating flow inside a Diesel injector including dynamic needle movement

Abstract: We perform large-eddy simulations (LES) of the turbulent, cavitating flow inside a 9-hole solenoid common-rail injector including jet injection into gas during a full injection cycle. The liquid fuel, vapor, and gas phases are modelled by a homogeneous mixture approach. The cavitation model is based on a thermodynamic equilibrium assumption. The geometry of the injector is represented on a Cartesian grid by a conservative cut-element immersed boundary method. The strategy allows for the simulation of complex, moving geometries with sub-cell resolution. We evaluate the effects of needle movement on the cavitation characteristics in the needle seat and tip region during opening and closing of the injector. Moreover, we study the effect of cavitation inside the injector nozzles on primary jet break-up.

LES of temporally evolving turbulent cavitating shear layers ( Book Chapter)

Abstract: We present LES results of temporally evolving cavitating shear layers. Cavitation is modeled by a homogeneous equilibrium mixture model whereas the effect of subgrid-scale turbulence is accounted for by the Adaptive Local Deconvolution Method (ALDM). We quantitatively compare LES results with experimental data available in the literature. In terms of computational performance, we present a strong scaling study of our MPI-parallelized in-house Fortran code INCA on Cray XE6 “Hermit” at the High Performance Computeing Center Stuttgart (HLRS).

Large-eddy simulation of cavitating nozzle and jet flows

Abstract: We present implicit large-eddy simulations (LES) to study the primary breakup of cavitating liquid jets. The considered configuration, which consists of a rectangular nozzle geometry, adopts the setup of a reference experiment for validation. The setup is a generic reproduction of a scaled-up automotive fuel injector. Modelling of all components (i.e. gas, liquid, and vapor) is based on a barotropic two-fluid two-phase model and employs a homogenous mixture approach. The cavitating liquid model assumes thermodynamic- equilibrium. Compressibility of all phases is considered in order to capture pressure wave dynamics of collapse events. Since development of cavitation significantly affects jet break-up characteristics, we study three different operating points. We identify three main mechanisms which induce primary jet break-up: amplification of turbulent fluctuations, gas entrainment, and collapse events near the liquid-gas interface.

Numerical investigation of shedding partial cavities over a sharp wedge

Abstract: In this contribution, we examine transient dynamics and cavitation patterns of periodically shedding partial cavities by numerical simulations. The investigation reproduces reference experiments of the cavitating flow over a sharp wedge. Utilizing a homogeneous mixture model, full compressibility of the two-phase flow of water and water vapor is taken into account by the numerical method. We focus on inertia-dominated mechanisms, thus modeling the flow as inviscid. Based on the assumptions of thermodynamic equilibrium and barotropic flow, the thermodynamic properties are computed from closed-form analytical relations. Emphasis is put on a validation of the employed numerical approach. We demonstrate that computed shedding dynamics are in agreement with the references. Complex flow features observed in the experiments, including cavitating hairpin and horse-shoe vortices, are also predicted by the simulations. Furthermore, a condensation discontinuity occurring during the collapse phase at the trailing portion of the partial cavity is equally obtained.

On instationary mechanisms in cavitating micro throttles

Abstract: The current investigation presents numerical simulations of cavitating flows in a simplified model of a mushroom valve chamber of a piezo common rail injection system. Two discharge throttles with different step diameters are investigated. The developed models are able to predict relevant features of cavitating flow in fuel injectors. Special attention is put on the investigation of wave dynamics and related instationary mechanisms in the discharge throttle and the valve chamber. To this respect, a compressible flow solver with a homogeneous mixture model and barotropic description of the diesel-like-fluid is utilized. Highly unsteady phenomena are observed in both investigated designs. The structure of the cavitating flow is further analyzed with an emphasis on the interaction between collapsing vapor clouds in the throttle step and reentrant motion in the discharge throttle. Furthermore, numerical simulations reveal significant influence of the throttle step diameter on the cavity dynamics.

LES of Temporally Evolving Turbulent Cavitating Shear Layers

Abstract: We present LES results of temporally evolving cavitating shear layers. Cavitation is modeled by a homogeneous equilibrium mixture model whereas the effect of subgrid-scale turbulence is accounted for by the Adaptive Local Deconvolution Method (ALDM). We quantitatively compare LES results with experimental data available in the literature. In terms of computational performance, we present a strong scaling study of our MPI-parallelized in-house Fortran code INCA on Cray XE6 “Hermit” at the High Performance Computeing Center Stuttgart (HLRS).

Implicit LES of Noise Reduction for a Compressible Deep Cavity Using Pulsed Nanosecond Plasma Actuator

Abstract: High-subsonic cavity flows have been intensively studied due to their practical importance in aeronautical applications, such as, weapon bays, measurement windows and wheel wells [1].

Assessment of Implicit Subgrid-Scale Modeling for Turbulent Supercritical Mixing

Abstract: Space transportation systems predominantly rely on cryogenic rocket combustion engines, which have successfully been used for decades. However, satisfying the increasing requirements in terms of rocket performance and reliability is very challenging due to decreasing budgets and the request for short development cycles. Therefore, the importance of computational methods in the development process increases steadily, raising the demand for computational fluid dynamics (CFD) tools that are able to simulate the flow at rocket combustor conditions.