Wolfgang Schroeder

Analysis of regional compliance in a porcine model of acute lung injury

Lung protective ventilation in acute lung injury (ALI) focuses on using low tidal volumes and adequate levels of positive end-expiratory pressure (PEEP). Identifying optimal pressure is difficult because pressure-volume (PV) relations differ regionally. Precise analysis demands local measurements of pressures and related alveolar morphologies. In a porcine model of surfactant depletion (n=24), we combined measuring static pressures with endoscopic microscopy and electrical impedance tomography (EIT) to examine regional PV loops and morphologic heterogeneities between healthy (control group; CON) and ALI lungs ventilated with low (LVT) or high tidal volumes (HVT). Quantification included indices for microscopy (Volume Air Index (VAI), Heterogeneity and Circularity Index), EIT analysis and calculation of regional compliances due to generated PV loops. We found that: (1) VAI decreased in lower lobe after ALI, (2) electrical impedance decreased in dorsal regions and (3) PV loops differed regionally. Further studies should prove the potentials of these techniques on individual respiratory settings and clinical outcome.

In vivo microscopy in a porcine model of acute lung injury.

Regional inhomogeneity and alveolar mechanics in a porcine model of acute lung injury (ALI) was evaluated using confocal laser scanning microscopy (CLSM). CLSM was performed through thoracic windows of the upper and lower lobes. Image quantification was conducted by use of a volume air index (VAI). Twelve anesthetized, mechanically ventilated pigs were randomized to non-injury (control group, n = 6) or ALI induced by surfactant depletion (ALI group, n = 6). CLSM was performed at baseline, after 1 h at 5 mbar and after 2 h at 15 mbar positive end-expiratory pressure (PEEP). Haemodynamics, respiratory mechanics and calculation of pulmonary ventilation-perfusion distribution by MIGET were determined. At baseline, VAI was not different. In the upper lobes, VAI significantly decreased in ALI compared to control group, with no changes after PEEP application. In the lower lobes, VAI significantly decreased in ALI compared to control group. Incremental PEEP significantly increased VAI in ALI, but not in control group. Haemodynamics were significantly compromised in the ALI group. A significant deterioration in oxygenation and ventilation-perfusion distribution could be seen being restored after PEEP adjustment. The VAI may help to assess regional inhomogeneity of the acutely injured lung.

Numerical Investigation of Combustion Noise and Sound Source Mechanisms in a Non-Premixed Flame Using LES and APE-RF

In this study, combustion noise and sound source mechanisms of an unconfined turbulent non-premixed flame, i.e., the DLR-A flame is investigated. A hybrid LES/CAA approach is thereby employed in which a low Mach number variable density large-eddy simulation (LES) is combined with the acoustic perturbation equations for reacting flows (APE-RF). In the first step of the hybrid analysis the flamelet/progress variable (FPV) model is employed as combustion model followed by the acoustic simulation in the second step using the acoustic perturbation equations for reacting flows (APE-RF). In the acoustic analysis, special emphasis is placed on the impact of the thermoacoustic source contributions within the pressure-density relation of the APE-RF system on the radiated acoustic field and the applicability of these source formulations in terms of a hybrid CFD/CAA approach. The flamelet/progress variable database has been extended in terms of acoustic source terms. The unsteady heat release rate, the source describing the effect of non-isomolar combustion, and the species diffusion term are described by two independent parameters, i.e., the mixture fraction and the progress variable. From the findings in the present study, the analysis of the acoustic field of low Mach number reacting flows induced by the thermoacoustic sources such as the unsteady heat release leads to a very stiff problem formulation, since the related sources require highly resolved regions in the source area, which restricts the possible time step during temporal integration of the equations. The numerical bottleneck is not so restrictive when a source term formulation based on the density distribution is used. Spectra obtained from the simulated acoustic field, using two different source term formulations involving derivatives of the density are in good agreement with the experimental data even in the higher frequency range.

Noise Prediction for a Turbulent Jet Using an LES/CAA Method

Acoustic simulations of a Mach number 0.9, Reynolds number 3,600 and a Mach number 0.9, Reynolds number 400,000 round jet are performed based on a two-step approach using a large eddy simulation (LES) for the flow field and a solution of the acoustic perturbation equations (APE) for the acoustical field. The present simulations are compared with available experimental and numerical results at similar flow conditions. Both jet simulations are in good agreement with the noise characteristics of jets at such varying Reynolds numbers showing the jet noise physics to be successfully captured in the present simulations. The dominant source term in the APE system for jet noise is shown to be the Lamb vector. The maximum amplitudes of the sound pressure levels for the high Reynolds number jet are overpredicted compared with the direct LES approach. 1 A comparison with data from the literature shows the APE method to be less susceptible to the extent of the source region than the Lighthill volume method.

High-Frequency Measurements of Acoustic and Entropy Disturbances in a Hypersonic Wind Tunnel

With a combination of various pressure and heat flux sensors mounted on a range of probes the disturbance level of a Mach 6 Ludwieg tube is determined and characterized. Wide spectra of fluctuation values are obtained up to 1 Mhz. The measurements are carried out for different flow conditions and probe positions to identify the sensitivities of free stream disturbances effects on surface pressure and heat flux density fluctuation. Furthermore, modal analysis is presented where the measured flow quantities contribute to acoustic and entropy modes in the freestream. The cone surface data are being used to rebuild the freestream disturbance spectra by using high-fidelity numerical simulations. The results are compared with modal analysis using hot wire data.

Cut-Cell Method Based Large-Eddy Simulation of a Tip-Leakage Vortex of an Axial Fan

The viscous flow around a rotating axial fan at a Reynolds number of 9.36 × 10 based on the outer casing diameter is investigated by large-eddy simulation (LES) with special focus on the tip-leakage flow region. A massively parallelized finite-volume flow solver for compressible flows based on hierarchical Cartesian grids is used. The immersed boundaries of the fan geometry are handled by a fully conservative cut-cell method. A 72◦ segment, which includes one of the five fan blades, is resolved with approx. 250 million cells, for which a rotational periodic boundary condition for Cartesian meshes has been developed. Results of the instantaneous and the mean fan flow field are discussed and compared to Reynolds-averaged Navier-Stokes (RANS) results of a 360◦ simulation. The main differences are observed for the turbulent kinetic energy in the wake region generated by the tip-gap vortex. Furthermore, the influence of the tip-gap size on the vortical structures is investigated. It is shown that a reduction of the tip-gap size leads to a change of the shape and size of the tip-gap vortex. Additionally, more separation and counter-rotating vortices are generated inside the tip-gap, which, however, result in a lower turbulent kinetic energy.

Noise Sources of Trailing-Edge Turbulence Controlled by Porous Media

To reduce trailing-edge noise an investigation of a noise reduction technique based on porous media is presented. Large-eddy simulations (LES) and solutions of the acoustic perturbation equations (APE) are used to investigate the trailing-edge noise of a flat plate at a freestream Mach number 0.06 and a Reynolds number of 135000 based on the chord length and the freestream velocity. The acoustic fields are determined in a three dimensional domain to include the impact of the spanwise coherence length on the noise generation. The porous surface at the trailing edge covers an area in the spanwise times streamwise direction of 512 times 800 in inner wall units. The two-point correlation of the velocity components shows that the modified velocity field by the porous surface has a smaller correlation length and a smooth variation of the turbulence length at the trailing edge. The porous surface reduces the overall sound pressure level from 3dB to 8dB. The sound spectra possess a strong tone at the Strouhal number of fh/U∞ = 0.2 and the broadband spectrum follows the −2 power slope of the frequency. Due to the uniform porous surface the peak of the tone was decreased by 10dB.

Sharp resolution of complex moving geometries using a multi-cut-cell viscous flow solver

In many engineering problems the sharp resolution of the flow field near irregularly shaped boundaries is essential, e.g., for the flow in complex geometries such as internal combustion engines or for particulate flows with irregular particle shapes or inter-particle and wall-particle collisions. Especially when moving boundaries are involved, immersed boundary methods have been increasingly used for the simulation of such flows during the past decades. Among the different immersed boundary variants the cut-cell method is the only strictly mass-conserving approach and is capable of providing a sharp resolution of arbitrarily complex boundary configurations. In cut-cell methods, Cartesian cells that are intersected by the boundary surfaces are reshaped to retain only the fluid fraction of the original cell volume. However, computing the intersections of Cartesian cells with complex or non-smooth boundaries is tedious and difficult to be realized in a generic and robust fashion. In this study, a new multi-cut-cell method is presented in which complex intersections of a single Cartesian cell with multiple surfaces are handled by a generic and conceptionally simple procedure. In this strict finite-volume approach, an accurate representation of different boundary conditions within a single cell is realized. Sharp features of the boundaries and independently moving objects are tracked by a multiple-level-set formulation which preserves non-smooth regions of the boundary. The accuracy of the new method is demonstrated for several three-dimensional flow configurations involving moving boundaries, such as the turbulent flow field in a realistic internal combustion engine and colliding particles.

Thermoacoustics of a turbulent premixed flame

Quantitative analyses of noise induced by turbulent combustion processes are essential for the design of efficient combustors. To understand the noise generating mechanisms detailed thermoacoustic source mechanisms for the acoustic perturbation equations are deduced from the governing equations of compressible reactive fluids. A generic burner configuration operated with a turbulent premixed flame is experimentally and numerically investigated to identify relevant source mechanisms and to show the dependence of the noise radiation on the operating conditions. Besides direct combustion noise mechanisms by heat release fluctuations indirect mechanisms by acceleration of entropy inhomogenities and non-isentropic mixing processes are identified as major noise sources.

Experimental and Computational Investigation of Oblique Shock-Vortex Interaction

Experimental and numerical results of vortex breakdown caused by the interaction of slender vortices and oblique shocks (oblique shock–vortex interaction, OSVI) are presented. Wind tunnel experiments are conducted at freestream Mach numbers of Ma∞ = 2.0 and 2.9. To obtain information about the global structure of the flow field and unsteady flow phenomena, the flow is visualized using a color–schlieren method along with a digital video camera. For a more detailed analysis of the shock–vortex interaction the differential interferometry is applied. The numerical investigation is based on solutions of the Euler and Navier–Stokes equations for unsteady three–dimensional supersonic flow. The experimental and numerical results show the strong time–dependent behavior of the flow structure and evidence the necessity of a locally normal shock in the vicinity of the vortex core to cause vortex breakdown.