Dietmar Rempfer

Application of the proper orthogonal decomposition to datasets of internal combustion engine flows

The proper orthogonal decomposition (POD) is applied to both computational fluid dynamics and particle imaging velocimetry data of simplified motored engine flows using two different methods. The first method is to apply the POD to ensembles of velocity fields obtained by considering the flow field taken at fixed crank-angle positions over a number of cycles. As a result, sets of POD modes are found, each of which describe the structure of the flow at a given piston position. These sets give some indication of the instability mechanism involved in tumble breakdown. The second method we use represents a novel approach of applying the POD to flows within a time-varying domain. The velocity fields are stretched to a fixed domain and normalized so that all phases of the flow are equally weighted. In this way, ‘phase-invariant POD modes’ are created. The phase-invariant modes show desirable properties for forming a suitable basis for future low-dimensional models which should describe the breakdown process mor...

Turbulence over a compliant surface: numerical simulation and analysis

In this paper we present results from a numerical investigation of turbulent channel flow in the presence of a compliant wall. The compliant wall is modelled as a homogeneous spring-supported plate. The simulation code is validated both by comparison with an alternative code and by reproducing results of linear stability theory. Our results demonstrate that with the wall compliance we used in the simulation there is little change in the very long-time behaviour of the turbulent skin friction drag and little modification to the near-wall turbulent coherent structures. The values of pertinent statistical quantities of the turbulence near the compliant walls converge to those near a rigid wall and the statistical effect of the wall compliance on the turbulent channel flow is small.

Optimized sensor placement for urban flow measurement

In this paper, we discuss a novel approach to the description of atmospheric flows in urban geometries. Our technique is based on the method of proper orthogonal decomposition (POD). We devise a method that enables us to compute the time-varying coefficients of a Karhunen–Loeve expansion of the urban flow field using knowledge of instantaneous velocity data taken at a minimum number of locations simultaneously. Using the POD basis functions and these velocity data, we solve a set of linear equations which gives us an estimate of the exact expansion coefficients. This method allows us to compute estimates for all coefficients thereby enabling us to reconstruct a close approximation to the flow field which is optimal in a certain sense. A quantitative comparison of the approximate coefficients with the coefficients of an exact Karhunen–Loeve expansion shows that the method works very well. Our method provides a practical approach to reconstructing the flow field using a minimum amount of information.

Divergence-Free Wavelet Analysis of Turbulent Flows

In this paper we study the application of divergence-free wavelet bases for the analysis of incompressible turbulent flows and perform several experiments. In particular, we analyze various nominally incompressible fields and study the influence of compressible perturbations due to experimental and computational errors. In addition, we investigate the multiscale structure of modes obtained from the Proper Orthogonal Decomposition (POD) method. Finally, we study the divergence-free wavelet compression of turbulent flow data and present results on the energy recovery. Moreover, we utilize wavelet decompositions to investigate the regularity of turbulent flow fields in certain non-classical function spaces, namely Besov spaces. In our experiments, we have observed significantly higher Besov regularity than Sobolev regularity, which indicates the potential for adaptive numerical simulations.

Three-dimensional modeling of a portable medical device for magnetic separation of particles from biological fluids

A portable separator has been developed to quantitatively separate blood-borne magnetic spheres in potentially high-flow regimes for the human detoxification purpose. In the separator design, an array of biocompatible capillary tubing and magnetizable wires is immersed in an external magnetic field that is generated by two permanent magnets. The wires are magnetized and the high magnetic field gradient from the magnetized wires helps to collect blood-borne magnetic nano/micro-spheres from the blood flow. In this study, a 3D numerical model was created and the effect of tubing-wire configurations on the capture efficiency of the system was analyzed using COMSOL Multiphysics 3.3(R). The results showed that the configuration characterized by bi-directionally alternating wires and tubes was the best design with respect to the four starting configurations. Preliminary in vitro experiments verified the numerical predictions. The results helped us to optimize a prototype portable magnetic separator that is suitable for rapid sequestration of magnetic nano/micro-spheres from the human blood stream while accommodating necessary clinical boundary conditions.

A comprehensive in vitro investigation of a portable magnetic separator device for human blood detoxification

A portable magnetic separator device is being developed for a proposed magnetically based detoxification system. In this paper, the performance of this device was evaluated via preliminary in vitro flow experiments using simple fluids and a separator unit consisting of one tube and two metal wires, each at the top and bottom of the tube. The effects of the following factors were observed: mean flow velocity U(o) (0.14-45 cm s(-1)), magnetic field strength micro(o)H(o) (0.125-0.50 T), wire size R(w) (0.125, 0.250 and 0.500 mm), wire length L(w) (2, 5 and 10 cm), wire materials (nickel, stainless steel 304 and 430) and tube size (outer radius R(o) = 0.30 mm and inner radius R(i) = 0.25 mm; R(o) = 0.50 mm and R(i) = 0.375 mm; and R(o) = 2.0 mm and R(i) = 1.0 mm). Our observations showed that the experimental results fit well with the corresponding theoretical results from the model we previously developed at a low flow velocity area (for example, U(o) < or = 20 cm s(-1)), strong external magnetic field (for example, > or = 0.30 T) and long wire length (for example, L(w) = 10 cm). The experimental results also showed that more than 90% capture efficiency is indeed achievable under moderate systemic and operational conditions. Pressure drop measurements revealed that the device could work well under human physiological and clinical conditions, and sphere buildup would not have any considerable effect on the pressure drop of the device. The breakthrough experiments demonstrated that a lower flow rate V, higher applied magnetic field micro(o)H(o) and diluted sphere suspension, i.e. lower C(o), would delay the breakthrough. All the results indicate the promise of this portable magnetic separator device to efficiently in vivo sequestrate nano-/micro-spheres from blood flow in the future magnetically based detoxification system.

Proper orthogonal decomposition in Squire's coordinate system for dynamical models of channel turbulence

The convergence rate and the structures of the proper orthogonal decomposition (POD) reconstruction are re-examined. The relatively slow convergence rate of the wall-normal velocity and the over-prediction of the Reynolds shear stress from POD-based representations of near-wall turbulence suggest that coherent structures are too well correlated. This is a consequence of the directional preference of the eigenfunctions toward the most energetic data (the streamwise motion at

A Parametric Study of a Portable Magnetic Separator for Separation of Nanospheres from Circulatory System

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Energy analysis of turbulent channel flow using biorthogonal wavelets

) which may be relaxed by extracting the POD eigenfunctions from the streak modes and the streamwise vortex modes separately. The rate of convergence and representation of structures may be improved by performing the POD in Squires coordinate system (PODS). The statistical reconstruction of the localized (

Theoretical Approaches to the Effect of Wall Compliance on Turbulent Flow

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