Özgür Ertunç

Homogeneity of turbulence generated by static-grid structures

Homogeneity of turbulence generated by static grids is investigated with the help of hot-wire measurements in a wind-tunnel and direct numerical simulations based on the Lattice Bolztmann method. It is shown experimentally that Reynolds stresses and their anisotropy do not become homogeneous downstream of the grid, independent of the mesh Reynolds number for a grid porosity of 64 %, which is higher than the lowest porosities suggested in the literature to realize homogeneous turbulence downstream of the grid. In order to validate the experimental observations and elucidate possible reasons for the inhomogeneity, direct numerical simulations have been performed over a wide range of grid porosity at a constant mesh Reynolds number. It is found from the simulations that the turbulence wake behind the symmetric grids is only homogeneous in its mean velocity but is inhomogeneous when turbulence quantities are considered, whereas the mean velocity field becomes inhomogeneous in the wake of a slightly non-uniform grid. The simulations are further analysed by evaluating the terms in the transport equation of the kinetic energy of turbulence to provide an explanation for the persistence of the inhomogeneity of Reynolds stresses far downstream of the grid. It is shown that the early homogenization of the mean velocity field hinders the homogenization of the turbulence field.

Mass Flow Rate Controlled Fully Developed Laminar Pulsating Pipe Flows

Pressure gradient driven, laminar, fully developed pulsating pipe flows have been extensively studied by various researchers and the data for the resultant flow field are available in a number of publications. The present paper, however, concentrates on related flows that are mass flow driven, i.e., the flows where the mass flow rate is prescribed as m=mM+mAfm(t) and fm(t) is periodically varying in time. Sinusoidal and triangular mass flow rate pulsations in time are analytically considered in detail. Results of experimental investigations are presented and are complemented by data deduced from corresponding analytical and numerical studies. Overall, the results provide a clear insight into mass flow rate driven, laminar, fully developed pulsating pipe flow. To the best of the authors’ knowledge, flows of this kind have not been studied before experimentally, analytically and numerically.

On the high contraction ratio anomaly of axisymmetric contraction of grid-generated turbulence

The present paper deals with the anomalous increase in second-order moments of longitudinal velocity fluctuations reported in experiments carried out to measure properties of axisymmetric strained turbulence when the strain is provided by axisymmetric contractions (nozzles). Experimental evidence is provided that the increase is due to inaccuracies in the measurements. It is shown that the imperfect spatial resolution of X-wire probes, the mass flow rate fluctuations in the flow facility, and the electronic noise contaminate the hot-wire measurements of not only the longitudinal but also the transverse velocity fluctuation components. These kinds of contaminations start to dominate the measurements, especially when the contraction ratio of the nozzle is high and/or the turbulence level is low and turbulence length scales are small. A series of measurement and correction methods is proposed employing two single normal wires and one inclined wire to detect and eliminate all three types of contaminations of ...

Pressure strain rate modeling of homogeneous axisymmetric turbulence

Recently several second order closure models have been proposed for closing the second moment equations, in which the velocity–pressure gradient tensor and the dissipation rate tensor are two of the most important terms. In the literature, the velocity–pressure gradient tensor is usually decomposed into a so called rapid term and a return-to-isotropy term. Models of these terms have been used in global flow calculations together with other modeled terms. However, their individual behavior in different flows has not been fully examined, because they are unmeasurable in the laboratory. Recently, the experimental data for axi-symmetric flow measurements of Ertunç (2008) (J.L. Lumley, 1975) have given us the opportunity to do this kind of study. In this article, we make direct comparisons of five representative rapid pressure strain rate models and eight return-to-isotropy models with the experimental data for axi-symmetric flow measurements of Ertunç (2008) (J.L. Lumley, 1975) and the theoretical RDT results of Sreenivasan (U. Schumann, 1977). The purpose of these direct comparisons is to explore the performance of these models at different contraction rates and identify the ones which give the best performance. The paper also describes the modeling procedure, model constraints, and the various evaluated models. The detailed results of the direct comparisons are discussed, and a few concluding remarks on turbulence models are given.

The penetration of acoustic cavitation bubbles into micrometer-scale cavities

The penetration of acoustically induced cavitation bubbles in micrometer-scale cavities is investigated experimentally by means of high-speed photography and acoustic measurements. Micrometer-scale cavities of different dimensions (width=40 μm, 80 μm, 10 mm and depth=50 μm) are designed to replicate the cross section of microvias in a PCB. The aim here is to present a method for enhancing mass transfer due to the penetration of bubbles in such narrow geometries under the action of ultrasound. The micrometer-scale cavities are placed in a test-cell filled with water and subjected to an ultrasound excitation at 75 kHz. A cavitation bubble cluster is generated at the mouth of the cavity which acts as a continuous source of bubbles that penetrate into the cavity. The radial oscillation characteristics and translation of these bubbles are investigated in detail here. It is observed that the bubbles arrange themselves into streamer-like structures inside the cavity. Parameters such as bubble population and size distribution and their correlation with the phase of the incident ultrasound radiation are investigated in detail here. This provides a valuable insight into the dynamics of bubbles in narrow confined spaces. Mass transfer investigations show that fresh liquid can be continuously introduced in the cavities under the action of ultrasound. Our findings may have important consequences in optimizing the filling processes for microvias with high aspect ratios.

Numerical simulations of swirling pipe flows- decay of swirl and occurrence of vortex structures

The present work aims at better understanding of the physics underlying swirling flows in pipes by means of numerical simulations. Direct numerical simulations have been carried out by using two different inlet swirl conditions. In one case, rotating honeycomb is used as the means to generate swirl whereas in the other case a solid body rotation is provided at the inlet. The inlet swirl intensity is varied in order to scan the underlying physics. Reynolds number 1730 is selected so that the flow remains in laminar regime. The results are compared with those obtained from the experiments using a similar experimental set-up. It is shown that the increase in the inlet swirl intensity leads to a faster decay of swirl downstream of the pipe. Similarly certain specific vortex structures are observed in the radial velocity contours. These structures are thought to be analogous with those found in the Taylor-Couette flow between a stationary outer cylinder and inner rotating cone. The reported investigations reveal dependence of swirl decay on the inlet swirl intensity and occurrence of vortex structures.

Torque-Matched Aerodynamic Shape Optimization of HAWT Rotor

Schmitz and Blade Element Momentum (BEM) theories are integrated to a gradient based optimization algorithm to optimize the blade shape of a horizontal axis wind turbine (HAWT). The Schmitz theory is used to generate an initial blade design. BEM theory is used to calculate the forces, torque and power extracted by the turbine. The airfoil shape (NREL S809) is kept the same, so that the shape optimization comprises only the chord and the pitch angle distribution. The gradient based optimization of the blade shape is constrained to the torque-rotational speed characteristic of the generator, which is going to be a part of the experimental set-up used to validate the results of the optimization study. Hence, the objective of the optimization is the maximization of the turbines power coefficient Cp while keeping the torque matched to that of the generator. The wind velocities and the rotational speeds are limited to those achievable in the wind tunnel and by the generator, respectively. After finding the optimum blade shape with the maximum Cp within the given range of parameters, the Cp of the turbine is evaluated at wind-speeds deviating from the optimum operating condition. For this purpose, a second optimization algorithm is used to find out the correct rotational speed for a given wind-speed, which is again constrained to the generators torque rotational speed characteristic. The design and optimization procedures are later validated by high-fidelity numerical simulations. The agreement between the design and the numerical simulations is very satisfactory.

Evolution of transitional structures from puff to slug through multiple splitting in a pipe flow at low Reynolds number

During laminar-to-turbulent transition in low Reynolds pipe flows, three main types of flow structures occur: traveling waves and the turbulent flow structures, namely puffs and slugs. In the present work, detailed experiments on the probability of occurrence and propagation speed of puffs, splitting puffs and slugs were conducted with the transition pipe-flow facility of LSTM-Erlangen. During the investigations, fully developed laminar pipe flow was triggered by an iris diaphragm with a pre-defined amplitude and lapse time. Different types of single and multiple puffs are classified and the probability of their occurrence as well as their propagation speed at the end of pipes with different lengths are evaluated.

High-speed visualization of acoustically excited cavitation bubbles in a cluster near a rigid boundary

In the present work, high-speed visualizations at one million frames/s have been used to study the oscillation characteristics of acoustic cavitation bubbles. The bubbles are generated by acoustic cavitation using an ultrasound transducer with an excitation frequency of 75 kHz near a rigid surface and the medium used is deionized water. The cavitation bubbles tend to collect in clusters near solid boundaries, where they are visualized using a high-speed camera. The collective oscillations give rise to many interesting phenomena like bubble collapse, coalescence, fragmentation and bubble translation. The image sequences provided here contribute to the better understanding of the entire lifecycle of acoustic cavitation bubbles.Graphical abstract

Numerical Simulation of Non‐Thermal Food Preservation

Food preservation is an important process step in food technology regarding product safety and product quality. Novel preservation techniques are currently developed, that aim at improved sensory and nutritional value but comparable safety than in conventional thermal preservation techniques. These novel non‐thermal food preservation techniques are based for example on high pressures up to one GPa or pulsed electric fields. in literature studies the high potential of high pressures (HP) and of pulsed electric fields (PEF) is shown due to their high retention of valuable food components as vitamins and flavour and selective inactivation of spoiling enzymes and microorganisms. for the design of preservation processes based on the non‐thermal techniques it is crucial to predict the effect of high pressure and pulsed electric fields on the food components and on the spoiling enzymes and microorganisms locally and time‐dependent in the treated product. Homogenous process conditions (especially of temperature f...