Fundamental modeling of turbulent flows in multi-physics environments

Dr. Javier Urzay, Center for Turbulence Research, Stanford University

Ever since the pioneering developments made at the beginning of the 20th century, the field of turbulence research has evolved rapidly and the multi-physics complexity of the relevant questions has grown exponentially. This talk will cover recent research performed at CTR on problems that involve turbulence along with additional multi-physical phenomena. For instance, the recently established PSAAP center at Stanford offers us a new platform to study fundamental flows of interest for solar energy, including particle-laden flows under thermal radiation. At high Reynolds numbers, the turbulence interactions with particles and radiation lead to intricate dynamics in the subgrid that influence particle dispersion and thermal efficiency in particle-laden solar collectors. These interactions are in need of new LES modeling approaches. At CTR we find other staggering challenges in subgrid-scale modeling for turbulent combustion, in that thermal expansion modifies the classic picture of the energy cascade with strong consequences on the turbulent transport of reactants. Other examples cited in the presentation will involve supersonic flows, wall-bounded flows, and plasma optics.

Keywords: turbulence, LES, particle-laden flows, combustion.

On the accuracy of the mesurements of velocity gradient statistics with hot-wire probes

Prof. Dr. Petar Vukoslavčević, member of Montenegrin Academy of Sciences and Arts, University of Montenegro, Faculty of Mechanical Engineering

In order to determine the most important statistical properties of turbulent flows like vorticity vector, strain rate tensor and vorticity-velocity correlations, it is necessary to simultaneously measure the instantaneous velocity gradient tensor. The first successful measurements of this tensor terms in turbulent flows were made by multi-sensor hot-wire probes. The operational principle of these probes is based on simultaneous measurements of velocity components at two or more points, closely separated in the flow coordinate directions. It is assumed that the velocity components vary linearly over the whole probe sensing area. A minimum of three arrays, with at least three hot-wire sensors each, are necessary to simultaneously measure the three velocity components at reference points and determine all six crosstream gradients. The attempt to measure streamwise velocity gradients was made by use of an additional array displaced in the upstream direction, but the most frequent approach to estimate these gradients was the application of the well known Taylor’s hypothesis of frozen turbulence. The measurement accuracy depends on a great number of parameters like: sensor response (cooling law and frequency response), number and arrangements of sensors within arrays, number of arrays, array configurations, spatial resolutions of sensors, arrays and probe and validity of Taylor’s hypothesis.
The hot-wire probe has been tested in various uniform turbulence free flows. Under these conditions it is possible to test successively the accuracy of sensor cooling law only. It was also possible to test how the sensor arrangements within an array affect the accuracy of velocity measurements assuming that the velocity variation over the array sensing area can be neglected.
Till recently little has been known about how the accuracy of the gradient based turbulence statistical properties depend on probe spatial resolution, number and array configuration. To clarify this problem a highly resolved turbulent channel flow Direct Numerical Simulation (DNS) with numerical grid of one viscous length, uniform in all direction, was used. At the channel wall and centerline one viscous length was about 0.6 and 0.25 Kolmogorov microscale.
The influence of various probe parameters on the measurement accuracy can be tested by performing virtual experiments. The probe sensors were simulated as points on numerical grid with response equal to the velocity at the appropriate grid points. The spatial resolution of real twelve-sensor vorticity probe configurations was tested first. The dependence of measurement accuracy of various turbulence statistics on probe spatial resolution is presented and compared with experimental results. To study the influence of the arrangements of probe arrays on the measurement accuracy, arrays are simulated as points located on the DNS mesh. Various array configurations, used so far, are compared assuming equal spatial resolution. The presented results show that the measurement accuracy can be strongly affected by array configurations. It becomes clear, for the first time, that the streamwise gradient statistics obtained by an array displaced upstream is badly in error, even with a probe with the spatial resolution close or better then the best spatial resolution of any of the probes used so far. The investigation of the validity of Taylor’s hypothesis and preliminary results of frequencies response analysis will be also presented.

Keywords: hot-wire, multy-sensor probe, velocity gradient statistics, vorticity, probe spatial resolution.

Progress in understanding viscous drag reduction

apl. Prof. Dr.-Ing. habil. Jovan Jovanović, M.Sc. Veronika Krieger, Institute of Fluid Mechanics, Friedrich-Alexander University Erlangen-Nuremberg

A significant portion of the drag which counteracts the motion of a body through a fluid is generated in the thin viscous region close to the solid boundary where the flow is nearly always turbulent. The viscous contribution to the total drag amounts to about 50% on commercial aircraft, 90% on underwater vehicles and almost 100% for pipe flows. The outstanding question of how wall-bounded turbulent flows can be rationally controlled with reasonable cost in order to reduce viscous drag has been investigated over past decades by employing different techniques such as polymer and surfactant additions, riblets, large-eddy breakup devices and compliant surfaces. Although substantial work has been done only marginal success has been achieved for wide engineering applications. In our work we demonstrate that reasoning about near-wall turbulence in the functional space which emphasizes the level of anisotropy of the velocity fluctuations and exploring the kinematic constrains for turbulence correlations without appeal to the dynamic equations of fluid flow not only provides an understanding of the causative physics behind remarkable and yet unclarified effects of turbulent drag reduction that are met in nature and engineering but also logically leads to the design of the surface topology capable of producing significant reduction of viscous drag which far exceeds what has been achieved so far. For this purpose grooves are suggested as a surface modification in order to obtain high drag reduction. Inside a groove and around it the velocity fluctuations in the tangential and normal directions are suppressed due to the side walls and therefore it is expected that turbulence in the groove will tend towards the axisymmetric state and reach the one-component limit at the walls, which is required in order to minimize the energy dissipation and induce the drag reduction effect.

Keywords: turbulence, viscous drag reduction, flow control.

Experimental validation of turbulence and transition models for engineering applications

R. Aragall, PhD student, C. Walter, PhD student, Prof. Dr.-Ing. habil. Gunther Brenner, Institute of Applied Mechanics, TU Claustahl

The presentation will give an overview about the experimental and computational study of turbulent and transitional flows at ITM/TU Clausthal. The first focus will be on the experimental investigation of transition in flows of non-Newtonian liquids in the context of the deep drilling technology. Secondly, results related to the validation of advanced turbulence models in turbomachinery are presented.
An important issue in deep and geothermal drilling is the well bore cleaning, i.e. the safe and efficient transport of drill cuttings from the bottom hole assembly and the cutting tools up to the surface over distances of several thousand meters. In that context, simulations and modelling becomes increasingly important in order to optimize and control such processes. These models rely on empirical correlations of pressure drop and transport characteristics and have to take into account multiple phases, non-Newtonian rheology and turbulence. At TU Clausthal, an experimental rig has been set up to examine such flows at moderate Reynolds numbers up to 5000, which are typical for drilling process. The correct estimation of the transition to turbulence is an important prerequisite in correctly predicting important process features. Optical techniques (PIV/LDA) are used to detect the onset of transition and to define new correlations for the pressure drop depending on geometrical, rheological and operating parameters.
The second part of the presentation focuses on the prediction of instable and unsteady flows in radial turbomachines. Such instabilities are potentially the cause for dynamic loads, vibrations and finally failure of machine parts. The question is, in how far advanced and scale resolving turbulence models may be used to reliably quantify such dynamic loads. In that context a contribution to the validation of computational models is made. The experiments base on TR-PIV and allow the quantification of velocity and their spectra inside a channel of the rotor at nominal and off-design conditions.

Keywords: turbomachinery, transient flows, computational fluid dynamics, particle image velocimetry, non-Newtonian fluids.

Modeling of pulverized coal combustion for in-furnace NOx reduction

Dr. Srđan Belošević, Res. Prof., Univ. of Belgrade, Institute of Nuclear Sciences “Vinča”, Lab. for Thermal Engineering and Energy

Pulverized coal-fired utility boilers should enable high efficiency of energy conversion, operation flexibility and emission reduction of pollutants like nitrogen oxides. Modification of combustion process is a cost-effective NOx control technology. For optimization of complex processes, such as turbulent reactive flow in coal-fired furnaces, mathematical modeling is regularly used. Numerical experiments were done by an in-house developed 3D differential comprehensive combustion code, with fuel- and thermal-NO formation/destruction reactions model. Various operating conditions in an utility boiler furnace were examined, such as fuel and preheated air distribution over the burners and tiers, operation mode of the burners, grinding fineness of coal and combined effect of different parameters. The NOx emission reduction of up to 30% and pulverized coal diffusion flame control can be achieved by proper combustion modifications in the case-study furnace. Such an approach to pollutants control enables evaluation of alternative solutions to achieve efficient and low emission operation of power plant furnaces.

Keywords: modeling, turbulent two-phase reactive flow, pulverized coal, combustion modifications, NOx reduction.

Anisotropic particles in turbulence: Numerical study of fiber flocculation in the turbulent air fow of an asymmetric planar diffuser

Dr. Jelena Andrić, Department of Applied Mechanics, Chalmers University of Technology

Particle-level simulations are employed to investigate the flocculation of anisotropic rod-like fibers in an asymmetric planar diffuser with a turbulent Newtonian fluid flow. Both the fiber inertia and the non-creeping fiber-flow interactions are taken into account. The fibers are modeled as chains of rigid, cylindrical segments. The equations of motion account for hydrodynamic forces and torques exerted by the fluid on the fiber segments. The Reynolds-averaged Navier-Stokes equations together with the eddy viscosity turbulence model are used to describe the fluid motion. A stochastic model is employed to account for the turbulent fluctuations and therefore capture the fiber dispersion. The fibers are assumed to interact through short-range attractive forces that cause them to interlock in flocs whenever the fiber-fiber contacts occur during the flow. It is found that the formation of fiber flocs is governed by both the turbulent dispersion and the lateral motion of fibers, which is triggered by the fiber inertia and the flow gradients.

Keywords: particle-level fiber model, fiber flocculation, turbulent flow.

Presentation of the facilities, methods and results of turbulence investigation in the VTI's wind tunnels

Dr. Slavica Ristić, Principal research Fellow, Suzana Linić, PhD student, Institute Goša, Dr. Marija Samardžić, VTI

Wind tunnels are the aerodynamic laboratories which task is to enable high quality and stabile air flow in controlled volume, a test section, during certain times, in order to study the effects of streaming around various aeronautical or non-aeronautical models (from airfoils and bluff bodies to complex motorized or robotized constructions). Generally, the air inside the wind tunnels is forced along the installation, by the power unit or artificial pressure difference, from the source to the test section. Representatives of the desired flow quality are the uniformity of the velocity and pressure fields along and across the test section, low turbulence level and low flow direction angularities or swirling.
The main requirement that leads to quality and reliable measurement results is a high flow quality in the test section, with the turbulence intensity in the first line. On the other hand, the known turbulence intensity, with other parameters, enables the exchange of the scientific and technical information, comparison of the experimental results from different wind tunnels and data scaling from the model to the real scale. The turbulence intensity reduction is of the high importance for the quality measurements.
This lecture will present the Experimental Aerodynamics Laboratory of the VTI in Belgrade. The Experimental Aerodynamics Laboratory was established in 1952. The most important wind tunnels are: the T-35 large subsonic wind tunnel and T-38 trisonic wind tunnel. The water tunnel will be presented,too. Special attention will be paid to the equipment and methods of turbulence measurements in the test section stream and around different test models.
Wind tunnel facilities maintain equipment and devices for sampling, acquisition and data reduction for various test types, from forces and moments measurements, over the pressure distribution measurements to the advanced measurements, followed with the appropriate flow visualization techniques. The modern instrumentation enables tests and measurements of static and dynamic model characteristics for testing of the models as are: half-models, 2D and 3D models. The measuring devices involve, for the separated or combined measurements: eight external and internal balances, for measuring of the forces and moments; electronic absolute and differential pressure sensors and Scanivalves, for pressure and pressure distribution measurements; stability derivatives measurements (in pitch, yaw, roll and plunging); flow visualization; air intake measurements; minimum drag measurements; high angle of attack measurements with models on bent stings; store loads measurements; test section calibration measurements; hot film and hot wire anemometry; Laser Doppler anemometry; holographic interferometry; external flow field measurements; aerodynamic noise measurements.

Keywords: wind tunnel, turbulence measurement, turbulence visualization, Laser Doppler anemometry.

Experimental investigation of an annular diffuser for different inflow profiles

J. Walter, PhD student, apl. Prof. Dr.-Ing. habil. D. Wurz, Prof. Dr.-Ing. M. Gabi, Karlsruhe Institute of Technology

Axial fans are used in power plants for fresh air supply. In order to achieve a high efficiency of the fan and the following annular diffuser, flow separation in the diffuser has to be prevented. Experiments are performed on an annular diffuser in order to investigate the influence of the fan outlet profile on its separation behavior. Two different profiles are generated, one homogeneous profile and one profile with the characteristics of a turbulent outlet of a fan. The latter one is generated by the superposition of screens in the inlet zone. The tests are conducted at a high Reynolds number of approximately 5*105. The mean velocity profiles and wall shear stresses are measured with hydraulic methods (Prandtl and Preston tubes). In order to determine the turbulent statistics of the fluid hot wire measurements are conducted. The results show that there is a lack of momentum at the outer wall of the diffuser and high shear stresses at the inner wall for the homogeneous profile. For the typical fan outlet profile it can be seen that there is an opposite effect with high wall shear stresses at the outer wall while the boundary layer of the inner wall lacks momentum.

Keywords: annular diffuser, flow separation, fan outlet profile, hot wire measurements.