On the influence of plasma DBD actuator on the flow in a rectangular channel

Previously, the vortical structures generated by plasma DBD actuator working in unsteady regime were investigated in detail. The generalized model describing the behaviour of these vortices in dependency on input power parameters was introduced. This paper should reveal how the wall-jet-like-flow generated by that actuator will affect the developed flow in a rectangular channel with cross-section dimension of 250 × 100 mm. The actuator is considered in spanwise configuration where the induced flow has the same or opposite orientation as the main flow. The flow control will be tested both for steady regime and for unsteady regime. The dynamic of that complex phenomenon will be studied and results in qualitatively and quantitatively meaning will be presented. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the dynamics of the vortex structures generated by plasma DBD actuator

The plasma actuator is used to generate pseudo-periodical vortices moving almost parallel to the wall under various settings of high-voltage high-frequency power AC. Low-frequency amplitude modulation is desired to generate vortices, otherwise wall-jet-like flow is present. It will be shown how the dynamics of generated coherent vortices alternates depending on generator setting, frequency spectrum will be introduced in dependency on frequency of amplitude modulation and duty cycle. Measurements will be performed using TR-PIV technique or HW anemometry across wall-jet flow. The generated flow patterns are to be applied for control of boundary layers. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase-Averaged Vortex Train Flow Generated by Plasma DBD Actuator

It will be shown how plasma actuator can generate wall-jet-like flow or train of periodical vortices depending on the generator setting. For generation the high-frequency high-voltage AC is used. Low-frequency modulation of the supply voltage is required to generate vortices. Data acquisition will be performed using time-resolved PIV technique. Phase-averaging will be studied from two different perspectives. Firstly, sampling of phases will be ensured using trigger that is contained in the PIV software and, secondly, phase-averaged flow will be computed from two main modes of POD analysis. The generated flow patterns are to be applied for control of a boundary layer. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Generation of the vortex train using plasma barrier discharge actuator

The plasma actuator is used to generate periodical train of vortices moving along the surface. For generation the high-frequency high-voltage AC is used forming more-or-less steady wall-jet-like flow by the dielectric barrier discharge or corona discharge. Low-frequency modulation of the supply voltage is applied to generate vortices. Parameters of the vortex train are studied as function of the generator setting using TR-PIV technique. The generated flow patterns are to be applied for control of a boundary layer. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Analysis of a Simple Impact Drill Model using Set-Oriented Numerical Methods

Set-oriented numerical methods provide a relatively new way for obtaining global insight into a non-linear or even non-smooth dynamical system. The need for analysis of systems with dynamical behaviour of high complexity arose from investigations on a piezoelectric drilling device, in which vibrational energy is transmitted from an actuator to the drill stem via a free-flying, impacting mass. Detailed understanding of the dynamics inside of this device is missing to the present day. Here, the use of a set-oriented approach will be suggested and demonstrated on a basic model for the free-flying mass. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of vortex induced vibrations and its control by suction and blowing

The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the Kármán vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle α = 100°. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations.     

Three-dimensional numerical simulation of nonisotropic thermal density flow in a strongly curved open channel

The results from a 3D nonisotropic algebraic stress/flux turbulence model are presented to investigate the structure of thermal density flow and the temperature distribution in a strongly curved open channel (180° bend). The numerically simulated results show that (i) several secondary flows take place at the bend cross-section 90° of the curved open channel, the feature which is not found for the isothermal flows and thermal density flow in a straight channel, and (ii) the thermocline in a curved channel is thicker than that in a straight channel due to the secondary flows-induced strong mixing process taking place in the former. Such features may be ascribed to the complex interaction of the buoyant force, the centrifugal force and the Reynolds stresses taking place only in curved channels. The simulated results are in good agreement with available experimental data, which indicates that the developed model can be applied for predicting the motion of the nonisotropic thermal density flow in the curved open channel.     

Boundary Layer Heat Transport in turbulent Rayleigh-Bénard Convection in Air

We study the convective heat transfer in a large-scale Rayleigh-Bénard (RB) experiment which is called the “Barrel of Ilmenau”. We present the results of flow visualization and Particle Image Velocimetry (PIV) measurements of the near wall flow field in a plane perpendicular to the surface of the heated bottom plate. The experiment was run in a smaller rectangular inset that was placed inside the larger barrel. The Rayleigh number amounts to Ra = 1.4 × 10¹⁰. The aspect ratios were Γ_x = 1 in flow direction and Γ_y = 0.26 perpendicular to the vertical flow plane. The measurements have been undertaken using a 2 W continuous wave Laser in combination with a light sheet optics and various cameras. Due to the slender geometry of the cell, the mean wind is confined in one direction where the Laser light sheet is aligned parallel. The flow was seeded with droplets of 1...2 µm size generated using an ordinary fog machine. Flow visualization as well as the PIV data clearly show the intermittent character of the boundary layer flow field that permanently switches between “laminar” and “turbulent” phases. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stokes flow in a junction of two-dimensional orthogonal channels

The purpose of this study is to calculate Stokes flow structures in relation to flow rate distribution in a junction of four orthogonal channels. Particular attention was paid to the existence of recirculation flow inside the channels for low Reynolds numbers. The presence of this recirculation flow may be observed, but only when the flow rate is relatively low in at least one of the branches.     

Stokes flow in a junction of two-dimensional orthogonal channels

The purpose of this study is to calculate Stokes flow structures in relation to flow rate distribution in a junction of four orthogonal channels. Particular attention was paid to the existence of recirculation flow inside the channels for low Reynolds numbers. The presence of this recirculation flow may be observed, but only when the flow rate is relatively low in at least one of the branches.     

Transfer Function and the State Space Model Identification of a Funnel Shaped Piezoelectric Structure for the Controller Design Purposes

For the controller design purposes in order to suppress vibration magnitudes of a funnel shaped piezoelectric shell structure the mathematical model of the structure was identified in the form of the transfer function and the state space representation. The excitation of the structure with different signal types as well as the measurement of the responses were achieved using the piezoelectric actuator and sensor patches attached to the funnel. From the frequency responses obtained as a ratio of the Fast Fourier Transforms between the appropriate sensor and the actuator signals, discrete-time transfer functions were determined by best curve fitting of the model-based frequency response. For the state space model identification the subspace based identification approach was used. Obtained models were used for the optimal LQ controller design. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Experimental investigation of a flexible high-water protection system in a down-scale water channel flow

As part of an interdisciplinary major project regarding the development of a flexible and efficiently installable high-water protective system we investigated the behaviour of different system parts of flexible channel walls (thin, plastic slides/membranes) and types of self-installing water barricades subjected to different water channel flows experimentally and numerically. In the experiments we studied the strain of different flexible channel walls and the efficiency of various water barricades in a down-scale model by optical deformation, flow velocity and time measurements, respectively. We varied the water height as well as the representative flow velocity of the water channel flow and we also examined the influence of water waves onto these high-water protective system parts. Selected results will be shown and described concerning the stress on the used model-slide. In addition we applied similarity theory not only for general data presentation but also to generate findings regarding a future practical implementation in technical applications. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Statistical modeling of compressible turbulent channel flow

Several questions that are relevant to turbulence modeling are addressed on the basis of recently obtained direct numerical simulation results of turbulent supersonic channel flow. In particular, this concerns the turbulence frequency production mechanism, wall damping effects on turbulence model parameters, and the relevance of compressibility effects. Limited support is found for usually applied models for the turbulence frequency production and wall damping effects. In contrast to that it is shown that turbulence frequency production mechanisms and wall damping effects may be explained very well on the basis of a frequency scaling that characterizes mean flow changes. The influence of compressibility is found to be relevant. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phase resolved measurements of the statistical flow behavior of a backward-facing step flow controlled by a sinusoidal Lorentz-force

In order to gain deeper inside into the behavior of a Lorentz-force controlled flow behind a backward-facing step, measurements with a laser Doppler velocity profile sensor were undertaken. By varying the frequency of the sinusoidal Lorentz-force the influence on the turbulence statistics was studied. Moreover, phase resolved measurement data is presented. The results confirm the characteristic frequency determined on basis of the momentum thickness beforehand. The aim of this study is to understand the influence of the different incitation signals and to reduce the reattachment length. The statistical results show a dependency of the turbulence degree on the excitation frequency as well as a deformation of the flow profile behind the step. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Axisymmetric flow over a backward-facing step in an annular pipe

The incompressible flow of a Newtonian fluid over a backward-facing step is investigated numerically. The geometry is an annular pipe in which the radius of the inner cylinder decreases suddenly. Keeping the radial expansion ratio fixed axisymmetric flows are computed for outlet radius ratios from 0.1 to 1 (ratio of the inner to the outer outlet radius). The Reynolds number at which the flow separates from the outer cylinder decreases as the outlet radius ratio decreases for constant inlet geometry. The growth with Reynolds number of the recirculation zone on the inner outlet cylinder just behind the step is strongly reduced when the recirculation zone on the outer cylinder is established. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transportation of dry fine powders by coordinated friction manipulation

The transportation of dry fine powders is an emerging technologic task, as in biotechnology, pharmaceutical or coatings industry particle sizes of processed powders are getting smaller and smaller. Fine powders are primarily defined by the fact that adhesive and cohesive forces outweigh the weight forces. This leads to mostly unwanted agglomeration (clumping) and adhesion to surfaces, what makes it more difficult to use conventional conveyor systems (e. g. pneumatic or vibratory conveyors) for transport. A rather new method for transporting these fine powders is based on ultrasonic vibrations, which are used to reduce friction and adhesion between powder and the substrate. One very effective set-up consists of a pipe, which vibrates harmoniously in axial direction at low frequency combined with a pulsed radial high frequency vibration. The high frequency vibration accelerates the particles perpendicular to the surface of the pipe, which in average leads to lower normal and thereby smaller friction force. With coordinated friction manipulation the powder acceleration can be varied so that the powder may be greatly accelerated and only slightly decelerated in each excitation period of the low frequency axial vibration of the pipe. The amount of powder flow is adjustable by vibration amplitudes, frequencies, and pulse rate, which makes the device versatile for comparable high volume and fine dosing using one setup. Within this contribution an experimental set-up consisting of a pipe, a solenoid actuator for axial vibration and a piezoelectric actuator for the radial high frequency vibration is described. An analytical model is shown, that simulates the powder velocity. Finally, simulation results are validated by experimental data for different driving parameters such as amplitude of low frequency vibration, pipe material and inclination angle. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nonlinear stability characteristics of the pressure-gradient driven flow between walls in a rotating system under the influence of the Coriolis force

The paper addresses the question of the disturbance propagation when the plane channel flow on a rotating system is under the influence of the Coriolis force. The problem is approached through the method of Stewartson and Stuart (1971) to handle bifurcation characteristics. It starts from solving the linear (eigenvalue) problem and extends the same to handle growing distubances in the vicinity of the neutral surface. Solution for the problem addressed over the range of flow parameters characterised by the dominant physical mechanism affecting transition, viz. the Tollmien-Schlichting mechanism or the Coriolis force mechanism, will be presented.. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)