On the flow around Glauert-Goldschmied body in the narrow channel and separation control strategy

Previously, there was tested a wire dielectric barrier discharge (DBD) plasma actuator to generate sufficiently strong ionic wind to affect freely developed boundary layer in the narrow channel. This paper will report about next step – to installation of actuator inside a streamlined body. The experiment will take place inside perspex rectangular (250 × 100 mm) channel and main task is to find the appropriate place for actuator fitting for three different flow regimes. Hence, the separation point and recirculation area will be investigated via PIV anemometry for base case and for active flow control methods (e.g. plasma actuator) as well as frequency spectrum of the flow will be evaluated to describe the nature of the flow. The measurement plane will be perpendicular to the bottom of the channel and in longitudinal level. These essential information will be used for actuator design, actuator embedding and to tune actuator frequency in order to suppress the recirculation area as much as possible. (© 2016 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)     

Unsteady Flow and its Influence to Aerodynamic and Aeroacoustic of VAWT's

A study is reported of the influence of unsteady flow on the aerodynamics and aeroacoustics of vertical axis wind turbines by numerical simulation. The combination of aerodynamic predictions with a discrete vortex method and aeroacoustic predictions based on Ffowcs Williams-Hawkings equation is used to achieve this goal. The numerical results show that unsteady flow of the turbine has a significant influence on the turbine aerodynamics and can lead to a decrease in generated noise as compared to the conventional horizontal axis wind turbine at the similar aerodynamic performance. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Transcritical Flow Over a Hole

Transcritical flow over a localized obstacle generates upstream and downstream nonlinear wavetrains. In the weakly nonlinear long-wave regime, this flow has been modeled with the forced Korteweg-de Vries (fKdV) equation, where numerical simulations and asymptotic solutions have demonstrated that the upstream and downstream nonlinear wavetrains have the structure of unsteady undular bores, connected by a locally steady solution over the obstacle. Further, it has been shown that when the obstacle is replaced by a step of semi-infinite length, it is found that a positive step generates only an upstream-propagating undular bore, and a negative step generates only a downstream-propagating undular bore. This result suggests that for flow over a hole, that is a step down followed by a step up, the two wavetrains generated will interact over the hole. In this paper, this situation is explored by numerical simulations of the fKdV equation.     

Localization of acoustic sources in subsonic jets

The sound in the acoustic far–field of a round jet is generated by a multitude of unsteady flow structures with different length and time scales. Most likely, different components of the noise spectrum are created by different structures in the jet which emit sound in different directions. Based on Lighthill's acoustic analogy, we present a method for relating flow structures of the jet with far–field noise spectra and their associated directivity patterns. The method allows to determine what kind of noise (with respect to frequency and emission direction) is generated at a given streamwise location. We illustrate the method with numerical results for a round isothermal jet. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Hydromagnetic rotating flow of a fourth‐order fluid past a porous plate

The rotating flow in the presence of a magnetic field is a problem belonging to hydromagnetics and deserves to be more widely studied than it has been to date. In the non‐linear regime the literature is scarce. We develop the governing equations for the unsteady hydromagnetic rotating flow of a fourth‐order fluid past a porous plate. The steady flow is governed by a boundary value problem in which the order of differential equations is more than the number of available boundary conditions. It is shown that by augmenting the boundary conditions based on asymptotic structures at infinity it is possible to obtain numerical solutions of the nonlinear hydromagnetic equations. Effects of uniform suction or blowing past the porous plate, exerted magnetic field and rotation on the flow phenomena, especially on the boundary layer structure near the plate, are numerically analysed and discussed. The flow behaviours of the Newtonian fluid and second‐, third‐ and fourth‐order non‐Newtonian fluids are compared for the special flow problem, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

Numerical solution of transonic and subsonic flow of compressible inviscid and viscous fluid

The work presents two numerical solutions of compressible flows problems with high and very low Mach numbers. Both problems are numerically solved by finite volume method and the explicit MacCormack scheme using a grid of quadrilateral cells. Moved grid of quadrilateral cells is considered in the form of conservation laws using Arbitrary Lagrangian–Eulerian method. In the first case, inviscid transonic flow through cascade DCA 8% is presented and the numerical results are compared to experimental data. The second case, numerical solution of unsteady viscous flow in the channel for upstream Mach number M_∞=0.012 and frequency of the wall motions 100 Hz is presented. The unsteady case can represent a simplified model of airflow coming from the trachea, through the glottal region with periodically vibrating vocal folds to the human vocal tract. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the effect of the wall roughness and free stream turbulence on the boundary layer development

Development of a flat plate boundary layer (grad P = 0) was investigated in the closed type wind tunnel (0.5 × 0.9 × 2.7 m³). The plate was either aerodynamically smooth or covered with a thin plate (10 mm thick) with the surface made from the sand paper (grits 80). FST is generated by square mesh plane grids/screens set across the flow as to produce homogeneous, close to isotropy turbulence with various scales of velocity and length. The surface roughness was of transient type. The comparison is made of the effect of various grouping of the wall roughness and FST on the boundary layer development. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical study of vortex eccentricity in a gas cyclone

This paper presents a numerical study of the vortex eccentricity in a gas cyclone and its effect on the performance of the cyclone. The gas flow in the cyclone was modeled as an unsteady flow by the Navier–Stokes equations with the Reynolds Stress Model (RSM) as the turbulence model. The particles were modelled by the Lagrangian particle tracking (LPT) approach in an unsteady gas flow. Gas cyclones with the same dimensions and total flow rates but different numbers of inlets were simulated with the inlet velocity varying from 12 to 20 m/s. The vortex eccentricities in different cases were analyzed in terms of radial deviation and angular deviation. In addition, the frequency of the precessing vortex core (PVC) was analyzed by the fast Fourier transform (FFT). The results show that the vortex center in the single inlet cyclone has a great eccentricity and its precession center is also different from the geometric center, which reduces the particle collection efficiency. The increase in the symmetry of the inlet causes only a very small increase in the pressure drop in the simulated cases, but it can significantly reduce the vortex eccentricity, particularly by eliminating the eccentricity of the PVC center. The improvement of the vortex eccentricity can generally increase the collection efficiency for particles greater than 2.0 µm. The increase of the collection efficiency is mainly because the symmetrical vortex can restrain the short-circuiting flow of particles. The results can improve the understanding of the vortex flow in gas cyclones which may guide the optimization of gas cyclones.     

Two-dimensional ultrasound Doppler velocimeter for velocity field measurements of liquid metal flows

We present a novel fluid flow measurement system based on the pulsed-wave ultrasound Doppler velocimetry being able to determine two-dimensional velocity fields. It applies for the measurement of unsteady liquid metal flows driven by electromagnetic forces concerning the research field of magnetohydrodynamics. The application of advanced processing techniques enable high data acquisition rates and concurrently a high spatial resolution facilitating to resolve transient liquid metal flow structures which could not been acquired so far. An experimental setup utilizing liquid metal in a cubic vessel exposed to a stationary rotating magnetic field was used to validate the reliability of the measurement system. The swirling fluid motion in its horizontal section could be resolved into a velocity field grid of 24 × 24 vectors while achieving frame rates of about 30 fps. Results from a further study driving liquid metal in a cylindrical vessel by a pulsed rotating magnetic field are presented. (© 2011 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.     

On some unsteady flows of a non-Newtonian fluid

Some properties of unsteady unidirectional flows of a fluid of second grade are considered for flows impulsively started from rest by the motion of a boundary or two boundaries or by sudden application of a pressure gradient. Flows considered are: unsteady flow over a plane wall, unsteady Couette flow, flow between two parallel plates suddenly set in motion with the same speed, flow due to one rigid boundary moved suddenly and one being free, unsteady Poiseuille flow and unsteady generalized Couette flow. The results obtained are compared with those of the exact solutions of the Navier–Stokes equations. It is found that the stress at time zero on the stationary boundary for the flows generated by impulsive motion of a boundary or two boundaries is finite for a fluid of second grade and infinite for a Newtonian fluid. Furthermore, it is shown that for unsteady Poiseuille flow the stress at time zero on the boundary is zero for a Newtonian fluid, but it is not zero for a fluid of second grade.     

Homotopy analysis of unsteady boundary layer flow adjacent to permeable stretching surface in a porous medium

This communication deals with the unsteady boundary layer flow of a viscous fluid in porous medium started due to the impulsively stretching of the plane wall. The wall is assumed to be porous so that suction or injection is possible. Complete analytic solution which is uniformly valid for all the dimensionless times 0 ⩽ τ < 0 in the whole spatial region 0 ⩽ η < ∞ is obtained by a purely analytic technique, namely the homotopy analysis method. Results are discussed through graphs.     

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)     

Discrete Adjoint based Sensitivity Analysis for Optimal Active Flow Control of High-Lift Configurations

Blowing and suction type of active flow control techniques can be used to delay the flow separation on the flap and to enhance the aerodynamic performance of high-lift configurations. Effective separation control and maximum enhancement in the mean lift coefficient are achieved by finding the optimal actuation parameters. The optimal set of actuation parameters can be obtained by combining the gradient based algorithms with discrete adjoints. In the present work, an unsteady discrete adjoint incompressible RANS solver is developed for the optimal active separation control. The adjoint solver is applied to the test case of active flow control on the flap of a 2D high-lift configuration. Sensitivity gradients are presented to demonstrate the accuracy of unsteady adjoint RANS solver. (© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interactive computing methods for aeroelastic analysis of turbomachinery

An interaction viscous‐inviscid method for efficiently computing steady and unsteady viscous flows is presented. The inviscid domain is modeled using a finite element discretization of the full potential equation. The viscous region is modeled using a finite difference boundary layer technique. The two regions are simultaneously coupled using the transpiration approach. A time linearization technique is applied to this interactive method. For unsteady flows, the fluid is assumed to be composed of a mean or steady flow plus a harmonically varying small unsteady disturbance. Numerically exact nonreflecting boundary conditions are used for the far field conditions. Results for some steady and unsteady, laminar and turbulent flow problems are compared to linearized Navier‐Stokes or time‐marching boundary layer methods. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Quenching of reaction by cellular flows

We consider a reaction-diffusion equation in a cellular flow. We prove that in the strong flow regime there are two possible scenarios for the initial data that is compactly supported and the size of the support is large enough. If the flow cells are large compared to the reaction length scale, propagating fronts will always form. For small cell size, any finitely supported initial data will be quenched by a sufficiently strong flow. We estimate that the flow amplitude required to quench the initial data of support L₀ is The essence of the problem is the question about the decay of the L^∞-norm of a solution to the advection-diffusion equation, and the relation between this rate of decay and the properties of the Hamiltonian system generated by the two-dimensional incompressible fluid flow. Received: September 2004 Revision: September 2005 Accepted: September 2005     

Determining the limits of geometrical tortuosity from seepage flow calculations in porous media.

Recent investigations have found a distinct correlation of effective properties of porous media to sigmoidal functions, where one axis is the Reynolds number Re and the other is the effective property dependent of Re, Θ_i = S_i(Re). One of these properties is tortuosity. At very low Re (seepage flow), there is a characteristic value of tortuosity, and it is the upper horizontal asymptote of the sigmoidal function. With higher values of Re (transient flow) the tortuosity value decreases, until a lower asymptote is reached (turbulent flow). Estimations of this parameter have been limited to the low Reynolds regime in the study of porous media. The current state of the art presents different numerical measurements of tortuosity, such as skeletization, centroid binding, and arc length of streamlines. These are solutions for the low Re regime. So far, for high Re, only the arc length of stream lines has been used to calculate tortuosity. The present approach involves the simulation of fluid flow in large domains and high Re, which requires numerous resources, and often presents convergence problems. In response to this, we propose a geometrical method to estimate the limit of tortuosity of porous media at Re → ∞, from the streamlines calculated at low Re limit. We test our method by calculating the tortuosity limits in a fibrous porous media, and comparing the estimated values with numerical benchmark results. Ongoing work includes the geometric estimation of different intrinsic properties of porous media. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)