Suction Velocity Research Articles

CFD Investigation of Separation Control on a Vertical Axis Wind Turbine: Steady and Unsteady Suction

High-fidelity two-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) simulations are employed to investigate the influence of boundary layer suction through a slot located near the leading edge of a vertical axis wind turbine operating in dynamic stall. The analysis includes both steady and unsteady suction with different frequencies. The results shows that: (i) when the suction slot is located within the chordwise extent of the laminar separation bubble, dynamic stall can be avoided with minimal suction amplitude; (ii) the most promising suction location is the most upstream suction location studied, at 8.5%c where c is the blade chord length; (iii) the suction only needs to be applied during the azimuthal angles when dynamic stall occurs; (iv) the oscillation frequency of the suction velocity has insignificant influence on the obtained turbine power gain; (v) applying unsteady suction is interesting as it reduces the energy consumption of the suction system, thus the net power gain.

Computation of Critical Submergence Depth to Avoid Surface Vortices at Vertical Pumps Intakes

The pumping station became widely used in many fields. Free surface vortices at intakes of pumps are not favorable. It may cause noise, excessive vibration, damage to the pumping structure, reduction in efficiency and flow for hydro-turbines, etc. One of the important problems encountered during the pump intake design is the depth of submergence and other design parameters to avoid strong free-surface vortices formation. This study aims to compute the critical submergence depth with some geometrical and hydraulic limitations by using Computational Fluid Dynamic (CFD) package. The mathematical model was validated with a laboratory model that had been conducted. The model of three intake pipes was investigated under five different submergence depth (S), three different spaces between intake pipes (b), and five different suction velocities (v). The results showed the best operation cases when the space between intake pipes (b) equal to 4D, the submergence depth of water is equal or greater than 1.25 from the bell mouth diameter of intake pipe (D), and the suction velocity less than 2 m/s. The worst case was when the space between the suction pipe (b) was (2D), in this case, the vortex appeared at submergence depth (S/D = 2) with suction velocity 3 m/s.

Experimental and numerical study of the effect of perimeter jet enhancement on the capture velocity of a rectangular exhaust hood

Using a radial jet at the outer edge of a circular exhaust hood can effectively reduce the velocity attenuation in front of the exhaust hood. The velocity distribution, especially the centerline velocity, is a key parameter in the effective design and use of an exhaust hood. In this paper, based on the derivation of the prediction formula of the suction velocity, the study of the suction velocity of jet-enhanced rectangular exhaust hood that considers the jet enhanced, viscous and geometrical effects is presented via the experimental and numerical methods. To understand the suction velocity distribution, the centerline velocity is investigated by varying the jet velocity, jet angle, jet slot width, exhaust rate and aspect ratio of the hood. It is found that the critical jet velocity condition is a most effective and energy-saving working condition for this kind of hood. Based on the critical jet velocity working condition, reducing the jet angle instead of increasing the exhaust rate is an airflow-rate-saving way to further increase the centerline velocity at a relatively large distance away from the hood opening. Calculation equations for the prediction of the centerline velocity of rectangular jet-enhanced exhaust hood are proposed. The error of the formula is proven to be within 10%, and its application range is proposed. The outcome of this work can help to efficiently design and use this type of energy-saving exhaust hood in residential and industrial local ventilation applications.

Performance Evaluation of Hybrid Air Purification System with Vegetation Soil and Electrostatic Precipitator Filters

This paper describes designing, manufacturing, and evaluating an eco-friendly modular-type air purification system to enhance the removal efficiency of fine particulate matter (PM) in urban public spaces, especially in hotspots. This system consists of artificial soil based-vegetation and electrostatic precipitator (ESP) filters. Unlike the so-called passive removal method, which adsorbs fine PM only by the leaves of plants, the vegetation soil filter based on multi-layered different artificial soils adopts an active removal method in which air purification is performed in the soil itself, bypassing external air by using the air circulation fan in the soil. The ESP filter is designed and evaluated to have a high fine PM removal efficiency, even at high suction velocity, to remove large amounts of outdoor fine PM. Throughout the experimental measurements on the hybrid air purification system with vegetation soil and ESP filters, it is observed that the vegetation soil filter has a 78.5% reduction efficiency for PM2.5 and a 47% for PM10 at the inlet air velocity of 0.15 m/s. The ESP filter also has a 73.1% reduction efficiency for PM2.5 and 87.3% for PM10 at an inlet air velocity of 3 m/s. Based on the performance evaluations of the vegetation soil filter and the ESP filter, it is noted that each individual module will be applied to an air purification tower with vertical expansion and installed in a high concentration area of fine PM in a downtown area to contribute to the fine PM reduction in the community.

Multivariate Jeffrey Fluid Flow past a Vertical Plate through Porous Medium

An analysis is suggested to study the impact of Hall currents in Jeffrey fluid which is chemically reactive through a porous medium limited by a semi-infinite vertical permeable plate within the existence of heat generation. An evenly distributed magnetic field turns vertically on the porous surface which absorbs the Jeffrey fluid with a changed suction velocity with time. The analytical expressions are solved by means of three terms harmonic and non-harmonic functions. Statistical calculations are carried out for the point of resultant outcomes which are shown graphically and the impacts of the parameters velocity, temperature and concentration are listed. In addition, the results of skin-friction coefficient (τ), Nusselt number (Nu) and Sherwood number (Sh) are taken in to consideration. It is revealed that the impact of the Hall parameter on the channel velocities and skin friction coefficient is subjected to the estimation of the wall suction parameter.

Experimental justification of poor thermal and flow performance of solar chimney by an innovative indoor experimental setup

Power generation of solar chimneys effectively depends on their thermal performance. The manufactured samples, available in the literature, have many functional weaknesses and their obtained output power is much lower than expected. Due to the variation of operating conditions (i.e. solar radiation, wind velocity, ambient temperature etc.) of outdoor experimental setups, the thermal performance and heat transfer coefficient cannot be properly evaluated for those systems. Herein, a new indoor test setup is employed to study the steady-state thermal performance of the solar chimney collector while the effects of environmental changes on the tests are minimized. The flexibility of the custom-designed experimental setup allows the investigation of effects of angle of collector, height of chimney, and the area of collector on the convection heat transfer coefficient, velocity and temperature distribution at various points, and the ratio of energy transferred to that introduced to the collector (RET). Our analyses show that the poor performance of common solar chimneys can be justified by existence of secondary flows beneath the collector. The results are important in designing the next generation of solar chimneys with optimized geometry and enhanced thermal efficiency. Results show that, high thermal and frictional losses, due to formation of secondary flows, cause less than 9% of the input energy to enter the chimney and generate power. The collector angle is known the most responsible parameter for controlling formation of the secondary flows. For the divergent collector, increasing the collector angle from 0 to 14 reduces the thermal losses by 1.72% and enhances the heat transfer coefficient by 41.6%. On the other hand, higher amounts of secondary flows in convergent collectors, increases thermal losses by 98.8%. Based on the obtained results, increasing the collector area, as commonly suggested by many researches, while increasing the suction velocity, significantly reduces thermal efficiency of the chimney; This issue along with increasing the manufacturing costs, indicate that increasing the area acts marginally and cannot be a good idea to improve chimney performance most of the times.

Effect of Flow Separation Control with Suction Velocity Variation: Study of Flow Characteristics, Pressure Coefficient, and Drag Coefficient

Flow separation is expected to have the effect of increasing aerodynamic drag due to decreased pressure distribution at the rear of the vehicle. The faster the flow separation occurs, the lower the pressure distribution is in the area, thereby reducing vehicle performance. Therefore, flow modification is needed with expected effects on the separation delay and the reduction in wake and vortex formation. This modification can be done through the application of suction active control in the separation area. The research is intended to analyze the effect of suction active control on flow characteristics, pressure distribution and aerodynamic drag on vehicle models with suction velocity variations. The test model used is an Ahmed model modified by changing the orientation of the flow. The study used a numerical computational approach with a standard k-epsilon turbulence model at 19.4 m/s upstream velocity. Results revealed that the use of flow active control was able to reduce wake and vortex formation through separation delay and to increase the minimum pressure coefficient by 73% on the model with Usc₂ suction velocity of 0.5 m/s, gaining the highest drag coefficient reduction of 10.897% in the same model.

Influence of boundary layer flow suction on trailing edge noise generation

The effect of uniform inclined flow suction on an equilibrium turbulent boundary layer developing over a flat plate is experimentally investigated for aeroacoustic purposes. Simultaneous measurements of streamwise velocity with hot-wire anemometry, and surface pressure fluctuations using flush-mounted microphones were performed at various locations downstream of the flow control treatment. The paper discusses the effects of flow suction on the turbulent quantities describing the boundary layer, and the associated hydrodynamic pressure field to assess the effects of flow suction on trailing edge noise generation. Two parameters were varied, the inclination of the flow suction velocity (α) and the flow suction severity (σ). The former is the angle of flow suction with respect to the free-stream flow (α = 30°, 50°, 70° and 90°), while the latter is the ratio of momentum deficit within the boundary layer to the momentum of flow suction (σ = 2.5–9.1). Flow suction reduces the height of the boundary layer, which results in an increase of mean shear. The flow energy content within the boundary layer is reduced by suction with the most significant amount of reduction is observed within the logarithmic region. A moderate energy increase is observed in the buffer layer. These effects strengthen with growing suction severity until σ ≈ 6. Above this point, no further changes are observed in the turbulent quantities. The reduction in the size and energy content of the logarithmic layer is responsible for the reduction of the surface pressure fluctuations at mid-frequencies, while the increased energy content in the buffer layer increases the pressure spectral content at high frequencies. The estimates of the far-field trailing edge noise show that flow suction leads to a noise reduction at mid-frequencies with penalties observed at low and high frequencies. Flow suction at an angle of α = 70° and σ ≈ 6 exhibits the best performance in reducing the estimated far-field overall noise levels. Above this point, further increasing the flow suction severity does not provide any additional noise reduction benefits.

Mixed Convection on MHD Flow with Thermal Radiation, Chemical Reaction and Viscous Dissipation Embedded in a Porous Medium

In this paper, a theoretical analysis has been made to study the effect of mixed convection MHD oscillatory Couette flow in a vertical parallel channel walls embedded in a porous medium in the presence of thermal radiation, chemical reaction and viscous dissipation. The channel walls are subjected to a constant suction velocity and free stream velocity is oscillating with time. The channel walls are embedded vertically in a porous medium. A magnetic field of uniform strength is applied normal to the vertical channel walls. The nonlinear and coupled partial differential equations are solved using multi parameter perturbation techniques. The effects of physical parameters, viz., the radiation absorption parameter, Prandtl number, Eckert number, dynamic viscosity, kinematic viscosity, permeability of porous medium, suction velocity, Schmidt number and chemical reaction parameter on flow variables viz., temperature, concentration and velocity profile have been studied. MATLAB code is used to analyze theoretical facts. The important results show that an increment in the radiation absorption parameter and permeability of porous medium results in an increment of the temperature profile. Moreover, an increment in the Prandtl number, Eckert number and dynamic viscosity results in a decrement of the temperature profile. An increment in suction velocity results in a decrement of the velocity profile. An increment in the Schmidt number, chemical reaction parameter and kinematic viscosity results in a decrement of the concentration profile.

UNSTEADY FLOW OF CASSON LIQUID FILM ON A STRETCHING SHEET WITH RADIATIVE HEAT TRANSFER

Flow of thin uniform Casson liquid film on an unsteady porous stretching sheet is studied both analytically and numerically. Combined effects of porosity, transverse magnetic field, thermal radiation, suction/injection, and thermocapillarity are considered for analysis. Results showed that the film thickness enhances with the Casson parameter, porosity parameter, Hartmann number and injection velocity but the opposite phenomenon is observed for suction velocity, thermocapillary and thermal radiation parameters. The behavior of the temperature field has been discussed with different physical parameters.

MHD Flow of a Stratified Dusty Fluid in Porous Medium Past An Infinite Porous Vertical Plate

In this article, MHD flow of a stratified dusty viscous fluid through a porous medium past an infinite porous vertical plate with time dependent suction has been investigated assuming that the free stream oscillates about a constant mean and suction velocity as exponentially decreasing function of time. The liquid velocity, particle velocity and skin friction for liquid and dust particles have been obtained. The effect of magnetic field, Grashoff number, permeability and stratification factor on velocity profiles and skin friction have been discussed with the help of graphs and tables.

Evaluation of KTT-2 Pneumatic Peat Harvesting Machine’s Nozzle Suction Capacity

Milled peat harvesting with pneumatic harvesting machines has a number of doubtless advantages over mechanical harvesting: operating time decreases down to 1 day, additional milling of wet peat deposit during the harvesting is not needed, list of necessary machinery decreases. However, in the late 1980s, due to the imperfection of the design of pneumatic machines, this method began to be used less and less frequently. Market appearance of KTT-2 and MPTU-30 pneumatic harvesting machines promoted their introduction into operating processes of peat extractive companies. According to feedback from operators, these machines work well with high-moor peat but work much worse with low- moor peat which is of higher density. One of basic parameters of pneumatic harvesting is suction velocity, that’s why field experiment for evaluation of KTT-2 machine suction nozzle performance was held. Experiment results enabled us to plot velocity diagram of suction flow and find maximum suction velocity: 26.74 m/s. Velocity diagram analysis showed distance from the nozzle – air flow speed relation. We calculated size of suction flow’s active area and offered methods of active area sizing.

Thermal radiation and chemical reaction effects on a porous media flow with heat generation

The present study investigates the effects of thermal radiation and chemical reaction on the unsteady free-convection flow over a vertical infinite porous plate under the action of a transverse magnetic field, suction velocity and heat sources. The incompressible Newtonian fluid model is considered and the convective, diffusive and reactive processes are incorporated to describe the time evolution of fluid temperature and mass concentration within the flow. These led to a coupled system of partial differential equations alongside the relevant conditions. A decoupled finite difference scheme is formulated for the dimensionless model, and the resulting linear system are solved, using the Gauss Seidel algorithm, for the field variables. The stability and convergence of the method are proved, and the results are numerically verified using the method of manufactured solutions. The numerical results are presented and the effects of the various flow parameters on the field variables are discussed. It is observed that an increase in the radiation parameter decreases the temperature, while an increase in the chemical reaction parameter leads to an increase in the concentration

Mathematical Modelling and Exact Solution of Fully Developed Natural Convection Flow in a Vertical Permeable Micro-Channel in the Existence of Thermal Radiation

Analytical solution to the steady natural convection flow in a vertical permeable micro-channel in the existence of thermal radiation is presented. Specifically, the effects of velocity slip and temperature jump is considered. Due to the presence of thermal radiation, the momentum and energy equations are coupled system of ordinary differential equations. The effect of various parameters controlling the physical situation such as thermal radiation, temperature difference, Knudsen number, and fluid wall interaction are discussed with the aid of line graphs and tables. The results of the analyses indicated that the rate of heat transfer on the permeable micro-channel walls can be enhanced with the increase in the temperature difference parameter. Furthermore, the skin friction on the cold wall could be reduced with the increase in suction/injection parameter. For both asymmetric heating situations, increase in suction velocity (negative value of S) results to increase volume flow rate.

Computational approach to the turbulent boundary layer of a rotating porous cylinder with strong suction based on an algebraic turbulence model

Many important technical systems operate according to the principles of momentum transfer, heat transfer and mass transfer near the surface of a cylinder rotating in a confined space. Numerical study of the Taylor–Couette flow with an imposed inward radial throughflow in the rotating inner cylinder demonstrates the possibility of flow stabilization to prevent large-scale vortices up to high rotation rates. The fluid is involved in rotation by the inner rotating cylinder only in a thin layer (boundary layer) near the cylinder surface with strong radial throughflow. Turbulence in the boundary layer may occur before the centrifugal instability, and there is a way to control the thickness of the layer as well as shear intensity and turbulence intensity near the surface of the rotating cylinder. The present work develops a compact and robust approach for the turbulent boundary layer calculation on the surface of a rotating cylinder based on an algebraic turbulence model. The generalized Cebeci–Smith model was supplemented with Richardson number-based corrections to address the centrifugal force action. It also generalized the correction used for accounting for wall suction. Analytical corrections and empirical coefficients of the model were tuned to reflect the coupled influence of the specific flow conditions. The approach, comprehensively verified in the preliminary research, was adapted to the desired geometric configuration and checked. The modified algebraic turbulent model was incorporated into an iterative calculation method developed especially for the case of axisymmetric boundary layers. The results of the algebraic turbulence model with different combinations of rotational speed and suction velocity agree well with the simulation results of the Reynolds stress turbulent model. The method proposed provides efficient practical engineering calculations and can be applied up to approaching the centrifugal instability in the boundary layer.

Numerical Solution of the Flow of the Upper Convicted Maxwell Fluid in Boundary Layer of Stretching Sheet in the Presence of MHD

In this study, the solution of the upper limit of Maxwell liquid flow in the boundary layer stretch Sheet MHD side extension with integrated bvp5c digital solution from MATLAB. An approximate solution has been derived for suction velocity, and injection velocity, skin friction. Use the graph to discuss the results obtained to track the impact of different flow parameters.

Transport and Deposition of Large Aspect Ratio Prolate and Oblate Spheroidal Nanoparticles in Cross Flow

The objective of this paper was to study the transport and deposition of non-spherical oblate and prolate shaped particles for the flow in a tube with a radial suction velocity field, with an application to experiments related to composite manufacturing. The transport of the non- spherical particles is governed by a convective diffusion equation for the probability density function, also called the Fokker–Planck equation, which is a function of the position and orientation angles. The flow is governed by the Stokes equation with an additional radial flow field. The concentration of particles is assumed to be dilute. In the solution of the Fokker–Planck equation, an expansion for small rotational Peclet numbers and large translational Peclet numbers is considered. The solution can be divided into an outer region and two boundary layer regions. The outer boundary layer region is governed by an angle-averaged convective-diffusion equation. The solution in the innermost boundary layer region is a diffusion equation including the radial variation and the orientation angles. Analytical deposition rates are calculated as a function of position along the tube axis. The contribution from the innermost boundary layer called steric- interception deposition is found to be very small. Higher order curvature and suction effects are found to increase deposition. The results are compared with results using a Lagrangian tracking method of the same flow configuration. When compared, the deposition rates are of the same order of magnitude, but the analytical results show a larger variation for different particle sizes. The results are also compared with numerical results, using the angle averaged convective-diffusion equation. The agreement between numerical and analytical results is good.

Association study between basis weight distribution of melt-blown web and air velocity distribution on the collector

Basis weight distribution of melt-blown fibrous web is attracting considerable interest because it directly affects the application performance of nonwovens. Basis weight distribution is strongly determined by the air velocity distribution on the top surface of the collector. However, how the mentioned air velocity distribution affects basis weight distribution is still unknown. Measurement of the mentioned two distributions is the first step to investigate the influence mechanism. Thus, this work measures air velocity distribution on the collector under different processing conditions. Then the fibrous web is fabricated under same conditions. After fabrication, the basis weight distribution of fibrous web is analyzed by using image analysis method. The effects of processing conditions, including velocity of air jet and suction, die-to-collector distance, and pore size of the collector, on the air velocity distribution and basis weight distribution are quantitatively analyzed. The results demonstrate that higher suction speed and air jet velocity, smaller die-to-collector distance, and larger pores of the collector result in higher air velocity on the top surface of the collector. On the other hand, higher suction speed and air jet velocity, larger die-to-collector distance, and smaller pores of the collector create evenly distributed basis weight. In addition, the correlation between basis weight distribution and air velocity distribution is discussed based on the Spearman correlation analysis method. High correlation demonstrates that air velocity and basis weight display similar distribution on the collector. Namely, both of air velocity and basis weight in the central zone are larger than those in the fringe zone on the collector.

Analysis of Two Finite Dierence Schemes for a Channel Flow Problem

Aims/ Objectives: To investigate the influence of a model parameter on the convergence of two finite difference schemes designed for a convection-diffusion-reaction equation governing the pressure-driven flow of a Newtonian fluid in a rectangular channel.Methodology: By assuming a uni-directional and incompressible channel flow with an exponentially time-varying suction velocity, we formulate a variable-coefficient convectiondiffusion- reaction problem. In the spirit of the method of manufactured solutions, we first obtain a benchmark analytic solution via perturbation technique. This leads to a modified problem which is exactly satisfied by the benchmark solution. Then, we formulate central and backward difference schemes for the modified problem. Consistency and convergence results are obtained in detail. We show, theoretically, that the central scheme is convergent only for values of a model parameter up to an upper bound, while the backward scheme remains convergent for all values of the parameter. An estimate of this upper bound, as a function of the mesh size, is derived. We then conducted numerical experiments to verify the theoretical results.Results: Numerical results showed that no numerical oscillations were observed for values of the model parameter less than the theoretically derived bound.Conclusion: We therefore conclude that the theoretical bound is a safe value to guarantee non-oscillatory solutions of the central scheme.

Dynamic feedback control through wall suction in shear flows

AbstractFlow control is of interest in many open and wall‐bounded shear flows in order to reduce drag or to avoid sudden large fluctuations that may lead to material failure. An established means of control is the application of suction through a porous wall. Here, we combine suction with a feedback strategy whereby the suction velocity is adjusted in response to either the kinetic energy or the shear stress at the bottom wall. The control procedure is then used in an attempt to stabilize invariant solutions and to carry out direct numerical simulations with a prescribed value of the friction coefficient.