Showing papers in "Journal of Applied Fluid Mechanics in 2019"

A Numerical Analysis of Laminar Forced Convection and Entropy Generation of a Diamond-Fe3O4/Water Hybrid Nanofluid in a Rectangular Minichannel

Abstract: The convective heat transfer and entropy generation of diamond-Fe3O4/water hybrid nanofluid through a rectangular minichannel is numerically investigated under laminar flow conditions. Nanoparticle volume fractions for diamond-Fe3O4/water hybrid nanofluid are in the range 0.05-0.20% and Reynolds number varies from 100 to 1000. The finite volume method is used in the numerical computation. The results obtained for diamond-Fe3O4/water hybrid nanofluid are compared with those of diamond/water and Fe3O4/water conventional nanofluids. It is found that 0.2% diamond-Fe3O4 hybrid nanoparticle addition to pure water provides convective heat transfer coefficient enhancement of 29.96%, at Re=1000. The results show that diamond-Fe3O4/water hybrid nanofluid has higher convective heat transfer coefficient and Nusselt number when compared with diamond/water and Fe3O4/water conventional nanofluids. For diamond-Fe3O4/water hybrid nanofluid, until Re=600, the lowest total entropy generation rate values are obtained for 0.20% nanoparticle volume fraction. However, after Re=800, diamond-Fe3O4/water hybrid nanofluid with 0.20% nanoparticle volume fraction has the highest total entropy generation rate compared to other nanoparticle volume fractions. A similar pattern emerges from the comparison with diamond/water and Fe3O4/water conventional nanofluids. For 0.2% nanoparticle volume fraction, diamond-Fe3O4/water hybrid nanofluid and diamond/water nanofluid have their minimum entropy generation rate at Re=500 and at Re=900, respectively. Moreover, this minimum entropy generation rate point changes with nanoparticle volume fraction values of nanofluids.

Monitoring the Performance of Centrifugal Pump under Single-Phase and Cavitation Condition: A CFD Analysis of the Number of Impeller Blades

Abstract: In this current study, the transient numerical calculations using CFD are carried out under different number of impeller blades for the flow field within a centrifugal pump under single-phase and cavitation condition. Both qualitative and quantitative analyses have been carried out on all of these results in order to better understand the flow structure within a centrifugal pump under both single-phase and cavitation. Also, the investigation using different number of impeller blades relating to the static pressure, velocity magnitude and vapour volume fraction variations have been analysed. Fluctuations pressure in both time and frequency domains at the impeller and volute of the pump also investigated. As a result, the pressure and velocity were gradually increased from inlet to outlet of the pump. Pressure at the impeller outlet was higher than the pressure at other parts due to high interaction between impeller and volute tongue region. The distribution of volume fraction first occurs at the inlet eye of impeller. Furthermore, the cavitation increases as the number of impeller blades and flow rate increase. The length of the cavity was increased when low pressure at the inlet impeller (eye) decreased at Z=5 blades cavitation was affected highly at the suction of impeller compared to other number of blades particularly at high flow rate.

An Experimental Study on Vibration Signatures for Detecting Incipient Cavitation in Centrifugal Pumps Based on Envelope Spectrum Analysis

Abstract: Vibration signatures have been studied for monitoring the condition of centrifugal pumps by many researchers, however, there is limited published information on the application of vibration analysis to incipient pump cavitation. The paper will review the state of the art in the field and develops an effective signal processing approach based on envelope spectral analysis to close this gap. A purpose-built test rig was employed for recording vibration signals from a centrifugal pump at a wide range of operating conditions. The collected data was then processed using time domain and frequency domain analysis methods. The study has shown that the vibration energy concentrated mainly in the frequency range between 8-15 kHz. At the flow rates less than 300(l/min), i.e. in design flow rate range, the vibration amplitudes remain constant and does not show a notable change by the flow rate increase. However, a notable increase in the vibration level is evident when the flow rate exceeds 300 (l/min). Analysis the result of filtered vibration signatures has revealed that vibration signal parameters: peak value, root mean squared (RMS), crest factor along with vibration spectrum allow development of cavitation (for the flow rates higher than 300 (l/min)) to be diagnosed reliably. However, conventional signal processing methods may not produce a clear separation of the incipient cavitation from the healthy baseline. Therefore, envelope spectrum analysis has been carried out on recorded vibration signatures to detect the onset of cavitation from the baseline and satisfactory results have been perceived.

CFD Analysis of Aerodynamic Drag Effects on Vacuum Tube Trains

Abstract: Aerodynamic aspects of train shapes suitable for Vacuum Tube Train System are investigated in this paper. Three feasible geometries for the vacuum tube train system have been considered and modelled in three dimensions and have been computationally studied using the commercial software Ansys Fluent. Aerodynamic drag loads on these geometries have been calculated under different tube pressures and speeds of the train, which provide insight on various operating parameters that need to be considered while designing the vacuum tube train system. The present computational research shows that, the suitable vacuum pressure, and different shapes of head and tail of the train have significantly effects the drag force of the vacuum train in the tunnel. Overall, the elliptical train shape with a height to base ratio of 2:1 is more efficient for aerodynamic drag reduction of the vacuum tube train at the vacuum tube pressure of 1013.25 Pa.

Effect of Nozzle Pressure Ratio and Control Jets Location to Control Base Pressure in Suddenly Expanded Flows

Abstract: In this paper, computational fluid dynamic (CFD) analysis and experiments have been carried out to study the effect of nozzle pressure ratio, i.e. the ratio of inlet pressure to atmospheric pressure, and the pitch circle diameter of the control jets to regulate the base pressure. The variables considered for the analysis as well as the experiments are the nozzle pressure ratio (NPR), the Mach number (M) and the pitch circle diameter (PCD) of the control jets. The area ratio considered for the study is kept constant at 4.84 while the length to diameter (L/D) ratio of an enlarged duct is set constant at 5. The inertia parameter considered for the study is Mach number. The Mach numbers considered for study are 1.5, 2.0, and 2.5. The nozzle pressure ratio considered for study are 2, 5 and 8. Three different pitch circle diameters of control jets considered for study are 13.1 mm, 16.2 mm and 19.3 mm. From the numerical simulations and the results of the experimental tests, it is found that the control jets are very beneficial to increase the base pressure at higher NPR when the jets issuing from the nozzles are under-expanded. The control jets were able to increase the base pressure value from 160% to 400% at nozzle pressure ratio 8. It is concluded that the parameter D3 is the most effective pitch circle diameter of the control jets to increase the base pressure.

Channel Blockage and Flow Maldistribution during Unsteady Flow in a Model Microchannel Plate heat Exchanger

Abstract: This paper describes the problem of channel blockage as a result of flow maldistribution between the channels of a model mini channel plate heat exchanger consisting of one pass on each leg. Each leg of the heat exchanger contains 51 parallel and rectangular minichannels of four hydraulic diameters namely 461 μm, 571 μm, 750 μm and 823 μm. In addition, a more complex geometry has been investigated where for the sake of breaking the development length the inclined transverse cuts have been incorporated. The moment of liquid phase transition through the exchanger (the working medium: water) was recorded for the mass fluxes ranging from 18.67 to 277.76 kg/m2s in 51 parallel channels with the use of a fast speed camera. The Reynolds numbers Re in the individual channels were from 10.76 to 90.04. The relationship between the mass flux and the size of the minichannels in the presence of the maldistribution is discussed here. The existence of the threshold in the mass flux below which the phenomenon occurs has been shown. Two mechanisms of channel blocking have been recorded and described in detail. A miniscale variation of one of them containing the extended geometry was created as well.

Effect of Slot-Guidance and Slot-Area on Air Entrainment in a Conical Ejector Diffuser for Infrared Suppression

Abstract: A numerical study has been carried out on a new design of ejector diffuser (infrared suppression device). New design conceptualizes exploiting the shape of the slot openings. A circular arc is provided to guide the entrained fluid at the slot openings. Performance of guided-slot ejector diffuser (GED) has been compared with conventional (non-guided-slot) ejector diffuser (NGED) in terms of local and cumulative mass entrainment ratios, temperature distribution and static pressure recovery. Three slot-area variations are also studied, namely (i) increasing slot-area ranging 1 ≤ A0 ≤ 2.02, ( 0 A is area of 1 st slot) (ii) constant slot-area A0 = 1 and (iii) decreasing slot-area ranging 0.49 ≤ A0 ≤ 1. Simulations have been carried out at fixed Reynolds number Re = 1.3  10. It is observed that GED has 3.5% higher cumulative mass entrainment ratio than NGED. GED forms cold annulus region below ejector diffuser wall from 1 slot onwards which results in wall temperatures being close to ambient temperature (300 K). Higher mass entrainment rate and lower wall temperatures make GED a better infrared suppression device but static pressure recovery is better in NGED (Cp = 0.79) compared to GED (Cp = 0.43). Slot-area study reveals that the performance of increasing slot-area for GED and NGED is superior then constant and decreasing slot-area configurations. The cumulative mass entrainment is 20% higher while static pressure recovery is 45% more for the increasing slot-area GED when compared to the decreasing slot-area GED.

Modeling of Erosion Wear of Sand Water Slurry Flow through Pipe Bend using CFD

Abstract: In the present study, erosion wear of a 90 pipe bend has been investigated using the Computational fluid dynamics code FLUENT. Solid particles were tracked to evaluate the erosion rate along with k-ɛ turbulent model for continuous/fluid phase flow field. Spherical shaped sand particles of size 183 μm and 277 μm of density 2631 kg/m are injected from the inlet surface at velocity ranging from 0.5 to 8 ms at two different concentrations. By considering the interaction between solid-liquid, effect of velocity, particle size and concentration were studied. Erosion wear was increased exponential with velocity, particles size and concentrations. Predicted results with CFD have revealed well in agreement with experimental results. The magnitude and location of maximum erosion wear were more severe in bend rather than the straight pipe.

Numerical Flow Simulation and Cavitation Prediction in a Centrifugal Pump using an SST-SAS Turbulence Model

Abstract: The paper handles the subject of the modelling and simulation of the flow inside a centrifugal pump through non-cavitating and cavitating conditions. Operating under cavitation state is so perilous to a pump and can considerably reduce its lifetime service. Hence, to provide highly reliable pumps, it is essential to comprehend the inner flow of pumps. The investigated centrifugal pump comprises five backward curved-bladed impeller running at 900 rpm. The modelling process started with an unsteady numerical analysis under non-cavitating conditions to validate the numerical model and the solver comparing with the available testing data. Due to high Reynolds numbers, turbulence effects have been taken into account by unsteady RANS methods using an SST-SAS turbulence model. The obtained pump performances were numerically compared with the experimental ones, and the outcome shows an acceptable agreement between both. The temporal distribution of the internal flow parameters such as pressure and velocity was then studied. Furthermore, basic investigations of cavitating flow around 3D NACA66-MOD profile using a recently developed and validate cavitation model was established. The verification of the numerical simulation validity was based on comparing calculated and experimental results and presented good agreement. Finally, a 3D simulation of the inception of the cavitating pocket inside the centrifugal pump is performed to analyze the impact of the cavitation in the decrease of the head and efficiency.

On the Reynolds-Averaged Navier-Stokes Modelling of the Flow around a Simplified Train in Crosswinds

Abstract: Currently, there are different computational fluid dynamic (CFD) techniques used to obtain the flow around trains. One of these techniques is the Reynolds-averaged Navier-Stokes (RANS), which is commonly and widely used by industry to obtain the mean flow field around trains in different operating conditions. In order to assess the performance of RANS turbulence modelling for train aerodynamics, five different common RANS modelling have been used in this paper to obtain the flow and the surface pressure around a simplified train model subjected to crosswind; the standard k- model, the realisable k- model, the Re-Normalisation Group (RNG) k- model, the standard k- model and Shear Stress Transport (SST) k- model. The train model was stationary and subjected to crosswind with a 90 yaw angle. The effects of mesh size and spatial discretization scheme on the aerodynamic characteristics of the train were also investigated. The results obtained from the different RANS models were compared to those from published experimental data. In general, all the RANS models provided the pressure distribution trend. However, all k- models overestimate the surface pressure on the train body except the bottom face. The standard k- model underestimates the surface pressure on the train body except the streamwise face. It was shown that the simulation using SST k- model together with a second order discretization scheme provides the closest results to the experimental surface pressure. It could be concluded from the present study that the SST k-model with a second order discretization scheme and y+ around 1.0 is the most appropriate RANS model for simulating the flow around trains subjected to crosswinds.

Pulsatile Flow Investigation in Development of Thoracic Aortic Aneurysm: An In-Vitro Validated Fluid Structure Interaction Analysis

Abstract: Thoracic aortic aneurysm (TAA) is a severe cardiovascular disease with a high mortality rate, if left untreated. Clinical observations show that aneurysm growth can be linked to undesirable hemodynamic conditions of the aortic aneurysm. In order to gain more insight on TAA formation, we developed a computational framework in vitro to investigate and compare the flow patterns between pre-aneurismal and post-aneurismal aorta using a deformable wall model. This numerical framework was validated by an in vitro experiment accounting for the patient-specific geometrical features and the physiological conditions. The complex flow behaviors in the preaneurismal and post-aneurismal aorta were evaluated experimentally by particle image velocimetry (PIV). Our experimental results demonstrated flow behaviors similar to those observed in the fluid-structure interaction (FSI) numerical study. We observed a small vortex induced by the non-planarity of pre-aneurismal aorta near the aortic arch in pre-aneurysmal aorta may explain the aneurysm formation at the aortic arch. We found that high endothelial cell action potential (ECAP) correlates with the recirculation regions, which might indicate possible thrombus development. The promising image-based fluid-structure interaction model, accompanied with an in vitro experimental study, has the potential to be used for performing virtual implantation of newly developed stent graft for treatment of TAA.

Double-Diffusive Bioconvection in a Suspension of Gyrotactic Microorganisms Saturated by Nanofluid

Abstract: This paper focuses on analytical and numerical investigation of double-diffusive bioconvection in a porous media saturated by nanofluid using the modified mass flux condition. Normal mode technique is employed to solve the governing equations of the Brinkman-Darcy model. The Galerkin weighted residual method (singleterm and six-term) is used to obtain numerical solution of the mathematical model. It is found that due to the presence of gyrotactic microorganisms, Rayleigh number is decreased substantially which shows that convection sets in earlier as compared to nanofluid without microorganisms and this destabilizing effect is more predominant for faster swimming microorganisms. modified Darcy number number, Soret parameter, and porosity postpone the onset of the bioconvection, whereas nanoparticle Rayleigh number, bioconvection Rayleigh number, nanoparticle Lewis number, Dufour parameter, Péclet number, and Lewis number pre-pone the onset of bioconvection under certain conditions.

Investigation of Parameters Affecting Axial Load in an End Suction Centrifugal Pump by Numerical Analysis

Abstract: The total force produced in the axial direction on a pump is called axial load and is caused by the pressure difference between the front and rear of the impeller and the hydrostatic force in the suction direction. In a centrifugal pump, 3D computer-aided analysis programs are used to design and reduce R&D and manufacturing costs. In this study, parameters affecting axial load of the centrifugal pump with a single suction and closed impeller were investigated by using the Computational Fluid Dynamics (CFD) method. In this context, the flow rate and the some physical properties such as the back gap of the impeller, wear ring and balancing holes, of the centrifugal pump were investigated to determine how much affected the axial load. The results showed that the wear ring and the balancing holes give rise to effective results on the axial load, while the back gap of the impeller does not affect the large extent. With the design changes made with these parameterizations, the axial force was reduced by up to 60%, whereas the efficiency was decreased by 5%. The loss of efficiency due to this decrease in axial force is negligible. However, higher efficiency values were also found at a different point from the working point where the axial load is lowest.

Detection of Cavitation through Acoustic Generation in Centrifugal Pump Impeller

Abstract: The most common device which transport fluid in industries, agriculture as well as domestic water supply is the centrifugal pump. Based on fluid transfer conditions, several let-downs are occur in the centrifugal pump, cavitation is one among them. The flow pattern at the eye of impeller deviates from the ideal case with the occurrence of cavitation. Due to cavitation, vibration occurs on blades that generates noise in pump. In this study, the acoustics generated in centrifugal pump impeller due to cavitation is detected with the sound pressure by using 3-Dimensional, steady and unsteady state computational fluid dynamics (CFD) analysis of an industrial centrifugal pump impeller. Harmonic force analysis with blade row model helps in finding the sound pressure. The acoustics generated with unsteady-state is compared with cavitation at steady-state CFD analysis. The Reynolds averaged Navier-Stokes equations model as well as Shear Stress Transport (SST) turbulence model are used for the CFD simulation. The results show that the sound pressure calculated increases with the increase in cavitation (i.e. formation of vapour bubbles and sudden drop in head) which shows that high noises are generated by centrifugal pump impeller at lower net positive suction head (NPSH) at a particular discharge.

Performance Improvement and Two-Phase Flow Study of a Piezoelectric Micropump with Tesla Nozzle-Diffuser Microvalves

Abstract: The Present article aims to design a piezoelectric micropump using a combinational form of microvalves with sufficient diodicity in low-pressure gradients. The goal is to enhance the capability of piezoelectric micropumps with Tesla-type valves in order to deliver insulin. Tesla-type valves are in the category of passive valves which have sufficient diodicity in case of high-pressure gradients. However, low mass flow rates are often required in drug delivery devices. In this paper, the performance of MT135 Tesla-type valve in low pressure-gradient flows has been investigated and a range of reunion angles, which have not been studied before has been examined by numerical solutions. Inspired by nozzle-diffuser valve types, some changes in the bypass path of the microvalve have been exerted to boost the diodicity of the valve in low-pressure conditions that resulted in 9.97% increase of diodicity. At last but not least, the velocity gradients in singlephase flow of water has been attained and performance of micropump toward other kinds of flows has been investigated by a volume of fluid (VOF) model including water as the primary phase and air as the secondary one. To complete the analysis, a VOF model consisting of an arbitrary kind of Casson fluid with the primary phase of water was reached and discussed.

Flow Control of Non-Newtonain Fluid using Riga Plate: Reiner-Phillipoff and Powell-Eyring Viscosity Models

Abstract: This article investigates the controlling effects of electromagnetic field generated by Riga plate on the boundary layer flow of non-Newtonian fluid. Two classical viscosity models of non-Newtonian fluids namely; PowellEyring and Reiner-Phillipoff fluid models have been considered to study the different behaviors of nonNewtonian fluid flow. Numerical solution of the problem in the presence of strong suction is obtained using the nonlinear shooting method. The results are studied in terms of modified Hartmann number, non-Newtonian fluid parameters and the Bingham number. Linear regression is performed on the numerical results to present the correlation expression for the skin friction.

Numerical Simulation of Flow around a High-Speed Train Subjected to Different Windbreak Walls and Yaw Angles

Abstract: The prediction of flow around a high-speed train subjected to different windbreak walls and yaw angles has been investigated using steady Shear Stress Transport (SST) k-ω turbulence model at the Reynolds number of 1.0×10 based on the height of the scaled train model. The results show that an effective windbreak wall provide a favourable shielding effect for the train behind it, and force the primary positive pressure on the windward of the train to transfer on the wall. Consequently, the airflow cannot directly act on the train body, and the train is basically in an environment with small negative pressure. The inclined slope (the earth embankment type) windbreak wall shows poor anti-wind performance that should not be used along the new high-speed railways. When designing the windbreak wall, the influences of yaw angles should be taken into account.

Analysis of the Gas Flow in a Labyrinth Seal of Variable Pitch

Abstract: The paper discusses the results of investigations performed for the segments of straight-through labyrinth seals of constant length. Increasing the number of teeth of a segment resulted in a reduction of the pitch length to obtain the slot seals. The phenomena occurring during gas flow in labyrinth and slot seals differ significantly. They are described with different calculation models. The analysis presented in this paper is related to the change of the tightness and the nature of the flow from a straight-through labyrinth seal to a slot seal. The paper includes the results of experimental research and CFD calculations. Models applied for the Neumann and Scharer labyrinth seals as well as the model of the Salzman and Fravi slot seals were discussed. For the Neumann and Scharer models, correction coefficients for the tested geometry were proposed. Based on the assumptions for the said models and the obtained results, the phenomena responsible for the minimization of the leakage were discussed. The leakage rate in segments of different gap heights depending on the number of teeth and the pressure ratio upstream and downstream of the segment has been analyzed. Based on the experimental data, an optimum number of teeth in the segment for minimum leakage was determined. CFD calculations allowed determining the minimum leakage geometry. The experimental data contained in this paper confirm that the determined optimum pitch range is independent of the pressure drop.

Numerical Investigation on the Effects of Internal Flow Structure on Ejector Performance

Abstract: Recent work on ejector performance enhancement indicates that more information on ejector internal flow structure is needed to have a clearer picture of factors and conditions affecting operation and performance of these devices. This paper relies on experimental studies and CFD simulations to identify flow structures occurring under typical ejector refrigeration conditions and primary nozzle geometry and position. Effects on parameter distributions and the resulting operation of the device are given particular attention. The CFD model used for this purpose was validated by using in-house data, generated from an experimental prototype and over a wide range of conditions. The experiments for the selected condition were predicted very satisfactorily by numerical model. The study then focused on the role of the primary nozzle geometry and the distance of the nozzle from the beginning of the mixing chamber (NXP), in locally shaping the flow structure and the related consequences on ejector operation. Simulations on NXP for given operating conditions have shown that an optimum value was always found, and slightly varied the operating conditions within the range considered. Primary nozzle shape changes in terms of outlet diameters for given upstream conditions directly affected the expansion level of the flow. The simulations showed that an optimum range of nozzle exit diameters could be found, for which ejector performance was highest. Moreover, under these conditions it was observed that pressure fluctuations inside the ejector were reduced.

Bluff Body Drag Control using Synthetic Jet

Abstract: The paper presents the results of an experimental investigation wherein the bullet form drag force as a function of oscillating actuator frequency, various voltage and for different orifice/slot configuration are studied. In order to perform the experiment, an axisymmetric bullet shape model with ellipsoidal nose was used in wind tunnel. The synthetic jet actuator was used to flow control at sharp cut end. The experiment was conducted in a wind tunnel with a working diameter of 1000 mm and a maximum velocity of 45 m/s. The measurements were carried out for the Reynolds number from 88000 to 352000 and for relatively large Strouhal numbers up to St = 4.5 based on model external diameter and free stream velocity. While synthetic jet was switched on, drag coefficient has been reduced by -6% and increased by +22% in relation to the case with the synthetic jet was switched off. The synthetic jet has more impact for relatively low free stream velocity and for single axisymmetric orifice.

Effects of Two-Way Turbulence Interaction on the Evaporating Fuel Sprays

Abstract: This article discusses the importance of using different turbulence modulation models in simulation of evaporating sprays. An in-house CFD code has been modified to take into account the effect of considering turbulence modulation by standard or consistent models. These models may predict an augmentation (consistent model) or a reduction (standard model) in the turbulence kinetic energy of continuous phase. Calculations are done in a Eulerian-Lagrangian framework and the effect of injected droplets on turbulent kinetic energy and its rate of dissipation is included in the equations of the continuous phase. Results are shown to be valid by comparing them to Sandia spray A configuration experimental data. Results show that considering the effect of existing droplets in a turbulent combustion chamber can play a major role in having a more accurate CFD simulation. These models can alter the velocity field drastically when droplets are injected into the chamber with a high velocity. As a result, spray characteristics such as evaporation rate is also altered. It can be concluded that modulation models should be used in the simulation of evaporating sprays in order to attain more accurate and realistic results.

Experimental Study on the Friction Drag Reduction of Superhydrophobic Surfaces in Closed Channel Flow

Abstract: Due to the importance of copper and its alloys in marine applications, the main objective of this research is to provide a simple, effective and low cost manufacturing approach to fabricate a superhydrophobic riblet copper surface with high drag reduction capability in laminar and turbulent flow regimes. Therefore, the riblets are produced by wire cut technique on the copper substrate and then by using a wet chemical method, a superhydrophobic coating is produced on the riblet surface. A pressure drop measurement system consists; pump, closed channel flow with a fabricated surfaces on the lower wall, connections and pressure drop transmitter is employed to measure the pressure drop in the close channel flow, for Reynolds number from 300 to 2769, in order to evaluate the ability of the fabricated surface to reduce the friction drag. The experimental results revealed that combining the abilities of the riblet and superhydrophobic surfaces increases the surface’s ability to reduce friction drag. In addition, the riblet surface and superhydrophobic riblet surface on average decreased the friction drag by 10.33% and 42.65% correspondingly in water flow ranging from laminar to turbulent flow regime. Finally, according to the experimental results, the drag reduction performance of riblet surface is improved from 18.9% to 56.9% after superhydrophobic coating.

Influence of the Presence of the Lorentz Force and its Direction on the Suppression of Secondary Flow in Two Different Orifices: A Numerical Study using OpenFOAM

Abstract: The present analysis emphasized the presence of Lorentz force and its directional effect on the fluid flow and its structure in the channel with two differently shaped orifices. The flow through orifice causes the generation of the bubbles or eddies in the downstream flow. In this study, the numerical code is developed in the open source CFD tool kit OpenFOAM. The magnetohydrodynamics (MHD) principle is adopted to achieve the present objectives. Direct numerical simulation (DNS) has been carried out to predict the flow features at fixed Reynolds number of Re = 1000 and blockage ratio of 1:4 with the varying magnetic field. The magnetic field is varied in term of Hartmann number (Ha) in the direction normal to the flow of fluid. The induced Lorentz force considerably occupies the wake flow area downstream of the throat and hence suppressed down the vortices in the flow. The results obtained has the promising effect of suppressing down the vortex flow past two different orifices produced by the electromagnetic pressure gradient. The present study shows the MHD based flow can be significantly employed for the flow past orifice or any arbitrary obstacle in order to achieve the flow without wake region. The current analysis suggests the method of vortex control by producing Lorentz force using magnetic field without modification of geometry or additional use of devices into the system.

An Experimental Investigation of a Passively Flapping Foil in Energy Harvesting Mode

Abstract: Energy extraction through flapping foils is a new concept in the domain of renewable energy, especially when the system is fully driven by incoming free-stream flow, a phenomenon known as flow-induced vibration. To investigate this concept, a water tunnel test-rig was designed and fabricated, where a flat plate foil made from plexiglass performs two-degrees of freedom pitch and plunge motion under the influence of incoming water flow. For this study a power-takeoff system was not introduced, hence energy harvesting performance was evaluated through real-time force and motion measurements with the help of sensors. The energy harvester performed self-sustained flapping motions when the free-stream velocity reached a threshold value, known as the cut-off velocity, which for this test-rig is 0.40 m/s (without sensors) and 0.50 m/s (with sensors). To support these self-sustained flapping motions, inertial mass blocks were placed to provide the necessary inertia especially when the flat plate foil performed the pitching or stroke reversal action. Different inertial mass units (mib = 0.45, 0.90 & 1.35 kg/block) were tested to analyze their effect on the flat plate foil kinematics and its energy harvesting performance. Other parameters such as pitching amplitude (θo = 30, 43 & 60) and free-stream velocity (U∞ = 0.57 m/s, 0.65 m/s and 0.78 m/s) were varied at fixed pivot location (xp = 0.65 chords (c)) to augment the varying inertial mass unit study. In the first section at fixed mib of 0.45 kg/block and xp = 0.65c from leading edge, energy harvesting performance (C̅p & η) was observed to increase with increase in pitching amplitude, while it degraded as the free-stream velocity increased. Best energy harvesting performance of η = 52.5% and C̅p = 1.124 was achieved with mib = 0.45 kg/block, θo = 60 and U∞ = 0.57 m/s. Varying mib also had a considerable effect on the energy harvesting performance of the test-rig, where the mib = 0.90 kg/block case showed a 36.5% and 21.13% decline in performance compared to the mib = 0.45 and 1.35 kg/block cases, respectively at θo = 60 and U∞ = 0.57 m/s. This shows that the energy harvester is sensitive to changes in inertial loads, affecting the forcemotion synchronization which eventually affects its performance.

An Analysis on the Flow Field Structures and the Aerodynamic Noise of Airfoils with Serrated Trailing Edges Based on Embedded Large Eddy Flow Simulations

Abstract: This paper presents the numerical analysis on the aerodynamic flows and noise of airfoils with serrated trailing edges at 5 =1.6 10 Re  . Flow simulations were performed with an embedded large eddy simulation (ELES) method. Two modified airfoils with serrated trailing edges (same widths, different lengths) were studied and compared with the baseline airfoil baseline NACA-0018 airfoil. It is seen that the unsteady lift and drag coefficients of the baseline airfoil A0 have a peak at about 2270Hz, which is close to the tonal noise frequency experimentally observed. Under the flow conditions studied in this research, the longer saw tooth serrations changed the flow fields near the trailing edge, which provides the potential of suppressing the tonal noise. Predictions based on acoustic analogy indicate that the longer saw tooth serrations decreases the overall sound pressure levels. This paper provides a basic understanding of the noise reduction mechanism in the airfoils with serrated trailing edges.

Angle of Attack Investigations on the Performance of a Diverterless Supersonic Inlet

Abstract: An extensive experimental investigation to study the effects of angle of attack (AOA) on the performance of a body-integrated supersonic inlet has been carried out. The present inlet, known as Diverterless Supersonic Inlet (DSI), is utilized with a three-dimensional bump to provide both supersonic flow compression and boundary layer diversion. Experiments were conducted at the presence of a typical fore-body including an elliptical nose to further contemplate the effects of fore-body geometry on the approaching flow. All tests were conducted at a constant free stream Mach number, M∞ = 1.65 zero degrees angle of sideslip (AOS), and at various angles of attack (AOA) ranging from -2 to 6 degrees. The results showed that the present DSI had acceptable performance characteristics for all ranges of AOA tested. It should be noted that the present DSI does not have any moving, adjustable or auxiliary mechanisms as such systems or mechanism are used to improve the performance of an inlet.

Comparison of Interface Description Methods Available in Commercial CFD Software

Abstract: This study addresses the characteristics of the interpolation functions and interface reconstruction routines for the VOF – Volume of Fluid method available in the commercial CFD software ANSYSFLUENT. This software was used because it has both implicit and explicit VOF approaches along with diverse interpolation functions. Some of these functions were compared from different viewpoints: the quality of the reconstructed interface; the ability to preserve the initial mass inside the system (numerical diffusion); and the computing time. To undertake the qualitative and quantitative comparisons, a test problem that combines the classical dam break and slosh tank benchmark problems was used. No analytical solution available was found for this problem, in which the most interesting feature is a high interaction between the velocity field and volume fraction, thus making it ideal for addressing the issue of interface smearing. ANSYS-FLUENT permits using 5 interpolation functions for transient simulations: PLIC, CICSAM, HRIC (explicit and implicit) and the UPWIND scheme, and four when performing steady state ones: BGM, modified HRIC, COMPRESSIVE and UPWIND schemes. Both transient and steady state solutions were analyzed in this study, using all the above schemes, except the UPWIND one for steady state simulations. It was found that, for thinner grids, PLIC, CISAM and the explicit HRIC schemes had similar performances concerning the quality of the reconstructed interface and mass conservation. On the other hand, PLIC shows the best results for coarser grids, being the only to conserve mass for all tests. The computation time was similar for all transient simulation (within each grid). Concerning the steady state simulations, which are, in fact, distorted transient simulations, the BGM and the COMPRESSIVE schemes produced similar results, but BGM consumed more computational time.

Study of Conjugate Heat Transfer from Heated Plate by Turbulent Offset Jet in Presence of Freestream Motion using Low-Reynolds Number Modeling

Abstract: The present study deals with conjugate heat transfer from a heated flat plate by a turbulent offset jet in presence of freestream motion. The turbulent convection in fluid and conduction in solid is solved in a coupled manner by simultaneously satisfying the equality of temperature and heat flux at the solid-fluid interface. The computations have been carried out using low-Reynolds number (LRN) k −ω SST model in the fluid region. The capability of LRN modeling have enabled to solve the entire boundary layer including the thin viscous sublayer due to which Moffatt vortices (secondary recirculation regions) have been captured near the corner of the wall where the turbulence Reynolds number is low. The bottom surface of solid plate is maintained at a constant temperature higher than the jet inlet temperature whereas the jet inlet temperature is same as that of the ambient. The present investigation reports the effects of offset ratio of jet (OR), Reynolds number of flow (Re), solid to fluid thermal conductivity ratio (K), solid slab thickness (S) and freestream velocity (U∞) on conjugate heat transfer arises due to solid and fluid interaction. The offset ratio is varied in the range OR = 3 − 11, Reynolds number in the range Re = 10000 − 25000, solid to fluid thermal conductivity ratio in the range K = 1 − 2000, solid slab thickness in the range S = 1−20 and freestream velocity in the range U∞ = 0.1 − 0.25. The effects of various parameters on the near-wall velocity profile, solid-fluid interface temperature, local Nusselt number variation along the plate, heat flux variation along the plate, etc. have been discussed in detail.

Interaction between Natural and Ventilated Cavitation around a Base Ventilated Hydrofoil

Abstract: In more recent years, supercavitation has attracted intensive attention due to its potentials in drag reduction for underwater vehicles. Ventilation is acknowledged as an efficient way to enhance cavitation when vehicles work under low speed. That means natural and ventilated cavitation may coexist in the flow and the interaction between the natural cavitation and ventilated cavitation has to be considered. In this paper, ventilated cavitating flow with natural cavitation around a base-ventilated hydrofoil is solved by a multi-phase cavitation solver based on OpenFOAM. The Partially-Averaged Navier-Stokes method is utilized for resolving turbulence. Lengths of the natural cavities are investigated under non-ventilation and ventilation conditions. Cavity shape evolution and interface deformation have also been studied under different angle of attack. Results show that ventilation cavitation at the base of the hydrofoil tends to depress the natural cavitation on the hydrofoil surface. As the increase of the attack angle, the shedding cavity of natural cavitation have a great impact on the interface shape of the ventilation cavity. Furthermore, the research also finds that the re-entry jet is the reason for natural cavitation shedding process and the interface deformation of the ventilated cavity arises from the vortex structures induced by the shedding natural cavitation.