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Accepted Manuscripts

BASIC VIEW  |  EXPANDED VIEW
research-article  
Chia-Yuan Chen, Bivas Panigrahi, Kok-Shen Chong, Wei-Hsien Li, Yi-Li Liu and Tsung-Yi Lu
J. Fluids Eng   doi: 10.1115/1.4039368
In the current semiconductor industrial scenario, wafers are rinsed in an overflow rinsing tank while being mounted on several lifters prior to most of its manufacturing process. However, a major drawback of this overflow rinsing methodology is that some of the processing fluid stagnate due to the generated vortices in the regions between the side and middle lifters which entrap some of the flushed particles that further adhere and deteriorate the surface of the wafers. In this work, the hydrodynamics of the flow field inside the wafer rinsing tank with this original lifter orientation set-up was studied and compared through numerical simulation and flow visualization using particle image velocimetry method, and a strong agreement was found between them in terms of instantaneous velocity calculation. A new lifter orientation set-up was initiated and it was evidenced by the numerical simulation that with this new set-up the generated vortices which are situated opposite to the lifters tilting direction can be displaced significantly in terms of magnitude and distribution. This work presents a new wafer cleaning concept which shows its great potentials in improvement and implementation to the current in-line wafer batch fabrication process without modifying the original design of the rinsing tank.
TOPICS: Computer simulation, Semiconductor wafers, Flow visualization, Manufacturing, Particulate matter, Vortices, Design, Flow (Dynamics), Hydrodynamics, Fluids, Semiconductors (Materials)
research-article  
Armin Bodaghkhani, Yuri Muzychka and Bruce Colbourne
J. Fluids Eng   doi: 10.1115/1.4039369
This paper describes a numerical simulation of the interaction of a single nonlinear wave with a solid vertical surface in three-dimensions. A coupled Volume of Fluid (VOF) and Level Set Method (LSM) is used to simulate the wave-body interaction. A Cartesian-grid method is used to model immersed solid boundaries with constant grid spacing for simplicity and lower storage requirements. Mesh refinement is implemented near the wall boundaries due to the complex behavior of the free surface around the body. The behavior of the wave impact, the water sheet, and the high-speed jet arising from the wave impact are all captured with these methods. The numerical scheme is implemented using parallel computing due to the high CPU and memory requirements of this simulation. The maximum wave run-up velocity, instant wave run-up velocity in front of the vertical surface, the sheet break-up length, and the maximum impact pressure are computed for several input wave characteristics. Results are compared with a laboratory experiment that was carried out in a tow tank, in which several generated waves were impacted with a fixed flat-shaped plate model. The numerical and experimental data on sheet breakup length are further compared with an analytical linear stability model for a viscous liquid sheet, and good agreement is achieved. The comparison between the numerical model and the experimental measurements of pressure, the wave run-up velocity, and the break-up length in front of the plate model shows good agreement.
TOPICS: Simulation, Waves, Pressure, Computer simulation, Dimensions, Stability, Fluids, Storage, Water, Nonlinear waves
research-article  
Kamal Selvam, Emir Öngüner, Jorge Peixinho, El-Sayed Zanoun and Christoph Egbers
J. Fluids Eng   doi: 10.1115/1.4039294
Velocity fluctuations are widely used to identify the behavior of developing turbulent flows. The pressure on the other hand, which is strongly coupled with the gradient of the mean velocity and fluctuations, is less explored. In this study, we report the results of an experimental study for the development of pipe flow at high Reynolds numbers, where the wall pressure fluctuations are measured along the axial direction. It is found that the pressure fluctuations increase exponentially along the pipe with a self-similarity scaling. The exponential growth of the pressure fluctuations along the pipe saturates after reaching a critical position. It qualitatively agrees with the critical position for fully developed turbulence, which was obtained from earlier velocity fluctuations at various locations along the pipe centerline. Additional tuft visualizations have been carried out near the pipe wall, reaffirming that fluctuations increase as the flow moves downstream along the pipe. Results also show that the exponential growth of the pressure fluctuations is robust to different sizes of ring obstacles placed close to the pipe inlet. Finally, it is found that the pressure fluctuations decrease as a function of Reynolds number, contrary to the boundary layer flow.
TOPICS: Pressure, Turbulence, Pipe flow, Fluctuations (Physics), Pipes, Reynolds number, Flow (Dynamics), Visualization, Fully developed turbulent flow, Boundary layers
research-article  
Pascal Bader, Manuel Pschernig, Wolfgang Sanz, Jakob Woisetschlaeger, Franz Heitmeir, Walter Meile and Günter Brenn
J. Fluids Eng   doi: 10.1115/1.4039257
Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to- turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In the present paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser-Doppler anemometry. Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow "freeze", while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid re-transition to turbulent flow is observed. The findings of this work also describe the mechanism of re-transition.
TOPICS: Flow (Dynamics), Boundary layers, Turbulence, Fluctuations (Physics), Skin friction (Fluid dynamics), Heat transfer, Lasers, Transport phenomena, Flat plates, Flow measurement, Turbomachinery
Technical Brief  
Eduard Amromin
J. Fluids Eng   doi: 10.1115/1.4039252
According to several known experiments, an increase of the incoming flow air content can increase the hydrofoil lift coefficient. The presented theoretical study shows that such increase is associated with the decrease of the fluid density at the cavity surface. This decrease is caused by entrainment of air bubbles to the cavity from the surrounding flow. The theoretical results based on such explanation are in a good agreement with the earlier published experimental data for NACA0015.
TOPICS: Hydrofoil, Inflow, Flow (Dynamics), Cavities, Fluid density, Bubbles
research-article  
Noor Afzal and Abu Seena
J. Fluids Eng   doi: 10.1115/1.4039259
The maxima of Reynolds shear stress, and turbulent burst mean period time are crucial points in the intermediate region (termed as mesolayer) for large Reynolds numbers. The three layers (inner, meso, outer) in a turbulent boundary layer have been analyzed from open equations of turbulent motion, independent of any closure model like eddy viscosity or mixing length etc. Little above critical point, the matching of mesolayer predicts the log law velocity, peak of Reynolds shear stress domain and turbulent burst time period. The instantaneous velocity vector after subtraction of mean velocity vector, yield the velocity fluctuation vector, also governed by log law. The static pressure fluctuation $p'$ also predicts log laws in the inner, outer and mesolayer. The relationship between $u'/U_e $ with $u/U_e$ from structure of turbulent boundary layer is presented in inner, meso and outer layers. The shape factor in a turbulent boundary layer shows linear behavior with non-dimensional mesolayer length scale. It is shown that the Prandtl transposition theorem, connects the velocity of normal co-ordinate $y$ with offset a to $y+a$ then the turbulent velocity profile vector and pressure fluctuation log laws are altered; but skin friction log law, based on outer velocity $U_e$, remains independent of $a$ the offset of origin. But if skin friction log law is based on bulk average velocity $U_b$ then skin friction log law depends on $a$, the offset of origin. These predictions are supported by experimental and DNS data.
TOPICS: Theorems (Mathematics), Pressure, Fluctuations (Physics), Boundary layer turbulence, Shear stress, Turbulence, Skin friction (Fluid dynamics), Shapes, Eddies (Fluid dynamics), Viscosity, Reynolds number
research-article  
Puxuan Li, Steve Eckels, Garrett Mann and Ning Zhang
J. Fluids Eng   doi: 10.1115/1.4039256
The setup of inlet conditions for a Large Eddy Simulation (LES) is a complex and important problem. Normally there are two methods to generate the inlet conditions for LES, i.e. synthesized turbulence methods and precursor simulation methods. This study presents a new method for determining inlet boundary conditions of LES using particle image velocimetry (PIV). LES shows sensitivity to inlet boundary conditions in the developing region, and this effect can even extend into the fully developed region of the flow. Two kinds of boundary conditions generated from PIV data, i.e. steady spatial distributed inlet (SSDI) and unsteady spatial distributed inlet (USDI), are studied. PIV provides valuable field measurement,but special care is needed to estimate turbulent kinetic energy and turbulent dissipation rate for SSDI. Correlation coefficients are used to analyze the autocorrelation of the PIV data. Different boundary conditions have different influences on LES, and their advantages and disadvantages for turbulence prediction and static pressure prediction are discussed in the paper. Two kinds of LES with different sub-grid turbulence models are evaluated: namely Dynamic Smagorinsky-Lilly Model (Lilly model) and Wall Modeled Large Eddy Simulation (WMLES model). The performance of these models for flow prediction in a square duct are presented. Furthermore, the LES results are compared with PIV measurement results and Reynolds-Stress Model (RSM) results at a downstream location for validation.
TOPICS: Particulate matter, Turbulence, Boundary-value problems, Large eddy simulation, Flow (Dynamics), Pressure, Ducts, Kinetic energy, Simulation, Stress, Energy dissipation
research-article  
Grunde Olimstad, Morten Osvoll and Pål Henrik Enger Finstad
J. Fluids Eng   doi: 10.1115/1.4039250
For low flow and high head applications, pump types such as progressive cavity or gear pumps are often used. However, centrifugal pumps are much more robust and wear resistant, and are beneficial if they can handle the rated head and flows. By challenging the limitations of low specific speed(Nq), centrifugal pumps can be made to handle a combination of low flow and high head which previously required other pump types. Conventional centrifugal pumps have specific speed down to 10, while in this paper a design with specific speed of 4.8 is presented. The paper describes several iterative steps in the design process of the low Nq pump. These iterations were done one physical pumps which were successively tested in a test rig. Motivation for each step is explained theoretically and followed up by discussion of the measured results. Four different geometries of the pump were tested, all of them manufactured by rapid prototyping in nylon material. A substantial question is how low the specific speed of a centrifugal pump can be. Limitations of low Nq pumps are discussed and new findings are related to volute cavitation. In addition, limitations due to, disk friction, volute losses, leakage flow and pump stability are discussed and shows to limit the design space for the pump designer.
TOPICS: Design, Centrifugal pumps, Pumps, Flow (Dynamics), Friction, Wear, Gear pumps, Nylon fabrics, Cavitation, Rapid prototyping, Disks, Cavities, Leakage flows, Stability
research-article  
Jinhong Hu, Jiandong Yang, Wei Zeng and Jiebin Yang
J. Fluids Eng   doi: 10.1115/1.4039258
The transient behaviors of a prototype pump turbine are very important to the safe operation of a pumped-storage power plant. This is because the water hammer pressure during transient events affects the pressure surges in the spiral case and draft tube. In addition, the transient pressure pulsations in the vaneless space are important in the evaluation of the life of the runner. Although several detailed studies have been conducted on the water hammer pressure of a hydropower plant, very few have considered the transient pressure pulsations that occur in the pump turbine. The objective of the present study was to determine the characteristics of the transient pressure pulsations of a 300-MW prototype Francis pump turbine during load rejection and power failure. For this purpose, the frequency features in the steady state were first analyzed using Fast Fourier Transform (FFT). A Savitzky-Golay filter was then used to extract the water hammer pressure and pulsating pressure from the acquired raw pressure signals. Further, a 1-D Method of Characteristics mathematical model of the pump-turbine was established and used to simulate the transient variations of the flow discharge during transient events, to enable the division of the transient operation conditions into several domains. Finally, the characteristics of the transient pressure pulsations in the spiral case, vaneless space, and draft tube were investigated in the time and frequency domains. This paper also discusses the causes of the pressure pulsations that occur under different modes of operation of a pump turbine.
TOPICS: Transients (Dynamics), Engineering prototypes, Pressure, Pump turbines, Simulation, Water hammer, Failure, Fast Fourier transforms, Filters, Signals, Steady state, Surges, Hydroelectric power stations, Stress, Pumped storage power plants, Flow (Dynamics)
research-article  
Dimar Detert Oude Weme, Martijn van der Schoot, Niels P. Kruyt and Eric van der Zijden
J. Fluids Eng   doi: 10.1115/1.4039251
The effect of trimming of radial impellers on the hydraulic performance of low specific-speed centrifugal pumps is studied. Prediction methods from literature, together with a new prediction method that is based on the simplified description of the flow field in the impeller, are used to quantify the effect of trimming on the hydraulic performance. The predictions by these methods are compared to measured effects of trimming on the hydraulic performance for an extensive set of pumps for flow rates in the range of 80% to 110% of the best efficiency point. Of the considered methods, the new prediction method is more accurate (even for a large impeller trim of 12%) than the considered methods from literature. The new method generally overestimates the reduction in the pump head after trimming, and hence results less often in impeller trims that are too large when the method is used to determine the amount of trimming that is necessary in order to attain a specified head.
TOPICS: Impellers, Centrifugal pumps, Flow (Dynamics), Pumps
research-article  
Anurag Sharma and Bimlesh Kumar
J. Fluids Eng   doi: 10.1115/1.4039253
In this work, we have performed the flume study to analyze the structure function of turbulent boundary layer with and without downward seepage. Sediment transport experiments were done in the laboratory for no seepage (NS), 10% seepage (10%S) and 15% seepage (15%S) cases. Measures of stream wise velocity variance were found increasing with seepage, which lead to increase in sediment transport with seepage. Results show that the variance of stream wise velocity fluctuation follows logarithmic law with distance away from the bed, within inner layer. This observation is also valid for even-order moments obtained in this work. The results shows that the (2p-order moments)1/p also follow logarithmic law. The slopes Ap in the turbulent boundary layer seem fairly unaffected to no seepage and seepage flow, but follows non- universal behavior for no seepage and seepage runs. The computed slope based on the Gaussian statistics does not agree well with the slope obtained from the experimental data and computed slope are reliable with sub-Gaussian performance for no seepage flow and super-Gaussian behavior for seepage flow.
TOPICS: Seepage (Hydrology), Boundary layer turbulence, Flow (Dynamics), Sediments, Statistics as topic, Flumes
research-article  
Peng Wang, Hongyu Ma and YingZheng Liu
J. Fluids Eng   doi: 10.1115/1.4039254
Due to the practical space limitation, the control valve in industrial utilities is usually immediately followed by a short flow passage, which would introduce considerable complexity into highly unsteady flow behaviors, along with the flow noise and structure vibration. In the present study, the unsteady behaviors of the steam flow inside a control valve with a T-junction discharge, when the valve operates under the choked condition, is numerically simulated. Towards this end, the detached eddy simulation is used to capture the spatiotemporally varying flow field in the serpentine flow passage. The results show periodic fluctuations of the aerodynamic forces on the valve spindle and periodic fluctuations of the pressure and flow rate at the two discharge outlets. Subsequently, proper orthogonal decomposition analysis is conducted using the velocity field and pressure field, identifying respectively the dominant coherent structures and energetic pressure fluctuation modes. Finally, the extended-POD method is used to delineate the coupling between the pressure fluctuations with the dominant flow structures superimposed in the highly unsteady flow field. The fourth velocity mode at St = 0.1, which corresponds to the alternating oscillations of the annular wall-attached jet, is determined to cause the periodic flow imbalance at the two discharge outlets, whereas signatures of the first three modes are found to be dissipated in the spherical chamber. Such findings could serve as facts for vibration prediction and optimization design. Particularly, the POD and extended-POD techniques were demonstrated to be effective methodologies for analyzing the highly turbulent flows in engineering fluid mechanics.
TOPICS: Flow (Dynamics), Simulation, Eddies (Fluid dynamics), Valves, Junctions, Principal component analysis, Steam, Pressure, Fluctuations (Physics), Unsteady flow, Vibration, Public utilities, Noise (Sound), Fluid-dynamic forces, Design, Optimization, Fluid mechanics, Aerodynamics, Turbulence, Oscillations
research-article  
Hanxiang Jin and Alexandrina Untaroiu
J. Fluids Eng   doi: 10.1115/1.4039249
Hole-pattern annular gas seals have two distinct flow regions: an annular jet-flow region between the rotor and stator, and cylindrical indentions in the stator that serve as cavities where flow recirculation occurs. As the working fluid enters the cavities and recirculates, its kinetic energy is reduced, resulting in a reduction of leakage flow rate through the seal. The geometry of the cylindrical cavities has a significant effect on the overall performance of the seal. In this study, the effects of elliptical shape hole pattern geometry on the leakage and dynamic response performance of an industry-relevant hole-pattern seal design are investigated using a combination of CFD, hybrid bulk flow-CFD analysis, and a design of experiments technique. The design space was defined by varying the values of five geometrical characteristics: the major and minor radius of hole, the angle between the major axis and the axis of the seal, the spacing between holes along the seal axis, and hole spacing in the circumferential direction. This detailed analysis allowed for a greater understanding of the interaction effects from varying all of these design parameters together as opposed to studying them one variable at a time. Response maps generated from the calculated results demonstrate the effects of each design parameter on seal leakage as well as the co-dependence between the design parameters. The data from this analysis was also used to generate linear regression models that demonstrate how these parameters affect the leakage rate and the dynamic coefficients, including the effective damping.
TOPICS: Experimental design, Performance evaluation, Shapes, Design, Flow (Dynamics), Cavities, Leakage, Geometry, Stators, Computational fluid dynamics, Damping, Rotors, Fluids, Kinetic energy, Jets, Leakage flows, Regression models, Dynamic response
research-article  
Hossein Mahdizadeh, Soroosh Sharifi and Pourya Omidvar
J. Fluids Eng   doi: 10.1115/1.4039248
In this study, a second-order accurate Godunov-type finite volume method is used for the solution of the two-dimensional water hammer problem. The numerical scheme applied here is well-balanced and is able to treat the unsteady friction terms, together with the convective terms, within the differences between fluxes of neighboring computational cells. In order to consider the effect of unsteady friction terms during the water hammer process, the k-e and k-w turbulence models are employed. The performance of the proposed method with the choice of different turbulence models is evaluated using experimental data obtained from one low and one high Reynolds-number turbulent test cases. In addition to velocity and pressure distributions, the turbulence characteristics of each variant of the model, including eddy viscosity, dissipation rate and turbulent kinetic energy during the water hammer process are fully analyzed. It is found that the inclusion of the convective inertia terms leads to more accurate pressure profiles. The results also show that using a relatively high CFL number close to unity, the introduced numerical solver with both choices of turbulence models provides reasonable and acceptable predictions for the studied flows.
TOPICS: Inertia (Mechanics), Pressure, Flow (Dynamics), Friction, Wave propagation, Turbulence, Eddies (Fluid dynamics), Viscosity, Kinetic energy, Reynolds number, Flux (Metallurgy), Energy dissipation, Transients (Dynamics), Water hammer, Algorithms, Pipe flow, Approximation, Finite volume methods
research-article  
Saranya VS, Priyank Kumar and Sudip Das
J. Fluids Eng   doi: 10.1115/1.4039247
Studies were made to understand the flow features around an open cavity at Mach 2.0 corresponding to Re=0.55×106 based on the cavity depth. Experiments were carried out using a blowdown type Supersonic Wind Tunnel having a test section size of 50mm×100 mm. Oil flow and schlieren flow visualisation were made to understand the steady flow features inside the cavity. Unsteady pressures were measured at several locations to obtain the fluctuating flow field details and the pressure spectrum. Impinging wall modifications of the cavity were made with an objective to reduce the Rossiter's mode frequencies and its amplitude. Partial ramping of the impinging wall with variations in height and angles were made. With adoption of a specific combination of the impinging wall height and angle, the first two modes of the multiple tonal characteristics could be reduced significantly. The present adopted method could result in 74% reduction of RMS pressure and a noise reduction of 11 dB.
TOPICS: Flow (Dynamics), Cavities, Pressure, Wind tunnels, Noise control, Flow visualization
research-article  
Pablo Ouro, Thorsten Stoesser and Luis Ramirez
J. Fluids Eng   doi: 10.1115/1.4039235
This paper presents large-eddy simulations of symmetric and asymmetric (cambered) airfoils forced to undergo deep dynamic stall due to a prescribed pitching motion. Experimental data in terms of lift, drag, and moment coefficients are available for the symmetric NACA 0012 airfoil and these are used to validate the large-eddy simulations. Good agreement between computed and experimentally observed coefficients is found confirming the accuracy of the method. The influence of foil asymmetry on the aerodynamic coefficients is analysed by subjecting a NACA 4412 airfoil to the same flow and pitching motion conditions. Flow visualisations and analysis of aerodynamic forces allow an understanding and quantification of dynamic stall on both straight and cambered foils. The results confirm that cambered airfoils provide an increased lift-to-drag ratio and a decreased force hysteresis cycle in comparison to their symmetric counterpart. This may translate into increased performance and lower fatigue loads when using cambered airfoils in the design of vertical axis turbines operating at low tip-speed ratios.
TOPICS: Design, Turbines, Blades, Airfoils, Large eddy simulation, Drag (Fluid dynamics), Stress, Flow visualization, Fluid-dynamic forces, Cycles, Flow (Dynamics), Aerodynamics, Fatigue
research-article  
Wayne Strasser and Alex Strasser
J. Fluids Eng   doi: 10.1115/1.4039234
A computational study was carried out to investigate the effects of internal geometry changes on the likelihood of solids buildup within, and the efficiency of, an industrial dust collector. Combustible solids held up in the unit pose a safety risk. The dust collector serves multiple functions, so the design requires a delicate balance. Particles should be separated from the incoming mixture and collected in the bottom of the unit. This particulate material should freely flow into a high-speed ejector (Mach 0.4) underneath. Gas must also flow freely to the top outlet, but sufficient gas must flow down to the ejector so that its motive gas augments the transport of particles back to the reactor ("recirculation"). Computational design evaluations included 1) rod spacing, 2) ledge removal, and 3) rod cover plates. Testing on particle size distribution and density were carried out in-house to provide inputs to the CFD model. Rod spacing reduction had a mixed effect on flow distribution. Plates were found to induce a negative effect on recirculation and a mixed effect on combustible solids accumulation. Removal of the ledge, however, offered slightly more recirculation along with completely alleviating stagnant solids accumulation. It is shown that, without consideration of detailed fluid physics, general separator design principals might be misguiding.
TOPICS: Dust, Solids, Flow (Dynamics), Design, Ejectors, Plates (structures), Particulate matter, Safety, Bulk solids, Computational fluid dynamics, Fluids, Physics, Density, Testing, Geometry, Particle size, Risk
research-article  
Paul Ziade, Mark A. Feero, Philippe Lavoie and Pierre E. Sullivan
J. Fluids Eng   doi: 10.1115/1.4039233
The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large-eddy simulation. Two angles-of-attack were considered: 5 and 12 degrees. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 degrees, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. Linear stability analysis suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.
TOPICS: Separation (Technology), Stability, Shear (Mechanics), Airfoils, Boundary layers, Flow (Dynamics), Bubbles, Spectroscopy, Reynolds number, Emission spectroscopy, Large eddy simulation, Uncertainty
research-article  
Timothy Lee and Vincent Tremblay-Dionne
J. Fluids Eng   doi: 10.1115/1.4039236
The aerodynamic properties and flowfield of a NACA 0015 airfoil over a wavy ground were investigated experimentally via surface pressure and particle image velocimetry measurements. Flat-surface results were also obtained to served as a comparison. For the wavy ground, there exhibited a cyclic variation in the sectional lift coefficient Cl over an entire wave length. The maximum Cl observed at the wave peak (produced by the wavy ground-induced RAM pressure) and minimum Cl occurred at the wave valley (resulting from the unusual suction pressure developed on the airfoil's lower surface due to the converging-diverging flow passage developed underneath it) reduced with increasing ground distance. By contrast, the pitching-moment coefficient showed an opposite trend to the variation in Cl and had an almost all-negative value. Meanwhile, two peak values in the drag coefficient over each wave length were observed. The wavy ground effect-produced gains in the mean Cl and lift-to-drag ratio were at the expense of longitudinal stability. Additional measurements considering different wave lengths and amplitudes are needed to further quantify the impact of wavy ground on wing-in-ground effect (WIG) airfoils and wings.
TOPICS: Aerodynamics, Airfoils, Waves, Pressure, Wings, Drag (Fluid dynamics), Stability, Flow (Dynamics), Particulate matter, Suction
research-article  
Timothy Lee and Lok Sun Ko
J. Fluids Eng   doi: 10.1115/1.4039232
The ground effect on the aerodynamic loading and leading-edge vortex flow generated by a slender delta wing was investigated experimentally. Both the lift and drag forces were found to increase with reducing ground distance (up to 50% of the wing chord). The lift increment was also found to be the greatest at low angles of attack ? and decreased rapidly with increasing ground distance and ?. The ground effect-caused earlier wing stall was also accompanied by a strengthened leading-edge vortex with an increased rotational speed and size compared to the baseline wing. The smaller the ground distance the stronger the leading-edge vortex and the earlier vortex breakdown became. Meanwhile, the vortex trajectory was also found to be located further inboard and above the delta wing in ground effect compared to its baseline-wing counterpart. Finally, for WIG (wing-in-ground effect) craft with delta-wing planform the most effective in-ground-effect flight should be kept within 10% of the wing chord.
TOPICS: Aerodynamics, Vortex flow, Wings, Vortices, Chords (Trusses), Trajectories (Physics), Drag (Fluid dynamics), Flight

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