Applied Mechanics and Materials Vol. 225

Abstract: In this paper, the effects of changing under-relaxation factors for different variables on the numerical solution of 2D incompressible laminar flow over a backward-facing step (BFS) are studied using PHOENICS commercial Computational Fluid Dynamics (CFD) software. This is conducted by changing under-relaxation factors for velocities and pressure during the 2D simulation. Ten different batches of under-relaxation factor for pressure ranging from 0.1 to 1.0 were used while the values of under-relaxation factor for velocities were manipulated between 0.1 and 1.0. For each batch of the computation, the error percentage of pressure and velocities were obtained. Based on this work, it is found that the recommended values of under-relaxation factor for pressure to achieve lower error percentage are between 0.6 and 0.8. Based on findings of the study, the appropriate values of under-relaxation factor for pressure and velocities can be selected to achieve the levels of error percentage permitted for computational studies.

Abstract: The approach to problems of wall interference in wind tunnel testing is generally based on the so-called classical method, which covers the wall interference experienced by a simple small model or the neo-classical method that contains some improvements as such that it can be applied to larger models. Both methods are analytical techniques offering solutions of the subsonic potential equation of the wall interference flow field. Since an accurate description of wind tunnel test data is only possible if the wall interference phenomena are fully understood, uncounted subsequent efforts have been spent by many researchers to improve the limitation of the classical methods by applying new techniques and advanced methods. However, the problem of wall interference has remained a lasting concern to aerodynamicists and it continues to be a field of active research until the present. The main objective of this paper is to present an improved classical method of the wall interference assessment in rectangular subsonic wind tunnel with solid-walls.

Abstract: Designers of the aero engines are in quest of maintaining the pressure as high as possible at the face of the compressor, with air velocity not higher than 0.8 Mach. Reduction of the flow from supersonic to such speed is combined with pressure reduction. This paper presents results from coupled techniques to solve for the flow field of double wedge spiked supersonic intake. The selected spike has 4° forebody wedge angle and the second ramp angle is 8°. The external part of the flow was solved analytically while the internal part was solved numerically by finite volume technique. The analysis was carried out at different Mach numbers (1.4, 1.8, 2.2, 2.4, and 3) and different angles of attack (0°, 6°, and 12°). The procedure is validated and the results are presented in terms of the pressure recovery at the face of the compressor. The results have shown that generally the pressure recovery decreases by increasing of incidence angle. The non-zero incidence was found to produce noticeable difference in pressure distribution at the face of the compressor. This became considerably effective at incidences leading to detached shocks at the leading edge of the spike.

Abstract: Past researches have studied the stator blade guide vane angle implementation respective to the rotor blade angle. And outlet guide vane is recommended to encounter the air swirl problem occurs in the axial flow fan compared to inlet guide vane. This paper presents an investigation on the distance between rotor-stator for an outlet guide vane with fixed stator angle. Two specimens were studied: 30° rotor and 34° stator, and 40° rotor and 44° stator. The stator angle was obtained from previous study, which is the optimized angle for the presented stator blade profile. The distance of 50mm and 500mm between rotor and stator is investigated in this study. This would be the constraint length of motor spacing between rotor and stator. This investigation is to explore the possibility of positioning the guide vane after the motor as a separate fixture to reduce manufacturing and assembly cost of guide vane. To investigate the flow structure and to analyze qualitatively commercial CFD package, FLUENT is exploited. The computational model was validated against experimental data. The experimental analysis is done in reference to AMCA 210-07 standard test procedure and the data presented for rotor without stator guide vane model. The performance curve of the axial fan was plotted to compare the effect of the guide vane distance between rotor and stator. The efficiency curve also obtained from measured power input to motor. The results shows the 50mm-distanced stator perform better than the 500mm-distanced stator. This is because the high swirling of flow from the rotor is well-corrected by the 50mm stator compared to the 500mm stator.

Abstract: Flow over the ONERA M6 wing with vortex generators using more accurate higher order numerical schemes is studied using computational methods. In this paper, the effect of delta vortex generator orientation on the wing and its implication on wing performance is computed more accurately using second order upwinding scheme. Turbulence modeling used is k-omega sst. It has been found that numerical results are comparable and close to the experiment. The analysis results show that the co-rotating clockwise position of vortex generators is more effective than co-rotating (anticlockwise) or counter rotating position. The vortex generators have been found to control the boundary layer separation and give improvement in lift at high angle of attack.

Abstract: An active flow control technology known as synthetic jet actuator (SJA) is a zero-net mass-flux device to create pulsed jet that produces momentum to its surroundings and uses a vibrating diaphragm inside the cavity to generate an oscillatory flow through a small orifice. The performance of SJA depends on the design of an orifice and cavity, and oscillating membrane. SJA design based on piezoelectric diaphragm used in this project because of their size, lightweight, no need for external air supply, without the pipe complex, fast response time and low power consumption. This paper describes the cavity effect to SJA designs and experiments were performed to determine the air jet velocity produced through the orifice using a hot-wire anemometer at a different cavity thickness. The results demonstrate that the jet velocity increase would be better if the cavity thickness is reduced. However, more studies are needed to optimize the size of cavity and orifice for appropriate applications.

Abstract: Numerical simulation is of interest for most fan designers to optimize the fan designs. Computational fluid dynamic (CFD) has become an essential tool in almost every branch of fluid dynamics and one of the major tools for fan designs. As the fan designers relying on the numerical simulation, the accuracy of tools such as CFD in predicting the performance has become a subject of interest. This paper validates the CFD modeling of an axial fan design against experimental result. The experimental rig and test procedure are developed with reference to “AMCA standard 210”. The analysis is conducted on 1250mm diameter axial fan with two different blade pitch angle 30° and 40°. Prior to encounter the swirling effect and deflection of velocity vector due to rotor blade, a stator blade with the same profile as rotor blade is used as the outlet guide vanes in opposite direction. The computational model is created according to the experimental condition and applied realistic boundary conditions. The model is simulated using commercial CFD package, ANSYS FLUENT. The results obtained are compared against experimental data (AMCA standard 210) over wide range of flow rate. Provided the modeling strategy is chosen appropriately with correct configuration of mesh density and turbulent model then, the results correlates closely with experimental data. This is shown in this investigation. The guide vane incidence angle determination is also done in this paper for 30° and 40° blade pitch angle. The outcome of this paper would provide confidence for designers in numerical simulation for predicting performance of axial fan. In addition, numerical simulation creates a platform for systems to be optimized with a lower cost and high efficiency outcomes.

Abstract: Flexible cantilever beams in channel flow loses its stability through a flutter mechanism at sufficiently high Reynolds number. This sustained transfer of flow work to cantilever beam oscillation could be extracted to generate electrical power, which may be utilized to power wireless sensors for structural health monitoring or supply additional power in unmanned or micro air vehicles. Such flexible beam system is known to exhibit some form of hysteresis, with super- and sub-critical velocities for onset of flutter, which may not be captured by current potential flow modeling. We present numerical simulation of a viscous, Navier-Stokes solver coupled to a cantilever beam, to capture this flutter velocity hysteresis. Numerical result under a linearly varying flow velocity shows an approximately 11% hysteresis, suggesting potential of present model for identifying more accurate boundaries of beam flutter. The prediction of cantilevered piezoelectric beam response under varying or unsteady flow velocities is necessary for reliable design of energy harvesting devices and warrants further experimental investigations.

Abstract: The unsteady flow fields above NACA 0015 airfoil pitching with/without upstream turbulence generator are investigated in a water tunnel by mean of particle image velocimetry (PIV). The turbulence was generated by a square bar mesh situated at the inlet of the test section. The airfoil pitching waveform is performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). By using PIV, the instantaneous vortex structures above the pitching airfoil can be revealed. It allows us to study the free stream turbulence effects on dynamic stall over an airfoil at pitching waveform the same as VAWT. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. The dynamic stall process is delayed to higher incidence angles on increasing the turbulence intensity.

Abstract: Single normal hot-wire measurements of the streamwise component of velocity were taken in boundary layer flows subjected to pressure gradients at matched friction Reynolds numbers Reτ ≈ 3000. To evaluate spatial resolution effects, the sensor lengths are varied in both adverse pressure gradient (APG) and favorable pressure gradient (FPG). A control boundary layer flow in zero pressure gradient ZPG is also presented. It is shown here that, when the sensor length is maintained a constant value, in a contant Reynolds number, the near-wall peak increases with (adverse) pressure gradient. Both increased contributions of the small- and especially large-scale features are attributed to the increased broadband turbulence intensities. The two-mode increase, one centreing in the near-wall region and the other one in the outer region, makes spatial resolution studies in boundary layer flow more complicated. The increased large-scale features in the near-wall region of an APG flow is similar to large-scales increase due to Reynolds number in ZPG flow. Additionally, there is also an increase of the small-scales in the near-wall region when the boundary layer is exposed to adverse pressure gradient (while the Reynolds number is constant). In order to collapse the near-wall peaks for APG, ZPG and FPG flows, the APG flow has to use the longest sensor and conversely, the FPG has to use the shortest sensor. This study recommends that the empirical prediction by Huthins et. al. (2009) to be reevaluated if pressure gradient flows were to be considered such that the magnitude of the near-wall peak is also a function of the adverse pressure gradient parameter.