Wei Hua Ho

Location of the vortex formation threshold at suction inlets near ground planes by computational fluid dynamics simulation

Abstract Vortices can develop in the intakes of turbojet and turbofan aero engines during high power operation near solid surfaces such as during takeoff or ground tests. These vortices can stall the compressor and cause significant damage. The factors determining the formation of the vortex include engine thrust, distance from ground, and ambient vorticity. A computational fluid dynamics study was conducted to determine whether these effects could be predicted numerically, and the results were compared to previous wind-tunnel studies. This is the first reported study of numerical predictions of the vortex formation threshold. Two factors, which had not been previously studied either experimentally or numerically, were investigated: the effects of increasing the size of the suction inlet and increasing the thickness of the ground boundary layer. Both were found to encourage vortex formation. The threshold of vortex formation predicted numerically agrees with previous wind-tunnel studies, within the scatter of the experimental data. The numerical results reproduce all trends identified in the experimental studies. The minimum shear required to form a vortex was found, lying between 0.001 and 0.05/s (Rossby number of the order of 105).

Validated Unsteady Computational Fluid Dynamic Analysis of an Oscillating Bio-Inspired Airfoil

An unsteady, two-dimensional numerical study was conducted to investigate the aerodynamic and flow characteristics of a bio-inspired corrugated airfoil oscillating at 2Hz with an amplitude of 10°. The upstream flow was set such that the chord Re = 14,000. The computational results were validated against experimental results from a 2D particle image velocimetry (PIV) experiment on the same airfoil geometry. Complex flow structures such as the formation and shedding of trailing edge vortices have been revealed to have significant impacts on the lift and drag characteristics of the airfoil in oscillating motion. The shed vortices provide a low pressure region on the top surface of the airfoil throughout the period of oscillation, thus increasing lift of the airfoil. In particular, vortices formed and shed from the rear-most corrugation appear to have the largest effect. The pitch-down motion produces a lower absolute peak lift as compared the pitch-up motion which may be explained by the disruption of the high pressure zone on the top surface of the airfoil by a vortex forming in the corrugations. This results in a relatively lower high pressure region on the advancing side as compared to the pitch-up motion. In addition to the lift calculations, drag calculations indicate that net thrust is being produced during the oscillations and more thrust is produced on the pitch-up than the pitch-down motion.

Validated CFD Simulations of EWH Energy Storage Based on Tank Orientation

Thermal energy storage is known to be responsible for the largest energy-consuming factor in households’ energy demand. A computational study was performed to investigate if there is any difference in energy requirements between horizontal and vertical mounting configurations. The model used in the study is that of a Trendline 150 l dual mounting electric water heater which is a common size tank used in typical South African households. The simulations investigated the length of time it took for the tank to cool down from a starting temperature of 353 K with cold water (at 283 K) flowing into the tank at two flow rates of 13.8 l/min and 7.5 l/min, respectively. The heating element was “turned on” during the entire period of the simulations. It was found that a vertically mounted tank cooled down less at the higher flow rate of 13.8 l/min and the reverse for the lower flow rate of 7.5 l/min.

Effect of Leading Edge Protuberance on Thrust Production of a Dynamically Pitching Aerofoil

The paper presents a computational analysis of the characteristics of a NACA 634-021 aerofoil modified by incorporating sinusoidal leading-edge protuberances at Re = 14,000. The protuberances are from the tubercles of the humpback whale flipper with leading edge acting as passive-flow control devices that improve performance and manoeuvrability of the flipper. They are characterized by an amplitude and wavelength of 12% and 50% of the aerofoil chord length respectively. Three-dimensional CFD on the modified aerofoil oscillating about a point located on the centreline at quarter-chord has been performed with the frequency and amplitude of oscillation being 4Hz and 10 deg respectively. In addition to the lift and thrust coefficients, near wall flow visualisations and the shedding of vortices during oscillations are presented to illustrate the unsteady flow features on the performance of the oscillating flipper. The results show an improvement in the thrust production when compared to previous studies on similar symmetric aerofoil without the leading edge modifications.

Unsteady CFD Analysis of an Oscillating Aerofoil Inspired by Dragonfly Wings

An unsteady two-dimensional CFD analysis of an oscillating (dynamically pitching) aerofoil inspired by dragonfly wings was conducted to investigate its aerodynamic and flow characteristics. The aerofoil morphology was an idealised geometry based on the crosssection near the mid-span of a dragonfly wing. The aerofoil was made to oscillate at 2Hz with amplitude of 10° with an upstream flow such that the chord Reynolds number was 14,000. The methodology mirrored a previous study but with slight differences due to the difference in geometry. Complex flow structures near to the aerofoil surface revealed significant effect on lift and drag characteristics. Lift and drag hysteresis indicate that there is net lift generated but no net thrust. Instantaneous lift and drag shows there is a difference in both the negative and positive peak lift and drag values between when the aerofoil is pitching up and when it is pitching down. This is consistent with previous studies. Comparisons with previous studies on oscillating smooth aerofoils do not indicate that such corrugated aerofoils exhibit any advantages. It is possible that any performance enhancements will only manifest itself when operating in tandem with other aerofoils in close proximity such as between the fore and hind-wing of a dragonfly.

A Consolidated Study Regarding the Formation of the Aero-Inlet Vortex

Under certain flow conditions, when an air inlet is aspirated in close proximity to a solid surface, an inlet vortex will form between the inlet and the surface under certain flow conditions. This phenomenon can manifest itself during the operation of aircraft engines either when the aircraft is on the runway prior to take-off or during engine ground run, or when the engine is in a test cell during post maintenance tests. The vortex can pitch debris into the intake causing foreign object damage (F.O.D.) to the engine blades or result in compressor stall. The take-off problem can be partially solved by keeping the runway clear of debris and scheduling the throttle appropriately whenever possible. However throttle scheduling will not be appropriate during engine tests both on the ground and in a test cell. The characteristics of the vortex depends on a number of geometric and flow conditions such as the position of the engine relative to the surface, intake flow capture ratio and upstream flow. To eliminate these vortices at the design stage of aircraft configuration or new test cell, it is essential to be able to predict the onset of the vortex or at least understand the factors affecting their formation. In addition, it is also very important to understand the characteristics of such a vortex to be able to determine the potential damage if complete prevention is not possible.

Effect Of Turbulence Intensity On Vortex Formation Threshold In A Jet Engine Test Cell.

Vortical structures can develop in the intakes of aircraft engines during operation in the proximity of solid surfaces. Take-off and testing in a ground facility are clear examples of such scenarios. When such a vortex is formed and ingested into the engine, potentially catastrophic damage can occur. The vortex can cause the compressor to stall, resulting in severe damage to the engine. Procedures have been put in place to prevent such damage from occurring on the runway. However to prevent such vortices from forming, especially in the test cells, it is necessary to be able to predict the onset of the vortex or at least to understand the factors affecting the formation of such vortices. This paper extends the scope of previous investigations by investigating the effects that increasing upstream turbulence intensity has on the vortex formation threshold. The results show that an increase in upstream turbulence intensity increases the range of conditions over which a vortex forms. All three regimes show signs of shifting the threshold of vortex formation to lower ratio of inlet velocity over upstream average velocity (Vi/Vo) for a given ratio of inlet height over inlet diameter (H/Di).