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http://hdl.handle.net/10204/907
Fri, 22 May 2015 17:46:59 GMT2015-05-22T17:46:59ZOptimisation of a novel trailing edge concept for a high lift device
http://hdl.handle.net/10204/7889
Title: Optimisation of a novel trailing edge concept for a high lift device
Authors: Botha, JDM; Dala, L; Schaber, S
Abstract: A novel concept (referred to as the flap extension) is implemented on the leading edge of the flap of a three element high lift device. The concept is optimised using two optimisation approaches based on Genetic Algorithm optimisations. A zero order approach which makes simplifying assumptions to achieve an optimised solution: and a direct approach which employs an optimisation in ANSYS DesignXplorer using RANS calculations. The concept was seen to increase lift locally at the flap. The solution to the zero order optimisation showed a decreased stall angle and decreased maximum lift coefficient against angle of attack due to early stall onset at the flap. The DesignXplorer optimised solution matched that of the baseline solution very closely. Computational Aeroacoustic simulations were performed using the DES (Detached Eddy Simulation) model, in 2D, on the baseline and DesignXplorer optimised solution. The DesignXplorer optimised concept steadied the shear layer that bounds the spoiler cove thus reducing noise from this vicinity by 10dB at frequencies over 7 000Hz.
Description: 29th Congress of the International Council of the Aeronautical Sciences (ICAS), St Petersburg, Russia 7-12 September 2014Mon, 01 Sep 2014 00:00:00 GMThttp://hdl.handle.net/10204/78892014-09-01T00:00:00ZDynamic transition from Mach to regular reflection of shock waves in a steady flow
http://hdl.handle.net/10204/7806
Title: Dynamic transition from Mach to regular reflection of shock waves in a steady flow
Authors: Naidoo, K; Skews, BW
Abstract: The steady, two-dimensional transition criteria between regular and Mach reflection are well established. Little has been done on the dynamic effect on transition due to a rapidly rotating wedge. Results from experiments and computations done on steady and unsteady shock wave transition from Mach reflection to regular reflection, MR RR, are described. The measured motion and the initial shock incidence was used to mimic the experiment with a two-dimensional numerical code. The maximum rotation speed achieved at transition was approximately 2500°s(sup-1). Rapid wedge rotation was shown to have a significant measurable effect on transition. The code was also applied to the dependence of dynamic MR RR transition on other variables in the parameter space. These include rotation about the leading or trailing edge, initial incidence and rotation speed at two free-stream conditions. Impulsively started rotation in these cases was used with the rotation specified by M(subE)= c/a(sub8) where is constant angular velocity (negative anticlockwise), c the distance from the edge considered to the pivot point and a8 the free-stream sound speed. For the Mach numbers and range of rotation speeds tested, both the wedge and shock angle at transition decreased with increased rotation speed. The sensitivity of the transition angle to changing the rotation point from the trailing edge to the experimental model pivot point was investigated briefly at a free-stream Mach number of M=2.98 with M(subE)=-0.1. The wedge angle at transition increased by 1.5° and the shock angle at transition decreased by 1.5°, a significant variation. The effect of the initial incidence was also investigated. By reducing the initial wedge angle from 24.5 to 23.5° for these initial conditions the shock angle at transition decreased by approximately 1.8°, also a marked sensitivity. The flow field development for impulsive rotation about the wedge trailing and leading edges at M=1.93 for M(subE)=-0.075 was analysed in some detail. The flow field development is very sensitive to the rotation centre, more especially at large rotation rates. Four phases of the Mach stem development were identified in both cases. For rotation about the wedge leading edge the Mach stem height remains constant until the expansion waves arrive at the triple point. This is followed by an increase in Mach stem height, which then remains constant for a short period after which it decreases until transition to RR. For rotation about the wedge trailing edge the impulsive start generates a disturbance on the incident wave which propagates down the wave, through the triple point and down the Mach stem. The stem height is constant until the arrival of the incident wave disturbance. This causes a sudden decrease in Mach stem height. Subsequently, the Mach stem height remains constant for a short time, before it decreases until transition to RR. Similar effects in the variation of stem height with wedge angle occur at the higher Mach number of 2.98. It was demonstrated that MR can be maintained for a while at zero wedge incidence with a sufficiently large rotation rate of M(subE)=-0.1, with M=1.93, for both leading and trailing edge pivot points.
Description: Copyright: 2014 Cambridge University Press. This is an ABSTRACT ONLY. The definitive version is published in Journal of Fluid Mechanics, vol. 750, pp 385-400Tue, 01 Jul 2014 00:00:00 GMThttp://hdl.handle.net/10204/78062014-07-01T00:00:00ZControl of cavity acoustics by surface waviness in landing configurations
http://hdl.handle.net/10204/7705
Title: Control of cavity acoustics by surface waviness in landing configurations
Authors: Dala, L
Abstract: The aviation industry’s interest towards cavity control methods rose accordingly due to the acoustic attenuation potential of such methods. However cost effective control devices which are also efficient over a wide range aircraft operations are yet to be developed. This paper investigates the use of a sinusoidal surface modification application upstream of a cavity as a passive acoustics control device in approach conditions. Optimum sinusoidal amplitude and frequency were previously determined by the means of a two-dimensional computational fluid dynamics analysis for a cavity with a length to depth ratio of 4. A complete three-dimensional CFD analysis of this configuration as well as a base-line case without control device was carried out in this study. The overall sound pressure level was reduced with the surface modification at the majority of the points investigated.
Description: Copyright: 2014 IJRAME Aero Team, Hyderabad. This is an ABSTRACT ONLY. The definitive version is published in International Journal of Research in Aeronautical and Mechanical Engineering, vol. 2(8), pp 24-45Fri, 01 Aug 2014 00:00:00 GMThttp://hdl.handle.net/10204/77052014-08-01T00:00:00ZClassification of gap flow regimes in two side-by-side circular cylinders
http://hdl.handle.net/10204/7691
Title: Classification of gap flow regimes in two side-by-side circular cylinders
Authors: Atkins, M; Dala, L; Tongbeum Kim, T
Abstract: The behavior of the flow around two circular cylinders arranged in a side-by-side configuration strongly depends on the transverse center-to-center gap spacing ratio (T/D) where D is the cylinder diameter. Based on the observed wake structure [13, 15], the flow pattern that emerges when the gap spacing is varied is conventionally classified into three flow regimes: single bluff body, biased flow and symmetric flow. The single bluff body flow regime is defined when the gap spacing is small e.g., T/D < 1.2. The wake pattern is an asymmetrical single vortex street with vortices shed alternately from the outer surfaces of the cylinders. The non-dimensional shedding frequency, i.e., Strouhal number S = (f2D/U8) is approximately 0.2 where f is the shedding frequency and U8 is the free stream velocity. In this regime, the characteristic length of 2D agrees with the Strouhal number that is typical calculated for single bluff bodies with a diameter of 2D [12]. The biased flow regime is observed at an intermediate gap spacing ranging between e.g., 1.2 = T/D = 2.2. The flow passing between the cylinders - gap flow - is biased towards one cylinder (Fig.1.(b)). The wake pattern behind that cylinder has a narrow near-wake (n) with a higher vortex shedding frequency Sn = (fnD/U8) ~ 0.3 while the other cylinder has a wider near-wake (w) and a lower shedding frequency Sw = (fwD/U8) ~ 0.1 [15]. The wake is asymmetrical and bi-stable because the gap-flow switches direction between cylinders. The time intervals between the switching or flopping events are several orders of magnitude greater than the vortex shedding period [9, 16]. Due to the relatively large timescale of the switches, the switching might be described as the behavior of a dynamical system with two quasi-stable states (9).
Description: 15th International Symposium on Transport Phenomena and Dynamics of Rotating Machinery, ISROMAC-15, Honolulu, HI, USA, 24-28 February 2014. Abstract attached.Sat, 01 Feb 2014 00:00:00 GMThttp://hdl.handle.net/10204/76912014-02-01T00:00:00Z