Integrating CFD and Experiment in Aerodynamics - CFD4Aircraft

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The Synergy of CFD and Experiments in Aerodynamics Research at Cranfield University, Shrivenham A. J. Saddington, K. Knowles and N. J. Lawson Aeromechanical Systems Group, Department of Aerospace, Power and Sensors, Cranfield University, RMCS, Shrivenham, Swindon, Wiltshire, SN6 8LA, UK k.knowles@rmcs.cranfield.ac.uk Keywords: LDV, PIV, CFD, supersonic, jet, transonic, cavity, racecar Abstract This paper discusses how the combined use of computational fluid dynamics and experimentation has been applied to three particular fluid dynamics problems: high-speed turbulent jet flow, transonic cavity flows and open-wheeled race car aerodynamics. In each case knowledge gathered from one analysis technique has been used to assist in the application of the second technique thereby enabling greater understanding of the flow physics being studied than would have been possible through the isolated use of one or other methodology. 1 Introduction For some time now, aerodynamics research carried out at Cranfield University’s Shrivenham Campus has made use of both experimental and numerical methods. The combination of these two disciplines has enabled greater understanding of the flow physics being studied than would have been possible through either a purely experimental or purely numerical approach. Using examples of recent research projects, this paper will discuss the synergistic role that CFD and experiments have had in our aerodynamics research programme. Early combined CFD and experimental research was primarily driven by the need for CFD validation e.g. [1]. With computational resources limited, the experimental results were primarily used to validate the CFD models. The main objective was to give globally similar results to those measured in the experiments. The CFD was then used to reveal limited extra information on the flowfield. More recently, improved computational resources have enabled refinements to the CFD models. Whilst CFD validation is still important, these models have provided additional insight into the physical processes involved, which were not discernable from the experimental measurements. 2 High-speed jet research The rate at which a supersonic jet mixes with the surrounding ambient fluid is important for many aerospace applications, notably in jet aircraft and rocket propulsion. For supersonic jets the generation of streamwise vortices appears to be beneficial in improving mixing [2] and schemes have been investigated using vortex generators, tabs [3] and other intrusive devices. This work [4,5] presents a numerical and experimental study of mixing in underexpanded, supersonic turbulent jets issuing from axisymmetric and castellated nozzles into quiescent conditions. Experimental measurements using laser doppler velocimetry (LDV) and pitot probe measurements [6] along the jet centreline at a nozzle pressure ratio (NPR = nozzle total to ambient static pressure ratio) of 4 indicated that the 1
castellated nozzles entrained more mass flow into the jet than a simple convergent nozzle. The CFD models verified that this was the case but crucially also provided a physical explanation of the entrainment mechanism that would have been very difficult to deduce from the available experimental data. 2.1 Nozzle design Three castellated nozzles with convergent profiles and an exit diameter, D of 29.4 mm were investigated (Fig. 1). Each nozzle had four regularly spaced castellations. The difference between the three nozzles was confined to the geometry of the gap between each castellation. The first nozzle (regular) had castellations cut by a radial line from the centre of the nozzle, as shown by the inner region of Fig. 2. The ‘outer region’ of Fig. 2 indicates alternative tooth designs (for all four teeth); radial position is as the ‘inner region’. The second nozzle (divergent chamfered) had castellations cut such that the gap between each nozzle was divergent in profile, as indicated by the ‘*’ on the outer region of Fig. 2. The third nozzle (convergent chamfered) had castellations cut such that the gap between each profile was convergent in profile, as indicated by the ‘**’ on the outer region of Fig. 2. The plain axisymmetric nozzle was of the same overall geometry as the castellated ones but with the gaps between the teeth ‘filled in’ [7]. The mixing enhancement obtained from the three castellated nozzle geometries was to be compared with the axisymmetric convergent nozzle [7]. Figure 1: Nozzle geometry showing the internal profile. Figure 2: Nozzle castellation configurations. Hatched boxes indicate locations of teeth in nozzle lip: (*) alternative profile for divergent chamfered castellations; (**) alternative profile for convergent chamfered castellations. 2.2 Experiments Experiments were conducted in a nozzle test cell at Shrivenham (Fig. 3). Data were collected for a NPR of 4. Threedimensional LDV measurements were taken of the flow from the castellated nozzles. Seeding of the jet was by direct injection from a TSI six-jet seeder into the nozzle plenum chamber using JEM Hydrosonic seeding fluid. Measurement traverses were made along the nozzle centreline, x. Probe access limited data collection to the first 10D from the nozzle exit plane. The LDV measurements were estimated to be accurate to ±1% of velocity based on the sample time and frequency. Additional comparative data were taken from centreline LDV measurements of the plain axisymmetric nozzle [4] and pitot probe measurements of the convergent chamfered castellated nozzle [6, 8] made previously under the same test conditions. 2
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