CFD Analysis of Micro-Ramps for Hypersonic Flows Mogrekar Ashish 1, a, Sivakumar, R. 2, b

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1 Applied Mechanics and Materials Submitted: ISSN: , Vols , pp Revised: doi: / Accepted: Trans Tech Publications, Switzerland Online: CFD Analysis of Micro-Ramps for Hypersonic Flows Mogrekar Ashish 1, a, Sivakumar, R. 2, b 1 M.Tech, VIT Chennai, India 2 Professor, SMBS, VIT Chennai, India a mogrekar.ashish2012@vit.ac.in, b sivakumar.r@vit.ac.in Keywords: SBLI, Micro-ramp, Hypersonic, flow control device. Abstract. Air intake is a crucial component for supersonic and hypersonic air breathing propulsion devices. The intake must provide the required mass flow rate of air with minimal loss of stagnation pressure. A major difficulty in the stable operation of an intake is associated with shock wave boundary layer interaction (SBLI). This causes boundary layer separation and adverse pressure gradients which lead to total pressure loss, flow unsteadiness and flow distortion in the intake system. Passive control devices such as micro-ramp, thick-vanes provide better boundary layer control and reduce parasitic drag. The proposed study aims to perform CFD analysis of micro-ramp for hypersonic flows and validate the results with the available experimental data. Two micro ramp models namely MR80 and MR40 are considered for this study. Results obtained show the presence of micro ramp successfully delayed the flow separation and helped to suppress SBLI. Introduction Shock wave- boundary layer interaction (SBLI) is the problem encountered by the air- breathing propulsion system. The SBLIs cause boundary layer separation and adverse pressure gradient. These result in total pressure loss, flow unsteadiness, increased aerodynamic drag and heat fluxes, fluctuating pressure loads and flow distortion in the intake section leading to the reduction of the overall propulsive efficiency of the hypersonic vehicle and premature structural fatigue of aerostructures [1, 2, 3] The quality of the flow field is also reduced significantly. Flow control mechanisms have been developed to prevent the boundary layer separation and to keep the overall propulsive efficiency high even at off-design conditions [3]. These can be applied either at the beginning or throughout the interaction itself. One of the popular flow control method is bleeding the flow at the shock impingement location to suppress the separations. This thins out the boundary layer and reduces the pressure loss. The problem is one tenth of the incoming mass flow has to be removed to make the system effective. To compensate for this loss of mass flow, the size of the inlet has to be increased which results in increase in weight and drag [4]. Passive devices such as micro-ramps, micro-vanes, thick-vanes and fences have emerged as a good flow control methods because of robustness, easy implementation, light weight, cost-effectiveness and simplicity [5]. Micro-ramp is a micro vortex generator, which completely eliminates bleeding. The height of the micro-ramp varies from 20 to 40% of the boundary layer thickness [6]. Since the micro-ramp is small in size, it has higher structural robustness and cannot be easily damaged in the flow field. Also it reduces the parasitic drag. From the experiments conducted on micro-ramp at Mach2.5, it was found that the counter-rotating streamwise vortices generated by the micro-ramp, that travel downstream helped to suppress the SBLIs effect and improve the state of the boundary layer by producing upwash and downwash motion [7]. From the literature survey, it is found that the majority of studies on passive control devices were done for supersonic freestream conditions. Prior to Saad et al. [2], no literature on the effects of passive control devices for hypersonic flow conditions is available. It is not possible to extrapolate the results obtained from the supersonic flow to hypersonic conditions. The hypersonic flow is primarily characterized by a very high level of energy and the effect of heating of the body is very large. Hence, a separate study is required to determine the effectiveness of passive control devices for hypersonic flow conditions. The limitations on measurements in such flows provide the motivation for the present study. The study aims to carry out highly accurate CFD simulations of passive flow control devices, namely, the micro-ramp in order to provide detailed insights into the flow field. This would allow a better understanding of these devices and hence better utilization of All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-10/05/16,13:40:31)

2 Applied Mechanics and Materials Vols the same in practical applications. The results obtained for the micro-ramp study will be compared with the experimental results of Saad et al., [8]. Computational Methodology The micro ramp model considered for our CFD analysis is shown in Fig. 1. The micro ramp is placed over a flat plate with 360 mm x 60 mm x 5 mm dimensions. Assembly of micro ramp model placed in wind tunnel is shown in Fig. 2. Two micro ramp models MR80 and MR40 shown in Table 1 are considered. A base line model without micro ramp named MR00 is also used for comparing results further. In order to develop an oblique shock for SBLI, a shock generator is placed at ceiling of the tunnel which produces 34 oblique shock. In the present study, the geometry of the wind tunnel is truncated near the flat plate to simplify the computational time. Further, computations were performed in one-half of the cross-section as the geometry was symmetry about the vertical (y = constant) mid-plane. The flow over the micro ramp model is compressible and three-dimensional. Air is the working fluid. The SST k-ω turbulence model was used in the present calculations. The operating conditions are shown in Table 2. Table 1 Dimensions of micro ramp models. Dimensions (mm) MR80 MR40 Height, h Chord, c Width, w Fig. 1 Micro ramp model. Fig. 2 Assembly of Micro ramp and flat plate with Boundary Condition. The computational domain was meshed with tetrahedral elements and prism layers were generated (Fig. 3) on micro ramp model to capture near wall regions. Grid independence study was also performed by adapting cells near the shock regions. The solution did not change with the mesh count. To resolve boundary layers, the wall y+ for all the cases was kept less than 5. Table 2 Operating conditions. Total pressure [kpa] 650 Total temperature [K] Mach number 5.0 Free stream pressure [kpa] Free stream temperature [K] Unit Reynolds number [m-1] 13.3 x 106 Boundary Conditions Fig. 3 Zoomed view of meshing. Figure 2 shows the boundary condition details. The micro ramp with flat plate is considered as a solid made of aluminum grade Pressure far field boundary conditions, namely free-stream static pressure, static temperature, Mach number and Z components of flow directions were imposed on the top, bottom and back boundaries. The front boundary is a symmetry boundary hence normal velocity and normal gradients of all the variables were set to zero. The 3-D

3 1964 Dynamics of Machines and Mechanisms, Industrial Research simulations were carried out using the commercial software ANSYS FLUENT 14. The coupled solver was used for all the calculations with explicit time stepping. Results and Discussion Figure 4 shows the Mach contours obtained from CFD and color Schlieren image for baseline case (without Micro ramp). Typical SBLI shock products such as separation shock, reattachment shock, separation bubble, and expansion fan were clearly visible from the contours plot. The boundary layer developed from the leading edge of flat plate is seen. An oblique shock generated by shock generator impinges on the boundary layer developed and it creates a large pressure difference across it. This pressure difference causes the boundary layer separation. A separation bubble is formed at the shock impingement location; this separation bubble causes flow to deflect. The flow deflection causes boundary layer to start to separate and at one point, there will be no contact with wall which is called as a separation point. The series of expansion waves are reflected from the separation region which leads the flow to move back against the wall till it reaches the reattachment point, as the flow direction changes, compression waves are generated, which moved towards the reattachment point. At the reattachment point, another shock wave called the reattachment shock induces. Fig. 4 Flow structure comparison for MR00. The comparison of surface pressure distribution for MR00 and MR80 with experimental results is shown in Figures 5 and 6 respectively. The pressure distribution for centerline region was found the most affected region for upstream interaction; hence the graph was plotted between pressure gradients and span-wise locations for centerline pressure distribution. Note that X= 0 is the location where shock was impinged. The (p/p ) is the ratio of absolute pressure to freestream pressure and (x-x s /δ) is distance from the leading edge of flat plate. For all cases, the pressure rise is observed to start at x= -8, whereas separation is starting at X= -1.4, this shows that the upstream interaction influence in this SBLI case is significantly large. The maximum pressure gradient is observed at X= -1.4 and X= 1.4. The points with maximum pressure gradient are represented as separation point and reattachment point. Fig. 5 Pressure comparison for MR00. Fig. 6 pressure comparison for MR80.

4 Applied Mechanics and Materials Vols Fig. 7 Pressure comparison for MR40. Fig. 8 Pressure comparison for MR00, MR40 and MR80. Figure 7 shows the surface pressure distribution graph for MR40. It is observed that the pressure rise is delayed upstream the location of shock impingement which helps to suppress the flow separation. Fig. 8 shows the surface pressure comparison graph for uncontrolled case (baseline) and flow controlled cases (MR40 and MR80). It is found that the presence of flow control device helped in controlling the flow separation and successfully delayed the pressure rise upstream hence reducing the upstream influence compared to the uncontrolled case. The leading edge shock generated by MR80 was stronger than MR 40. This can be observed as the pressure rise downstream the micro ramp in MR80 is higher. At X= -1.4, a slight increase in pressure gradient is observed which indicates the reduction of separation. The results obtained from CFD are strongly supporting the experimental results. Conclusions The micro ramp models placed in hypersonic flow at a free stream Mach number of 5 have been analyzed using CFD. Flow structures obtained using CFD simulation are compared with available experimental data. The obtained contours show strong agreement with Schlieren images. Typical SBLI shock products such as separation shock, reattachment shock, separation bubble, and expansion fan were clearly visible. The centerline region of flat plate found to be the most affected region by SBLI interaction. Obtained results clearly show the presence of micro ramp successfully delayed the pressure rise upstream the shock impingement and thus help to suppress SBLI. The greater size micro-ramp, MR80 proved to be slightly more effective especially in delaying the pressure rise.the surface pressure distributions for centerline location are compared with experimental data. The numerical predictions obtained match well with the experimental values. Acknowledgements The authors would like to thank Dr. V. Babu, Professor, Department of Mechanical Engineering, IIT Madras for his help and valuable suggestions during the course of this work. References [1] Paul L. Blinde, Ray A.Humble, Bas W. van Oudheusden and Fulvio Scarano, (2009), Effects of micro-ramps on a shock wave/turbulent boundary layer interaction, Shock Waves, 19, [2] Mohd R. Saad, Hossein Zare-Behtash, Azam Che-Idris and Konstantinos Kontis, (2012), Micro- Ramps for Hypersonic Flow Control, Micromachines 3, [3] Frank K. Lu, Qin Li and Chaoqun Liu, (2012), MIcrovortex generators in high-speed flow, Progress in Aerospace Sciences, 53, [4] S. Lee, E. Loth and H. Babinsky, (2011), Normal shock boundary layer control with various vortex generator geometries, Computers & Fluids, 49,

5 1966 Dynamics of Machines and Mechanisms, Industrial Research [5] Roschelle R. Martis, and Ajay Misra, (2013), Effect of height of microvortex generators on swept shock wave boundary layer interactions, CEAS Aeronaut J, DOI 10,1007/s y, [6] Yonghua Yan and Chaoqun Liu, (2013), Study on the initial evolution of ring-like vortices generated by MVG, CEAS Aeronaut J, DOI 10, 1007/s , [7] Babinksy H, Li. Y, Pitt Ford CW,(2009), Microramp control of supersonic oblique shock wave/ boundary layer interactions, AIAA Journal, 47, [8] Mohd R. Saad, Experimental Studies On Shock Boundary Layer Interactions Using Micro- Ramps At Mach 5, University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences, 2013.

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