Shock Tunnel Investigations On Hypersonic Separated Flows

Abstract

Knowledge of flow separation is very essential for proper understanding of both external and internal aerothermodynamics of bodies. Because of unique flow features such as thick boundary layers, merged shock layers, strong entropy layers, flow separation in the flow field of bodies at hypersonic speeds, is both complex as well as interesting. The problem of flow separation is further complicated at very high stagnation enthalpies because of the real gas effects. Notwithstanding the plethora of information available in open literature even for simple geometric configurations the experimentally determined locations of flow separation and re-attachment points do not match well with the results from the computational studies even at hypersonic laminar flow conditions. In this backdrop the main aim of the present study is to generate a reliable experimental database of classical separated flow features around generic configurations at hypersonic laminar flow conditions. In the present study, flow visualization using high speed camera, surface convective heat transfer rate measurements using platinum thin film sensors, and direct skin friction measurements using PZT crystals have been carried out for characterizing the separated flow field around backward facing step, double cone and double wedge models. The numerical simulations by solving the Navier-Stokes equations have also been carried out to complement the experimental studies. The generic models selected in the present study are simple configurations, where most of the classical hypersonic separated flow features of two-dimensional, axi-symmetric and three dimensional flow fields can be observed. All the experiments are carried out in IISc hypersonic shock tunnel (HST2) at Mach 5.75 and 7.6. For present study, helium and air have been used as the driver and test gases respectively. The high speed schlieren flow visualization is carried out on backward facing step (2 and 3 mm step height), double cone (semi-apex angles of 150/350 and 250/680) and double wedge (semi-apex angles of 150/350) models by using high speed camera (Phantom 7.1). From the visualized shockwave structure in the flow field the flow reattachment point after separation has been clearly identified for backward facing step, double cone and double wedge models at hypersonic Mach numbers while the separation point could not be clearly identified because of the low free stream density in shock tunnels. However the flow visualization studies helped clearly identifying the region of flow separation on the model. Based on the results from the flow visualization studies both the physical location and distribution of platinum thin film gauges was finalized for the heat transfer rate measurements. Surface heat transfer rates along the length of two backward facing step (2 and 3 mm step height) models have been measured using platinum thin film gauges deposited on Macor substrate. The Eckert reference temperature method is used along the flat plate for predicting the heat flux distribution. Theoretical analysis of heat flux distribution down stream of the backward facing step model has been carried out using Gai’s dimensional analysis. The study reveals for the first time that at moderate stagnation enthalpy levels (~2 MJ/kg) the hypersonic separated flow around a backward facing step reattaches rather smoothly without any sudden spikes in the measured values of surface heat transfer rates. Based on the measured surface heating rates on the backward facing step, the reattachment distance was estimated to be approximately 10 and 8 step heights downstream of 2 and 3 mm step respectively at nominal Mach number of 7.6. Convective surface heat transfer experiments have also been carried out on axi-symmetric double cone models (semi-apex angles of 15/35 and 25/68), which is analogous to the Edney’s shock interactions of Type VI and Type IV respectively. The flow is unsteady on the double cone model of 25/68 and measured heat flux is not constant. The heat transfer experiments were also carried out on the three-dimensional double wedge model (semi-apex angles of 15/35). The separation and reattachment points have been clearly identified from the experimental heat transfer measurements. It has been observed that the measured heat transfer rates on the double wedge model is less than the double cone model (semi-apex angles of 150/350) for the identical experimental conditions at the same gauge locations. This difference could be due to the three-dimensional entropy relieving effects of double wedge model. PZT-5H piezoelectric based skin friction gauge is developed and used for direct skin friction measurements in hypersonic shock tunnel (HST2). The bare piezoelectric PZT-5H elements (5 mm × 5 mm with thickness of 0.75 mm) polarized in the shear mode have been used as a skin friction gauge by operating the sensor in the parallel shear mode direction. The natural frequency of the skin friction sensor is ~80 kHz, which is suitable for impulse facilities. The direct skin friction measurements are carried out on flat plate, backward facing step (2 mm step height) and double wedge models. The measured value of skin friction coefficient (integrated over an area of 25 sq. mm; sensor surface area) at a distance of 23 mm from the leading edge of the sharp leading edge backward facing step model is found to be ~ 0.0043 while it decreases to ~ 0.003 at a distance of 43 mm from the leading edge at a stagnation enthalpy of ~ 2MJ/kg. The measured skin friction matches with the Eckert reference temperature within ± 10%. The skin friction coefficient is also measured on the double wedge at a distance of 73 mm from the tip of the first wedge along the surface and is found to be 4.56 × 10-3. Viscous flow numerical simulations are carried out on two-dimensional backward facing step, axi-symmetric double cone and three-dimensional double wedge models using ANSYS-CFX 5.7 package. Navier-Stokes Simulations are carried out at Mach 5.75 and 7.6 using second order accurate (both in time and space) high resolution scheme. The flow is assumed to be laminar and steady throughout the model length except on the double cone (semi-apex angles of 250/680) model configuration, which represents the unsteady flow geometry. Analogous Edney Type VI and Type IV shock interactions are observed on double cone, double wedge (semi-apex angles of 150/350) and double cone (semi-apex angles of 250/680) models respectively from the CFD results. Experimentally measured convective heat transfer rates on the above models are compared with the numerical simulation results. The numerical simulation results matches well with the experimental heat transfer data in the attached flow regions. Considerable differences are observed between the measured surface heat transfer rates and numerical simulations both in the separated flow region and on the second cone/wedge surfaces. The separation and reattachment points can be clearly identified from both experimental measurements and numerical simulations. The results from the numerical simulations are also compared with results from the high speed flow visualization experiments. The experimental database of surface convective heating rates, direct skin friction coefficient and shockwave structure in laminar hypersonic flow conditions will be very useful for validating CFD codes