Liquid crystals have become an accurate and convenient means of measuring surface temperature and heat transfer for the gas turbine and heat transfer research communities. The measurement of surface shear stress using liquid crystals is finding increasing favour with aerodynamicists and developments in these techniques ensure that liquid crystals will continue to provide key thermal and shear stress data in the future. The increasing use of three-dimensional finite element computational models has allowed industry to capitalize on the advantages of the full surface data generated. The paper reviews the use of these complex materials in research with a special emphasis on recent developments in the field. The aim is to provide the reader with an up to date background in this measurement technology and allow the researcher to decide whether liquid crystals would be suitable in specific applications.

https://iopscience.iop.org/article/10.1088/0957-0233/11/7/313/pdf

Liquid crystal measurements of heat transfer and surface shear stress

Numerical simulation of hypersonic flow over highly blunted cones with spike

Digital particle image thermometry/velocimetry (DPIT/V) is a relatively new methodology that allows for measurements of simultaneous temperature and velocity within a two-dimensional domain, using thermochromic liquid crystal tracer particles as the temperature and velocity sensors. Extensive research has been carried out over recent years that have allowed the methodology and its implementation to grow and evolve. While there have been several reviews on the topic of liquid crystal thermometry (Moffat in Exp Therm Fluid Sci 3:14–32, 1990; Baughn in Int J Heat Fluid Flow 16:365–375, 1995; Roberts and East in J Spacecr Rockets 33:761–768, 1996; Wozniak et al. in Appl Sci Res 56:145–156, 1996; Behle et al. in Appl Sci Res 56:113–143, 1996; Stasiek in Heat Mass Transf 33:27–39, 1997; Stasiek and Kowalewski in Opto Electron Rev 10:1–10, 2002; Stasiek et al. in Opt Laser Technol 38:243–256, 2006; Smith et al. in Exp Fluids 30:190–201, 2001; Kowalewski et al. in Springer handbook of experimental fluid mechanics, 1st edn. Springer, Berlin, pp 487–561, 2007), the focus of the present review is to provide a relevant discussion of liquid crystals pertinent to DPIT/V. This includes a background on liquid crystals and color theory, a discussion of experimental setup parameters, a description of the methodology’s most recent advances and processing methods affecting temperature measurements, and finally an explanation of its various implementations and applications.

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Digital particle image thermometry/velocimetry: a review

In supersonic flow, a spike attached to the nose reduces the drag of a blunt body. In this paper, supersonic flows around a spiked blunt body are numerically simulated to examine the effects of the spike length, Mach number, and angle of attack. Three-dimensional thin-layer compressible Navier-Stokes equations are solved using a highresolution upwind scheme with LU-ADI time-integration algorithm. The computed results show that the drag of the spiked blunt body is significantly influenced by the spike length, Mach number, and angle of attack. Scales of the separated region are not significantly influenced by the freestream Mach number. For the spiked blunt body at angle of attack, the flowfield becomes complex with spiral flows. The computed results are in reasonable agreement with experimental data.

Numerical Investigation of Supersonic Flows Around a Spiked Blunt Body

When flying at hypersonic speeds, it is a fundamental requirement to reduce the high drag resulting from a blunt nose cone in the ascent stage to increase the payload weight on the one hand and decrease the amount of energy needed to overcome the Earth's gravity on the other. However, an aerospike can be attached on the front of the nose cone to obtain a high drag and heat load reduction. Different Mach numbers at different altitudes have been chosen to investigate the effect of the aerospike on the nose cone's surrounding flowfield. The drag and the heat load reduction is numerically evaluated at Mach numbers of 5.0, 7.0, and 10.0. Different lengths of the aerospike are investigated between 1 and 4 times the diameter of the dome of the nose cone. Additional modifications to the tip of the spike to obtain different bow shocks are examined, including a sharp front, a blunt spike, and an aerodome mounted on the tip of the spike. To solve the very complicated flowfield, the flow solver CFD-FASTRAN is used.

Numerical Investigation of a Spiked Blunt Nose Cone at Hypersonic Speeds

Nomenclature substrate specific heat capacity, Jkg^K" fin leading-edge diameter, 10 mm hue;Eq. (7) heat transfer coefficient, Wm~K" intensity; Eq. (10) substrate thermal conductivity, Wm^K" mean refractive index of liquid crystal pitch of helical structure, m convective heat flux, Wm~ red, green, and blue coordinates red, green, and blue chromaticity coordinates saturation; Eq. (9) model surface temperature, K adiabatic wall temperature, K model initial temperature, K time, s semi-infinite penetration depth, m thermal diffusivity, k/ pc, ms~ c D H h / k n P qu) R,G, B r, g, b S T Ta 7} t x* a y A = wavelength of reflection, m p = substrate density, kgm~ (/) angle of illumination

Liquid Crystal Thermography for Heat Transfer Measurement in Hypersonic Flows: A Review

The stability of oscillatory motions of vehicles flying at hypersonic Mach numbers is of considerable relevance to their initial design. Methods are needed for quick and accurate predictions of stability and control that are applicable over a wide range of body shapes, angles of attack, and flow conditions without the need to resort to computationally time-consuming numerical flowfield calculation methods. The purpose of this paper is to present experimental and theoretical data concerning the static and dynamic pitching stability of pointed and blunted 10 deg semiangle cones and a double-flared hyperballistic shape. Although Newtonian theory gives inadequate accuracy of prediction, inviscid embedded Newtonian theory, which accounts for the reduced dynamic pressure and lower flow velocity in the embedded flow downstream of the strong bow shock, is shown to provide surprisingly good agreement with experimental data over a wide range of conditions. Comparisons with experimental results show that the broad flow features associated with nose bluntness, angle of attack, and center of gravity position and their effect on static and dynamic stability are well described in regimes not containing flow structural change. However, in some cases discrepancies exist between the predictions and experimental observations, and these have been attributed to a variety of viscous-flow phenomena involving boundary-layer transition and flow separation, including complex lee-surface vortical flows.

Comparison of predictions and experimental data for hypersonic pitching motion stability

Robin East BSc(Eng), PhD, CEng, MRAeS, MA/AA, is Reader in Aeronautics in the Department of Aeronautics and Astronautics at the University of Southampton. His research interests include hypersonic aerodynamics with specific application to re-entry vehicle aerodynamics and stability, high temperature gas dynamics including heat transfer and combustion and development of techniques for gas dynamic testing in intermittent facilities. His teaching interests include thermodynamics, propulsion and gas dynamics. Geoffrey R Hutt BSc(Hons), PhD, AMRAeS, MAIAA is a lecturer in the Aeronautics and Astronautics Department at the University of Southampton. His initial research activities were in the construction and evaluation of a pilot intermittent cryogenic wind tunnel, following which he has undertaken a PhD research programme in re-entry vehicle technology. His teaching interests include mechanics of flight and aerospace systems instrumentation and measurement techniques. The accurate measurement of model position using non-intrusive optical techniques is of importance in dynamic wind tunnel testing. Techniques are described which relate to studies undertaken in intermittent hypersonic wind tunnels in which flow durations vary between 20 ms and 900 ms and the flo~ speed is approximately seven times the speed of sound. Associated high speed data acquisition is an essential feature of these experiments; however, even more fundamental is the problem of acquiring a reliable measurement of the model motion, from which its aerodynamic character can be deduced. In the majority of wind tunnel test programmes, the model being examined is held fixed and subject to the stream velocity of the wind tunnel test section flow. The measurement of aerodynamic loads on the model may be made from strain gauged supports or deduced from integrated model pressure distributions. By virture of its constraints, the static model will only permit static force derivatives to be deduced. However, if the dynamic stability of the vehicle is to be measured, the time dependent aerodynamics must also be explored.

Optical Techniques for Model Position Measurement in Dynamic Wind Tunnel Testing

This paper presents experimental data highlighting the effects of a forward facing aerodynamic spike on the pitch stability of a blunted cone in hypersonic flow. Spike length and angle of attack variations on stability derivatives are described. Suitable application of the inviscid embedded Newtonian pitch stability prediction technique shows how the influence of spike aerodynamics may be successfully modelled. The paper extends the knowledge of aerodynamic spike flow effects, which have to date concentrated on aerodynamic drag reduction qualities at zero degrees angle of attack.

Static and dynamic pitch stability of a blunted cone with forward facing aerodynamic spike in hypersonic flow

Re-entry vehicles when penetrating the earth’s atmosphere may undergo large amplitude oscillatory motions, the aerodynamic stability of which must be determined for the vehicle shape to be evaluated. Bodies travelling close to or above flow Mach* number M = 1 generate shock waves which are discontinuities of flow properties such as pressure, temperature and density. The Schiieren technique has been used to visualize the strong density gradient that exists across the shock front and a comparison made between the shock wave-forms of a model undergoing large amplitude motions with those of the model held fixed at different angles of attack. Any difference in the shock positions could then be attributed to the model motion and hence be related to its stability. Tests using a hypersonic wind tunnel have investigated three axi-symmetric shapes, which have been photographed both in motion with a high-speed cine camera and when held fixed with a conventional 35 mm camera. The Schiieren technique reveals t...

R. A. East

Papers

Liquid Crystal Thermography for Heat Transfer Measurement in Hypersonic Flows: A Review

Comparison of predictions and experimental data for hypersonic pitching motion stability

Optical Techniques for Model Position Measurement in Dynamic Wind Tunnel Testing

Static and dynamic pitch stability of a blunted cone with forward facing aerodynamic spike in hypersonic flow

Application of Schiieren Photography to the Investigation of Large Amplitude Oscillatory Motions of Shapes Travelling at Flow Mach Number M = 6.85