Thermal Streaks Research Articles

Nonlinear Evolution of Instabilities in a Laminar Separation Bubble at Hypersonic Speeds

The development of both convective stationary perturbation and global instabilities in the vicinity of a laminar separation bubble above an axisymmetric compression corner in hypersonic flow was investigated using numerical simulations. The flow configuration of interest corresponded to the cone–cylinder–flare model used in experimental measurements in the Boeing/U.S. Air Force Office of Scientific Research Mach-6 Quiet Tunnel (BAMQT) at Purdue University. For a flare angle of 10 deg and unit Reynolds number of 11.5×106 m−1, their surface flow visualizations identified the presence of streamwise elongated thermal streaks near the reattachment location that had a dominant azimuthal wave number m=36. Previous linear stability analyses predicted that the amplification characteristics of small-amplitude, unsteady, and convective instabilities within this flow were consistent with the surface pressure fluctuations measured in the experiment. However, their accompanying investigation of global instabilities showed that the separation bubble was weakly unstable at the 10 deg flare angle, with the most unstable global mode corresponding to a stationary disturbance with m≈5–6 (i.e., well below the measured wave number of m=36. Besides characterizing the global instability for selected flare angles, the present numerical simulations quantify the details of the stationary equilibrium state associated with supercritical bifurcation resulting from the nonlinear saturation of the unstable global mode. Although velocity perturbations associated with the saturated global mode are dominated by the fundamental spanwise wavelength associated with the linear global instability, the surface heat flux downstream of the reattachment is dominated by m=36, in agreement with the experimental measurements.

Thermal streak spacing in fully developed duct flow with different Reynolds and Prandtl numbers

The present study investigates the behaviour of thermal streaks on a heated foil which is cooled with turbulent flow in a square duct channel. Real-time infrared thermography is used to visualize and measure the spacing between the thermal streaks. A stainless-steel foil with a thickness of 25 microns is cooled by water. The experiments were performed in a range of Reynolds numbers from 5000 to 20000 and Prandtl numbers from 3 to 7. The mean temperature, root-mean-square of temperature and autocorrelation function have been calculated and used to measure the average thermal streak spacing and power spectra in the spanwise and streamwise directions. The root mean square temperature was 0.3 °C to 0.5 °C which corresponds to roughly 10% of the mean temperature difference between foil and water. The uncertainty in mean temperature difference and root mean square temperature was around 5% and 10%, respectively. The measured thermal streak spacing was 100 wall unit to 180 wall unit under the present experimental range. The uncertainty in measured thermal streak spacing was around 2.5%. The effects of Reynolds number, Prandtl number and heat flux on the thermal streak spacing and also on the statistics of the temperature field have been presented and discussed in this paper. A new correlation has been proposed to predict the dimensionless thermal streak spacing. The error in the prediction is estimated within ± 15 %.

Free-stream coherent structures in parallel compressible boundary-layer flows at subsonic and moderate supersonic Mach numbers

As a first step towards the description of coherent structures in compressible shear flows, we present an asymptotic description of nonlinear travelling-wave solutions of the Navier–Stokes equations in the compressible asymptotic suction boundary layer (ASBL). We consider free-stream Mach numbers $M_\infty$ in the subsonic and moderate supersonic regime so that $0\leqslant M_\infty \leqslant 2$ . We extend the large-Reynolds-number asymptotic theory of Deguchi & Hall (J. Fluid Mech., vol. 752, 2014, pp. 602–625) describing ‘free-stream’ coherent structures in incompressible ASBL flow to describe a nonlinear interaction in a thin layer situated just below the free stream. Crucially, the nonlinear interaction equations for the velocity field in this layer are identical to those obtained in the incompressible problem, and thus the asymptotic analysis supporting free-stream coherent structures in compressible ASBL is easily deduced from its incompressible counterpart. The nonlinear interaction produces streaky disturbances to both the velocity and temperature fields, which can grow exponentially towards the wall. We complete the description of the growth of the velocity and thermal streaks throughout the flow by solving the compressible boundary-region equations numerically. We show that the velocity and thermal streaks obtain their maximum amplitude in the unperturbed boundary layer. Increasing the free-stream Mach number enhances the thermal streaks and suppresses the velocity streaks, whereas varying the Prandtl number suppresses the velocity streaks, and can either enhance or suppress the thermal streaks depending on whether the flow is in the subsonic or moderate supersonic regime. Such nonlinear equilibrium states have been implicated in shear transition in incompressible flows; therefore, our results indicate that a similar mechanism may also be present in compressible flows.

Investigation of turbulent flow in square duct with heated foil thermometry

Fully developed turbulent flow of water in a square duct has been studied with thermography of the 25 μm heated steel foil that covers part of the duct wall. Thermal fluctuations observed in the specific case fall in-between the ideal isothermal boundary condition, which is achieved with a thick heater cooled with water, and ideal isoflux boundary condition, which could be theoretically achieved with an infinitely thin heated foil. Results in the present paper are obtained at fixed Reynolds number 10000 and heat flux at the thin foil around 6 kW/m2, which was considered to be the upper limit, where the water can be treated as a passive scalar at Prandtl number 5.4 with temperature increments around 5 °C above the inlet value. Main experimental results are 2D temperature fields on the outer side of the foil, which were averaged over time to predict the mean foil temperature, temperature fluctuations and the corresponding spectra. These parameters provide accurate description of the turbulence near the heated foil. Simple geometry of the experiment and fully developed velocity field are convenient for accurate numerical simulation, which was performed with wall-resolved Large Eddy Simulation approach. Average magnitude of the measured and computed temperature fluctuations was roughly 0.5 °C and 0.6 °C, respectively, which corresponds to about 10% of the temperature increment. Measured thermal streak spacing was around 60 wall units, and was together with the power spectra of the temperature fluctuations in close agreement with the LES numerical predictions that included the foil with conjugate heat transfer. Despite the square cross-section of the duct, which contains around dozen thermal streaks on each side of the duct, the spectra and the streak spacing remain similar as in the channel flow between the parallel planes.

Evaluation of Impact of Hot-Mix Asphalt Density Differentials on Thermal Streak Phenomenon by Passive Infrared Thermography

AbstractThe impact of hot-mix asphalt (HMA) density differentials on thermal streaks and their temporal evolution were examined by means of infrared thermography under laboratory conditions. A seri...

Nonlinear unsteady streaks engendered by the interaction of free-stream vorticity with a compressible boundary layer

The nonlinear response of a compressible boundary layer to unsteady free-stream vortical fluctuations of the convected-gust type is investigated theoretically and numerically. The free-stream Mach number is assumed to be of $O(1)$ and the effects of compressibility, including aerodynamic heating and heat transfer at the wall, are taken into account. Attention is focused on low-frequency perturbations, which induce strong streamwise-elongated components of the boundary-layer disturbances, known as streaks or Klebanoff modes. The amplitude of the disturbances is intense enough for nonlinear interactions to occur within the boundary layer. The generation and nonlinear evolution of the streaks, which acquire an $O(1)$ magnitude, are described on a self-consistent and first-principle basis using the mathematical framework of the nonlinear unsteady compressible boundary-region equations, which are derived herein for the first time. The free-stream flow is studied by including the boundary-layer displacement effect and the solution is matched asymptotically with the boundary-layer flow. The nonlinear interactions inside the boundary layer drive an unsteady two-dimensional flow of acoustic nature in the outer inviscid region through the displacement effect. A close analogy with the flow over a thin oscillating airfoil is exploited to find analytical solutions. This analogy has been widely employed to investigate steady flows over boundary layers, but is considered herein for the first time for unsteady boundary layers. In the subsonic regime the perturbation is felt from the plate in all directions, while at supersonic speeds the disturbance only propagates within the dihedron defined by the Mach line. Numerical computations are performed for carefully chosen parameters that characterize three practical applications: turbomachinery systems, supersonic flight conditions and wind tunnel experiments. The results show that nonlinearity plays a marked stabilizing role on the velocity and temperature streaks, and this is found to be the case for low-disturbance environments such as flight conditions. Increasing the free-stream Mach number inhibits the kinematic fluctuations but enhances the thermal streaks, relative to the free-stream velocity and temperature respectively, and the overall effect of nonlinearity becomes weaker. An abrupt deviation of the nonlinear solution from the linear one is observed in the case pertaining to a supersonic wind tunnel. Large-amplitude thermal streaks and the strong abrupt stabilizing effect of nonlinearity are two new features of supersonic flows. The present study provides an accurate signature of nonlinear streaks in compressible boundary layers, which is indispensable for the secondary instability analysis of unsteady streaky boundary-layer flows.

Entrainment of short-wavelength free-stream vortical disturbances in compressible and incompressible boundary layers

The fundamental difference between continuous modes of the Orr–Sommerfeld/Squire equations and the entrainment of free-stream vortical disturbances (FSVD) into the boundary layer has been investigated in a recent paper (Dong & Wu, J. Fluid Mech., vol. 732, 2013, pp. 616–659). It was shown there that the non-parallel-flow effect plays a leading-order role in the entrainment, and neglecting it at the outset, as is done in the continuous-mode formulation, leads to non-physical features of ‘Fourier entanglement’ and abnormal anisotropy. The analysis, which was for incompressible boundary layers and for FSVD with a characteristic wavelength of the order of the local boundary-layer thickness, is extended in this paper to compressible boundary layers and FSVD with even shorter wavelengths, which are comparable with the width of the so-called edge layer. Non-parallelism remains a leading-order effect in the present scaling, which turns out to be more general in that the equations and solutions in the previous paper are recovered in the appropriate limit. Appropriate asymptotic solutions in the main and edge layers are obtained to characterize the entrainment. It is found that when the Prandtl number $\mathit{Pr}<1$, free-stream vortical disturbances of relatively low frequency generate very strong temperature fluctuations within the edge layer, leading to formation of thermal streaks. A composite solution, uniformly valid across the entire boundary layer, is constructed, and it can be used in receptivity studies and as inlet conditions for direct numerical simulations of bypass transition. For compressible boundary layers, continuous spectra of the disturbance equations linearized about a parallel base flow exhibit entanglement between vortical and entropy modes, namely, a vortical mode necessarily induces an entropy disturbance in the free stream and vice versa, and this amounts to a further non-physical behaviour. High Reynolds number asymptotic analysis yields the relations between the amplitudes of entangled modes.

Secondary instabilities of Görtler vortices in high-speed boundary layer flows

Görtler vortices developed in laminar boundary layer experience remarkable changes when the flow is subjected to compressibility effects. In the present study, five $\mathit{Ma}$ numbers, covering incompressible to hypersonic flows, at $\mathit{Ma}=0.015$, 1.5, 3.0, 4.5 and 6.0 are specified to illustrate these effects. Görtler vortices in subsonic and moderate supersonic flows ($\mathit{Ma}=0.015$, 1.5 and 3.0) are governed by the conventional wall-layer mode (mode W). In hypersonic flows ($\mathit{Ma}=4.5$, 6.0), the trapped-layer mode (mode T) becomes dominant. This difference is maintained and intensifies downstream leading to different scenarios of secondary instabilities. The linear and nonlinear development of Görtler vortices which are governed by dominant modal disturbances are investigated with direct marching of the nonlinear parabolic equations. The secondary instabilities of Görtler vortices set in when the resulting streaks are adequately developed. They are studied with Floquet theory at multiple streamwise locations. The secondary perturbations become unstable downstream following the sequence of sinuous mode type I, varicose mode and sinuous mode type II, indicating an increasing threshold amplitude. Onset conditions are determined for these modes. The above three modes can each have the largest growth rate under the right conditions. In the hypersonic cases, the threshold amplitude $A(u)$ is dramatically reduced, showing the significant impact of the thermal streaks. To investigate the parametric effect of the spanwise wavenumber, three global wavenumbers ($B=0.5$, 1.0 and $2.0\times 10^{-3}$) are specified. The relationship between the dominant mode (sinuous or varicose) and the spanwise wavenumber of Görtler vortices found in incompressible flows (Li & Malik, J. Fluid Mech., vol. 297, 1995, pp. 77–100) is shown to be not fully applicable in high-speed cases. The sinuous mode becomes the most dangerous, regardless of the spanwise wavelength when $\mathit{Ma}>3.0$. The subharmonic type can be the most dangerous mode while the detuned type can be neglected, although some of the sub-dominant secondary modes reach their peak growth rates under detuned states.

Measurements of surface thermal structure, kinematics, and turbulence of a large-scale solitary breaking wave using infrared imaging techniques

The surface temperature fields of large-scale solitary breaking waves are measured using infrared imaging techniques in a laboratory surf and swash zone. The surface velocity fields obtained by cross-correlating the images are decomposed into wave and turbulent motions using two filtering methods in the spatial and temporal domains. The techniques presented here provide new quantitative descriptions for the evolution of the surface thermal structures, kinematics, and turbulence that are induced by unsteady and highly foamy turbulent coastal flows. Novel organized streaks of thermal structures, which exhibit a finger-like shape, are found on the water surface of the crest roller behind the head of the rebounding jet. These thermal streaks evolve with time and become isotropic when returning to the surrounding bulk water temperature. The Froude-scaled maximum flow speed, accelerations, and vorticity are O(1), and the scaled turbulent kinetic energy (TKE) is O(−1); these results are similar to previous findings from numerical results and periodic surf-zone breakers. Significant and concentrated structures of these quantities occur in the moving wave crest during the uprush phase; however, these structures only develop during the late stages of the backwash phase. The TKE increases shoreward from the surf to the swash zones. The ratio of the averaged variance of the turbulent velocity in the wave breaking zone does not agree with the canonical prediction for plane-wake turbulence; however, the ratio is similar to that of boundary-layer turbulence and decreases in the bore region and the swash zone, indicating an increase in the turbulence anisotropy shoreward from the surf to the shallower swash flow.

Nonlinear Response of a Compressible Boundary Layer to Free-stream Vortical Disturbances

The nonlinear response of a compressible boundary layer to unsteady free-stream vortical fluctuations of the convected-gust type is investigated. The amplitude of the disturbances is strong enough for nonlinear interactions to be induced within the boundary layer. Attention is focused on the induced low-frequency streamwise-stretched components of the boundary-layer disturbances, known as streaks or Klebanoff modes, which may become unstable and break down to fully developed turbulence. The streaks are described using the mathematical framework of the so-called boundary-region equations, i.e. the asymptotic limit of the Navier-Stokes equations for low-frequency disturbance, developed by Leib et al.1 and extended by Ricco & Wu 2 and Ricco et al.3 to linear perturbations in compressible boundary layers, and nonlinear disturbances in incompressible boundary layers, respectively. In the present work, compressibility is taken into account through aerodynamic heating effects due to the mean-flow velocity being comparable with the speed of sound. Both velocity and thermal (temperature) streaks are generated as well as are a new mean flow and fluctuations of higher frequencies. Nonlinearity attenuates the fluctuations of the streamwise velocity and a similar stabilizing effect is observed on the temperature.This represents the preliminary study which is necessary to carry out the secondary instability analysis on the new unsteady compressible three-dimensional flow generated by the nonlinear interactions.

Compressible laminar streaks with wall suction

The response of a compressible laminar boundary layer subject to free-stream vortical disturbances and steady mean-flow wall suction is studied. The theoretical frameworks of Leib et al. [J. Fluid Mech. 380, 169–203 (1999)10.1017/S0022112098003504] and Ricco and Wu [J. Fluid Mech. 587, 97–138 (2007)10.1017/S0022112007007070], based on the linearized unsteady boundary-region equations, are adopted to study the influence of suction on the kinematic and thermal streaks arising through the interaction between the free-stream vortical perturbations and the boundary layer. In the asymptotic limit of small spanwise wavelength compared with the boundary layer thickness, i.e., when the disturbance flow is conveniently described by the steady compressible boundary region equations, the effect of suction is mild on the velocity fluctuations and negligible on the temperature fluctuations. When the spanwise wavelength is comparable with the boundary layer thickness, small suction values intensify the supersonic streaks, while higher transpiration levels always stabilize the disturbances at all Mach numbers. At larger spanwise wavelengths, very small amplitudes of wall transpiration have a dramatic stabilizing effect on all boundary layer fluctuations, which can take the form of transiently growing thermal streaks, large amplitude streamwise oscillations, or oblique exponentially growing Tollmien-Schlichting waves, depending on the Mach number and the wavelengths. The range of wavenumbers for which the exponential growth occurs becomes narrower and the location of instability is significantly shifted downstream by mild suction, indicating that wall transpiration can be a suitable vehicle for delaying transition when the laminar breakdown is promoted by these unstable disturbances. The typical streamwise wavelength of these disturbances is instead not influenced by suction, and asymptotic triple deck theory predicts the strong changes in growth rate and the very mild modifications in streamwise wavenumber in the limit of larger downstream distance and small spanwise wavenumber.

1047 UNSTEADY MEASUREMENT OF CONVECTIVE HEAT TRANSFER TO A WATER FLOW IN A CIRCULAR PIPE USING HIGH-SPEED INFRARED THERMOGRAPH

Unsteady measurements of convective heat transfer was performed to a water flow in a horizontal circular pipe with inner diameter of D = 20.4 mm for Reynolds number from 1000 to 30000. The test surface for the heat transfer measurement was fabricated from a titanium foil of 22 μm thick coated with black paint, which was heated electrically under conditions of constant heat flux. The time-spatial distribution of the heat transfer coefficient was evaluated from the temperature fluctuation on the test surface measured using a high-speed infrared thermograph (〜800Hz). The time-averaged heat transfer coefficient measured here agreed well to the conventional empirical correlation. As a result of the present measurement, the dynamic feature of the thermal streaks in a circular pipe was revealed, which elongated along the streamwise direction and flows downstream with some meanderings. The statistic values of the spatio-temporal characteristics, such as the mean spanwise wavelength of thermal streaks, was investigated and compared with those of the previously published literature.

I224 円管内乱流熱伝達の時空間変動特性(一般講演(13))

Unsteady measurement of convective heat transfer was performed to a water flow in a horizontal circular pipe using a high-speed infrared thermograph. The test surface was fabricated from a 22μm thick titanium foil coated with black paint. The removable turbulence promoter was attached at entrance of the pipe. From the time-series data of the heat transfer distribution, we could obtain the rms value of fluctuating heat transfer coefficient and mean spanwise wavelength of the thermal streaks and the mean period of heat transfer fluctuation.

E124 高速度赤外線カメラを用いた円管内水流の熱伝達測定(OS-18: 熱・物質移動の計測および現象解析(2))

In order to acquire the basic data for the thermal conjugation problems between fluid and solid, we are planning to measure spatio-temporal heat transfer from a solid wall to a liquid flow using a high-speed infrared thermograph. The purpose of this work is to verify the feasibility of this measurement to a turbulent water flow in a circular tube. The test surface was fabricated from a 22μm thick titanium foil coated with black paint. As a result, the time-averaged heat transfer coefficient measured here agreed well to the conventional empirical correlation. Also, statistic values of the time-spatial distribution, such as the rms value of the fluctuating heat transfer coefficient and the mean spanwise wavelength of the thermal streaks were reasonably agreed to those reported in previously published literature.

Correlation between small-scale velocity and scalar fluctuations in a turbulent channel flow

Direct numerical simulations of a turbulent channel flow with passive scalar transport are used to examine the relationship between small-scale velocity and scalar fields. The Reynolds number based on the friction velocity and the channel half-width is equal to 180, 395 and 640, and the molecular Prandtl number is 0.71. The focus is on the interrelationship between the components of the vorticity vector and those of the scalar derivative vector. Near the wall, there is close similarity between different components of the two vectors due to the almost perfect correspondence between the momentum and thermal streaks. With increasing distance from the wall, the magnitudes of the correlations become smaller but remain non-negligible everywhere in the channel owing to the presence of internal shear and scalar layers in the inner region and the backs of the large-scale motions in the outer region. The topology of the scalar dissipation rate, which is important for small-scale scalar mixing, is shown to be associated with the organized structures. The most preferential orientation of the scalar dissipation rate is the direction of the mean strain rate near the wall and that of the fluctuating compressive strain rate in the outer region. The latter region has many characteristics in common with several turbulent flows; viz. the dominant structures are sheetlike in form and better correlated with the energy dissipation rate than the enstrophy.

Near-wall similarity between velocity and scalar fluctuations in a turbulent channel flow

The degree of near-wall similarity between velocity and scalar fields is examined with the use of direct numerical simulation databases for a smooth wall turbulent channel flow with passive scalar transport. Particular attention is given to correlations between velocity and scalar fluctuations as well as those between their derivatives. In the vicinity of the wall, the largest correlation is between the wall-normal vorticity fluctuation and the spanwise scalar derivative, mainly due to the significantly reduced effect of the fluctuating pressure gradient. Other correlations peak at locations where the effect of the pressure fluctuation is smallest. The near-wall similarity between momentum and thermal streaks is not perfect because momentum streaks are often intensified or weakened by the fluctuating pressure gradient.

Oscillatory thermal structures induced by unconfined slot jet impingement

Low-frequency temporal measurements of impingement surface temperature in a nominally steady turbulent submerged impinging slot jet flow using infrared (IR) thermography are presented and analyzed. Data of surface temperature oscillations were recorded at an exit Reynolds number of 22,500 and at nozzle spacings ( Y) of 0.5, 1.0, 2.0, 4.5 and 5 nozzle hydraulic diameters ( D h) from the impingement surface. Mean temperature maps were used to identify specific locations (lines) of interest on the impingement surface for transient experiments. Low-magnitude, periodically repeating thermal streaks were observed for the Y/ D h = 5.0 jet impingement, suggesting the presence of near-wall streamwise counter-rotating vortex pairs along the impingement line. For the Y/ D h = 0.50 jet impingement, temperature streaks at the impingement line were not detected; however, distinct thermal streaks were observed at locations corresponding to the local minimum and secondary maximum in heat transfer. To better understand the trends in the temperature oscillations, the time series were analyzed using a proper orthogonal decomposition (POD) method. Approximately 80% of the variance in temperature fluctuations at the impingement line could be reconstructed from five modes for the Y/ D h = 5.0 jet impingement. For the near-wall ( Y/ D h = 0.50) jet impingement, the dominant two modes accounted for 80% of the variance in temperature fluctuations at the locations of minimum and secondary peak in heat transfer.

Measurements of Time-Space Distribution of Convective Heat Transfer to Air Using a Thin Conductive Film

A measurement technique of time-space characteristics of heat transfer to air has been developed using a thin conductive film and a high-speed infrared thermograph. In this work, a titanium foil of 2 m thick was used as a test surface, and measured temperature on it by employing an infrared thermograph of 120 Hz. The accuracy of the measurements was confirmed by comparing the heat transfer coefficient of a laminar boundary layer to that of a numerical analysis. In order to verify the applicability of this measurement technique to practical measurements, unsteady heat transfer on the wall of a turbulent boundary layer was examined. It was possible to restore the timespace distribution of the heat transfer coefficient up to 30 Hz in time and 4.5 mm in spatial wavelength by solving the heat conduction equations inside the wall, even though the heat transfer coefficient was low ( h = 10 – 20 W/mK). The results showed that the time-space behavior of the heat transfer was clearly revealed, which was reflected by the streaks formed in the near-wall region of the turbulent boundary layer. The statistical values of the turbulent boundary layer, that is, rms value of the fluctuating heat transfer coefficient and mean spacing of the thermal streaks, agreed well to those of previous data of DNS and experiments. INTRODUCTION Convective heat transfer generally has a nature of nonuniformity and unsteadiness, which is reflected by a three-dimensional flow near a wall. However, most experimental studies concerning the convective heat transfer have been performed in a time-averaged manner or using one-point measurements. This frequently results in poor understandings on the mechanisms of the heat transfer. Measurement techniques for time-space characteristics of the heat transfer have been developed using liquid crystal (Iritani et al., 1983) or using infrared thermography (Hetsroni and Rozenblit, 1994, and Nakamura and Igarashi, 2004, 2006), by employing a thin test surface having low heat capacity. However, the major problem of these measurements is attenuation of the temperature fluctuation due to the heat capacity of the test surface. Also, lateral conduction through the test surface attenuates the amplitude of the spatial temperature distribution. These attenuations are considerably large, especially for the heat transfer to air for which the heat transfer coefficient is low. The present author investigated the frequency response and the space resolution of the heat transfer from a test surface by solving heat conduction equations (Nakamura, 2007). Figure 1 (a) and (b) shows the upper limit of the fluctuating frequency, fmax, and the lower limit of the spatial wavelength, bmin, respectively, which are detectable using infrared thermograph, where h is fluctuating amplitude of the heat transfer coefficient and TIR0 is noise equivalent temperature difference (NETD) of infrared thermograph for a black body. If an extremely thin conductive film, as indicated in Fig. 1, is used as the heated surface, the unsteady heat transfer to air is observable up to f 50 Hz and b 1 mm (at Tw – T0 = 30C and h = 3 – 5 W/mK) by employing an infrared thermograph of nowadays ( TIR0 = 0.025C). In this work, the measurement technique using a thin conductive film and a high-speed infrared thermograph was applied to measure the unsteady heat transfer to air caused by flow turbulence. The attenuation due to the heat capacity and the lateral conduction was restored by solving the heat conduction equations inside the wall. NOMENCLATURE bc : cutoff wavelength in space bmin : lower limit of spatial wavelength detectable c : specific heat fc : cutoff frequency fmax : upper limit of fluctuating frequency detectable h : heat transfer coefficient l : wall-friction length = /u q : heat flux Re : Reynolds number based on momentum thickness T : temperature T0, Tw : freestream and wall temperatures TIR0 : noise equivalent temperature difference of infrared thermograph for a black body t : time u0, u : freestream and wall-friction velocities x, y, z : streamwise, vertical, and spanwise coordinates : thickness : thermal conductivity : kinematic viscosity of fluid : density of fluid Superscripts and Subscripts ( ) : mean value ( )rms : root-mean-square value

Heat transfer measurement of turbulent spots in a hypersonic blunt-body boundary layer

This paper presents data on turbulent-spot propagation in the hypersonic boundary-layer flow over a blunted cylindrical body. Data are based on the measurement of time-dependent surface heat transfer rates using gauges positioned as arrays in either the axial or transverse directions. These are used to provide data on individual spots, including sectional profiles, characteristic spot planform geometries, propagation speeds, growth rates and some information on the development of an internal thermal cell structure and corresponding thermal streaks in the base or calm region of the spot.

Transient thermal structure, turbulence, and heat transfer in a reattaching slot jet flow

The role of turbulent fluctuations on mean heat transfer coefficient in a reattaching slot jet flow is studied experimentally. Convective heat transfer rate and near-wall fluid flow are examined in the recirculation, reattachment, and post-reattachment regions for two nozzle-to-surface spacings of 0.25 and 0.75 times the width of the nozzle bottom plate. In the reattachment region, results indicate a strong correspondence between variances of near-wall velocity fluctuation and peak heat transfer rate for both spacings. Thermal structures that vary in the spanwise direction are identified in the recirculation region from low-frequency transient infrared thermographs of the heated surface. While these thermal structures are confined to regions in the vicinity of nozzle bottom plate for the low nozzle spacing, they span the entire recirculation region at larger spacings. Thermal streaks are observed past reattachment for the larger nozzle spacing, suggesting a periodic breakup and re-formation of the jet curtain. The scaling of heat transfer distribution is affected by the flow structure in the geometrically non-similar area of the recirculating flow beneath the nozzle. A correlation for peak Nusselt number is presented.