The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

Gifford, Andrew R.; Diller, Thomas E.; Vlachos, Pavlos P.

doi:10.1115/1.4002595

Cited by 19 publications

(9 citation statements)

References 26 publications

(3 reference statements)

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“…Properties of the generated turbulence were calculated from the measured flow field with no stagnation model present at downstream distances 17.5M and the results compared well with the published correlations (Baines & Peterson 1951). These calculations were performed using the methods described by Gifford et al (2011). Details of the grid geometry and turbulence properties are given in table 1.…”

Section: Water Tunnel Facility and Turbulence Generationmentioning

confidence: 99%

“…Gifford, Diller & Vlachos (2011) used timeresolved digital particle image velocimetry (TRDPIV) and a thin-film heat flux sensor to study heat transfer augmentation in the stagnation region of a flow containing large-scale free-stream turbulence. Subsequent tracking of the vortices present showed that the rate of heat transfer at the wall was directly influenced by the strength and proximity of the vortex.…”

Section: Introductionmentioning

confidence: 99%

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The role of large-scale vortical structures in transient convective heat transfer augmentation

2013

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The physical mechanism by which large-scale vortical structures augment convective heat transfer is a fundamental problem of turbulent flows. To investigate this phenomenon, two separate experiments were performed using simultaneous heat transfer and flow field measurements to study the vortex-wall interaction. Individual vortices were identified and studied both as part of a turbulent stagnation flow and as isolated vortex rings impacting on a surface. By examining the temporal evolution of both the flow field and the resulting heat transfer, it was observed that the surface thermal transport was governed by the transient interaction of the vortical structure with the wall. The magnitude of the heat transfer augmentation was dependent on the instantaneous strength, size and position of the vortex relative to the boundary layer. Based on these observations, an analytical model was developed from first principles that predicts the time-resolved surface convection using the transient properties of the vortical structure during its interaction with the wall. The analytical model was then applied, first to the simplified vortex ring model and then to the more complex stagnation region experiments. In both cases, the model was able to accurately predict the time-resolved convection resulting from the vortex interactions with the wall. These results reveal the central role of large-scale turbulent structures in the augmentation of thermal transport and establish a simple model for quantitative predictions of transient heat transfer.

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Section: Water Tunnel Facility and Turbulence Generationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The role of large-scale vortical structures in transient convective heat transfer augmentation

2013

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“…The circulation (Γ) was calculated around iso-contours of the swirling strength. The vortextracking algorithm described previously (Gifford, 2011) was used to compile the trajectory of each vortex.…”

Section: Identification and Tracking Of Coherent Structuresmentioning

confidence: 99%

Aerodynamics of the flying snake Chrysopelea paradisi: how a bluff body cross-sectional shape contributes to gliding performance

Holden

Socha

Cardwell

et al. 2014

Journal of Experimental Biology

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A prominent feature of gliding flight in snakes of the genus Chrysopelea is the unique cross-sectional shape of the body, which acts as the lifting surface in the absence of wings. When gliding, the flying snake Chrysopelea paradisi morphs its circular cross-section into a triangular shape by splaying its ribs and flattening its body in the dorsoventral axis, forming a geometry with fore-aft symmetry and a thick profile. Here, we aimed to understand the aerodynamic properties of the snake's cross-sectional shape to determine its contribution to gliding at low Reynolds numbers. We used a straight physical model in a water tunnel to isolate the effects of 2D shape, analogously to studying the profile of an airfoil of a more typical flyer. Force measurements and time-resolved (TR) digital particle image velocimetry (DPIV) were used to determine lift and drag coefficients, wake dynamics and vortexshedding characteristics of the shape across a behaviorally relevant range of Reynolds numbers and angles of attack. The snake's crosssectional shape produced a maximum lift coefficient of 1.9 and maximum lift-to-drag ratio of 2.7, maintained increases in lift up to 35 deg, and exhibited two distinctly different vortex-shedding modes. Within the measured Reynolds number regime (Re=3000-15,000), this geometry generated significantly larger maximum lift coefficients than many other shapes including bluff bodies, thick airfoils, symmetric airfoils and circular arc airfoils. In addition, the snake's shape exhibited a gentle stall region that maintained relatively high lift production even up to the highest angle of attack tested (60 deg). Overall, the crosssectional geometry of the flying snake demonstrated robust aerodynamic behavior by maintaining significant lift production and near-maximum lift-to-drag ratios over a wide range of parameters. These aerodynamic characteristics help to explain how the snake can glide at steep angles and over a wide range of angles of attack, but more complex models that account for 3D effects and the dynamic movements of aerial undulation are required to fully understand the gliding performance of flying snakes.

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“…Over the years a number of investigators [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] have studied the influence on turbulence on laminar boundary layer development and transition and their work support the conclusion that turbulence level, turbulence scale, and Reynolds number have significant influences. Mayle [24] discussed the influence of turbulence combined with favorable and adverse pressure gradient on spot production.…”

Section: Introductionmentioning

confidence: 92%

Heat Transfer Measurements in a Compressible Flow Vane Cascade Showing the Influence of Reynolds Number, Mach Number, and Turbulence Level on Transition and Augmentation of Laminar Heat Transfer by Free-Stream Turbulence

Stahl¹,

Moualeu²,

Long³

et al. 2014

50th AIAA/ASME/SAE/ASEE Joint Propulsion Conference

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This research is focused on the effects of large changes in Reynolds number that typically occurs during the flight of high altitude UAV's. This paper documents the influence of Reynolds number, turbulence level, and exit Mach number on the vane surface Stanton number. Reynolds number is based on true chord and exit conditions and ranges from 90,000 through 1,000,000. Low and high inlet turbulence levels were developed for the study and determined to be 0.8% and 9.0%. Tests were run at exit Mach numbers of 0.7, 0.8 and 0.9. These surface heat transfer measurements were acquired in the University of North Dakota's transonic cascade test facility. This facility uses a closed loop to allow the regulation of system pressure to control the test condition Reynolds number. The Mach number is adjusted using a "roots" blower driven by a variable frequency drive. Heat transfer measurements were acquired using a constant heat flux foil fabricated using a 0.023 mm Inconel foil backed with 0.05 mm of Kapton and adhered to the heat transfer vane using a high temperature acrylic adhesive. The linear cascade is configured in a four vane three full passage arrangement. The low turbulence condition is developed using the existing flow conditioning section coupled to a 4.7 to 1 area ratio nozzle. The high turbulence condition uses a mock aero combustor to generate a turbulence level of around 9.0%. These data show the influence of Mach number, Reynolds number and turbulence level on transition and heat transfer augmentation and are expected to be useful in grounding heat transfer predictive methods applicable to small or high altitude gas turbine engines. NomenclatureC vane true chord length, m Cp specific heat ratio, (J/kg/K) h surface heat transfer coefficient, (W/m 2 /K) Lu energy scale, Lu = 1.5 u' 3 / Ma Mach number Re C Reynolds number based on true chord and exit conditions S surface distance from stagnation point, m St Stanton number, h/V EX Cp Tu turbulence intensity, Tu = |u'|/U IN u' rms fluctuation velocity, m/s U IN ideal cascade inlet velocity, m/s V EX ideal cascade exit velocity, m/s Y Normal distance from the wall, m Y + normal distance in wall units, Y + = Y (/)/ Greek Letter Symbols  turbulent dissipation rate, m 2 /s 3  kinematic viscosity, m 2 /s  density, kg/m 3  surface shear stress, N/m 2 Subscripts EX refers to conditions at the nozzle exit plane IN refers to conditions at the nozzle inlet plane INTRODUCTIONUnmanned aerial vehicles (UAVs) have a growing mission in supporting military engagements abroad as well as ensuring domestic security in the US. UAV's have been engaged to help assess the damage of natural disasters, patrol borders and assess crop health. The yearly market for unmanned aerial vehicles (UAV's) is expected to grow to at a rate of 12% through 2018 to a value of $18.6 billion according to Market Research Media. A growing number of large to medium UAV's are being designed to use turbofan engines due to the ability to achieve high thrust to weight ratios and high thermal and propu...

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The Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence

Cited by 19 publications

References 26 publications

The role of large-scale vortical structures in transient convective heat transfer augmentation

The role of large-scale vortical structures in transient convective heat transfer augmentation

Aerodynamics of the flying snake Chrysopelea paradisi: how a bluff body cross-sectional shape contributes to gliding performance

Heat Transfer Measurements in a Compressible Flow Vane Cascade Showing the Influence of Reynolds Number, Mach Number, and Turbulence Level on Transition and Augmentation of Laminar Heat Transfer by Free-Stream Turbulence

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