scholarly journals Local Measurement of Loss Using Heated Thin Film Sensors

1998 ◽  
Author(s):  
Mark R. D. Davies ◽  
Francis K. O’Donnell
Keyword(s):  
Thin Film ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Entropy Generation ◽  
Free Stream ◽  
Unit Area ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Thin Film Sensors ◽  
Wall Shear

A calibration equation is derived linking the non-dimensional entropy generation rate per unit area with the non-dimensional aerodynamic wall shear stress and free stream pressure gradient. It is proposed that the latter quantities, which can be measured from surface gauges, be used to measure the profile entropy generation rate. It is shown that the equation is accurate for a wide range of well-defined laminar profiles. To measure the dimensional entropy generation rate per unit area requires measurement of the thickness of the boundary layer. A general profile equation is given and used to show the range of accuracy of a further simplification to the calibration. For flows with low free stream pressure gradients, the entropy generation rate is very simply related to the wall shear stress, if both are expressed without units. An array of heated thin film sensors is calibrated for the measurement of wall shear stress, thus demonstrating the feasibility of using them to measure profile entropy generation rate.

Local Measurement of Loss Using Heated Thin-Film Sensors

1999 ◽  
Vol 121 (4) ◽  
pp. 814-818 ◽  
Author(s):  
M. R. D. Davies ◽  
F. K. O’Donnell
Keyword(s):  
Thin Film ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Entropy Generation ◽  
Free Stream ◽  
Unit Area ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Thin Film Sensors ◽  
Wall Shear

A calibration equation is derived linking the nondimensional entropy generation rate per unit area with the nondimensional aerodynamic wall shear stress and free-stream pressure gradient. It is proposed that the latter quantities, which can be measured from surface sensors, be used to measure the profile entropy generation rate. It is shown that the equation is accurate for a wide range of well-defined laminar profiles. To measure the dimensional entropy generation rate per unit area requires measurement of the thickness of the boundary layer. A general profile equation is given and used to show the range of accuracy of a further simplification to the calibration. For flows with low free-stream pressure gradients, the entropy generation rate is very simply related to the wall shear stress, if both are expressed without units. An array of heated thin film sensors is calibrated for the measurement of wall shear stress, thus demonstrating the feasibility of using them to measure profile entropy generation rate.

scholarly journals Measurements of Aerodynamic Wall Shear Stress With Heated Thin Film Gauges

1997 ◽  
Author(s):  
M. R. D. Davies ◽  
J. E. Fitzgerald ◽  
J. T. Duffy ◽  
F. K. O’Donnell
Keyword(s):  
Thin Film ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Pressure Gradient ◽  
Pipe Flow ◽  
Free Stream ◽  
Suction Surface ◽  
Linear Cascade ◽  
Wall Shear ◽  
Laminar Pipe Flow

Previous publications have demonstrated the method of heated thin film gauge aerodynamic wall shear stress calibration in a laminar flow with a favourable free stream pressure gradient. Further evidence, derived from calibrating a gauge in laminar pipe flow and flow over a wedge, supports both the format of the calibrating equation and the value of the calibration constants. The pipe flow calibration is extended into turbulent flow and it is shown that the format of the calibrating equatinn remains unchanged whilst the value of the first constant changes markedly. The calibration constants are applicable to any such gauges mounted on an aluminium substrate in air flow operated at the same overheat temperature. The calibration constants are then applied to allow measurement of the wall shear stress in a low pressure gradient region of the suction surface of a linear cascade turbine blade. Finally, these measurements are compared favourably with those taken from a calibrated Preston tube mounted on the same blade.

Turbulent dynamics of sinusoidal oscillatory flow over a wavy bottom

2018 ◽  
Vol 858 ◽  
pp. 264-314 ◽  
Author(s):  
Asim Önder ◽  
Jing Yuan
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Oscillatory Flow ◽  
Free Stream ◽  
Stream Velocity ◽  
Half Cycle ◽  
Primary Vortex ◽  
Vortex Dipole ◽  
Coherent Vortices ◽  
Wall Shear

A direct numerical simulation study is conducted to investigate sinusoidal oscillatory flow over a two-dimensional wavy wall. The height and wavelength of the bottom profile, and the period and amplitude of the free-stream oscillation, are selected to mimic a wave-driven boundary layer over vortex ripples on a sandy seabed. Two cases with different Reynolds numbers$(Re)$are considered, and the higher-$Re$case achieves a fully developed turbulent state with a wide separation between the energy-containing and dissipative scales. The oscillatory flow is characterized by coherent columnar vortices, which are the main transport agents of turbulent kinetic energy and enstrophy. Two classes of coherent vortices are observed: (i) a primary vortex formed at the lee side of the ripple by flow separation at the crest; (ii) a secondary vortex formed beneath the primary vortex by vortex-induced separation. When the free-stream velocity weakens, these vortices form a counter-rotating vortex dipole and eject themselves over the crest with their mutual induction. Turbulence production peaks twice in a half-cycle; during the formation of the primary vortex and during the ejection of the vortex dipole. The intensity of the former peak remains low in the lower-$Re$case, as the vortex dipole follows a higher altitude trajectory limiting its interactions with the bottom, and leaving minimal residual turbulence around the ripples for the subsequent half-cycle. Flow snapshots and spectral analysis reveal two dominant three-dimensional features: (i) an energetic vortex mode with a preferred spanwise wavelength close to the ripple wavelength; (ii) streamwise vortical structures in near-wall regions with a relatively shorter spanwise spacing influenced by viscous effects. The vortex mode becomes strong when the cores of the vortices are strained to an elliptical form while moving towards the crest. Following the detachment of the vortices from the ripple, the vortex mode in the higher-$Re$case breaks down the spanwise coherence of the columnar vortices and decomposes them into intermittent patches of turbulent vortex clusters. The distribution of wall shear stress over the ripple is also analysed in detail. The peak values are observed near the ripple crest around the ejection of the vortex dipole and the maximum free-stream velocity. In the former, both the vortex mode and streamwise vortices have strong footprints on the wall, yielding a bimodal wall-shear-stress spectrum with two distinctive peaks. In the second high-stress regime, decaying coherent vortices impose strong inhomogeneity on the wall shear stress as their wall-attached parts sweep the ripples. These spanwise variations in the wall shear provide insights into the instability of two-dimensional sand ripples.

scholarly journals Entropy Generation in MHD Conjugate Flow with Wall Shear Stress over an Infinite Plate: Exact Analysis

Entropy ◽  
2019 ◽  
Vol 21 (4) ◽  
pp. 359 ◽  
Author(s):  
Arshad Khan ◽  
Faizan ul Karim ◽  
Ilyas Khan ◽  
Tawfeeq Alkanhal ◽  
Farhad Ali ◽  
...  
Keyword(s):  
Shear Stress ◽  
Wall Shear Stress ◽  
Entropy Generation ◽  
Infinite Plate ◽  
Mhd Flow ◽  
Bejan Number ◽  
Grashof Number ◽  
Entropy Generation Number ◽  
Mass And Heat Transfer ◽  
Wall Shear

The current work will describe the entropy generation in an unsteady magnetohydrodynamic (MHD) flow with a combined influence of mass and heat transfer through a porous medium. It will consider the flow in the XY plane and the plate with isothermal and ramped wall temperature. The wall shear stress is also considered. The influences of different pertinent parameters on velocity, the Bejan number and on the total entropy generation number are reported graphically. Entropy generation in the fluid is controlled and reduced on the boundary by using wall shear stress. It is observed in this paper that by taking suitable values of pertinent parameters, the energy losses in the system can be minimized. These parameters are the Schmitt number, mass diffusion parameter, Prandtl number, Grashof number, magnetic parameter and modified Grashof number. These results will play an important role in the heat flow of uncertainty and must, therefore, be controlled and managed effectively.

The Effect of Reynolds Number, Compressibility and Free Stream Turbulence on Profile Entropy Generation Rate

2002 ◽  
Author(s):  
Philip C. Griffin ◽  
Mark R. D. Davies ◽  
Francis K. O’Donnell ◽  
Ed Walsh
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Laminar Boundary Layer ◽  
Entropy Generation ◽  
Free Stream ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Suction Surface ◽  
Free Stream Turbulence ◽  
Layer Velocity

Detailed aerodynamic data from the suction surface boundary layer on a turbine blade arranged in a linear subsonic cascade was acquired under high free stream turbulence conditions (∼ 5.2%) generated using a perforated plate placed upstream of the cascade. In addition, data was also obtained from a transonic turbine cascade utilizing the same blade profile but of much smaller chord at free stream turbulence levels of 3.5%. Velocity profiles from the laminar, transitional and turbulent boundary layers were measured at various locations along the airfoil suction surface for the incompressible regime at ReC of 76,000. For the compressible test cases, boundary layer velocity profiles were measured at two locations towards the aft section of the blade at ReC of 163,000 and MEx of 0.37 respectively. For both cases the boundary layer velocity profiles were acquired by traversing a single normal hot wire probe normal to the blade surface. In addition the extent of the transition region over the blade surface was determined for both compressible and incompressible regimes by the use of an array of heated thin film sensors over a range of Reynolds and exit Mach numbers. It was observed that an earlier transition ensued at high free stream turbulence conditions in comparison to a previous investigation at comparable ReC and lower turbulence level (0.8% Tu). In addition comparisons were made to existing incompressible data at ReC = 185,000 and 0.8% free stream turbulence intensity. One of the primary observations resulting from an earlier transition was a thicker turbulent boundary layer, but in addition it was also noted that shear strain rates in the laminar boundary layer were significantly higher than those obtained at the 0.8% turbulence intensity. Further analyses also elucidated the presence of fluctuating components of velocity in the laminar boundary layer and were attributed to the effects of the free stream turbulence. This leads to the notion of a hybrid boundary layer, possessing both laminar and turbulent characteristics. These findings have implications regarding the profile loss of the blade, that is the loss generated in blade boundary layers and wakes normally associated with phenomena such as viscous shear, Reynolds stress production, shock wave formation and heat transfer across temperature differences and can be quantified in terms of the amount of entropy generated. For the purposes of this study entropy creation is solely restricted to that arising due to fluid dynamic phenomena, thereby assuming an adiabatic and quasi-isothermal flow. The entropy generation rate per unit volume is obtained directly from the boundary layer velocity profile; further integration gives rise to the entropy generation rate over the boundary layer at a point or over the entire suction surface length. Even though the number of quantitative measurement points on the transonic cascade was limited due to the very thin boundary layer present, no effects attributable to compressibility were observed on the entropy generation rate at the Mach number in question. Increased free stream turbulence had a greater effect on the generated entropy due to increased viscous shear in the laminar boundary layer and increased Reynolds stress production. In contrast, free stream turbulence did not have any significant effect on the turbulent boundary layer in the context of this study, as it was observed that the amount of entropy generated in the turbulent boundary layer was approximately equivalent for both turbulence levels at comparable Reynolds number.

scholarly journals Numerical studies for effect of geometrical parameters on water jet pump performance via entropy generation analysis

2021 ◽  
Vol 15 (3) ◽  
pp. 8319-8331
Author(s):  
Muhammad Penta Helios ◽  
Wanchai Asvapoositkul
Keyword(s):  
Shear Stress ◽  
Aspect Ratio ◽  
Entropy Generation ◽  
Water Jet ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Geometrical Parameters ◽  
Jet Pump ◽  
Pump Performance ◽  
Stress Layer

This paper presented an implementation of entropy generation analysis in the main flow field of a water jet pump via the CFD method. This study aimed to identify the inefficient location of energy conversion and to analyse entropy generation sources in each region of the water jet pump. The 2D-axisymmetric model and realisable k-ε (RKE) turbulence model at steady-state conditions were performed to validate jet pump performance and to assess the entropy generation. Likewise, the effects of the projection ratio  and throat-aspect ratio as independent parameters were investigated. As a result, the throat is the most inefficient part due to the high total entropy generation rate, following by diffuser part. Also, the entropy generation rate was assessed dominant than viscous dissipation due to the turbulent dissipation, which was caused by a turbulent shear stress layer of mixing the streams. In conclusion, the projection ratio influenced the growth of the shear stress layer as well as the entropy generation. Further, the throat-aspect ratio affected the distribution of entropy generation in the throat section. An appropriate combination of both parameters has an impact on the jet pump performance improvements reducing the entropy generation rate in the future.

scholarly journals Turbine Blade Aerodynamic Wall Shear Stress Measurements and Predictions

1998 ◽  
Author(s):  
J. E. Fitzgerald ◽  
A. J. Niven ◽  
M. R. D. Davies
Keyword(s):  
Thin Film ◽  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Turbine Blade ◽  
Incompressible Flows ◽  
Pressure Gradients ◽  
Suction Surface ◽  
Stress Measurements ◽  
Wall Shear

The correct prediction of the aerodynamic wall shear stress is a good test of a numerical codes ability to predict profile loss. Its measurement with heated thin film gauges is significantly easier than attempting a complete measurement of a turbine blade boundary layer. A modified form of previously published heated thin film gauge calibrations allow wall shear stress measurement in laminar incompressible flow with favourable pressure gradients and turbulent incompressible flows with small pressure gradients. In this paper, measurements are presented of the distribution of aerodynamic wall shear stress over the suction surface of a turbine blade in a linear cascade. Gauge voltage signal analyses show a laminar separation bubble between about 53% and 63% of suction surface length that is confirmed by surface flow visualisation. By-pass transition is detected by downstream gauges. Wall shear stress measurements are presented at two cascade incidence angles and for tripped and natural transition. The commercial code FLUENT is used to predict the surface pressure distribution, the aerodynamic wall shear stress distribution in the laminar region and the turbulent surface shear distribution for the tripped boundary layer. Comparisons are made between measurements and predictions.

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