Heat transfer and film cooling measurements on aerodynamic geometries relevant for turbomachinery

A measurement technique for recording convective heat transfer coefficient and adiabatic film cooling effectiveness in demanding environments with highly curved surfaces and limited optical access, such as turbomachinery, is presented. Thermography and tailor-made flexible heating foils are used in conjunction with a novel multistep calibration and data reduction method. This method compensates for sensor drift, angle dependence of surface emissivity and window transmissivity, heat flux inhomogeneity, and conductive losses. The 2D infrared images are mapped onto the 3D curved surfaces and overlapped, creating surface maps of heat transfer coefficient and film cooling effectiveness covering areas significantly larger than the window size. The measurement technique’s capability is demonstrated in a sector-cascade test rig of a turbine center frame (TCF), an inherent component of modern two-spool turbofan engines. The horseshoe vortices were found to play a major role for the thermal integrity of turbine center frames, as they lead to a local increase in heat transfer, and at the same instance, to a reduction of film cooling effectiveness. It was also found that the horseshoe vortices lift off from the curved surface at 50% hub length, resulting in a pair of counter-rotating vortices. The measurement technique was validated by comparing the data against flat plate correlations and also by the linear relation between temperature difference and heat flux. This study is complemented with an extensive error and uncertainty analysis. Article highlights This paper presents an accurate measurement technique for heat transfer and film cooling on 3D curved surfaces with limited optical access using flexible tailor-made heating foils, infrared thermography and a high-fidelity multistep calibration process. Graphical abstract

Investigation of Local Scouring around Hydrodynamic and Circular Pile Groups under the Influence of River Material Harvesting Pits

Mining activities can endanger the stability of hydraulic structures. Numerical modeling of local scouring around hydrodynamic and circular bridge pile groups, due to the action of clear water conditions via non-cohesive sediment, was performed using a computational fluid dynamics (CFD) model, a large eddy simulation (LES) turbulence model, and a van Rijn sedimentary model with FLOW-3D software. The pile groups were positioned upstream and downstream of a sand mining pit. The results showed that the scour depth around the downstream pile group was greater than that of the upstream one. Using hydrodynamic piers reduced the scour depth upstream of all piers and the material harvesting pit. The maximum reduction in scour depth was observed in front of the fifth pier, with a 29% reduction in scour depth. Additionally, for all models, as the material harvesting pit was moved downstream, the downstream turbulence was enhanced and stronger flow reversal and horseshoe vortices were detected in from of the downstream pile group. The flow patterns around the pile group showed that the presence of hydrodynamic piers in the upstream pile group leads to a decrease in the maximum flow velocity, whereas, when such piers were positioned in the downstream pile group, the velocity increases.

Effect of Rear Wing on Time-Averaged Ground Vehicle Wake With Variable Slant Angle

The slant angle plays a crucial role in the flow property of hatchback ground vehicles. An optimum slant angle is obligatory for better handling the ground vehicles when fitted with a rear wing. In this regard, the variation of time-averaged flow properties around a wing-attached hatchback ground vehicle (Ahmed body) due to a variable slant angle is accessed by this paper. The design includes a scaled Ahmed body as a reference ground vehicle and a rear wing with NACA 0018 profile. The computational studies are executed with Reynolds-averaged Navier–Stokes based k-epsilon turbulence model with nonequilibrium wall function. The vehicle's model is scaled to 75% of the actual model, and analyses are conducted with Reynolds number 2.7 × 106. After the study, it is observed that a 15 deg slant angle is the critical angle for the wing attached state in which the drag coefficient is maximum. After this angle, a sudden reduction of coefficients is observed, where 25 deg is critical for without wing condition. Besides this, the two counter-rotating horseshoe vortices in the separation bubble and side edge c-pillar vortices also behave differently due to the wing's presence. The turbulent kinetic energy variation and the variation in coefficients of surface pressure are also affected by the rear wing attachment. This paper will assist in finding the optimum slant angle for hatchback ground vehicles in the presence of a rear wing. Thus the study will help in increasing stability and control for hatchback ground vehicles.

Experimental Scours by Impinging Twin-Propeller Jets at Quay Wall

Experiments were conducted to investigate the seabed scour holes due to the interaction between the twin-propeller jet and quay wall. Vertical quay wall was modelled by using a polyvinyl chloride (PVC) plastic plate in a water tank. The relationship between the positions of the propeller and the vertical quay wall was designed according to the actual working conditions of a ship entering and leaving a port. Propeller-to-wall distance and rotational speed were changed to observe the various scour conditions. The scour depth was measured by using an Acoustic Doppler Velocimeter (ADV). Primary scour hole was found within the jet downstream and secondary scour hole occurred beneath of the propeller. Third scour hole was found close to the quay wall due to horseshoe vortices. The maximum scour position of this third scour hole was found at the jet centre near the quay wall. Temporal formation of scour holes can be divided into three stages: axial scour formation, obstructed scour expansion and equilibrium stages. The quantitative relationships for six characteristic parameters of the scour pit were established including the maximum scour depth (εmax,q), maximum scour depth position (Xm,q), maximum scour width (Wm,q), length of main scour pit (XS,q), maximum deposition height (ZD,q), and location of maximum deposition height (XD,q).

Experimental Study of Inclined Bridge Pier Scouring

The bridges are one of important structures in any country. The failure of bridges occurs due to many factors including design flaws and manufacturing construction errors. Among all imperfections scouring around the pier is the most detrimental. So, the estimation of local scouring around a bridge pier is of fundamental importance for the safe design of bridges. Although numerous researches have been done on local scouring around a single bridge pier. The present study investigates the effect of angle of inclination of dual bridge pier configuration on local scouring around bridge piers. Principally rectangular shaped dual bridge piers were installed in sand bed of laboratory flume at angle of inclination of 0°,7°,12°,15° and 19° with vertical respectively. Three different flow rates 9, 14 and 18L/sec were considered during each trial. The duration of each trial was kept around 2 hours. The scour depth was measured separately around both piers with the help of point gauge under clear water condition. The value of scour depth around upstream pier was larger as compared to downstream pier because of the lower strength of horseshoe vortices around downstream pier. From the experimental results, it can be concluded that there is an inverse relationship between the angle of inclination and scour depth, an increase in the angle of inclination leads to decrease in scour depth around both piers. The value of scour depth was maximum when piers were at 0° and minimum at 19°. It was also found that scour depth increases with the increase in flow rate.

Large Eddy Simulation of Flow Over a Hemispherical Obstacle Within a Cylindrical Tube

Motivated by recent experiments on the dustiness of nanoscale powders, this research addresses the modeling of powder aerosolization within the Venturi Dustiness Test (VDT) apparatus. As a first step in such modeling, we investigate the flow over a hemispherical (powder) hill, at the operating Reynolds number Re ∼ 20,000, in the powder holding tube attached to the VDT. The powder holding tube is idealized as a cylindrical tube, obstructed by a hemispherical (powder) hill. The upstream flow field is characterized by the presence of a horse-shoe vortex, formed due to the separation of the boundary layer from the wall of the tube. A stagnation point occurs at the lower front surface of the bump. Strong near-wall vorticity is generated midway up the bump, and the flow is separated near the top of the bump. Kelvin-Helmholtz vortices are produced due to the strong shear-layer vorticity, and these then travel downstream with the flow. The upstream near-wall flow is dominated by a horseshoe vortex forming a necklace pushed out into the wake area. The flow detaches from the bump at the separation line, leading to vortex ‘roll-up’. These rolled-up vortices merge with the horseshoe vortices to form a large entangled hairpin vortex. The arch-type vortices shed with a frequency consistent with the Strouhal estimate fSt ∼ 6700 Hz.

Spatial and statistical analysis of coherent vortical structures within a stress-driven free-surface turbulent shear flow

<p>The water surface under high wind condition is characterized by elongated high-speed streaks and randomly emerged low-speed streaks, which are attributed to underneath coherent vortical motions. These vortical structures within aqueous turbulent boundary layer plays a critical role in turbulent exchange, their characteristics and statistics are therefore of interest in this study. Direct numerical simulation of an aqueous turbulent flow bounded by a stress-driven flat free surface was performed. Simulation results of cases with high wind condition (surface friction velocity = 1.22 cm/s) as well as weak wind condition (surface friction velocity = 0.71 cm/s) are analyzed. To identify the underlying vortical structures, an indicator of swirling strength derived from local velocity-gradient tensor is adopted. A formal classification scheme, based on the topological geometry of the vortex core, is then applied to classify the identified structures. Surface layers with the two wind conditions reveal similar results in statistics and spatial distribution of vortical structures. Two types of characteristic vortices which induce the surface streaks are extracted, including quasi-streamwise vortex and reversed horseshoe vortex (head pointing upstream), most inclining at about 10 to 20 degrees. Quasi-streamwise vortices are the dominant structure, and both high- and low-speed streaks are fringed with such vortices; they adjoin the surface streaks as counter-rotating arrays in either staggered or side-by-side spatial arrangement. The length of quasi-streamwise vortices, however, are significantly shorter than the corresponding surface streaks, only 10% of the extracted quasi-streamwise vortices are longer than 150 wall units. Reversed horseshoe vortices, associated with downwelling motions and surface convergence, are located beneath the high-speed streaks. In contrast to the turbulent boundary layer next to a flat wall, typical forward horseshoe vortices (head pointing downstream) associated with upwelling motions are barely found within the free-surface turbulent shear flow.</p><p>This work was supported by the Taiwan Ministry of Science and Technology (MOST 107-2611-M-002 -014 -MY3).</p>

Local Scour Near Flexible Flow Deflectors

Rigid flow deflectors are usually used on water flow beds to protect engineering structures such as breakwater in coasts and to regulate flow routes in open channels. To reduce its side-effects, i.e., local scour at the toe of deflectors, a flexible flow deflector is proposed, and the corresponding local scour was investigated in this study. A flume experiment was conducted to investigate local scour. To show the advantage of flexible deflectors, a control experimental test was also conducted using a traditional rigid deflector under the same blockage area configuration and the same flow conditions. The flow field near the flexible deflector was also measured to reveal the local flow field. The results show that the bed-scour develops near the toe edges of both flexible and rigid deflectors, but the maximum and averaged scour depths for the flexible deflector are smaller. This advantage of flexible deflector in scour depth is mainly caused by its prone posture, which induces the upward stretching and enlarging horizontally rotating vortex and the upward shifted vertically rotating vortex. The former dissipates more turbulent energy and the latter results in smaller bed shear stress, which lead to smaller scour depth directly. In addition, the up- and down-swaying movement of the flexible deflector can also assistant to dissipate more turbulent energy, thereby damping the intense of the horseshoe vortices and thus weakening scour depth as well. The results of this study provide an elementary understanding on the mechanisms of a flexible flow structure and an alternative deflector-device to reduce scour depth.

Двойственность вихревой структуры, возникающей при сверхзвуковом обтекании области сопряжения затупленного тела и пластины вязким газом

Results of numerical simulation of the supersonic gas flow past a fin-body mounted on the flat plate with developing laminar boundary layer are presented. The calculations cover flow cases with the freestream Mach number of 6.7 and two different Reynolds numbers: Re=12500, 15600. The structure of the horseshoe vortices arising in the body leading-edge region is analyzed. It has been revealed that for both the Reynolds numbers there exist two stable solutions, which correspond to different metastable states of the flow. The solutions differ in the number of the vortices forming in the separation region, as well as in the separation region length. In the smaller Reynolds number case, both solutions are steady-state, whereas in case of the larger Re value, one of the solutions remains steady-state, and the other one becomes quasi-periodic.

On the Morphodynamic Alterations around Bridge Piers under the Influence of Instream Mining

Sand mining in an active alluvial channel can compromise the streambed stability of the hydraulic structures nearby. This experimental study is aimed at investigating the effects of rectangular mining pit on the morphodynamics around circular tandem piers in a movable bed. A rectangular pit is excavated upstream of two circular piers embedded in the sand-bed in a tandem arrangement. The results are then compared to a case having only the piers without any mining pit. Turbulent stresses and mean velocities in the near-bed region rise significantly at the upstream region of the piers in the presence of a pit. Also, stronger flow reversal and horseshoe vortices have been detected at the base of the pier front. Due to these alterations in the nature of turbulence, erosion of channel beds upstream of the piers, increased scour depths, scour volume, and lateral erosion of the scour hole have been observed. Dynamic evolution of the local scour at various time scales has been studied using a wavelet cross-correlation method. Spatial evolution of local scour is found to be faster when a pit is excavated in the channel. Thus, mining activities near the piers can lead to significant changes in the flow-field, causing excessive scour around piers.