Primary Vortex Pair Research Articles

Numerical investigation into heat transfer augmentation in a square minichannel heat sink using butterfly inserts

Heat transfer augmentation in a square minichannel heat sink through introduction of various butterfly insert configurations is numerically investigated for a range of Reynolds numbers (200 – 900). The present study also examines the individual effects of insert, pitch of the insert, and perforations in the wings of the butterfly insert on the thermohydraulic performance of minichannel heat sinks. Incorporating any kind of insert in a typical square minichannel is found to have a positive influence on the overall thermohydraulic performance, yielding performance evaluation criteria consistently over unity for the whole range of tested Reynolds numbers. The effect of pitch is tested for unperforated inserts, wherein a reduction of pitch distance from 10 mm to 5 mm is found to increase heat transfer performance by 15.08 - 46.39% for the studied range of Reynolds numbers. Perforations in the wings of butterfly inserts are found to result in a reduction of friction factor, with a 4.01 – 20.78% decrease, when compared with an unperforated insert having an identical pitch. However, this reduction is achieved at the cost of a lower heat transfer performance (5.07 – 16.17%). Therefore, unperforated butterfly inserts are deemed to be superior in terms of the overall thermohydraulic enhancement. In all enhanced minichannel configurations, a redistribution of velocity and temperature fields due to the formation of a primary vortex pair and thinning of thermal boundary layers due to a second pair of secondary vortices are thought to be the major contributors behind the observed thermohydraulic enhancement. • Introduction of a butterfly insert with 10 mm pitch is found to enhance heat transfer performance in MCHS by 42.02% to 109.31% for the tested range of Reynolds numbers. • A reduction in pitch from 10 mm to 5 mm in an MCHS with unperforated butterfly inserts is observed to yield an augmentation of average Nusselt number by 15.08% to 46.39% for the studied range of Reynolds numbers. • Perforation on insert wings is found produce a reduced enhancement in average Nusselt number, i.e., 5.07%–16.17%, for the investigated range of Reynolds numbers. • Observed thinning and disruption of thermal boundary layers and enhanced fluid mixing at the central region of the minichannel due to a pair of primary vortices are thought to be the major reasons for enhanced thermal performance. • A maximum performance evaluation criterion of 1.66 is observed for the case of unperforated butterfly inserts with 5 mm pitch at a Reynolds number of 900.

Relation Between Geometry and Wake Characteristics of a Supersonic Microramp

Microramp vortex generators are robust, reliable, and simple devices for passively controlling the boundary layer in several aerospace applications. Various past studies have investigated the effectiveness of microramps in controlling the flow separation induced by shock-wave/boundary-layer interactions. Building upon the past knowledge, this paper reports a systematic investigation of the relation between the microramp geometry and the downstream flow characteristics. A simplified flow model of the microramp wake is provided to explain and predict the influence of changing the geometry on the circulation of the primary vortex pair. The model also provides scaling relations for the evolution of the wake characteristics (that is, wake velocity, wake location, and added incompressible momentum), incorporating the effect of all geometry parameters. Extensive experimental data have been used to validate the model.

Role of secondary shear-layer vortices in the development of flow asymmetry on a cone–cylinder body at high angles of incidence

Vortex asymmetry develops on slender bodies at high angles of incidence, leading to large side-forces and yaw moments even in symmetrical flight conditions. In the present experimental study, the flow over a slender body consisting of a conical forebody and a cylindrical aftbody is studied at high angles of incidence at low speeds. Experiments consisted of force measurements using a six-component strain gauge balance and velocity field using Particle Image Velocimetry. Results show that a pair of counter-rotating vortices develop nearly symmetrically over a polished cone but become asymmetric over the cylinder. Several secondary shear-layer vortices develop at the cone–cylinder junction and over the cylinder due to sharp turning angle, surface imperfections, and joints. These secondary shear-layer vortices then merge with the primary vortex pair, leading to the flow asymmetry. The flow characteristics and side-forces vary significantly when the conical forebody is rotated about its axis of symmetry and set to different azimuthal positions. The results strongly suggest that a change in the geometry at the cone–cylinder junction and surface imperfections present on the cylindrical aftbody play an important role in determining the overall flow-asymmetry and the magnitude of side-forces generated on the slender body at high angles of incidence.

Experiments on asymmetric vortex pair interaction with the ground

The interaction between the asymmetric vortex pair and the ground was experimentally investigated in a closed-circuit water channel for the current study. Particle image velocimetry tests were conducted to achieve the velocity measurements of flow fields at various crossflow planes. The vortex pair, with the reference circulation Reynolds number ReΓ ≈ 4490, was tested with various initial heights (h0/b0 = 1.5, 1.0 and 0.5) and circulation ratios (λ = 0.36:1, 1:0.36, 0.6:1, 1:0.6), with it being found that according to different interaction effects, the trajectory and merger of the vortex pair could be significantly affected. As the primary vortex pair approached the ground, a high turbulent kinetic energy region was observed between the secondary and primary vortical structures, which suggests violent interaction and accelerated circulation decay of the primary vortex. In addition, depending on the distribution of secondary vorticity and image vortices to the moving coordinate system, merging-promoting or -inhibiting effects were observed, which eventually affected the evolution of the primary vortex pair.

POD analysis on vortical structures in MVG wake by Liutex core line identification

The Liutex core line method, first combined with the snapshot proper orthogonal decomposition (POD), is utilized in a supersonic micro-vortex generator (MVG) wake flow at Ma = 2.5 and Reθ = 5 760 to reveal the physical significance of each POD mode of the flow field. Compared with other scalar-based vortex identification methods, the Liutex core line identification is verified to be the most appropriate approach that is threshold-free and provides full information of a fluid rotation motion. Meanwhile, the Liutex integration is employed to quantitatively track the evolution of the vortices in MVG wake and is applied to the determination of the effective control section of the MVG wake for the optimization study of MVG design. The physical mechanism of each POD mode for multi-scale and multi-frequency vortical structures is investigated by using Liutex core line identification to give some revelations. For the mean mode (mode 0) indicating the time-averaged velocity flowfield of the MVG wake flow, a pair of primary counter-rotating streamwise vortices and another pair of secondary vortices is uniquely identified by two pairs of Liutex core lines with Liutex magnitude. In contrast, mode 1 is featured by a fluctuated roll-up motion of streamwise vortex, and the streamwise component of the MVG wake is demonstrated to be dominant in terms of the total kinetic energy contribution. Meanwhile, a dominant shedding frequency of St = 0.072 is detected from the temporal behavior of mode 2, which has the organized arc-shaped vortex structures shedding from MVG induced by the K-H instability. Additionally, mode 4 subjects to low-frequency oscillations of the wall vortices and thus takes a relatively lower frequency of St = 0.044.

Impingement of a counter-rotating vortex pair on a wavy wall

In this paper, we investigate the impingement of a two-dimensional (2-D) vortex pair translating downwards onto a horizontal wall with a wavy surface. A principal purpose is to compare the vortex dynamics with the complementary case of a wavy vortex pair (deformed by the long-wavelength Crow instability) impinging onto a flat surface. The simpler case of a 2-D vortex pair descending onto a flat horizontal ground plane leads to the well known ‘rebound’ effect, wherein the primary vortex pair approaches the wall but subsequently advects vertically upwards, due to the induced velocity of secondary vorticity. In contrast, a wavy vortex pair descending onto a flat plane leads to ‘rebounding’ vorticity in the form of vortex rings. A descending 2-D vortex pair, impinging on a wavy wall, also generates ‘rebounding’ vortex rings. In this case, we observe that the vortex pair interacts first with the ‘hills’ of the wavy wall before the ‘valleys’. The resulting secondary vorticity rolls up into a concentrated vortex tube, ultimately forming a vortex loop along each valley. Each vortex loop pinches off to form a vortex ring, which advects upwards. Surprisingly, these rebounding vortex rings evolve without the strong axial flows fundamental to the wavy vortex case. The present research is relevant to wing tip trailing vortices interacting with a non-uniform ground plane. A non-flat wall is shown to accelerate the decay of the primary vortex pair. Such a passive, ground-based method to diminish the wake vortex hazard close to the ground is consistent with Stephan et al. (J. Aircraft, vol. 50 (4), 2013a, pp. 1250–1260; CEAS Aeronaut. J., vol. 5 (2), 2013b, pp. 109–125).

Reynolds number and slant angle effects on the flow over a slanted cylinder afterbody

The cylinder with a slanted base is a simplified, canonical bluff body geometry that shares similarities to aircraft fuselages, which are known to produce a strong vortex pair due to their upswept afterbody. This work will examine in detail the surface flow and near-field characteristics of the flow over the slanted cylinder for slant angles of $20^{\circ }$ , $32^{\circ }$ and $45^{\circ }$ with spatially dense measurements. Principal flow features of the mean flow field are identified, showing the connection between the main counter-rotating vortex pair observed in the wake and the separation bubble observed at the leading edge of the slant. A full reconstruction of the three-dimensional mean flow field using stacked stereoscopic particle image velocimetry reveals intricate details of this flow and clearly shows the direct connection between the two features. To our knowledge, this is the first direct measurement of the full three-dimensional flow topology for this geometry. The separation bubble length is found to be directly proportional to the slant angle and inversely proportional to the Reynolds number. Furthermore, the circulation within the primary vortex pair increases with increasing slant angle. This strengthening of the vortices is correlated to the form drag of this body in the vortex-dominated regime. A bi-stable steady-state wake is also observed in this flow at a low Reynolds number for the slant angle of $45^{\circ }$ , where the formation of either a separated wake flow state or a vortex-dominated flow state is dependent upon initial conditions, i.e. the presence of overshoot of the free-stream velocity during wind tunnel start-up.

Transitional Flow Dynamics Behind a Micro-Ramp

Micro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

Direct numerical simulation of a counter-rotating vortex pair interacting with a wall

The influence of a horizontal wall on the evolution of the long-wave instability in equal strength counter-rotating vortex pairs is studied with direct numerical simulation. The two vortices descend under mutual induction and interact with a ground plane, as would aircraft trailing vortices in ground effect. Both the linear and nonlinear development of the pair is studied for three initial heights above the wall, representative of three modes of interaction identified by experiment. A study of two vortex core sizes over a range of Reynolds numbers ( ) is used to verify parameter independence. The vortex system undergoes complex topological changes in the presence of a wall, with the separation of wall generated vorticity into hairpin-like vortex tongues and axial flow development in the primary pair. The secondary structures are rotated and stretched around the primary vortices, strongly influencing the resultant flow evolution and are comparable with those observed for oblique ring–wall interaction. Of the three modes, the small-amplitude mode shows the formation of four principal tongues per long wavelength of the Crow instability, with the secondary vortex remaining connected. The large-amplitude mode undergoes a re-connective process to form two non-planar secondary vortex structures per wavelength, and a simulation of the large ring mode in its first formation develops six tongues per wavelength. These secondary structures ‘rebound’ from the wall and interact at the symmetry plane prior to dissipation, governing the bending, stretching and trajectory of the primary vortex pair.

Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$ kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

Control of circular cylinder flow using distributed passive jets

A wind tunnel study has been carried out to investigate flow control around a hollow circular cylinder using passive jets driven by naturally occurring pressure differences. Flow enters the cylinder through spanwise holes along the stagnation line and exits through a spanwise distribution of holes at$\pm 65^{\circ }$. The diameter of the entry and exit holes were 1 % and 0.5 % of the cylinder diameter, respectively. Reynolds numbers were at the upper end of the subcritical regime and ranged from$3\times 10^{4}$to$2.8\times 10^{5}$. Jet spacings of 10 % and 20 % of the cylinder diameter were investigated, and the ratio of the average jet exit velocity to the cross-flow velocity at the boundary layer edge was found to rise to approximately 0.35 and 0.4, respectively, above a Reynolds number of$1.5\times 10^{5}$. Findings based on using the surface oil flow technique revealed a repeating, organised cellular pattern downstream of adjacent jet exit holes consisting of a primary counter-rotating vortex pair structure, followed by a secondary weaker pair. Downstream of adjacent exit holes, and centred midway between them, there exists a separation bubble which delays final flow separation compared with the flow directly downstream of a jet. The variation in the angular position of boundary layer separation across the span had the effect of suppressing von Kármán vortex shedding. This resulted in a drag coefficient, at the upper end of the Reynolds-number range studied, 14.5 % lower than that found using trip wires to initiate boundary layer transition.

Evaluation of Ramp-Type Micro Vortex Generators Using Swirl Center Tracking

The evolution of the primary vortex pair downstream of micro vortex generators placed in a supersonic stream is compared using a swirl center tracking technique, for standard, slotted, and split-ramp-type vortex generators. Evolution of vortex strength, determined using circulation, and distribution of vortices, determined using contours of helicity, are also compared. Results show that secondary vortices emerge stronger, and liftoff heights drop, with the introduction of slot (slotted micro vortex generator) or gaps (split-ramp micro vortex generator). Near-surface contours of axial velocity and integral properties of the boundary layer are then compared to evaluate the effects of the evolution of the vortical structures on the flowfield. Results shows that the split-ramp device results in the most energetic near-surface flow but the worst outgoing boundary-layer integral properties, whereas the slotted-ramp devices with taper fare well on both counts. The computations performed in this work involve supersonic flow past single micro vortex generators at Mach 2.5 and use an immersed-boundary method to render the control devices. The flow solver is suitable for high-speed turbulent flows and has been extensively validated in earlier works. The turbulence model used is Menter’s shear-stress transport version.

On Reynolds number dependence of micro-ramp-induced transition

The variation of transitional flow features past a micro-ramp is investigated when the Reynolds number is decreased approaching the critical regime. Experiments are conducted in the incompressible flow spanning from supercritical to subcritical roughness-height-based Reynolds number ($Re_{h}=1170$, 730, 460 and 320) with tomographic particle image velocimetry. The effect of $Re_{h}$ on three-dimensional flow behaviour is analysed in a domain encompassing 73 ramp heights in the streamwise direction. Above the critical $Re_{h}$, the primary vortex pair and induced central low-speed region in the mean flow field are active over longer range when decreasing $Re_{h}$. In the instantaneous flow, at $Re_{h}<1000$, the hairpin vortices induced by Kelvin–Helmholtz (K–H) instability progress gradually from close to the micro-ramp into the region where the overall shear layer is destabilized, indicating the correlation between the K–H instability and the onset of transition. The breakdown of K–H vortices as observed at $Re_{h}=1170$, does not occur at lower $Re_{h}$. Decreasing $Re_{h}$, the secondary vortex structures make their first appearance significantly downstream, postponing the formation of sideward disturbances, which destabilize the local shear layer by ejection events. Two major types of eigenmodes with symmetric and asymmetric spatial distribution of velocity fluctuations in the near wake are clearly identified by proper orthogonal decomposition. The symmetric and asymmetric modes correspond to the presence of vortex shedding and a sinuous wiggling motion respectively. It is found that $Re_{h}$ is the key factor determining the importance of the symmetric mode. At $Re_{h}=1170$, the disturbance energy of the symmetric mode decays before the onset of transition, suggesting that it is relatively insignificant in the process. However, decreasing $Re_{h}$ to 730 and 460, the symmetric mode produces continuous growth of high level disturbance energy, leading to transition.

Study on wake structure characteristics of a slotted micro-ramp with large-eddy simulation

In this paper, a novel slotted ramp-type micro vortex generator (slotted micro-ramp) for flow separation control is simulated in the supersonic flow of Ma = 1.5, based on large eddy simulation combined with the finite volume method. The wake structure characteristics and control mechanisms of both slotted and standard micro-ramps are presented and discussed. The results show that the wake of standard micro-ramp includes a primary counter-rotating streamwise vortex pair, a train of vortex rings, and secondary vortices. The slotted micro-ramp has more complicated wake structures, which contain a confluent counter-rotating streamwise vortex pair and additional streamwise vortices, with the same rotation generated by slot and the vortex rings enveloping the vortex pair. The additional vortices generated by the slot of the micro-ramp can mix with the primary counter-rotating vortex pair, extend the life time, and strengthen the vortex intensity of primary vortex pair. Moreover, the slot can effectively alleviate, or even eliminate the backflow and decrease the profile drag induced by the standard micro-ramp, therefore improving the efficiency of separation control.

Novel Vortex Generator for Mitigation of Shock-Induced Flow Separation

A novel vortex generator, termed as “slotted vortex generator,” is proposed in this study. Computations are conducted for supersonic flow at a freestream Mach number of 2.5 past single vortex generators of three different heights. For each device height , three values of the slot radius , , and are used. Comparisons made with a standard wedge-type vortex generator using streamwise-velocity profiles, near-surface streamwise-velocity contours, pitot pressure deficit contours, etc., indicate that the new device has less device drag and produces fuller near-surface streamwise velocities downstream of the device. It is observed that the primary counter-rotating vortex pair formed due to the vortex generator lifts off at a slower rate when a slotted vortex generator is used. Computations of an impinging oblique-shock/boundary-layer interaction at Mach 2.5 for a flow turning angle of 7 deg, with flow control using (separately) an array of slotted and standard vortex generators, indicate that the slotted vortex generators with are most effective in reducing the region with reversed flow when placed closer to the shock/boundary-layer interaction region.

Visualization of supersonic flow around a sharp-edged, sub-boundary-layer protuberance

Innovations in conventional surface and planar laser scattering visualizations revealed complex structures in the Mach 2.5 flow past a sharp-edged, sub-boundary-layer ramp with swept sides that is one type of micro vortex generator (MVG). The incoming flow separated over the leading edge despite the ramp angle being below the threshold for incipient separation. The separation produced a weak trailing horseshoe vortex system. The flow over the top of the MVG separated off the slant edges to produce a large primary vortex pair. Extra details were revealed at the trailing edge with at least two pairs of singularities. Vortex filaments spring from these singularities. Symmetry breaking from the confluence of the two primary vortices was observed as an unsteady wake to result in a train of possibly ring or hairpin vortices trailing downstream.

Non-invasive determination of external forces in vortex-pair-cylinder interactions

Expressions for the conserved linear and angular momenta of a dynamically coupled fluid + solid system are derived. Based on the knowledge of the flow velocity field, these expressions allow the determination of the external forces exerted on a body moving in the fluid such as, e.g., swimming fish. The verification of the derived conserved quantities is done numerically. The interaction of a vortex pair with a circular cylinder in various configurations of motions representing a generic test case for a dynamically coupled fluid + solid system is investigated in a weakly compressible Navier-Stokes setting using a Cartesian cut-cell method, i.e., the moving circular cylinder is represented by cut cells on a moving mesh. The objectives of this study are twofold. The first objective is to show the robustness of the derived expressions for the conserved linear and angular momenta with respect to bounded and discrete data sets. The second objective is to study the coupled dynamics of the vortex pair and a neutrally buoyant cylinder free to move in response to the fluid stresses exerted on its surface. A comparison of the vortex-body interaction with the case of a fixed circular cylinder evidences significant differences in the vortex dynamics. When the cylinder is fixed strong secondary vorticity is generated resulting in a repeating process between the primary vortex pair and the cylinder. In the neutrally buoyant cylinder case, a stable structure consisting of the primary vortex pair and secondary vorticity shear layers stays attached to the moving cylinder. In addition to these fundamental cases, the vortex-pair-cylinder interaction is studied for locomotion at constant speed and locomotion at constant thrust. It is shown that a similar vortex structure like in the neutrally buoyant cylinder case is obtained when the cylinder moves away from the approaching vortex pair at a constant speed smaller than the vortex pair translational velocity. Finally, the idealized symmetric settings are complemented by an asymmetric interaction of a vortex pair and a cylinder. This case is discussed for a fixed and a neutrally buoyant cylinder to show the validity of the derived relations for multi-dimensional body dynamics.

Pulsed-Jet Ejector Thrust Augumentor Characteristics

An experimental study is carried out to investigate the characteristics of a pulsed-jet ejector thrust augmentor that is suitable for use in vertical take-off and landing aircraft. Phase-locked particle image velocimetry is used to obtain the detailed spatio-temporal evolution of the ejector flowfield. Experiments were carried out to determine the conditions formaximum thrust augmentation. In the presence of the ejector, the pulsed-jet primary vortex induces a secondary vortex at the ejector wall. The peak vorticity magnitude of the primary vortex was found to be at a maximum at the condition of maximum thrust augmentation. The presence of this primary and secondary vortex pair results in enhancedmass entrainment andmixing, which, in turn, produces increased thrust augmentation. The nonmonotonic variation of the thrust augmentation ratio with the ejector area ratio (ejector inlet area/nozzle exit area) is largely determined by the peak vorticity magnitude of the primary-vortex ring and the secondary vortex.

A numerical evaluation on the effect of sister holes on film cooling effectiveness and the surrounding flow field

A numerical study was performed to evaluate the effectiveness of the novel sister hole film cooling technique. Two secondary coolant holes bound the primary coolant hole slightly downstream of its midpoint, intended to minimize the primary vortex pair and improve cooling performance. An unstructured hexahedral mesh was generated and the realizable k–e turbulence model with near-wall modeling was used in these simulations. Blowing ratios of 0.2, 0.5, 1.0, and 1.5 were simulated to evaluate the applicability of sister holes in practical applications. It was found that sister holes significantly improved cooling performance over the entire computational domain, particularly at high blowing ratios. These results arose by countering the primary vortex pair with a secondary pair from these sister holes, ultimately maintaining flow adhesion where the coolant stream would have otherwise separated.

A Numerical Study on Improving Large Angle Film Cooling Performance through the Use of Sister Holes

The present study evaluates a novel sister hole cooling technique using large inclination angle cylindrical holes simulated numerically. Here, a 55° inclination angle has been applied to simulate more realistic turbine conditions. Two sister holes bound the primary injection hole and are shifted slightly downstream to promote flow adhesion. As a means of determining the validity of the technique, adiabatic effectiveness and vortex flow structures were evaluated at four blowing ratios: 0.2, 0.5, 1.0, and 1.5. The results indicate that the sister hole technique dramatically reduces the primary kidney vortex pair offering significant improvements in effectiveness at all blowing ratios.