Vortex Roll-up Research Articles

The bypass transition mechanism of the Stokes boundary layer in the intermittently turbulent regime

This numerical study focuses on the coherent structures and bypass transition mechanism of the Stokes boundary layer in the intermittently turbulent regime. In particular, the initial disturbance is produced by a temporary roughness element that is removed immediately after triggering a two-dimensional vortex tube under an inflection-point instability. The present study reveals a complete scenario of self-induced motion of a vortex tube after rollup from the boundary layer. The trajectory of the vortex tube is reasonably described based on the Helmholtz point-vortex equation. The three-dimensional transition of the vortex tube is attributed to the Crow instability, which leads to a sinusoidal disturbance that eventually evolves into a ring-like structure, especially for the weaker vortex. Further investigation demonstrates that three-dimensional or quasi-three-dimensional vortex perturbations in the free stream play a critical role in the boundary layer transition through a bypass mechanism, which is featured by the non-modal and explosive transient growth of the subsequent boundary layer instabilities. This transition scenario is found to be analogous to the oblique transition in the steady boundary layer, both of which are characterised by the formation of streaks, rollup of hairpin-like vortices and burst into turbulent spots. In addition, the streamwise propagation of turbulent spots is discussed in detail. To shed more light on the nature of the intermittently turbulent Stokes boundary layer, a conceptual model is proposed for the periodically self-sustaining mechanism of the turbulent spots based on the present numerical results and experimental evidence reported in the literature.

Separated Flow Response to Rapid Flap Deflection

The transient response of a massively separated flow over an airfoil to rapid flap actuation is presented. A NACA 0006 airfoil is oriented at a fixed incidence of 20 deg for a Reynolds number of . The experiments are performed in a water tunnel with a wing spanning the width of the test section to produce a nominally two-dimensional flowfield. The airfoil is bisected about the midchord position, resulting in a 50%-chord trailing-edge flap. The flap is rapidly deflected in a smoothed-ramp profile over a range of deflection speeds and amplitudes. The flap maneuver is completed in a fraction of a single convective time. Focus is given to a deflection amplitude of 2 deg to minimize geometric deviation from the nondeflected configuration. The desired response to such a flap motion is the evocation of vortical transients conducive to lift enhancement. Through this study, two distinct transient responses are observed that are directionally dependent on flap actuation. In motions resulting in an increase in airfoil camber, the lift is increased instantaneously to modest values before relaxation to a separated steady state. In effect, this mode expedites convergence to the steady-state value of the final airfoil configuration and is devoid of the “antilift” spike associated with the discrete actuation of conventional fluidic actuators. In motions resulting in a decrease in airfoil camber, the lift profile is characterized by an initial reduction before a surge in lift, culminating in a global peak and followed by relaxation. Both deflection modes prove disruptive to the leading-edge shear-layer dynamics through trailing-edge actuation and are cause for rollup of a leading-edge vortex. Ridges of the finite-time Lyapunov exponent field are used to determine that the net decrease in camber motion induces significant entrainment near the trailing edge, leading to a smaller recirculation region and reattachment of the flow above the suction surface trailing-edge region. The net increase in camber motion does not generate this entrainment, and therefore yields a significantly larger recirculation region.

Computational Predictions of Velocity Ratio and Ejection Angle on Purge Flow in a Linear Turbine Cascade with Upstream Disturbance

Secondary air bled from the compressor which bypasses the combustion chamber is used to seal the turbine components from incoming hot gas. Interaction of this secondary air or purge flow with the mainstream can alter the flow characteristics of turbine blade passage. An in depth analysis of secondary loss generation by purge flow in the presence of upstream disturbances has huge relevance. The objective of present study is to understand the aerodynamic and thermal effects caused by the purge coolant flow in the presence of an upstream wake. A linear turbine cascade is selected for the computational study and a stationary cylindrical rod which resembles the trailing edge of nozzle guide vane is kept 20 mm before the leading edge to generate the upstream wake (or disturbance). Purge flow disturbances includes strong formation of Kelvin-Helmholtz vortices at trailing edge and additional roll-up vortices at leading edge. Detailed analysis is carried out by varying the velocity ratios as well as the ejection flow angle. Higher velocity ratio and perpendicular coolant ejection reduces the mainstream axial momentum which enhances the passage cross flow. Even though the mass averaged total pressure loss is linearly dependent on the velocity ratio, a reduction in the ejection angle brings down the loss coefficient at the blade exit. A lower ejection angle will improve the film cooling effectiveness also. The presence of purge flow causes an increase in the overturning and underturning.

Smoke streak visualization of steady flows over a spinning cone at angle of attack in flight

A smoke streak visualization technique is employed to observe the steady flow field over a cone of a half angle of $$\theta _c = 30^\circ $$, spinning at an angular speed of $$\varOmega = 5 \text { revs/s}$$, at angles of attack of $$0 \le \alpha \le 36^\circ $$, in an axial flow of $$U_\infty = 1 \text { m/s}$$. The Reynolds number is $${\sim }10^4$$ based on the generator of the cone. Streaklines highlight increasing asymmetry of the flow and formation of vortices at increasing angles of attack. The rotation direction of the cone shapes the streaklines ingested into the boundary layers. Good agreement in streakline patterns between potential flow theory and experiment is observed over the cone at $$\alpha = 0$$, the agreement getting better near the tip of the cone. No signs of boundary layer instabilities are noted as the Reynolds number is not sufficiently high, except at the highest angle of attack $$\alpha =36^\circ $$ where a wave pattern is observed on the rolled-up streamwise vortex. Streaklines over a $$60^\circ $$ cone at angle of attack $$\alpha $$ in an axial flow of 1 m/s.

Active flow control for power enhancement of vertical axis wind turbines: Leading-edge slot suction

Vertical axis wind turbines (VAWTs) suffer from a poor power performance at low tip speed ratios, where their blade aerodynamics are dominated by unsteady separation and dynamic stall. Therefore, to enhance their aerodynamic performance, separation control is highly desired. The present study intends to suppress the flow separation on VAWTs using boundary layer suction through a slot located near the blade leading edge. High-fidelity computational fluid dynamics simulations extensively validated with experiments are employed. A characterization of the impact of the suction amplitude, 0.5% ≤ AS ≤ 10%, and the suction location, 8.5 ≤ XS/c ≤ 28.5, is performed. The dependency of the obtained power gain on operating conditions, i.e. tip speed ratio, 2.5 ≤ λ ≤ 3.5, Reynolds number, 0.51 × 105 ≤ Rec ≤ 2.78 × 105, and turbulence intensity, 1% ≤ TI ≤ 25%, is studied. The results show that applying suction along the chordwise extent of the laminar separation bubble (LSB) can prevent its bursting, eliminate/postpone its formation, avoid the formation of the dynamic stall vortex and trailing-edge roll-up vortex, and delay the incipient trailing-edge separation. This will significantly increase the blade lift force, decrease the drag force, delay the stall angle and suppress the aerodynamic load fluctuations. For the reference turbine and for AS = 0.5% and XS/c = 8.5%, the power coefficient at λ of 2.5, 3.0 and 3.5 is enhanced by 247%, 83% and 24%, respectively. The suction location is critical while a minimum amplitude, e.g. AS = 0.5%, suffices. The optimal suction location is insensitive to TI, weakly sensitive to λ while comparatively more sensitive to Rec.

Flow pattern analysis in a rotating scraped surface plate heat exchanger

The flow pattern inside a scraped surface plate heat exchanger used for ice slurry production is studied both experimentally and numerically. Isothermal planar velocity fields are reported in the range 2.2 · 104⩽Rerot⩽1.7 · 105 using a phase averaged PIV technique. Unsteady RANS modelling of the flow, using the rotating frame methodology is used to obtain a time-averaged description of the flow structure, which was successfully validated against the PIV data. The flow structure is characterized by the presence of a roll-up vortex generated by the scraper blade, which is the main factor for both the radial and vertical displacements of the flow. Increasing the Rerot number leads to a more uniform flow, reducing the presence of stagnant zones. However, the main pattern structure does not change significantly when increasing the scraping velocity.

Experimental investigation of entropy waves generated from acoustically excited premixed swirling flame

Entropy waves play an important role in the production of indirect combustion noise and thermoacoustic instability. The characteristics of entropy waves from swirling flames have not been systematically investigated. Here, a premixed methane/air swirl burner was built with upstream acoustic excitation from a loudspeaker. A joint infrared imaging and tunable diode laser absorption spectroscopy (TDLAS) thermometry measurement was carried out to investigate the entropy waves generated in this burner. The infrared imaging technique provides qualitative images of the distribution, propagation and dissipation of entropy waves while the TDLAS technique provides quantitative measurement of temperature fluctuation. Time resolved measurements of the swirling flame bulk velocity, CH* chemiluminescence, gas temperature and infrared images were simultaneously obtained, demonstrating that entropy waves were generated from the premixed swirling flame under external acoustic excitation. Entropy waves were shown to be greatly influenced by the amplitude and frequency of acoustic waves. They also showed dissipation as the entropy waves propagate downstream according to the attenuated temperature fluctuation and infrared radiation intensities. Simultaneous high speed infrared imaging and particle image velocimetry measurements showed that the temperature non-uniformities arise from engulfment and mixing through periodic vortex roll-up.

Theoretical Prediction of Roll Moment Due to Sideslip for Thin Low-Aspect-Ratio Wings

The physics associated with leading-edge flow reattachment and tip vortex roll-up in crossflow are incorporated into a simple theory that enables the improved prediction of the lateral static stability derivative of thin low-aspect-ratio wings at low Reynolds numbers. The theory is validated against experimental stability derivative measurements of a total of 13 flat-plate wings. Accurate prediction of is obtained to lift coefficients as high as 0.7. Results indicate that the tip vortex, and its expedited roll-up over thin low-aspect-ratio wings at low Reynolds numbers, is destabilizing in terms of . In addition, leading-edge flow reattachment at low lift coefficients, whether in the form of a separation bubble for wings with unswept leading edges or an elongated separation region on wings with swept leading edges, may be stabilizing or destabilizing in terms of based on whether the wing has forward or backward quarter-chord sweep.

Turbulence-Model Comparison for Aerodynamic-Performance Prediction of a Typical Vertical-Axis Wind-Turbine Airfoil

In this work, different turbulence models were applied to predict the performance of a DU-06-W-200 airfoil, a typical choice for vertical-axis wind turbines (VAWT). A compromise between simulation time and results was sought, focusing on the prediction of aerodynamic forces and the developed flow field. Reynolds-averaged Navier–Stokes equation (U-RANS) models and Scale-Resolving Simulations (SRS), such as Scale-Adaptive Simulation (SAS) and Detached Eddy Simulation (DES), were tested, with k − ω -based turbulence models providing the most accurate predictions of aerodynamic forces. A deeper study of three representative angles of attack (5 ° , 15 ° , and 25 ° ) showed that U-RANS models accurately predict aerodynamic forces with low computational costs. SRS modeling generates more realistic flow patterns: roll-up vortices, vortex packets, and stall cells have been identified, providing a richer unsteady flow-field description. The power spectrum density of velocity at 15 ° has confirmed a broadband spectrum in DES simulations, with a small peak at a Strouhal number of 0.486. Finally, indications regarding the selection of the turbulence model depending on the desired outcome (aerodynamic forces, airfoil flow field, or VAWT simulation) are provided, tending toward U-RANS models for the prediction of aerodynamic forces, and SRS models for flow-field study.

Modified near-wakes of axisymmetric cylinders with slanted base

Near-wakes of axisymmetric cylinders with slanted base were investigated in wind tunnel experiments for an upsweep angle of 28°. Effects of splitter-plate, cavity, and flaps on the afterbody vortices and separated flow were studied by means of surface pressure and Particle Image Velocimetry measurements. The splitter plate causes more diffused afterbody vortices due to the turbulence ingestion from the separation region. When the slanted base is replaced with a deep cavity, there is weaker roll-up of vorticity due to the lack of streamwise flow and a solid surface. Varying the splitter plate length in the range tested did not have significant influence on the flowfield, apart from affecting the strength of the splitter-plate vortex. With the splitter plate and cavity, unsteadiness is dominated by the flow separation region, as opposed to the afterbody vortices in the baseline case. A pair of vertical flaps attached to the side-edges of the splitter plate can reduce the unsteadiness at the measurement plane immediately downstream of the splitter plate.

Vortex pairing in jets as a global Floquet instability: modal and transient dynamics

The spontaneous pairing of rolled-up vortices in a laminar jet is investigated as a global secondary instability of a time-periodic spatially developing vortex street. The growth of subharmonic perturbations, associated with vortex pairing, is analysed both in terms of modal Floquet instability and in terms of transient growth dynamics. The article has the double objective to outline a toolset for the global analysis of time-periodic flows, and to leverage such an analysis for a fresh view on the vortex pairing phenomenon. Axisymmetric direct numerical simulations (DNS) of jets with single-frequency inflow forcing are performed, in order to identify combinations of the Reynolds and Strouhal numbers for which vortex pairing is naturally observed. The same DNS calculations are then repeated with an added time-delay control term, which artificially suppresses pairing, so as to obtain time-periodic unpaired base flows for linear stability analysis. It is demonstrated that the natural occurrence of vortex pairing in nonlinear DNS coincides with a linear subharmonic Floquet instability of the underlying unpaired vortex street. However, DNS results suggest that the onset of pairing involves much stronger temporal growth of subharmonic perturbations than that predicted by modal Floquet analysis, as well as a spatial distribution of these fast-growing perturbation structures that is inconsistent with the unstable Floquet mode. Singular value decomposition of the phase-shift operator (the operator that maps a given perturbation field to its state one flow period later) is performed for an analysis of optimal transient growth in the vortex street. Non-modal mechanisms near the jet inlet are thus found to provide a fast route towards the limit-cycle regime of established vortex pairing, in good agreement with DNS observations. It is concluded that modal Floquet analysis accurately predicts the parameter regime where sustained vortex pairing occurs, but that the bifurcation scenario under typical conditions is dominated by transient growth phenomena.

Aerodynamic factors affecting rebreathing in infants.

The rebreathing of expire air, with high carbon dioxide and low oxygen concentrations, has long been implicated in unexplained Sudden Infant Death Syndrome (SIDS) when infants are placed to sleep in a prone (facedown) position. This study elucidates the effect of the aerodynamic parameters Reynolds number, Strouhal number, and Froude number on the percentage of expired air that is reinspired (rebreathed). A nasal module was designed that served as a simplified geometric representation of infant nostrils and placed above a hard, flat surface. Quantitative and flow visualization experiments were performed to measure rebreathing, using water as the working medium, under conditions of dynamic similarity. Different anatomic (e.g., tidal volume, nostril diameter), physiological (e.g., breathing frequency), and environmental (e.g., temperature, distance from the surface) factors were considered. Increases in Strouhal number (simultaneously faster and shallower breathing) always produced higher rebreathed percentages, because rolled-up vortices in the vicinity of the nostrils had less time to move away by self-induction. Positively and negatively buoyant flows resulted in significant rebreathing. In the latter case, consistent with a warm environment and a high percentage of rebreathed CO2, denser gas pooled in the vicinity of the nostrils. Reynolds numbers below 200 also dramatically increased rebreathing because the expired gas pooled much closer to the nostrils. These results clearly elucidated how the prone position dramatically increases rebreathing by a number of different mechanisms. Furthermore, the results offer plausible explanations of why a high-temperature environment and low birthweight are SIDS risk factors. NEW & NOTEWORTHY A fundamentally new aerodynamics-based approach to the study of rebreathing of expired air in infants is presented. Rebreathing is implicated in unexplained Sudden Infant Death Syndrome (SIDS) when infants sleep in a prone position. This is the first time that aerodynamic parameters are systematically varied and their effects on rebreathing quantified. The study provides us with a deeper understanding of the effects of breathing frequency, tidal volume (birthweight) and environmental conditions.

On Dynamics of Acoustically Driven Bistable Fluidic Valves

The dynamics of an actively controlled fluidic diverter with novel actuation method are presented. This bistable fluidic valve is based on the Coanda effect and is able to switch the main flow solely by means of acoustic excitation. The switching is explained through a combination of experiments and large eddy simulations (LES). The switching time and minimum energy required are characterized for a range of pressure ratios, acoustic excitation frequencies, and input powers. It is shown that the switching mechanism depends on the excitation of natural instabilities inside the free shear layer. An enhanced roll-up of vortices at the excitation frequency increases the rate of entrainment and results in a transverse pressure gradient sufficient to counteract the Coanda effect leading to jet detachment and switching.

Experimental investigation of flow development and coherent structures in normal and oblique impinging slot jets

The development of normal and oblique impinging slot jets is investigated experimentally using planar particle image velocimetry. The study is performed for two jet Reynolds numbers, $${Re} = 3000$$ and 6000, and four jet orientation angles relative to the wall ( $$\theta = 90^\circ , 60^\circ , 45^\circ , 30^\circ$$ ), with the nozzle-to-plate spacing fixed at four slot widths. Within the range of impingement angles considered, the flow is characterized by a stagnation region, followed by a region of flow reorientation into a wall jet. The development of the wall jet downstream of the impingement region is shown to be closely related to the evolution of coherent structures forming due to the Kelvin–Helmholtz instability. For normal jet impingement at $${Re} = 3000$$ , these shear layer rollers remain coherent past the reorientation region and induce the formation and shedding of wall-bounded vorticity; the shed vorticity pairs with the primary shear layer vortices and ejects from the wall, resulting in deflection of the mean wall jet from the surface. Both the primary and the induced structures break down farther downstream, marking final stages of transition to turbulence. For $${Re} = 6000$$ , breakdown of the shear layer vortices occurs in the reorientation region, leading to earlier transition into a turbulent wall jet. Consequently, wall-bounded vorticity roll-up and ejection are less significant, and the deflection of the wall jet away from the surface is reduced. The analysis presented quantitatively relates the development of coherent structures to salient changes in the time-averaged flow statistics.

Numerical Research on the Vortex Center on the Forward-Swept 3-D Wind Turbine Blades at Low Rotational Speed

This paper studies the CFD simulation of forward three-dimensional (3-D) horizontal axis wind turbine (HAWT) blades. Using logarithmic grid and Q-criterion to learn the vortex dynamics around the blades at low rotational speed. The computational fluid dynamics (CFD) simulation uses Q-criterion to probe vortices and logarithmic grid to emphasize the micro-gridding effect of the turbulent boundary layer. The visualization & measurement of the simulation results give the coefficient of pressure (Cp). For forward 3-D wind turbine blade, at low rotational speed, the strongly accelerated laminar region surrounds the lower blade, and the decelerated tip blade region coalesce each other give rise to a reverse limiting streamline, eroding the laminar region further until a little is left on the tip of the blade. The "reverse limiting streamline" grows inward radially, the area is narrowing closing to the leading edge of the blade tip. The second side of the rolled-up vortex appears the velocity ratio (Uc/Ulocal) of the second vortices are higher than the main vortex cores. For radius R=1.547 m, U=12 m/s, at 210 RPM, CL and CD values reach a maximum with fully laminar tip conditions. While at 120 RPM, the CL and CD values reach a minimum in the absence of laminar tips. The results show the detailed vortex dynamic pattern surround the blades, give more understanding to design laminar 3-D blade toward a noiseless wind turbine system.

Effects of Sidewalls and Leading-Edge Blowing on Flows over Long Rectangular Cavities

The present work investigates sidewall effects on the characteristics of three-dimensional (3-D) compressible flows over a rectangular cavity with aspect ratios of and at using large-eddy simulations. For the spanwise-periodic cavity flow, large pressure fluctuations are present in the shear layer and on the cavity aft wall due to spanwise vortex roll-ups and flow impingement. For the finite-span cavity with sidewalls, pressure fluctuations are reduced due to interference to the vortex roll-ups from the sidewalls. Flow oscillations are also reduced by increasing the Mach number from 0.6 to 1.4. Furthermore, secondary flow inside the cavity enhances kinetic energy transport in the spanwise direction. Moreover, 3-D slotted jets are placed along the cavity leading edge with the objective of reducing flow oscillations. Steady blowing into the boundary layer is considered with momentum coefficient and 0.0194 for and 1.4 cases, respectively. The three-dimensionality introduced to the flow by the jets inhibits large coherent roll-ups of the spanwise vortices in the shear layer, yielding 9–40% reductions in root mean square (rms) pressure and rms velocity for both spanwise-periodic and finite-span cavities.

Closed-loop control of a free shear flow: a framework using the parabolized stability equations

In this study the parabolized stability equations (PSE) are used to build reduced-order-models (ROMs) given in terms of frequency and time-domain transfer functions (TFs) for application in closed-loop control. The control law is defined in two steps; first it is necessary to estimate the open-loop behaviour of the system from measurements, and subsequently the response of the flow to an actuation signal is determined. The theoretically derived PSE TFs are used to account for both of these effects. Besides its capability to derive simplified models of the flow dynamics, we explore the use of the TFs to provide an a priori determination of adequate positions for efficiently forcing along the direction transverse to the mean flow. The PSE TFs are also used to account for the relative position between sensors and actuators which defines two schemes, feedback and feedforward, the former presenting a lower effectiveness. Differences are understood in terms of the evaluation of the causality of the resulting gain, which is made without the need to perform computationally demanding simulations for each configuration. The ROMs are applied to a direct numerical simulation of a convectively unstable 2D mixing layer. The derived feedforward control law is shown to lead to a reduction in the mean square values of the objective fluctuation of more than one order of magnitude, at the output position, in the nonlinear simulation, which is accompanied by a significant delay in the vortex pairing and roll-up. A study of the robustness of the control law demonstrates that it is fairly insensitive to the amplitude of inflow perturbations and model uncertainties given in terms of Reynolds number variations.

A comparative three-dimensional study of impulsive flow emanating from a shock tube for shock Mach number 1.6

A three-dimensional transient impulsive flow emanating from an open end of a 165-mm driver section shock tube is investigated numerically in the present study for a shock Mach number of 1.6. Here, the main objective was to find out an appropriate model to simulate the early evolution of the high Mach number transient flows by comparing the flow field obtained from two numerical models with both qualitative and quantitative experimental observations. The 3-D numerical simulations were performed by solving RANS equations using the shear stress transport K-ω model and large eddy simulation of the flow with the help of ANSYS CFX software. The experiments were performed with the simultaneous 2-D and 3-D particle image velocimetry systems and high-resolution smoke flow visualizations. The 2-D PIV results were used for comparing the numerical results obtained on a plane, and the 3-D PIV results were used to compare the azimuthal variations across the vortex ring. It is observed that the velocity field of the transient jet, Kelvin–Helmholtz (K–H) vortices at the trailing jet and the interaction of K–H vortices with the primary vortex ring were resolved well in LES quite similar to the experiments. However, the SST K-ω model resolved only the velocity field of the transient jet in the axial region similar to the experiments and dissipated the K–H vortices at the jet boundary. Though both models resolved the shear layer originated from the triple point, they did not predict the rollup and formation of vortices and their interactions observed in a 24 MP camera. This demands the use of a higher spatial resolution. It is also noticed that a substantial adverse pressure gradient experienced by the vortex ring and trailing jet during the early stage of evolution due to the presence of precursor/incident shock was responsible for the pairing up and merging of the shear layer vortices to form a stronger counter-rotating vortex ring. This adverse gradient also plays a dominant role in the azimuthal expansion (rapid increase in diameter) of the vortex ring.

Effect of inlet flow turbulence on the combustion instability in a premixed backward-facing step combustor

This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH* chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re, the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re, the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics.

Simulation of turbulent flow in a rapid compression machine: Large Eddy Simulation and computationally efficient alternatives for the design of ignition delay time experiments

Rapid compression machines (RCMs) are widely used by the fuel research community to provide engine-relevant conditions to study ignition delay time (IDT), which is a crucial target for validating chemical kinetic mechanisms for fuels. Creviced piston heads are routinely used to ensure temperature homogeneity within the combustion chambers of RCMs. However, due to the exponential dependence of kinetic rate coefficients on temperature, homogeneity of the temperature field is vital for a clear interpretation of results and identification of chemical kinetic mechanisms. The overall aims of this work are to (1) support operators of RCMs in ensuring that their devices achieve sufficient levels of in-chamber temperature homogeneity and (2) validate performance of a previously-developed correlation for temperature inhomogeneity in RCMs at different conditions. Large Eddy Simulation (LES) is conducted to resolve 3D unsteady structure of turbulent flow, throughout compression and long post-compression times, at two representative operating conditions in the NUI Galway RCM. Results show that at higher pressures, 3D LES temperature and velocity fields are well approximated by a previous 2D laminar model. At low pressures, 2D laminar and 3D LES agree well for flow during compression, but predict different evolution of roll-up vortices and temperature distribution after compression. However, across all conditions there is a satisfactory agreement between 2D laminar and 3D LES results for global temperature inhomogeneity. Moreover, the previously developed correlation (based on 2D laminar simulations) for temperature inhomogeneity is found to agree with 3D LES to within ±20%. Therefore, the correlation may be used for preliminary design and evaluation of RCMs and RCM experiments. We propose a framework for design of RCM experiments based on the use of correlations, supported by 2D laminar simulations, and finally 3D LES of selected cases for confirmation.